The Modification Using Sr And Casting Process Design Of S-Car Steering Housing

S-Car casting is a kind of automotive power steering gear housing projects which has been accepted and put into production in Ningbo BLAC Co. Ltd. Through Sr metamorphism to make mechanical properties of the alloy meet the requirements, combined with a reasonable casting process design and computer-aided simulation of the casting process, the quality of S-Car casting is collaboratively assured.German brands AlSilOMg(Cu) alloy has good overall performance, commonly used in the production of gravity casting housing member. However, due to the sheet coarse eutectic silicon separating on a(Al) matrix, it results in the poor plasticity of AlSilOMg(Cu) alloys, which can not meet the performance requirements of S-Car parts. Sr is often used to modify the alloy’s microstructure morphology to improve the plasticity. Currently, the mechanism of Sr metamorphism is still controversial. And for different alloys, Sr metamorphic processes are also different in guiding production. The Sr modification and metamorphic behaviors in AlSilOMg(Cu) are studied in this article. This article gives that, at700℃pouring temperature,20℃to25℃ambient temperature,83%to89%relative humidity conditions, within the range of0.003%～0.07%(wt.%), Sr prohibits metamorphism on AlSilOMg(Cu) alloy under all above circumstance, enabling the morphology of eutectic silicon changing from sheet-like to fine fibrous or particulate, and it is a potent modificator. With the increase of Sr content, the less sheet-like eutectic silicon, and fibrous eutectic silicon becomes increasingly small; corresponding to the mechanical properties, the elongation(Agt) of AlSilOMg(Cu) alloy under cast increases from1.42%to4.46%, and its strength value increases slightly, not performing deterioration phenomenon. Through XRD analysis after modification, new phases are not found in the alloy. By optical microscopy and SEM analysis of surface scanning, the author agrees the theory of impurity-induced deterioration of twinning about Sr, Sr is considered as impurities adsorbed on the surface of a silicon crystal, inducing twin crystal, leading highly branched silicon growth, thus contributing to a(Al) nucleation and phase growth by making the typical morphology of dendrites become columnar, and the secondary dendrite arm spacing becomes smaller. Mg is of small content in the alloy, which almost exists as Mg2Si.distributes in the grain boundaries, so the role of Sr on Si makes Mg2Si finer, making precipitates dispersed in the matrix after T6heat treatment when the Sr content (wt.%) is0.04%. The yield strength after T6heat treatment (Rp) increases from100.93Mpa (before T6) to212.05MPa. Since Sr and Al together form a poorly dense oxide film, the addition of Sr makes AlSilOMg(Cu) alloy increase the content of hydrogen, and in order to meet the requirements of compactness of the casting, the content of Sr is not more than0.0262%. The decay of Sr content in standing heat process is not obvious, while the hydrogen content increases, so the melt holding time should not be long, and Sr decay and suction phenomena are caused due to the oxidation of Sr lead by oxide inclusions, molten aluminum shouldn’t be agitated when waiting for pouring. Al-10Sr is added to AlSi10Mg(Cu), which is considered as intermediate alloy. Considering that in remelting process the burning rate of Sr is about45%, the final optimum of adding Al-10Sr (wt.%) to the AlSilOMg (Cu) melting is0.13%～0.37%.After analyzing the structure of S-Car detailedly, the casting process is designed, and ultimately open bottom pouring gating system with Tilt of SPR is adopted, then the side and the top totally four feedings are used. The usage of Pro/E, MAGMA computer-aided software helps designing the mold cavity, pumping core design, casting gating system and simulating the pouring process. Through continuous optimization of process parameters, finally we determine that the pouring temperature is700℃, the casting time is5～6s, mold temperature is400℃or so.While degassing to refine the molten AlSilOMg(Cu) aluminum,0.13%～0.37%(wt.%) of Al-10Sr intermediate alloy are added to modify the matrix. After holding15～30min, under700±5℃pouring temperature and about400℃mold temperature, together with5～6s pouring time, the quality of S-Car castings is good.