Study On Grain Refinement Process And Mechanism Of Mg-Al Based Alloy

Zr has significant refinement on magnesium,which is mainly used on Mg-Zn and Mg-RE alloys,but can not be used on the most widely used Mg-Al alloy.So it is necessary to find a new type of grain refiner.The mismatch,melting point and electronegativity between the elements are discussed in this paper,and we choose Mg-0.6Ca-xPb and Mg-0.6Ca-xSn master alloys. Finally Mg-0.6Ca-1.8Pb and Mg-0.6Ca-1.9Sn are selected through analysis of organization and phase.Then hot extrusion processing is conducted,further the extrused alloys is respectively added into the AZ31 alloy.Analysis of organization and phase, refinement mechanism are studied in the paper.The results show that:Mg-0.6Ca-xPb and Mg-0.6Ca-xSn alloys are successfully produced by using resistance furnace,protecting by SF6+CO₂. Mg-0.6Ca has coarse microstructure,whose secondary dendrite arm spacing and average grain size is separately 25.2μm and 274μm, which decline with the increase of Pb content.When Pb reaches 1.8%, the organization is the tiniest, whose secondary dendrite arm spacing and average grain size is separately 11.8μm and 127μm. The refinement is mainly due to Ca₂Pb,Mg₂Ca phase,which can impede grain growth.Ca₂Pb phase can also be heterogeneous nucleation.The organization shows fluctuationg with the increase of Sn content. When Sn reaches 1.9%, the organi-tion is relatively the tiniest, whose secondary dendrite arm spacing and average grain size is separately 8.6μm and 94μm. The refinement is mainly due to CaMgSn,Mg₂Sn phase,which can impede grain growth.CaMgSn phase can also be heterogeneous nucleation.Mg-0.6Ca-1.8Pb and Mg-0.6Ca-1.9Sn are selected to solute and extrude, thereby the state of the second phase change into dispersion from reunite,the grain size has decreased substantially. The second phase are respectively Ca₂Pb,Mg₂Ca,Mg_1.86Pb. CaMgSn,Mg₂Sn.Hot extrusion process does not change the phase type.The produced master alloys have good refinement on AZ31.Secondary dendrite arm spacing and average grain size of AZ31 is separately 38μm and 182μm.When Ca, Pb reach 0.73%, Secondary dendrite arm spacing and average grain size separately decline to 34.8μm and 90μm.The refinement is mainly due to Ca₂Pb,Al2Ca phase, which can be heterogeneous nucleation and impede grain growth.When Ca, Sn reach 0.8%, Secondary dendrite arm spacing and average grain size separately decline to 33.3μm and 93μm. The refinement is mainly due to CaMgSn,Mg₂Sn phase,which can impede grain growth.CaMgSn phase can also be heterogeneous nucleation.