Study On The Microstructure Evolution Of Light Alloies During The Semi-solid Slurry Preparation And Reheating Process

As the new forming technology which has advantages of both liquid and solid forming, semi-solid forming process can offer several potential advantages, such as high efficiency, energy conservation, near-net-shape forming as well as excellent performance parts and many other features. Semi-solid forming technology can fabricate near-net-shape products using light alloy materials, bad formability materials and shape complexity components, and therefore attracts more and more attention. This paper studies the microstructure evolution of A380 casting aluminum alloy,6061 deforming aluminum alloy and high wt%Si Mg-Si alloy during the semi-solid slurry preparation and reheating process.This paper selects A380 casting aluminum alloy,6061 deforming aluminum alloy and AZ91 magnesium alloy as the raw material, studied the microstructure evolution of semi-solid slurry preparation and reheating process of these light alloies. Slurry preparation stage, prepared semi-solid slurry of A380 and 6061 aluminum alloy by liquidus casting, and prepared slurry of high wt%Si Mg-Si alloy by repeated melting. Also studied the metamorphic refinement process of the slurry of the light alloies by adding small amounts of Mg-Sr or K2TiF6, studied the best preparation conditions and microstructure evolution of semi-solid slurry of A380,6061 aluminum and high wt%Si Mg-Si alloy. Reheating stage is mainly about the reheating process of the prepared slurry of light alloies, studied the optimal holding temperature, the best holding time and the microstructure evolution of the slurry during reheating process.Slurry preparation stage, the optimum microstructure with the average grain diameter of 20.4μm and roundness value of 4.4 of semi-solid slurry of A380 casting aluminum alloy can be obtained when the temperature at 600℃ and holding time for 60 min. There are a-Al, Si and Al9Si phases in the alloy. When add 0.02wt% K2TiF6 refiner, there is no significant refining effect on the slurry, and also lead to abnormal coarse Si phase. But if add 0.02wt% Mg-Sr modifier, there is significantly refining effect on the microstructure of the Si phase, and Sr2Si3 phase will appear; the optimum microstructure with the average grain diameter of 41.4 μm and roundness value of 2.4 of semi-solid slurry of 6061 deforming aluminum alloy can be obtained when the temperature at 670℃ and holding time for 60 min. There are α-Al and Al9Si phases in the alloy. When add 0.02wt% K2TiF6 refiner or 0.02wt% Mg-Sr modifier, there is significantly refining effect on the microstructure of the a-Al phase, and the refinement of K2TiF6 are more uniform; Mg-Si alloy, silicon content of 10wt%, was successfully prepared by repeated melting method, and there are Mg, Mg17Al12 and Mg2Si phases in the alloy. The prepared Mg-Si alloy has poor corrosion resistance ability, but the improment of the microhardness of the magnesium matrix is obvious. The microhardness of the magnesium matrix of the prepared Mg-Si alloy, whose designed silicon content is 8wt%, can reach HV58.4, and the microhardness of the Mg2Si phase is HV143.9, when the microhardness of the magnesium matrix of the raw material is HV36.4. The closer to Mg2Si phase, the higher the hardness is, and the more uniformly dispersed Mg2Si phase, the higher the microhardness of the magnesium matrix is. The hardness of the Mg2Si phase in slurry of different silicon content is different, and the hardness of the Mg2Si phase is the hignest in the different phases of the prepared Mg-Si alloy.Reheating stage, the optimum microstructure of the reheating A380 slurry, whose average grain diameter is 49.74μm and roundness value is 1.75, can be obtained when the reheating temperature at 583℃ and holding time for 20min; The optimum microstructure of the reheating 6061 slurry, whose average grain diameter is about 110μm and roundness value is about 1.55, can be obtained when the reheating temperature at 635℃ or 645℃ and holding time for 20min; The optimum microstructure of the reheating Mg-8wt%Si slurry, whose average grain diameter is about 35μm and roundness value is about 1.36, can be obtained when the reheating temperature at 570℃ or 580℃ and holding time for 30min, or the reheating temperature at 590℃ and holding time for 20～30min.