2. General information on welding#

2.1. What is welding?#

The welding of two metal parts is the obtaining of continuity by fusion or local diffusion thanks to a contribution of heat and possibly a contribution of metal. Many welding processes can therefore be described physically as a brief passage at very high temperature, of a small quantity of metal, followed by free or forced cooling, most often by conduction in the mass of the welded parts and in the elements of the workstation. Over time, this temperature change determines the final metallurgical structure of the welded joint, which is sometimes different from the base metal.

2.2. Phenomenon involved during welding#

From a mechanical point of view, the very localized thermal gradients brought into play during the welding of two materials cause local expansions and contractions resulting in the appearance of fields of deformation and incompatible stresses, which remain after returning to ambient temperature.

For some materials, the thermal history associated with the welding operation causes changes in solid-state microstructures that influence the state of residual stresses and distortions of the welded joint. These changes in microstructures occur when the temperature reaches a threshold called the transformation start temperature. These transformations stop when the temperature exceeds the temperature at the end of transformation.

Two types of stress can then be distinguished: stresses of thermal origin generated throughout the duration of the welding process and stresses of metallurgical origin (possibly) generated during the duration of the transformations.

Therefore, a numerical simulation of welding must take into account the phenomena coupled together, thermal, possibly metallurgical and finally mechanical.

2.3. Review of the various interactions#

2.3.1. Thermal - Metallurgical Interaction#

  • Thermal meaning => Metallurgic: the thermo-physical properties (conductivity, calorific capacity, diffusivity…) depend on the phases present.

  • Metallurgical meaning => Thermal: metallurgical transformations are accompanied by latent heat effects that modify temperature distributions.

2.3.2. Thermal - Mechanical Interaction#

  • Thermal Sense => Mechanic: this influence results, on the one hand, from variations in mechanical characteristics with temperature and, on the other hand, from expansions and contractions of thermal origin.

  • Mechanical sense => Thermal: the evolution of irreversible deformation as well as internal variables of work hardening leads to the dissipation of energy in the form of heat. However, this rise in temperature of mechanical origin is often negligible compared to that resulting from the heat input delivered by the welding process, taking into account the relatively low deformations and rates of deformation that are involved. This hypothesis is no longer valid in the case of processes involving high dispersions, for example, friction welding.

2.3.3. Metallurgical-Mechanical Interaction#

  • Metallurgical meaning => Mechanics: the influence of metallurgical history on mechanical history results mainly from four factors. The first lies in the dilations and contractions caused by metallurgical transformations. The second factor is the phenomenon of transformation plasticity. The third is the phenomenon of the restoration of work hardening. Finally, the last factor corresponds to the particular behavior associated with the multi-phase aspect of the material.

  • Mechanical meaning => Metallurgic: the application of a constraint changes the energy stored in the material and the atomic structure of the network. The presence of mechanical stress can therefore play a role in metallurgical transformations.

2.4. What do we mean by melt zone, ZAT, revenue zone?#

When talking about welding, the design engineer will often hear about a molten zone, ZAT, and a tempered zone, which is three distinct areas in a welded component. For this reason, we give a very general definition of these areas below.

  • The Melted Zone (Z.F): this is the zone where the temperature is higher than the solidus of the steel considered or of the filler metal. The composition of the molten metal results from the chemistry of the filler metal, the chemistry of the base metal, and interactions with the environment. This zone is separated from the Heat Affected Zone (ZAT) by the melting line that characterizes all the points that have reached solidus temperature.

  • The Heat Affected Zone (ZAT): very generally, it is **the**solid zone**of the welded assembly where the**behavior of the material is strongly influenced by the temperature field**due to welding. The term ZAT has a very particular meaning when the base metal or filler metal material undergoes structural transformations: it is the area where the transformation takes place. This Z.A.T is itself broken down into three main parts: the zone where the transformation is complete, the zone where the transformation is partial and finally the**revenue zone (Z.R) ** where the maximum temperature reached is slightly lower than the temperature at the start of transformation but where the behavior of the material is slightly modified compared to that of the base metal.

  • The zone not affected thermally: in a very general way, it is the**solid zone**of the welded assembly where the**behavior of the material is little influenced by the temperature field. For materials undergoing structural transformations, this is the part of the welded assembly where the maximum temperature reached is much lower than the initial transformation temperature and where the welding operation does not cause a change in the metallurgical and mechanical characteristics of the base metal.

2.5. Why is it important to simulate the welding process?#

The evolution of welding processes applied to steels has now made it possible to achieve good reproducibility of the operation and a good quality of welded assemblies. All the same, welds are the weak points of structures. The fields of deformations and stresses induced by the welding operation have a decisive role on the quality and mechanical strength of the welded component; the residual stresses induced by welding accentuate all the risks of damage by fatigue, by creep, by corrosion and by fragile rupture…

This is why it is important to have robust and reliable tools to simulate this process.