| This paper employs Mg-10Gd-3Y-0.7Zr alloy obtained in the conventional casting,low-pressure die casting using varied step plate including forming sand mould,semipermanent mould, graphite mould and metal mould. The samples are subjected tothe OM, SEM observation to investigate the influence of the solidification conditions onthe microstructural composition and mechanical properties of Mg-10Gd-3Y-0.7Zr alloy.Structural analysis indicates that the as-cast microstructure of both conventionalcasting and low-pressure die casting Mg-10Gd-3Y-0.7Zr alloy consists of primary α-Mgand eutectic Mg24(Gd,Y)5, the net-like eutectic phase is continuously distributed alongthe grain boundary. The as-cast microstructure of Mg-10Gd-3Y-0.7Zr alloy is featured bythe equiaxed solidification. As the cooling rate increases, the grains of alloy becomesfiner.Comparied with the alloy obtained by conventional casting, the alloy oflow-pressure die casting has better mechanical properties. Among those step plate of6mm~18mm thickness, the UTS varied between200MPa~230MPa, the YTS variedbetween130MPa~150MPa, the hardness varied between75HV0.1~85HV0.1.The superior heat treatment of Mg-10Gd-3Y-0.7Zr alloy is525℃for7hours, and240℃for12hours. After heat treatment the net-like eutectic phase can not be observedin the as-cast alloy, and is assumed to have been thorougly soluted in the matrix, thesquare-shape Mg5(Gd,Y) phase is observed distributing uniformly around the grainboundaries. The UTS, YTS and extensibility of the conventioal-casted alloy can achieve210MPa~230MPa,120MPa~140MPa and4.3%~13.8%respectively after T6heattreatment. The enhancement in the mechanical property is more obvious in thelow-pressure die-casted alloy, and the18mm thick semipermanent has an optimizedmechanical properties, the UTS, YTS and extensibility is307.6MPa,202.1MPa and9.9%,which is an increase of50.7%,33.7%and90.6%respectively. This result also indicatesthe strengthening of the Mg-10Gd-3Y-0.7Zr alloy comes mainly from ageing effect. |
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