Preparation And Study On Microstructure And Properties Of Mg-Al Alloys With Sn And Ca Addition

Mg-Al alloy is the most widely used die-cast magnesium alloy. However, the limited strength and toughness and the poor elevated strength are the major obstacles to its wider structural applications. Mg-7Al alloy was investigated in this work, and Sn, Ca alloying and heat treatment methods were used to improve the strength and toughness of the alloy. The alloying ability and the structural stability of the binary phases in Mg-Al-Ca-Sn alloys were studied by first principle calculation. The influence of Sn and Ca on the microstructure, mechanical properties and corrosion behavior of the Mg-Al alloy were studied by OM, XRD, DSC, SEM, TEM, tensile test, weight loss test and electrochemical test, and the strengthening mechanism and corrosion resistance mechanism were researched.The calculated heats of formation and cohesive energies of the binary phases Mg17Al12, Al2 Ca, Mg2 Sn and Mg2 Ca in Mg-Al-Ca-Sn alloy showed that Al2 Ca and Mg2 Sn had the stronger alloying ability and structural stability. The mechanism of structural stability was investigated by calculating the densities of states(DOS), Mulliken electron occupation number, metallicity and charge density difference of these compounds. The elastic constants of these phases were calculated, which indicated the Al2 Ca had the strongest resistance deformation ability and Mg2 Sn performed the best ductility. The calculations of thermodynamic properties showed that the Gibbs free energy of Al2 Ca and Mg2 Sn was lower than that of Mg17Al12, which indicated Al2 Ca and Mg2 Sn were more stable than Mg17Al12 phases. Hence, the heat resistance of Mg-Al-based alloys could be improved by adding Ca and Sn in theory.The grains were refined, the growth of Mg17Al12 phase was suppressed and a few of dispersed Mg2 Sn particles with granular morphology were formed after 2wt.% Sn added into the Mg-7Al alloys. AT72 alloy exhibited better tensile properties at both room and elevated temperature than Mg-7Al alloys due to the refinement of grains, increase of second phases, optimal morphology and precipitation of Mg2 Sn with better thermodynamic properties. The grain size of the alloy had a little variation, and the alloy was strengthened due to Al and Sn dissolved into the matrix after T4(400℃?20 h) heat treatment. The precipitation strengthening effect of particle Mg2 Sn phases was more apparent due to the dissolution of Mg17Al12. The ultimate tensile strength, yield strength and elongation of AT72-T4 alloy reached to 276 MPa, 202.6 MPa and 10.6%, which were increased by 18.2%, 7.0% and 24.7% compared with those of die-cast alloys, respectively. The strength and ductility of AT72-T6(400℃?20h+200℃?15h) alloy were reduced because of the discontinuous precipitation of brittle Mg17Al12 phases and the coarsening of Mg2 Sn phases.The significant influences of Sn addition on the microstructure, mechanical properties and corrosion behavior of die-cast Mg-7Al-1Ca were obtained. The addition of Sn effectively refined grains, changed the morphology of microstructure from dendritic to equiaxed and increased the amount of intermetallic phases. The addition of Sn also suppressed the formation of(Mg,Al)2Ca(C36) phase, leading to the formation of the new ternary CaMgSn phase and binary Mg2 Sn phase. The mechanical properties and corrosion properties were improved with Sn addition. The results showed that Mg-7Al-1Ca-0.5Sn alloy had the optimal tensile properties and impact toughness at room temperature, with the value of σb=258.36 MPa, σ0.2=188.08 MPa, δ=10.21% and Ak=25J. Mg-7Al-1Ca-1.0Sn alloy exhibited the optimal tensile properties at elevated temperature and hardness, with the values of σb=175.47 MPa, σ0.2=132.65 MPa, δ=15.39% and 61.9HB. The results of the potentiodynamic polarization measurement and weight loss test showed that the corrosion resistance of the alloys was improved due to the increased corrosion potential and the reduced corrosion current density with the minor addition of Sn. However, excessive Sn addition leads to the decrease of the corrosion resistance due to the discontinuous intermetallic phases. However, the effect of heat treatment on the mechanical properties of Mg-7Al-1Ca-x Sn alloy was not significant due to the growth of the grains.