Effect Of Ca And Sm On Microstructure And Hot Cracking Susceptibility Of Mg-6Al-Zn-xCa-ySm Alloy

Magnesium alloys have been widely used in automotive and aerospace industries because of their high performance-price ration.However,magnesium alloys show high susceptibility to hot cracking during the solidification and application temperature.They become the bottleneck for commercial applications.The alloying of alkaline earth element Ca and rare earth element rare Sm can further improve properties of the alloys.Meanwhile,Ca and Sm can play role of flame retardant and melt purificication,so that the comprehensive properties of magnesium can be significantly improved.Thereforce,it is of great significance to study the hot cracking mechanism and alloying for improving the range of magnesium alloys in practical applications.For hot cracking susceptibility of magnesium alloys,most of previous inspections were focused on only for Mg-Al series,but seldom concurn about Mg-Al-Zn series.Therefore,this paper uses AZ61 magnesium alloy as the matrix,using rare earth element Sm and alkaline earth element Ca composite alloying.The effect of Ca and Sm on microstructure and hot cracking susceptibility of Mg-6Al-Zn-xCa-ySm alloy was studied by OM,XRD,SEM,EDS,solidification curve measurement,solidification load measurementand,and so on.1 The result show that with the content of Sm is fixed,the Al₂Ca phase and the?-Mg₁₇Al₁₂ phase in the Mg-6Al-Zn-xCa-ySm alloy changed from intermittent mesh to continuous mesh with the increase of Ca content.When the Ca content exceeds 2%,the composite structure?Mg,Al?₂Ca+?-Mg₁₇Al₁₂ phase is produced at the grain boundary of the Mg-6Al-Zn-xCa-ySm alloy.When the Ca content is fixed,as the Sm content increases,the Al₂Sm in alloy gradually aggregates into a block shape by the dispersed particles.At the same time,the composite structure Al₂Sm+?-Mg₁₇Al₁₂2 phase appears in the grain boundary and crystal.2 The hot cracking susceptibility tendencies were as a function of Sm and Ca follows the"V”shape curve.When the alloy contains a small amount of Ca and Sm,the Al₂Sm pahse can serve as propitious sites for the heterogenous nucleation of alloy.It leads to an increase in the nucleation rate of the alloy and refines the grains.The particle Al₂Sm dispersed and distributed in matrix refine grain increase the numbers and the thickness of liquid film which can reduce the hot cracking susceptibility of alloy.At this time,it follows the liquid film theory.When the alloy contains a large amount of Ca and Sm,the segregation of rare earth elements and alkaline earth elements at the grain boundaries weakens the refinement effect,resulting in a decrease in the number of alloy liquid films and an increase in dendritic bridges.Increased hot cracking susceptibility due to lower strength of the dendritic bridge and increased solidification shrinkage stress.Meanwhile,the formation of thermal cracking is the result of the combination of liquid membrane theory and dendritic bridge theory.