Heat source

Equivalent heat sources in thermal joining

Structural welding simulation in general and Simufact Welding in particular investigate the influence of heat and fixtures on the welded assembly.

One of the most important model parameters is the representation of the heat source. For thermal joining the heat source is usually represented by a so called equivalent heat source, a mathematical representation of volume heat flux inside the model and of surface heat flux on boundaries of components. This approach has been well-established for the last 35 years when it comes to calculation of welding stresses and distortions.

The shape and the heat flux distribution of a heat source are not results of the simulation, but input parameters that need to be calibrated in order to get best possible results. Depending on the quality of the calibration, equivalent heat sources allow consideration of melt pool geometry (its depth, width and length), the geometry of heat affected zone, positions of isothermal surfaces and also a number of indirect effects, like material flow and influence of surface quality.

Geometrical parameters of an equivalent heat source, as well as intensity distribution inside the heat source itself, can be calibrated using metallurgical cross-sections or temperature measurements during the process, for instance thermo-couples or pyrometric measurements. As an alternative, the resulting heat flux can be calculated using a process simulation like Simweld.

Example for equivalent heat sources
Example for equivalent heat sources. left: metallurgical cross-section, center: resulting equivalent heat source depth and width, right: resulting melt pool

 

The calculation of welding distortions induced by a moving heat source is based on incremental displacement of the heat source location. Necessary time increments can be estimated through the length of a melt pool as well as heat source moving velocity. This approach considers the effects of shrinkage of melt pool and of plastic upsetting of the material around the melt pool on distortions, which are effects usually used in flame straightening. When it comes to calculations of large models with long weld paths, this approach leads to a huge number of needed increments and to very long calculation times. Additionally, the finite element mesh along the weld path has to be relatively fine.

In such cases Simufact Welding offers a number of simplifications, beginning with the simplification of the heat source itself and continuing towards overall model simplification. Please read Simufact´s scalable approach providing more details.

 

Heat source in resistance spot welding

In a resistance spot welding model, the heat is created due to Joule’s heating, based on voltage, current and resistivity parameters of materials and surfaces involved. In such models, no equivalent heat sources are needed. Instead, the heat generation is calculated out of material and process properties, the user has to provide information on electrical current and forces applied on electrodes.

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