Study On Microstructure And Mechanical Properties Of AZ91D Magnesium Alloy By Squeeze Casting Near The Liquidus Line

AZ91D is the most widely used cast magnesium alloy with good fluidity and wide semi-solid temperature range,which is theoretically suitable for near liquidus squeeze casting.In this article,mechanical agitation tools were used to prepare AZ91D magnesium alloy slurry,and then poured the slurry into the squeeze casting machine to produce castings when the slurry temperature decreased near the liquidus.The effect of pouring temperature on microstructure and mechanical properties of AZ91D magnesium castings were characterized by X-ray diffraction instrument,electronic metallographic microscope,scanning electron microscope,electronic universal tensile tester and Vickers hardness tester.At the same time,suitable pouring temperature range for near liquidus squeeze casting AZ91D magnesium alloy process and the defects of castings were investigated,thus the optimum pouring temperature was determined.The results show that:When the pouring temperature ranged from 630?to 610?,the microstructure was composed of a large number of short rod?₂-Mg and a small number of rosette?₁-Mg.When the pouring temperature ranged from 610?to 595?,the?₂-Mg phase gradually disappeared,the ratio of?₁-Mg phase increased significantly and its'morphology was spherical,which appeared typical semi-solid forming characteristics.When the pouring temperature decreased from 595?to 585?,the size of?₁-Mg increased and the morphology changed back to rosettes.With the decrease of pouring temperature,the precipitation position of Mg₁₇Al₁₂ phase gradually transferred from?₂-Mg grain boundary to?₁-Mg grain boundary or within the?₁-Mg grain,which forming the local enrichment phenomenon of alunimum element.The suitable casting temperature range for near liquidus squeeze casting of AZ91D magnesium alloy is 610??595?.When the pouring temperature dereased from 630?to 605?,the average grain size of the casting gradually decreased.to the conctrary,the grain shape factor,casting tensile strength and vickers hardness gradually increased.When the casting temperature decreased from 605?to 585?,the average grain size of the casting gradually increased.the grain shape factor,casting tensile strength and vickers hardness gradually decreased.The optimum pouring temperature for near liquidus squeeze casting of AZ91D magnesium alloy is 605?.The defects types of near liquidus squeeze AZ91D magnesium alloy castings are mainly shrinkage and segregation,and their distribution is related to pouring temperature and mold filling path of alloy slurry.When the pouring temperature is610?,the main form of casting defects is normal segregation,which is distributed in the near sprue.When the casting temperature is 605?,the casting defects are mainly intergranular segregation and distributed in the far sprue.When the pouring temperature is 595?,a small number of shrinkage defects appeare in the near spure.When the pouring temperature is 585?,the shrinkage defeats near the spure increase obviously,and they are mainly distributed in eutectic structure.Segregation defect in castings can be eliminated by proper heat treatment process.