FSW of AM20 Magnesium Alloy
4.3 Effect of Individual Parameters on Microstructure and Mechanical Properties
4.3.4 Metallographic Analysis
Metallographic analysis was performed to examine the weld bead geometries, grain size variation at different zones, grain orientation and macro/microstructures of the joints. Optical microscopy was used to observe the microstructure and determine the grain size at different zones of the weld. The detail of the metallographic study has been discussed in the following sub-sections.
FSW of AM20 Magnesium Alloy
4.3.4.1 Effect of Process Parameters on the Weld Macrostructure
The transverse cross section of a welded specimen representing the bead geometry is shown in Fig. 4.18(a) which shows the width of the NZ at different layers.
Due to significant grains refinement, cup shape NZ is clearly distinct from other zones.
The weld bead width of the specimens were measured at three different locations in the NZ namely, bottom, middle and upper, layers along the thickness direction. The upper and bottom layers are 0.5 mm from the top and bottom of the workpiece, respectively.
The measured weld bead values of all the specimens are shown in Table 4.14. In Fig.
4.18(b), clear difference between the NZ and TMAZ of the welded specimens can be seen.
Fig.4.17 SEM micrograph of the tensile tested specimens (a) Exp. No. E1 and (b) it’s magnified view, (c) Exp. No. E4 and (d) it’s magnified view.
Fig.4.18 (a) Macro image of the FSWed joint E4 and different zones (a-NZ, b-TMAZ, c- HAZ and d-BM/unaffected zone), (b) TMAZ and NZ interface of specimen E4.
Chapter 4 Table 4.14 Width of the NZ at upper, middle and bottom layers
Exp. No. E1 E2 E3 E4 E5 E6 E7 E8 E9 E10
NZ upper (mm) 8.67 10.31 9.51 11.24 10.75 9.18 10.52 9.80 9.48 9.15 NZ middle (mm) 6.83 7.69 7.75 7.81 8.26 7.22 7.37 7.66 7.43 7.19 NZ bottom (mm) 5.61 6.48 6.16 6.02 6.50 6.09 6.03 6.42 6.15 5.52 From the measured bead geometry it was found that the width of the NZ varies from specimen to specimen because it depends on various process parameters. It was observed that the bottom NZ size vary from 5.5 to 6.5 mm which is nearly equivalent to the diameter of the tool pin. The size of the upper layer is significantly higher compared to other two layers because of the combined effect of shoulder and pin. The effect of shoulder is diminished with thickness of the workpiece. The average NZ size at the bottom layer is 3.85% higher than the pin diameter due to the stirring effect.
4.3.4.2 Feature of Weld Microstructure
The microstructures produced in experiment E4 at various zones of the weldment along with the BM are shown in Fig. 4.19(a-h). Optical micrographs were taken at bottom, middle and top of the NZ, Fig. 4.19(a-c), advancing and retreating side of TAMZ, Fig. 4.19(d, e) and advancing and retreating side of HAZ, Fig. 4.19(f, g) of the weld to comparing the variation of the grain size and grain orientation. From the microstructre, it was found that the grains at the NZ are appreciably different compared to other zones. The coarser needle-like grains of the BM become fine grains at the NZ due to the stirring action of the rotating tool and recrystalization.
The average grain diameters of various zones were measured by applying Heyn line intercept method (Rose et al., 2012, ASTM E112-04, 2006, Rajakumar et al., 2012).
The grains in the NZ (around 22.5 μm) are finer than TMAZ (advancing side 33.21 μm and retreating side 34.52 μm). There is insignificant variation in the grain size across different layers of the NZ. The average grain size of advancing side is 3.79% smaller than the retreating side of the TMAZ. This is due to the plasticized metal of TMAZ is extruded from the advancing side, encounters dynamic recrystallisation and deposited on the retreating side and thus the grains are comparatively finer in advancing side than the TMAZ of the retreating side. This also explains why all the tensile specimens were failed at the TMAZ of the retreating side. Some of the grains also stretched along the moving tool direction because of shear force (Sirong et al., 2010). Also because of the pulling
FSW of AM20 Magnesium Alloy
action of the rotating tool the grains become elongated and eliptical shape in TMAZ, as shown in Fig. 4.19(d, e) compared to NZ (Rose et al., 2012).
Fig.4.19 Microstructure of Exp. No. E4 at the different zones and the average grain diameter, (a) upper NZ, 22.43 μm (b) middle NZ, 22.5 μm (c) bottom NZ, 22.5 μm (d)
advancing side of TMAZ, 33.21 μm (e) retreating side of TMAZ, 34.52 μm (f) advancing side of HAZ, (g) retreating side of HAZ, (h) base material.
The microstructure of the as recived BM is elongated needle like structure, shown in Fig. 4.19(h). It is closely related to the structure of a warm rolled Mg sheet (ASM handbook 9). During FSW process the grain was recrystalized and became equiaxed. In HAZ, the effect of the tool is less compared to other zones. In this zone, the grains are affected only by the heat generated during the process. The microstructure at the HAZ of the advancing side (Fig. 4.19f) has less needle like elongated grains compared to the
Chapter 4 retreating side (Fig. 4.19g). This may be due to the temprature difference in advancing and retreating side of the weld.
4.3.4.3 Effect of Process Parameters on Grain Size
The variations of average grain size values at different weld zones are given in the Table 4.15 against all the experiments. The variations of grain diameters at NZ with process parameters are depicted in Fig. 4.20. From the micrographs, it is seen that there is a noticiable variation in average grain size at the NZ. It is observed from the Fig.
4.20(a-d) and Fig. 4.21(a) that, grains became finer with an increase of shoulder diameter. As the shoulder diameter increases more heat is generated (shown in Fig.
4.15a) which leads to proper grain mixing and refinement. With increasing shoulder