FSW of AM20 Magnesium Alloy with Metallic Foil Alloying
5.2 Analysis of the Experimental Results
5.2.5 Phase Analysis of the Joint
5.2.5.1 XRD Analysis
The purpose of the XRD analysis is to detect the formation of different phases at the NZ area and compared with EDX analysis. XRD analysis was performed on each specimen for different experiments. In FSW experiments without an interlayer, alloying element are not added so no IMCs are formed hence, XRD analysis is not performed.
However for experiments with Zn alloying element (Exp.5) and Al alloying element (Exp.2) XRD analysis is quite relevant and useful for finding IMCs at the NZ area of the welds and the results are shown in Fig. 5.7. From the analysis it was observed that different IMCs are formed at the NZ. While considering the melting point of alloying foils, Al was completely plasticized and Zn completely melted and mixed with Mg BM at the NZ during the FSW processes which are shown in the mapping Fig. 5.10 and 5.11.
The reaction phase at the NZ may consist of Mg-Al and Mg-Zn binary phases.
Chapter 5 XRD analysis of Exp.2 with Al interlayer as alloying element is shown in the Fig. 5.7(a). The formed minor IMCs at the NZ area are Al12Mg17 and AlMg with a higher percentage of unreacted Mg phases. Similar IMCs were also reported by other researchers (Chang et al., 2011, Peng et al., 2005) in FSW and different alloying processes. Al12Mg17 (Zhang et al., 2013) (less intensity) and AlMg (less intensity) IMCs are less compared to pure Mg phase (3 peaks with very high intensity). These minor IMCs with less intensity behave as a reinforcing particle (Zhao et al., 2008) and play an important role for improvement of material properties like tensile strength. The intensity of Mg peaks is higher due to higher percentage of pure Mg at the weld zone. The intensity and percentage of IMCs depends on the controlled amount of alloying elements.
It was observed that IMCs are formed and distributed uniformly at the NZ as shown in Fig. 5.10. Thin and uniform distributed IMCs add towards the improvisation of joint strength in case of Exp.2.
The XRD analysis of Exp.5 is represented in the Fig. 5.7(b). At the joint interface Mg7Zn3 (Xu et al., 2016, Hosseini et al., 2013) and MgZn (Xu et al., 2015, 2016) with less intensity, MgZn2 (Liu et al., 2013) and Mg4Zn7 (Xu et al., 2015) with very less intensity are observed along with very high intensity pure Mg phases. However MgZn2
and Mg4Zn7 are more brittle due to a higher percentage of Zn compared to Mg7Zn3 and MgZn with dominating effect for higher percentage and high intensity. The formed minor IMCs with less intensity can improve the mechanical properties. The different IMCs are formed due to easy melting and higher diffusion affinity of Zn compared to other elements (Shiri et al., 2013). So, Zn easily mixed with Mg alloy by the stirring action of the FSW tool and formed a higher number of IMCs compared to Al alloying.
More uniform and thin IMCs are formed in the case of Exp.5 (Fig. 5.11) which gives highest tensile properties compared to other set of experiments.
Fig.5.7 XRD analysis of welded specimen at the NZ cross-section of (a) specimen Exp.2 with Al alloying (b) specimen Exp.5 with Zn alloying
FSW of AM20 Mg Alloy with Metallic Foil Alloying 5.2.5.2 EDX Analysis
The FESEM-EDX analysis was carried out on the cross-section of weld center to detect various IMCs formed by the alloying elements. The analysis was performed on all the welded specimens. Out of these, only the best specimen on the basis of tensile results with Al and Zn alloying are represented in this section. Specimens without alloying elements are not considered because the NZ contain same composition as that of BM.
Figure 5.8(a) indicates a uniform distribution of Al alloying elements on the Mg matrix throughout the weld zone. The EDX analysis was performed as point and area spectrum indicated in the Fig. 5.8(a). Spectrum-1 as shown in the Fig. 5.8(b) indicates higher percentage of Al with less Zn on the Mg base matrix. The percentage composition revealed that the formed IMCs is Al12Mg17 (formed at 450 °C) (ASM Handbook, 1992) with some pure Mg phase. Spectrum-2 and 3 as furnished in Fig. 5.8(c-d), respectively, indicate higher percentage of Mg matrix and weight percentage indicates formation of Al12Mg17 and AlMg (formed around 450 °C) (ASM Handbook, 1992, Peng et al., 2005) IMCs, respectively, with pure Mg phase. Similar IMCs had also been reported by other researchers (Chang et al., 2011, Peng et al., 2005). The presence of these IMCs improves the ductility of the joint due to Al alloying in Mg matrix and increases the mechanical properties of the joints (Aonuma et al., 2009). The cause for improved ductility lies in low brittleness with Al alloying element that in turn increases the tensile strength compared to without alloying cases. The addition of appropriate amount of Al in Mg substrate increases the contents of Al-base solid solution namely Al12Mg17 and AlMg IMCs that eliminates stress concentration and hinders crack propagation resulting in improved tensile properties (Liu et al., 2013).
Similarly, experiment with Zn alloying specimen Exp.5 is represented in Fig. 5.9 for the EDX analysis. Figure 5.9(a) indicates that the Zn alloying elements distributed uniformly at the weld zone. Different point and area spectrums indicate variation of weight percentage of elements with different compositions resulting different IMCs formation. Spectrum-1 as shown in Fig. 5.9(b) indicates higher percentage of Zn with minor Mn and Al element in Mg matrix. The weight percentage indicates that the formed IMCs are Mg7Zn3 with some pure Mg phase and agree well with prior work (Xu et al.,
Chapter 5 2016). These binary phases are formed due to eutectic reaction of Zn and Mg by the FSW process at 340 °C (Xu et al., 2016, ASM Handbook, 1992).
Fig.5.8 FESEM-EDX analysis of specimen Exp.2 (a) weld zone of Exp.2, (b) spectrum 1 of Exp.2, (c) spectrum 2 of Exp.2, (d) spectrum 3 of Exp.2
Fig.5.9 FESEM-EDX analysis of specimen Exp.5 (a) weld zone of Exp.5, (b) spectrum 1 of Exp.5, (c) spectrum 2 of Exp.5, (d) spectrum 3 of Exp.5
Spectrum-2 as reflected in Fig. 5.9(c) has highest percentage of Zn including Mn
FSW of AM20 Mg Alloy with Metallic Foil Alloying
et al., 2013, ASM Handbook, 1992) and Mg4Zn7 (ASM Handbook, 1992) (may formed at temperature of around 416 °C) as these IMCs are formed at higher temperature with pure Mg phases. Likewise spectrum-3 as shown in the Fig. 5.9(d) represents formation of MgZn IMCs at 325 °C (Xu et al., 2016, ASM Handbook, 1992) including pure Mg phase at the weld zone. Due to proper IMCs formation, thin and uniform distribution (Fig. 5.11) of these IMCs improvises the tensile strength and Exp.5 gives highest tensile strength compared to other experiments. Alloying using the FSW process is one of the new techniques for getting enhanced metallurgical and mechanical properties.