Microstructural Evolution During Partial Remelting Of Ti-Al-2024Al Bulk Alloy Prepared By Cold Pressing Of Mixed Powder

A new method, powder thixoforming, has been proposed to fabricate a new in situ composite which takes the Ti@Al3Ti core-shell structured particles(Ti@Al3Tip)as reinforcement. This new composite is expected to overcome the brittleness of the common particle reinforced Al matrix composite. This new technology combines the advantages of powder metallurgy and thixoforming techniques. A bulk alloy is first obtained by using the blending and pressing procedure of powder metallurgy, and then a composite component is prepared by using the partial remelting and thixoforming procedures of thixoforming. The partial remelting can not only achieve a semisolid ingot for thixoforming, but also produce the Ti@Al3Ti core-shell structured particles through the reaction between the Ti powder and Al melt. In order to offer a solid theoretical foundation to the thixoforming after the bulk alloy being partial remelted,the microstructural evolution during partial remelting of the bulk alloy prepared by cold pressing of the Ti-Al-2024 Al powder mixture was investigated. And the kinetics relationship between the Ti powder and Al matrix has also been investigated. The reaction between the Ti powder and Al melt was simulated by using dropping experiment.The results indicate that an ideal semisolid microstructure with small and spheroidal primary particles can be obtained after the bulk alloy prepared by cold pressing of the Ti-Al-2024 Al powder mixture being partially remelted in 640℃. The microstructural evolution of the matrix can be divided into four stages: a rapid coarsening of the powder grains within 2024 Al powder; a formation of primary α-Al particles surrounded with a continuous liquid film; increasing of liquid amount and a slight coarsening of the primary α-Al particles; and a slowly coarsening of the primary α-Al particles. A desired semisolid microstructure can not be obtained when the remelting temperature is too low or too high and the appropriate temperature is640℃. The evolution process of pores in the bulk alloy can also be divided into three stages: during the early remelting stage(0-15min), the pore percentage increases rapidly with the time due to the existence of the Kirkendall effect. In the middle remelting stage(15-30min), the pore percentage decreases with the time due to the improved liquid filling capacity to the pores. During the late remelting stage(after30min), the pore percentage increased rapidly due to volume expansion from the reaction between the Ti powder and Al melt. The pore percentage decreased with the partial remelting temperature rise because the ability of the liquid filling pores is higher than that of pore formation from the Kirkendall effect. With the increasing ofreheating time, pore size gradually decreases in the effect of liquid filling and the shape of the pores becoming more rounded in order to reduce the solid-gas interface energy.A kind of shell-core structured particles-Ti@Al3Ti, a dense intermetallic compound Al3 Ti layer wrapped the residual Ti core, can be formed through the reaction between Al and Ti after being heated for 15-20 min at 640℃. Thickness of the reaction layer increases with the remelting time. To a Ti powder with given size, voids and cracks will generate in the Al3 Ti reaction layer when the thickness increases to a certain value due to the combining effect of Kirkendall effect, volume dilation and the brittle nature of Al3 Ti phase. When the heated for 210 min, the Ti particles completely react with Al to become into agglomerates composed of Al3 Ti particles with different sizes. The mathematical relationship between the thickness and remelting time can be expressed as:88.0(28)(35)1.0 tx. The amount of volume expansion caused by the TTi Ali3? phase transition is about 216%. The stress caused by the volume expansion can be calculated byTi Alus. The thickness of Al3 Ti reaction layer increases with the temperature in a linear law.The experimental results of the dropping experiment indicate that the growth direction of the reaction Al3 Ti layer is bidirectional. But the growth rate towards to the Ti plate is faster than that that towards to the Al melt due to the diffusion rate of Ti atoms to the Al melt through the Al3 Ti reaction layer is faster than that of the Al atoms.The thickness variation of Al3 Ti reaction layer with the reaction time follows a similar law in the two experiments and the relationship for the dropping experiment and bulk alloy experiment can be expressed as0.06+1.97t+-0.12t=X2a, 0.17+4.21t+-0.43t=X2b, respectively. It can be easily found that the reaction rate of the former is faster than the latter due to the contact area of the Ti particles with Al melt for the bulk alloy experiment is larger than that for the dropping experiment. In addition, the growth of the reaction layer in the dropping experiment is controlled by atom diffusion in the initial stage, but it changes into grain boundary diffusion in the latte stage and the reaction rate gradually decreases. The thickness variation of the reaction layer with the temperature in the dropping experiment follows a linear law.