Study On Grain Refinement Of Mg-Al Castings By Al3Ti3B Master Alloy

The work mainly focused on the effects of Al3Ti3B master alloy on grainrefinement of ingot of Mg-Al alloys. Some typical Mg-Al alloys, such asAZ91D and AZ31B alloys in Mg-Al-Zn system, AM60 alloy in Mg-Al-Mnsystem were investigated systematically for relations among addition amount,holding time, microstructure and mechanical properties.Apparent grain refinement is found in the Mg-Al castings through addingsmall quantity Al3Ti3B master alloy to AZ91D, AM60 and AZ31B. In theexperiments, the average grain size of AZ91D decreases from 260μm to 30μm with merely 0.6wt% while for AM60 and AZ31B show a decrease theiraverage grain sizes from 300μm to less than 60μm and 30μm with 0.6wt%and 0.9% Al3Ti3B, respectively.At the same time, the secondary dendrite arm space also decreases, whichlead to the homogeneous distribution of solute, and refinement of the eutecticMg17Al12 phase. The reduction of secondary dendrite arm space leads tohigher efficiency of heat treatment.With the decreasing of secondary dendrite arm space or grain size, theUTS, YS and elongation increase in the as-cast alloys. The phenomena areexplained by assuming that (1)the density of grain boundaries became higherwhich acts as obstacles to the crack propagation with a smaller grain size;(2)with the decline of secondary dendrite arm space, the cast structurebecomes cleaner and sounder and residual Hall-Petch hardening appeared.The main factors for grain refinement of Al3Ti3B master alloy are theheterogeneous nucleus of AlB2 and TiB2 and the increasing of undercoolingdegree between the solid/liquid interface during solidification and resultingmore nucleus. Because of the congregate of B-particles, the Al3Ti3B'scapability of refinement is declined.