Interpass lack of fusion

Lack-of-fusion defects occur when the individual layers in multi-pass welding don't fuse properly with each other. The usual culprits are slag residues or dirt between the layers, which prevent clean melting. Also, if the interpass temperature is too low, you won't get a proper bond. The result is a weld that has voids and cold spots inside. It won't withstand any load and is prone to cracking. So: between each layer, thoroughly remove the slag, preferably with a chipping hammer and wire brush. And make sure the part doesn't get too cold between layers, then it will fuse cleanly.