The Law Of Vacuum Die Casting To Improve The Porosity Defects And Mechanical Properties Of Castings

Aluminum alloy vacuum die-casting method is an important direction of advanced die-casting technology,which is of great significance to the high-quality production of large and complex castings.However,there is a lack of quantitative research on how to reduce and eliminate residual gas-induced defects in castings through vacuum die-casting.The relationship and law of vacuum degree to improve residual gas-induced defects and product mechanical properties is still unclear.The purpose of this paper is to study the improvement of gas-induced defects by vacuum die-casting and the mechanical properties of castings.Taking vacuum die-casting aluminum alloy engine as the research object,using electron microscope,metallurgical microscope,nanoindentation experiment and numerical simulation analysis methods,three kinds of methods are studied.The influence of different cavity vacuum on the hole defects of die castings,and the influence of the number,size and morphology of hole defects on the tensile properties and micro hardness of the castings,and the following conclusions are drawn:(1)The cavity vacuum is increased from 1013mbar to 100mbar,the number of pores on the sample surface is significantly reduced,the large-diameter pores gradually disappear,the average diameter of the pores in a certain area is also reduced,and the main concentration range is gradually reduced;the vacuum degree increases,and the pore defects are reduced from large areas.The irregular pores gradually become smaller and more rounded micropores;the vacuum degree increases,the density of the casting becomes larger,and the porosity becomes lower.The casting sample formed under 1013mbar absolute pressure has a density of 2.52 g/cm~3 and a porosity of 8.5%;when the absolute pressure is 100 mbar,the casting has a porosity of 3.7%and a density of 2.66g/cm~3.(2)Use the in-situ nanoindentation system to test the hardness value of the material matrix nanoindentation around the pores of 20?m-280?m and the distance from the edge of the hole.For holes with different diameters,the larger the hole diameter,the greater the impact on the surrounding hardness.And when the distance from the hole is the same,the larger the hole diameter,the smaller the hardness.(3)Use ABAQUS to establish the 2D/3D model data of nanoindentation near the pore defect,and analyze the existence of the hole defect in the 2D finite element model in the form of a semicircle,a 3/4 circle,and a full circle directly under the indenter.The pore size changes and the distance from the edge of the hole is changed.When there is a difference,the nanoindentation experiment simulates the displacement-load curve.It is found that when the hole defect tends to be directly below the indentation point,the indenter receives a smaller load to reach the maximum indentation depth.At the same time,the closer to the edge of the hole,the indenter The reaction force is also smaller.(4)Analyzed the displacement load curve of the hole directly under the indenter.When the distance between the indentation point and the edge of the hole is the same,the diameter of the hole defect is 10?m,20?m,and 40?m.The farther away from the hole,the greater the reaction force on the indenter,namely The hole defect has a limited range of influence on the material;the larger the hole diameter,the greater the impact on the indentation test,that is,the greater the weakening effect on the material,the smaller the contact stiffness and the lower the hardness of the material near the hole.(5)Analyzed the influence of different pore defects on the hardness of nanoindentation,and found that when the pore defects have different morphologies,the hardness value of nanoindentation is different.The stress concentration first occurs at the irregular sharp corners and the most easily deformed area.The greater the radian,the less effective the support for the material and the lower the hardness value when the shape is approximately square.(6)Vacuum die-casting technology can significantly reduce the number and size of gas-induced defects in castings,and reduce the influence of pores on hardness of the material,which is of great significance to the improvement of the mechanical properties of castings.