LOW Z-FORCE OCTASPOT SWEPT FRICTION STIR SPOT WELDS WELDING CONVENTIONAL TOOL AND PROCESS DEVELOPMENT APPROACH. A Thesis by.

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1 LOW Z-FORCE OCTASPOT SWEPT FRICTION STIR SPOT WELDS WELDING CONVENTIONAL TOOL AND PROCESS DEVELOPMENT APPROACH A Thesis by Tze Jian Lam B.S.M.E., Wichita State University Submitted to the Department of Mechanical Engineering and the faculty of Graduate School of Wichita State University in partial fulfillment of the requirements of the degree of Master of Science May 2010

2 Copyright 2010 by Tze Jian Lam All Rights Reserved

3 LOW Z-FORCE OCTASPOT SWEPT FRICTION STIR SPOT WELDS WELDING CONVENTIONAL TOOL AND PROCESS DEVELOPMENT APPROACH The following faculty members have examined the final copy of this thesis for form and content, and recommend that it be accepted in partial fulfillment of the requirement for the degree of Master of Science with a major in Mechanical Engineering. George E. Talia, Committee Chair Dwight A. Burford, Committee Member Brian Driessen, Committee Member iii

4 DEDICATION To my parents, my sister, my brothers, my relatives, and my friends iv

5 ACKNOWLEDGMENTS As a graduate research assistant in the Advanced Joining and Processing Laboratory (AJ&PL) of the National Institute for Aviation Research at Wichita State University, I would like to thank Dr. Dwight Burford, Director of AJ&PL, for giving me the opportunity and support to lead the project of Low Z-Force Octaspot Swept Friction Stir Spot Welds Welding Conventional Tool and Process Development Approach (CFSP07-WSU-03). This project was funded by the National Science Foundation s (NSF) Center for Friction Stir Processing (CFSP), which is part of the Industry University Cooperative Research Center (IUCRC) program. This project work is also my thesis, as part of the requirements for completing my Master of Science degree in Mechanical Engineering at Wichita State University. I would like to thank my advisor and committee chair, Dr. George Talia, for his guidance, and principal investigator and committee members, Dr. Dwight Burford and Dr. Brian Driessen, as well as Dr. Christian Widener for their efforts and help with this thesis. Also, I would like to recognize the hard work of students in NIAR s AJ&PL, especially James Gross, who developed much of the early low Z-force welding program. I would like to thank Kristie Bixby for her editorial efforts with this thesis. I thank the Graduate School for supporting me financially throughout my Master s degree. And I also thank my parents and family members for their encouragement in my studies. v

6 ABSTRACT An investigation was conducted to develop low Z-force (normal/forge load) friction stir spot welds (FSSWs) using conventional tooling and process development approaches. Low Z- forces can be achieved by studying the relationship between pin tool features, geometries, processing parameters, and resultant strength of coupons produced by friction stir spot welding (FSSW). The effects of geometrical and feature changes of pin tool designs including shoulder diameters, shoulder features, probe diameters, probe shapes, and probe features on the joint properties of inch-thick bare 2024-T3 aluminum alloy were evaluated. Welding tools included Psi, Counterflow, Modified Trivex, and V-flute pin tools. A Box-Behnken design of experiments (DOE) approach was used to investigate the effects of three process parameters: spindle speed, Z-force (forge load), and travel speed. The goal of the investigation was to maintain the ultimate tensile load (UTL) in unguided lap shear coupons tested in tension while reducing the Z-force required for producing a sound joint. This goal was achieved on a specially built MTS Systems Corporation ISTIR PDS FSW gantry system. In addition to singlespot unguided lap shear tests, the performance of low Z-force FSSW joints was evaluated by optical metallographic cross-section analyses, which were then correlated with process parameters, UTL, and pin tool designs. The maximum Z-force spikes encountered during the initial plunge were reduced by an order of magnitude, and the Z-force processing loads were reduced by half for Octaspot swept FSSW, most effectively by controlling the plunge rate under force control. Additional reductions in Z-force were achieved by refining the conventional FSSW tool shoulder and probe designs. Therefore, it was demonstrated that weld forces can be reduced to the point where it would be feasible to perform robotic low Z-force FSSW for at least some applications. vi

7 TABLE OF CONTENTS Chapter Page 1. INTRODUCTION LITERATURE REVIEW FSSW Process Controls Development of Process Parameters Tool Geometry Variation of FSSW Material Flow OBJECTIVE TEST PROCEDURE Pin Tool Designs Additional Pin Tool Designs Material Preparation Weld Setup Weld Programs Mechanical Properties Testing RESULTS AND DISCUSSIONS Achieving Low Z-Force Concave Shoulder Tool Study (Phase 1) Concave Shoulder Diameter Study Psi Tool (0.30 Inch and 0.40 Inch) Counterflow Tool (0.30 Inch and 0.40 Inch) Probe Design Study with 0.30-Inch-Diameter Concave Shoulder Modified Trivex Tool Duo V-Flute Tool Tri V-Flute Tool Achievement in Concave Shoulder Study (Phase 1) Concave Shoulder Diameter Study Probe Design Study Optimization Weld Parameters (DOE 2) Surface Preparation Surface Finish...55 vii

8 TABLE OF CONTENTS (continued) Chapter Page 5.9 Scroll Shoulder Tool (0.30 Inch) Study (Phase 2) Achievement Duo V-Flute Scroll Featureless Probe Shape Study (Phase 3) Featureless Trivex Featureless Pentagon Featureless Octagon Achievement in Featureless Probe Shape Study (Phase 3) Probe Diameter Study (Phase 4) CONCLUSIONS AND FUTURE WORK...74 REFERENCES...78 APPENDICES...83 A. Detailed Calculation for Table B. Duration of Octaspot Swept FSSW...85 C. UTL Results...88 viii

9 LIST OF TABLES Table Page 1. Ratio of Probe Physical Unit Volume to probe Swept Unit Volume Pin Tool Matrix Average UTL and Corresponding Z-Force Applied Using Concave Shoulder Psi Tool Average UTL and Corresponding Z-Forces Applied using Concave Shoulder Counterflow Tool Compilation of DOE 1 UTL Results for Probe Design Study of 0.30-Inch-Diameter Concave Shoulder Compilation of DOE 2 UTL Results for Probe Design Study of 0.30-Inch-Diameter Concave Shoulder Hooking Defect of Featureless Trivex Pin Tool Hooking Defect of Featureless Pentagon Pin Tool Hooking Defect of Featureless Octagon Pin Tool Summary of Hooking Defect and Ratio of Probe Physical to Swept Unit Volume Weld Radius Compensation for Probe Radius Reduction Average UTL and Standard Deviation of DOE 1 for Probe Diameter Study Z-Force Reduction and Corresponding Pin Tools and Weld Parameters...73 ix

10 LIST OF FIGURES Figure Page 1. Friction Stir Welding (FSW) Process (courtesy of TWI) Friction Stir Spot Welding (FSSW) Process (courtesy of Kawasaki) Typical FSW Butt Joint with Fixed Pin Tool Typical FSW Lap Joint with Fixed Pin Tool Schematic Representation of Pin Tools MTS System Corp. ISTIR PDS Five-Axis FSW Machine at AJ&PL NIAR WSU ABB IRB 7600 Six-Axis Articulated Robot at AJ&PL NIAR WSU Schematic Diagram of Process Controls of Octaspot FSSW Different Probe Shapes with Same Effective Swept Area Octaspot Travel Path Schematic Cross-Sectional Representation of Plunge and Swept FSSW Flat Scrolls Shoulder on Duo V-Flute Pin Tool: (a) 0.40-Inch Diameter and (b) 0.30-Inch Diameter Wiper Shoulder on Duo V-Flute Pin Tool: (a) 0.40-Inch Diameter and (b) 0.30-Inch Diameter Pin Tools with Five-Degree Concave Shoulder Inch-Diameter Probe Shapes: (a) Concave Shoulder Trivex, (b) Pentagon, and (c) Octagon Reduced Shoulder and Probe Diameter Sizes of Duo V-Flute Single-Spot Unguided Lap Shear Specimen Experimental Weld Setup Worm Hole Defect in Octaspot FSSW...27 x

11 LIST OF FIGURES (continued) Figure Page 20. Kissing Bond Defect in Plunge FSSW Sheet Lifting (left) and Hooking (right) in Lap FSW Command and Feedback Plot for Typical Octaspot FSSW (Hybrid Weld Program) Command and Feedback Plot of 0.40-Inch-Diameter Psi Tool Welded with Position Control Command and Feedback Plot of 0.30-Inch-Diameter Psi Tool Welded with Position Control Command and Feedback Plot for Low Z-Force Swept FSSW Low Z-Force Cross-Sectional Metallographic (1.2X) Joint Interface of Figure 26 (100X): (a) Left Side and (b) Right Side Command and Feedback Plot for Low Z-Force Swept FSSW Main Effects Plot of 0.30-Inch-Diameter Concave Shoulder Psi Tool Main Effects Plot of 0.40-Inch-Diameter Concave Shoulder Psi Tool Low Z-Force Swept FSSW with 0.30-Inch-Diameter Psi Tool at 1,100 lbf Low Z-Force Swept FSSW with 0.30-Inch-Diameter Psi Tool at 900 lbf Low Z-Force Swept FSSW with 0.30-Inch-Diameter Psi Tool at 700 lbf Low Z-Force Swept FSSW with 0.40-Inch-Diameter Psi Tool at 1,100 lbf Low Z-Force Swept FSSW with 0.40-Inch-Diameter Psi Tool at 900 lbf Low Z-Force Swept FSSW with 0.40-Inch-Diameter Psi Tool at 700 lbf Main Effects Plot of 0.30-Inch-Diameter Concave Shoulder Counterflow Tool Main Effects Plot of 0.40-Inch-Diameter Concave Shoulder Counterflow Tool...38 xi

12 LIST OF FIGURES (continued) Figure Page 39. Low Z-Force Swept FSSW with 0.30-Inch-Diameter Counterflow Tool at 1,100 lbf Low Z-Force Swept FSSW with 0.30-Inch-Diameter Counterflow Tool at 900 lbf Low Z-Force Swept FSSW with 0.30-Inch-Diameter Counterflow Tool at 700 lbf Low Z-Force Swept FSSW with 0.40-Inch-Diameter Counterflow Tool at 1,100 lbf Low Z-force Swept FSSW with 0.40-Inch-Diameter Counterflow Tool at 900 lbf Low Z-Force Swept FSSW with 0.30-Inch-Diameter Modified Trivex Tool at 1,100 lbf Low Z-Force Swept FSSW with 0.30-Inch-Diameter Modified Trivex Tool at 900 lbf Low Z-Force Swept FSSW with 0.30-Inch-Diameter Modified Trivex Tool at 700 lbf Joint Interface of Figure 46 (100X): (a) Right Side and (b) Left Side Plug Pull-Out Failure Mode Low Z-Force Swept FSSW with 0.30-Inch-Diameter Duo V-Flute Tool at 1,100 lbf Low Z-Force Swept FSSW with 0.30-Inch-Diameter Duo V-Flute Tool at 900 lbf Low Z-Force Swept FSSW with 0.30-Inch-Diameter Duo V-Flute Tool at 700 lbf Joint Interface of Figure 51 (100X): (a) Right Side and (b) Left Side Low Z-Force Swept FSSW with 0.30-Inch-Diameter Tri V-flute Tool at 1,100 lbf...46 xii

13 LIST OF FIGURES (continued) Figure Page 54. Low Z-Force Swept FSSW with 0.30-Inch-Diameter Tri V-flute Tool at 900 lbf Low Z-Force Swept FSSW with 0.30-Inch-Diameter Tri V-flute Tool at 700 lbf Joint Interface of Figure 55 (100X): (a) Right Side and (b) Left Side Low Z-Force Swept FSSW with 0.30-Inch-Diameter Counterflow Tool at 700 lbf Low Z-Force Swept FSSW with 0.30-Inch-Diameter Psi Tool at 700 lbf Low Z-Force Swept FSSW with 0.30-Inch-Diameter Modified Trivex Tool at 700 lbf Low Z-Force Swept FSSW with 0.30-Inch-Diameter Duo V-Flute Tool at 700 lbf Low Z-Force Swept FSSW with 0.30-Inch-Diameter Tri V-Flute Tool at 700 lbf UTL Results Comparison of Low Z-Force Octaspot Swept FSSW for Five Pin Tools with 0.30-Inch-Diameter Concave Shoulder in DOE UTL Results Comparison of Low Z-Force Octaspot Swept FSSW for Five Pin Tools with 0.30-Inch-Diameter Concave Shoulder in DOE 1 and DOE UTL Results Comparison of Low Z-Force Octaspot Swept FSSW for Four Pin Tools with No Surface Preparation Low Z-Force FSSW with 0.30-Inch-Diameter Concave Shoulder with Half-Degree of Tilt Angle Low Z-Force FSSW with 0.30-Inch-Diameter Concave Shoulder with One-Degree of Tilt Angle Low Z-Force FSSW with 0.30-Inch-Diameter Flat Scrolls Shoulder with Half-Degree of Tilt Angle...56 xiii

14 LIST OF FIGURES (continued) Figure Page 68. UTL Results Comparison of Low Z-Force Octaspot Swept FSSW for 0.30-Inch-Diameter Scroll Shoulder Duo V-flute in DOE Featureless Trivex Cross-Sectional Metallographic M Right Side of Figure 69 with Inch Hooking Defect Left Side of Figure 69 with Inch Hooking Defect Featureless Pentagon Cross-Sectional Metallographic M Right Side of Figure 72 with Inch Hooking Defect Left Side of Figure 72 with Inch Hooking Defect Featureless Octagon Cross-Sectional Metallographic M Right Side of Figure 75 with Inch Hooking Defect Left Side of Figure 75 with Inch Hooking Defect Metallographic Image of CFSP09307_6_M Metallographic Image of CFSP09307_6_M Metallographic Image of CFSP09307_6_M Metallographic Image of CFSP09307_6_M Metallographic Image of CFSP09307_12_M Right Side of Nugget in Figure xiv

15 LIST OF ABBREVIATIONS/NOMENCLATURES AJ&PL CFSP CNC DFT DOE FSP FSW FSSW GKSS HAZ HCl HF HNO 3 HRS IRB ISTIR IUCRC LOP NIAR NSF PDS Advanced Joining and Processing Laboratory Center for Friction Stir Processing Computer Numerically Controlled Discrete Fourier Transformation Design of Experiment Friction Stir Processing Friction Stir Welding/Weld Friction Stir Spot Welding/Weld Gesellschaft zur Förderung der Kernenergie in Schiffbau und Schiffstechnik (German: Society for the Promotion of the Nuclear Energy in Shipbuilding and Naval Technology) Heat-Affected Zone Hydrochloric Acid Hydrofluoric Acid Nitric Acid High Rotational Speed Industrial Robot Intelligent Friction Stir Welding for Research and Production Industrial University Cooperative Research Center Lack of Penetration National Institute for Aviation Research National Science Foundation Process Development System xv

16 LIST OF ABBREVIATIONS/NOMENCLATURES (continued) PFSW RPT SEM TMAZ TWI UTL Plunge Friction Spot Welding/Weld Retractable Pin Tool Scanning Electron Microscope Thermomechanically Affected Zone The Welding Institute Ultimate Tensile Load xvi

17 CHAPTER 1 INTRODUCTION Friction stir welding (FSW) was patented by The Welding Institute (TWI) in England in 1991 [1]. FSW is a solid-state joining technology, which differs from conventional fusion welding in that the joining process occurs below the melting temperature of the welded material [2,5]. This new joining process is especially beneficial on materials such as 2XXX and 7XXX series aluminum alloys, which are relatively difficult to join by conventional fusion welding. The use of aluminum alloys in automotive and aerospace industries gained popularity because of their high strength-to-weight ratio, resistance to corrosion, energy savings, etc. [3,4]. In recent years, research and development of FSW technology has made significant progress toward understanding the fundamentals of this joining technology [5]. The FSW process consists of four stages: rotate, plunge, translate, and retract. FSW was introduced as a linear weld with a non-consumable pin tool, which rotates about its own axis, plunges into a weld specimen to a specified depth, translates in a linear or curvilinear path along the joint line, and retracts at the end of weld path (Figure 1). With this process, welding can occur in a butt or lap joint configuration. One of FSW s main variants is friction stir spot welding (FSSW), which is similar to FSW only without the translation of a pin tool. FSSW is mainly applied in lap joint configurations with only three stages: rotate, plunge, and retract (Figure 2). The simplest form of FSSW, called poke or plunge FSSW, was patented by Mazda in 2003 [6] as plunge friction spot welding (PFSW) [20]. Other variants of FSSW are Squircle [7], Octaspot [25-28, 30-33], Stitch-FSW [5] or Stitch-FSSW from Gesellschaft zur Förderung der Kernenergie in Schiffbau und Schiffstechnik (GKSS) [4,8,9], and swing-fsw [5] or swing- FSSW from Hitachi [4,10,11,12], which increases the joint shear area. Another variant of FSSW 1

18 relates to the exit hole that is left when the pin tool retracts; thus, a process called refill FSSW solves the issue by refilling the exit hole. The process of refill FSSW has been patented in Japan [13] and in the United States [24]. Another variant of FSW, friction stir processing (FSP), was developed to exploit the benefit of the FSW process to change the microstructure of cast materials to a void-free and fully recrystallized fine grain microstructure found in the weld nugget of FSW [2,5,14]. Figure 1. Friction Stir Welding (FSW) Process (courtesy of TWI). 2

19 (a) Rotate (b) Plunge (c) Retract Figure 2. Friction Stir Spot Welding (FSSW) Process (courtesy of Kawasaki). The microstructures of FSW and FSSW weld zones use the same terms: weld nugget, thermomechanically affected zone (TMAZ), heat-affected zone (HAZ), and unaffected zone or parent material (Figure 3). The weld nugget, also called the stir zone, is the zone that the probe has occupied and significantly processed, producing a fine, fully recrystallized grain structure. The TMAZ is the zone that receives some limited plastic deformation and is significantly affected by the thermal cycle of the process, while the HAZ experiences a thermal cycle that is only significant enough to change the properties and microstructure of the material. Finally, the unaffected zone experiences a minimal thermal cycle, which is not significant enough to change the microstructure or mechanical properties [2]. Also, a small amount of asymmetry occurs transverse to the weld direction. The advancing side of the weld panel (left side of Figure 1) occurs when the tool rotation direction is the same as the tool travel direction, whereas, the retreating side of weld panel (right side of Figure 1) is found on the side where the tool rotation direction is opposite the tool travel direction. The advancing side of a transverse metallographic sample is shown in Figure 3. The right side of this figure has a clear distinctive line between the TMAZ and HAZ, but on the retreating side, there is no such clearly discernible line between the TMAZ and HAZ. The weld 3

20 nugget properties, such as fatigue, deformation, and tensile load, are generally superior to the surrounding parent material due to the nugget s fine grain microstructure [2]. In a typical FSW lap joint configuration, the weld zones mentioned above can also be observed, as shown in Figure 4. Retreating Side Parent Material HAZ TMAZ Nugget TMAZ HAZ Advancing Side Parent Material Figure 3. Typical FSW Butt Joint with Fixed Pin Tool. Retreating Side Parent Material HAZ TMAZ Nugget TMAZ HAZ Advancing Side Parent Material Figure 4. Typical FSW Lap Joint with Fixed Pin Tool. Conventional FSW tools are non-consumable pin tools, which consist of a body, a shoulder, and a probe or pin. These tools are also known as fixed-pin tools, where the length of the probe is fixed (Figure 5a). Bobbin tools, also known as self-reacting pin tools, consist of three parts: an upper shoulder, a probe, and a lower shoulder (Figure 5c). Self-reacting pin tools eliminate the potential for lack of penetration (LOP) in the weld and apply minimal net force normal to the part assembly, since the down force of the upper shoulder is opposed by the upward force of the lower shoulder. Similarly, FSSW typically uses fixed pin tools but also uses refill or retractable FSSW pin tools, which consist of an independently moveable probe and shoulder with an optional containment ring (Figure 5b). The probe of FSW or FSSW tools typically consists of different features such as threads, flutes, and/or flats, which help to channel the flow of material. In order to promote material movement, the shape of the probe can be in the 4

21 form of a circle, triangle, square, pentagon, etc.. The shoulder captures material displaced by the probe and exerts a forging force (normal load) to consolidate the material. The body of the pin tool is inserted into the pin tool holder, which is attached to the forge spindle of the FSW machine. The probe of both retractable and self-reacting pin tools is attached to an independent pin axis in an FSW machine in order to control pin force and pin position separately from the forge axis. Body Body Containment Ring Upper Shoulde Shoulder Probe Probe Lower Shoulde (a) Fixed Pin Tool (b) Retractable Pin Tool (c) Self-Reacting Pin Tool Figure 5. Schematic Representation of Pin Tools. Applications and designs lead to various definitions of pin tools such as fixed pin tool or conventional pin tool, retractable pin tool or refill pin tool (RPT), and self-reacting pin tool or bobbin pin tool. A fixed pin tool is where the probe and shoulder do not move relative to each other (Figure 5a), whereas in a retractable pin tool, the probe and shoulder can move relative to each other along the axis of tool rotation (Figure 5b) [15]. A fixed pin tool leaves an exit hole at the end of the weld, whereas a retractable pin tool is designed not to produce an exit hole. The relative motion of the probe and shoulder in an RPT tool set enables it to refill the exit hole. A self-reacting pin tool has an additional lower shoulder attached to the probe, and both the upper shoulder and lower shoulder create a nominally zero net force while clamping the weld material 5

22 to keep it from escaping from the joining region (Figure 5c) [17]. The design of a self-reacting pin tool requires no backing anvil, eliminates the lack of penetration defect, and increases the travel speed due to heating from both shoulders [16,17]. FSW machines are usually gantries for stiffness, with three to five-axes of motion for two- or three-dimensional welding, position and load control capability, and intelligence and sensing capability. The multi-purpose gantry FSW machine used in this study was an MTS Systems Corporation s ISTIR Process Development System (PDS) (Figure 6), which is capable of a wide range of process development parameters such as high Z-force (normal load) up to 20 kip with the stiffness of the gantry system. Figure 6. MTS System Corp. ISTIR PDS Five-Axis FSW Machine at AJ&PL NIAR WSU. This FSW machine can be programmed using position control, load control, or a combination of both. The intelligence and sensing capability enables the capture of data on weld parameters and feedback forces that can be analyzed to ensure weld quality. Articulated-arm robots equipped for 6

23 FSSW, such as the ABB IRB 7600 (Figure 7), is desirable for this manufacturing process because of high flexibility and low capital investment. However, articulated-arm robots have a lower degree of stiffness and normal force, both of which present challenges for the transition of FSSW technology to articulated robots, such as methods to decrease the required Z-force. Figure 7. ABB IRB 7600 Six-Axis Articulated Robot at AJ&PL NIAR WSU. 7

24 CHAPTER 2 LITERATURE REVIEW 2.1 FSSW Process Controls The process controls of FSSW have improved over the decades with the advancement of computing, sensing, and measuring. Advancement in machining technology directly benefits FSSW because, from its inception, FSW was developed using computer numerically controlled (CNC) machines. The more capabilities of FSSW machines mean that the more varieties of process controls can be developed. Position control is normally applied to FSSW. This is the simplest process control and requires the least amount of processing monitoring by the machine controller. A position control weld program uses a command of known weld depths to maintain a constant tool depth throughout the weld (Figure 8a) [4,18]. Another process control of FSSW is load control, which involves a force-feedback process. It requires more intelligence for measuring, sensing, feedback, and command controls for loading and positioning. A load-control weld program begins by establishing a nominal load command based on a feedback load obtained from a preliminary weld using a position-control weld program. This load is typically maintained at a constant load throughout the weld (Figure 8b) [4,18]. Variation of FSSW process control can be a combination of position control and load control, known as hybrid control. The hybrid-control weld program operates with the position control as the initial command control, beginning with the plunge step controlled to a specified weld depth. Once the plunge phase is complete, the program switches to load control as command control to maintain a predetermined constant forging load during the weld (Figure 8c) [26]. 8

25 Value Load Value Load Value Load Position Position Position Time Time Time Command Feedback (a) Position Control (b) Load Control (c) Hybrid Control Figure 8. Schematic Diagram of Process Controls of Octaspot FSSW. The feedback reaction force of a position-control weld program increases during the plunge stage, due to displacement of material when the probe is plunged into the joint material, and increases significantly when the tool shoulder comes in contact with the top surface of the joint material. As the pin tool is moved in the Octaspot path, if the weld coupon has irregular thickness or if the backing fixture is uneven, a position control weld will produce poor weld quality due to not maintaining a sufficient forging force (Figure 8a). Since the Z-force acts as the forging force, which is an important factor to ensure a fully consolidated weld, load control as command control can ensure a constant load level throughout the weld. However, the increased position of the pin tool that travels causes lifting because the predetermined load is low (Figure 8b) [Note: the position values in Figures 8b and 8c can be either negative, when the tool plunges into the material (predetermined load too high), or positive, when the tool rises above the material (predetermined load too low)]. In the hybrid-control weld program, position control is utilized to ensure that a sufficient weld depth is reached in the plunge stage of the weld; then the program is changed to load control to ensure a consistent forging force for the rest of the weld. However, the high-reaction load due to the control position during the plunge phase is not favorable for low Z-force FSSW research. A low Z-force weld program based on load control is 9

26 used to eliminate high-reaction loads, known as Z-force spikes, in the plunge step of the weld (Figure 8a and 8c). 2.2 Development of Process Parameters The process parameters of Octaspot swept FSSW are similar to FSW and include spindle speed (rpm), travel speed (ipm), plunge speed (ipm), tilt angle (degree), dwell time (sec), and forge load or normal load (lbf). The process parameters of plunge FSSW include spindle speed, plunge speed, and dwell time, whereas an Octaspot swept FSSW has a closed-loop path (Figure 10) involving the additional process parameters of travel speed and tilt angle. A hybridcontrol weld program (Figure 8c) includes the initial plunge under position control and the tool movement under load control. In a low Z-force weld program, plunge depth is defined by a constant-plunge spindle speed and a constant-plunge dwell time, both introduced to reach specific plunge depth within a range of low forge load. For a low Z-force weld program developed from the hybrid-control weld program of Octaspot swept FSSW, all process parameters are held constant. These include the tilt angle, dwell time, plunge speed, plunge dwell time, and plunge spindle speed. For this research, the effects of variation and interaction of process parameters such as normal load, weld spindle speed, and travel speed are of main interest for characterizing the weld properties of low Z-force Octaspot swept FSSW. Each process parameter has its own role; therefore, the investigation of certain, more significant process parameters is more desirable for research that is constrained by time and budget. Since three factors (k = 3) or process parameters were selected for this study, two general design of experiments (DOEs), either two-level with k factors (2 k ) or three-level with k factors (3 k ) designs are appropriate DOEs. A three-level DOE with 27 runs has a higher resolution than a 10

27 two-level DOE with 8 runs. In addition, a three-level DOE can be a second-order model. However, a three level DOE will increase the cost and time. A model that provides a response surface can be used to optimize the process parameters for maximizing the ultimate tensile load (UTL) of lap shear coupons. Therefore, statistical development of process parameters using a response surface method, such as a Box-Behnken DOE, can significantly reduce time and cost compared to a full factorial DOE. For example, a Box-Behnken or Central Composite DOE has only 15 or 16 runs, compared to 27 runs in a three-level full design with three factors 3 3. Compared to a Box-Behnken DOE, a Central Composite DOE contains points on the corners of the design space cube, which can represent factor-level combinations that are either expensive or impossible to test because of physical process constraints [19]. In certain situations, these corner points can be extreme process parameters, which ultimately can damage the pin tools. The three process parameters chosen as the main interest of investigation in this study were selected because FSSW was treated as a thermo-mechanical controlled process. Weld spindle speed and travel speed are controlled variables in a weld program, and they directly affect thermal input to the work piece [2, page 71]. The term cold weld is typically associated with a weld that is made with a relatively high travel speed and low spindle speed. A hot weld is typically described as a weld with a relatively low travel speed and high spindle speed. These relative terms of cold and hot welds do not correlate with peak temperature [2, page 37]. One would assume a hot weld should reach a higher peak temperature compared to a cold weld, but the high conductivity of aluminum tends to disperse the heat of a hot weld because of the slow travel speed, thus resulting in a lower peak temperature. A final controlled variable chosen to be investigated in this study was normal load because a controlled path extrusion [2 pp 301,20] 11

28 FSSW need a consistent normal load to produce a good FSSW joint. All other process parameters were kept constant in this research but may be investigated in future work. 2.3 Tool Geometry FSW and FSSW tools have similar characteristics, such as body, shoulder, and probe (Figure 5), which may have a range of different features and shapes. Features on the probe, such as flats, flutes, and threads, can promote the flow of material around the probe. A concave shoulder traps material that is displaced by the probe. A shoulder with a flat face and scrolls will tend to capture the material displaced by the probe and redirect it inward toward the probe. Probes with different cross-sectional shapes are shown in Figure 9. These shapes serve to change the ratio of the physical volume of the probe to the swept volume of the probe. Table 1 provides the volume per unit length (or unit volume) of each probe. (a) (b) (c) (d) (e) (f) (g) Figure 9. Different Probe Shapes with Same Effective Swept Area: (a) Rectangular, (b) Triangular, (c) Square, (d) Pentagon, (e) Hexagon, (f) Octagon, and (g) Circular TABLE 1 RATIO OF PROBE PHYSICAL UNIT VOLUME TO PROBE SWEPT UNIT VOLUME Probe Shape a b c d e f g Probe Physical Unit Volume Ratio of Probe Physical to Swept Unit Volume Detailed calculation refers to Appendix A. 12

29 In plunge FSSW, the plunge stage creates a hooking defect at the lap joint interface due to displacement of the probe s volume of material. In addition, features on the probes, such as threads, which provide an augering effect that causes material to recirculate toward the shoulder, further increase the lifting and hooking, and create a large weld nugget. However, the Octaspot swept FSSW process includes a closed-loop path that consumes the hooking feature and simultaneously creates a larger stir zone compared to the plunge FSSW process. The shoulder of a pin tool has three main functions: (1) to capture material displaced by the probe, (2) to apply Z-force or forging force, and (3) to create frictional heat. Shoulder features, such as flat scroll or Wiper [21] (Figure 12 and Figure 13), are designed to capture material and direct it toward the probe. A concave shoulder (Figure 14), which has a small pocket of volume, captures the displaced material and keeps it pressed against the probe. For thin-gage material, an optimum shoulder diameter is favorable to create adequate frictional heat and avoid a large heat-affected zone. A large shoulder diameter creates a wider HAZ, compared to a small shoulder diameter. Since low Z-force is the primary goal of this research, the pin tool shoulder diameter needs to be reduced for low process forces but yet provide sufficient forging force to ensure consolidation of the weld nugget. A large shoulder diameter requires more Z-force compared to a small shoulder diameter to create similar forging pressure for sufficient consolidation of the weld nugget. 2.4 Variation of FSSW Existing fastening methods such as rivets and resistance spot welds have been widely applied in the automotive and aerospace industries for decades. FSSW has been introduced recently as an alternative fastening method for thin-gauge materials. The simplest type of FSSW, referred to as plunge FSSW or poke FSSW, is an attractive alternative replacement for existing 13

30 discrete fastening methods because it can be produced rapidly and with a simple motion. Plunge FSSW has shown many benefits and already has been implemented in the automotive industry [22]. Besides plunge spots, refill FSSW can fill the exit hole and leave a nearly flush surface with an opposing pin and shoulder [23,24]. Swept FSSW, such as the Squircle disclosed by TWI [7] and developed at Wichita State University (WSU) as an Octaspot, has been shown to be up to 250 percent stronger than rivets and resistant spot welds in a single-spot lap shear [25]. Plunge and refill FSSW differ from swept FSSW. Swept FSSW has an additional closed-loop translation movement (Figure 10). This closed-loop translation increases the joint shear area and has been demonstrated at WSU to have better mechanical properties compared to plunge or refill FSSW [7,26,27,33]. 1) Plunge 2) Move Out 3) Begin Sweep 4) Perimeter Undulation 5) Complete Sweep 6) Move In & Retract Figure 10. Octaspot Travel Path.[25,27] 2.5 Material Flow Plunge FSSW cross-sections tend to exhibit an upward flow of material from the bottom sheet causing an uplift of the faying surface, called hooking. The hooking caused by the vertical translation of material creates a thinning of the effective thickness of the top sheet. In contrast, 14

31 Plunge (Poke) Spot swept FSSW consumes the hook by sweeping around the perimeter, giving it better control of the faying surface geometry and increasing the effective shear area of the nugget (Figure 11). Swept Spot Figure 11. Schematic Cross-Sectional Representation of Plunge and Swept FSSW.[26] For single-pass linear FSW lap welds, placing the advancing side or retreating side in the load path significantly affects the mechanical properties measured by the unguided lap shear coupons [26,29]. Hooking is typically observed on the advancing side of lap welds and sheet lifting along the retreating side of lap welds (Figure 21). Both defects can be significantly affected by probe design. Prior related work involving the Counterflow tool was found to produce excellent unguided lap shear mechanical properties on both the advancing side and retreating side when placed directly on the loading path [29]. In making an Octaspot swept FSSW, the advancing side is typically placed directly on the loading path because it produces a clearly distinctive line between the TMAZ and HAZ [26]. This distinctive line on the advancing side is placed on the outside of the Octaspot swept FSSW weld nugget to ensure that there is no sheet thinning or hooking around the joint. In this study, the retreating side of an Octaspot swept FSSW was placed inside the weld nugget and not directly subjected to a tensile lap-shear test load. The hooking defect on the advancing side and joint interface oxide remnant line (sheet lifting) on the retreating side can be eliminated by appropriate probe designs. 15

32 CHAPTER 3 OBJECTIVE Friction stir spot welding development work has commonly been used on a gantry-type system because of the wide range of Z-forces, also known as forging forces or normal forces, required to produce a sound FSSW. However, articulated robots, which are limited to lower Z-forces, are preferred for implementation in manufacturing plants because of their potential to produce three-dimensional structures with more flexibility and lower capital costs than a conventional gantry system. Thus, for robotic applications, an investigation into low Z- force FSSW using conventional tools and process development is crucial for the development of this technology. Lower Z-forces can be achieved by studying the relationship between pin tool features, geometries, and process parameters measured by UTL, and optical metallographic cross-sections. FSSW must maintain a significant joint strength with lower Z-force and be comparable to existing FSSW joint strength. The weld cycle time must be minimized to achieve a lower manufacturing time and thus be competitive with other fastening technologies. This research helps to indentify the portability issues associated with moving FSSW technology from gantries to robots and provides a path for implementation of FSSW utilizing articulated robots in the automotive and aerospace industries. 16

33 CHAPTER 4 TEST PROCEDURE 4.1 Pin Tool Designs A conventional fixed-pin tool design used for a lap-joint weld requires an adequate probe length to penetrate through the first sheet of material and partially breaking the surface interface of the second sheet material to create a joint. Whereas, a lap-joint weld with different material thicknesses to be welded required a two-piece pin tool, a body, and a detachable probe with different probe lengths or a retractable pin tool. In this study, a conventional pin tool with a fixed probe length will be utilized to lap weld bare aluminum alloy 2024-T3 sheet with a thickness of inch. Since AJ&PL has ongoing research involving short, continuous, linear FSW and Octaspot swept FSSW lap weld joints using a similar thickness of material, a few existing pin tool designs were utilized in this research. A comparison of existing data with low Z-force data on mechanical properties such as single-spot unguided lap shear weld UTL were analyzed based on Z-forces and pin tool designs. Each pin tool has a few unique features designed on the probe such as threads, flutes, and flats. A new pin tool design has two opposing flutes and resembles the letter V in the alphabet; hence, it is named the V-flute (Figure 12). Typical shoulder designs are concave, flat, and convex. In this experiment, pin tools were designed with a five-degree concave shoulder with no features. The material displaced by the probe in the plunge process was captured mostly under the concave shoulder. Another pin tool shoulder was designed with grooved features on a flat shoulder, hence named flat scrolls, and was used in this experiment to capture displaced material, scooping and directing it toward the center of the pin tool (Figure 12). Another variant of the flat scrolls without the exiting pin tool shoulder lip, called the Wiper (Figure 13a), was considered 17

34 in the design stage. However, a reduction of the shoulder diameter from 0.40 inch to 0.30 inch (Figure 13b) prevented its use, and the flat scrolls design with a similar shoulder feature (Figure 12b) was used instead. (a) Figure 12. Flat Scrolls Shoulder on Duo V-Flute Pin Tool: (a) 0.40-Inch Diameter and (b) 0.30-Inch Diameter. (b) (a) Figure 13. Wiper Shoulder on Duo V-Flute Pin Tool: (a) 0.40-Inch Diameter and (b) 0.30-Inch Diameter. (b) Five pin-tool designs were included in this research. Three pin tools were extensively investigated for short linear lap FSW, plunge FSSW, and Octaspot swept FSSW. Two preferred pin tools for Octaspot swept FSSW were the Counterflow [28,29,30,31] and Psi tool [25,30,31,32,33] designs developed at WSU, whereas a Modified Trivex tool [26,30,31] has been shown to be successful for plunge and Octaspot FSSW (Figure 14a to 14f). In addition, a new pin tool design named the V-flute [30] Tri V-flute and Duo V-flute (Figure 14g to 14j) was included in this research. A Tri V-flute pin tool has three sets of V- flutes and a Duo V-flute has two sets of V-flutes. The two designs were developed to study the effects of multiples V-flutes on UTL joint strengths for an Octaspot swept FSSW. Two pin tool shoulder diameters of 0.30 inch and 0.40 inch were included in this research to investigate the effects of shoulder sizes on Z-force applied, corresponding to the UTL of joint 18

35 strength. The pin tool probes had base diameters of inch and a seven-degree taper angle. All the pin tools included in this research had a five-degree concave shoulder. (a) (c) (e) (g) (i) (b) (d) (f) (h) (j) Pin Tool Shoulder Diameters: Top row 0.40 inch and bottom row 0.30 inch. Probe Design: Counterflow Tool (a) and (b), Psi Tool (c) and (d), Modified Trivex Tool (e) and (f), Tri V-Flute (g) and (h), and Duo V-Flute (i) and (j). Figure 14. Pin Tools with Five-Degree Concave Shoulder. Although all pin tools were designed with a seven-degree tapered cylindrical probe, each of the pin tools shown in Figure 14 has at least one or more features on the probe for its identity and functionality. The features on the probe add an additional factor, which leads to the study of different probe designs on the mechanical properties of the weld. The Counterflow tool has a combination of two features: thread and counterflow flutes on the probe (Figure 14a and 14b). The Psi tool has a combination of two features: inclined flats and vertical flutes on the probe (Figure 14c and 14d). The Modified Trivex tool has an offset thread feature on the edges of a seven-degree tapered Wankel triangular-shaped probe (Figure 14e and 14f). The new pin tool design included in this research, the V-flute, has a seven-degree tapered cylindrical probe designed with the feature of two opposing flutes. The Tri V-flute pin tool was designed with three sets of opposing flutes (Figure 14g and 14h), and the Duo V-flute was designed with two sets of opposing flutes (Figure 14i and 14j). 19

36 The matrix of the pin tools had a combination of two shoulder sizes and two shoulder features, and the five probe designs created a total of 20 pin tools (Table 2). Thus, this research was divided into two phases: that involving the concave shoulder (phase 1) and that involving the flat scrolls (phase 2). Phase 1 involved the pin tool matrix with two different shoulder diameters to study the effects of shoulder diameter on Z-forces and five probe designs to study the effects of probe designs on mechanical properties. However, the Modified Trivex, Tri V- flute, and Duo V-flute tools with 0.40-inch-diameter shoulders in phase 1 and all 0.40 inchdiameter shoulders in phase 2 were not made because the 0.40-inch-diameter shoulder required a higher Z-force. In phase 2, the pin tool matrix was reduced to one probe design (Duo V-flute ) to study the effects of the flat scrolls shoulder feature and the concave shoulder feature on the 0.3-inch-diameter shoulder on mechanical properties (UTL). TABLE 2 PIN TOOL MATRIX Shoulder Diameter Counterflow Trivex Concave PSI Tri V-Flute 0.3 inch 0.4 inch Scroll Counterflow Trivex PSI Tri V-Flute 0.3 inch 0.4 inch Duo V-Flute Duo V-Flute Phase 1 study Phase 2 study Pin tools not made 20

37 4.1.1 Additional Pin Tool Designs Further investigation led to a phase 3, which consisted of three probe shape designs with no features on the probe: Wankel s triangular-shaped probe, called Trivex (Figure 15a); the pentagon-shaped probe, called Pentagon (Figure 15b); and the octagon-shaped probe, called Octagon (Figure 15c) with a inch-diameter probe base and 0.30-inch-diameter fivedegree concave shoulder. This additional investigation studied the relationship between the ratio of physical volume to swept volume and the hooking defect of Octaspot swept FSSW. The Duo V-flute pin tool design was selected to further reduce the Z-force from a 0.40 inch-diameter shoulder with a inch-diameter probe (Figure 16a), to a smaller inch-diameter shoulder with a inch-diameter probe (Figure 16b), to a phase 4 study, which was the final design of a small 0.25-inch-diameter shoulder with a small 0.10-inch-diameter probe (Figure 16c). This additional investigation, which studied the relationship between two pin tools, as shown in Figure 16b and 16c, reduced the effects of shoulder and probe diameters on Z- force and UTL of Octaspot swept FSSW. (a) (b) (c) Figure Inch-Diameter Probe Shapes: (a) Concave Shoulder Trivex, (b) Pentagon, and (c) Octagon. (a) (b) (c) Figure 16. Reduced Shoulder and Probe Diameter Sizes of Duo V-Flute. 21

38 4.2 Material Preparation The weld coupon used in this study was a lap joint configuration with 1.0-inch overlap in inch-thick, bare 2024-T3 aluminum alloy. The specimen coupon configuration is shown in Figure 17. Both top and bottom sheets were 2024-T3 aluminum alloy, 1.0 inch wide and 4.0 inches long. Grain direction was parallel to the mechanical tensile shear test direction. The Octaspot path began in the center, moved to the positive X-axis, circulated 450 degrees, and returned to center from the positive Y-axis. Figure 17. Single-Spot Unguided Lap Shear Specimen. Prior to FSSW, the surface oxide layer of the weld coupon at the joint interfaces and tool contact interface was removed with a dual-action (DA) sander, also known as a random orbital sander, with a 180-grit disk. The weld coupon was also wiped with methyl ethyl ketone (MEK) to remove any remaining sanded oxide particles. Surface oxide was removed, unless it was indicated that there was no prewelding preparation or only MEK wipes were used for cleaning. Surface oxide can remain in the FSSW nugget if its dispersion is insufficient. A separate investigation could be initiated to correlate the effects of surface preparation and UTL of FSSW. 22

39 4.3 Weld Setup All FSSW setups were made with a five-axis ISTIR PDS FSW machine from the MTS Systems Corporation. Welding was supported with a 0.50-inch-thick steel backing plate with a inch machined step for lap welds (Figure 18). Steel bars were spaced 0.75 inch apart, clamped with finger clamps spaced 6.0 inches apart, and tightened with a torque wrench to 40 ftlbf, providing approximately 900 lbf down force. The weld fixture position was set up so that the lower sheet was on the positive X-axis side of the machine, and the start of the first spot through the fifteenth spot from negative to positive was on the Y-axis (Figure 18). In this setup, the metallographic cross-section of each spot was consistently processed (Note: Steel backing support was removed from time to time to accommodate other projects) T3 0.5 x Steel Bar + Y-axis into slide 0.75 ~ 900 lbf ~ 900 lbf 4130 Steel Backing Support 0.04 Spacer + X-axis T3 4.4 Weld Programs Figure 18. Experimental Weld Setup. Weld programs used on the MTS FSW machine were written using a combination of load control and position control. This capability of the MTS software provides an advantage to researchers to further investigate FSSW with low Z-force with innovative weld schedules tested in this research. The first weld program utilized position control, which commanded the pin tool to plunge into the weld coupon at a specified depth. The second weld program utilized a hybrid weld program with a partial initial plunge using position control and then switched to load control for the remainder of the weld. In addition to controlling maximum weld forces, load- 23

40 control FSSW has been shown to have more consistent ultimate tensile load results with lower standard deviations [26]. However implementation of a full load-control weld program has a few obstacles with which to be concerned, such as uncontrolled plunge depth and weld program modification. Modification of the weld program to load control introduced additional parameters, such as plunge dwell time and plunge spindle speed. However, the weld program was modified with minimal changes, and most of the constant values remained the same. Process parameters vary in a weld program and depend on the types of FSSW. In plunge FSSW, the main process parameters are spindle rotational speed, plunge speed, plunge depth, and dwell time. In a hybrid weld program written for Octaspot FSSW, additional process parameters included in the hybrid weld program are travel speed, tilt angle, spot radius, and Z- force. A low Z-force weld program modified from a hybrid weld program introduced a new process parameter, plunge spindle speed, and substituted dwell time with plunge dwell time and removed the plunge depth. Selecting which process parameters to hold constant and which to be varied requires a literature review on process parameters. The process parameters selected to be varied in this research were spindle speed, travel speed, and forge load. The process parameters matrix used a Box-Behnken DOE approach to determine the process parameters window and the significance of each process parameter with response to ultimate tensile load of Octaspot - FSSW. Since the hybrid weld program has a position control in the plunge section, feedback of the normal load spiked up to 3,000 lbf at the time of pin tool shoulder contact with the weld coupon. Prior to changing the hybrid weld program to the low Z-force weld program, several solutions were suggested to reduce the spike of the Z-force feedback. Pre-welding solutions suggested for reducing normal load, such as preheating and predrilling the weld coupon, were 24

41 not practical and not tested. However, modification of the process parameters, such as reducing the plunge speed, reducing the plunge depth, increasing the spindle speed, and increasing the dwell time, were more practical solutions. The position-control weld program was utilized to approximate the Z-force value for the load-control weld program from feedback force data. Three selected process parameters were varied using the Box-Behnken DOE approach and run with the low Z-force weld program to investigate the effect of process parameters and pin tools designs on the mechanical properties of low Z-force Octaspot FSSW. 4.5 Mechanical Properties Testing There are two different types of mechanical properties tests: destructive and nondestructive. Destructive tests, such as the tensile shear test, fatigue test, cross-tension test, crosssectional optical metallographic test, cross-sectional hardness test, impact or dynamic or crash test, and corrosion test have been established and used to determine mechanical properties. Nondestructive tests, such as the phased-array ultrasonic test, X-ray test, surface hardness test, laser test, surface optical metallographic test, scanning electron microscopy (SEM), and discrete Fourier transformation (DFT) software that analyzes feedback forces, can be very time and cost effective for quality assurance. In this research, destructive testing using the tensile shear test of a single spot on unguided lap shear coupons was used to evaluate the UTL mechanical properties of low Z-force FSSW. The 2024-T3 aluminum alloy required a minimum of four days or 100 hours of post-weld natural aging treatment to allow the weld nugget to stabilize [2 pp74,34]. The microstructure of the weld nugget went through a by-product heat-treatment process after FSSW, since weld nuggets require time for grain growth and recrystalization to reach a stable temper. 25

42 In addition to the tensile shear test, optical metallographic analyses of FSSW crosssections were used to qualitatively evaluate the welds. Repeated welds were milled close to the center and mounted into clear epoxy resin for polishing. The orientation of the Octaspot weld path with respect to the machine axis was as follows: starts from the center, moves out to the positive X-axis, travels counter clockwise 450 degrees, and returns to center from the positive to the negative Y-axis (Figure 10 and Figure 17). Keller s reagent is a chemical etching was used to enhance the difference of the weld nugget, TMAZ, HAZ, and parent material due to different grain structures. Keller s reagent consists of 2.5% nitric acid (HNO 3 ), 1.5% hydrochloric acid (HCl), 1% hydrofluoric acid (HF), and 95% distilled water. Finally, pictures of the optical metallographic were documented and examined to reveal certain weld defects, nugget size, and joint interface defects. Weld defects, such as lack of consolidation or lack of fill, which looks like wormholes (Figure 19), and kissing bonds, known as lack of penetration into the second sheet, leads to nugget shear failure (Figure 20). In a lap weld, sheet lifting is shown on the left side of Figure 21, and hooking as shown on the right side of Figure 21, known as the upward or downward movement of the joint interface, both hooking and sheet lifting create a sheet-thinning defect on the upper or lower sheet of the welded coupon. Sheet thinning defects do appear in Octaspot swept FSSW since it is a lap joint configuration, and changes of the loading path to a thinner sheet leads to premature failure in mechanical testing. Optical metallographic digital images and failure analysis of low Z-force Octaspot FSSW coupons on the tensile shear test were categorized and documented. 26

43 Figure 19. Worm Hole Defect in Octaspot FSSW. Figure 20. Kissing Bond Defect in Plunge FSSW. Figure 21. Sheet Lifting (left) and Hooking (right) in Lap FSW. 27

44 CHAPTER 5 RESULTS AND DISCUSSIONS 5.1 Achieving Low Z-Force Previous research has been performed using a 0.40-inch-diameter probe shoulder. This data was beneficial in taking steps toward effective low Z-force FSSW. Octaspot swept FSSW using the hybrid weld program consisted of position control in the plunge process and switching to load control in the sweep stage. Feedback from the Z-force (forge force) of the position control welds had two distinctive Z-force spikes, the probe spike and the shoulder spike, as the material was in contact with the pin tool during the initial plunge, which reached up to 2,000 lbf (1,100 lbf spike of Z-force in addition to 900 lbf command force (Figure 22)). The Z-force spike can be as high as 1,500 lbf to 2,000 lbf in addition to the command Z-force. The high Z-force spike created by the pin tool shoulder was undesirable for this low Z-force study because it is beyond the force capability of most robotic arms. Forge Force (lbf) FSW07079_01_9 Plunge Swept Shoulder (Spike) Probe Time (sec) Forge Force Cmd, lbf Forge Force Fbk, lbf Forge Fbk, in Forge Position (in) Figure 22. Command and Feedback Plot for Typical Octaspot FSSW (Hybrid Weld Program). 28

45 Therefore, a few possible solutions to reduce these spikes were considered: predrilling before FSSW, preheating before FSSW, decreasing initial plunge depth, increasing spindle speed, increasing dwell time, and decreasing plunge rate. Most of these possible solutions were tested using the existing hybrid weld program, and the data from feedback forces was compared directly with existing FSSW data. Predrilling and preheating before FSSW were not investigated because the additional steps required for drilling and heating would increase the cycle time to complete a spot weld. The remaining solutions were unsuccessful when implemented with the existing hybrid weld program. Plunge depth was decreased from inch to inch, but Z- force spike was not eliminated. Spindle speed increased up to 2,000 rpm created a hotter weld and decreased the Z-force spike but was unable to eliminate it. Plunge rate decreased from 17 ipm to 1 ipm, which created a slower weld at the plunge stage and a distinctive probe spike and shoulder spike. Dwell time increased from 1 second to 5 seconds before the swept stage tended to reduce the Z-force spike. The Z-force spike was not eliminated, but trends of lower Z-force spike were observed from the feedback forces plots. Therefore, the final option was to modify the hybrid weld program to a load control weld program. Existing data of the 0.40-inch-diameter shoulder weld using the hybrid weld program was used as a benchmark for Z-force and UTL comparison. A position-control weld program was used to determine an appropriate Z-force for a corresponding hybrid weld program. The position-control weld program was also used to estimate a required Z-force to maintain the tool depth while in the swept stage of the weld. The 0.40-inch-diameter shoulder created a spike up to 3,000 lbf, which decreased to an average of 1,700 lbf during the sweep stage of the FSSW (Figure 23), whereas the 0.30-inch-diameter shoulder spiked up to 3,500 lbf and continuously dropped to an average of 800 lbf at the end of 29

46 the sweep stage (Figure 24). The reduction of Z-force at the end of the sweep stage for the inch-diameter shoulder showed that a lower Z-force could be achieved simply by reducing the shoulder diameter. The data also suggest that all position control aspects of the weld program should be eliminated and performed under load control in order to eliminate the Z-force spike in Octaspot FSSW. Using the estimated average load of 900 lbf from position control and applying it to the load control weld program successfully produced Octaspot FSSW with a small Z-force spike. The Z-force spike was lowered to 1,000 lbf; with command force of 900 lbf with additional shoulder spike of 100 lbf (Figure 25) lbf CFSP08302_1_ Forge Force (lbf) Average 1700 lbf Forge Position (in) Time (sec) Forge Force Fbk, lbf Forge Fbk, in Forge Cmd, in 0 Figure 23. Command and Feedback Plot of 0.40-Inch-Diameter Psi Tool Welded with Position Control. 30

47 CFSP08301_1_5 Forge Force (lbf) lbf 800 lbf Forge Position (in) Time (sec) Forge Force Fbk, lbf Forge Fbk, in Forge Cmd, in Figure 24. Command and Feedback Plot of 0.30-Inch-Diameter Psi Tool Welded with Position Control. 0 CFSP08301_11 Forge Force (lbf) Shoulder (Spike) Forge Position (in) Time (sec) Forge Force Cmd, lbf Forge Force Fbk, lbf Forge Fbk, in Figure 25. Command and Feedback Plot for Low Z-Force Swept FSSW. The load-control weld program, known as the low Z-force weld program, successfully created Octaspot FSSW with a low Z-force of 900 lbf, desirable joint interface, and a fully consolidated weld nugget. The weld joint interface of low Z-force Octaspot FSSW is shown in 31

48 Figure 26, using a Psi tool with a 0.30-inch-diameter shoulder, and corresponds to the Z-force feedback shown in Figure 25. The weld also exhibited a desirable joint interface with minimal or no hooking, as shown in Figure 27. Figure 26. Low Z-Force Cross-Sectional Metallographic (1.2X). (a) (b) Figure 27. Joint Interface of Figure 26 (100X): (a) Left Side and (b) Right Side. Using the same tool and lowering the commanded Z-force to 700 lbf, the command and feedback force plot shows no spike of Z-force and only fluctuation of 50 lbf (Figure 28). The load-control weld program significantly reduces the spike of Z-force, and a combination using a low-commanded Z-force below 700 lbf can eliminate the Z-force spike. The surface faying interface has minimal to no hooking for the swept FSSW welded with 700 lbf of commanded Z- force. 32

49 Forge Force (lbf) CFSP08301_ Time (sec) Forge Force Cmd, lbf Forge Force Fbk, lbf Forge Fbk, in Forge Position (in) Figure 28. Command and Feedback Plot for Low Z-Force Swept FSSW. 5.2 Concave Shoulder Tool Study (Phase 1) To further investigate strategies for reducing the Z-force, three essential process parameters Z-force, spindle speed, and travel speed were studied in further detail while all other parameters were held constant. Process parameters were investigated using Box-Behnken DOE to show correlations between UTL and these three process parameters. The first DOE had a process parameter low and high range of 4 ipm to 8 ipm for travel speed, 700 lbf to 1,100 lbf for Z-force, and 800 rpm to 1,200 rpm for spindle speed with a midpoint (three levels). The UTL of unguided single spot lap shear was used to correlate with the process parameters. 5.3 Concave Shoulder Diameter Study In this part of the research, two pin tool designs were used to study the effects of shoulder diameter on Z-force process parameter. Pin tool designs included Psi and Counterflow with two five-degree concave shoulder diameter sizes of 0.30 inch and 0.40 inch. 33

50 5.3.1 Psi Tool (0.30 Inch and 0.40 Inch) Low Z-force specimens were welded with two pin tool diameters, 0.30 inch and 0.40 inch, using a concave shoulder with a Psi tool probe. UTL increased for the 0.30-inchdiameter shoulder specimens as the Z-force decreased (Figure 29). On the other hand, as the UTL decreased, the Z-force decreased for the 0.40-inch-diameter shoulder (Figure 30). UTL UTL Figure 29. Main Effects Plot of 0.30-Inch-Diameter Concave Shoulder Psi Tool. UTL UTL Figure 30. Main Effects Plot of 0.40-Inch-Diameter Concave Shoulder Psi Tool. 34

51 Cross-sectional metallographic analysis using optical microscopy provided more evidence to support this trend. The 0.30 inch-diameter concave shoulder with the Psi pin welded at Z-force of 1,100 lbf over-plunged, which created sheet thinning in the top sheet and an exit hole (Figure 31). The Z-force of 900 lbf metallographic shows it to be slightly over-plunged with minimal flash (Figure 32), and the Z-force of 700 lbf metallographic shows adequate plunge depth and minimal flash (Figure 33). Figure 31. Low Z-Force Swept FSSW with 0.30-Inch-Diameter Psi Tool at 1,100 lbf. Figure 32. Low Z-Force Swept FSSW with 0.30-Inch-Diameter Psi Tool at 900 lbf. Figure 33. Low Z-Force Swept FSSW with 0.30-Inch-Diameter Psi Tool at 700 lbf. The 0.40-inch-diameter concave shoulder of the Psi Tool welded at Z-force of 1,100 lbf and 900 lbf created a wide weld nugget, a wide flow arm with adequate plunge depth, and no flash (Figure 34 and Figure 35), whereas a Z-force of 700 lbf created an unconsolidated weld nugget that showed insufficient Z-force (Figure 36). 35

52 0.050 in Figure 34. Low Z-Force Swept FSSW with 0.40-Inch-Diameter Psi Tool at 1,100 lbf in Figure 35. Low Z-Force Swept FSSW with 0.40-Inch-Diameter Psi Tool at 900 lbf. Figure 36. Low Z-Force Swept FSSW with 0.40-Inch-Diameter Psi Tool at 700 lbf. From the metallographic inspection shown in Figures 34 to Figure 36, the pin tool with the wider shoulder diameter created a deeper flow arm and wider TMAZ and HAZ zones (Vshaped nugget, as shown in Figure 34 and Figure 35), with sufficient Z-force of 1,100 lbf and 900 lbf, respectively. The 0.40 inch-diameter shoulder showed less sensitivity at a higher and wider range of Z-force from the metallographic observation and ultimate tensile load. At Z-force of 700 lbf, the pin tool shoulder without sufficient Z-force created a worm hole, and lack of contact of the shoulder with the weld coupon created no flow arm with minimal TMAZ and HAZ zones (U-shaped nugget, as shown in Figure 36), whereas the 0.30-inch-diameter shoulder 36

53 showed higher sensitivity at a similar range of Z-force compared to the 0.40-inch-diameter shoulder in metallographic observation and UTL. Table 3 shows the results of average UTL for respective shoulder diameters and Z-forces, which indicates that at a high Z-force of 1,100 lbf, the average UTL of the 0.30-inch-diameter shoulder is low at 816 lbf, and at a low Z-force of 700 lbf, the average UTL of the 0.40-inchdiameter shoulder is low at 1,050 lbf. Table 3 agrees with the main effect plots shown in Figure 29 and Figure 30. TABLE 3 AVERAGE UTL AND CORRESPONDING Z-FORCES APPLIED USING CONCAVE SHOULDER PSI TOOL Shoulder Diameter\ Z-Forces Applied Average UTL 700 lbf 900 lbf 1,100 lbf 0.30 inch 1,119 lbf 1,113 lbf 816 lbf 0.40 inch 1,050 lbf 1,192 lbf 1,216 lbf Average UTL results refer to Appendix C Counterflow Tool (0.30 Inch and 0.40 Inch) Counterflow tool welded joints with two shoulder diameter sizes of 0.30 inch and 0.40 inch. The main effects plot shows a similar trend to the results of the Psi tool when welded with similar weld parameters. The 0.30-inch-diameter concave shoulder shows better performance at low Z-force compared to the 0.40-inch-diameter shoulder (Figure 37 and Figure 38). 37

54 UTL UTL Figure 37. Main Effects Plot of 0.30-Inch-Diameter Concave Shoulder Counterflow Tool. UTL UTL Figure 38. Main Effects Plot of 0.40-Inch-Diameter Concave Shoulder Counterflow Tool. Metallographic cross-sectional analysis of the 0.30-inch-diameter concave shoulder of the Counterflow tool shows similar trends as that which occurred with the Psi tool. Figure 39 shows that over-plunging created sheet thinning at the exit hole, and the top sheet indicating flash at Z-force of 1,100 lbf is similar to what is shown in Figure 31. Figure 40 shows slightly over-plunging with minimal amount of flash at Z-force of 900 lbf, which is similar to what is shown in Figure 32. Figure 41 shows a good weld nugget with adequate plunge at Z-force of 700 lbf, similar to what is shown in Figure

55 Figure 39. Low Z-Force Swept FSSW with 0.30-Inch-Diameter Counterflow Tool at 1,100 lbf. Figure 40. Low Z-Force Swept FSSW with 0.30-Inch-Diameter Counterflow Tool at 900 lbf. Figure 41. Low Z-Force Swept FSSW with 0.30-Inch-Diameter Counterflow Tool at 700 lbf. Figure 42 and Figure 43 showing welds with a 0.40 inch-diameter concave shoulder with the Counterflow tool at Z-force of 1,100 lbf and 900 lbf, respectively, show wider HAZ and TMAZ zones (V-shaped nuggets), similar to what occurred with the 0.40 inch-diameter Psi tool. A 0.40-inch-diameter shoulder creates excessive heat and wider TMAZ and HAZ zones, which changes the properties of the parent material and can significantly reduce overall strength of the weld nugget. 39

56 0.050 in Figure 42. Low Z-Force Swept FSSW with 0.40-Inch-Diameter Counterflow Tool at 1,100 lbf in Figure 43. Low Z-force Swept FSSW with 0.40-Inch-Diameter Counterflow Tool at 900 lbf. Table 4 shows the results of average UTL for respective shoulder diameters and Z-forces. At a high Z-force of 1,100 lbf, the average UTL of a 0.30-inch-diameter shoulder is low at 1,013 lbf. At low Z-force of 700 lbf, the average UTL of a 0.40-inch-diameter shoulder is not available. Table 4 agrees with the main effect plots shown in Figure 37 and Figure 38. TABLE 4 AVERAGE UTL AND CORRESPONDING Z-FORCES APPLIED USING CONCAVE SHOULDER COUNTERFLOW TOOL Shoulder Diameter\ Z-Forces Applied Average UTL 700 lbf 900 lbf 1,100 lbf 0.30 inch 1,165 lbf 1,184 lbf 1,013 lbf 0.40 inch NA 1,178 lbf 1,166 lbf Average UTL results refer to Appendix C The Counterflow tool with different shoulder diameters of 0.30 inch and 0.40 inch has similar results and trends compared to the Psi tool. The 0.40 inch-diameter Counterflow tool was unable to plunge at 700 lbf Z-force because the tip of the probe has a larger surface area 40

57 compared to the Psi tool. The 0.30-inch-diameter shoulder pin tool for both the Psi tool and Counterflow tool showed a better performance and achieved comparable UTL to the inch-diameter shoulder pin tool at lower Z-force using a low Z-force weld program. Since the shoulder diameter investigation confirmed that the small shoulder can achieve lower Z-force, the remaining pin tool design of the Modified Trivex, Duo V-flute, and Tri V-flute, all with 0.40-inch-diameter shoulders was not investigated. Phase 2 of the study of shoulder features used one tool selection based on phase 1 results with the 0.30-inch-diameter shoulder only. The remaining phase 1 of this project was an investigation into different probe designs affecting the Z-force and mechanical properties of swept FSSW using a 0.30-inch-diameter concave shoulder. 5.4 Probe Design Study with 0.30-Inch-Diameter Concave Shoulder In the remainder of phase 1 (probe design study), the Modified Trivex, Duo V-flute, and Tri V-flute pin tools with 0.30-inch-diameter concave shoulders were included in the study and welded with the low Z-force weld program with similar weld parameters as the Psi and Counterflow tools. The different probe designs affected the nugget joint area and joint interface morphology, both of which significantly affect UTL Modified Trivex Tool The Modified Trivex tool welded joints with similar weld parameters had a significantly lower average UTL of 911 lbf in the first DOE. The weld nugget metallographic inspection at 1.2X magnification (Figure 46) was not enough to reveal hooking defects. At 100X magnification, the surface faying interface was revealed (Figure 47), showing that a hooking defect created sheet thinning on the upper sheet, which carried a significantly lower UTL. All four metallographic cross-section specimens had hooking defects. 41

58 Besides metallographic analysis and UTL values, failure mode was another indication of the hooking defect. All coupons tested in DOE 1 showed plug pull-out failure (Figure 48). The Wankel s triangular probe shape of the Modified Trivex tool promoted volumetric side material movement due to the probe s small physical to swept unit volume ratio. The triangularshaped probe tip area of the Modified Trivex was smaller compared to the circular-shaped probe tip area of the Psi and Counterflow tools (Figure 14) (see Figure 9 and Table 1). This finding led to an investigation of correlations of the probe s physical to swept unit volume ratio with different probe shapes to reduce volumetric side material movement. Volumetric side material movement may promote hooking defects in a lap joint weld. Metallographic analysis for DOE 1 shows that the 0.30-inch-diameter concave shoulder over-plunged at 1,100 lbf Z-force creates excessive flash (Figure 44), slightly over-plunged at 900 lbf Z-force creates some flash (Figure 45), and an adequate plunge at 700 lbf Z-force (Figure 46). This trend is similar to that of Psi and Counterflow tools. Figure 44. Low Z-Force Swept FSSW with 0.30-Inch-Diameter Modified Trivex Tool at 1,100 lbf. 42

59 Figure 45. Low Z-Force Swept FSSW with 0.30-Inch-Diameter Modified Trivex Tool at 900 lbf. Figure 46. Low Z-Force Swept FSSW with 0.30-Inch-Diameter Modified Trivex Tool at 700 lbf. (a) Figure 47. Joint Interface of Figure 46 (100X): (a) Right Side and (b) Left Side. (b) in Figure 48. Plug Pull-Out Failure Mode. 43

60 5.4.2 Duo V-Flute Tool The Duo V-flute tool welded joints with similar weld parameters as in first DOE had a similar trend of weld nugget as that of the 0.30-inch-diameter Psi, Counterflow, and Modified Trivex tools. Figure 49 metallographic shows that over-plunging at 1,100 lbf Z-force created excessive flash, Figure 50 shows that slightly over-plunging at 900 lbf Z-force created some flash, and Figure 51 shows adequate plunging at 700 lbf Z-force. Since the magnification of 1.2X is not enough to reveal hooking defects, the 100X magnification of metallographic analysis shown in Figure 52 indicates that no hooking occurred at the faying surface interface. Figure 49. Low Z-Force Swept FSSW with 0.30-Inch-Diameter Duo V-Flute Tool at 1,100 lbf. Figure 50. Low Z-Force Swept FSSW with 0.30-Inch-Diameter Duo V-Flute Tool at 900 lbf. 44

61 Figure 51. Low Z-Force Swept FSSW with 0.30-Inch-Diameter Duo V-Flute Tool at 700 lbf. (a) (b) Figure 52. Joint Interface of Figure 51 (100X): (a) Right Side and (b) Left Side Tri V-Flute Tool The Tri V-flute tool welded joints with similar weld parameters as the first DOE had a similar weld nugget trend as that of the 0.30-inch-diameter Psi, Counterflow, Modified Trivex, and Duo V-flute tools. Figure 53 metallographic shows that over-plunging at 1,100 lbf Z-force created excessive flash, Figure 54 shows slightly over-plunging at 900 lbf Z-force created some flash, and Figure 55 shows adequate plunging at 700 lbf Z-force. Since the magnification of 1.2X is not enough to reveal hooking defects, the 100X magnification of metallographic analysis shown in Figure 56 indicates that no hooking occurred at the faying surface interface. 45

62 Figure 53. Low Z-Force Swept FSSW with 0.30-Inch-Diameter Tri V-flute Tool at 1,100 lbf. Figure 54. Low Z-Force Swept FSSW with 0.30-Inch-Diameter Tri V-flute Tool at 900 lbf. Figure 55. Low Z-Force Swept FSSW with 0.30-Inch-Diameter Tri V-flute Tool at 700 lbf. (a) (b) Figure 56. Joint Interface of Figure 55 (100X): (a) Right Side and (b) Left Side. 46

63 5.5 Achievement in Concave Shoulder Study (Phase 1) Concave Shoulder Diameter Study Position control was found to be not suitable for low Z-force application on Octaspot swept FSSW due to the sudden increase in Z-force spikes. Using position control, the 0.30-inchdiameter shoulder shows a promising decrease of Z-force at the end of Octaspot swept FSSW. The weld program was modified to load control, and Z-force spikes were reduced significantly and even eliminated. The 0.30-inch-diameter shoulder welded with the load-control weld program performed better than the 0.40-inch-diameter shoulder at low Z-force. Although performance of the 0.30-inch-diameter shoulder was better, sensitivity of the Z-force increased significantly, thus affecting variation in plunge depth. The average UTL of low Z-force weld coupons with a 0.30-inch-diameter shoulder were comparable with average UTL of high Z-force spike weld coupons with a 0.40-inch-diameter shoulder. Since the 0.40-inch-diameter shoulder requires higher Z-force, it was delineated from further study Probe Design Study Using the same weld parameters as in the DOE 1 for five different probe designs showed no significant increase of weld nugget area for Octaspot swept FSSW using the low Z-force weld program (Figure 57 to Figure 61). Since the path and radius of Octaspot was similar, the weld nugget size was similar across the five pin tool designs. The variation of weld nugget size was due to different depths of polishing. Probe designs were analyzed for two main categories: shape and features. The Modified Trivex has a Wankel s triangular-shaped smaller probe tip area compared to the other four pin tools that have a circular-shaped probe. The Psi tool with three inclined flats slightly reduced the probe-tip area. A small probe-tip area is recommended to further reduce the Z-force required for FSSW. Although a small probe-tip area reduced the 47

64 plunging Z-force, the small ratio of the probe s physical to swept unit volume increased the swept volume, which in turn promoted side material movement, thus creating the hooking defect. Figure 57. Low Z-Force Swept FSSW with 0.30-Inch-Diameter Counterflow Tool at 700 lbf. Figure 58. Low Z-Force Swept FSSW with 0.30-Inch-Diameter Psi Tool at 700 lbf. Figure 59. Low Z-Force Swept FSSW with 0.30-Inch-Diameter Modified Trivex Tool at 700 lbf. Figure 60. Low Z-Force Swept FSSW with 0.30-Inch-Diameter Duo V-Flute Tool at 700 lbf. 48

65 Figure 61. Low Z-Force Swept FSSW with 0.30-Inch-Diameter Tri V-Flute Tool at 700 lbf. Table 5 shows the first DOE UTL minimum, maximum, average, and standard deviation values for five different probe designs. Figure 62 shows the average UTL with standard deviation comparison between a 0.30-inch-diameter concave shoulder with five different probe designs and high Z-force spike 0.40-inch-diameter benchmark UTL value. From the first DOE, the average UTL of all tools was comparable to the average UTL of high Z-force, except the Modified Trivex tool. Figure 62 shows no significant effect of the five probe designs to variation in UTL of Octaspot FSSW due to a large standard deviation. TABLE 5 COMPILATION OF DOE 1 UTL RESULTS FOR PROBE DESIGN STUDY OF 0.30-INCH- DIAMETER CONCAVE SHOULDER Pin Tools\UTL Min Max Average Standard Deviation Psi 624 lbf 1,208 lbf 1,036 lbf 163 lbf Counterflow 905 lbf 1,261 lbf 1,133 lbf 102 lbf Modified Trivex 785 lbf 1,036 lbf 911 lbf 61 lbf Duo V-Flute 600 lbf 1,231 lbf 1,016 lbf 175 lbf Tri V-Flute 847 lbf 1,240 lbf 1,063 lbf 127 lbf UTL results refer to Appendix C 49

66 Low Z-force Swept FSSW 0.30 inch Shoulder Diameter 1400 Ultimate Tensile Load (lbf) Psi Counterflow Trivex Duo V-flute Tri V-flute High Z-force 0 DOE 1 Figure 62. UTL Results Comparison of Low Z-Force Octaspot Swept FSSW for Five Pin Tools with 0.30-Inch-Diameter Concave Shoulder in DOE 1. Since the first DOE was designed to accommodate the wide variation of probe designs and shoulder diameters, optimization of the weld parameters was required to achieve higher and precise UTL. Other factors include metallographic analysis to avoid over-plunging by reducing the maximum Z-force from 1,100 lbf to 950 lbf and reduction in cycle time by increasing the maximum travel speed from 8 ipm to 13 ipm. Statistical analysis of weld parameters also played a minor role in weld parameter selection. The main effects plots of DOE 1 for all tools were analyzed. The main effects plot shown in Figure 29 indicates that Z-force had a quadratic trend line with a maximum point between 700 lbf and 1,100 lbf, travel speed had a quadratic trend line with a minimum point between 4 ipm and 8 ipm, and spindle speed had a linear trend line with a maximum point at 800 rpm and minimum point at 1,200 rpm. The main effects plot shown in Figure 37 indicates that Z-force had a quadratic trend line with a maximum point between 700 lbf and 1,100 lbf, travel speed had a quadratic trend line with a minimum point at 4 ipm and maximum point at 8 ipm, and spindle speed had a quadratic trend line with a maximum point between 800 rpm and 1,200 rpm. From these two main effects plots, extrapolation of weld 50

67 parameters did not stand true but indicated that an increase in travel speed will further increase UTL. Increase of travel speed is another factor that contributed to the reduction of cycle time of welding. Weld program optimization also contributed to a reduction in weld cycle time, and dwell time was reduced by an increase in acceleration rate of spindle rotational. Dwell time was reduced from 7 seconds to 5 seconds and finally to 2 seconds by optimization of weld programs (refer to Appendix B). 5.6 Optimization Weld Parameters (DOE 2) The first DOE was created to find the process window, and the second DOE was created to achieve maximum UTL for all tools. Using Box-Behnken DOE, weld parameters in both a low and high range were selected, travel speed was 7 ipm to 13 ipm, Z-force was 750 lbf to 950 lbf, and spindle speed was 800 rpm to 1,100 rpm with a mid-point. The coupons were naturally aged for a minimum of four days before tensile testing. The weld panels contained a total of 21 spot welds, with six coupons having repeated weld parameters for metallographic analysis. In addition to optimizing the weld program, the weld cycle time was further reduced as a result of optimizing the weld parameters in DOE 2. The low travel speed range (4-8 ipm) was increased to a high travel speed range (7-13 ipm). This increase of travel speed reduced its weld time significantly: an increase in travel speed from 4 ipm to 13 ipm reduced the weld time from 15 seconds to 7 seconds (refer to Appendix B). A total cycle time of five welds was analyzed and compared and resulted in a total reduction of five seconds (refer to Appendix B). The fastest Octspot swept FSSW was completed in a total time of nine seconds for each weld: two seconds dwell time and seven seconds weld time. 51

68 Table 6 shows that the reduction of the standard deviation for all tools increases the precision and repeatability of Octaspot FSSW by optimization through the response surface method. Only the Psi tool had the highest standard deviation, almost double or triple compared to the other tools. Hooking defects in all metallographics of the Modified Trivex tool contributed to the low UTL values in DOE 2. The Wankel s triangular-shaped Modified Trivex tool with a small probe physical to swept unit volume ratio had a large side volumetric displacement. Side volumetric displacement created the hooking defect in all welded coupons. TABLE 6 COMPILATION OF DOE 2 UTL RESULTS FOR PROBE DESIGN STUDY OF 0.30-INCH- DIAMETER CONCAVE SHOULDER Pin Tool\UTL Min Max Average Standard Deviation Psi 731 lbf 1,234 lbf 1,143 lbf 129 lbf Counterflow 1,049 lbf 1,204 lbf 1,117 lbf 45 lbf Modified Trivex 874 lbf 1,024 lbf 958 lbf 44 lbf Duo V-Flute 976 lbf 1,255 lbf 1,176 lbf 76 lbf Tri V-Flute 1,021 lbf 1,245 lbf 1,173 lbf 63 lbf UTL results refer to Appendix C Figure 63 shows that the Psi tool was able to match the average UTL of the Counterflow tool on the second DOE with a slightly reduced standard deviation, whereas the average UTL of the Counterflow tool dropped 16 lbf but the standard deviation was reduced by half. The average UTL of the Duo V-flute and Tri V-flute increased, and the standard deviation was reduced by half. The precision and repeatability of Octaspot swept FSSW increased as the range of weld parameters decreased by optimization of the process parameters. Metallographic cross-sections show minimal to no over-plunging, with minimal to no hooking defects, except for the Modified Trivex tool. 52

69 Low Z-force Swept FSSW 0.30 inch Shoulder Diameter 1400 Ultimate Tensile Load (lbf) Psi Counterflow Trivex Duo V-flute Tri V-flute High Z-force 0 DOE 1 DOE 2 Figure 63. UTL Results Comparison of Low Z-Force Octaspot Swept FSSW for Five Pin Tools with 0.30-Inch-Diameter Concave Shoulder in DOE 1 and DOE 2. Four tools showed the least significant effect of probe design to variation in UTL for Octaspot swept FSSW, except the Modified Trivex tool. The radical probe shape of the Modified Trivex tool with a small physical to swept unit volume ratio contributed to the variation of UTL due to the hooking defect. However, features on the probe, such as vertical flutes, inclined flats, threads, and opposing inclined flutes, contributed minimally to the variation of UTL in both DOEs. The features on the probes, such as threads, flutes, and flats, had a minimal effect on the tensile load due to the identical closed-loop path achieving closer values of the tensile load results. Features such as inclined flutes are preferred, due to the clean shear on the advancing side and minimal or no hooking at the joint interface. In Octaspot swept FSSW, the shape of the probe with the small ratio of physical to swept unit volume may promote the side displacement of material, which creates the hooking defect. 53

70 5.7 Surface Preparation Surface oxide of the weld coupon at the joint interface and pin tool contact interface was removed using a dual-action sander. Different operators removed a varied amount of surface oxide, which may have contributed to the reduction of the weld quality. However, all pin tools, except the Modified Trivex, were welded with surface oxide remaining on the weld coupons. MEK wipes remove dirt and oil without removing surface oxide. Figure 64 shows that there was no reduction in average UTL, the amount of surface oxide removed are not significant to the variation of UTL for Octaspot swept FSSW. The standard deviation of UTL for all tools increased slightly except for the Psi tool, where the standard deviation decreased. Therefore, reducing the surface preparation step in the manufacturing process can significantly save time as well as the cost of labor and consumables without sacrificing UTL Low Z-force Swept FSSW 0.30 inch Shoulder Diameter 1200 Ultimate Tensile Load (lbf) Psi Counterflow Trivex Duo V-flute Tri V-flute High Z-force 0 DOE 2 No Prep Figure 64. UTL Results Comparison of Low Z-Force Octaspot Swept FSSW for Four Pin Tools with No Surface Preparation in DOE 2. 54

71 5.8 Surface Finish The surface finish of each spot weld is very important, as the excessive expulsion of aluminum, known as flash generation, creates debris and requires post-weld touchup. Since the shoulder diameter was reduced from 0.40 inch to 0.30 inch for a lower Z-force, the small shoulder was unable to capture material displaced by the probe, thus creating a flash (Figure 65). Modifying the tilt angle weld parameter from half a degree to one degree significantly reduced the amount of flash (Figure 66). In the manufacturing process, the elimination of debris and postweld touchup can significantly reduce cost and manufacturing time. Figure 65. Low Z-Force FSSW with 0.30-Inch-Diameter Concave Shoulder with Half-Degree of Tilt Angle. Figure 66. Low Z-Force FSSW with 0.30-Inch-Diameter Concave Shoulder with One-Degree of Tilt Angle. 5.9 Scroll Shoulder Tool (0.30 Inch) Study (Phase 2) In the second phase, the flat scrolls shoulder was studied for comparison with concave shoulder pin tools. The effects of flat scrolls were also observed on the surface finish, crosssectional metallographic, and UTL. Only one pin tool probe design, Duo V-flute, was selected for this study due to budget and time constraints. From the concave shoulder diameter study and probe design study results in phase one, it was not necessary to study the effects of the features of flat scrolls on all pin tool designs. 55

72 5.9.1 Achievement Duo V-Flute Scroll Weld parameters used for the Duo V-flute Scroll pin tool were selected from a inch-diameter concave shoulder DOE 2, at 800 rpm to 1,100 rpm, 7 ipm to 13 ipm, and 750 lbf to 950 lbf. Using the same weld parameters and probe features, the flat scrolls feature compared fairly against the concave feature. The surface finish of Octaspot swept FSSW welds of flat scrolls is not as smooth as the concave shoulder because the flat scrolls feature extends out of the shoulder lip (Figure 67). The amount of flash generated using the flat scroll shoulder was less compared to the concave shoulder welded with similar weld parameters (Figure 65). The flat scrolls shoulder captured and directed material inward, generating a smaller amount of flash. The concave shoulder tool required changing the tilt angle to one degree to reduce the amount of flash generation (Figure 66), whereas the flat scrolls shoulder generated no flash with a halfdegree of tilt angle. Figure 67. Low Z-Force FSSW with 0.30-Inch-Diameter Flat Scrolls Shoulder with Half-Degree of Tilt Angle. The average UTL of the Duo V-flute Scroll was 1,147 lbf, with a standard deviation of 50 lbf. The average UTL of the Duo V-flute Scroll was comparable to the Psi, Counterflow, Duo V-flute, and Tri V-flute concave shoulder tools. The standard deviation of the flat scrolls shoulder was lower compared to the concave shoulder (Figure 68). 56

73 Low Z-force Swept FSSW 0.30 inch Diameter Shoulder 1400 Psi Ultimate Tensile Load (lbf) DOE 2 Counterflow Trivex Duo V-flute Tri V-flute High Z-force Duo V-flute Scroll Figure 68. UTL Results Comparison of Low Z-Force Octaspot Swept FSSW for 0.30-Inch-Diameter Scroll Shoulder Duo V-flute in DOE 2. The Z-force feedback plot shows no Z-force spike to a maximum spike of 100 lbf, in addition to the commanded Z-force. The fluctuation of Z-force about 50 lbf was observed in all 15 welds. Metallographic analysis showed some degree of hooking at the surface interface up to inch. The Duo V-flute with concave shoulder showed no sign of the hooking defect, but the Duo V-flute with flat scrolls shoulder had hooking defects. The flat scrolls features might promote a more aggressive flow arm directing material inward, thus creating hooking defects. Recent discovery that the pin tool holder had worn out and because the pin tool had run out up to inch might also be another cause for the hooking defect. Flat scroll features do not reduce the Z-force compared to the concave shoulder with the same shoulder diameter Featureless Probe Shape Study (Phase 3) The results of low average UTL coupled with metallographic analysis of the Modified Trivex tool confirmed the hooking defect in all 15 welds of the DOE. The concave pin tool study of different features on probe designs led to a study of probe shape, which showed that all four pin tools with circular-shaped probe designs performed better than the triangular-shaped 57

74 Trivex pin tool. Therefore, a full parametric study of probe shape was initiated to investigate the relationship of the probe physical to swept unit volume ratio and the hooking defect. It was hypothesized that increasing the number of sides of the probe to make it closer to a circular shape, or a ratio of probe physical to swept unit volume closer to 1, might reduce the hooking defect (Figure 9 and Table 1). Due to budget and time constraints, three pin tools were selected for this study: octagon, pentagon, and Trivex (Figure 15). The following weld parameters from DOE 2 were utilized: 800 rpm to 1,100 rpm, 7 ipm to13 ipm, and 750 lbf to 950 lbf Featureless Trivex The Featureless Trivex pin tool achieved average UTL of 915 lbf with a standard deviation of 50 lbf. The corresponding UTL and hooking defect ranged from to inch for the respective metallographic cross-sectional shown in Table 7. UTL values do not directly represent the severity of the hooking defect but more likely represent the set of process parameters. Metallographic analysis showed a similar size of the weld nugget area, except for the presence of the hooking defect (Figure 69, Figure 70, and Figure 71). Since these metallographic cross sectionals were taken using an inverted microscope, Figure 69 is flipped horizontally on purpose to match the eye view through the microscope. Therefore, the left side of the Figure 69 metallographic image corresponds to the right side of the metallographic sample. The threaded Trivex probe in the phase 1 design study showed that it achieved a similar average UTL of 958 lbf with a standard deviation of 44 lbf. Thus, the threaded features on the edge of the Trivex probe did not significantly enhance the UTL value. 58

75 TABLE 7 HOOKING DEFECT OF FEATURELESS TRIVEX PIN TOOL Featureless Trivex Corresponding UTL Hooking Defect (1/1000 inch) (lbf) Left Right M M M M M Figure 69. Featureless Trivex Cross-Sectional Metallographic M19. Figure 70. Right Side of Figure 69 with Inch Hooking Defect. 59

76 Figure 71. Left Side of Figure 69 with Inch Hooking Defect. 60

77 Featureless Pentagon The Featureless Pentagon pin tool achieved an average UTL of 1,093 lbf with a standard deviation of 66 lbf. The corresponding UTL and hooking defect ranged from 0 inch to inch for the respective metallographic cross-sectional shown in Table 8. As with the Trivex pin tool, severity of the hooking defect did not represent the UTL trend and depended on the set of process parameters. Metallographic analysis showed the presence of the hooking defect, but this was less severe than with the Trivex -shaped pin tool (Figure 72, Figure 73, and Figure 74). These metallographic images were taken using an inverted microscope; therefore, the left side of Figure 72 corresponds to the right side of the metallographic sample. TABLE 8 HOOKING DEFECT OF FEATURELESS PENTAGON PIN TOOL Featureless Pentagon Corresponding UTL Hooking Defect (1/1000 inch) (lbf) Left Right M M17 1, M18 1, M19 1, M20 1, Figure 72. Featureless Pentagon Cross-Sectional Metallographic M19. 61

78 Figure 73. Right Side of Figure 72 with Inch Hooking Defect. Figure 74. Left Side of Figure 72 with Inch Hooking Defect. 62

79 Featureless Octagon The Featureless Octagon pin tool achieved an average UTL of 1,033 lbf with a standard deviation of 23 lbf. The corresponding UTL and hooking defect ranged from to inch for the respective metallographic in the cross-sectional shown in Table 9. UTL values do not directly represent the severity of the hooking defect but are more representative of the set of process parameters. Metallographic analysis showed a similar size of weld nugget area, except for the presence of the hooking defect (Figure 75, Figure 76, and Figure 77). Since these metallographic images were also taken with an inverted microscope, the right side of the image is the left side of the metallographic sample. TABLE 9 HOOKING DEFECT OF FEATURELESS OCTAGON PIN TOOL Featureless Pentagon Corresponding UTL Hooking Defect (1/1000 inch) (lbf) Left Right M M M M M Figure 75. Featureless Octagon Cross-Sectional Metallographic M19. 63

80 Figure 76. Right Side of Figure 75 with Inch Hooking Defect. Figure 77. Left Side of Figure 75 with Inch Hooking Defect. 64

81 5.11 Achievement in Featureless Probe Shape Study (Phase 3) Table 10 shows the trend and summary of the hooking defect and its relationship to the ratio of probe physical to swept unit volume. The hooking defect values were recorded from five metallographic samples within the DOE weld parameters range. The hooking defect was averaged from four circular probes: Counterflow, Psi, Duo V-flute, and Tri V-flute. All hooking defect images were taken with an inverted microscope and measured using PaxIt image software. The depth of samples in a mount may vary, and the different amount of grinding and polishing of different mounted samples can affect the measurement of the hooking defect. Therefore, a direct comparison of the hooking defect across different pin-tool metallographic samples becomes less accurate, and metallographic samples within the same pin tool but mounted in different setting cups will skew the hooking defect values. The hooking defect was recorded in a two-dimensional or one cross-sectional segment. All metallographic samples were polished as close to the center of the spot weld or slightly passed the center. In this study, metallographic analysis found that the hooking defect could be three-dimensional, which may vary around the weld nugget. TABLE 10 SUMMARY OF HOOKING DEFECT AND RATIO OF PROBE PHYSICAL TO SWEPT UNIT VOLUME Probe Shape UTL +/- Standard Deviation (lbf) Ratio of Probe Physical to Swept Unit Volume Hooking Defect (1/1000 inch) Circular with Features ~ 1,150 +/ ~0-5 Featureless Octagon 1,033 +/ Featureless Pentagon 1,093 +/ Featureless Trivex 915 +/ Threaded Trivex 958 +/ UTL results refer to Appendix C 65

82 The hypothesis of reducing the hooking defect by increasing the ratio of probe physical to swept unit volume turned out to be false for this particular DOE set. Increasing the ratio of probe physical to swept unit volume was similar to increasing the number of sides from triangular, pentagon, and octagon, and did not show any trend supporting this hypothesis. However, the hypothesis is still plausible because from the Trivex -shaped tool to the Pentagon-shaped tool, the hooking defect was reduced. Therefore, a full parametric investigation should be able to confirm this hypothesis. Hooking defects can be reduced or eliminated by features on the probe, such as the flutes, threads, flats, or combinations of more than one feature with the proper set of process parameters. Locations of features on the probe are also crucial to eliminating the hooking defect because threads at the edge of the Trivex pin tool did not reduce the hooking defect. A combination of features is also important because the probe with threads alone creates sheet thinning, but with additional features, the Counterflow flute reduces sheet thinning in the linear lap weld Probe Diameter Study (Phase 4) In phase 1, the concave shoulder pin tool study, a probe with a inch diameter was unable to plunge at certain weld parameters of low Z-force of 700 lbf, low spindle speed of 800 rpm, and high travel speed of 13 ipm. Probe designs with a small probe tip area, such as the Psi tool with three inclined flats and the Trivex tool with a triangular-shaped probe were able to plunge at the lower extremes of the set of weld parameters mentioned previously. Therefore, a reduction of probe diameter from inch to inch will further reduce the required Z-force to plunge below 700 lbf. During the design step of reducing probe diameter size, it was determined that shoulder diameter could be reduced from 0.30 inch to 0.25 inch 66

83 (Figure 16c). This pin tool with the Duo V-flute probe was designed to reduce the Z-force below 700 lbf. Although the pin tool was designed to achieve a lower Z-force, another main objective of this study was to maintain static UTL of 1,100 lbf. In Octaspot, the spot radius was held constant with the same probe diameter, but the total weld radius had to be increased to compensate for the smaller probe diameter (Table 11). Increasing the spot radius to inch with a inch probe radius created a total weld radius of inch, slightly higher than the current probe s total weld radius of inch. TABLE 11 WELD RADIUS COMPENSATION FOR PROBE RADIUS REDUCTION Pin Tool Probe Probe Radius Spot Radius Total Weld Radius Current Probe inch inch inch Small Probe inch inch inch Small Probe inch inch inch Since this study used a new pin tool, new weld parameters range were selected to achieve a lower Z-force with a higher spindle speed range of 1300 rpm to 2000 rpm, travel speed range of 7 ipm to 13 ipm, and Z-force range of 450 lbf to 700 lbf. In addition to investigating spindle speed, travel speed, and Z-force weld parameters, tilt angle and spot radius were included. Earlier investigations in phase 1 showed that an increase in the tilt angle improved the surface finish for a shoulder-diameter modification from 0.4 inch to 0.3 inch. The weld spot radius was increased to compensate for the decrease of probe radius to remain comparable to the weld radius to achieve a UTL of 1,100 lbf. The average UTL results shown in Table 12 consist of a work order from CFSP09307_4, 5, and 6, with weld parameters of 1,300 rpm to 2,000 rpm, 7 ipm to 13 ipm, and 450 lbf to 700 lbf and optimization in work order CFSP09307_7 with weld parameters of 1,500 rpm to 1,800 67

84 rpm, 8 ipm to 12 ipm, and 500 lbf to 600 lbf. The average UTL with wide standard deviation was unable to provide significant results to confirm that the increase of tilt and spot radius increased the average UTL. Metallographic results provided additional information to explain the slightly lower average UTL and wide standard deviation. TABLE 12 AVERAGE UTL AND STANDARD DEVIATION OF DOE 1 FOR PROBE DIAMETER STUDY Work Order Average UTL (lbf) Standard Deviation (lbf) Tilt, Spot Radius (degree, inch) CFSP09307_ , 0.08 CFSP09307_ , 0.10 CFSP09307_ , 0.10 CFSP09307_7 1, , 0.10 UTL results refer to Appendix C One metallographic image (Figure 78) showed that a high spindle speed of 1,650 rpm with a combination of slow travel speed of 7 ipm and high Z-force of 700 lbf caused the tool to over plunge and the sheet to thin. Another metallographic image (Figure 79) showed that at a low spindle speed of 1,300 rpm with a mid-travel speed of 10 ipm and low Z-force of 450 lbf, it was not possible create a consolidated weld nugget. An unconsolidated weld is one of the main reasons for a low average UTL and wide standard deviation. Good process parameters at a spindle speed of 1,650 rpm, travel speed of 10 ipm, and Z-force of 575 lbf corresponded to a good metallographic image that showed no significant defects (Figure 80) with a high UTL value of 1,114 lbf. Metallographic images with obvious defects, shown in Figure 81, correspond to a spindle speed of 2,000 rpm, travel speed of 13 ipm, and Z-force of 575 lbf, which yielded a UTL of 662 lbf. Obvious defects due to an improper combination of weld parameters was another contribution to lower average UTL and wide standard deviation. Metallographic images with 68

85 microdefects, which can only be detected at higher magnification with slightly reduced UTL to 1,000 lbf, were more difficult to analyze. Figure 78. Metallographic Image of CFSP09307_6_M21. Figure 79. Metallographic Image of CFSP09307_6_M17. Figure 80. Metallographic Image of CFSP09307_6_M19. Figure 81. Metallographic Image of CFSP09307_6_M23. Although this DOE consists of weld parameters that can achieve a low Z-force of about 575 lbf with a high spindle speed up to 2,000 rpm, metallographic analysis showed defects, a surface oxide line defect, and a confirmed lower UTL value. Since the FSSW process is highly 69

86 dependent on the amount of heat input, spindle speed and travel speed can significantly affect the amount of Z-force that should be applied to obtain a sound FSSW. The concave shoulder diameter study in phase 1 revealed that shoulder diameter is another factor affecting the amount of Z-force required to produce a sound FSSW using the same weld parameters. Therefore, a second DOE was initiated with a lower spindle speed range of 800 rpm to 1,100 rpm, travel speed range of 7 ipm to 13 ipm, and Z-force range of 600 lbf to 800 lbf, similar to that used in the concave shoulder diameter study DOE except for the Z-force. UTL results showed that the Z- force at 600 lbf and a combination of low spindle speed of 800 rpm or 950 rpm and travel speed of 10 ipm or 13 ipm, respectively, were unable to produce a sound FSSW and was confirmed with a metallographic image that looks similar to Figure 79. In the DOE 2, weld parameters in combination with a Z-force of 700 lbf produced FSSW with a UTL above 1,000 lbf up to 1,100 lbf. Results showed that reduction of the shoulder diameter from 0.3 inch to 0.25 inch did not significantly reduce the amount of Z-force to produce a sound FSSW. Optimization of the weld parameters by changing the Z-force range from 650 lbf to 750 lbf with a similar spindle speed and travel speed range yielded an average UTL of 1,091 lbf with a standard deviation of 45 lbf. Reduction of the shoulder diameter can increase the sensitivity of weld parameters: a Z-force range of at least 300 lbf for a 0.3-inch-diameter shoulder was reduced to 100 lbf for a 0.25-inch-diameter shoulder. The amount of heat input through frictional heat of the shoulder and probe can also affect the Z-force. A pin tool with a 0.1-inch-diameter probe with a 0.25-inch-diameter shoulder produced less frictional heat than a inch-diameter probe with a 0.3-inch-diameter shoulder, which can increase the Z-force required to produce a sound FSSW. An optimum shoulder diameter can reduce sensitivity of weld parameters, lower the Z-force, and produce a sound FSSW. 70

87 In the DOE 2, the metallographic image in Figure 82 confirmed its respective UTL value of 1,111 lbf showing no sign of defects. However, at high magnification, the metallographic image in Figure 83 provided further details, which the UTL results were unable to detect, such as microdefects and a surface oxide line. These are defects that cannot be detected by static shear testing because the defects are protected by the weld nugget, which is strengthen by fine, dynamically recrystallized grains. Some metallographic images from both DOEs showed some microdefects and surface oxide lines in the weld nugget. These microdefects and surface oxide lines can be a source of crack initiation, which can be detrimental in fatigue tests. Further investigation will be required to eliminate voids, and surface oxide line defects using more aggressive probe flute depths, and inadequate overlap of the probe radius and spot radius. Figure 82. Metallographic Image of CFSP09307_12_M20. 71

88 Figure 83. Right Side of Nugget in Figure 82. Although the Z-force was reduced by changing the weld parameter, pin tools have a unique range of weld parameters, which dictate the Z-force range. Pin tools with different shoulder and probe diameters and different probe designs with flutes, threads, flats, and shapes can also contribute to and limit the range of weld parameters. The second DOE using a inch-diameter shoulder and 0.1-inch-diameter probe welded with a similar spindle speed and travel speed of 0.30-inch-diameter shoulder and inch-diameter probe achieved a reduction of 150 lbf Z-force. The change from a 0.4-inch-diameter shoulder to a 0.3-inch-diameter shoulder achieved a reduction of 250 lbf Z-force. The Counterflow tool reached the same UTL value, regardless of shoulder size, with a similar Z-force. Pin tools have unique weld parameters because 0.4-inch-diameter and 0.3-inch-diameter shoulders have the same UTL with the same weld parameters, regardless of the difference in 72