Partially Remlting And Solidification Of 2024 Alloy Casted By Liquid-liquid/Solid Mixing

Slurry/billet consists of solid particles and liquid in semi-solid processing. The liquid can be employed for the solidification shrinkage. So the free defects will be obtained in theory. But the problems in the secondary solidification of remained liquid,such as precipitated phase and composition segregation, are still existed. These will have obvious effects on the property casting, which limits the widespread application of the semi-solid processing. Thus, the systematic study of secondary solidification is necessary. The ingots with non-dendritic grain of 2024 alloy is prepared by Self-Inoculation Method(SIM), through the liquid-solid mixing way. The microstructure and solidification behavior of 2024 alloy after partial remelting is studied by using the controllability of the thermal simulator during heating and cooling.The semi-solid slurry of 2024 alloy is prepared by controlled diffusion solidification(CDS) technology through the liquid-liquid mixing way. So the suitable slurry used for rheoforming can be obtained. Then, rheo-diecastings combined CDS technology are achieved with the high pressure cast. The results indicate that:Compared to the ingots without isothermal holding, both the small dendritic grain and rose-shape grain decreases, and the tendency to be structural separation for grains is observed under isothermal holding treatment. Original microstructure of ingots has only an effect on particles size and shape of secondary solidification α3, however, the effect on secondary solidification of remelting liquid is unobvious. As the heating temperature rises, the remelting liquid phase of 2024 alloy ingots increases in the process of partial remelting. In addition, it shows that the effects of heating temperatures on the α3 particles are not obvious. The effects of heating rate on the solidification microstructure of ingots with isothermal holding treatment after remelting are obvious, such as α1 particles and α3 particles. Calculating data shows that changing heating rate may optimize the casting microstructure.Average particle size of α3 in the samples with isothermal treatment is larger than that of samples without isothermal treatment, while particle shape factor of α3 is less than that of samples without isothermal treatment at same cooling rates. It shows that the particles of samples with isothermal holding treatment are more globular. Variation tendency of both the shape factor and average particle size of α3 in all samples after remelting is uniform with the cooling rate. With cooling rates increasing, secondarysolidification α3 for large particles decrease, whereas increase for small particles.Shape factor of α3 particles decreases, and particle roundness enhances. Secondary solidification mode includes two ways: The one is that secondary solidification α2depends on α1 particles surface and grows into toes structure at the same time. The other is that α3 independently nucleates from remelting liquid and finally grows into fine near-spherical grains. At later solidification stage, particle grains of α3 were easy to merge and coarse.The resulting alloy can be prepared by CDS technology, which can obtain the slurry with non-dendritic grain. Both the water-quenched and as-cast microstructures of the resulting alloy slurry are significantly affected by high-temperature melting(pure Al), as well as by low-temperature melting(Al-10%Cu alloy).