Inertia friction welding is the process in which stored kinetic energy in a flywheel is converted to heat by relative sliding movement between surfaces of axi-symmetric components to achieve a weld in the solid-state. The work in this paper relates to the production of dual-alloy shafts for aeroengines. Frictional characteristics determine the conditions at the weld interface and these are controlled by rotational velocity and applied axial pressure. So-called representative and predictive methods have been developed to evaluate friction conditions during the process and these are discussed in this paper. Weld data for the dissimilar weld between a high strength steel and a nickel-based super-alloy were provided by Rolls-Royce and MTU Aero Engines. The finite element software package DEFORM-2D is used to develop coupled thermo-mechanical axi-symmetric models. In previous work, methods employed to evaluate the efficiency of mechanical energy utilised during a weld, a parameter of great importance for numerical analysis, are not clear. Previous predictive approaches have employed test/weld data in one way or another to obtain the interface friction coefficient. This paper proposes a formula that incorporates the value of the mechanical energy efficiency of the welding machine into the calculation of coefficient of friction for representative modelling. It also introduces a predictive approach based on sub-layer flow theory to predict frictional behaviour during the welding process that is independent of test/weld data.

Dividing the weld path into multiple seams that are welded sequentially in a specific order is one of the most important and cost effective distortion and residual stresses mitigation strategies. This is particularly important for welding fabrication of thin structures. The number of sequence options resulting from reordering seams increases explosively with the increase of the number of weld seams. Investigating all sequences using experimental, analytical or computational approaches requires substantial expenses in terms of time, resources and cost and in many situations is practically impossible. Therefore, welding industries tend to opt for small improvement and abandon the search for the absolute optimum welding sequence, e.g. the welding sequence that produces the absolute minimum welding induced distortion. This paper presents an optimization procedure to improving the effectiveness of the search for an optimized welding sequence. The optimization procedure is based on the principles of genetic algorithms (GA), in which the finite element (FE) analysis is used to produce the distortion information and direct the evolution of the GA within the optimization. The capability of the optimization procedure to identify an optimum welding sequence was demonstrated for the keyhole plasma arc welding (KPAW) of two Ti-6Al-4V thin structures, being a flat plate and a simplified replica of a portion of an aero-engine casing. The optimization procedure developed in this study was capable of minimizing the welding induced distortion by up to 55% and of improving the effectiveness of the search for an optimized welding sequence by up to 98%.

This article presents a comprehensive piece of research work focused on the development, validation and application of finite element modelling capability for the prediction and optimization of robotic keyhole plasma arc welding of Ti-6Al-4V thin structures. Experimental and computational investigations were carried out to characterize, develop, optimize and validate various aspects of the finite element modelling. The experimental investigations cover the determination of welding parameter envelopes using a robotic welding cell and the measurements of thermal history, distortion, residual stress and weld pool profile. The computational investigations include the development and validation of finite element models as well as the development and validation of a fully automated welding sequence optimization tool using a genetic algorithm approach. The work provides useful guidance and generic methodologies for optimum design of thin and complex lightweight structures and has formed a basis for the development of a framework on structural integrity assessment and component lifing of thin structures fabricated by welding. The optimization tool has significant potential to be conveniently modified to suit other optimization objectives and/or welding processes.

Inertia friction welding is the process in which stored kinetic energy in a flywheel is converted to heat by relative sliding movement between the components' surfaces. The process is widely used in joining high strength aero-engine alloys. Heat transfer interactions play a fundamental role in determining weld quality as temperature has a significant impact on the mechanical, thermal and metallurgical behaviours of materials. Heating and cooling rates influence thermal stresses induced by variations in material properties due to variations in temperatures. Heat transfer modes involved in the IFW process, apart from conduction, were hardly mentioned in previous work while their significance has not been identified. In this paper the heat transfer modes and their significance in the modelling of the IFW process of the high strength steels SCMV and AerMet 100 has been identified and a methodology by which experimental, numerical and empirical approaches were collectively utilised to optimize the heat transfer analysis is presented. The methodology was validated for optimizing the heat transfer analysis of the IFW process of the high strength steels that are the subject of this study.