Microstructure And Properties Of A201 Aluminum Alloy With High Solid Faction Semisolid Die-casting Process

High solid fraction semisolid die-casting has smooth slurry flow during mold filling process,which can effectively reduce die-casting defects,and high quality die-castings can be obtained.In this paper,the microstructural evolution during reheating of semisolid A201 aluminum alloy is investigated.The influence of process parameters on the microstructure of high solid-fraction rheological slurry of A201 aluminum alloy was studied using the SEED method.The temperature field change and slurry cooling behavior during the preparation of the slurry were investigated.The nucleation and growth mechanisms of the primary α-Al particles under the SEED conditions were discussed.A hot cracking evaluation mold for semisolid die-casting was designed,and the hot tearing susceptibility(HTS)of A201 alloy under different die-casting processes was studied.The A201 alloy impeller was successfully produced by high solid faction semisolid die-casting process.The main research contents and results of this thesis are as follows:DSC analysis shows that in the process of preparing high solid fraction semisolid slurry,it is possible to obtain a larger working window by reducing the heating or cooling rate,and to increase the controllability of the slurry preparation process.During the reheating process,the shape factor of the solid particles of A201 slurry does not change significantly with the increase of holding time,but the particles continue to coarsen.The grain coarsening rate K during isothermal holding is 940μm3/s according to the LSW model.High solid fraction rheological slurry of A201 aluminum alloy was prepared using the SEED method.The results show that.By appropriately lowering the pouring temperature and reducing the rotation time,the SEED process is able to produce slurry within which the primary α-Al particles are fine,globular and uniformly distributed.The desirable high solid fraction slurry of A201 alloy with average primary α-Al size of 68μm can be obtained at a pouring temperature of 660℃and a rotation time of 65s.The slurry cooling behavior during the SEED process was systematically investigated.The slurry preparation process can be divided into two stages:the chilling stage and the slow cooling stage.Due to the chilling effect of the crucible wall,three temperature intervals are formed inside the melt.As time increases,the temperature difference between the three gradually decreases.By reducing the heat transfer coefficient of the outer wall of the crucible,the temperature field distribution inside the slurry can be significantly changed,the temperature difference from the edge of the slurry to the center is reduced,and the uniformity of the slurry is obviously improved.Besides,the primary α-Al grains are more globular and uniform,and the dendritic layer at the edges is substantially eliminated.The study provides an experimental basis for the improvement of the design of the SEED crucible,which would improve the quality of semisolid slurry.The nucleation and growth mechanisms of the near-spherical or spherical grains under the SEED process were analyzed.The nucleus of the SEED slurry mainly comes from two parts:First,under low pouring temperature,a large number of nuclei are formed due to the chilling effect of the crucible on the melt;the second is that the alloy melt rapidly cools to the under-cooled state,and the melt undergoes an overall outbreak of nucleation.In the chilling stage,the interfacial tension plays a major role in the interface stability of spherical primary α-Al.At the same time,there is a large temperature gradient and strong convective mixing in the melt,which inhibits the constitutional supercooling,so the spherical primary α-Al can maintain the interface stability;In the slow cooling stage of weak convection,under the conditions of high-density grains and low-intensity cooling,the overlapping of the concentration field and temperature field between the grains occurs,which reduces the concentration gradient and temperature gradient at the interface front reduced,and increases the uniform distribution of solute,thereby increasing the sphere interface stability and enabling the primary a-Al to grow stably.Used the designed hot tearing evaluation mold,the hot tearing susceptibility of A201 aluminum alloy under different semisolid die-casting processes was tested.The results show that the hot cracking tendency decreases with increasing casting pressure.When the casting pressure is increased from 30 MPa to 90 MPa,the HTS index of A201 aluminum alloy is reduced from 38 to 0.3.In this study,the semisolid microstructure contains mainly near-spherical or spherical grains,and the liquid can feed effectively even when the pressure is low.The main reason for the decrease in the hot cracking tendency is that the increasing casting pressure accommodates the stress buildup during solidification.When the mold temperature is raised from 50℃to 200℃,the HTS index is reduced from 35 to 3.Taking the turbocharger impeller as an example,the engineering application prospects of high solid fraction semisolid die-casting of high-end complex parts of A201 aluminum alloy were studied.By increasing the mold temperature,the filling ability of the A201 semisolid slurry to the impeller blade is improved,and increasing casting pressure effectively improves the internal compactness of the A201 semisolid impeller.The A201 semisolid impeller has higher strength and heat resistance than other impeller products.After T71 heat treatment,the yield strength,tensile strength and elongation of A201 impeller are 402 MPa,475 MPa and 7.3%,respectively.