| Semisolid metal processing is regarded as one of the near-net shape forming technologies with wide application in the21st century due to such advantages as high efficiency, high quality and low cost, energy-saving and environmental protection and so on. The aluminum alloy parts of semi-solid forming have a lot of excellent properties such as light weight, high strength/weight ratio, good heat conductivity, excellent machinability and recycle etc. Semisolid aluminum parts have widely been applied as structural components in the fields of aerospace, automotive, and other commercial application, etc. Especially with the development of the automobile industry, the lightweight of vehicle has drawn more and more attention. In recent years, the study of the semisolid metal processing of such silicon containing aluminum alloys as6061, A390, ADC12has aroused broad attention in the field of materials.The key to semisolid metal processing is fabricating the semi-solid metals with fine, uniform and spheroidal microstructure for thixoforming or rheoforming. Several fabrication techniques have been applied for a wide variety of alloys. Thereinto the method of near-liquidus semi-continuous casting has attracted extensive study for its advantages such as simple process, high efficiency, fine non-dendritic structure and conducive industrialization, etc.In despite of lots of work for semisolid forming of aluminum alloy, but so far, it can hardly be found literatures about the processing of semisolid billet of the near-eutectic and hypereutectic aluminum alloys by means of near-liquidus. In addition, the average grain size obtained by this method is about50μm and the grain morphology is non-dendritic microstructure. But there still exist a certain amount of rosette grains to be further improved in the alloys. In this paper, the semi-solid billet of silicon containing aluminum alloy was fabricated by near-liquidus semi-continuous casting, and then deformed and heat treated for further improvement of the microstructure, which is called optimization Strain Induced Melt Activation (SIMA) method here. Evolution of the semisolid microstructures of6061. A390and ADC12were studied by means of experiments, theoretical analysis and multiscale simulation. The main results achieved are as follows:(1) A nucleation model for the liquid-solid phase transformation was established concerning the semi-continuous casting process according the theory of homogeneous nucleation taking into account the grain growth, as follows nnuclei＝IV(1-Q(t))twhere nnuclei is the nucleation number. I is defined as the nucleation rate equal to the number of the nucleation in the unit time and the unit volume. Q (t) is equal to the fraction of the untransformed volume.t is solidification time.Thus obtaining the model for predicting the grain size The predicting results of the grain size for6061alloy are consistent with the experimental results. It indicates that homogeneous nucleation is the primary nucleation mechanism for liquid-solid phase transformation of alloys in near-liquidus semi-continuous casting process.(2) The microstructural evolution of the alloys was simulated by multiscale simulation method during the near-liquidus semi-continuous casting process. The simulated morphology and size of the grains are consistent with the experimental results. The simulated results show that the concentration of the melt near the liquid/solid boundary becomes higher because of the solute discharged in the course of solidification, which leads to constitutional supercooling. The degree of the constitutional supercooling results in the different styles of grain growth in the solidification. The grains grow with near-spheroide when the constitutional supercooling is small. The grains grow with rosette character when the composition supercooling is large.(3) The effects of silicon content on the grain size of Al-Si alloy show different law in hypoeutectic and hypereutectic range.In the hypoeutectic range, the average size of primary α-Al phase increases with the decrease of Si content. Near the eutectic point, it coexist the primary α-Al phase, the primary Si and the eutectic micro structure, and the obtained grains are fine. In hypereutectic range, the average size of primary Si and that of α-Al phase increase with Si content. (4) Near-liquidus semi-continuous casting can be used to obtain semisolid billets of hypereutectic Al-Si alloy with fine, uniform, globular and non-dendritic microstructures. which is suitable for semisolid metal processing. The primary Si is near-spheroide and uniformly distributed. The average size of primary Si is about10μm.(5) The optimal Strain Induced Melt Activation (SIMA) method is proposed in this paper to further improve the microstructure of semi-solid alloys fabricated by near-liquidus semi-continuous casting. When the reduction of the deformation is less than50%, the grain size of the heat treated samples decreases sharply with increasing the degree of the deformation. The larger the degree of the deformation is, the better the roundness of the grain is. When the reduction of the deformation is more than50%, the grain size is smaller, the roundness is better after heat treatment and the grain does not significantly change with increasing the degree of deformation. Therefore, the better reduction is larger than50%. The semisolid billet with fine and near-spheroide microstructure can be obtained at elevated temperature in shorter isothermal holding time or at low temperature in longer isothermal holding time. The appropriate matching of isothermal holding temperature and corresponding isothermal holding time is shown in figure5.17(See p.103).(6) It is demonstrated that near-liquidus semi-continuous casting is a simple, feasible and effective method to prepare the6061、ADC12and A390semi-solid billets. The optimum process parameters for fine and near-spheroide microstructure are less than10℃above the liquidus temperature for the pouring temperature, and the range of150mm/min-170mm/min for the casting velocity. |
PDF Full Text Request