Research On Characterization And Control Mechanism Of Shrinkage Porosity And Solid-liquid Segregation In Semi-solid Die Casting

With the increasing awareness of people around the world on "energy saving and emission reduction",achieving "replace steel with aluminum" in the transportation industry has become an important development trend in the automotive industry.The semi-solid forming technology has gradually become an important way to achieve "replace steel with aluminum" because that it can improve the casting performance and heat treatment performance of aluminum alloys.However,in the current actual production of aluminum alloy semi-solid casting,there still be a large number of casting defects frequently.Due to the lack of systematic analysis and statistics of these defects,it has led to longer research and development cycles,increased production costs and even shortened the product service period.This research mainly focuses on shrinkage porosity and solid-liquid segregation which are typical semi-solid casting defects in thick-walled products(thickness×10mm).Through a large number of analysis and mechanical tests,the effects of different process parameters on two types of defects were obtained.This has great guiding significance for the future development and production of similar thick-walled products(such as automobile wheels,calipers,etc.).The main research results are as follows:It is concluded that the main influencing factors of shrinkage porosity defects are:intensification pressure,biscuit thickness and die temperature.With the increase of boost pressure,the increase of the biscuit thickness,and the increase of the die temperature,the shrinkage porosity defect area gradually decreases.When the intensification pressure is maintained at 1000 bar,the biscuit thickness is maintained at 68 mm,and the die temperature is maintained at 230?,there are semi-solid castings without shrinkage porosity defects.The solid-liquid segregation includes radial solid-liquid segregation and axial solid-liquid segregation.Radial segregation is mainly about that in the radial direction,from the edge to the core of castings,the liquid fraction decreases significantly and then fluctuates slightly,finally increases slightly at the position of the core.Axial segregation is mainly about that the liquid fraction at the middle position in the axial direction is lower than the upper and lower positions.Through the research methods of control variables,the main influencing factors are:intensification pressure,filling length,plunger speed,die temperature.The intensification pressure has a significant effect on the axial solid-liquid segregation,but has no significant effect on the radial solid-liquid segregation.At lower intensification pressure(100bar,300bar),the difference in the liquid fraction of the castings axis is exactly great,and when the intensification pressure rises above 700bar,the solid-liquid segregation in the axis decrease significantly.The filling length has a significant effect on the radial liquid distribution.At a certain height in the axial direction,the shorter the filling length,the higher the overall liquid fraction.The die temperature has a significant effect on the solid-liquid segregation in the axial direction,while it has little effect on the solid-liquid segregation in the radial direction.When the die temperature is low,the axial liquid distribution difference is large,while the high die temperature will significantly decrease the solid-liquid segregation in the axial direction.The plunger speed has little effect on the overall solid-liquid segregation,but it will affect the liquid distribution in the middle and upper part.The solid fraction of slurry has no significant effect on solid-liquid segregation,and has no significant effect on liquid distribution.The last chapter of the article studies the effect of solid-liquid segregation on mechanical properties and the effect of boost pressure on mechanical properties.Radial solid-liquid segregation will cause a significant difference in mechanical properties in the radial direction.From the edge to the core,the hardness decreases gradually,the tensile strength is gradually reduced,and the minimum is taken at the core.Elongation increases slightly and then decreases significantly.Axial solid-liquid segregation will lead to differences in mechanical properties in the axial direction.the lower part of the overall hardness value is higher than the upper part,while the tensile strength and yield strength of the middle part is slightly lower than the upper and lower parts,and the elongation of the upper and middle parts is slightly higher than the lower part.