Heat Treatment Process And Strengthening Mechanism Research On Semi-Solid Casting Al-Si-Cu-Mg Aluminum Alloy

Compared with conventional liquid casting,the semi-solid casting has a series of advantages,such as smooth filling,good air entrapment and shrinkage resistance.Therefore,heat treatment processes can be applied to strengthen the cast aluminum parts.In this study,the heat treatable Al-Si-Cu-Mg alloys were studied and semi-solid casting technique was used to produce industrial workpieces for the study.The majoe studies include:?I?microstructure and mechanical property evolution during heat treatment;?II?formation mechanism of solid state diffusion-induced pores;???heat treatment strengthening mechanism of Al-Si-Cu-Mg alloys.The characteristics,morphologies and distribution of eutectic phases in the studied hypo-eutectic Al-Si-Cu-Mg alloys were analyzed by FESEM and TEM.The second phases existing in the 319S aluminum alloy are the block Al₂Cu,the fine Al₂Cu,the hexagon Al₅Cu₂Mg₈Si₆ and the snake-like Al₈Mg₃FeSi₂,while the main eutectic phases existing in the 357 aluminum alloy are Mg₂Si and Al₂Cu.Ostwald Ripening effect was detected in the 357 alloy during heat treatment.The ripening experienced three stages,i.e.eutectic silicon fragmentizing,growing up and spheroidizing.These proceses were statistically analyzed.The results show that both temperature and time have positive influences on ripening process.At high temperature of 540? where is closed to the solidus.In addition,at low temperature of 170?,the ripening progress was also promoted.In this study,the mean area of a eutectic silicon particle increased from 4.0?m² to 7.2p?m² by 80%.The mechanical properties of cast parts and energy cost of heat treatment were considered together to modify heat treatment parameters.The results show that the 357 alloy can achieve tensile strength of 220?359MPa,yield strength of 106?307MPa and elongation of,15,5?9.2%.By optimizing heat treatment parameters while meeting requrirement of mechanical propertiesto,energy consumption can be reduce by 64.8%.Mechanical properties of rheo-cast and thixo-cast 319S alloy was also studied.The mechanical properties of the former was lower than the later under the same heat treatment process by 91.2%on ultimate strength,92.9%on yield stress and 88.6%on elongation.The study shows that only optimization of heat treatment parameters cannot make mechanical properties of the cast parts by the rhoe-and thixo-casting processes se,while other strengthening technologies shall be used due to the different microstructure produced by these two different mechods.In-situ comparison technique was occupied to analyze the formation mechanism of solid atom diffusion-induced pores.The results show that non-equilibrium counter-diffusion of alloying elements at the temperatures of lower than phase melting points is the formation mechanism of the pores.The Fick diffusion law,the LSW theory and the Kirkendall effect shall be all used to describe the formation mechanism of the pores.Besides,it's proved that soaking temperature and holding time acceleratedgrowing process of the pores.Two suggestions to avoid formation of the pores during heat treatment were proposed,i.e.to add a pre-solution treatment or/and to minize the size and distribution density of the second phases located at grain boundaries formed during solidification process.Effeccts of natural aging was also studied.The results show that natural ageing treatment was one significant way to improve the mechanical properties of the studied Al-Si-Cu-Mg alloys.In T4 heat treatment,the hardness of the 357 alloy was improved by 20%.In T6 heat treatment,the precipitate number of 319S in per area alloy increased by 40%.Therefore,proper multi-stage ageing treatment is one ideal method to modify the comprehensive properties of the Al-Si-Cu-Mg alloys.