通过能够准确检测和记录镁合金凝固过程中温度、收缩位移和收缩应力细微变化的实验装置, 研究了Mn对Mg-6.5Zn-xMn系合金凝固过程中热裂倾向性的影响规律. 结果表明, 所提出的热裂倾向性评价指标最大收缩速率(v_max)和应力累积系数(k)越大, 热裂倾向越大, 且其在高固相率出现时热裂倾向更显著; 随Mn含量的增加, Mg-6.5Zn-xMn系合金的v_max增大, 但其出现向低固相率迁移; k在Mn含量为0.35%时达到最大, 且在高固相率时出现, 导致其热裂倾向性最大. 该系合金的热裂纹在凝固后期(高固相率)萌生并扩展, 晶粒间存在明显的补缩通道; 低熔点相于凝固后期在晶粒表面形成液膜, 且液膜越厚, 晶粒越细, 热裂倾向性越小; 枝晶分离后相互接触的枝晶臂搭接形成的晶间搭桥加强了合金凝固后期晶间结合力, 但晶粒收缩受阻拉断晶间搭桥会形成热裂.

Mg-Zn-Mn alloy has high hot cracking tendency (HCT), but few researches focus on its hot cracking behavior and mechanism. The effect of Mn on the HCT of Mg-6.5Zn-xMn alloys was studied by the designed equipment which can measure and record the subtle changes of temperature, shrinkage displacement and shrinkage stress during solidification in this study. The results indicate that the larger the maximum contract rate (v_max) and the stress accumulating coefficient (k), which are put forward to evaluate HCT, the higher the HCT is, and there is higher HTC when v_max or k presents at high fraction of solid. The v_max of Mg-6.5Zn-xMn alloy increases with the increase of Mn content, however its position move towards to lower fraction of solid, and the k reaches the maximum value and presents at high fraction of solid at 0.35%Mn, which means the greatest HCT in this composition. The hot cracks of these alloys initiated and propagated at final stage of solidification (with higher fraction of solid), and the intergranular feeding channels could be observed. The thicker the liquid film around grains formed by the low melting point phases and the finer the grains, the less the HCT of the alloy is. After dendritic separation, interdendritic bridging formed by the jointing of dendrite arms could enhance the adhesive force between grains at final stage of solidification. However, the break of interdendritic bridging due to the hindrance to grain contraction would result in the hot cracks.