3.1 Hardness distribution
The detailed microstructures and mechanical properties are described elsewhere [6,17], thus the hardness distribution is briefly shown in this section. Figure 4 shows the hardness profiles measured on the top and bottom surfaces of the tensile specimen along the dotted line in Fig. 2. Softening occurred in the SZ due to the heat input during FSW, and the minimum hardness appeared at the thermo-mechanically affected zone (TMAZ). It should be noted that the minimum hardness appears at about 9 mm away from the interface on the top surface and about 6.5 mm on the bottom surface. In addition, the hardness near the interface (0~1 mm) on the bottom surface was nearly comparable to the minimum hardness at 6.5 mm away from the interface. The hardness mapping measured on the side surface of Al side is shown in Fig. 5 [6]. It should be noted that the area of lower hardness (blue area) in the TMAZ has vertical asymmetricity, where lower hardness appears further from the interface on the top side. Furthermore, minimum hardness appears near the interface (0~3 mm) only on the bottom side.
3.2 DIC analysis: Top and bottom surfaces
The ultimate tensile failure load of the joint was 201 MPa, where tensile failure occurred in Al side along TMAZ as shown in Fig. 6, indicating that the interface strength was higher than the TMAZ with lower hardness [6]. Firstly, DIC analysis was performed on the top surface of the dissimilar weld of the tensile specimen. Figure 7 shows the analytical area on the top surface including the interface between Al and steel, where y axis corresponds to the loading direction. The analytical conditions are summarized in Table 1. The image before loading was used as a reference image, and the images at 4.0 kN (86 MPa), 5.0 kN (107 MPa) and 6.0 kN (128 MPa) are used for the DIC analysis. The DIC analytical results showing y-axis displacement and strain are revealed in Fig. 8. Figure 9 shows the y-axis strain along the dotted line in Fig. 8(b) at 6.0 kN. It should be noted that the maximum local strain concentration occurred at about 9 mm away from the interface, which corresponds to the locally-softened area of TMAZ as shown in Fig. 4. The local strain distributions at 0, 1, 9, 12 and 14 mm away from the interface are shown in Fig. 10. The areas in the figure were squares with the size of 1.85 mm × 1.85 mm. At the beginning of the loading (4.0 kN and 5.0 kN), strain concentration occurred at about 1 mm away from the interface in very narrow area. However, the wide strain concentration occurred at 9 mm from the interface at 6.0 kN, which corresponds to the softened TMAZ (Fig. 4) and the actual fracture location (Fig. 6).
Subsequently, DIC analysis was performed on the bottom surface, where the analytical conditions are summarized in Table 1. y-axis displacement and strain are shown in Fig 11. Furthermore, the local strain distributions at 0, 3, 5.6, 11.5 and 19 mm away from the interface are shown in Fig. 12. Local strain concentration occurred just beside the interface from 4.0 to 6.0 kN, while large strain occurred at about 5.6 mm away from the interface at 6.0 kN. That location corresponds to the locally-softened area (Fig. 4) and the actual fracture location (Fig. 6). Consequently, the local strain concentration occurred at the locally-softened area in TMAZ, and just beside the interface on the bottom surface.
The DIC analytical result on the side surface is shown in Fig. 13 as the strain distribution at 6.0 kN. It should be noted that the strain asymmetricity between top and bottom sides is observed on the side surface along TMAZ. In addition, local strain concentration near the interface occurred only on the bottom side as shown by the arrow in the figure, and it is consistent with Figs.10 and 12. In our previous study, the strain distribution was measured by DIC using friction stir welded Al-Mg-Sc plates. In that case, local strain concentration occurred only at TMAZ where minimum hardness appeared, and static failure occurred at the maximum straining area. In the present case, local straining occurred at TMAZ and just beside the interface on the bottom side. However, local straining area was much narrower near the interface due to the constraint by steel with higher elastic modulus. Therefore, it is assumed that the final failure predominantly occurs at wider local-straining area in TMAZ.
3.3 Local stress-strain curves
The local straining figures shown in Figs. 10 and 12 were obtained at every 0.5 kN on the top surface, and every 1 kN on the bottom surface to draw local stress-strain curves. The local strain was defended as the average value of each square shown in Figs. 10 and 12. Figures 14a and 14b are the local stress-strain curves at SZ, TMAZ, HAZ and base metal (BM) on the top and bottom surfaces, respectively. The black solid line is a stress-strain curve of the bulk sample of the base metal, where elastic modulus of 68.3 GPa was measured by DIC in the present study and 0.2% proof stress of 287 MPa was given by a material supplier. On the top surface (Fig. 14a), the stress-strain relationships in SZ, HAZ and BM are linear, where the local elastic moduli are 65.4, 53.5 and 54.0 GPa, respectively. On the other hand, local yielding occurred only at TMAZ, where the hardness was minimum. The elastic modulus at TMAZ is 60.8 GPa. The local 0.2% proof stress at TMAZ is estimated about 120 MPa, which is 58 % lower than the base metal. The hardness drop at TMAZ was about 48 % as shown in Fig. 4, therefore the drop of local yield stress was larger than the hardness. However, the value of the local yield stress still seems reasonable because the deference in the reduction rates is small. Similar to the top surface, the local yielding is prominent only at TMAZ on the bottom surface (Fig. 14b), where the local proof stress is also about 120 MPa. The strain value at the maximum load of 6.0 kN is also slightly larger than the elastic line at SZ, but it seems a measurement error because the strain values at the maximum load on the bottom side are larger than the top side at all locations including base metal. The local elastic moduli at SZ, TMAZ, HAZ and BM on the bottom side are 63.8, 50.7, 52.9 and 60.6, respectively. It should be noted that the elastic modulus at the SZ was the largest on both top and bottom surfaces. It was confirmed that many steel fragments were dispersed in the SZ by microstructural observations [6,17]. Therefore, higher local elastic modulus at the SZ could be attributed to such steel fragments with higher elastic modulus than Al.