Grain Refinement Of AZ91D Magnesium Alloy And Microstructural Evolution During Partial Remelting

Magnesium alloys,as the lightest structural metallic materials,have large application potential,Semisolid forming has been consinder as the most promising forming technology for magnesium alloys.However,the microstrctures of as-east magnesium alloys are relatively coarse,which seriously impairs their mechanical properties and also affects semisolid mierostructures.Among the grain refining technologies of magnesium alloy,addition of carbon containing grain refiner is the most effective method.In this thesis,the effects of addition amounts of MgCO₃ and SiC addition temperature,holding time,cooling rate and casting ingot size on the microstructures of AZ91D magnesium alloy were investigated.The mechanism of grain refining was discussed based on the nucleation and crystal growth theory. Meanwhile,the microstructural evolutions of the grain-refined AZ91D alloys were studied during partial remelting.The microstructure of AZ91D alloy was significantly refined by MgCO₃ and SiC.Its grains was decreased from 311μm to 53μm after being treated by 1.0% MgCO₃ at 790℃and held for 10min.The refing effect decreased as the holding time of treated melt increased.Increasing cooling rate of melt from the treated temperature to pouring temperature could decrease grain size.After being treated by 0.2%SiC at 760℃～780℃and held for 30min,the grain size dereased to 70μm.The effects of the other factors on the microstructure were similar to those of MgCO₃. Comparatively,the refining decay of SiC was weaker and its refining effect was more stabal.Its maneuverability and dependability were better.The grain refining of MgCO₃ and SiC was attributed to heterogeneous nucleation and depressing growth.Al₄C₃ was formed through reaction between the refiner and alloy element.The Al₄C₃ could act as the heterogeneous nuclei.Part of C element agglomerated at grain boundaries to form Al₄C₃,which could depress grain growth.In addition,the formed Mg₂Si could also depress grain growth if SiC was used.The semisolid microsturcture available for semisolid forming could be obtained when the MgCO₃ and SiC treated AZ91D alloys were partially remelted.The microstructural evolution could be divided four stages,the initial coarsening stage due to dissolution of eutectic of residual eutectics,spheroidization due to partial melting of primary particles and the final coarsening stage due to mergence and Ostwald ripening.The primary particles were small and more spheroidal for the refining treated alloy.The finer the as-cast microstruture,the smaller and more spheroidal the primary particles in the semisolid microstructure.The primary particle size increased with the increase of casting ingot size.