Joint strength of Inconel 718 alloy and its improvement by post-weld heat treatment – joint performance and its controlling factors in friction welding of Inconel 718 alloy

Abstract

The influences of welding parameters on tensile properties of friction-welded joints of Inconel 718 alloy (subjected to a post-weld heat treatment (PWHT) consisting of a solution treatment at 1253 K and double ageing treatments at 993 and 893 K) have been investigated to reveal the controlling factor of the joint performance. All joints obtained were fractured near the bond interface at smaller elongations and area reductions than the base metal on tensile tests, although most of them showed tensile strengths comparable with that of the base metal. The observations of fractured surface and its cross-sectional microstructure suggested that an interfacial fine grain zone including numerous fine Laves phase particles 30–100 nm in size was responsible for a low ductility fracture at shorter friction time and lower friction pressure. As the friction time and pressure were increased, the fine grain zone disappeared, and a reduction in hardness near the bond interface became significant, causing a rather ductile fracture near the bond interface. With an increase in friction time, coarse Laves phase particles, a few micrometres in size remaining near the bond interface increased and they acted as a crack nucleation site of ductile fracture. An increase in the solution treatment temperature during the post-weld heat treatment enhanced the dissolution of the coarse Laves phase in the low-hardness region, and enabled us to obtain joints that were free from unacceptable grain growth and fractured in the base metal at a solution treatment temperature of 1323 K.