Resistance Welding Guide for Sheet Metal Fabrication
Resistance welding uses electrical current and mechanical pressure to join metal sheets without filler material. It's the dominant joining method in automotive and appliance manufacturing due to its speed, consistency, and automation potential.
Types of Resistance Welding
| Type | Application | Typical Thickness | Welds/Min |
|---|---|---|---|
| Spot welding | Sheet metal overlap joints | 0.5-3.0mm each side | 30-120 |
| Seam welding | Continuous leak-tight joints | 0.5-3.0mm | 1-3 m/min |
| Projection welding | Pre-formed projections, studs | 0.8-6.0mm | 15-60 |
| Flash butt welding | End-to-end joining of bars | 10-200mm² cross section | 2-10 |
Process Parameters
- Weld current: 6,000-15,000A for 1.0mm steel. Higher for thicker material or more conductive metals (Al, Cu).
- Weld time: 8-20 cycles (50/60Hz). Shorter for thin sheets, longer for thick.
- Electrode force: 200-500 N for 1mm steel. Insufficient force causes expulsion. Excess force causes indentation.
- Hold time: 5-30 cycles after current stops. Allows nugget to solidify under pressure.
Nugget Quality Criteria
For structural welds, minimum nugget diameter = 4√t (where t = sheet thickness in mm). Nugget shear strength should exceed 3.5 × t × weld diameter × UTS of material. Destructive peel testing is the standard quality verification method.
Electrode Maintenance
- Electrode tip face diameter grows 10-30% before dressing is needed
- Dress frequency: every 500-2000 welds depending on coating and material
- Tip material: Cu-Cr-Zr alloy for steel, Cu-Al2O3 dispersion-strengthened for coated steels
- Water cooling (4-8 L/min) essential. Maintain outlet temp below 50°C.
Common Defects
- Expulsion: Molten metal ejected from weld zone. Causes: high current, low force, contaminated surface.
- Under-sized nugget: Insufficient current or weld time. Check with ultrasonic or peel test.
- Surface indentation: Excessive electrode force or small tip diameter. Reduce force by 10-15%.