Research Of Rheo-diecasting Process Of Wrought Aluminum、Agnesium Alloy By Self-inoculation Method

The semi-solid slurry of2024and AZ61wrought alloy was prepared by a novelSelf-Inoculation Method (SIM). The morphology of primary phase much fine and roundduring the semi-solid slurry isothermal hold in the semi-solid state, the suitable semi-solidslurry for rheoforming can be obtained. In this work, Self-Inoculation Method (SIM)combines with the High Pressure Die Casting (HPDC) machine. The results indicate that:（1） The semi-solid slurry of2024wrought aluminum alloy was isothermal holding fora short time within the temperature range of semi-solid range, finer and more uniformglobular primary α-Al particles could be obtained. The microstructural morphology of theprimary α-Al particles without entrapped liquid was of fine and globular grains. Theoptimal parameters were as follows: the average equivalent diameter is70.80μm, and74.15μm; and the shape factor is1.32and1.42; and the solid fraction is43.57%and45.14%after the semi-solid slurry was held at625℃for3min and5min, respectively.（2） The average equivalent diameter is44.98μm; and the shape factor is1.37; and thesolid fraction is37.73%after the semi-solid slurry of AZ61wrought magnesium alloy washeld at600℃for3min, obtained the suitable parameters for rheoforming.（3） The whole melt is placed in undercooling state when the semi-solid slurry holdsisothermally within semi-solid temperature range in preheated accumulator. Temperatureand composition field in the liquid becomes uniform, providing a relatively stablesolid/liquid interface, the latent heat release, and then the primary phase growth. In thisprocess, the morphology of the residual of dendrites will evolve during isothermal holdingof the semi-solid slurry in the semi-solid state; the tip of dendrites coarsened, neck,fragmented. It leads to the formation of fine and spherical microstructure. Agglomerationor coalescence and coarsening are the key factors to influence the morphology of primaryparticles. The quenched microstructure grows in the semi-solid state, shows that a mixtureof globular, equiaxed, and few dendrite particles has evolved into fine and sphericalprimary particles and dispersed in the slurry. Ostwald ripening processes through thedissolution smaller globules rather than diffusion solid matters from the area with highcurvature to areas with low curvature.（4） The solidification of rheo-diecastings prepared with self-inoculation method wascomposed of following two distinct stages: primary solidification process and secondarysolidification. The process of secondary solidification consisted further of following threestages:(1) Direct growth of secondary primary (α2) phase from the surface of the primaryα-Al phase particles without re-nucleation,(2) The α3–Al phase will independently nucleate from residual liquid and grow into fine equiaxed morphology.(3) Eutecticreaction at the end. The eutectic solidification is developed by coupled growth. Theeutectic α-Al attaches to θ-Al2Cu phase and grows with bridge-growth mechanism. Theα-Al and θ-Al2Cu phases precipitate alternately after each other, and α-Al+θ-Al2Cu phasewill take a laminar eutectic structure.（5） The microstructure of secondary solidification influences the tensile properties.But whatever the parameters condition, the crack originated at the defects, and propagatedalong the boundary of the solid particle or dendrite of secondary solidification and thematrix. Compared with the material produced by permanent mold, but nothing haschanged.