Research On Gas-induced Defects And Mechanical Properties Of Aluminum Alloy Vacuum Die Casting

With the rapid development of lightweight automobiles,more and more attention has been paid to light alloy represented by magnesium-aluminum alloy.As the main forming technology of light alloy materials,die casting forming technology has become the main reason that restricts the die casting to become large,thin and complicated due to the existence of medium cavity gas in the die casting process.Gas-induced defects refer to the gas involved in the filling process or the gas precipitated from the molten metal remaining in the casting,resulting in round or oval pores in the die casting.In this paper,the research objective is to study the law of vacuum die casting to improve the air-induced defects and mechanical properties of castings.The research object is engine cylinder and shock absorber.Through the visualization experiment of die casting,vacuum die casting experiment and test analysis experiment,the influence of cavity gas on die casting gas-induced defects and the influence of cavity air pressure(vacuum degree)on die casting gas-induced defects were studied,and through the study of the influence of gas-induced defects on mechanical properties of castings,the rule of vacuum die casting to improve macro and micro mechanical properties of castings was deeply studied.Some progress has been made in the following areas and the following conclusions have been reached:(1)According to the visual experiment of die casting and the numerical simulation results,the influence of die casting cavity gas on gas-induced defects mainly occurs in the areas where gas is wrapped by metal liquid or is seriously excluded,mainly including the backflow area,the cylinder near the tail area and the final filling area.(2)According to the visual experimental results,in the shock absorber tower casting,the porosity of the backflow region,the final filling region and the near-end region of the cylinder are relatively high and the porosity is relatively large.(3)As the absolute pressure of the die casting cavity decreases(1013 mbar to 100mbar),the average porosity of the die casting decreases from 8.5% to 3.7%,and the number and size of gas-induced defects are significantly reduced.(4)According to the dynamic analysis results of Magma simulation and formation of gas-induced defects,vacuum die casting will hinder the generation of separation flow to a certain extent,reduce the entrainment phenomenon and reduce the influence of cavity gas on gas-induced defects,thus reducing the gas-induced defects of die castings.(5)According to the tensile test results,vacuum die casting can improve the macro tensile properties of die castings,and with the decrease of absolute pressure(increase of vacuum degree),the fracture surface morphology of the sample is smoother,the surface pore size decreases,the pore distribution is more uniform,and the tensile strength and elongation are improved.(6)According to the results of nanoindentation experiment,vacuum die casting can improve the microhardness of die castings.The existence of pores will reduce the microhardness of the surrounding tissues.To a certain extent,the larger the pores,the greater the influence on the surrounding tissues,i.e.at the same distance,the larger the diameter of the pores,the lower the hardness of the tissues.