1996 Volume 14 Issue 3 Pages 502-509
The effects of surface treatment of skin steel sheets on resistance spot weldability were examined about electro-conductive vibration damping steel sheets (VDSS) which contained metal particles in resin layer. Zn-Fe coating was applied as a surface treatment. Though electric resistance between the skin sheets decreased, pin-hole became easy to be formdd due to the existence of the coated layer. And, that was promoted by increase in coating thickness. The pin-hall had a shape of the hollow which spread parallel to the sheet surface. This was very different from a globular type of pin-hall which was seen in VDSS made of bare skin sheets. When the pin-hall occurred a ralatively long period of high voltage (about 20 V) was observed on voltage wave form. Those results suggested that the pin-hole was generated by arc electric discharge, however that of VDSS made of bare skin sheets was generated by joule heating. Arc discharge was occurred by opening an electric circuit between metal particle and skin sheet due to a evaporation of coated layer.
On the other hand, it is well known that electrode face is deformed by a number of spot welds on coated steel sheets. Using such deformed electrodes, another type of defect that was local fusing of skin sheet adjacent to electrode was observed. This phenomenon was caused by delay of exclusion of resin.
From above test results, it was clear that the coated steel sheet used as the skin sheets was promoting weld defects. However, good weldability of VDSS made of coated skin sheets was achieved by optimizing adding condition of metal particles.