Projection Welding: Process, Parameters and Defects
What Is Projection Welding?
Projection welding is a resistance welding process that concentrates current and electrode force at one or more raised features on a component. These projections may be embossed in sheet, machined into a solid part, or formed by the natural crossing of wires. As current flows, resistance heating softens and collapses the projection, producing a localized weld while the electrodes continue to apply force.
The process is widely used to attach weld nuts, bolts, studs, brackets and electrical contacts. It suits repeatable production because the component geometry determines the weld location. Several projections can sometimes be welded in one cycle, provided the tooling, current distribution and projection dimensions are consistent.
How Projection Welding Works
A projection weld develops in a short sequence. First, the electrodes clamp the parts and establish electrical contact. Current then flows through the restricted contact area at the projection. Heat rises rapidly, the projection begins to soften, and the welding head follows its collapse. Current stops after the required nugget forms, but force remains during the hold stage while the joint solidifies.
Four elements govern this sequence: current, force, time and projection geometry. Cooling, electrode alignment, surface condition and machine follow-up determine whether those inputs reach the joint consistently. A setting that works with one nut design or coating cannot be assumed to work with another.
Types of Projection Welding
| Projection type | Typical part | Main production concern |
| Embossed projection | Sheet-metal brackets, stamped assemblies | Projection height, base diameter and consistency after forming |
| Solid projection | Weld nuts, bolts, studs and bosses | Projection geometry, thread protection and head follow-up |
| Annular projection | Tubes, bosses and leak-resistant circular joints | Uniform ring contact, alignment and circumferential collapse |
| Cross-wire projection | Wire grids, baskets and electrical components | Wire diameter, crossing alignment and surface condition |
Embossed and solid projections are not interchangeable design ideas. An embossed feature depends on the sheet supporting the applied force without premature collapse. A solid nut or stud can carry more load, but poor location or an oversized projection can distort threads, mark the reverse surface or shift the weld nugget.
Projection Welding Parameters
Welding Current
Current supplies the heat that forms the weld. Too little current leaves an undersized nugget or an unfused projection. Too much current can cause expulsion, electrode pickup, excessive indentation or damage to nearby threads. Current demand rises with weld area, material conductivity, coating and the number of projections energized at the same time.
Electrode Force
Force brings the surfaces into controlled contact and contains the molten metal. Low force encourages expulsion and erratic resistance. Excessive force can flatten an embossed projection before sufficient heat develops, spreading current over a larger area and producing a weak joint. Dynamic follow-up matters as much as the initial force because the head must move while the projection collapses.
Weld Time, Squeeze Time and Hold Time
Squeeze time allows the machine to reach force before current starts. Weld time controls how long heat is generated. Hold time keeps the joint compressed while it solidifies. Short squeeze time may start current before stable contact is achieved; long weld time can overheat the assembly; short hold time can leave a hot joint vulnerable to separation or cracking.
Projection Geometry and Part Fit-Up
Projection height, angle, diameter and position define the initial current density. Variation from the forming die, worn tooling or part handling changes the heat pattern even when the machine settings stay fixed. Flat mating surfaces, accurate holes and a repeatable location pin are essential for weld nuts and bolts. Geometry should follow the applicable component and welding standards rather than a copied shop sketch.
How to Develop a Projection Welding Schedule
Start with the material grade, coating, thickness, projection design and required joint strength. Confirm that the machine can supply the necessary current and force without operating at its limit. Use clean production parts and the actual tooling, then establish a trial window rather than searching for one “perfect” setting.
A practical development sequence is:
- Verify projection dimensions, flatness, hole position and part orientation.
- Set squeeze time and force so every projection contacts the mating part evenly.
- Increase current or weld time in controlled steps until the required nugget develops.
- Check for expulsion, thread damage, reverse-side marking and unequal collapse.
- Perform destructive tests and record the acceptable current-force-time window.
- Challenge the window with normal production variation, including coating and part tolerances.
For multiple projections, equal current sharing is the real test. One projection contacting first may take most of the current while another remains cold. Tooling parallelism, projection tolerance and component stiffness often fix this problem more effectively than another parameter change.
Projection Welding Equipment and Tooling
Projection welding typically uses a stationary resistance welding machine with a rigid frame, controlled force system, transformer or inverter power source, cooling circuit and flat electrodes. Portable equipment exists for specialized work, but repeatable multi-projection production favors a rigid machine with accurate alignment and responsive follow-up.
Electrode material, face size and cooling should match the work. Copper alloys are common for steel applications, while harder inserts may be used where concentrated wear occurs. The locating pin for a weld nut is commonly insulated so current follows the intended projection path rather than damaging the pin or threads. Electrode faces must remain flat and parallel.
Projection Welding vs Spot Welding
| Decision point | Projection welding | Spot welding |
| Weld location | Defined by a formed or solid projection | Defined by electrode position |
| Typical parts | Nuts, studs, brackets, wires and stamped features | Overlapping sheets and body panels |
| Multiple welds | Several projections may weld in one cycle | Usually one electrode pair creates one spot at a time |
| Tooling demand | Projection and locating geometry are critical | Tip shape, access and electrode alignment are critical |
| Surface marking | A broad electrode can reduce marking on one face | Electrode indentation is common on both faces |
Projection welding is usually the better choice when the component already includes a controlled feature such as a weld nut or embossed dimple. Spot welding is simpler for plain overlapping sheet. Neither is automatically stronger; joint design, nugget development and test requirements determine strength.
Projection Welding vs Seam, Arc and Laser Welding
| Process | Choose it when | Trade-off |
| Projection welding | Discrete, repeatable attachments are produced at volume | Requires controlled projections, access from both sides and dedicated tooling |
| Seam welding | A continuous or leak-resistant seam is required | Roll-electrode access and heat buildup must be managed |
| Arc welding | Joint geometry varies or a continuous structural weld is needed | More operator or automation path control and greater total heat input |
| Laser welding | Precision seams, one-sided access or low total heat input drive the decision | Higher capital cost, tighter fit-up and laser safety controls |
The original part should decide the process. A weld nut on an automotive bracket is a natural projection-welding application. A sealed tank seam belongs to seam welding. A long structural fillet suits arc welding, while a precision enclosure seam may favor laser welding. Comparing only machine price or headline speed usually leads to the wrong answer.
Common Projection Welding Defects and Solutions
| Defect | Likely cause | Corrective action |
| Incomplete weld | Low current or time, excessive force, poor contact | Verify projection geometry and contact; restore the validated current-force-time window |
| Expulsion or spatter | High current, low force, short squeeze time or poor follow-up | Increase contact stability; correct force and timing before reducing current blindly |
| Unequal projection collapse | Part tilt, inconsistent projections or electrode misalignment | Inspect tooling, flatness, location and projection dimensions |
| Electrode sticking | Excess heat, coating pickup, wrong electrode material or poor cooling | Restore cooling, dress or replace electrodes and review the schedule |
| Thread damage | Expulsion, current through the pin, misalignment or excessive heating | Use an insulated locator; correct alignment, force, time and current |
| Weak torque or push-off result | Small nuggets, one-sided current sharing or contamination | Section the welds, verify all projections and tighten part/process controls |
Projection Welding Quality Control
Quality control should connect machine signals with physical test results. Monitor current, voltage, force, displacement or dynamic resistance when the controller supports it, but do not accept the waveform by itself. A stable trace can repeat a consistently weak weld if projection geometry or material has changed.
For weld nuts, destructive torque, push-off or peel testing is commonly used during setup and at defined production intervals. Cross-sections reveal nugget position, fusion area and unequal projection collapse. Visual checks should cover expulsion, indentation, thread condition, alignment and coating damage. Test frequency and acceptance values belong in the control plan, drawing or applicable standard.
Where Projection Welding Is Used
Automotive and transport manufacturers use projection welding for weld nuts, studs, brackets and selected electrical assemblies. Appliance and general fabrication plants use it for fasteners, supports and stamped sheet components. Cross-wire welding serves guards, racks, baskets and wire products. Electrical manufacturers use small resistance joints for contacts, terminals and lead-frame assemblies.
Aerospace use should not be presented as a broad default application. Where projection welding is considered for a flight or engine component, the material, design authority, qualification plan and traceability requirements govern the decision. The same caution applies to pressure equipment and safety-critical structures.
Conclusion
Projection welding succeeds when the projection and the machine schedule are treated as one system. Stable current, force and time cannot compensate for a tilted nut, worn die or uneven projection height. Validate the joint with destructive tests, establish a production window and monitor both the process signal and the part condition.
If your component is difficult to access from both sides, needs a continuous seam or cannot accommodate a formed projection, laser welding may deserve a sample comparison. SENFENG can review the part drawings, material, joint access and cycle target to determine whether a handheld or automated laser process is relevant. Projection welding remains the better tool when a fastener or stamped projection already fits the production design.
FAQ
What is projection welding used for?
It is commonly used to attach weld nuts, bolts, studs, brackets and electrical contacts, and to join crossed wires in repeatable production.
What are the main projection welding parameters?
The main variables are welding current, electrode force, weld time, squeeze and hold time, projection geometry, electrode condition and cooling.
How is projection welding different from spot welding?
Projection welding localizes heat with a raised feature on the part. Spot welding localizes the weld mainly through the electrode tips placed on overlapping sheets.
How is a projection weld tested?
Use the test required by the component, such as torque, push-off, peel or cross-section testing. Visual inspection alone cannot confirm every projection fused correctly.
Can several projections be welded at once?
Yes, when the machine has enough current and force and the tooling keeps every projection in equal contact. Uneven projection height can cause one weld to form before the others.
