Effect Of Heat Treatment On Microstructure And Properties Of Mg-xZn-0.3Sm Magnesium Alloys

Semi-solid die casting was one of the commonly used semi-solid forming techniques for magnesium alloys.The forming temperature of magnesium alloy semi-solid die casting was lower,which will significantly weaken the oxidation and combustion tendency of magnesium alloy.In addition,there were fewer internal defects in semi-solid die castings.These characteristics enable the product to further improve its performance through heat treatment process.At present,the strengthening effect of commonly used magnesium alloys after semi-solid die casting was not obvious Thus,it was necessary to develop a new type of magnesium alloy suitable for semi-solid die-casting forming.Mg-Zn-RE alloy not only had a large solidification range,but also had obvious solid solution strengthening and age strengthening effects.Our research team ha d studied it for many years.Based on the previous studies,the author explored the structural changes of Mg-xZn-0.3Sm alloy in different states,and optimized the heat treatment process.In this paper,the effects of Zn element on the microstructure morphology,phase composition,and alloy hardness changes of Mg-xZn-0.3Sm magnesium alloy were studied by Optical Microscope,X-ray Powder diffractometer,Scanning Electron Microscopy,X-ray Energy Dispersive Spectroscopy,hardness tester,etc.The optimal heat treatment process of Mg-9Zn-0.3Sm alloys as-cast and semi-solid treatment was further explored.The conclusions were as follows:（1）The effect of Zn content on the microstructure and hardness of Mg-0.3Sm alloys.Different amounts of Zn were added to Mg-0.3Sm alloy.With increasing Zn content,the grains gradually became fine,and the hardness of the alloy increases first and then decreases.The Mg-0.3Sm alloy with the Zn content of 9 wt.%had the highest hardness（59.73 HV）.In this case,the phase of the alloy mainly containedα-Mg matrix and three eutectic phases(Mg Zn₂,Mg₇Zn₃ and Mg₂Zn₁₁).（2）The influence of Zn content,holding temperature,and holding time on the semi-solid microstructure of Mg-xZn-0.3Sm（x=5 wt.%,7 wt.%,9 wt.%,11wt.%）alloys.After isothermal heat treatment of Mg-xZn-0.3Sm（x=5 wt.%,7wt.%,9 wt.%,11 wt.%）alloy,it is found that the alloy with the Zn content of9wt.%has the smallest,roundest,and most uniformly distributed solid particles.The Mg-9Zn-0.3Sm alloy has the optimal isothermal heat treatment temperature and time of 590℃and 30 minutes,respectively,the average primary phase particle size of 44.68μm,shape factor of 1.47,and the solid phase ratio of55.98%.During the isothermal remelting process,the alloy microstructurally undergoes many stages,which is the initial coarsening stage,separation of solid particles,spheroidization stage,and later mergence and growth stage.（3）The effect of solution treatment and aging treatment on microstructure and hardness of Mg-9Zn-0.3Sm magnesium alloy.The Mg-9Zn-0.3Sm magnesium alloy reached the peak aging state after undergone solution treatment at 320℃×20 hours and aging treatment at 160℃×16 hours.Under this process condition,the alloy had the biggest hardness（89.38 HV）.Mg-9Zn-0.3Sm magnesium alloy reached the peak age strengthening state and mainly consisted ofα-Mg phase,Mg Zn₂ phase,Mg₇Zn₃ phase and Mg₂Zn₁₁ phase.The non-dendrite Mg-9Zn-0.3Sm magnesium alloy undergone solid solution at 330℃for 20 hours and age strengthening at 160℃for 16 hours.The eutectic structure was uniformly dispersed in the alloy.The alloy had the hardness of 81.16 HV.