Microstructure Evolution Mechanism And Defect Control Of ADC12 Aluminum Alloy Near-liquidus Squeeze Casting

Squeeze casting is an advanced technology in which high pressure is applied to molten metal.It has two characteristics such as slow filling and solidification under high pressure.It leads to some advantages including high mechanical properties,dense microstructure,good surface smoothness,accurate dimensional precision,available heat treatment and good weldability of squeeze casting.Squeeze casting is divided into direct and indirect squeeze casting.Indirect squeeze casting is easy to industrialize because of promotion of casting equipment.Indirect squeeze casting is widely used in automotive and aero-space industries.This paper proposes a concept of near-liquidus squeeze casting or NLSC for short,which is used for automobile bearing parts.In this paper,ADC12 aluminum alloy was used to prepare for NLSC experiment.Analysis and test means of microstructure were carried out by hydrostatic device,optical microscope,scanning eletron microscope and transmission electron microscope.Phase composition and fracture mode of support formed by NLSC were analyzed.Microstructure evolution and the effect of parameters on fracture appearance and mechanical property were researched.Under the parameters of pouring temperature 600?,pressure 150 MPa,injection velocity 0.15m/s and holding time15 s,the fine and uniformity globular was obtained without preparation of semisolid billets.The globular coefficient of globular structure was more than 0.7,the grain size was 20?m,the solid fraction was 79%,the eutectic structure was small particles,the tensile strength was295 MPa.Mode of nucleation and growth of NLSC globular structure were elaborated.The globular structure mechanism of growth and spheroidizing were analyzedfrom the perspective of thermodynamics.Equation of critical radius of globular structure was derived.Constitutive equations of thermal drop and cooling rate in sleeve and cavity were obtained.Critical radius was inversely proportional to super-cooling degree.Pressure affected critical radius by super-cooling.The parameters affected the globular structure by solidification time.High pouring temperature and low pressure prompted the growth of globular structure due to extend the solidification time.Macroscopic and microcosmic observation was processed for defect of NLSC.The formation mechanism of defect and segregation were researched.The model of segregation across the section was built.Decrease of solidification time difference can reduce or eliminate the shrinkage porosity.Low pouring temperature and high pressure can decrease the solidification time difference.The segregation across the section was divided into four regions.The center segregation of four regions had different microstructure and element distribution.Increase the cooling rate can reduce or eliminate the center segregation.The designing principles of pouring system and overflow system were given.Design of pouring system,overflow system and cooling system were used for control the defect.Progressive solidification can reduce the shrinkage porosity.The equation of diameter and velocity of cooling system with modulus was deduced.The best optimal process and its effect degree for defect were obtained from the orthogonal test of five process parameters.