Study On The Microstructure And Mechanical Properties Of Squeeze Casting Mg-12Zn-4Al-0.5Ca Alloy

The most widely used magnesium alloys in industry are Mg-Al series. However, due to the low melting point of the intermetallic phase Mg17Al12, the elevated temperature mechanical properties of these alloys are poor. Mg-RE series alloy, although exhibit excellent heat-resistance, the high-cost also limit their wider application. Therefore, developing a new heat-resistant while low-cost magnesium alloy is of great value.In this study, a new heat-resistant magnesium alloy of Mg-12Zn-4Al-0.5Ca(ZAX12405) was designed. Systematic research for both squeeze casting, semisolid slurry preparation using gas bubble stirring(GISS) method and rheo-squeeze casting process of ZAX12405 alloy have been done. The following results can be drawn:Microstructure of as-cast ZAX12405 alloy prepared by three different casting process were all consisted of α-Mg, τ-Mg32(Al, Zn)49 and φ-Al2Mg5Zn2. Ca was mainly soluted in the α-Mg and eutectic phase. Both GISS processing and squeeze casting were useful to weaken the aggregation tendency of Ca at the grain boundary.During the squeeze casting of ZAX12405 alloy, increasing the applied pressure led to significant cast densification and a certain extent of grain refinement in the microstructure; lowering the pouring temperature refined the microstructure of ZAX12405 alloy, but deteriorated the cast densification; increasing the dwell time promoted cast densification and mechanical properties just before the solidification process ended. For squeeze-cast ZAX12405 alloy, cast densification was considered more important than microstructure refinement for the promotion of mechanical properties. A combination of highest applied pressure(120MPa), medium pouring temperature(650°C) and dwell time(30s) brought the highest mechanical properties, under which the UTS, YS and Ef of the alloy reached 211 MPa, 113 MPa and 5.2% at room temperature. Comparing with the mechanical properties of gravity casting ZAX12405 alloy(UTS 151 MPa, YS 115 MPa, Ef 1.35%), the UTS and Ef of squeeze-cast ZAX12405 alloy increased 40% and 300%, respectively.During the semisolid slurry preparation of ZAX12405 alloy using GISS method, lowering the temperature led to the increase of solid volume fraction and the average shape factor of α-Mg; increasing the gas flow rate led to the continuously increase of average shape factor and decrease of average particle size of α-Mg. Hence optimized processing parameters for GISS was: pouring temperature 580°C and gas flow rate 8L, under which the average particle size and shape factor reached 43μm and 0.56 respectively. The mechanism for formation of non-dendrite microstructure in this study was considered a mixture of dendrite fragmentation mechanism and copious nucleation mechanism.There were two typical kinds of grains in the microstructure of rheo-squeeze casting ZAX12405 alloy: near globular or rosette-like α1-Mg and fine dendrite α2-Mg. Compared with squeeze-cast and gravity-cast ZAX12405 alloy, rheo-squeeze casting alloy further improved both ambient and elevated temperature mechanical properties, especially for elongation. The ambient temperature mechanical properties of rheo-squeeze casting ZAX12405 alloy reached: UTS 217 MPa, YS 112 MPa and elongation 7.5%. At the temperature of 200°C, the UTS and the elongation were 138 MPa and 38% respectively. The refinement of grain size, the disappearance of intermetallic phase with nearly net-worked morphology, the decrease of the volume fraction of the second phase and the restraint of casting defects like hot-tearing and shrinkage porosities were considered several main reasons for the promotion of ambient and elevated temperature mechanical properties.