Semisolid Microstructural Evolution during Partial Remelting of a Bulk Alloy Prepared by Cold Pressing of the Ti-Al-2024Al Powder Mixture

Abstract

A new method, powder thixoforming, has been proposed to fabricate an in situ Al ₃Ti _p/2024Al composite. During partial remelting, the microstructural evolution of the bulk alloy prepared by cold pressing of the Ti, Al, 2024Al powder mixture was investigated, and the formation mechanism of the Al ₃Ti particles produced by the reaction between the Ti powder and the Al alloy melt is also discussed in detail. The results indicate that the microstructural evolution of the 2024 alloy matrix can be divided into three stages: a rapid coarsening of the powder grains; a formation of primary α-Al particles surrounded with a continuous liquid film; and a slight coarsening of the primary α-Al particles. Simultaneously, a reaction layer of Al ₃Ti can be formed on the Ti powder surface when the bulk is heated for 10 min at 640 °C The thickness ( X) of the reaction layer increases with the time according to the parabolic law of $X=-0.43t^2+4.21t+0.17$ . The stress generated in the reaction layer due to the volume dilatation can be calculated by using the equation ${\sigma }_{A{l}_{3}Ti}=-\frac{E_{A{l}_{3}Ti}}{6\left(1-{\upsilon }_{A{l}_{3}Ti}\right)}\frac{{t^2}_{A{l}_{3}Ti}}{t_{Ti}}\left(\frac{1}{R}-\frac{1}{R_0}\right)$ . Comparing the obtained data with the results of the drip experiment, the reaction rate for the Ti powder and Al powder mixture is greater than that for the Ti plate and Al alloy mixture, respectively.