The Semi-solid Non-dendritic Microstructure Evolution Of ZA52-xNd Magnesium Alloys

Magnesium and magnesium alloys have many prominent characteristics and its application range is very wide, but Mg is an active metal, it is oxidized quickly in molten state and easily burn, general forming methods are hard to avoid. When using semi-solid forming, because of small proportion of liquid and low-temperature homework, the dangers of combustion significantly reduced. Therefore, it is profitable to develop the semi-solid magnesium alloys. This paper selected ZA52-xNd (x=0.3, 0.6, 0.9%) alloys by using isothermal heat treatment to prepare semi-solid slurry then observe and analyze the evolution process of Non-dendritic microstructure.ZA52 alloy treated at 600℃from 10min to 60min, the fraction of solid phase of semi-solid remelting microstructure did not change significantly and shape factor curve flattened after 30min. For ZA52 magnesium alloy holding at a certain temperature, when holding time extended to the certain time later the degree of particles suffered a little influence from holding time. At 600℃treated for 30min the average equivalent diameter of globular particle was 75.55μm and shape factor was 1.65.When adding different content of Nd to ZA52 magnesium alloy, as-cast microstructure got a different degree refined, but ZA52-0.6Nd alloy received the most refined and uniform distributed microstructure; tensile strength and elongation sharply declined with the increasing content of Nd, when adding 0.9% Nd, tensile strength reduced from 206 MPa of ZA52 alloy to 157MPa; microhardness rised first and droped later, when Nd content was 0.6% the highest value reached to 86.46 HV; added Nd to ZA52 magnesium alloy the fracture mechanism transformed from cleaves fracture to quasi- cleaves fracture, corrosion resistant performance of alloys was improved.Semi-solid microstructure of each alloys compared under the same condition, the results show that: adding different content of rare earth Nd to ZA52 alloy then conducted by isothermal heat treatment, dendritic microstructure was fused and transformed to globular microstructure, but the different content of Nd made greater difference of microstructure.Compared the microstructure of ZA52-0.6Nd alloy in different experimental parameters, it was found that the best semi-solid isothermal heat treatment temperature of ZA52-0.6 Nd alloy was 600℃and the transformation process of microstructure by the extension of holding time at 600℃was from as-cast dendritices crystal to big lump microstructure to small block microstructure and nearly spherical microstructure and then to spherical microstructure, and got coarsened spherical microstructure finally. In different temperature holded for 30min respectively, with holding temperature improved, the average equivalent diameter and shape factor of primary phase particles took the same trend of increasing first and decreasing later. ZA52-0.6Nd alloy treated at 600℃for 30min, the average equivalent diameter of globular particle was 71.37μm and shape factor was 1.45, the fraction of solid phase was 75.64%. But the microstructure that treated for 60min was the most rounding, which shape factor reduced to 1.28.