在浇注温度700 ℃、模具温度200 ℃条件下,通过热裂曲线和凝固曲线测试以及OM、XRD、SEM等手段分析了Ca加入量对Mg-5Al-xCa (x=0.5、1.0、2.0、3.0、4.0、5.0,质量分数,%)合金热裂敏感性的影响。结果表明,当Ca加入量增加到4.0%时,合金的热裂敏感性随着Ca加入量的增加而减小;Mg-5Al-4.0Ca合金的热裂敏感性最小,其热裂敏感性系数仅为0.824;Ca加入量增加到5.0%时,合金的热裂敏感性又出现上升,其热裂敏感性系数上升到0.96。加入适量的Ca会降低Mg-5Al-xCa合金凝固时α-Mg的析出温度,并抑制Mg17Al12相形成,缩小合金凝固温度范围,增加组织中的共晶含量,有利于合金凝固后期的补缩,从而降低合金的热裂敏感性。但加入过多的Ca会增加含Ca脆性相的数量,并使合金组织发生粗化,从而导致合金的热裂敏感性增加。
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-5Al-xCa (x=0.5, 1.0, 2.0, 3.0, 4.0, 5.0, mass fraction, %) alloys at the pouring temperature 700 ℃ and mold temperature 200 ℃ 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-5Al-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 α-Mg in Mg-5Al-xCa alloys, inhibit precipitation of Mg17Al12 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.
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