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Welding defects can cause serious problems: welding is a very important process and it must be executed with precision, but the welding joint could turn out to be a potential point of weakness in the structure. Let’s see together which are the most common welding defects, what they involve and how to avoid them.

Welding visual defects

The UNI EN 6520-1 standard specifies that there are two types of errors: imperfections,that is a deviation from the optimal welding, and defects, not acceptable as they can compromise the strength of the welding joint.

Welding defects that are visible or detectable through non-destructive testing are divided into six categories: shape and size defects, cavities, solid inclusions, lack of fusion and penetration, cracks.


Shape and size defects

This is an error indicating that a weld has not been carried out correctly and has an irregular weld seam. The most frequent are:


  • excess of overmetal: has a very high weld seam, resulting from the use of too high current and incorrect preparation of the joint. Due to the thick shape of the overmetal, tensions are concentrated at the edges, which can damage the joint.
  • incomplete filling: as opposed to excess, it presents excessive penetration or insufficient material ratio
  • marginal carvings: in manually welded joints, due to incorrect use of the torch, grooves may be found at the edge of the cord
  • arc strike on the weld: in this case, the welder triggers the arc directly on the material generating a localized fusion that can be accompanied cracks
  • splashes of melted material: deposits of material that can cause corrosive phenomena


They are formed in the absence of material, when the gas is trapped in the melting bath due to very rapid cooling. Depending on the size may be pores, blowholes or worm holes.

To avoid the formation of cavities, the speed must be slowed down, allowing the gas to escape.

Solid Inclusions

Similar to the cavities but, in this case, what remains trapped is an external material: it can be slag from the solidification of molten metal or, if used, from the electrode. It is very common in TIG welding, as tungsten from the torch can get trapped.

Lack of fusion and penetration

The lack of fusion and penetration, although for different reasons, have similar results: the absence of continuity between the edges and the melted area, occurring more in case of very complex geometries.
They arise due to too high pass speed or too low current usage.

The glues are very similar, where a layer of oxide material is placed between the flap and the molten area. This is a typical defect of MAG welding, as it uses active oxidizing gases.

These types of defects are difficult to detect by non-destructive testing: the only way is to put in place preventive measures to avoid their formation.

Hot and cold cracks

Cracks are the biggest defects of a weld: depending on the size and the stresses which the joint is subjected, they can cause a breakage.

They are divided into two categories:

  • hot cracks: these are generated by a high presence of impurities in the base material at high temperatures. To avoid the formation of cracks, it is necessary to maintain a not too low processing speed and the right intensity.
  • cold cracks: are formed during a too fast cooling due to the hydrogen present in the melting tank. To avoid these cracks, preheat the weld and check that there is no humidity.

Welding control is an extremely important action: defects, visual or otherwise, can compromise the final job and lead to breakage.

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