Hot Cracking Behavior of Mg-5A1-xCa Alloys

Mg-Al-Ca base alloys have great potential for application because of its low cost and good high temperature creep properties, but its higher hot cracking susceptibility greatly limits the application of the alloy. The effect of Ca addition on the hot cracking susceptibility of Mg-5A1-xCa (x=0.5, 1.0, 2.0, 3.0, 4.0, 5.0, mass fraction, %) alloys at the pouring temperature 700 degrees C and mold temperature 200 degrees C was studied by using hot cracking curve test, solidification curve test, OM, XRD and SEM. The results showed that the hot cracking susceptibility of alloys decreased with increasing Ca addition until to 4.0%, and the Mg-5AI-4.0Ca alloy had minimal hot cracking susceptibility, which cracking susceptibility coefficient was only 0.824. But when Ca addition increased to 5.0%, the hot cracking susceptibility of the alloy increased, and the cracking susceptibility coefficient increased to 0.96. The appropriate Ca addition can reduce the precipitation temperature of alpha-Mg in Mg-5A1-xCa alloys, inhibit precipitation of Mg(17)A1(12) phase, narrow solidification temperature range of the alloy and increase eutectic content, which are helpful for the alloy having a stronger ability of compensation at the solidification end to decrease hot cracking susceptibility. But excessive Ca addition will increase the numbers of brittle phases containing Ca and coarsen the microstructure, resulting in the increase of hot cracking susceptibility.