Study On Composition And Heat Treatment Process Of High Performance Cast Al-Si-Cu-Mg Alloy

Recently,the development of high specific power diesel engines with high pressure and working temperature and direct injection gasoline engines bring forward higher request for strength and toughness of cast Al alloys.The composition and heat treatment were designed by using material thermodynamics calculation and phase diagram calculation in this study,which improved the efficiency and reduced the cost.Effects of Cu,Mg,Ti and Zr on the microstructure and properties were systematically investigated by optical microscopy and scanning electron microscopy.The evolution law and mechanism of the effects on the microstructure and second phase morphology of the alloy were revealed,and the composition was optimized.The effects of different heat treatment on the mechanical properties were studied by orthog onal experiment,quality index and quality chart.The heat treatment of the alloy was optimized,and the mechanical properties of Al-Si-Cu-Mg alloy were significantly improved,reaching the expected research goal.The main conclusions can be summarized as follows:（1）The content of Cu and Mg not only affects the type and content of phases in the as-cast structure,but also greatly affects the equilibrium solidus temperature and Q phase formation temperature.The content of Al₂Cu and Q depend on Cu and Mg,respectively.Reducing Cu and increasing Mg can increase the Q phase dissolution temperature,while increasing Cu and Mg will decrease the equilibrium solidus temperature.The designed composition of Al-Si-Cu-Mg alloy meet the relationship:9.5≤Cu+20.4Mg≤10.7 and 1.5%≤Cu≤2.5%,0.35%≤Mg≤0.45%,because of the small variation range of Mg content,it is determined to be 0.4%.（2）The as-cast alloy mainly containsα-Al,eutectic Si,θ-Al₂Cu,Q-Al₅Cu₂Mg₈Si₆,α-Al₁₅（Fe,Mn）₃Si₂ and（Al,Si）₃（Ti,Zr）.The content of Cu and Mg has great influence on the morphology,distribution and dissolution of the second phase s.Increasing Mg will not only promote the formation of large Q phase,but also increase the dissolution temperature of Q phase,so that it can not be completely dissolved during solution treatment,which significantly reduced the properties of the alloy.（3）Al-5Ti-1B can not only refine grain,but also refine eutectic Si.With the addition of 0.1%Ti,the grain refinement and the size,morphology and distribution of eutectic Si reached the best state,and also the mechanical properties was best.Excessive Ti can lead to the aggregation and growth ofα-Al and eutectic Si.The main reason is that Al₃Ti and Ti B₂ particles are prone to aggregation and precipitation.Grain refinement has no obvious effect on the strength,mainly improves the elongation.（4）The addition of Zr will form needle-like or plate-like（Al,Si）₃（Ti,Zr）phase in the microstructure.Increasing the content of Zr can increase the size and quantity of（Al,Si）₃（Ti,Zr）phase.The mechanical properties of the alloy with 0.1%Zr content were the best.The addition of excessive Zr will form a large number of insoluble coarse lath-like（Al,Si）₃（Ti,Zr）phases,which severely split the alloy matrix during the tensile test,resulting in a significant decline in mechanical properties.（5）The phase transformation sequence and temperature of each phase in the as-cast structure of Al-Si-Cu-Mg alloy were determined by phase diagram calculation and DSC thermal analysis experiment.Combined with the thermodynamic calculation and analysis results of single-stage and two-stage solid solution on the concentration distribution of Cu element between secondary dendrites and the content of precipitated phases in the alloy,the T6 heat treatment process was designed as follows:495℃×2h+530℃×10h WQ+155℃×12h AC.（6）The secondary solution temperature has the greatest influence on the comprehensive mechanical properties of Al-9Si-2Cu-0.4Mg alloy,and the optimized solution process is 500℃×3h+530℃×10h.Increasing the aging temperature will reduce the peak strength and shorten the peak aging time of the alloy.When over-aging occurs,the strength decreases and the elongation increases.The comprehensive mechanical properties of the alloy reach the best when aging at 155℃×18h,Rm=417MPa,Rp_0.2=324MPa and A=5.2%.