Optimization and Influence of Process Parameters in FSW of Al/Cu Dissimilar Joint
6.3 Influence of Individual Process Parameters on Mechanical and Microstructures Properties
6.3.5 Phase Analysis of Al/Cu Weld
The SEM equipped with EDX analysis was performed on the fracture surface of the tensile tested specimen. The analysis revealed that the fracture surface mainly contain Al-Cu mixture in the form of IMCs. There was a negligible amount of other elements in the weld. The weight percentage of Al or Cu particles in the NZ varied from point to point due to presence of various IMCs. Figure 6.13 shows EDX analysis of specimen Al/Cu.11 with various spectrums. Specimen Al/Cu.11 was considered for SEM-EDX
Optimization and Influence of Process Parameters in FSW of Al/Cu Dissimilar Joint analysis as tensile facture was occurred at the NZ. The chemical compositions, in weight percentage, of various spectrums (Fig. 6.13a-e) are given in Table 6.5, these values agree well with (Carlone et al., 2015) published results. The variation of Al-Cu particles indicates formation of different IMCs. Using Al-Cu binary phase diagram (Tan et al., 2013), it is found that IMCs namely, Al2Cu ( ), AlCu (ȵ), Al4Cu9 (γ), Al2Cu3 (ɛ) were developed by Al-Cu reactions. The presence of these IMCs decreases the mechanical properties (Jana et al., 2010) of the joints. Most of the cases it was observed that the IMCs contains Cu rich particle that induce high hardness and brittle fracture (Firouzdor et al., 2012). The formation of various IMCs at the NZ can be confirmed further by XRD analysis.
Fig.6.13 SEM-EDX analysis of (a) tensile tested specimen Al/Cu.11, (b) spectrum-23,
Chapter 6 Table 6.5 Energy dispersive X-ray result in weight %
Point Al Cu Other Composition Spectrum-23 41.3 58.7 - Al2Cu3
Spectrum-24 65.0 34.3 Fe Al2 Cu Spectrum-25 36.9 63.1 - Al4 Cu9
Spectrum-26 42.2 57.0 Si AlCu
The XRD analysis has been done at the NZ of the welded specimens. Three specimens were taken from a welded sample i.e., Al side of the weld (2 mm from weld center), weld center and Cu side of the weld (2 mm from weld center) as shown in Fig.
6.14. This study is performed to find out the formation of various IMCs at the respective welding zones. The XRD analysis of the welded specimen Al/Cu.8 is shown in Fig.
6.15(a-c). The NZ contains various IMCs having different color and compositions. Some studies (Tan et al., 2013) found that due to Al-Cu reaction Al-rich phase Al2Cu and Cu- rich phase Al4Cu9 are most commonly formed adjacent to Al and Cu sides, respectively.
From the Fig. 6.15(a-c), it is also found that Al-rich IMCs (2 peaks of AlCu and 2 peaks of Al2Cu) dominate the Al side and Cu-rich phases (1 peak of AlCu, 4 peaks of Cu9Al4, 2 peaks of Al2Cu3) dominate the Cu side. Whereas weld center, contains nearly same amount of Al and Cu, consists of AlCu (6 peaks) and Al2Cu (1 peak) IMCs. It is observed that the presence of uniform and continuous IMCs layer are beneficial (Xia-wei et al., 2012) for the dissimilar bonding. Excessive IMCs lead to the higher hardness (Xue et al., 2010) and sometimes decrease the mechanical properties (Ouyang et al., 2006).
However thin, uniform continuous IMCs layer is necessary (Xue et al., 2010, 2011) for achieving sound FSWed Al/Cu joint (refer Fig. 6.16a).
Fig.6.14 Schematic of extracted XRD specimens from the weld
Optimization and Influence of Process Parameters in FSW of Al/Cu Dissimilar Joint
Fig.6.15 XRD analysis of welded specimen Al/Cu.8 at the cross section of Al/Cu joint (a) Al side of weld, (b) center of weld, (c) Cu side of weld
Specimen Al/Cu.8 Specimen Al/Cu.2
Fig.6.16 Elements distribution mapping with Al and Cu intensity by line scan of the specimens Al/Cu.8 and Al/Cu.2, (a) overall mapping of Al/Cu.8, (b) overall mapping of
Al/Cu.2, (c) Al and Cu individual of Al/Cu.8, (d) Al and Cu individual of Al/Cu.2, (e) line scan of Al/Cu.8 and (f) line scan of Al/Cu.2
Chapter 6 Few specimens were considered from the weld center for mapping and line scanning. To observe elemental distribution across the NZ mapping was done over the entire specimen. The details of observation of two specimens Al/Cu.8 and Al/Cu.2, corresponding to sound and poor welds are presented in Fig. 6.16. The distributions of Al and Cu particles in the matrix (Fig. 6.16a, b) are represented in red and green colors, respectively. For both the specimens, it is observed that major elements are Al and Cu (Fig. 6.16c, d). It is observed in Fig. 6.16(e) that the flow of the IMCs material in the NZ of specimen Al/Cu.8 is thin, continuous and uniform. However the flow of the IMCs material in the NZ of specimen Al/Cu.2 (refer Fig. 6.16f) is discontinuous and non- uniform with bulk in size.
From the line scan analysis, it was also observed that Al element has higher diffusion rate compared to the Cu and welded region contains mixture of Al and Cu which is shown in Fig. 6.16(e, f) for specimens Al/Cu.8 and Al/Cu.2, respectively. The variation of Al and Cu in case of specimen Al/Cu.2 confirms that stirring action induced remixing of materials and a quite non-homogeneous distribution compared to specimen Al/Cu.8. The intensity of Al is higher compared to the Cu as higher percentage of Al is available at the NZ due to the tool offset. Discontinuous reaction layers are observed in case of specimen Al/Cu.2 at the interface where some fragments of Cu particles were pushed towards Al and vice versa.