The different processes for joining metal parts can be systematically subdivided into different categories depending on their action principle. Their bond can be form-closed, frictional or positive-substance bond (figure 3). Very often, it is not possible to make a clear distinction between closing shape and frictional bond, as some processes render a clear distinction between operating principles impossible.

Figure 3: classification of weldings by their action principle

A positive substance bond is mostly inseparable, and the bond takes place only by using additional material or consumables. The most frequent types of joints in this category are adhesive, soldered, brazed and welded joints. When welding materials, one has to distinguish between fusion welding and pressure welding.

Fusion and pressure welding
Strong plastification

Fusion welding leads to a welding of the pieces by applying heat at the point of connection which fuses the pieces together and even joins a material. After the hardening of the mixed components, a solid joint occurs. Unlike fusion welding, pressure welding depends on the application of high pressures and/or high temperatures, resulting in a strong plastification and a local deformation of the pieces to be joined in the welding area so that a bond between both pieces is made. The energy required for the welding process is of a different kind for both types of procedure.

Figure 4: Process principle of metallic pressure welding based on movement

Proven energy sources here are gas, arc welding, light, electron or plasma beams.

Ultrasonic welding belongs in the category of pressure welding and uses motion and kinetic energy for welding pieces together.

Depending on the kind of motion, a distinction in metal welding between cold-pressed welding, friction welding and ultrasonic welding can be made. All three procedures show a high similarity. Ultrasonic metal welding is a combination of cold-press welding and friction welding because of its mode of action.

Figure 4 shows the different principles of cold-press, friction and ultrasonic welding. Cold-press welding takes place at room temperature. By applying high pressures to both pieces the materials weld together. A strong material deformation at the welding zone accounts for the bond.

The differences between the various metal welding processes

In the friction welder, one or both pieces rotate while they are pressed together. The frictional heat which emanates together with the static pressure causes the bond between the pieces. The back pressure required for joining the pieces in comparison to cold-press-welding is drastically reduced because of the additional rotational energy. The matching of the surfaces promotes plastification and local deformation of the pieces being welded.

A considerably lower welding pressure