How do I resistance spot weld coated steels?

Frequently Asked Questions

Coated steels, mainly having zinc coatings, are readily welded provided that the important factors are understood and the process is set up appropriately.

Compared with uncoated steels, the main differences are that (1) higher electrode force and welding current are required, and (2) the coating forms an alloy with the copper of the electrode tips and reduces electrode life.

Guideline welding conditions are available in standards such as BS1140 (British Standard specification for uncoated and coated low carbon steel sheet). Electrode force is usually increased by 10 to 20% compared to the equivalent setting for uncoated steel of a given thickness, otherwise a poorer process tolerance (narrower welding range) is likely. Longer weld times are also beneficial, allowing 2 to 3 cycles for the coating to be displaced before weld growth starts. Some users prefer a double pulse condition, in an effort to control the coating removal by using an initial low current pulse. However, a single pulse is normally quite satisfactory. Substantially higher welding currents are needed compared with uncoated steel to compensate for the low interface resistance caused by the coating. The actual increase depends on the coating type and material thickness combination.

Electrode wear is a greater problem with coated steel. Firstly, it is essential to provide good water cooling. The electrode body diameter should also be a minimum of 2.5 to 3 times the tip diameter to improve cooling. Truncated cone electrodes give the longest lives. Class 2, copper/chromium/zirconium electrode materials are commonly used but the higher conductivity copper/zirconium and aluminium oxide dispersion strengthened copper alloys can reduce sticking and improve life. Electrodes must be regularly dressed or changed to prevent deterioration of weld quality. The tip diameter should be restored by cutting the cone angle but little material is removed from the tips themselves, sufficient to remove pitting and erosion wear. Care should be taken to maintain the tip alignment.

It is possible to compensate for electrode wear by progressive increase in welding current, known as stepping. This can be effective in prolonging electrode life and can be used in conjunction with electrode dressing on automated equipment. It is, however, necessary to set the interval and percentage current increase by experiment as the rate of electrode wear and its effect on weld quality is dependent on the application (material and equipment).

Treatments which impart a high resistance film on the surface, such as heavy passivated layers, should be avoided where possible. These can lead to splash, surface burning and misshapen weld formation.

Further information