| Liquid die casting is a common method for preparing magnesium alloy components.However,the components formed by liquid die casting cannot be heat treated,so its mechanical properties cannot be improved again by means of heat treatment.Although magnesium alloy components formed by semi-solid die casting can be heat treated,at present the magnesium alloys currently used for semi-solid forming are weak in heat treatment and cannot fully take advantages of heat treatment strengthening.The Mg-Zn-Cu-based alloys are a kind of magnesium alloy system with significant age-strengthening effect,the heat treatment strengthening effect can be fully exerted when the alloys are produced by semi-solid forming technology.However,there are few studies on the heat treatment of as-cast and semi-solid Mg-Zn-Cu alloys after micro-alloying.Therefore,Cr element was added on the basis of ZC61 alloy in this work,and the following contents were studied by means of characterization methods such as OM,XRD,SEM,TEM and hardness test.The ZC61-x Cr alloy was prepared by permanent mold casting and its as-cast microstructure was studied.The results indicated that the microstructure of the alloys did not form new phase after the addition of Cr element,and the as-cast microstructure of all alloys was composed of?-Mg,MgZn2 and CuMgZn phase.The grain of ZC61alloy could be refined when the content of Cr was 0.1 wt.%.The non-dendritic ZC61-x Cr alloys were prepared by semi-solid isothermal heat treatment and the evolution of semi-solid microstructure was studied in detail.The results indicated that the addition of Cr element was beneficial to the formation of more ideal semi-solid microstructure on ZC61 magnesium alloy.The optimal process of isothermal heat treatment was at 585°C holding for 30 min when the Cr content was0.1 wt.%,and at the same time the average solid size,shape factor and fraction of solid phase were 37.5?m,1.33 and 67.2%,respectively.In the semi-solid non-dendritic microstructure after water quenching,the solid phase was consisted of?1-Mg and?2-Mg,and the composition of the liquid phase including liquid phase between solid particles,“small liquid pool”in solid particles,and liquid films.Studying on microstructure of as-cast ZC61 and ZCR6101 alloys after solution and aging treatment showed that ZCR6101 alloy had higher supersaturation than ZC61alloy without adding Cr element under the same process of solution treatment?at 405°C solution for 20 h?.The addition of Cr element could increase the hardness of the alloy and reduce the time to reach the peak-aged T6 under the same aging process?down 5h,at 160°C aging for 060 h?.The maximum hardness of the ZCR6101 alloy was 82.25HV.Solid solution and aging heat treatment on semi-solid microstructure of ZCR6101alloy after isothermal heat treatment at 585°C for 30 min were studied.The results indicated that the solid solution transformation process of non-dendritic ZCR6101alloy at 405°C solution treatment was as follows:merging of between primary?1-Mg particles,coarsening and coalescence between?2-Mg particles formed by secondary water quenching and growth of?1-Mg absorbed by?2-Mg.In the artificial aging process,there were some G.P.zones on the{0001}Mg basic plane in the non-dendritic ZCR6101 alloy at the initial stage of aging.There were two kinds of strengthening phases at the peak-aged T6 when the G.P.zone was dissolved(?1'phase?{0001}Mg,?2'phase?{0001}Mg).From the direction parallel to[112?0]Mg,the?1'phase,which precipitates more,was long strip and long block in morphology;and the?2'phase,which precipitates less,was mainly disc in the direction parallel to[0001]Mg. |
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