Projection welding is a type of resistance welding that uses small, raised features on one of the metal parts to concentrate electrical current and heat at precise locations. Instead of relying on the electrode tip to define where the weld forms (as in spot welding), the projections themselves control where current flows, where heat builds up, and where the metal fuses. This makes it possible to create multiple welds in a single press of the machine, with consistent quality across each joint.
How the Process Works
One of the two metal parts being joined has small raised features stamped, formed, or machined into its surface. These projections can be round dimples, elongated ridges, rings, or the built-in corners of fasteners like weld nuts. When the parts are pressed together, the projections create the only points of contact between the two surfaces. Electrical current passes through these contact points, and because the current is squeezed into such small areas, resistance generates intense, localized heat.
As the projections heat up, they soften and collapse under the applied pressure. A weld nugget forms at each projection site as the molten or near-molten metal fuses the two pieces together. The entire cycle happens in a fraction of a second. Because the projections absorb and concentrate the heat, the surrounding material stays relatively cool, which produces clean welds with minimal surface marking.
Types of Projections
The shape and style of the projection depends on what’s being welded.
- Embossed projections are stamped dimples formed directly into sheet metal. These are the most common type for joining two sheets together. The dimple is sized relative to the thickness of the thinner sheet.
- Solid projections are machined features, often a V-shape or angled ridge, cut into a solid component like a bracket or block. These create an initial line of contact with the mating part and typically produce a solid-state, forge-style weld rather than a fully molten nugget.
- Annular projections are ring-shaped features found on fasteners like weld nuts and studs. The ring concentrates current around the full perimeter of the fastener, creating a strong, sealed joint in one shot.
Cross-Wire Welding
A specialized form of projection welding occurs when two round wires are placed together at 90 degrees. The crossing point acts as a natural projection, concentrating current at that single contact location without any stamped or machined features. This technique is widely used to manufacture wire mesh, fencing, grill grates, refrigerator shelves, and similar grid-based products.
How It Differs From Spot Welding
Spot welding and projection welding are both resistance welding processes, but they control heat in fundamentally different ways. In spot welding, the electrode tip determines the weld location and size. The electrodes must be precisely shaped, and each weld cycle produces one joint at a time. In projection welding, the part geometry does the work of locating and sizing the weld. The electrodes can be flat-faced because they only need to deliver current and pressure evenly across the projections.
This difference creates several practical advantages. Projection welding electrodes carry more current than spot welding electrodes and can join thicker materials. Because the projections control heat distribution rather than the electrode face, electrode wear is slower and more predictable. Multiple projections can be welded simultaneously in a single cycle, which dramatically speeds up production when a part needs several attachment points. The heat balance is also more uniform, since each projection generates its own controlled heat zone rather than depending on electrode positioning.
Projection Sizing and Design Rules
Getting the projection dimensions right is critical. Industry schedules for low-carbon steel tie projection diameter and height to the thickness of the thinner sheet being joined. For example, a sheet 0.031 inches thick calls for a projection diameter of 0.094 inches and a height of 0.022 inches. A thicker sheet at 0.125 inches needs a 0.281-inch diameter projection standing 0.060 inches tall. For sheets 0.250 inches and above, projection height plateaus at around 0.110 inches while diameter continues to scale up.
A few key design rules govern the process. The projection should always be placed on the thicker piece. Sizing is based on the thickness of the thinner piece. The maximum thickness ratio between the two parts being joined is 3 to 1. Tolerances are tight: for material under 0.050 inches thick, projection diameter must be held within ±0.003 inches and height within ±0.002 inches. Thicker material allows slightly more room, at ±0.007 inches on diameter and ±0.005 inches on height.
What Goes Wrong
Most projection welding defects trace back to three root causes: current problems, alignment problems, or projection consistency.
If welding current is too high, you get expulsion, where molten metal sprays out from the joint. This also accelerates electrode wear and leaves deep surface indentation. If current is too low, the projections never reach fusion temperature and the weld fails under load. Dirty or coated surfaces create inconsistent current flow and increase electrode degradation. Poor electrical connections anywhere in the circuit, from loose cables to oxidized contacts, can mimic low current conditions and produce weak joints.
Projection consistency matters just as much. If one projection is larger than designed, it heats slowly and may not collapse in time to form a proper nugget. If a projection is undersized or missing entirely, it overheats, expels metal, or collapses before a nugget can develop. When multiple projections are welded at once, the electrode pressure must force all of them into contact simultaneously and at equal force. If even one projection contacts first, it carries most of the current and overheats while the others starve.
Materials and Compatibility
Low-carbon steel is the easiest and most common material for projection welding. The process also works well with stainless steels, nickel alloys, and titanium. Advanced high-strength steels used in automotive manufacturing are regularly projection welded, though they require more careful parameter control due to their higher strength and different thermal behavior.
Coated materials like galvanized steel present additional challenges. The zinc coating creates surface contamination that accelerates electrode wear. Research on welding steel nuts to galvanized advanced high-strength steel found that both common electrode materials, a 75/25 tungsten-copper blend and a beryllium-free copper alloy, develop an oxidized surface layer and pitting as weld count increases. Electrode design and maintenance become more important with coated materials to maintain consistent weld quality over long production runs.
Aluminum, copper, and magnesium are more difficult to projection weld. Aluminum is prone to porosity and cracking. Copper welds only fair, with similar porosity concerns. Magnesium requires specialized techniques and equipment. High-carbon steels above 0.50% carbon also present challenges due to their tendency to form brittle weld zones.
Common Applications
Projection welding is a workhorse in automotive manufacturing. Weld nuts, weld studs, and brackets are attached to body panels and structural components using annular and embossed projections, often dozens per vehicle. The ability to create multiple welds in a single machine cycle makes it ideal for high-volume production lines where speed and repeatability matter.
Beyond automotive, projection welding is used in appliance manufacturing, construction hardware, electronics enclosures, and wire products. Any application that requires fasteners attached to sheet metal, wire grids assembled from crossing wires, or multiple consistent joints made in a single operation is a natural fit for the process.