Electrode life in spot welding is governed by the nugget formability of welded material and the expandability of electrode face diameter. The authors studied the effects of coating layer and electrode face diameter on nugget formation in two-stack welding using zinc and iron-zinc coated sheets. Here minimum welding current is difined as the current where nugget diameter is 4√t of sheet thickness (t).
In both coating types, minimum welding current increases as coating-layer zinc contentt increases. Therefore, the greater the coating layer, the higher the minimum welding current. This effect is apparent in 0-1.4 μm coating but rapidly diminishes beyond.
When coating thickness is constant, minimum welding current of zinc coated sheet is greater than that of iron-zinc coated sheet. The minimum welding current of iron-zinc coated sheet increases with zinc content in coating layer. This effect is weak up to 60% Zn but pronounced above 60%. Effects of coating thickness and type on minimum welding current increase with increase of electrode face diameter.
The effect of coating type correlates with coating layer melting point. Analysis of faying surface conditions shows that the lower the melting point, the faster the softening and melting of coating layer, and that the true contact area rapidly increases from the initial power application to the start of nugget formation.
In single-layer coating, coating layer melting point is considered to govern the increase in true contact area and affect minimum welding current, while increase in coating thickness and electrode face diameter enhances these effects. Similar assumption applies to multi-layer coating, though qualitatively, by combining the effects of melting point and thickness of each coating layer, without considering structural differences in the coating layer between single-and multi-layer coating.
