Study On Hot-Tearing Susceptibility Of AIN/AZ91D Composite

With the increasing emphasis on energy conservation,emission reduction,and environmental protection issues,magnesium alloys and their composite materials have been widely used in industries such as rail transit,aerospace,and electronic information due to their high specific strength,high specific stiffness,and excellent thermal and electromagnetic shielding properties.At present,magnesium-based composites are mostly prepared by casting,and the differences in reinforcement phases and their introduction methods lead to more complex and uncontroltable hot tearing behaviour of the composite,which seriously restricts the industrial promotion and application of high-performance magnesium-based composite.In order to further promote the production and application of in-situ AlN particle reinforced AlN/AZ91D magnesium-based composite（AlN/AZ91D composite）,two methods,i.e.a constrained rod（CRC）hot tearing testing mould and a CSC^*hot tearing model,were used to study the hot tearing sensitivity and mechanism of the AlN/AZ91D composite.Furthermore,numerical simulation methods are used to determine the optimal casting process parameters of the composite.The specific research content and main conclusions are as follows:（1）The hot tearing sensitivity of the AIN/AZ91D composite was evaluated.The evaluation results of the CRC hot tearing testing mould and the CSC^*hot tearing model are consistent,both indicating that the composite has a low hot tearing sensitivity,which is only one-third of the hot cracking sensitivity of the matrix AZ91D alloy.（2）The strength of the AlN/AZ91D composite in the brittle zone was explored.Compared to AZ91D alloy,the composite has a lower temperature for hot tearing,shorter"impact" time for shrinkage stress,higher peak stress,and milder macroscopic hot tearing,which indicates that the composite has higher strength in the brittle temperature range.（3）The effect of in-situ AlN particle reinforcement on the microstructure of the AlN/AZ91D composite was elucidated.The AlN particles formed in situ have a grain refinement effect on the matrix structure.It can also refine the β-Mg17Al12 phase and inhibit its formation,resulting in the β-Mg17Al12 phase changing from a continuous network distribution to a discontinuous network distribution.（4）A HTS^*simulation hot tearing evaluation model based on ProCAST numerical simulation software was constructed,and the casting process parameters were optimized using the model.The sensitivity of the AlN/AZ91D composite to hot tearing can be evaluated by use of the model,and the location of hot tearing occurrence can also be predicted.Under the process conditions of a metal mould temperature of 350℃ and a pouring temperature of 710℃,a CRC hot tearing specimen of the AlN/AZ91D composite without hot tearing can be obtained.（5）The mechanism of hot tearing sensitivity of the AlN/AZ91D composite was revealed.Grain refinement caused by in-situ AlN particles and the discontinuous network distribution of the β-Mg₁₇Al₁₂ phase is the fundamental reason for the low hot tearing sensitivity of the composite.Grain refinement affects the composites hot tearing sensitivity through solidification shrinkage and liquid phase replenishment.