Microstructure characteristics and solidification behavior of thixomolded Mg-9%Al-1%Zn alloy

Experimental investigation and theoretical analysis of the microstructure of thixomolded AZ91D were carried out to comprehensively understand the morphology transformation of solid particles and the solidification behavior. Typical microstructure of thixomolded AZ91D is composed of α-Mg and β-Mg17Al12, characterized with αun, αprim and eutectic. Four kinds of αun are classified according to the morphology and generation mechanism, such as spherical (α1), irregular (α2), entrapping liquid alloy inside (α3) and entrapping pool inside (α4). Under the effect of heating, shearing, collision, agglomeration or fragmentation, α2 and α4 can be the middle states of α1 and α3. Similarly, α4 and α3 can also break into α2 and become α1 at the end. Controlled by undercooling, αprim nucleates and spherically grows within the remaining liquid alloy of thixomolded AZ91D until instability growth. The investigated microstructure was theoretically proved according to the analysis of Mg-Al binary phase diagram.