Friction stir processing of newly-designed Mg-5Al-3.5Ca-1Mn (AXM541) alloy: Microstructure evolution and mechanical properties

Friction stir processing (FSP) of a new rare earth-free Mg-5Al-3.5Ca-1Mn (AXM541) alloy resulted in significant microstructural refinement and mechanical properties comparable to existing AXM series Mg alloys. Severe refinement of the Al-Ca (C36) phase during FSP led to uniform dispersion throughout the microstructure. The synergistic effect of high heat input and the presence of these 1-4 mu m size particles resulted in dynamic re crystallization via particle stimulated nucleation (PSN), with an average grain size of 4.5 mu m after FSP. Further, improved mechanical properties of the AXM541 alloy along the processing direction produced a tensile yield strength (TYS) of 322 +/- 14 MPa and a total elongation of 16 +/- 3%. The increase in strength was also attributed to the dispersion strengthening effect associated with Al-Ca and Al-Mn (D810) particles. Moreover, the AXM541 alloy showed a better mechanical response compared with other AXM alloys irrespective of high Ca/Al ratio, due to the effective refinement and redistribution of Al-Ca phases resulting from FSP. However, strain hardening ability and TYS of the AXM541 alloy could be further improved if the Mn/Al ratio could be tailored, as Al-Mn particles do not break down during FSP.