Study On Solidification Characteristics And Filling Law Of A357 Alloys

With the high speed development in science and technology, the demand for the complicated thin wall aluminium alloy component becomes more and more imperative. These complicated thin wall parts have the symbol of thin walls, complicated structure, large outline size, precise shape and high quality requirement. As a result materials treatment, precise forming and the control of quality digitization are significant to obtain the high quality casting.This paper regards A357 alloy as the research object. We apply the double-thermocouples to determine the dendrite coherency point during solidification process. In addition, establish the theoretical models used for calculate fraction of coherency solid and also study the effects of various kinds of melt treatments and cooling rate on the fraction of coherency solid and solidification behavior of alloy. The effects of filling pressure on the microstructure and performance of A357 alloy are also studied by utilizing low-pressure casting. The main conclusions are as follows:Grain refinement has no effect on the morphology of Si in A357 alloy, but the phase Si is refined and precipitated along the a-Al grain boundary with tiny fiber shape after modification and superheat treatment. With the increase of the cooling rate, both the grain size and secondary dendrite arm spacing of A357 alloy decrease, and the distribution of Si phase is changed from a chain to a short strip or short bar.With the increase of cooling rate, the f_s^coh value of A357 is increased. Grain refinement improves the f_s^coh value significantly, but modification treatment has little effect on f_s^coh, superheat treatment decreases the f_s^coh value. The further analysis indicates that grain size plays a decisive role in the f_s^coh value of A357 alloy: the smaller the grain size is, the easier f_s^coh increase. This help to improve the final quality of the alloy, while the secondary dendrite arm spacing and modification have no influence on f_s^coh.With the ascend liquid surface steady and filling, the increase of filling pressure is help for obtaining high quality pure casting with smooth surface and no impurity, and notably improves the interdendritic feeding ability, the microstructure compactness, and the tension properties of alloy. The morphology of the fracture gradually is changed from strip and clumpy cavity to tiny cavity. When the filling pressure reaches 50KPa, the gas trapping occurs in the mould cavity for that liquid surface elevating too rapidly which will induce casting defects, such as pore, etc., and finallly affected the mechanics performance.