Research On Microstructure And Formation Mechanism Of A390 Alloys Fabricated By Controlled Diffusion Solidification

Controlled Diffusion Solidification is a new casting method,in whose process, two precursor alloys will be smelted separately and mixed together at a designed temperature. CDS is able to control the temperature field and solution field to improve the alloy structures. In this research, CDS is used to prepare hypereutectic Al-Si alloy A390 and precursor alloy 1(A1)Al-25 Si, precursor alloy 2(A2) is Al-10Cu-7Si-Mg. The effects on microstructure and mechanisms are also investigated.The mixing temperature of A2(T2) is invariable, A1 temperature(T1) is 730 ℃, 740 ℃, 750 ℃, 770 ℃, the grain size of primary Si in A390 is 28.91 μm, 34.12 μm, 47.34μm, 59.17μm. And the ratio of length to width(R=L/W) of eutectic Si also rises. As A390 alloys is modified with P and Sr, the grain size of primary Si is 48.19μm and R of eutectic Si is 5.2. In terms of uniformity of grain size, most primary Si particles in alloys by CDS are in interval of 0-60 μm interval, compared with 40-60 μm interval for modified A390 alloys. However, primary Si with large size(above 100μm) is fewer in CDS alloys. For the grain distribution, primary Si particles in modified ingots are obviously more uniform. The tensile strength of specimens by CDS and modification is separately 179.42 MPa and 175.07 MPa.The rapid-cooling ingot mixed with two melts could be divided into three areas: mixing area A, near the mixing area B, regular solidification area C. It can be observed in An area that two melts permeate each other by “striation” form, and the “striation” is easily broken to small particles by violent strike. The octahedral grain structure is the typical morphology in area A. <1 0 0> directions grow fastest to become the angles of octahedral. {1 1 1} planes have the lowest growing velocity, so finally they can remain to form the planes of octahedral. Some large primary Si grains with complex shapes can be easily found in area C which could be explained by the appearance of high solution concentration area. When alloys solidify regularly, solutes would gather together in some direction of grains, at last they could not grow to uniform structure. The growth of eutectic Si particles is also restrained. their <1 0 0> directions can not grow completely due to Si solutes are not able to adhere to. Finally eutectic Si particles are surrounded by α phase, needle-like shape is hardly to observe.When CDS is in mixing process, melts are preserved at 640 ℃ to hold 1, 3, 5 min. According to the statistic results, the alloy structure become more uniform, although the grain size gradually increases. By tensile tests, the specimen hold 3 min at 640 ℃ express 218.79 MPa of tensile strength. If the CDS ingot is preserved at 500 ℃ for 8 h, then quenched immediately by water and last preserved at 170 ℃ for 12 h, the shape of eutectic Si obviously become globular, its tensile strength is 192.38 MPa. when treating the CDS ingot which is hold 3 min at 640 ℃ under this process route, tensile strength increases to 229.18 MPa.