Porosity: tiny voids in the weld metal. Not as serious as other defects. Can be caused by contaminants either in the metals or from the atmosphere due to lack of shielding. May also be caused by overheating.
Inclusions: voids filled with either tungsten from the GTAW process, or slag, the byproduct of the flux used in the SMAW and FCAW processes. Processes without tungsten or flux, such as GMAW or OAW, are immune from inclusions.
Cracks: serious defects which are almost never acceptable.
Segregation-induced cracking is caused when impurities (such as phosphorous, zinc, copper or sulfur), which have a lower melting point and therefore remain molten as the rest of the metal in the weld admixture cools and solidifies, migrate to the center of the weld and force apart the surrounding metal. Likelihood of this type of cracking can be reduced by using metals with fewer impurities, limiting the penetration and depth of fusion into a joint, using larger diameter electrodes with less amperage, and keeping weld passes wider than they are deep.
Overlap: when the molten weld pool spills out of the joint onto the base metal, usually due to loss of surface tension when the weld pool becomes too large. Overlap can create a stress concentration, effectively weakening the joint.
Undercut: when not enough weld metal is deposited and the thickness of the material is less than it should be at the weld toes.
Incomplete fusion: when either the base metal or weld metal fail to melt properly and the metals fail to fuse together. May be caused by not enough heat, incorrect electrode angle, erratic travel speed, buildup of slag or molten metal, or impurities on the base metal. Both slag/molten metal buildup and base metal impurities cause incomplete fusion by inhibiting heat transfer to the base metal. GTAW is the only type of arc welding for which welds may be fixed by reheating and blending the base and filler metals together; any other arc weld with incomplete fusion would require grinding out and re-welding.
Incomplete penetration: when a CJP groove weld has incomplete fusion. May be caused by bad joint design with shallow groove angles or small root openings. See "incomplete fusion" for other possible causes.
Excessive convexity (fillet welds) / excessive reinforcement (groove welds): when too much weld material has been deposited, resulting in an over-sized weld. Not only is weld metal wasted, transition at the toes of the weld is sharp, creating stress concentrations. See also post on stress concentration due to permanent backing.
Excessive penetration (a.k.a. burn through): a hole created by too much heat for the thickness of the material being welded. Can be avoided by reducing torch tip or electrode size, reducing machine settings like voltage or amperage, controlling overall heat put to the base metal, or using faster travel speeds. Regarding joint geometry, reducing root openings and/or reducing chamfers on bevels can also help.
Arc strikes / spatter: can cause heat affected zones (HAZ) to form in the base metal away from the weld, or for sensitive base metals, may even cause a base metal failure. Can be avoided with proper machine settings and welding technique.
