Effects Of Heat Treatment On Microstructure And Properties Of Rheocasting Al-Si-Cu-Mg Alloys

Al-Si-Cu-Mg alloy is a typical alloy series in cast aluminum alloy that can be strengthened by heat treatment process.Semi-solid processing(SSP)can manufacture complex parts and has advantages in short process,low cost and excellent product performance,which has been broadly applicated in automobile,engineering plant and communication equipment.At present,the study of the heat treatment process of Al-Si-Cu-Mg alloys is mainly focused on traditional castings or thixocastings while the rheocastings is lack of research.The microstructure of the alloy in as-cast condition is quite different in different casting processes,which usually leads difference of the evolution of the microstructure and properties during the same heat treatment conditions.Therefore,the present research results cannot provide effective guidance for the formulation of the heat treatment process of the rheocasting Al-Si-Cu-Mg alloy.In order to solve this problem,this work studied the effect of solution treatment parameters and aging treatment parameters on the microstructure and mechanical properties of rheocasting Al-Si-Cu-Mg alloy.The main results are as follows:1)The evolution of the microstructure and properties of the semi-solid rheocasting Al-Si-Cu-Mg alloy during heat treatment at 455?,475?,495? for0.5-8 hours was studied.Using metallographic method and Differential Scanning Calorimetry(DSC)to characterize the content of Al₂Cu phase in as-cast and solution heat treated rheocasting Al-Si-Cu-Mg alloy samples.The results showed that the Al₂Cu phase content decreased significantly within 2 hours during the solution heat treatment and stabilized after 2 hours.The average size,aspect ratio and roundness of Si particles are measured by metallographic method.The results showed that in the initial stage of solution heat treatment,the initial network and fibrous shape of Si particles gradually broke and became spheroidized as the solution time was extended.If the solution time is too long,the Si particles will mature,the Si particles will continue to grow,and the roundness will increase again.In addition,too high solution heat treatment temperature will aggravate the maturation and growth of Si particles.Combining with the changing of the hardness after solution treatment,the influence of the morphology of Si particles and Al ₂Cu phase on the hardness of the alloy was analyzed.It can be concluded that the optimum solution treatment parameters of test rheocasting Al-Si-Cu-Mg alloy is 475?/6h.2)Likewise,aiming at the semi-solid rheocasting Al-Si-Cu-Mg alloy after475?/6h solution treatment,the evolution of its microstructure and properties during aging treatment at 170?,190?,and 210? for 2-16 hours was studied.The metallographic method and Differential Scanning Calorimetry(DSC)results were used to characterize the content and morphology of the Al₂Cu phase in the sample.The results showed that in the early stage of aging,the content of Al ₂Cu phase increased significantly with the extension of time,and in the middle and late stages of aging,the content stabilized.The Al₂Cu phase was aggregated and connected and the size increased at the late stages of aging.Moreover,too high aging temperature will aggravate the occurrence of Al₂Cu phase growth.Characterized the tensile strength,yield strength,elongation and hardness of the alloy.The results showed that as the aging time increased,the alloy hardness first increased and then decreased,and as the aging temperature increased,the time for the formation of the hardness peak decreased.Under the conditions of 190? and 210?,the tensile strength and yield strength of the alloy also increased first and then decreased,but the time when the peak appears was different from the hardness peak.After aging at190?/10h,the alloy sample reached the maximum yield strength of 305.7MPa,the sample's tensile strength was 364.3MPa,and the elongation rate was 3.8%.3)By analyzing the fracture morphology of the tensile specimen by SEM,the main factors affecting the fracture toughness of the alloy were explained.And the reasons for the impact of different aging treatments on the mechanical properties of the alloy were explained according to the microstructure.