Simplified Thermo-Elastoplastic Numerical Modelling Techniques Applied to Friction Stir Welding of Mild Steel

Friction stir welding is a relatively new advanced joining technique that requires minimal power input, ultimately leading to less inherent residual stresses and distortion. The process involves a spinning tool which first plunges into the surface of the, to be welded assembly and then traverses along the joint. Frictional heat is generated, softening the material at temperatures significantly below the melting temperature of the parent material. As the tool traverses along the joint at a predetermined speed, the assembly is joined by means of a plastic straining process. This advanced welding technology has been validated for various aluminium alloys but it is only recently, due to advances in tool technology, that the possibility of joining mild steel using friction stir welding has become a viable option. This study looks into friction stir welding of mild steel and develops simplified numerical methods for the prediction of thermal gradients, residual stresses and deformation. In principle the process modelling requires a multi-disciplinary approach involving coupled thermo-fluid, microstructural-structural modelling process. Much of the latest thermo-mechanical studies of friction stir welding rely on a number of over simplifications particularly related to the heat flux distribution across the tool shoulder, and also on the backing plate boundary conditions. The objective of this paper is to scrutinise the effects of modelling in more detail and establish the most important factors leading to accurate yet computationally efficient prediction of thermal gradients and inherent residual stresses. The results show that both the heat input and heat loss modelling, due to heat dissipation to the surroundings, are crucial for the determination of the final inherent welding residual stresses. The heat generated is modelled through a predefined linear heat flux variation across the tool shoulder. However if a more precise and localized residual stress information is sought, a full thermo-fluid-structural analysis is required. This is time consuming and probably does not give significant information on manufacturing optimization. On the other hand, simplified global solutions offer the possibility to optimise friction stir welding parameters and boundary conditions during the preliminary stages of the development of the fabrication procedures, at relatively minimal time and processing power. This work is financed under the European Commission in Call FP7-SST-2012-RTD-1 High Integrity Low Distortion Assembly (HILDA) project.