The continued drive for increased efficiency, performance and reduced costs for 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 satisfactory 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 prone to heat affected zone and strain age cracking 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.

The paper describes a new finite element, derived from the classical constant strain element, in the context of localization analysis of elastic-plastic materials. The formulation is based on the “enhanced strain” concept, which is extended to include regularized displacement discontinuities. As a result, the localization band is embedded within the element and the conventional displacement topology is preserved. The major advantage, as compared to the interelement representation, is that advanced mesh (re)alignment strategies are totally avoided and unstructured meshes are sufficient. An interesting feature of the element is that the condition for existence of an internal element discontinuity is identical to the classical condition for band-shaped localization. The element is combined with a cohesive crack model that is based on the Rankine criterion and a well-established calibration technique to achieve an objective response in the post peak softening range in terms of the dissipated (fracture) energy...