Microstructure Evolution Of AZ80-0.2Y Magnesium Alloy Processed By Semi-solid Isothermal And Induction Heat-treatment

Semi-solid forming is an efficient near-net forming method,which combines the advantages of liquid-phase forming and solid-phase forming.The isothermal semi-solid process has a simple principle and easy to operate,but the billet has a coarse structure.Induction heating can rapidly obtain semi-solid billets which have fine and uniform microstructure.For large size billets,due to the skin effect,the temperature field is not uniform with high frequency heating.Therefore,research on the uniformity of the structure is conducive to industrial optimization and application.In this paper,AZ80-0.2Y alloy was taken as the research object and the isothermal semi-solid and induction heated billet microstructures are studied,and the homogeneity of the large-size billet under high power was analyzed.The main results were as follows:The dendrite arm of the as-cast AZ80-0.2Y magnesium alloy was melted by isothermal semi-solid treatment.With the prolongation of the isothermal time,the liquid phase increased,and the grain growth was also more pronounced.After isothermal semi-solid treatment,the structure of as-extruded AZ80-0.2Y was composed of solid spheres of uniform size.With the prolongation of the isothermal time,the grain grew,the liquid fraction increased,the grain became more spherical,and at higher temperatures,the evolution became faster.The intragranular liquid pool gradually converged to form a large spherical liquid pool as the time increases.Induction heating can quickly warm the material to a semi-solid state.After induction heating,the obtained semi-solid microstructure was uniform and fine,the grain growth was less,and a higher liquid fraction can be obtained.With the increase of the induction heating temperature,the grain grew,the liquid fraction increased.And the grain shape had a little change.The grain size obtained by high power induction was the smallest,followed by segmented heating,and the grain size obtained by low power heating was the largest.High-power heating and segmented heating achieved similar liquid fraction and both higher than low-power heating.The roundness of grain was the similar by three different heating methods.The microstructure involution has three stages: crystal grain coarsening,liquid phase appearance and liquid phase morphology change.Grain coarsening is mainly driven by interface energy;the liquid phase appears to be a rapid melting of the solute enrichment zone.The liquid phase mainly forms in the crystal;the liquid form as strip pool rather spherical pool because intragranular elements have insufficient diffusion distance during rapid heating.And with the increase of temperature,some strip pools are connected with grain boundaries.It is concluded that the temperature of the material center was low and the edge is high by using the finite element method to simulate the high-frequency induction heating temperature field of large-size billet.The greater the power is,the faster the heating rate and the more inhomogeneous temperature field is.As analysis the microstructure,the temperature of the sample edge is higher,where the shape of the grain is also more different than other positions.