Nickel based superalloy welding practices for industrial gas turbine applications

The continued drive for increased efficiency, performance and reduced costs fir industrial gas turbine engines demands extended use of high strength-high temperature capability materials, such as nickel based superalloys. To satisfy the requirements of the component design and manufacturing engineers, these materials must be capable of being welded in a satisafactory manner. The present paper describes the characteristic defects found as a result of welding the more difficult, highly alloyed materials and reviews a number of welding processes used in the manufacture and repair of nickel alloy components. These include gas tungsten arc (GTA) and electron beam (EB) welding, laser powder deposition and friction welding. Many of the more dilute nickel based alloys are readily weldable using conventional GTA processes; however, high strength, precipitation hardened materials are zone and strain age cracking prone to heat affected defect formation. A number of factors are found to affect the propensity for defects: composition (aluminium and titanium content), grain size, pre- and post-weld heat treatment, as well as the welding process itself (control of heat input and traverse speed). Process parameter identification is still largely empirical and a fuller understanding of the joining processes is dependent upon the development and application of more sophisticated numerical modelling techniques.