FSW of AM20 Magnesium Alloy
4.3 Effect of Individual Parameters on Microstructure and Mechanical Properties
4.3.1 Mechanical Properties Analysis
FSW of AM20 Magnesium Alloy
Fig.4.5 Response plot for GRG in Strategy-2.
4.3 Effect of Individual Parameters on Microstructure and Mechanical
Chapter 4 as excess flash and makes unfilled tear line defect. Less than 12 mm shoulder diameter leads to inferior weld quality due to insufficient heat generation. It was observed that larger shoulder diameter leads to higher heat generation and coarse grains at the NZ which deteriorate the tensile strength. Therefore, more than 24 mm and less than 12 mm shoulder diameter was not considered for this work. Tensile testing results, i.e., nominal stress vs. nominal strain curves are shown in Fig. 4.7(a). From the graph, it is found that the welded specimens are failed suddenly after the yield point, which may indicate that it is not a pure ductile failure. It may be combination of ductile and brittle failure which can be further confirmed by fractograph test (discussed in fractograph Section 4.3.3).
Among the different shoulder diameters, 24 mm gives comparatively better tensile properties.
Fig.4.6 Friction stir welded joints with varying shoulder diameter.
Variation of UTS, YS and percentage of elongation with shoulder diameter and plunge depth is shown in Fig. 4.7(b-c). From the figures, it is observed that with the increase in the shoulder diameter, above tensile properties found to follow an increasing trend. This is because with the increasing shoulder diameter contact surface area between tool and workpiece increases that leads to more frictional heat generation (shown in Fig.
4.15a), proper stirring and grain refinement therefore welding becomes stronger. The maximum UTS and YS of the weld were 132.1 MPa and 115.5 MPa which are 65% and 72% of BM, respectively. There is no published research article, which discuses specifically the effect of plunging depth on mechanical properties of Mg welded joints.
However, plunging depth is a critical process parameter to ensure good weld quality.
Fig. 4.7(c) depicts that with the increase in plunging depth, tensile properties found to be increasing upto a certain level and then decrease with further increase of plunging depth.
The downward pressure required for proper heat generation is determined by plunge depth. At a 0.03 mm plunging depth, due to lower downward pressure, heat generation was less compared to 0.12 mm plunging depth (shown in Fig. 4.15b) that leads to better
FSW of AM20 Magnesium Alloy
weld joint. When plunging depth increased to 0.21 mm, the heat generation was more but due to high flash formation, excessive local thinning occurred at the shoulder workpiece interface area which leads to poor weld strength. Therefore, among three levels of plunging depth, 0.12 mm plunging depth gives better tensile properties compared to 0.03 mm and 0.21 mm plunging depth.
Fig.4.7 (a) Stress vs. strain curve with varying shoulder diameter, (b) percentage of elongation, UTS and YS variation with shoulder diameter and (c) plunging depth.
Effect of welding speeds and tool rotational speeds are investigated and results are furnished in Fig. 4.8(a-b). From the figures it is observed that with the increase in these two parameters, UTS, YS and percentage of elongation found to follow decreasing trend. For a given tool rotational speed, plunge depth and tool geometry; higher welding speed significantly deteriorates per unit length heat input and stirring of plasticized material. Micro voids are formed because of improper stirring at higher welding speed which leads to lower welding strength. It is to be noted that too low welding speed also deteriorates weld strength due to larger heat input and slower cooling rates causing excessive grain growth and subsequently reduces tensile strength. On the other hand, tool rotational speed is responsible for friction heat generation and stirring of plasticized material. At higher tool rotational speed high heat was generated which promoted grain growth that contributed to the depletion in joint strength. Due to the combined effect of heat and shearing stress at higher rotational speed more flash was observed which further reduced weld strength.
Fig.4.8 Variation of percentage of elongation, UTS, YS with (a) welding speed and (b) tool rotational speed
Chapter 4 4.3.1.2 Effect of Process Parameters on Bending Properties
Butt welded joints specimens are often tested by bend test to check weld quality.
The effect of shoulder diameter on the bending angle is given in Fig. 4.9(a). It is observed from the figure that when the shoulder diameter increases bending angle also increases. A Similar trend was also observed in tensile test results. With the increasing shoulder diameter, ductility of the joint increases due to more frictional heat generation and proper stirring. The maximum bending angle among all the specimens without visual crack was 45°. Whereas, with the increase in the plunging depth as shown in Fig. 4.9(b), bending angle increases for 0.12 mm and further increase in the plunging depth leads to decrease in bending angle. At 0.03 mm plunging depth heat generation rate was less due to inadequate contact pressure which leads to poor weld joint. At 0.12 mm plunging depth heat generation rate was adequate which facilitated proper material stirring at the NZ and increase ductility. Grain growth takes place at 0.21 mm plunging depth due to high heat input which causes lower bending angle (Rajakumar et al., 2013).
Fig.4.9 Bending angle variation with (a) shoulder diameter and (b) plunging depth.
The variations of bending angles with the welding speeds and tool rotational speeds are depicted in Fig. 4.10(a-b) respectively. With the increase in the above parameters, it is observed that bending angle found to decrease. With increasing welding speed, heat input per unit weld length decreases leading to improper material stirring.
This results in low bending angle due to poor ductility of the welded material. Whereas, at higher tool rotational speed, grain growth is promoted due to the generation of high heat and results in joints with low ductility.
Fig.4.10 Bending angle variation with (a) welding speed and (b) tool rotational speed.
FSW of AM20 Magnesium Alloy
4.3.1.3 Effect of Process Parameters on Micro-hardness
In this section, a comprehensive study of the influence of welding parameters on the micro-hardness is performed. The 2D micro-hardness contour of a specimen is shown in Fig. 4.11. Irrespective of process parameters setting, it was found that the hardness of the upper layer is relatively higher than the middle and bottom layers. It was also found that the hardness of the NZ is considerably higher compared to TMAZ, HAZ and BM.
There are two general reasons for improvement of hardness (Rong-chang et al., 2008) at the NZ (i) substantially finer grain size at the NZ than that of other zones, and (ii) formation of small IMCs in dissimilar FSW. Among these two reasons grain refinement plays an important role in material strengthening in similar FSW process. According to the Hall-Petch law (Rong-chang et al., 2008, Hall, 1954) hardness increases as the grain size decreases.
Fig.4.11 2D micro-hardness contour of a specimen.
The variation of average micro-hardness values at NZ with welding speed and tool rotational speed is shown in Fig. 4.12(a-b). From the measured hardness data it was found that the lower welding speed give higher micro-hardness value, because at lower welding speed with constant rotational speed, ratio between tool rotational speed to welding speed is more compare to the higher welding speed. As a result grain becomes finer which leads to increase in hardness (Fig. 4.12a). The mentioned ratio also increases with increasing tool rotational speed at constant welding speed. Thus, with the increase in tool rotational speed, the measured hardness values were found to increase (Fig.
4.12b).
Chapter 4
Fig.4.12 (a) Effect of welding speed, (b) tool rotational speed on micro-hardness.
Effect of shoulder diameter and plunging depth is shown in Fig. 4.13(a-b). It was observed from the Fig. 4.13(a) that with an increase in shoulder diameter the hardness value also increases. More heat is generated on the workpiece at higher shoulder diameter which leads to proper mixing of the plasticized material and therefore the grain became finer and leads to higher hardness values. However similar decisive statement is lacking for the variation of hardness with plunging depth. It was found that as plunging depth increases from 0.03 mm to 0.12 mm the hardness value also increases due to grain refinement as heat generation rate increased Fig. 4.13(b). However further increase of plunge depth leads to grain growth due to higher heat generation and results in low hardness values.
Fig.4.13 (a) Effect of shoulder diameter, (b) plunging depth on micro-hardness.