The Solidification Behavior Of6061Aluminum Alloys Under Sub-Rapid Cooling Condition

The Al-Mg-Si alloys produeed by conventional easting proeess exhibit the drawbacks of coarse and uneven distribution of microstructure and precipitate phase. Different degrees-of structural refinement and the second phase can be obtained by the sub-rapid solidifieation, increasing uniformity of organization and composition, extended solid solubility of alloying elements and the formation of new metastable phase which can substantially improve the performance of aluminum alloys.In the present paper,sub-rapidly solidified Al alloy castings were prepared by using several different self-designed device,study the precipitation sequence of each phase of6061Al alloys in the conventional solidification condition;the effect of microstructure in6061Al alloys by different cooling rate and to refine and change the alloy microstructure and morphologies by increasing the cooling rate;the microstructure and characteristics of phase in casting6061Al alloys under sub-rapidly solidification.The results show that in conventional solidification the grains of Al alloys were coarse and uneven distribution, mainly consists of primary a-Al and a large number of β-Al5FeSi and Mg2Si phase distributed in the intergranular. During solidification solute elements Fe,Mg,Si enriched at grain boundaries to form intermetallic compounds β-Al5FeSi、Mg2Si phases, however there were only a small amount of solute elements exist in the grains (primary a-Al phase) and micro-segregation of elements were serious.The grains became more and more refine and reduce segregation even form metastable crystalline phases with the increase of cooling rate. With the increase of cooling rate in the near-equilibrium solidification the microstructure became more complex from three phases α-Al,β-Al5FeSi,Mg2Si phases to five phases α-Al,β-Al5FeSi,Mg2Si,AlSiMgCu(1) and AlSiMgCu(2) phases. The metastable crystalline phases Al6MgSi form in sub-rapid solidification.The cooling rate of6061Al alloy castings up to100-103K·s-1, grains at equiaxed zone refined obviously in sub-rapid solidification Al alloy castings. With the increase of cooling rate grain size became smaller, Highe cooling rate suppress the proliferation of solute elements formed a mainly consist of β-Al phase microstructure, precipitate tiny Al8FeSi, Al6MgSi, Al8FeMg3Si6phases in the grain boundary increase the uniformity of casting composition and organization.