Research On Sima Processing And Thixoforming Of Semi-solid 6061 Aluminum Alloy

Semi-solid forming is an emerging technology that combines the advantages of both liquid forming and solid forming.It can be used to produce metallic components with high integrity,improved mechanical properties,complex shape,and tight dimensional control.However,the application research of semi-solid forming of aluminum alloy was very slow.Therefore,in this paper,the commercial 6061 aluminum alloy was taken as the research object,From aspects of the preparation of billets by SIMA process,the compression behavior at room temperature to semi-solid temperature and the final thixoforming,the evolution of semi-solid microstructure and properties of 6061 aluminum alloy and its intrinsic mechanism were studied,which provides theoretical direction and experimental reference for thixotropic forming of aluminum alloy.The 6061 aluminum alloy semi-solid billets were first prepared by SIMA process with upsetting as a pre-deformation method.Increasing the amount of deformation is conducive to the refinement and spheroidization of solid grains,but the amount of critical deformation is 33.3%.During the isothermal heat treatment,the alloying and recrystallization occur first in the alloy microstructure.At the same time,the low-melting AlFeSi phase and Mg₂Si phase are equivalent to melt first in the trigeminal boundary,and then the solute atoms continue to migrate to the liquid phase.Solid grains were gradually surrounded by the liquid phase and spheroidized,and the non-dendritic semi-solid billets of 6061 alloy are finally formed.There are two main aspects of the effect of isothermal heat treatment on semisolid microstructure:heat treatment temperature and holding time.Increasing the heat treatment temperature will accelerate the process of recrystallization,melting and growth.However,when the temperature is too high,the rapid infiltration of the liquid phase will lead to insufficient recrystallization.With the increase of the holding time,the liquid fraction increases,and the average solid grain size decreases first and then increases,also the roundness gradually decreases.With the holding time and heat treatment temperature increase,the yield strength and tensile strength of the billet are gradually reduced,and the elongation increases first and then decreases.The compression test results show that the true stress-ture strain curve of 6061 alloy exhibits plastic deformation characteristics at 25-500??solid state temperature?,and shows typical characteristics of thixotropic deformation at 600-620??semi-solid temperature?,whose curve can be divided into three stages:stress surge,resistance deforming and steady thixotropic flow.The compressive peak stress and steady stress of 6061alloy have temperature sensitivity and strain rate sensitivity.Both the peak stress and the steady state stress show a decreasing trend as the temperature increases or the strain rate decreases.However,with the increase of temperature,the sensitivity of the steady state stress in the semi-solid state to the strain rate is weakened,mainly due to the effective lubricating effect of the liquid phase.The constitutive behavior of 6061 alloy over a wide temperature range was established by using hyperbolic sine-model with liquid fraction correction term.The apparent activation energy of plastic deformation is 248.639 kJ/mol and the apparent activation energy of semi-solid thixotropic deformation is 681.435 kJ/mol.By using two stage induction heating controlled by PID,the 6061 alloy semi-solid billet with high consistency between the center and the surface microstructure was obtained,and the thixo-squeeze casting was carried out.The mold-filling capacity of thixotropic forming is good.The primary spheroidized solid phase was uniformly distributed in the formed 3 mm thin plate.The primary spheroidized solid phase is uniformly distributed in the formed 3 mm thin plate,whose yield strength is 165 MPa,the tensile strength was 280 MPa,and the elongation was 9.5%.Compared to the liquid forming,semi-solid forming of 6061 alloy can reduce the tendency of the casting hot cracking,reduce the blow hole defects and improve plasticity.