Grain Refinment Of AM60B+XCa Magnesium Alloys And Microstructure Evolution During Semi-solid Isothermal Treatment

In the present work,based on AM60B alloy with alkaline earth additions of Ca,starting from the melting process,the effects of Ca content on the microstructure and mechanical properties of based alloy have been investigated by optical microscope(OM),scanning electron microscope(SEM),X-ray difractometer(XRD), etc.At the same time,I used semi-solid isothermal treatment with one-step and two-steps respectively to afford non-dendritic structure for thixorming,the mechanism of non-dendritic structure forming during semi-solid isothermal heat-treatment was discussed,and the effects of heating method,treatment temperature,holding time and ingot size on microstructure during isothermal heat treatment were investigated.Experimental results showed that:the additiont of calcium to AM60B resulted in refinement of as-cast dendritic microstrutures of alloy.The temperature of modification,holding time at temperature of modification and pouring temperature affected the refining effect of Ca elements.When the melt of AM60B alloy added Ca element at 750℃,held for 30min and poured into metal mold at 710℃,the usage of Ca for getting the best microstrutures and properties was 0.3%,the grain size of as-cast AM60B alloy was 110μm,and ultimate tensile strength of 166MPa and elongation of 4.35% were obtained.Further additions of Ca resulted in stabilization of the grain size which seemed to reach the saturation level,while the tensile strength and elongation of the alloy decreased,hardness increased with increasing Ca content.According to the change of Ca content in alloy,there were three different distributions:Ca was presented in magnesium matrix when it was low content;Ca dissolved in Mg₁₇Al₁₂ phase,and a new binary compounds which consisted of Al and Ca was emerged with increasing Ca content.The microstructural evolution of AM60B alloy during semi-solid isothermal treatment(SSIT) was investigated,the results indicated that it have experienced solution, melting , separation and globurizing, was results diffusion of solid solution atom and the combined effects of interfacial tension.It was found that the favorable undendritic microstructure could be obtained after isothermal holding at 615℃for 60min to 75min with one-step based on the conditions of this paper.As-cast structure and ingot size influenced microstructural evolution,the smaller the structure was,the easier the non-dendritic structure could be obtained,the bigger ingot size was,the longer time and higher temperature ware needed.There was a certain degree of match among process parameters of isothermal treatment with two-steps,in which the temperature and holding time had greater effects on preparation of non-dendritic.And two-steps technique could promote the liquefaction and granulation and contribute to get more even and fine undendritic microstructure.It was found that the favorable undendritic microstructure could be obtained after isothermal holding at 640℃for 5min and then at 615℃for 40min with two-steps based on the conditions of this paper.