Reduction of ASZ size and enhancement of mechanical properties of TLP bonded IN718 joint using post bond heat treatment process

Abstract

Transient liquid phase (TLP) bonding can be used to join Inconel 718 superalloy because the bond properties are similar to the parent material properties and little melting of base material. Presence of centerline eutectic and boride precipitates in the microstructure of the TLP bonded Inconel 718 joints affects mechanical properties of the joints. The eutectics and borides should be minimized to enhance the mechanical properties of the bond. This can be achieved to a great extent by prolonged isothermal holding. However, it can induce adverse effects by modifying the microstructure of the base material. Post bond heat treatment (PBHT) processes can be implemented to improve the mechanical properties without inducing any harmful effect of prolonged heating. In the present work, the effect of PBHT technique (i.e. solution treatment and double aging) on the microstructural evolution and improvement in mechanical properties were investigated. The ultimate tensile strength (UTS) and microhardness of TLP bond before and after the PBHT were analyzed and compared with the as-received workpiece. FESEM, FETEM and XRD analysis were used to quantify different precipitates formed during PBHT of TLP bond. The results showed that the PBHT improved the microstructure and mechanical properties of the bonded samples. After the solution treatment and double aging, a maximum UTS of 1210 MPa was obtained, which correspond to a 113.5% increase. The improvement of bond strength was attributed to increase volume fraction of γ' and γ" type strengthening phases during double aging and a decrease in the fraction of Nb-Mo type borides in the diffusion affected zone.