Solid-projection welds are essentially strain-assisted diffusion bonds. Because the projection welding typically collapse at very high temperatures, diffusion bonding can occur within the very limited available time (usually, less than 1 s).

Solid Projections

A considerably wider range of solid-projection welding processes is commonly used in production applications. Annular projection welding, like embossed-projection welding, is commonly used to provide a highly localized joint and to minimize thermal damage to other parts of the structure.

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Common variations of the projection welding:

Annular Projection Welds:

One of the most common applications of solid-projection welding is to attach either tubular components or members with circular bases to flat or curved substrates. This is commonly accomplished by annular projection welding, in which the projection is machined onto the circular base or tubular section.

Cross-wire welding is a variation of solid-projection welding in which the projection is formed by the contact point of two crossing wires. Upon resistance welding, heat is maximized at the location of the wire-wire contact, and the parts are subsequently forged together. Depending on the application, either highly localized joints (minimal forging) or heavily forged joints can be made.

Edge-to-sheet welds are typically a cross between embossed- and solid-projection welding. They are used to attach the end of a sheet to the flat face of an opposing sheet. The projections for welding are generally stamped into the face of the attaching sheet. During welding, the projection does collapse locally back into the base sheet. However, bonding is strictly solid state, similar to that of other solid-projection welding processes.

Material Effects

• Some of the material-related factors that affect diffusion bonding also affect solid-projection welding.
• The most notable of these is the ability of the metal to dissolve its own oxide.
• As a result, materials that do not energetically favor the solid solubility of oxygen, rather than the formation of the oxide at aluminum forging temperatures, will be relatively difficult to projection weld.
• The strength-temperature relationship also affects projection weldability. Materials that maintain their strengths at relatively high temperatures permit substantial heating to occur before projection collapse.
• This heat then becomes available to promote diffusion after projection collapse.
• Premature collapse results in lower temperatures in which diffusion can occur and in reduced current density, which prohibits further resistance heating.

• Bulk resistivity also plays a role in projection welding, but to a lesser degree. Increased bulk resistivity can reduce the effectiveness of the projection as a current concentrator. With increasing bulk resistivity, the tendency is for delocalized heating and general, rather than local, collapse of the projection. As a result, high-resistivity materials are more difficult to projection weld.
• Mild steels and low-alloy, nickel-base alloys are ideal materials for projection welding, because they readily dissolve their own oxides and have adequate strength-temperature and resistivity properties.
• Stainless steels and higher-alloy nickel base materials become slightly more difficult to weld, because of the formation of more-stable chromium and aluminum oxides, increased high-temperature properties, and higher resistivities.
• Projection welding is commonly applied to copper and copper-base alloys. In many applications, projections are virtually required for resistance welding, because of the high conductivity of these materials.
• On the other hand, aluminum and aluminum-base alloys are very difficult to projection weld. The aluminum oxide is so tenacious that solid-projection welding, in particular, is nearly impossible. In addition, it is very difficult to localize heat, because aluminum alloys soften at such low temperatures