Translated Abstract
Semi-solid forming balances good fluidity of liquid forming and high mechanical properties of solid forming. It can realize near-net-shape forming of complicated parts easily, which is viewed as one of the most promising metal forming technique in the 21st centry. Based on the application background of orbiting scroll for auto air-condition compressor, the flow behavior and microstructural evolution of semi-solid 2A50 alloy during electromagnetic stirring(EMS) preparetion, remelting and thixotropic squeeze casting process of orbiting scroll are investigated by theretical analysis, numerical simulaton and experimental research.
Three dimensional numerical model of coupled electromagnetic field, temperature field and flow field is established based on the ANSOFT and FLUENT software. Then numerical analysis of EMS process for 2A50 aluminium alloy is conducted. The effects of stirring current Is and stirring frequency fs on the magnitude and distribution of magnetic field intensity and EMS force are obtained; the evolution of temperature and flow field under different stirring intensity is discussed and the formation mechanism of vortex is revealed. Results show that both the magnetic field intensity and EMS force are increased with Is and decreased with fs; Is has no obvious influence on their homogeneity, while increased fs promotes skin effect and end effect. With increasing stirring intensity, the temperature is continually decreased and uniformly distributed; the maximum velocity increases and covers wider area. Excessive strong stirring intensity results in a wider and deeper vortex and the air enters inside, forming many gas pockets. This is primarily caused by the convection of the slurry that rises from the edge and fall to the center, which prevent the air to escape.
The corresponding EMS device is develop. The effects of stirring current Is, stirring frequency fs, holding temperature Th and preheating temperature of canister Tc on the grain morphology of semi-solid 2A50 alloy during EMS process are investigated. The nucleation and growth mechanism are also revealed. Results show that stirring intensity is increased effectively with Is and rotation velocity is double increased with fs; they shear and break the coarse dendrites. Then abundant nucleus drift apart, proliferation and grow equiaxially with the forced convection of the slurry, forming fine, isolated and spherical grains throughout the slurry. However, the grain morphology has no obvious improvement when Is 30A and fs 30Hz. Lower Th or Tc shortens efective stirring time, forming grain cluster with blurry boundaries; higher Th or Tc promotes grain coalescence and forms coarse grains. drifting and proliferation of nucleus, equiaxial growth and Ostwald ripening of grains are the primary nucleation and growth mechanism during EMS. Finally, the reasonable preparation process parameter are obtained as Is=30A, fs =30Hz, Th=645℃, Tc=350℃.
The EMS prepared semi-solid 2A50 alloy is used for the remelting experiments. The effects of initial grain morphology, remelting temperature Tr and soaking time tr on the grain morphology evolution during remelting experiments are obtained. Results show that if the initial grains are coarse dendrites, the remelting grains are coarsened by coalescence mechanism; if they are spherical grains, then they mainly are coarsened by Ostwald ripening mechanism. Effective liquid permeation directly decides the specific coarsening mechanism of the remelting grains. Properly increased Tr or tr thickens the liquid film and keep them coarsening by Ostwald ripening mechanism. The reasonable remelting parameters are Tr=590~595℃, tr=10~15min and the grain coarsening rate is 547μm3?s-1 at 590℃.
Aiming at the thixotropic squeeze casting process of orbiting scroll for auto air-condition compressor, the effects of pouring temperature Tp, filling velocity vp and mould temperature Tm on the filling and solidification behavior are obtained by the PROCAST software. Results show that with increasing vp, the temperature drop and filling time are decreased, but excessively high vp can generate ejection phenomena easily; the flow velocity, solidification time and shrinkage porosity are increased with increasing Tp; With increasing Tm, the temperature drop is decreased but the flow velocity is increased, meanwhile, the thermal center and shrinkage porosity is reduced obviously. Finally, the range of reasonable process parameter are Tp=590℃~595℃, vp=0.2m/s~0.3m/s, Tm=300℃~350℃.
The multi-directional squeeze casting equipment with six independent controlled hydraulic cylinder and the matched orbiting scroll mold are developed. The thixotropic squeeze casting experiments are conducted. The reliability of the optimized simulation parameters is verified. Then the effects of squeeze pressure P, pouring temprearture Tp and heat treatment process on the microstructural evolution and mechanial properties are investigated. Results show that with increasing P, the microporosity is reduced dramaticlly and the densification of grains is increased, which help to improve the mechanical properties. When P 100MPa, grain clusters forms and its mechanical properties tend to be stable. Liquid squeeze casting forms coarse dendritic and causes turbulent flow, which causes lower mechanical properties than semi-solid squeeze casting. T5 and T6 heat treatment can precipitate lots of strengthening phase, which enhance the tensile strength and hardness while reduce the elongation.
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