Solidification Microstructure Control Of Ternary Aluminium Alloys

Metallic materials in engineering applications are mostly multi-component and multi-phase alloys. It is of vital importance to control the microstructure of the multi-component and multi-phase alloys in the solidification process. In this work, we experimentally investigated the effects of the grain refinement, modification, solidification conditions, heat treatment conditions and melt superheat treatment on the microstructures and properties of Al-Si-Mg alloys. Heterogeneous nucleation in solidification processes of the multi-component alloys was theoretically analyzed. A concept of intergration of microstructural control and precise forming was proposed. The complex thin wall castings with qualified microstructure and properties were successfully produced by using the self-made counter-gravity casting equipment.Effects of Al-5Ti refiner and Al-10Sr modifier on the microstructures and the mechanical properties of as-cast Al-7Si-0.55Mg alloy were experimentally studied. The results showed that the solidification microstructure was signifipantly refined both in micro- and macroscale when the refiner was optimized to 0.08%~0.14% of Ti addition. The optimal Sr addition was 0.005%~0.01% when modified through the addition of Al-lOSr modifier. The eutectic Si phase was modified completely and the a-AI dendrite was also refined. And then, the mechanical properties of the alloys were increased. However, the mechanical properties were reduced when the optimal contents of Ti and Sr additions were combined in the melt. The reason was that Sr modification and heterogeneous nucleation of TiAl3 were weakened because of the formation of the new compound composed of part Sr and TiAla. The negative influence can be overcome and the best treatment result can be obtained by increasing Sr addition.The experiential nucleation model in solidification processes of the industrial alloys was established by introducing the experimental parameters. This model not only described nucleation rate of the alloys quantitatively, but also linked the nucleation rates with the final graindensity. The surface area of effective catalysts per unit volume melt (SV) and the contact angleof effective catalysts in the melt (f()) in solidification processes were calculated by using themodel. The matrix of heterogeneous nucleation is proposed to be the impurities in the melt and the effective catalysts are in the melt. The model can be used to estimate the nucleation parameters of Al-7Si alloy and Al-7Si-0.55Mg alloy in the definite melting condition.Effects of the cooling rate on the microstructure and the mechanical properties ofAl-7Si-0.55Mg alloy during solidification were experimentally investigated. The grain size and second dendrite arm spacing of the alloys were decreased and eutectic silicon was refined and dispersed as the cooling rate increased. DSC test results showed that the liquidus temperature and the binary eutectic temperature were decreased and the ternary/quarternary eutectic reactions and MgaSi phase were depressed as the cooling rate increased. The microhardness of the alloy at the higher cooling rate was distinctly raised.Effects of the heat treatment parameters on the phases and the mechanical properties of A357 alloy were also investigated. The tensile strength and the microhardness of the alloy were increased and the relative elongation was decreased slightly in the case of fast quenching and cooling.Superheat treatment on Al-7Si-0.55Mg melt was studied experimentally to improve the microstructure and mechanical properties without grain refinement, modification treatment and Be addition. After superheat treatment of the alloy melt, eutectic Si phase was modified. The modification efficiency was found to be" depended on the superheat temperature and the cooling rate in solidification. A superheat treatment at 800 ~850 modified the int'erdendritic eutectic and the efficiency was comparable to that achieved by Sr addition. The strength and elongation after T6 heat treatment were obviously raised aft...