Friction Stir Welding and Processing: An Overview

Transcription

1 Friction Stir Welding and Processing: An Overview Vikram A. Kolhe 1, Dnyandeo D. Shinde 2 vikram.kolhe@siem.org 1, dnyandeo.shinde@siem.org 2 Department of Mechanical Engineering 1, 2 Sandip Institute of Engineering & Management, Nasik 1, 2 Abstract Friction stir welding is a solid-state joining process that has gained acceptable progress in recent years. This method which was first used for welding of aluminum and its alloys is now employed for welding of other materials such as polymers and composites. Friction Stir Welding (FSW) is a solid state welding process in which the relative motion between the tool and the work piece produces heat which makes the material of two edges being joined by plastic atomic diffusion. Friction surfacing is a relatively new technology which is capable of producing coatings with zero dilution and good metallurgical bonding. This is attained because no melting is involved in this process. Friction surfacing is a solid state deposition process for producing wear and corrosion resistant coatings on metallic surfaces, which involves a rotating rod pushed against a horizontally moving plate. The rotating rod is the coating material and the plate is the substrate. Friction Stir Welding: Friction stir welding as shown in Fig.1 is a solid-state joining process that has gained acceptable progress in recent years. This method which was first used for welding of aluminum and its alloys is now employed for welding of other materials such as polymers and composites. Friction Stir Welding (FSW) is a solid state welding process in which the relative motion between the tool and the work piece produces heat which makes the material of two edges being joined by plastic atomic diffusion. This method relies on the direct conversion of mechanical energy to thermal energy to form the weld without the application of heat from conventional source [2]. Keywords Friction Stir Welding, Friction Surfacing, FSW Parameters. I. INTRODUCTION The friction based processing technologies encloses some of the most significant solid state manufacturing technologies for producing structural components in aluminum alloys. The FS allows the dissimilar joining of materials that would be metallurgical incompatible otherwise. It allows assembling in a single composite component, tailored material property combinations which are difficult to gather in a single monolithic material. The FS has been used in the production of long-life industrial blades, wear resistant components, anticorrosion coatings and in the rehabilitation of worn or damaged parts such as, turbine blade tips and agricultural machinery. Friction stir welding is a solid-state joining process that has gained acceptable progress in recent years. This method which was first used for welding of aluminum and its alloys is now employed for welding of other materials such as polymers and composites. Friction Stir Welding (FSW) is a solid state welding process in which the relative motion between the tool and the work piece produces heat which makes the material of two edges being joined by plastic atomic diffusion. This method relies on the direct conversion of mechanical energy to thermal energy to form the weld without the application of heat from conventional source [10]. Fig.1- Schematic diagram of Friction Stir Welding A. Process Parameters The rotational speed of the tools, the axial pressure and welding speed and the (weld time) are the principal variables that are controlled in order to provide the necessary combination of heat and pressure to form the weld. These parameters are adjusted so that the interface is heated into the plastic temperature range (plastic state) where welding can take place. During the last stage of welding process, atomic diffusion occurs while the interfaces are in contact, allowing metallurgical bond to form between the two materials. FSW/FS involves complex material movement and plastic deformation. Welding parameters, tool geometry, IJETT ISSN: September 2014 Volume 1 Issue 1 194

2 and joint design exert significant effect on the material flow pattern and temperature distribution, thereby influencing the microstructural evolution of material. In this section, a few major factors affecting FSW/FSP process, such as tool geometry, welding parameters, joint design are addressed [11]. The main FSW process parameters are the following: Tool geometry; Plunge speed and depth of probe in work pieces; Tool rotational speed and direction; Travel Speed along the joint line; Axial load; Dwell time at start of the weld; Clamping system (stiffness, precision and material of the anvil, for easy extraction of work pieces); Tilt and side tilt angles; Control during plunge, dwell and weld periods: Force control versus position control; Preheating/inter-pass temperature of work pieces. B. Influence of Tool Geometry Tool geometry is the most influential aspect of process development. The tool geometry plays a critical role in material flow and in turn governs the traverse rate at which FSW can be conducted. An FSW tool consists of a shoulder and a pin as shown schematically in Fig.2. importance in the welding of dissimilar metals where intermetallic compounds may be formed. The best procedure to lower the peak temperature would be to adopt lower speeds of rotation. D. Influence of pressure Pressure is an important parameter as it influences the temperature gradient in friction welding as well as the driving torque and power. The heating pressure chosen should be sufficient to maintain the surfaces in intimate contact to prevent atmospheric contamination. The optimum value of the pressure depends on the materials being welded and their size. Too low a value of pressure enlarges the heat affected zone due to lower power input while heavy pressure would extrude the plastic materials prematurely forcing cold material into contact with eventual lack of bonding defects. The forging pressure depends on the hot strength of the alloys being joined. The pressure chosen for this operation must be sufficient to consolidate. E. Influence of heating time or burn off Duration of heating is considered as the third basic parameter of the process though it depends on the rotary speed and pressure. For a given setting, duration of heating determines the energy input into the materials. Since the heating time is governed by the plastic deformation of the materials, this parameter is sometimes replaced by the upset or burn off. The monitoring of burn off has been found to be advantageous in the case of fully automatic machines since the monitoring of duration of heating does not take into account the surface irregularities on the joining faces. This technique also permits the upset length to be predicted to as close as 0.1 mm. Fig.2: An Actual Tool, with a Threaded - Pin. C. Influence of rotary speed It has been established that satisfactory weld can be made for a wide range of materials and sizes with peripheral speeds in the range of 75 to 105 m/min. Lower speeds are not recommended due to the high torques developed with work handling problems while higher speed produce a wider heat affected zone. Rotary speed has an important influence on the steady state temperature reached in the process. The rate of heat generation and heat dissipation balance each other at a steady value which depends on the process settings, particularly the rotary speed. This factor is of F. Selection of parameters A number of materials and their combinations such as steels of all types (plain carbon or alloy steels, low / medium and high carbon varieties, tool steels, stainless steels, maraging steels etc), copper and its alloys, aluminum and its alloys are being extensively friction welded with reproducible accuracy both in size tolerance and in mechanical properties. Table I serves as a guide to the selection of parameters for some of the material combinations being welded commercially. Though this table indicates the parameters for specific sizes, it could be well be used to work out the parameters for other sizes with slight modifications. Thus, the speed of rotation could be determined using the condition of constancy of peripheral velocity by the pressure values are to be corrected through a factor of two times the ratio of diameters. The upset values are to be determined experimentally for the actual production jobs. However, the upset specified in Table I would suffice for most of the sizes being friction welded in practice. Advantages of Friction Stir Welding: IJETT ISSN: September 2014 Volume 1 Issue 1 195

3 1. Low distortion, even in long welds 2. Excellent mechanical properties 3. Low shrinkage 4. Some tolerance to imperfect weld preparation (thin oxide layers) 5. Welding Preparation not usually required 6. Non-consumable tool 7. Welding of wrought materials to castings 8. No spatter, No filler wire required 9. No welder certification required 10. Energy efficient, Post weld processes not required 11. No porosity. II. FRICICTION SURFACING Fig.3 shows a simple schematic diagram of friction surfacing process. The frictional heat generated during the process plasticizes the consumable rod. The plasticized metal gets deposited on to the substrate creating a relatively thick coating with good inter-facial metallurgical bonding. The width of the coating depends on the diameter of the consumable rod and is normally in the range of 0.9 times the rod diameter [3]. configurations and along complex trajectories. Some examples of FS path case studies can be seen in Fig. 4 a displays a single FS curvilinear path, while Fig.2. depicts a continuous cylindrical build-up, as the consumable rod moves along a 3D helicoidal trajectory. Given the rough coating surface, FS is often followed by post processing operations in order to achieve the desired geometry and surface finish. The Fig. 3 depicts the milling surface Linear Friction Based Processing Technologies for Aluminum friction surfaced AA6082-T6 deposit presenting a fully bonded defect-free layer and a smooth surface finish. Low carbon steels are widely used for structural applications because of its ease in fabrication and the moderate strength it posses. However, its pure corrosion resistance at normal atmosphere is a matter of serious concern. Hard facing /coating techniques based on fusion welding and thermal spraying are generally employed to protect steel surface from corrosion. Fig 4: Examples of Friction Surfacing Trajectories Fig.3.: Friction Surfacing Process The FS allows the dissimilar joining of materials that would be metallurgical incompatible otherwise. It allows assembling in a single composite component, tailored material property combinations which are difficult to gather in a single monolithic material. This enables an advanced and detailed design, adjusting the required material properties according to different loading areas of a part and precluding the use of more expensive and specific materials capable of assembling all functional requirements. Although FS has limited largeoverlay capabilities, this process is ideal for localized repair and cladding. The FS has been used in the production of long-life industrial blades, wear resistant components, anti-corrosion coatings and in the rehabilitation of worn or damaged parts such as, turbine blade tips and agricultural machinery. Other applications feature the hard facing of valve seats, brake disks and tools such as punches, guillotine blades and drills. The FS can be performed over a great variety of substrate Fig.5: Surface Finish by Milling Fusion welding based coating techniques generally suffers from dilution and thermal spraying, results in mechanical bonding rather than metallurgical bonding. Friction surfacing is a relatively new technology which is capable of producing coatings with zero dilution and good metallurgical bonding. This is attained because no melting is involved in this process. Friction surfacing is a solid state deposition process for producing wear and corrosion resistant coatings on metallic surfaces, which involves a rotating rod pushed against a horizontally moving plate. The rotating rod is the coating material and the plate is the substrate. Fig.2. shows a simple schematic diagram of friction IJETT ISSN: September 2014 Volume 1 Issue 1 196

4 surfacing process. The plasticized metal gets deposited on to the substrate creating a relatively thick coating with good inter facial metallurgical bonding. The width of the coating depends on the diameter of the consumable rod and is normally in the range of 0.9 times the rod diameter [3]. Fig.6: Typical Coating Produced by Friction Surfacing. A. Process Parameters Coatings are evaluated based on thickness, width and bond strength/extension which depend on controllable process parameters, such as, i) forging force; ii) rotation speed and iii) travel speed. Substrate thickness, rod diameter and material properties define the thermo-mechanical system thus determining process parameters [4]: Forging force - improves bonding extension and results in wider and thinner deposits. However, excessive loads result in non-uniform deposition with a depression at the middle of the pass due to material expelling from the region beyond consumable rod diameter. Insufficient forging forces result in poor consolidated interfaces. Rotation speed - influences the bonding quality and coating width. While lower to intermediate rotation speeds enhance bonding quality, higher rotation speeds produce a more flat and regular deposit, with a more effective forging effect shaping the coating. Travel speed - strongly influences coating thickness and width, since it determines the rate at which material is deposited. As such, higher travel speeds result in thinner deposits. Faster travel speeds lead to shorter heat exposure periods, resulting in less grain growth and finer microstructures. Thinner deposits also cool more rapidly. The substrate heat affected zone decreases for higher travel speeds. Bonding at coating edges deteriorates for faster travel speeds. Tilt angle - A small tilting the consumable rod, in less than 3, has proven to reduce the unbounded extension of the deposit at the coating edges, by enabling a gradual increase of forging pressure applied by the consumable rod on the substrate, from the tip to the tail zone being thermo mechanically processed, at each instant. B. Advantages and limitations 1. Being a solid state process, FS allows depositing various dissimilar material combinations. Investigations report the deposition of stainless steel, tool steel and nickel-based alloys (Inconel) on mild steel substrates, as well as, stainless steel, mild steel and inconel consumables on aluminum substrates. 2. FS is best suited for applications with material compatibility issues. The process involves a hot forging action, which refines significantly the microstructure of the deposited material. The deposit is inherently homogenous and has good mechanical strength. 3. Since FS is mainly based on plastic deformation, this process presents some advantages over other coating technologies based on fusion welding or heat-spraying processes. Apart from avoiding defects commonly associated to fusion and solidification mechanisms, the heat input in FS is minimum and localized, preventing part distortion and minimizing the heat affected zone extension and dilution. This also makes FS suitable to process thermal sensitive materials, such as, aluminum alloys. Additionally, the absence of spatter, toxic fumes and emission of radiation makes this process cleaner and environmentally friendly. The absence of fusion and fast cooling rates enable to FS in a great variety of positions. III. CONCLUSION Friction Stir Welding has evolved as a mature and efficient solid state joining method, for the joining of aluminum alloys. However, there is a need to develop special materials/tool design for the joining of steels, zirconium and titanium alloys. In summary, significant progress has been made in understanding the problems and issues related to the friction stir welding & surfacing of materials. Much of the progress has been in better understanding of the physical processes of dissimilar alloy welding, and the structure and properties of various welds. However, cost effective and reliable welding of aluminum and other light weight alloys with harder alloys such as steels will require considerable further development. The need in the engineering industries for sophisticated materials tailored to accommodate demands for rigid, light weight structures may be powerful drivers for the further development of the friction stir welding & surfacing. REFERENCES [1] G.F. Batalha, A. Farias, R. Magnabosco, S. Delijaicov, M. Adamiak, L.A. Dobrzański, Evaluation of an AlCrN coated FSW tool, Journal of achievements in Materials and Manufacturing Engineering, Vol.55, issue 2, IJETT ISSN: September 2014 Volume 1 Issue 1 197

5 [2] R.S. Mishra, and Z.Y. Ma, Friction stir welding and processing,materials Science and Engineering R 50 (2005) [3] Pedro Vilaça, Joao Gandra and Catarina Vidal, Linear Friction Based Processing Technologies for Aluminum Alloys: Surfacing, Stir Welding and Stir Channeling, [4] H. Khalid Rafi, G. D. Janaki Ram, G. Phanikumar and K. Prasad Rao, Friction Surfacing of Austenitic Stainless Steel on Low Carbon Steel: Studies on the Effects of Traverse Speed, Proceedings of the World Congress on Engineering 2010 Vol II, WCE 2010, June 30 - July 2, 2010, London, U.K. [5] J.A. Lee, R.W. Carter, and J. Ding, Friction Stir Welding for Aluminum Metal Matrix Composites (MMC's), NASA / TM [6] A Thangarasu, N Murugan, I Dinaharan, And S J Vijay, Microstructure and microhardness of AA1050/TiC surface composite fabricated using friction stir processing, Sadhana,, Vol. 37, Part 5, October 2012, pp c Indian Academy of Sciences. [7] N. T. Kumbhar and K. Bhanumurthy, Friction Stir Welding of Al 6061 Alloy, Asian J. Exp. Sci., Vol. 22, No. 2, 2008; [8] Ichinori Shigematsu1, Yong-Jai Kwon; and Naobumi Saito, Dissimilar Friction Stir Welding for Tailor-Welded Blanks of Aluminum and Magnesium Alloys, Materials Transactions, Vol. 50, No. 1 (2009) pp. 197 to 203. [9] Dhananjayulu Avula, Ratnesh Kumar Raj Singh, D.K.Dwivedi, N.K.Mehta, Effect of Friction Stir Welding on Microstructural and Mechanical Properties of Copper Alloy, World Academy of Science, Engineering and Technology [10] An Overview Of R&D Work In Friction Stir Welding at Smu, V. Soundararajan, M. Valantand R. Kovacevic, and Research Center for Advanced Manufacturing (RCAM) Department of Mechanical Engineering Southern Methodist University, Dallas, Texas. [11] Dr. K.A. Asokkumar, BHEL, Tiruchirappalli, Friction Welding Process principles, Two day workshop on Friction Welding & Friction Stir Welding, Nov. 24 & 25, 2011 IJETT ISSN: September 2014 Volume 1 Issue 1 198