Lack of Fusion is commonly called LOF, IF, LORF, LOSF. LOF stands for Lack of Fusion, IF stands for Incomplete Fusion, LORF stands for Lack of Root fusion and LOSF stands for Lack of Sidewall Fusion.
Lack of fusion can occur when the weld metal fails to fuse completely with the sidewall of the joint or fails to penetrate adeequately the previous weld bead.
The principal causes are too narrow a joint preparation, incorrect welding parameter settings, poor welding techniques, magnetic arc blows and insufficient cleaning of joint
To prevent lack of fusion, the joint design should be sufficiently wide, maintain correct welding parameters, use correct welding techniques (electrode angle/good manipulation technique), avoid arc blows and clean weld joints.