Lack of fusion is the discontinuity in the weld where fusion has not occurred between weld metal and
parent metal or between adjoining weld beads so that we may have lack of side fusion, lack of inter-bead fusion or lack of fusion at the weld root. Incomplete fusion is produced during welding, most often unnoticed by a welder or an operator. After welding, it is most difficult, if not impossible, to detect it by the visual inspection or other non-destructive testing methods. It is most often detected in bend testing of the welded joint when the fracture occurs at the location of lack of fusion in spite of a relatively low load applied. The defect will usually run along the weld interface or individual beads, and thus indicate that there really is lack of fusion.
The main reason for the occurrence of lack of fusion is insufficient energy input at the weld area. Consequently, the parent metal in the weld groove or the previously made beads is not heated up to the melting temperature that is required for the parent metal to mix with the material and make a uniform weld. The lack of fusion is not due to the filler material used but exclusively to improper weld preparation, an unsuitable welding technology, including welding parameters, and weak performance of the qualified procedure. In practice, it has turned out that welders themselves are most often producers of the incomplete fusion. A well- qualified welder will melt the parent metal with an arc, mix it with the filler material, and thus make a weld.
The Operators must use a proper procedure in automatic and robotic welding. An unskilled or, often, the careless welder is bound, for various reasons indicated below, to produce the incomplete fusion in the weld. The lack of fusion can also be called a planar discontinuity of various sizes and shapes. If often happens that only one dimension, i.e. the one in the direction of weld progression, is particularly remarkable There seem to be two causes of lack of fusion, the first being an improper positions of the burner and the second an arc voltage too high, i.e., an arc too long. It can be said that the long arcs in welding with the two wires are the reason for the occurrence of lack of fusion. This statement can be substantiated by the high arc voltage and a very wide weld face. This indicates that the arc energy of the two arcs is distributed over a large area, the energy density is small, and the energy supplied is not sufficient to melt the parent metal. To improve the weld quality the arc length and, consequently, the arc voltage should be reduced and welding parameters should be recorded and stored.
