Resistance Welding
Resistance welding is a thermo-electric joining process that uses pressure and high electric current at low voltage to heat the faying surfaces of metal parts until they fuse locally, forming a weld nugget without filler metal. Common forms include spot, projection, seam, and butt/flash welding. The workpieces are clamped between electrodes, and current is passed for a controlled time while electrode force confines the weld zone and forges the joint as it solidifies.
On a CNC cell or automated welding station, resistance welding is programmed as a controlled schedule of force, current, and time so each weld point receives the same thermal input and electrode loading. CAM/CNC control can regulate these parameters point by point, ensuring repeatability across sheet, wire, brass, and steel parts in high-volume assemblies. The process is useful when minimal thermal distortion matters and no filler material is wanted, because heating is concentrated at the contact interface rather than spread through a larger arc zone. In production, weld quality is validated by shear tests, pull tests, visual inspection, and by monitoring current, force, displacement, and nugget size. Weld engineering starts by defining allowable deformation, nugget diameter, cosmetic limits, and tensile/shear strength targets, then correlates those requirements to the weld schedule and monitor limits. Material conductivity, thickness, surface contamination, and electrode wear all affect resistance, heat generation, and joint strength.
- Inadequate electrode force: Low clamp force destabilizes contact resistance, causing expulsion or overheating, while excessive force reduces resistance and underheats the joint, producing a weak or undersized nugget.
- Incorrect current/time schedule: Too little current or too short a time yields incomplete fusion and low pull strength; too much current or excess time causes oversize nuggets, indentation, spatter, and distortion.
- Poor surface condition or worn electrodes: Oxides, oils, coatings, or tip wear alter interface resistance and current density, shifting production from repeatable nuggets to inconsistent, rejected joints.
Why does resistance welding use low voltage but very high current?
Heat is generated by electrical resistance at the interface, so the process relies on very high current density through a small contact area rather than high voltage; systems commonly operate with currents in the kiloamp range.
What controls weld nugget formation most directly?
The primary variables are force, current, and time; these govern interface resistance, heat input, and the solidification conditions that form the nugget.
What process data should a shop monitor?
Peak current, weld time, electrode force, displacement or set-down, nugget diameter when applicable, and post-weld test results such as shear or pull strength.