Figure 3(a) shows the scanning electron microscopy (SEM) images on IMC layer, and it clearly distinguishes the interface between the Al/steel. The high energy density of welding heat source will melt aluminum quickly, but the molten pool will be restricted to a small size. Also there will be accelerated
diffusion of iron atoms in the molten pool of aluminum
as shown in Fig. 3(b). Fe rich IMCs at the interface could result in improved joint strength. With increasing weld time, the thickness of the IMC layer increases from 0 to about 2.8 ¹m. This thickening of IMC layer was also observed in later investigation
which was applied as post welding. The thickness of the intermetallic layer increased to about 5 ¹m after the post welding step. Peel tests and lap-shear
test were conducted between the aluminum sheet and steel sheet, and indicated that interfacial failure often occurred at the aluminum/steel interface. When tear failure occurred during a lap-shear test, fracture always followed in the interface on the aluminum side.