Area-Like Lack of Fusion
Large-scale absence of fusion between weld beads or to the base metal, leading to immediate loss of strength.
A lack of fusion is simply called: The weld only sticks to the base material without properly fusing with it. This happens when you weld too cold, travel too fast, or don't introduce the heat properly into the part. The result is a weld that sits on the material like a glued-on patch. Under load, something will definitely break off because the connection has no real load-bearing capacity. That thing is a classic reject defect. To avoid this: Make sure you have a clean, shiny weld pool that properly wets the base material. If the edge doesn't melt properly, you're in trouble. If in doubt, check the weld with a chisel or by ultrasonic testing.