Theoretical Background
4.1 Friction stir welding
4.1.3 Effect of tool pin profile during FSW
Chapter 4
Experimental Investigation
Figure 4.12. (a) Straight cylindrical pin tool C, (b) Straight cylindrical square thread pin tool CS, (c) Straight cylindrical V-thread pin tool CV, (d) Taper cylindrical pin tool T, (e) Taper cylindrical square thread pin tool TS and (f) Taper cylindrical V-thread pin tool TV.
Figure 4.13. Schematic illustration of tool profile (C) Straight cylindrical pin tool, (T) Straight cylindrical square thread pin tool, (CS) Straight cylindrical V-thread pin tool, (CV) Taper cylindrical pin tool, (TS) Taper cylindrical square thread pin tool and (TV) Taper cylindrical V-thread pin tool.
Chapter 4
Table 4.6. Design of Experiment of similar FSW joints.
Exp.
No.
Tool rotational speed (RPM)
Welding speed (mm/min)
Tool pin profile
1 600 98 C
2 1100 98 C
3 1500 98 C
4 600 98 CS
5 1100 98 CS
6 1500 98 CS
7 600 98 CV
8 1100 98 CV
9 1500 98 CV
10 600 98 T
11 1100 98 T
12 1500 98 T
13 600 98 TS
14 1100 98 TS
15 1500 98 TS
16 600 98 TV
17 1100 98 TV
18 1500 98 TV
The time–temperature history of advancing and retreating sides at 1100 rpm and CS pin profile tool is depicted in Fig 4.14. The temperature profile of each thermocouple point corresponding to the welding time relates the displacement of the tool from the starting point.
It observed that the temperature on the advancing side at all four locations is higher than the retreating side, although the temperature is measured in corresponding similar points. This temperature difference is the cause of asymmetric heat generation and plasticized flow of hot material from retreating side to advancing side. As moves away from the weld center line, the
Experimental Investigation
temperature difference is reduced and reaches equilibrium at certain distance due to similar rate of heat diffusion. However, the temperature in advancing side close to the weld center line is higher than that of the retreating side. It can be concluded that variations in dimensions and tool geometry of FSW tools along with tool rotational speed have prominent effects on thermal history [180].
Figure 4.14. Thermal history at 1100 rpm and CS pin profile tool (a) advancing side (b) retreating side.
Figure 4.15 illustrates the comparison of peak temperature between advancing and retreating sides for the experimental conditions shown in Table 4.6. It’s observed that the peak temperature obtained at 1500 rpm is higher than 600 and 1100 rpm due to increase in rate of heat generation with increase in tool rotational speed (from 600 to 1500 rpm).
Similarly, increase in surface area of pin promotes (threaded pin) the heat generation rate due to that peak temperature obtained at TC1 and TC5 in case of threaded pin is 10 to15 K higher that straight pin. These threaded pins are more effective in weld zone to enhance heat generation through plastic deformation and material flow. Cylindrical pin with square thread FSW tool produce maximum peak temperature at TC1 and TC5 due to high surface area as compare to other. However, the effect of tool rotation is more prominent in heat generation than tool geometry [12].
Chapter 4
Figure 4.15. Comparison of peak temperature of different experimental conditions at TC1 advancing side and TC5 retreating side for experimental condition (a) 600 rpm, (b) 1100 rpm and (c) 1500 rpm.
Figure 4.16 shows the axial force on the tool with respect to welding time at 1100 rpm, 98 mm/min welding speed and CS tool pin profile. There is a significant variation in the axial forces with respect to different phases of welding. The peaks of axial force is identified the pin plunge and shoulder plunge regions during plunging phase. During plunging heat is generated by the pin due to frictional and plastic deformation which drops the axial force and afterwards rises again when the shoulder of the tool is plunged into the hard and cold workpiece. When the tool has plunged into the workpiece and stayed for 45 seconds during dwell phase to preheat the workpiece the axial force again drops. During welding period axial force is approximately constant around 4 kN. The axial force continues to decrease as the tool plunge-out of the weld seam. The variation of force is attributable to the resistance offered by un-welded and colder material to tool movement. Therefore, this axial force data provides useful information about weld joint to estimate weld defect and quality [281].
Experimental Investigation
Figure 4.16. Variation of axial force at experimental condition of 1100 rpm, 98 mm/min welding speed and CS tool pin profile.
Figure 4.17 show the influence of FSW parameters on the average welding force. It’s found that, with increase in tool rotation speed there is a decrease in the axial force due to increase in heat input during welding. With an increase in heat input, the contact area below the shoulder and in the stir zone, the workpiece material becomes softer and reduces the resistance offered by workpiece in this region. Whereas, the effect of tool pin geometry is low as compare to tool rotational speed and welding speed. While it’s observed from Fig. 4.16, the variation is 0.5 to 0.8 kN between straight and threaded tool due to increase in material flow and heat generation than straight pin tool [282].
Figure 4.17. Comparison between average welding forces at various welding conditions.
Chapter 4
Figure 4.18. Tensile strength, yield strength and % of elongation of welded joint at various welding conditions (a) 600 rpm, (b) 1100 rpm and (c) 1500 rpm.
Figure 4.18 illustrate the tensile strength, yield strength and % elongation of welded joints corresponding to welding condition shown in Table 4.6. For these range of parameters, the tensile strength of weld in case of threaded pin is 2% higher than that of base material (base material tensile strength is 148.1 MPa). The maximum tensile strength of welds obtained at the rotational speed of 1100 rpm, welding speed of 98 mm/min, and with cylindrical pin with square thread FSW tool is 151.2 MPa which is 102 % of base material. The effect of rotational speed is concerned, the joints fabricated at a rotational speed of 1100 rpm are showing superior tensile properties compared to other joints, irrespective of tool profiles. Pin profile plays a crucial role in material flow and in turn regulates the welding speed of the FSW process [283]. In addition, the V-thread and square thread pin profiles produce extra stirring action in the flowing material due to thread in pin side surface. There is no such extra stirring action in the case of cylindrical and tapered plane pin profiles. This results in higher tensile
Experimental Investigation
strength. In case of threaded pin profiles irrespective of rotational speed, the tensile strength is higher.
Figure 4.19. Comparison between ultimate bending loads of various welding conditions (a) 600 rpm, (b) 1100 rpm and (c) 1500 rpm.
The fracture toughness of a specimen can also be determined using a three-point flexural test. This test is very sensitive to defects near the surface of the weld bead, such as root flaws. During the test a 1 mm/min cross-head speed is used and one specimen for each type of weld and base materials are tested. No root flaws or other defects are detected in most of the joints. From the Fig. 4.19(b) it is observed that at tool rotational speed of 1100rpm, with the straight cylindrical threaded tool, the temperature and material stirring increase enough, so the higher bending strength joint is produced. Since the temperature at the welding speed of 400 rpm is not enough to soften the base material, the materials were not sufficiently plasticized to be stirred and forged easily for all the pin profiles resulting lesser bending
Chapter 4 strength as shown in Fig. 4.19(a). Defect was in the root for all the joints fabricated. This defect is known as the tunnel-hole defect. Though the appearance of the welded surface seems to be good, tunnel defects could be observed at the advancing side and in the stirred zone of the weld. The plasticized metal under the shoulder cannot flow sufficiently during the welding process due to insufficient heat generation [284].
Figure 4.20. Hardness distributions on the transverse cross-section for joints fabricated at a rotational speed of 1100 rpm and 98 mm/min welding speed for different pin profiles (a) C, (b) CS (c) CV (d) T (e) TS and (f) TV.
Micro hardness distributions on the transverse cross-section of the joints for different tool pin profiles are shown in Fig. 4.20. It has been investigated that the hardness influenced by the tool pin profile due to the dominant influence on the plastic flow and deformation during welding. It’s also illustrates that, for the same welding parameters, the highest hardness increase was achieved in the nugget and TMAZ of the welds produced with the threaded tool, i.e., about 30% more of base material hardness. The increase in hardness is attributed to the formation of fine grains in the stir zone, and in addition, the reduced size of weaker regions, such as TMAZ and HAZ regions, results in higher tensile properties. Similarly, the weld region of the joint fabricated using square threaded pin profile tool contains finer grains compared to other joints. The higher number of stirring action experienced in the stir zone to
Experimental Investigation
produce finer grained microstructure and in turn yields higher strength and hardness.
Therefore, the complex intercalated microstructure and the orbital stacking of materials in the stir zone results in the fluctuation of hardness values in the stir zones which is influenced by tool pin profiles [285].
Figure 4.21 shows the macrographs of the welded joints with low magnification.
Figures 4.21(C) and (T) are fabricated using straight cylindrical and tapered cylindrical pin profiled tool, respectively, are having void defects. This is due to insufficient flow of plasticized material around the tool pin during welding process. In the case of threaded cylindrical and taper pin tool, the joints are found defect free as shown in Figs 4.21(CS), (CV), (TS) and (TV). Under these welding conditions, the heat generation is almost the same for tool shoulder. The screw thread will be beneficial to the heat generation, under the same weld parameter, the pin with screw thread will generate more heat than the pin without screw thread.
More heat input can improve the flow of the plastic material [179]. On the other hand, the screw thread on the pin exerts an extra downward force that will be beneficial to accelerate the flow of the plastic material. In addition, the pin profile has greater effect on the material flow behavior. This concluded that the heat input decides the formation and location of defect in the stir zone. In spite of the same heat input range, the different pin profiles alter the material flow, defect formation and its location [180].
Figure 4.21. Macrographs for weld joints in longitudinal cross section at 1100 rpm for different pin profile (a) C, (b) CS (c) CV (d) T (e) TS and (f) TV.
The grain size and its orientation for various tool pin profiles at 1100 rpm are analyzed using optical microscope and shown in Fig. 4.22. The grain size in nugget zone at six different
Chapter 4 tool pin profiles is 21.4 μm, 14.2 μm, 15.3 μm, 22.2 μm, 16.1 μm and 17.5 μm for C, CS, CV, T, TS and TV respectively at constant 1100 RPM and welding speed. The increase in degree of deformation during FSW results in the reduction of grain size according to the general principles of recrystallization. It is noted that the recrystallization grain size is reduced by threaded pin tool at constant tool rotational speed. The combination of lower temperature and shorter excursion time at the nugget bottom effectively retards the grain growth and results in smaller recrystallized grains [286].
Figure 4.22. Microstructure of nugget zone for joints fabricated at a rotational speed of 1100 rpm and 98 mm/min welding speed for different pin profiles (a) C, (b) CS (c) CV (d) T (e) TS and (f) TV.
The variation of average grain size in the stir zones under the various tool pin profiles and rotational speed are shown in Fig. 4.23. It’s observed that lower rotational speed resulted in lower peak temperature and consequently the reduction in grain size due higher plastic deformation and strain rate at low temperature [287-288]. During this process, the material experiences severe plastic deformation and thermal exposure which normally leads to formation of fine recrystallized grain structure. Similarly, at different tool pin profile average grain size decreases with increase in threaded profile in both straight and taper cylindrical pin.
At the circumference of the pin, the material flow is shown to be governed by the simple shear deformation induced by the rotating pin, which led to the formation of a fine equiaxed grains.
Experimental Investigation
Therefore, threaded pin generate higher degree of plastic deformation than straight and taper cylindrical tool which lead smaller grain at comparatively equivalent temperature [289].
Figure 4.23. Comparison between average grain sizes of nugget zone at various welding conditions.