Analysis of material flow in the sheet forming of friction-stir welds on alloys of mild steel and aluminum

The deep drawability of and material flow in friction-stir welds of dissimilar steel/aluminum were investigated in this study. Steel/Al tailor-welded blanks (TWBs) were found to possess sufficient joint strength and deep drawability with limiting drawing ratios (LDRs) as low as 1.7. However, the drawability of these TWBs was lower than that of the two base materials. Fracturing was observed near the weld line on the flat bottom of the aluminum alloy side. In addition, the aluminum alloy near the fractured area in the TWB cup was subjected to significant strain near the forming limit, correlating to a significant loss in their deep drawability. Numerical simulation results indicated that slightly enhancing the friction coefficient between the punch and the blank significantly mitigated the significant strain near the fracture area. Deep drawing tests verified that the LDRs of TWBs obtained using rosin were increased by nearly 19% compared with that obtained using conventional press oil. Therefore, this approach of enhancing the friction coefficient can be effectively applied to improve the formability of dissimilar steel/aluminum TWBs.