| Magnesium alloy is easy to burn at high temperature,and the formation of oxide inclusions will reduce the performance of product.The semi-solid forming process can prepare magnesium alloy products at lower forming temperature,which both reduces the inclusions and extends the life of the mold.However,magnesium alloys used by semi-solid forming are limited to AZ91 alloys currently.These alloys have poor heat treatment effect and are not beneficial to exert the advantages of heat treatment after semi-solid die casting.Therefore,studying a new magnesium alloy suitable for semi-solid forming is imperative.The heat treatment effect of Mg-Zn-Cu alloy is better,but the study of semi-solid microstructure evolution and optimization to the alloy are still not deep.In this paper,ZC61 alloys were added with Ce elements and following researches were studied by XRD,OM,SEM,TEM and tensile test.The microstructure and mechanical properties of ZC61+xCe alloys were studied.The results indicated that the as-cast microstructure of ZC61 alloy is mainly composed of matrix α-Mg,Cu Mg Zn and Mg Zn2 eutectic phase.With the increase of Ce content,a trace of Mg12Ce phase is formed,the grain size of the alloy is refined gradually,and the ultimate tensile strength（UTS）and elongation（EL）of the experimental alloys show a tendency of rising firstly and decreasing afterward.The finest grain can be obtained,and the UTS and EL of the alloy reach the maximum,when Ce content reaches 0.5%.The fracture mode of the alloy is changed from cleavage fracture to quasi-cleavage fracture,and then transformed to cleavage fracture.The effect of Ce content,holding temperature and holding time on the semisolid microstructure of the investigated alloys was studied by isothermal heat treatment.The results indicated that the solid particles were obviously refined and the solid fraction decreased sharply when Ce content reached 0.7%.After isothermal heat treatment at 600℃ for 25 min,Mg-6Zn-1Cu-0.5Ce alloy obtained the ideal semi-solid microstructure.And the particle size,shape factor and solid fraction were 57μm,1.16 and 68% respectively.Besides,it can be seen that the solid phase of semi-solid microstructure mainly consists of primary α1（Mg）phase and secondary solidified α2（Mg）phase.The process of semi-solid evolution is as follows: eutectic dissolution and α-Mg coarsening→separation→spheroidization→synthesis and growth.The particle separation mechanism includes the initial subgrain boundary infiltrationmechanism and the subsequent root remelting mechanism.When the solid and liquid phase reaches the equilibrium stage,non-dendritic particles occur the Ostwald ripening.The effect of heat treatment on the as-cast microstructure and mechanical properties of the alloy with 0.5%Ce was studied.The results indicated that the massive eutectic phases were dissolved into matrix and the residual eutectic phase distributed along grain boundaries existed in fine and dispersed granular form after solution treatment at 405℃ for 24 h.And after subsequent aging treatment,the precipitation of CuMgZn and MgZn2 generates at grain boundaries and in the grain interior,and the grain boundaries also precipitate obviously.With the increase of aging temperature,the number density of precipitates decreases,the trend of grain growth increases,and the UTS and micro-hardness decreases.Therefore,aging at160℃ for 20 h,the UTS and micro-hardness of the alloy reached to the maximum value of 256 MPa and 84 VHN,respectively.According to the analysis of TEM image in the peak aging stage,it can be seen that the rod-like β1’ phase which is perpendicular to the slip surface enhances the strengthen effect obviously.However,the strengthen effect of plate-like β2’ phase which is parallel to the slip surface is weak.Compared with three different fracture morphologies,the inter-granular fracture and the cleavage fracture are dominated in the as-cast condition.In the solid solution condition,the fracture mode is dominated by trans-granular fracture and quasi-cleavage fracture.In the case of aging treatment after solid solution,the fracture mode is trans-granular fracture and mixed fracture.The effect of heat treatment on the semi-solid microstructure of the alloy with0.5%Ce was studied.The results indicated that semi-solid structure after the solution treatment transformed as follows: eutectic phase dissolved→α2-Mg grain growth and the formation of fine particles→the coarsening and merging of fine particles→the combination of primary particles→the coarsening and growth of primary particles.As for semi-solid structure after aging treatment,the precipitation mainly precipitates in the secondary solidified zone.With the increase of aging temperature,the number density of precipitates decreases gradually,and the primary solid particles tend to grow up. |
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