Lack of fusion in flank zone

Lack of fusion at the root means: the weld metal hasn't properly fused with the base material there, no real metallurgical bond has been achieved. This happens when you weld too cold, travel too fast, or guide the electrode incorrectly. Such a spot is dangerous because it can simply tear under load – the component fails without warning. In the workshop, that means: first prepare the joint cleanly, no scale or slag residue left in. Then put the heat in properly, don't weld too fast, and direct the arc well on both edges so everything melts properly. When in doubt, keep an eye on the weld areas and rework if unsure.