Study Of Refinement And Modification On The Mistructrue Of Al-si Casting Alloys

By a suitable alloying means, the microstructure of Al-Si alloys could be optimized, and their mechanical properties could be improved significantly by using a traditional casting process. In this paper, complex master alloys including alloying elements Ti,B,RE,Sr, were produced by reaction bettween fluride salt and aluminum at high temperature. By means of optical microscopy, scanning electron microscopy, energy diffraction system, X-ray diffraction, differential scanning calorimetry and tensile test machine, the effect of the alloying elements on the mistructures and mechanical properties of A356 alloy was investigated, and the interactions between the refining phases and modification phases in complex master alloys were explored. Some conclusions can be made:(1)The microstructures of the alloys can be refined and modified.The size of initialα-Al dentrites can be 70-80μm, the secondary dendrite arm spacing can be 10-12μm and the aspect ratio of eutectic silicon phases can be 1.89-2.3.When using various master alloys, the optimat -ical alloying addition is: Al-10RE—3wt%; Al-10Sr—0.15-0.3wt%; Al-5Ti-1B—0.8-1.5wt%.(2) For the combination of refining and modification, the role of Al-5Ti-1B master alloy is to refine the initialα-Al dentrite size ,while that of Al-10Sr to modify the eutectic Si phase. The interaction betwwen Ti/B and Sr/RE occurs during solification.(3) For as cast A356 alloy treaed with complex combination of Al-5Ti-1B-1.5wt% and Al-10Sr-0.3% and Al-10RE-3%, the ultimate tensile strength can be more than 200MPa and elongation to failure can be over 6%.(4)When Ti/B mass ratio in a complex master alloys is less than 2.2, there exsist obvious interactions between AlB2 and Sr and/or RE, which generate SrB6 or REB6.Taking this interact -ion into account, Ti/ B mass ratio in a effctive complex master alloy should be more than 2.2.(5)The interaction between Sr and La forms La/Sr-enriched phases, in that the effective dose of Sr and La in composite master alloy are reduced and the modification of Si is deteriorated.