Friction stir processing of A-286 stainless steel: Microstructural evolution during wear

The effect of friction stir processing (FSP) on mechanical and wear behavior was investigated for A-286 stainless steel, an Fe-Ni-Cr based austenitic, precipitation hardened alloy. The alloy was characterized in different processed conditions, namely as rolled (AR)+aged and FSP+aged. High frequency reciprocating sliding wear behavior and wear mechanisms were investigated at room temperature. The Vickers microhardness and wear rates were measured and compared for each processing condition. It was determined that along with increasing microhardness in the stir zone, FSP resulted in improved wear resistance. Specifically, the wear rate in the stir zone was reduced from 1×10−6 to 6×10−7 mm3/N m due to FSP. Mechanistic studies were conducted to determine the effect of FSP on the microstructural evolution during wear. Scanning electron microscopy revealed increased coarse abrasion with the AR+aged alloy as compared to much finer-scale microabrasion with the FSP+aged alloy. This resulted in smaller and less abrasive wear debris, and hence lower wear rate. Furthermore, cross-sectional focused ion beam microscopy studies inside the stir zone of the FSP+aged alloy determined that increased microhardness was due to FSP-induced microscopic grain refinement resulting in Hall-Petch strengthening, and the corresponding wear rate decrease was due to even finer wear-induced grain refinement. With both effects combined, the level of surface fatigue wear was suppressed resulting in reduction of the wear rate. In contrast, the absence of FSP-induced grain refinement in the AR+aged alloy resulted in decreasing hardness and increasing wear rate. Overall, FSP of A-286 stainless steel alloy resulted in a lower wear rate suggesting it is a viable surface engineering technique to target and mitigate site-specific wear.