| A357 alloy has excellent casting characteristics, good specific strenghth and toughness, splendid fatigue and corrosion resistance. It has been used widely in the field of the aviation, military industry, etc. But the mechanical property of Al-Si alloy produced by most of our domestic enterprise has some gap to that of abroad, and can't meet the demand of aerospace currently. In order to increase the mechanical property of the alloy, the paper uses the optic microscope, XRD, SEM and mechanical property testing eqipments to study the effect of refiner and modifier to the alloy and its performance throngh adding the microelement Sc, the refiner of Al-Ti-B master alloy and the modifier of Sr in the melt. It can also optimize the heat treatment to improve the mechanical property of the alloy based on this.The research show that the addition of 0.2% Sc into A357 alloy can refine a(Al) intensively and modify eutectic Si to some extent. But the modification efficiency is much worse than that of adding Sr. When add Sc and Sr at the same time, the effect of refinement and modification to the cast structure of A357 will be remarkable, and can also improve the mechanical property of the alloy prominently.The addition volume of Al-Ti-B refiner to A357 alloy has great influence to its refining effect. Adding lwt% of Al-5Ti-1B or Al-3Ti-3B separately has excellent influence to the refning effect of the A357 alloy, but the effect of same addition volume of Al-3Ti-3B is better than that of Al-5Ti-lB. Two different Ti-B ratio above combining with same volume of Sr respectively can both improve the mechanical properties of A357 alloy prominently. But the Al-5Ti-1B with Sr function better than that of Al-3 Ti-3B with Sr.The compound function of Sc combing with Ti, B and Sr can enhance the ageing hardening effect. The ageing hardness will reach to the peak when adding 1wt% Al-5Ti-1B,0.02%Sr and 0.2%Sc to the A357 alloy. The best solution of the A357 alloy:single-ageing treatment of this alloy is 548℃/12h,and ageing for 6h at 175℃; The best two-stage ageing treatment of this alloy is ageing for 4h at first 120℃, and ageing for 5h at the following 175℃. Compared with solution of single -ageing treatment, the tensile strength and the elongation of A357 alloy in two-stage ageing treatment can be improved by 3.33% and 15% respectively.After the processing of two-stage ageing treatment to the A357 alloy which adding lwt%Al-5Ti-1B,0.2%Sc grain refiner and 0.02%Sr modifier, the tensile strength and the elongation of the alloy will reach to 360MPa and 6% respectively. |
