Role of thermal cycle in joining Ti-6A1-4V and Ti2AlNb-based alloys through diffusion bonding and post heat treatment

Dissimilar joints of Ti-6A1-4V and Ti(2)AINb-based alloys bonded by diffusion bonding (DB) were subsequently subjected to eight different post heat treatment (PHT) regimes by varying the solution and aging temperature. The evolution of microstructures and mechanical properties during the thermal cycles of DB and PHT processes were investigated. The solution treatment enlarges the width of the diffusion affected zone (DAZ), while the contribution of the aging to the DAZ width can be neglected. The microstructural transitional zone of the joint can be divided into beta + alpha and beta/B2 + O regions. No matter which thermal cycle was applied to the joint, the Ti concentration at the boundary between these two regions is determined to be around 73.24-74.22 at.% by comparing the element distribution and precipitate thickness distribution across the joint. Although it only has marginal effects on Ti-6AI-4V substrate, the PHT remarkably alters the phase composition of Ti(2)AINb substrate and the thicknesses of O phase within both substrate and joint. The acicular O phase seldom precipitates in Ti(2)AINb alloy by only applying the aging process while the solution treatment can restore the low-temperature hardening ability of this alloy by dissolving numerous lath O phases induced by DB. Thus, the age-hardening happens in both Ti(2)AINb substrate and the bonded part.