COMPUTATIONAL WELDING MECHANICS

Modeling a Weld Heat Source

Having specified the data available to characterize the weld heat source, the next step is to decide how to use the available data to compute the transient temperature field of the weld in the structure being welded. The most popular approaches are listed below. They are roughly in chronological order because the more recent generations have usually extended previous generations.

Most welding processes use a heat source such as an arc, plasma torch, laser or electron beam. The temperature field due to heat from the heat source melts the base metal creating a weld pool. In addition, filler metal from an electrode is often added to the weld pool. The weld pool moves along a weld joint and as the weld pool solidifies, it creates one pass of a weld joint. The temperature field driven by the weld heat source is the dominant driving force of the welding process. It causes phase transformations, thermal strain and thermal stress, distortion and residual stress. To analyze or predict the behavior of a weld in a structure, this transient temperature field must be computed with useful accuracy. The transient temperature outside of the weld pool depends primarily on the distribution of energy from the heat source and the conduction of heat away from the weld pool by conduction in the solid. Stress and strain usually have little effect on the transient temperature field.

We will assume that the heat source is an arc unless otherwise stated. However, almost everything we say would apply equally well to most other weld heat sources.

Because the physics of the weld heat source are often complex, thermal models of weld heat sources have been developed to reduce the complexity to more manageable levels. These models allow the heat equation or energy equation to be solved for an approximate solution while ignoring most of the physics of the welding process. Except in the interior of the weld pool itself, these approximate solutions can be remarkably accurate if they are given good data. If the physical phenomena in the interior of the weld pool are of interest, then a solution of such a thermal model is usually the starting point for solving the equations that model other physical phenomena in the weld pool, such as stirring of the liquid metal.

We will classify weld heat source models into categories of First Generation to Fifth Generation. First Generation is the oldest and simplest and Fifth Generation is the newest and most complex. Each older generation is a sub-class of every newer generation. By removing features from a younger generation one moves to an older generation. By adding features to an older generation, one moves from an older generation to a younger generation.