Microstructure And Mechanical Properties Of A356 Alloy Prepared By Rheo-high Pressure Die-casting Process

Semi-solid die-casting aluminium alloy technology has a wide range of applications in the field of automotive lightweight due to its advantages such as smooth filling,heat treatment and strengthening.Automotive lightweight is an important means to achieve the goal of energy saving and emission reduction and "Double carbon(carbon peaking and carbon neutrality)"goals.This study aims to fabricate limit block products(automotive parts)by rheo-high pressure die-casting process with proper heat treatments.The modified Gas Induced Semi-Solid(GISS)and Swirl Enthalpy Equilibration Device(SEED)slurry preparations were studied and the slurries with high liquid fraction and low liquid fraction were obtained,respectively.Combined with die-casting process,the parts were formed and heat treated.The effects of slurry-making and heat treatment processes on the structure and properties were analyzed by means of metallographic microscopy,scanning electron microscopy,transmission electron microscopy and mechanical properties.The main findings are as follows:(1)Semi-solid slurry of Al-7Si-0.5Mg aluminium alloy with fine and spherical grains uniformly distributed in liquid phase were prepared by GISS method.With the increase of the resting time from 85 s to 270 s,solid phase fraction of semi-solid slurry ranged from 0.67-0.77,the average grain size increased from 95 μm to 225 μm,and the average shape factor decreased from 0.80 to 0.33.When the amount of medium alloy added varied from 0.5 wt%-2.0 wt%,slurry microstructure obtained at around 1.5 wt%was superior.During the filling process of GISS die-casting,the solid phase fraction gradually decreases along the filling direction.Compared to traditional liquid die-casting,semi-solid die-casting improved mechanical properties of test bars,and after heat treatment(540℃ solution treatment for 30 minutes followed by natural aging for over 6 hours and artificial aging at 170℃ for 6 hours),tensile strength,yield strength and elongation of test bars reached 307 MPa,240 MPa and 8.8%respectively.(2)A356 high solid fraction slurry was prepared by SEED method and a limit block was obtained by semi-solid high pressure die-casting process.The filling of slurry is more stable in semi-solid die-casting than in liquid die-casting.In the parts,except for the surface,microstructure is more homogeneous and there is no obvious segregation phenomenon.By analyzing results of microhardness tests on parts under different solution and aging treatments,it was found that with the increase of solution time,microhardness increases first and then decreases;aging at 190℃ reaches the peak microhardness first than aging at 170℃.Microhardness at 150℃ increases slowly and does not precipitate enough strengthen phase.Product was heated at 1℃/min at 540℃ for 3 h+170℃ for 5 h,and its yield strength,tensile strength and elongation reaching 266 MPa,343 MPa and 8.9%respectively.The short solid solution(520℃ for 1 h+170℃ for 5 h)treatment achieved 92%and 93%of yield and tensile strength of bar at 540 ℃ for 3 h of solid solution,respectively.Comparing the two processes,it was found that the as-cast strength of GISS die-casting is higher than SEED die-casting,due to the finer grain size of GISS resulting in stronger grain refinement.However,after heat treatment,the microstructure of SEED die-casting is more dense,resulting in its strength surpassing that of GISS die-casting.Heat treatment resulted in improved aluminium alloy properties,relying on the combined effect of solid solution strengthening of Mg and Si elements and aging precipitation of Mg-Si strengthen phase.