Grain refinement of magnesium alloys via large-load friction stir processing
By Yoshihisa Kimoto (Osaka Research Institute of Industrial Science and Technology), Yoshiaki Morisada (Joining and Welding Research Institute, Osaka University), Hidetoshi Fujii (Joining and Welding Research Institute, Osaka University), Peter Cengeri (University of Vienna), Gerhard Krexner (University of Vienna), Michael Zehetbauer (University of Vienna)

Friction stir processing (FSP) can refine the grains in the stir zone (SZ). To produce ultrafine grains (typically smaller than 1 micrometer) in the SZ, 3 techniques are well-known so far: FSP under rapid cooling, FSP with ceramic nanoparticles, and FSP with metallic additive to form nanoprecipitates. As a new technique to produce ultra-fine-grained (UFG) structure, Nan Xu et al. has been performed large-load friction stir welding for AZ31 magnesium alloy and resulted in UFG structure in the SZ. In this study, conventional FSP and large-load friction stir processing (LL-FSP) has been examined for some magnesium alloys. For example, the conventional FSP for AZ31 under the conditions with rotation rate 1600 rpm, travel speed 400 mm/min and applied load 9.8 kN resulted in the grain size of 6.9 micrometer in the SZ. On the other hand, the LL-FSP under the conditions of 200 rpm, 10 mm/min and 25 kN has further refined the grain size of 3.5 micrometer.