Improving the formability and mechanical properties of TiAl alloy by direct forging of uncondensed powder

Abstract

The limited plasticity and poor formability of TiAl alloys constitute primary constraints on their widespread applicability. In this regard, an innovative forming method is proposed, characterized by the direct forging of unconsolidated pre-alloyed Ti–48Al–2Cr–2Nb powders. In this procedure, spherical powders with a broad particle size distribution are pre-encapsulated and subjected to vacuumed treatment, followed by direct high-temperature forging to attain the final desired shape. Throughout the process, the powders undergo pre-heating to various forming temperatures and are forged at different speeds, enabling a comparative analysis of the effect of forging temperature and speed on densification and the microstructural characteristics of the forged billet. Experimental results indicate that the powders can be nearly fully densified when direct forging is conducted within the α-phase region. By combining the finite element modeling and microstructural characterization, three mechanisms dominating the consolidation process of powders are revealed. Initial particle clustering and spatial rearrangement predominantly occur during the early deformation stage, enhancing densification. Subsequent fragmentation of particles, induced by severe plastic deformation, governs the entire deformation process, leading to the elimination of the prior particle boundaries and substantial densification improvement. The closure of the micrometer-scale voids is controlled by high-temperature diffusion and creep, which plays a pivotal role in weakening the detrimental effect of micro defects on mechanical properties. The above mechanisms act alternately or simultaneously, resulting in a homogeneous microstructure composed of α2+γ lamellae and bulk γ with small sizes, and a good synergy of strength and ductility. Therefore, this novel approach holds considerable potential as a cost-effective alternative strategy for producing TiAl components.