As with most procedural based technical systems, inevitable variability within the process leads to the potential for quality issues and welding is no different in this regard.
Welding defects can be numerous but most commonly it is possible to encounter porosity issues, contaminant entrapment, inadequate shielding, arc related issues to name but a few.
Numerous application-specific products are manufactured by the joining of two or more similar or dissimilar materials. Welding is one of the most common joining methods used in the case of metallic materials. The most common welding methods are gas metal arc welding (GMAW) or Manual Inert Gas (MIG) welding, gas tungsten arc or tungsten inert gas (TIG) welding, plasma arc welding, laser welding, electron beam welding. Generally, process selection is based on the product specification, process capability and cost. The above mentioned welding processes are routinely used in industries for the fabrication of almost all types of metallic products.
However, as with any other welding process, weld defects can occur. Most defects encountered in welding are due to an improper welding procedure. Once the causes are determined, the operator can normally and relatively easily correct the problem.
Porosity is a collective name describing cavities or pores caused by gas and non-metallic material entrapment in molten metal during solidification. There are many causes which include contamination, inadequate shielding, unstable arc, arc gap too short and poor welding technique in general. Porosity can be minimized in many different ways – by the proper selection of electrodes and/or filler materials, improved welding techniques, more attention to the work area during weld preparation and a slower speed to allow gasses time to escape.
The effects of porosity on performance depend on quantity, size, alignment, and orientation to stresses. When clustered at the weld’s center, porosity is not considered a dangerous fatigue promoter, or detrimental to fatigue resistance, although it may reduce the static stress carrying capacity of the weld.
Figure 1: Porosity
Undercutting is one of the more severe welding defects. It is essentially an unfilled groove along the edge of the weld. The causes are usually associated with incorrect electrode angles, incorrect weaving technique, excessive current and travel speed. Undercutting can be avoided with careful attention to detail during preparation of the weld and by improving the welding process. It can be repaired in most cases by welding up the resultant groove with a smaller electrode.
Figure 2: Undercut
Incomplete fusion or penetration is an internal planar discontinuity that is difficult to detect and evaluate, and very dangerous. It occurs when the weld metal does not form a cohesive bond with the base metal or when the weld metal does not extend into the base metal to the required depth, resulting in insufficient throat thickness.
Figure 3: Incomplete fusion
The protrusion of weld metal beyond the weld toe or weld root is caused by poor welding techniques and can generally be overcome by an improved weld procedure. The overlap can be repaired by grinding off excess weld metal and surface grinding smoothly to the base metal.
Figure 4: Overlap
Metal drops expelled from the weld that stick to surrounding surfaces. Spatter can be minimized by correcting the welding conditions and should be eliminated by grinding when present.
Figure 5: Spatter
References
1. Lahiri B.B., Bagavathiappan S., et al: Defect detection in weld joints by Infrared thermography, NDESAI 2011, December 2-3, Jamshedpur India, p.191;
2. Benton S.W.: Welding Defects, Accesed 03-2014;
3. Baughurst L., Voznaks G.: Welding defects, causes and correction, Australian Bulk Handling Review: July/August 2009, p.26-28