1 A NEW LOCAL-GLOBAL APPROACH FOR THE MODELLING OF WELDED STEEL COMPONENT DISTORSIONS B. SOULOUMIAC(1), F. BOITOUT(1), J.M. BERGHEAU(2) (1) ESI Software, 84 bd Vivier Merle, 69485 Lyon Cedex 03, France, Email : b.souloumiac@systus.com, f.boitout@systus.com (2) Laboratoire de Tribologie et de Dynamique des Systèmes, UMR5513 CNRS/ECL/ENISE 42023 Saint Etienne Cedex 2, France, Email : bergheau@enise.fr ABSTRACT Welding technology is the main joining technique used in industry for parts assembly in cars, ships, planes, trains or civil engineering machines. One major industrial concern is to limit the use of clamping tools. The result is a direct cost reduction but it also makes easier the automation of assembly lines. Optimisation of the welding sequence and process is a way to reach this goal. However, the experimental optimisation of the welding technique regarding distortions requires prototyping and measurements which are extremely expensive and time consuming and finally, very few solutions can be experienced. Finite element simulations can be used in that aim but the difficulty is, on one hand, that welding processes involve complex couplings between heat transfer, metallurgy and mechanical phenomena and, on the other hand, that where local models are sufficient to predict stresses, only global 3D models can correctly predict distortions. Considering the high gradients of temperature, phases, stresses and plastic strains located in small areas which are involved in welding simulations, the refinement of meshes near the welding line increases drastically models size and consequently leads to unreasonable computation times. In this paper, we present a new methodology to perform complex 3D welding simulations in order to calculate residual distortions and consequently optimise the welding sequence. The method rests upon a local 3D approach for the precise modelling of the physical phenomena induced by welding. The results are then transferred in a macroelement which is used in a global shell model for calculating distortions. The application of the method proposed on industrial applications is discussed. INTRODUCTION The welding technology is the main joining technique used in industry. Actually, the strong advantage of welding is that it is the most lightweight solution to connect parts. However, welding involves complex interactions between thermal, metallurgical and mechanical phenomena and therefore, leads to residual distortions and stresses which must be controlled [1]. Numerous welding problems, especially those concerning safety in the nuclear industry, are related to residual stresses. Numerical simulations offer a promising means of determining the metallurgical structure and the residual stresses inasmuch as direct measurements are costly and obviously cannot provide the different quantities at all points in the structure. Moreover, 2D or 3D local models give satisfying predictions with a relatively small number of degrees of freedom. So, refined approaches coupling heat transfer, metallurgy and mechanics have been implemented in finite element softwares like SYSWELD [2]. The optimisation of the welding technique regarding distortions requires prototyping and measurements which are expensive and time consuming and finally very few solutions can be experienced. Moreover, additional machining after welding or pre-distortion is often still