Study On Mg-Zn-RE Magnesium Alloys Via High Pressure Squeeze Casting And Heat Treatment

In practical engineering application,Mg-Al series alloys has low cost and good performance at room temperature,but its heat resistance is poor.For Mg-RE series alloys,it's difficult to achieve a large-scale application because of the high cost,though it has high strength and outstanding heat resistance.In contrast,Mg-Zn-RE series has great potential for further application due to its lower cost and excellent overall performance,especially heat resistance,as the result of a part of the RE elements is replaced by the Zn element,which can form various strengthening phases with the RE elements at the same time.At present,die-casting is the most popular process for forming magnesium alloys,but there are many defects in the castings,such as porosity and shrinkage,which will seriously affect the properties of the castings.Compared to die-casting,squeeze casting can effectively eliminate the formation defects,and improve the mechanical properties.In this study,a new low cost and heat-resistant rare earth magnesium alloy of Mg-6Zn-1.4Y-1Ce/La-0.6Zr?ZWE611?was designed.Squeeze casting?pressure?200MPa?was used to form the castings.The effects of Ce/La,squeeze pressure and heat treatment on microstructure,phase constituents and mechanical properties were studied.Firstly,with the addition of 1wt.%Ce/La elements,the new ZWE611 alloy was obtained from Mg-6Zn-1.4Y-0.6Zr?ZW61?alloy.The addition of Ce/La contributed to the grain refinement and the formation of T'-phase((Mg_1-xZn_x)₁₁?Ce,La?)with a high melting point.T'-phase would decrease the Zn/Y ratio in the residual melt,then change the phase transformation behavior of quasicrystal I-phase?Mg₃Zn₆Y?and W-phase?Mg₃Zn₃Y₂?.The phase constituents of ZWE611 alloy are?-Mg,T'-phase,I-phase and a small amount of W-phase.And their general precipitation sequence is:L??-Mg?T'-phase?W-phase?I-phase.Owing to the grain refinement and the increase of granular phases inside the grain,the strength of ZWE611 alloy at both room and elevated temperatures increased obviously than that of ZW61 alloy.However,the existence of the bulky and brittle T'-phase led to the decrease of the elongation.In addition,T'-phase and W-phase both have superior thermal stability,which could obviously enhance the strength at elevated temperature.The UTS at both room and 200?elevated tempertures of as-cast ZWE611 alloy are 197MPa and154MPa.Compared to ZW61 alloy,the increments are 9.44%and 14.93%,respectively.As a result of the joint action of T'-phase,W-phase and I-phase,the ZWE611 alloy is suitable for development as a heat-resistant magnesium alloy.Secondly,the effects of squeeze pressure on microstructure,phase transformations and mechanical properties were studied.When the high pressure applied,the thermodynamic and kinetic conditions would change,such as heterogeneous nucleation rate and growth rate,which led to the refinement of grain and second phases.At the same time,the increase of the solid solubility of the Zn elements in the?-Mg matrix led to the continuous decrease of the content of the second phases.With the grain refinement,the decrease of internal defects and the increase of density,the mechanical properties of the two alloys continuously increased with the increase of squeeze pressure.When the pressure of 800MPa was applied,the UTS of ZW61 and ZWE611 alloy are 243MPa and 237MPa,respectively.Compared to the gravity cast alloy,the increments are 35.00%and 20.30%,respectively.Finally,the influence of solid solution and aging treatment on microstructure and mechanical properties were investigated.The best heat treatment process of ZW61 alloy is as follows:solid solution at 460?for 8 h,and aging at 200?for 16 h.The I-phase is unstable at the temperature of 460?,and the transformation of I-phase to W-phase would occur.The Zn element into the?-Mg matrix led to the precipitation of a large amount of?₁?and?₂?phases,which could strengthen the alloys.Meanwhile,the small W-phase also could benefit the strength of the alloys.When the pressure of 800MPa applied,the YS and UTS of ZW61alloy after heat treatment are 136MPa and 285MPa,respectively.Compared to the as-cast alloy,the increments are 13.33%and 17.28%,respectively.For the squeeze casting ZWE611alloy with the applied pressure of 800MPa,the T'-phase and W-phase separated from the symbiosis state,and a more homogeneous microstructure was obtained after heat treatment.Similarly,the strengthening phases,such as MgZn phase,precipitated during the aging treatment,which could benefit the mechanical properties.