TWI Industrial Member Report Summary 17/1976
By S L Roswell
Background
The current emphasis on corrosion protection has led to a gradual replacement of mild steel by coated steels, particularly hot-dipped galvanised steel. The major problem when resistance spot welding this material is electrode wear. The present work has determined weldability lobes for various gauges and grades of hot-dipped galvanised steel and examined the effect of weld time on electrode life. The general shape of the weldability lobes are in agreement with other researchers and the life test results indicate that with thin gauge material (0.7mm, 190g/m2 coating weight) longer electrode lives are obtained at a short (4 cycles) than at a longer (8 cycles) welding time. Metallurgical investigations have shown that copper from the electrodes alloys with the free zinc on the sheet surface and results in liquid metal penetration around the steel grain boundaries. This effect appears to increase with increasing weld time.
An investigation into the corrosion properties of the weld zone has also been carried out when welding galvanised sheet. The results show that, although welds in mild steel had completely corroded after 12 months exposure in a marine environment, the rate of corrosion of resistance spot welds in galvanised steel was negligible during the first two years exposure in both rural and marine environments.