Evolution of rheocast microstructure of AZ31 alloy in semisolid state  

作　　者：Xing Bo Li Yuandong Ma Ying Chen Tijun Hao Yuan 

机构地区：[1]State Key Lab of Gansu Advanced Nonferrous Metal Materials,Lanzhou University of Technology [2]Key Lab of Nonferrous Metal Alloys and Processing,The Ministry of Education,Lanzhou University of Technology

出　　处：《China Foundry》2013年第4期221-226,共6页中国铸造（英文版）

基　　金：supported by the National Natural Science Foundation of China(50964010)

摘　　要：Semisolid rheoforming （SSR） is a promising technology for the production of Mg wrought alloy in foundry settings. In order to realize SSR, it is necessary to characterize the grain structure evolution during slurry preparation. In this paper, slurry of AZ31 alloy was produced by a novel rheocast process known as self-inoculation method （SIM）. Interrupted quenching technology was applied to investigate the primary a-Mg evolution during continuous cooling and isothermal holding. Results indicate that the initial microstructure of slurry produced by SIM is a mixture of irregular grains, which becomes ideally globular when the slurry slowly cools to 620 ~C and isothermally held for at least 30 s. The local solute diffusion leads to dendritic fragmentation and forms separated particles. During prolonged holding, the particle surface gradually becomes smooth because of protuberance melting and groove advancement. Coarsening of a-Mg grains in isothermal holding was analyzed using Lifshitz-Slyozov-Wagner theory. Results suggest that coalescence is most likely the dominant coarsening mechanism in the early stage while Ostwald ripening tends to be the principal one later. The EDS results indicate that a longer holding time leads to AI solute element segregation at the grain boundaries, but Zn distribution within liquid matrix has no obvious change.Semisolid rheoforming(SSR)is a promising technology for the production of Mg wrought alloy in foundry settings.In order to realize SSR,it is necessary to characterize the grain structure evolution during slurry preparation.In this paper,slurry of AZ31 alloy was produced by a novel rheocast process known as self-inoculation method(SIM).Interrupted quenching technology was applied to investigate the primaryα-Mg evolution during continuous cooling and isothermal holding.Results indicate that the initial microstructure of slurry produced by SIM is a mixture of irregular grains,which becomes ideally globular when the slurry slowly cools to 620℃and isothermally held for at least 30 s.The local solute diffusion leads to dendritic fragmentation and forms separated particles.During prolonged holding,the particle surface gradually becomes smooth because of protuberance melting and groove advancement.Coarsening ofα-Mg grains in isothermal holding was analyzed using Lifshitz-Slyozov-Wagner theory.Results suggest that coalescence is most likely the dominant coarsening mechanism in the early stage while Ostwald ripening tends to be the principal one later.The EDS results indicate that a longer holding time leads to Al solute element segregation at the grain boundaries,but Zn distribution within liquid matrix has no obvious change.

关 键 词：rheocast microstructure evolution mechanism AZ31 alloy quiescent state 

分 类 号：TG146.22[一般工业技术—材料科学与工程]