The Effect Of Light Rare Earth Element On Microstructures And Properties Of Die Casting Alloy

Mg alloy has attracted much attention because of its advantages such as good damping capacity,electromagnetic interference resistance and excellent heat dissipation.Mg alloy containing rare earth exhibits high strength and plasticity,good creep resistance and corrosion resistance at elevated temperatures.Up to now,the effect of light rare earth elements on microstructures and properties of die casting alloys has been rarely reported.This thesis mainly investigated the relationship between microstructures and mechanical properties of the as-cast and heat-treated die casting Mg-XLa-0.4Zr(La 0.5~3.0wt.%)alloys,corrosion properties under different conditions were compared,die casting property and heat conduction property were tested and analyzed primarily.Die casting properties of the alloys were tested by special mould.The evolution of mechanical properties with microstructures and precipitates of as-cast and heat-treated die casting alloys were studied through universal tensile testing machine,Metallurgical microscope,SEM,EDS and XRD.The corrosion properties of the die casting alloys were tested by electrochemical workstation,salt spray test and weight loss experiment.The thermal diffusivity and heat capacity were tested by laser thermal conductivity instrument and synchronous thermal analyzer,thus thermal conductivity rates of the die casting alloys were obtained.The flow property of the alloys decreased with the increasing La content.The die casting Mg-La-Zr alloy consisted of ?-Mg and eutectic phase(?-Mg+Mg17La2).The second phase concentrated at grain boundary,With the Increase of La content,the second phase became network,the average grain size decreased from 18.6?m to 8.7?m,with tensile strength of 153.2MPa and hardness of 58.35HBS(W).After solution treated at 560?for 8h,large numbers of eutectic phases among boundary formed into bulks,distributed in the grain and grain boundary,and rest became Mg17La2 phase,concentrated in the matrix in the form of Zr-rich partcles.After aging for 20 h at 200?,most of the eutectic phase became Mg17La2 phase and distributed in the grains.Hardness increased monotonously and the highest hardness was 60.04HBS(W).The quasi-cleavage fracture was the main fracture approach of the die casting alloy.The fracture approach was influenced by the La content,intergranular fracture occurred when the eutectic phase increase to some extent.The fracture approach was cleavage fracture after T4 treatment and quasi-cleavage fracture after T6 treatment.The corrosion resistance of eutectic phase was poor,the corrosion resistance of the alloys was influenced by the grain size,the corrosion resistance was best when La content was 3.2wt.%,compared with the die casting alloys,the heat treated samples were more corrosion resistant,alloys at any condition during polarization test showed no passivation,the corrosion rate was gentle at first and then increase,corrosion products were Mg(OH)2 and Mg2Cl(OH)3.The density has great influence on the corrosion resistance.The thermal conductivity of the die castings increased with the increasing La content,thermal conductivity can achieve 121.2W/(K·m)when La content was 3.0wt.% at 25?.