Effect Of Thermo-mechanical Treatment On Microstructure And Mechanical Properties Of Al-7Si-3Cu-0.5Mg Alloy

Hypoeutectic Al-Si-Cu-Mg alloys have been widely used in aerospace,automotive manufacturing,high-speed rail,construction,cable and other fields,due to their many advantages such as low density,high specific strength and good corrosion resistance and thermal cracking resistance.With the continuous development of lightweight requirements for high-end equipment in these fields,some parts have increasingly stringent requirements on the strength and ductility of Al-Si-Cu-Mg alloys.However,the refining limit of solidification microstructure under normal conditions made it impossible to further improve the strength and ductility of Al-Si-Cu-Mg alloy.Therefore,it was one of the core issues in the research field of lightweight structural materials to realize the significant synergic improvement of strength and ductility of Al-Si-Cu-Mg alloys and make them meet the service performance requirements of high-end equipment.Thermo-mechanical treatment（TMT）has potential in realizing the synergistic strengthening and ductility of aluminum alloys.However,the application of the TMT in Al-Si-Cu-Mg alloys was severely restricted by the following problems:（1）there is a contradiction between the dissolution of the second phase and the initial melting of the second phase,and the contradiction between the spheroidization of eutectic Si phase and the coarsening of eutectic Si phase during the heat treatment stage;（2）Al-Si-Cu-Mg alloy is prone to recovery and recrystallization at the stage of hot deformation,and the aging precipitated phases and eutectic Si phase of the alloy are easy to coarsening during high temperature service,resulting in poor thermal stability of the alloy structure.In this paper,based on the optimization of heat treatment process and the guarantee of thermal stability of microstructure,which are two important aspects of TMT,the application of TMT（heat treatment+hot rolling）on Al-7Si-3Cu-0.5Mg alloys was realized through optimization on two-stage solution process and the study on the effect of Zr on microstructure and mechanical properties of Al-7Si-3Cu-0.5Mg alloys.The microstructural evolution and mechanical properties of Al-7Si-3Cu-0.5Mg alloys during TMT were systematically studied,and the mechanism on synergistic improvement of strengthening and ductility was clarified.The results of the study are as follows:（1）The optimized two-stage solid solution process（495℃/8h+515℃/4h）was carried out according to the evolution of eutectic Si phase,alloy phases,porosity and hardness in the designed single-stage solution process.After two-stage solution,the second phases was adequate dissolved and the eutectic silicon phase was well modified,the initial melting of the second phase was reduced.Based on uniform elongation,two quality indexes（QI_U and QI_T）on the mechanical properties of cast aluminum alloys were optimized.The two optimized indexes showed that the mechanical properties of two-stage solid treated alloys（QI_U=537.1 MPa,QI_T=0.47）were better than those of495℃/12 h single-stage solid treated alloys（QI_U=504.5 MPa,QI_T=0.38）and515℃/16 h single-stage solid treated alloys（QI_U=527.4 MPa,QI_T=0.46）.（2）The effect of Zr on microstructure and mechanical properties of Al-7Si-3Cu-0.5Mg alloys was systematically studied.With the increase of Zr,the strength of Al-7Si-3Cu-0.5Mg alloys gradually increased,while the ductility gradually decreases.The enhancement on the strength of the alloy was mainly attributed to the refinement of grains and second phases,and the shortening of secondary dendrite spacing,as well as Q’-Al₅Cu₂Mg₈Si₆,θ’-Al₂Cu and D0₂₃-（Al,Si）₃Zr nano-scaled precipitates after heat treatment.The decrease of ductility was mainly due to the increase of the residual brittle Al-Si-Zr phase after heat treatment.Two optimized quality indexes showed that the optimum combination of strength and ductility was obtained in Al-7Si-3Cu-0.5Mg-0.3Zr alloy.The two optimized indexes showed that the best combination of strength and toughness was obtained when 0.3 wt%Zr was added,and the value of QIU and QIT are 549.5 MPa and 0.50,respectively.Compared with alloys without Zr,the value of QI_U and QI_T increased by 2.9%and 8.7%,respectively.The tensile strength and yield strength of the alloys were 414MPa and 311MPa,respectively,and the elongation to failure（Total elongation）and uniform elongation were 8.0%and 8.3%,respectively.（3）Based on the previous study on optimization of heat treatment and enhancement of alloy strength and ductility due to Zr addition,an optimized TMT was obtained.The grains of Al-7Si-3Cu-0.5Mg-x Zr alloy via the optimized TMT was further refined,the eutectic Si phase was further refined and spheroidized,the casting pores were eliminated,and the brittle Al-Si-Zr phase was broken.The optimum QI_Uvalue（573.9MPa）and QIT value（0.76）are obtained in Al-7Si-3Cu-0.5Mg-0.3Zr alloy after TMT.Compared with traditional T6 treated（515℃/16 h+175℃/8 h）Al-7Si-3Cu-0.5Mg alloy,the tensile strength and yield strength of the TMTed Al-7Si-3Cu-0.5Mg-0.3Zr alloy were increased by 8.8%and 28.5%,respectively.The total elongation and uniform elongation are increased by 27.3% and 21.2%,respectively.