CHAPTER 3 MATERIALS AND EXPERIMENTAL METHODS

Transcription

1 45 CHAPTER 3 MATERIALS AND EXPERIMENTAL METHODS This chapter is divided into six different sections. The section one explains the selection of materials and process flow in this research work. Section two and three explains the mechanical testing and microstructural analysis involved in this research work. The last three sections explain the friction welding, the pin-on-disc sliding wear and the hot compression testing process and their input process parameters. 3.1SELECTION OF MATERIALS AND PROCESS FLOW The received AZ31B magnesium alloy and magnesium nano-composite of magnesium used in the present investigation were synthesized by hybrid casting process (Disintegrated Melt Deposition) and hot extruded at 350 C with an extrusion ratio of 20 : 1. The experimental set-up disintegrated melt deposition is shown in Figure 3.1. The size of the alumina particle used is 50 nm. The cast-extruded pure magnesium alloy and pure magnesium 1.5 vol.% alumina (Al2O3) 1 wt.% calcium (Ca)magnesium nano composite were supplied by National University of Singapore. The addition of calcium has oxidation resistance at higher melting temperatures (Nguyen & Gupta 2009). Rods with a 20 mm diameter were obtained by hot extrusion. The rods were further sized according to specification requirements. The chemical composition and mechanical properties of the cast-extruded base material are provided in Tables 3.1 and 3.2.The work flow in this research is given in Figure 3.2.

2 46 Table 3.1 Chemical Composition (wt %) of as received AZ31B magnesium alloys (Nguyen 2009) Base Material Al Zn Mn Fe Si Cu Ni Mg Al 2 O 3 (Vol.%) Ca MGAL Bal MGNC Bal Table 3.2 Mechanical Properties of as received AZ31B magnesium alloys ( good agreement with Nguyen 2009) Materials Microhardness (HV) Yield Strength (MPa) Ultimate Tensile Strength (MPa) Elongation (%) MGAL MGNC

3 Figure 3.1 Schematic Diagram of DMD setup (Nguyen 2009) 47

4 48 Selection of materials Friction Welding Dry Sliding Wear Behavior Hot Deformation Behavior Preliminary Friction Welding for MGNC Selection of Sliding Wear Parameters Selection of Hot Deformation Parameters Mechanical Testing (Tensile Strength) Wear Testing using Pin-on-Disc apparatus Hot Compressions Testing Establish welding parameters using CCD Establish Wear Mechanisms (SEM, XRD) Establish Stress Strain Curves Conduct Friction Welding for MGAL specimens using the CCD parameters Constitutive modeling and establish apparent activation energy using Zener-Holloman Paramter Mechanical Testing (Tensile Strength and fracture study) Establish Deformation Mechanism (Optical microscopy, TEM analysis) Analyze the tensile strength using ANOVA and correlate with ANN Establish Processing Map (Stability DRX / DRV Instability Twining etc.,) Characterization (Optical microscopy, SEM and EDAX) Results and Discussion Conduct Friction Welding for Optimal Welding parameter for MGNC specimens. Conclusion Repeat the testing procedures for friction welded MGNC specimens Figure 3.2 Process Flow Diagram

5 MECHANICAL TESTING Mechanical testing was carriedout in order to obtain the tensile strength, nature of fracture during tensile testing and microhardness for the welded joints. The mechanical testing was carried for both MGAL and MGNC magnesium alloys. The testing procedures were carried out according to the ASM and ASTM standards Tensile Testing The smooth round bar tensile test of the friction welded samples was carried out in accordance with ASTM standard E8M-04. The samples were machined to a gauge diameter of 9 mm and gauge length of 45 mm. The universal testing machine used was 100 kn electromechanical controlled universal testing machine. The tensile specimens were prepared to evaluate yield strength, tensile strength and elongation. Figure 3.3 shows the tensile specimen before and after testing. Figure 3.3 Specimen before and after tensile testing

6 Fractrography Fractrography of the fractured surfaces of tensile test samples was conducted using a Scanning Electron Microscope (SEM). Fractrography study is important to depict the nature of failure as per the detailed discussion made in Chapter Microhardness The microhardness was measured across the weld at the mid-thickness region using Vicker smicrohardness tester. The microhardness was used for measuring the hardness of the weld region with 0.05 kg load for 20 s. Well polished welded samples of MGAL and MGNC were used for microhardness testing. 3.3 MICROSTRUCTURAL EVOLUTION Optical Microscopy The welded samples were sectioned exactly in center along the axis of the specimen as shown in the macrograph (Figure 3.4). For the hot deformed samples the samples were cut along the compression axis. While sectioning (sawing) utmost care was taken to avoid change microstructure due to cold working which complicates the microstructural interpretation. Two successive dry grinding was followed in order to avoid the twins during cold working. Firstly the samples were ground using silica carding abrasive belt grinding to remove the sawing marks. Secondly manual grinding was done using silicon carbide paper in stages of 600, 800, 1200 grit and finally fine grade 0 emery was used.

7 51 Figure 3.4 Macrograph of the weld specimen Mechanical disc polishing was done on cotton velvet by applying alumina slurry (alumina powder + distilled water) at 200 rpm. After polishing, rinsing was done to clean the polished surfaces using ethanol. Picric acid solution of 4g picric acid, 20ml acetic acid, 60ml ethanol and 20ml distilled water were used for etching the polished surface to reveal the grain boundaries (Michael 1999 and Jager 2006). The microstructural investigation included grain morphology, presence of twins and distribution of second phases Scanning Electron Microscopy (SEM) Scanning Electron Microscope (SEM) was used to obtain high quality images like fracture surfacesand wear surface. The SEM facility used for the research work is shown in the Figure 3.5.

8 52 Figure 3.5 JEOL SEM facility High Resolution Scanning Electron Microscopy (HRSEM) The Quanta 200 FEG scanning electron microscope (SEM) is a versatile high resolution scanning electron microscope with three modes of operation, namely, the high vacuum mode for metallic (electrically conducting) sample, low vacuum and environment scanning electron microscope (ESEM) modes for insulating, ceramic, polymeric (electrically insulating) and biological samples respectively. Apart from giving the high resolution surface morphological images, the Quanta 200 FEG also has the analytical capabilities such as detecting the presence of elements down to boron (B) on any solid conducting materials through the energy dispersive X-ray spectrometry (EDX) providing crystalline information from the few nano meter depth of the material surface. The resolution is 1.2 nm gold particle separation on a carbon substrate and magnification from a min of 12x to greater than 1, 00,000 X. The Quanta 200 field emission gun scanning election microscope facility is shown in Figure 3.6.

9 53 Figure 3.6 Quanta 200 FEG scanning electron microscope (SEM) High Resolution Transmission Electron Microscopy (HRTEM) This microscope (HRTEM) provides one of the best-in-class solutions to problems in diverse fields ranging from Materials Science to Biology. This microscope enables to view lattice resolution of 0.14 nm and point-to-point resolution of 0.19 nm with the following features like 200 kv acceleration voltage and provision to work with lower voltages according to the sample requirements. The features include Bright Field (BF) and Dark Field (DF) imaging, High Resolution Electron Microscopy (HREM), Selected Area Electron Diffraction (SAED) and Energy Dispersive X-ray Analysis (EDS. The deformed specimens of 3mm in diameter were sectioned in the centre parallel to the compression axis. The cut-surface was mounted and polished for metallographic examination using transmission electron microscopy (TEM). The cut disks were mechanically ground to less than 100 m in thickness, followed by dimple grinding the disk center to less than 20 m in thickness.

10 54 Finally, the samples were ion-milled to perforation at an ion accelerating voltage of 3 kv. After ion-milling the samples were loaded into the microscope to avoid oxide layer formation. 3.4 FRICTION WELDING The friction welding machine, which was capable of high precision joining, had a spindle driven by an AC motor. Friction and upset forces were read by a load cell and precisely controlled by a digital read out. The machine had a maximum upset force of 20 kn. The spindle motor rotated at a constant speed of 1500 RPM. The experimental set-up is shown in Figure 3.8. Maximum joint efficiency was chosen as optimal weld condition for the friction welding of AZ31B Mg alloys. The joint efficiency is defined as the ratio between the ultimate tensile strength of the weld material to the ultimate tensile strength of the base material. Figure 3.7 Friction welding experimental set-up

11 Selection of friction welding parameters Cast-extruded billets of MGNC samples were cut into rods of 18 mm in diameter and 75 mm long for conducting preliminary experiments as given in Table 3.3. After acquiring the preliminary data a detailed review was made and accordingly the friction welding parameters and the samples sizes were changed. For obtaining optimal welding condition the test was conducted for the magnesium alloy (MGAL) (Antonio, 2004). The revised friction welding parameters used for finding optimal condition are tabulated in Table 3.4. Table 3.3 Preliminary friction welding parameters. Test No. Friction pressure (MPa) Friction time (s) Upsetting pressure (MPa) Upsetting time (s) T T T T T T T T T

12 56 The friction welding parameters were chosen according to central composite design (CCD) matrix. In CCD, central composite face centered (CCF) design matrix with the star points being at the center of each face of factorial space was used, so = ±1. The chosen CCF design matrix with the levels is tabulated in Table 3.5 as given in the Design Expert statistical software package. The factorial three level coded values were chosen as maximum of +1 to minimum of -1. The selected parameters according to the chosen levels are tabulated in Table 3.6. Friction welding was carried out according to the parameters given in Table 3.6. Testing for optimality is discussed in detail in chapter 4. The friction welding was done for magnesium nano-composite using the optimal weld condition. Table 3.4 Friction welding parameters for obtaining optimal condition Diameter of the sample - 16 (mm) Length of the sample - 75 (mm) Parameter Rotational speed (N) (rpm) Upsetting time (UT) - 5 (s) Level Friction pressure (FP ) (MPa) Friction time (FT) (MPa) Upsetting pressure (UP) (s)

13 57 Table 3.5 Experimental design matrix Standard Run Coded Values FP (MPa) FT (s) UP(MPa)

14 58 Table 3.6 Experimental input parameters Standard Run Real Values FP (MPa) FT (s) UP(MPa)

15 Friction Welding Procedure The weld samples were cut according the sizes as motioned in the previous section Emery was applied to weld surface to make it rough for good bonging prior to welding. Then samples were held in the chucks of head stock and upsetting head. The welded parameters as per the plan of experiments discussed in section 3.5.1are fed into the programmable logical controller. Now the operation cycle was started. Once the welding was completed the welded sample were removed for mechanical and microstructural evolution as discussed in the earlier sections. 3.5 SLIDING WEAR TESTING Pin-on-disc experiment was used to obtain the dry sliding wear behavior of both MGNC and MGAL alloys. The experimental set-up of the pin-on-disc tester is shown in Figure 3.9. A pin holder loaded the stationary pins vertically onto a rotating OHNS steel disc, which had been oil-hardened to62 HRC. The pin is fastened rigid into friction arm of the set-up. Figure 3.8 Pin-on-disc experimental set-up

16 60 The experimental parameter for dry sliding is tabulated in Table 3.7. Prior to sliding, the disc was ground against 600-grit SiC paper for a few minutes to remove accumulated debris on the wear track, followed by cleaning with acetone. At the end of every sliding distance, the pins were carefully cleaned with acetone. The pins were weighed using a sensitive electronic balance with an accuracy of ±0.1 mg to determine the weight loss. On certain pins, material had been extruded from the pin surface and later re-solidified around the periphery. Since these extruded layers should rightly be considered as material worn out, they were carefully filed off prior to weighing so as not to add falsely to the weight. Table 3.7 Sliding wear parameters Diameter of the sample - 8 (mm) Length of the sample 15 (mm) Room Temperature C Normal load 10 N Sliding distance (D) (m) Sliding velocity (S) (m.s -1 ) HOT COMPRESSION TESTING Isothermal hot compression tests were conducted using servo-controlled universal testing machine with a maximum load capacity of 100 kn. The hot deformation of the material was investigated by hot compression tests at the temperature (T) range of C and in the strain rate ( ) range of s -1. The size of the specimens used in the test was 15mm height and15 mm diameter. For inserting a thermocouple to measure the specimen temperature and the adiabatic temperature rise during deformation, the specimens were

17 provided with 0.8 mm diameter hole machined at mid height to reach the centre 61 of the specimen. Dry graphite spray was used as the lubricant in all the experiments. All the samples were heated to the predetermined temperature for 20 minutes. The specimens were deformed up to a true strain of 0.5 and then quenched in water. The hot compression input parameter is given in Table 3.8. Table 3.8 Hot compression input parameters Temperature (T) ( C) Strain rate ( ) (s -1 )