15 GMAW Defects and Discontinuities
A discontinuity is an interruption of the typical structure of a material, such as a lack of its mechanical, metallurgical, or physical characteristics; any change to the normal pattern of work. A defect is a flaw in a part or product that is unable to meet minimum acceptance standards. The part or product can be rejected. Defects need to be either repaired or thrown away. All defects are discontinuities, but not all discontinuities are defects.
To ensure high-quality GMAW welding, follow proper welding procedures, maintain good welding technique, and implement effective quality control measures. To reduce all chances of discontinuities and defects, ensure regular inspections, proper material preparation, appropriate welding parameters, and skilled welder training.
The most common defects and discontinuities in GMAW include:
Overlap
Overlap is the protrusion of weld metal beyond the weld toe or weld root. It usually occurs when the arc does not melt the base metal sufficiently, causing the weld pool to flow onto unwelded base metal without fusing. Often, overlap occurs when the weld pool is allowed to become too large.
To prevent overlap, the size of the weld pool can be reduced by increasing the travel speed or reducing the wire feed speed. Also, heat input can be increased.
Porosity
Generally, surface porosity is the direct result of atmospheric contamination. Atmospheric contamination occurs if the shielding gas level is set either too low or too high. If the shielding gas level is too low, the air in the arc area is not fully displaced. If the shielding gas level to too high, it causes turbulence, which draws air into the weld zone. On occasion, porosity occurs if welding is performed in a windy area. Without a protective wind shield, the shielding gas envelope may be blown away, exposing the molten weld pool to the contaminating effects of the air.
Subsurface porosity is caused by moisture in the shielding gas, an excessive tip-to-work distance, or surface contamination such as rust, paint, dirt, or oil on the base metal.
To prevent porosity:
- Work with clean material and well-maintained equipment
- Don’t use excessive current or arc length
- Change things like gas flow rate and purity
- Reduce travel speed
- Don’t forget to turn gas on before you start welding
Crater cracks
Possible causes of cracks include moisture in the shielding gas, excessive tip-to-work distance, improperly filled concave craters, and contaminants such as rust, paint, dirt, or oil on the base metal.
Insufficient Penetration
Insufficient penetration (lack of penetration) is due to low heat input in the weld area, failure to keep the arc properly located on the leading edge of the weld pool, or traveling too fast. If the heat input is too low, increase the wire feed speed to increase the welding current.
Excessive Penetration
Excessive penetration (excessive melt-through) is caused by excessive heat in the weld zone or too wide a root opening. Excessive penetration results in a weld root bead that protrudes below the bottom of the joint.
Problems with excessive penetration can be minimized by reducing the wire feed speed, which lowers the current, or by increasing the travel speed. Too wide a root opening can be compensated for by increasing electrode extension and depositing the root pass with a weaving motion.
Whiskers
Whiskers are short lengths of electrode wire sticking through the weld joint. Whiskers are caused by pushing the wire past the leading edge of the weld pool. A small section of wire protrudes inside the joint and becomes welded to the deposited metal.
To remedy this defect, reduce the travel speed, increase the contact tip-to-work distance slightly, or reduce the wire feed speed.
