Mechanical properties of 718 inertia weld and its comparison with EBW

Abstract

Nickel base superalloy-718 is extensively used in aero gas turbine for critical rotating assemblies such as discs and shafts. These assemblies are subjected to severe service conditions at high temperature. Some of the engine builders employ electron beam welding process for joining of these rotating assemblies. However, fusion welds of nickel base superalloys are prone to microfissuring in the heat affected zone. These defects arise due to solidification cracking as a result of liquation along the grain boundaries. This inherent metallurgical problem in EB welding process can adversely affect the mechanical properties, particularly fatigue. Solid state bonding processes, viz. Inertia friction welding, are known to mitigate this problem. Keeping this in view, inertia welding studies on this alloy were taken up. This paper reports mechanical properties of inertia weld of 718 alloy in addition to microstructural examination results. Effect of post weld ageing treatment is also incorporated. Microstructural examination revealed that the heat affected zone or weld zone are free from microfissures. The weld zone microstructure is much finer than the base metal. Tensile strength and dutility [at RT, 300°C and 650°C] as well as RT LCF lives obtained on inertia welds, in PWHT condition, were at par with the corresponding parent material in STA condition. A comparative study on EBW indicated that the tensile strength [at RT and 300°C] is comparable to that of parent material, but the ductility and LCF lives of EBW of 718 alloy were below that of the corresponding parent material.