Simulations of joining processes use finite element analysis (FEA) and computational fluid dynamics (CFD) to predict residual stresses and distortion.
Residual stress information is often needed in order to reduce the conservatism in engineering critical assessments (ECAs). It is also important when assessing phenomena such as stress corrosion cracking (SCC) or the behaviour and effects of hydrogen in metallic structures. Predictions of distortion are a valuable tool in the optimisation of manufacturing processes.
The impact of changes in weld sequencing, clamping, tack welds, and weld process conditions can be investigated, with guidance from welding engineers, in order to recommend a process to minimise the distortion of a welded structure. Weld simulations are usually complex and require high-quality material property data, including high-temperature properties, and an excellent understanding of the simplifications that must be made to achieve useful results at reasonable cost and in a practical time-scale.
Finite element simulations are often applied to the prediction of residual stresses in structural components. The calculation of residual stresses involves complex non-linear analyses where several assumptions and approximations are required. The accuracy and resulting predictions, therefore, depend on the level of expertise of the analyst.
TWI has extensive experience of thermo-elasto-plastic modelling. In addition, thanks to TWI's comprehensive knowledge of welding processes, we are able to analyse the importance of residual stresses for each case. TWI is part of numerous international networks to advance the knowledge on measurement and prediction of residual stresses.
Prediction of weld distortion takes many forms, including basic rules-of-thumb, simple shrinkage models, and thermal elastic/plastic FEA of various degrees of complexity. Use of an appropriate level of simulation is a powerful and cost-effective way of reducing costs and increasing the quality of welded structures.
TWI has substantial experience in simulating welding processes. With access to a range of industry-standard commercial software, as well as bespoke in-house code, we can predict distortion due to a wide selection of welding processes, including arc, friction stir, laser, and electron beam. In addition, the effects of other processes, including post-weld heat treatment and proof testing, can be simulated.
Working closely with TWI's process groups provides numerical modellers with detailed insights into the welding processes, together with clear guidance about process changes to minimise distortion. This synergy also allows rapid and cost-effective assessment and validation of simulations via welding trials, including the use of state-of-the-art measurement equipment.
Benefits and applications
- Accurate residual stress profiles are used to reduce the conservatism of ECAs, allowing materials and cost savings without jeopardising the safety and performance of structures
- Reduced weld distortion based on numerical modelling can provide significant cost savings by reducing scrap and remediation
- Process simulation can be used to find process conditions to optimise a manufacturing process, for example to find the fastest process conditions that give acceptable distortion, or to balance welding speed and tool life in a friction stir welding process
- Simulation of repair welds, often combined with the use of mock-ups, can ensure that repairs are right-first-time and do not introduce unacceptable distortion or residual stress.
Weld process simulation is important for a wide range of industries. An example from the oil and gas sector is available in Industrial Member Report 942/2010 ('Comparison of measured and calculated residual stresses in steel girth and butt welds').
TWI has participated in several collaborative projects in the transport sector, including:
- MALCO, to develop low-cost no-distortion welding;
- OptWeld, to develop and demonstrate software for weld distortion prediction and minimisation; and work to develop a novel joining process for high-strength steels.
- For the ClampIT project ('Intelligent welding clamp design software using computer-aided optimisation') TWI developed a bespoke FEA solver specifically for distortion prediction.
- An example of distortion minimisation is provided by work carried out for the marine sector in this case study.
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