Numerical simulation of the welding process(SNS)

Since 2005, EC2-MS is developing skills and expertise in the field of Numerical Simulation of Welding. The aim of the NSW is to predict residual stresses and distortions that occur in metallic structures during the welding process.

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Residual stresses and distortions :

During a TIG welding operation, the non-uniform heat supplied and the subsequent cooling generate distortions and residual stresses. Distortions can cause tolerances problems, especially for thin structures. Thermal stresses are also likely to induce buckling phenomena during the course of welding in shell structures, and cause loss of structural integrity or dimension control. On the other hand, residual stresses can be a source of cracking, corrosion, and fracture problems in welded structures, such as welded pipes in boiling-reactor piping systems of nuclear power plants.

Although weld residual stresses or distortions might be attenuated by adequate heat treatments, their determination appears to be essential for costs-saving, manufacturing accuracy, or in order to perform lifetime analysis.

With the recent increasing of computer power, numerical simulation by the finite element method has become a highly effective mean of estimating transient stresses, as well as residual stresses and distortions generated by the welding process.

Numerical Computation of welding by the Finite Element Method

In many cases, accurate prediction of residual stresses and distortions remain very difficult since the welding process involves very complex thermal, metallurgical and mechanical phenomena in the Heat Affected Zone (HAZ), including:

• The determination of the transient temperature field in the welded pieces, given the heat input (deduced from the weld parameters), and accounting for all the non-linearity (temperature-dependant thermophysical properties, latent heat of phase transformations, boundary conditions)

• The determination, if any, of the microstructure evolution that is likely to appear during the weld (for instance bainitic and martensitic transformation occurring in low-carbon alloys), and of their mechanical consequences (metallurgical strain being added to the thermal strain, multi-phased plastic behaviour, transformed induced plasticity) that may have a strong influence on the result

• The determination of the transient mechanical evolution, accounting for the thermal and metallurgical fields, with a suitable constitutive equation for all the metallurgical components (Kinematic hardening, viscoplasticity…)

The computation of the thermal, metallurgical, and mechanical fields ought sometime to be solved in a fully-coupled way, in case of a process involving a strong thermal-mechanical interaction (friction welding, stir welding…). However, for fusion welding (TIG, MIG-MAG, laser…), a simple uncoupled thermal and subsequent mechanical analysis (taking into account metallurgical transformations) is sufficient.

NSW for industrial studies

Thank for the recent increasing power of computers, numerical simulation of welding has become available to perform industrial studies. It has become a highly interesting mean of predicting weld residuals stresses and distortions resulting from welding processes, or in order to control or optimize the assembly process.

EC2-MS has developed skills in the field of NSW through a great amount of studies performed in an industrial framework, although still in a RD context.

In connection with the laboratory of Mechanics of INSA de LYON (LAMCos), which is carrying out the “INZAT” research program initiated in 1994 at the instigation of nuclear industry, EC2-MS provides the transmission of a know-how in Numerical Simulation of Welding that has long remained the prerogative of state-of-the-art industries or laboratories .