Influence of pass number on microstructure, mechanical, and tribological properties of cold-rolled Al1050 during friction stir processing

In this investigation, cold-rolled Al1050 sheets underwent various passes of Friction Stir Processing (FSP), namely one, three, and five. Then, microstructural evolution was assessed by focusing on underlying restoration phenomena, including severe dynamic recovery (DRV) and continuous dynamic recrystallization (CDRX), and their corresponding effects on mechanical and surface properties were evaluated. Findings illustrated that initial cold-rolling on Al1050 and its high stacking fault energy are the primary driving forces for accelerating DRV, contributing to the supremacy of the DRV-induced softening over sub-grain strengthening caused by CDRX. Increasing the FSP pass number resulted in a drop in yield stress (YS), ultimate tensile strength (UTS), and microhardness values of the FSP-treated samples such that the UTS of one, three, and five passes FSPed samples decreased by ∼18 %, 32 %, and 35 %, respectively, compared to the cold-rolled Al1050. Conversely, the %elongation of the FSP-treated samples dramatically increased with increasing FSP pass number, which is evident from stress-strain curves and the fracture surfaces. Furthermore, increasing the FSP pass number increased the mean coefficient of friction (MCOF) and weight loss during the pin-on-disk test. Also, conducting various passes of FSP improved corrosion resistance compared to the base metal. However, increasing the FSP pass number reduced the corrosion resistance due to grain growth such that the one-pass FSPed sample illustrated better corrosion resistance than the five passes FSPed sample. Repeating the FSP led to excessive frictional heat input to the matrix, contributing to grain growth after increasing FSP passes, thereby decreasing mechanical properties and corrosion resistance.