Inadequate root penetration

Incomplete root fusion means the weld is not fully penetrated at the bottom, material is missing at the root. This usually happens when you have too little current, travel too fast, or the joint preparation is not right. The result is a weak connection that can crack under load. Make sure the root is clean and open and you set the right gap. If unsure, check the weld from the back or with a magnifying glass. Better to check once more than have a crack in the part later.