Incomplete fusion is a lack of penetration or fusion between the weld metal and the parent metal of the piece. Welds with incomplete fusion are weak and substandard welds at best (if not downright dangerous).
Here are a few of the most common causes for incomplete fusion, and solutions that will help you avoid the problem.
Cause: Not enough weld material deposited to fill the weld joint.
Solution: Keep making passes with your welder until you fill the weld joint completely with your weld metal. Don't stop until the job is done!
Cause: Gaps between your weld beads or spaces at the root (end) of a joint.
Solution: When you're making a welding pass, be sure to use enough weld metal to completely fill the space between the previous bead and the one you're working on. Also make sure that you deposit the weld metal all the way to the very end of the joint, and if there's a crater at the end, fill it.
Cause: Dirty surfaces.
Solution: Before you begin welding, make sure your metals are clean and free of rust and grease. Then, in between welding passes, clean your weld to ensure that no slag (trapped material) from a previous pass will be incorporated into subsequent passes.
