压铸镁合金AM60B的微观组织特征

Microstructure Characteristics of High Pressure Die Cast AM60B Magnesium Alloy

镁合金压铸件中的压室预结晶组织及离异共晶组织对铸件最终使用性能有着重要的影响。针对冷室压铸工艺,采用AM60B镁合金研究不同低速速度及高速速度下压铸实验件的凝固组织特征。通过OM、EBSD、XRD、SEM及EDS等实验手段进行分析和定量统计。结果表明:在低速压射阶段,压室中熔体的过热度消失,达到初生相形核条件;在高速充填型腔过程中,压室预结晶组织向铸件中心偏聚,使得其心部含量高于表层;同时,压铸件靠近浇口端压室预结晶组织含量高于远离浇口端;低速速度越小,压铸件中压室预结晶组织含量越高;高速速度越小,压铸件中压室预结晶颗粒越粗大,枝晶形貌越完整。在压铸凝固后期,初生α-Mg晶界处出现离异共晶组织,共晶β-Mg17Al12相在压铸件中心及表层分别呈网状和粒状分布,且压铸件中心及缺陷带位置的共晶组织含量高于表层。

The externally solidified crystals （ESCs） and divorced eutectics in die castings of magnesium alloy have a great influence on the final performance of castings. Under the cold-chamber high pressure die casting （HPDC） process, samples were produced at different slow shot phase and fast shot phase plunger velocities to investigate the microstructure characteristics of AM60B magnesium alloy. Based on the analysis and statistics of the experimental results obtained by using OM, EBSD, XRD, SEM and EDS, it is concluded that at the slow shot phase of the HPDC process, nucleation and crystals growth occur in the melt in the shot sleeve due to the loss of superheat; the ESCs are migrating to the central region of the castings during the fast shot phase of the melt into the die cavity at high speed, which lead to a higher distribution of the ESCs at the central region than the surface layer of the castings. Besides, the percentage of the ESCs is higher at a lower slow shot phase plunger velocity, and also is higher at the area near the gate than that of the area far from the gate of the castings. Coarse dendrites are observed at the cross section of the castings at a lower fast shot phase plunger velocity. At the last stage of solidification under HPDC process, fully divorced eutectics consisting of a-Mg and fl-MgxTA112 appear at the grain boundary of the primary a-Mg. Islands offl-MglTAlx2 phase are observed at the central region of the castings, while the fl-MglTAl12 phase reveals a more dispersed and granular morphology at the surface layer. The percentage of the eutectics is higher at the central region than that at the surface layer of the castings, whilst the defect band also contains a higher percentage of the eutectics than those at the adjacent regions.