Composition Design And Technology Study Of High Strength Semi-solid Aluminum Alloy For Automobile Parts

The objective of this paper is to obtain a high strength semi-solid aluminum alloy for automobile parts and semi-solid forming process.The effects of Si,Cu and Mg on the suitability,strength and microstructure of semi-solid state were studied by JMatPro material properties simulation software.The microstructure and properties of the alloy before and after heat treatment were studied by permanent casting experiment.The effect of injection parameters on semi-solid forming quality was studied by using Anycasting casting simulation software.The microstructure and properties of the alloy before and after heat treatment were studied by semi-solid die casting test using the method of preparing low solid fraction slurry.The effect of heat treatment on the microstructure and properties of the alloy was studied by the semi-solid die casting test using the method of preparing high solid fraction slurry.The following conclusions were reached:(1)The properties of Al-Si-Cu-Mg alloy were calculated by JMatPro.It is found that Si elements have great influence on the applicability of semi-solid rheological forming,while Cu and Mg elements have little influence.The Si element has the maximum process window at 6%,and the temperature sensitivity and solidification temperature range meet the evaluation criteria of semi-solid state.Cu and Mg elements have great influence on the alloy strength.When Cu element content is 4.5%and Mg element content is 0.8%,the alloy has higher strength and larger process window.The optimal alloy composition is Al-6Si-4.5Cu-0.8Mg,and the existing phases include αAl phase,eutectic Si phase,θ-Al2Cu phase,π-Al8FeMg3Si6 phase and Q-Al5Cu2Mg8Si6 phase.After the T6 heat treatment,the yield strength can reach 363MPa,the ultimate tensile strength can reach 364MPa,and the elongation can reach 0.7%.(2)Through the simulation calculation of Anycasting,the optimal injection parameters for the semi-solid die casting mold with low solid fraction are 0.3m/s at low speed and 3m/s at high speed.Al-6Si-4.5Cu-0.8Mg alloy was formed by semi-solid diecasting with low solid fraction.The yield strength,ultimate tensile strength and elongation of Al-6Si-4.5Cu-0.8mg alloy reached 353 MPa,391 MPa and 1.53%after T6 heat treatment.The eutectic Si phase of the alloy is coral-like/fibrous,the θ-Al2Cu phase is eutectic Al2Cu and massive Al2Cu coexist,and the Q-Al5Cu2Mg8Si6 phase is long strip,attached to the θ-Al2Cu phase.After solution treatment at 500℃ for 3h,θAl2Cu phase almost completely disappeared and eutectic Si phase spheroidized.(3)Al-6Si-4.5Cu-0.8Mg alloy was formed by semi-solid die-casting with high solid fraction.The optimal heat treatment parameters were solution treatment at 500℃for 3h,aging treatment at 180℃ for 10h,and Brinell hardness of 139.The yield strength of the alloy is 392MPa,the ultimate tensile strength is 418.5MPa,and the elongation is 1.63%.Compared with the as-cast,the yield strength of the alloy is increased by 206%and the ultimate tensile strength of the alloy is increased by 86%.The main strengthening phases are θ-Al2Cu phase and Q-Al5Cu2Mg8Si6 phase.After 10h of aging,θ’ phase and Q’ phase were uniformly dispersed in the matrix and played a major role in strengthening.θ’ phase is elongated and acicular,and Q’ phase is punctate.