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Lack of fusion in welded joints Gabriel RIHAR Welding Institute, Ptujska 19, SI-1000 Ljubljana Contact

Abstract.

For an efficient examination of welded joints, thorough knowledge not only of non-destructive testing methods but also of the defects to be detected is required. In the welding literature, however, there are very few data on lack of fusion, which is one of the most serious defects. There are also no efficient testing methods available for its detection.

Our studies were to provide as much data as possible on lack of fusion. It was investigated with which welding processes lack of fusion is most frequent, what its position in a welded joint is and what its physical properties are. It was also studied what kind of indications the lack of fusion gives with different non-destructive methods.

Testing was carried out with typical welded joints. Metallographic and mechanical testing was also used in order to make a comparison with non-destructive testing. Various types of lack of fusion were classified into characteristic groups, i.e. the types including voids and non-metallic inclusions and the types of pure lack of fusion which cannot be detected by non-destructive testing.
Key words: welding, NDT, weld defects, lack of fusion.

1 Introduction

It is known from the welding practice that the lack-of-fusion defects often occur in a welded joint. Many a failure of a welded structure thus occurs due to undetected lack of fusion in welded joints.

Lack of fusion is one of the most serious weld defects. It produces the notch effect. There are, however, no efficient non-destructive methods available for its detection.

If lack-of-fusion defects and cracks are compared, it is evident that much more attention is paid to cracks than to lack of fusion although lack of fusion is as serious a defect as a crack. In the welding literature numerous papers deal with cracks but very few with lack of fusion. Only some older documents of the International Institute of Welding were found to report on lack of fusion [1-4]. It was, therefore, decided to study lack of fusion more closely. It was to be found out where it occurs most frequently, under which conditions it forms and how it can be detected.

2 Definition of lack of fusion

In welds, incompletely fused spots, called lack of fusion, persist. A weld can lack union with the parent metal or with a previous weld bead. An adhesion joint forms, which can be rather strong in certain cases. It is much like a brazed joint or joint formed in metallisation. The purer lack of fusion is, the more difficult it is to detect it.

With regard to the position of the lack-of-fusion defects in a weld, three types of lack of fusion are distinguished [5]:

1. lack of side-wall fusion,
2. lack of inter-run fusion,
3. lack of fusion at the root of the weld.

As to the appearance of the fracture face, one distinguishes the lack of fusion due to unmelted oxide inclusions and the lack of fusion due to melted oxide inclusions. The lack-of-fusion defects due to unmelted oxide inclusions consist of oxides and non-metallic inclusions. Lack of fusion, of which three types, i.e. IIW references 4011, 4012, and 4013, are distinguished in a standard should not be mixed up with lack of penetration, i.e. IIW reference 402 [6]. The defects located at the surface are efficiently detected by a visual inspection. Lack of penetration inside the weld, however, can be detected by X-ray or ultrasonic inspection methods.

As to the possibility of detecting, different types of lack of fusion can be classified into two groups, i.e., the one in which lack of fusion includes voids or non-metallic inclusions which can be detected by non-destructive methods, and the one in which the lack of fusion shows no discontinuity in the material since it is a structural defect and thus cannot be detected by non-destructive methods.

3 Characteristics of lack of fusion

It was found in metallographic examinations that in a weld three types of lack of fusion can be found:
1. pure lack of fusion or lack of fusion due to melted oxide inclusions,
2. open lack of fusion,
3. lack of fusion consisting of non-metallic inclusions.

The pure lack of fusion is a structural defect. In this case the molten metal sticks to the parent metal which has not melted enough during welding. A joint between the solid phase and the liquid one forms. It is like a brazed joint. This type of lack of fusion cannot be detected by non-destructive testing methods but with a microscopic inspection. A straight fusion line indicates that there may be the lack of fusion between the parent metal and the weld. The inter-run lack of fusion is even more hidden. It can be detected only by an accurate microscopic inspection with a 50-times magnification. An example of the pure lack of fusion is shown in Fig. 1.

Fig 1: Pure lack of fusion between the final run and the parent metal. a) macrograph: x3.5; micrograph: x100.

Because of internal stresses produced during weld solidification and cooling, the faces sticking to each other will separate. A void having a width of only some hundredths of a millimeter forms. This gap in the weld is very muck like a crack. It can, however, be detected by non-destructive testing methods. Such a type of lack of fusion is difficult to distinguish from a crack. An example of the open lack of fusion is shown in Fig. 2.

Fig 2: Open lack of fusion between the central and final runs. a) macrograph, x3.5; b) micrograph: x100.

Where the lack of fusion is there are very often also oxides and non-metalic inclusions. Such a case is shown in Fig. 3. If the oxide layer does not melt, the inclusions are uniformly distributed across the entire surface of the lack-of-fusion defect. If they melt, however, the non-metallic inclusions become spherical.

Fig 3: Inclusions at the faces sticking together. a) The macrograph shows lack of fusion between the central and final runs; b) The micrograph shows inclusions at the faces sticked together.

4 Location of lack-of-fusion defects

The lack of fusion is a planar defect. It may appear at the edge of the parent metal or between runs. The lack of fusion between the parent metal and the weld metal shows a flat face. The lack of inter-run fusion, however, shows an irregular shape.

The lack of fusion is usually to be found at the weld inside. It rarely reaches the final runs or the root run. Location of typical types of lack of fusion are shown in Fig. 4.

Fig 4: Lack of fusion in a weld: lack of side-wall fusion (above), lack of inter-run fusion (below).

5 Detection of lack of fusion by non-destructive testing methods

As already mentioned, the non-destructive testing methods commonly used in testing of welded joints are ineffective in detection of the lack-of-fusion defects. In routine testing, the lack of fusion is usually not detected. When the lack of fusion is suspected in a weld, the testing methods used should be suitably adapted. Attention should be paid to all indications including those which do not exceed their levels of acceptability.

Account should be taken that the lack of fusion may appear in gas-shielded arc welding with a consumable electrode and gas welding. There is a great chance of the appearance of lack of fusion in downhill welding.

It is very useful to obtain data on the weld structure and welding conditions prior to welding. Information should be sought whether any mechanical and metallographic examinations of the welded joints to be examined have been performed.

6 Non-destructive testing methods for the detection of lack of fusion

The radiographic examination makes possible the detection of large, distinctive types of lack of fusion in which there are inclusions and voids of considerable size. The examination detects the inclusions but not the lack of fusion itself. Attention should be paid to all defects located at the fusion line and between individual runs. The radiographic testing methods are not suitable for detection of the lack of fusion.

For detection of the lack of fusion, the ultrasonic examination is the most suitable of all the available non-destructive testing methods. All types of lack of fusion, with the exception of the pure lack of fusion, can be detected. Our investigations showed that considerably large lack-of-fusion defects give weak ultrasonic indications which are in practice considered minor acceptable defects. A considerably poor reflection of ultrasonic waves is obtained due to:

• the sticked surface consists of a number of small defects which pass over to the pure lack of fusion;
• the lack of fusion located at the edges of a V weld coincides with the angle of reflection of ultrasonic waves.

Fig 5: Ultrasonic examination of the lack of fusion between the parent metal and the weld metal. a) examination from all four scanning directions; b) examination from the cover layer.

Because of the above, weak, repeating indications should also be considered when inspecting welds in which the lack of fusion is suspected. The ultrasonic waves should be directed, if possible, perpendicular to the sticking faces. It can be supposed that the lack of fusion may appear at the edge of the parent metal. In the case when the access of ultrasonic waves is possible from all four sides, the direct path as shown in Fig. 5a is used. The examination can be accomplished only from the cover layer. In this case the single bounce technique is used to examine the upper part of the weld (Fig. 5b).

The penetrant testing methods are used primarily when tightness of welded joints is to be ensured, e.g. in testing of tanks. They permit detection of the lack of fusion reaching the surface.

All indications at the edge of runs indicate a possibility of the lack of fusion. Special attention should be paid to fillet welds. The lack-of-fusion defect existing in a weld may open when loaded and produce leaking of a tank.

The lack of fusion is a typically welder's fault. By monitoring the welding process an experienced supervisor can find the formation of lack of fusion. Attention should be paid to the position of the weld pool which must not run ahead of the arc and fill the weld groove. Attention should be paid whether the welder sufficiently melts the joint edges. The lack of fusion can be successfully detected by the visual inspection during welding. The visual inspection after welding is not efficient.

7 Conclusions

In welds both the lack of fusion which is a structural defect and the lack of fusion in which the metal joint is interrupted by voids and non-metallic inclusions may be found. The pure lack of fusion can be detected only by the visual inspection during welding. The other types of lack of fusion may be detected by ultrasonic testing methods. The lack of fusion reaching the surface may be detected by the liquid penetrant or magnetic particle testing methods. Because of an unfavourable position of the lack of fusion and unfavourable properties of the sticking faces, weak indications are obtained with considerably large defects; therefore, acceptance criteria for the welds containing the lack of fusion should be made more strict.

References

1. N. Yamauchi, Y. Inaba, T. Taka: Formation Mechanism of Lack of Fusion in MAG Welding. IIW Doc. 212-529-82. International Institute of Welding, 1982.
2. Causes for Weld Defects. IIW Doc. XII-B-046-83. International Institute of Welding, 1983.
3. Gas-shielded Metal-Arc Welding of Steel. Directions for Execution of Process. Avoidance of Lack of Fusion. IIW Doc. XII-B-049-83. International Institute of Welding, 1983.
4. R. Killing and H. Hantsch: Beitrag zur Frage der Bindefehlerempfindlichkeit beim Metall-Aktivgasschweißen mit Fülldrahtelektroden. Schweißen und Schneiden, 45 (1993) 12, 689-693.
5. Multilingual Collection of Terms for Welding and Allied Processes / International Institute of Welding. Part 1. General Terms. Institut za varilstvo, Ljubljana, 1988.
6. EN 26520: Classification of imperfections in metallic fusion welds, with explanations.

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