Defects in metal arc gas (MAG) welds made in S235JR low carbon steel of 6 mm thickness were examined. A sample containing lack of fusion (LOF) and pores was examined by computed tomography – CT. The computed tomography examination was performed in order to define LOF size and position as well as dimensions and distribution of accompanying pores in the weld metal

B. Zorc

L. Kosec

M. Jovanović

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233 METALURGIJA 51 (2012) 2, 233-236M. JOVANOVIĆ, L. KOSEC, B. ZORCEXAMINATION OF WELD DEFECTS BY COMPUTED TOMOGRAPHYReceived – Prispjelo: 2011-04-18Accepted – Prihvaćeno: 2011-06-02Preliminary Note – Prethodno priopćenjeISSN 0543-5846METABK 51(2) 233-236 (2012)UDC – UDK 669.14.018.298:669.18=11Defects in metal arc gas (MAG) welds made in S235JR low carbon steel of 6 mm thickness were examined. A sample containing lack of fusion (LOF) and pores was examined by computed tomography – CT. The computed tomogra-phy examination was performed in order to defi  ne LOF size and position as well as dimensions and distribution of accompanying pores in the weld metal. Key words: MAG welding, LOF, computed tomography (CT), tomograph, porosityIspitivanje grešaka zavara kompjutorskom tomografi  jom. Ispitivane su greške u zavarenom spoju niskougljičnog čelika S235JR debljine 6 mm zavarenog MAG postupkom. Uzorak koji sadrži greške naljepljivanja i pore je ispitivan kompjutorskom tomografi  jom - KT. Programskom analizom tomografa je određena veličina i pozicija grešaka naljepljivanja kao i dimenzije i raspored pratećih pora u zavarenom spoju. Ključne riječi: MAG zavarivanje, naljepljivanje, kompjutorska tomografi  ja (KT), tomograf, poroznostINTRODUCTIONLack of fusion is a surface defect of a welded joint extending along the weld or on the boundary between individual weld beads due to incomplete metal fusion. LOF can occur in different materials; however, it is most frequent in steel welds. Molten ﬁ  ller material only sticks to the groove face or to the previous weld bead. In such places, the material cohesion is interrupted which results in a reduced load-bearing cross-section. A LOF is a hid-den defect which becomes extremely dangerous at dy-namic loading of welded structures since it represents an ideal initiator of a crack. The most often cause of a LOF is an incorrect selection of welding parameters or a weld-er’s mistake. The LOF occurrence due to welding tech-nology is most often ascribed to an unsuitable prepara-tion of a weld groove, to an incorrect inclination of a gun, to un unsuitable welding position and to the arc blow due to a magnetic ﬁ  eld [1]. The second group of LOF causes includes an insufﬁ  cient energy input into the weld. It has been proved, though, that the welding parameters selec-tion, such as intensity of welding current, wire feed rate and wire stick-out length are highly important as well [2]. Welding speed has the highest impact on the energy input [3]. A too high welding speed provides minor energy in-put to a unit of the weld length which results in insufﬁ  -cient fusion of parent material, greater LOF risk and lower quality of weld metal [4, 5]. On the other hand, a too low welding speed allows higher energy input to a M. Jovanović, L. Kosec, Faculty of Natural Sciences and Engineering, University of Ljubljana, Ljubljana, Slovenia. B. Zorc, Welding Institute, Ljubljana, Slovenia.unit of the weld length. The resulting molten pool is too big and overtakes the arc, disabling it to provide correct fusion of parent material. The molten pool of the ﬁ  ller material can oxidize on the surface so that LOF can occur as a result of puckering while pores and/or non-metal in-clusions can occur as well. The specimen for macrographic examination shows inclusions and pores caught between parent and ﬁ  ller materials [6-11]. There is no evidence of porosity and LOF connection in literature. However, research shows that porosity and LOF can be connected. Kou and Wang established that weld porosity mainly depends on the weld pool development mode [12]. LOF can be detected by different techniques of ultrasonic and/or radiograph-ic examination. EXPERIMENTSThe experiments were performed in the following steps: welding in a trough, performance of specimens for macrographic examination, computed tomography of samples, computer processing of tomograms using adequate tools. The main purpose of the examination was to ﬁ  nd out whether defects such as LOF and pores inside a weld metal can be detected by computed to-mography. A narrow gap between weld pieces was se-lected. A small angle prevented uniform melting of both weld pieces which resulted in a LOF. The LOF was con-ﬁ  rmed by a specimen for macrographic examination (Figure 1). Further investigations were directed to the assessment of the LOF type, size and form as well as to the presence of pores which accompany the LOF in a weld metal. Then, the tomograms were analyzed by rel-234   METALURGIJA 51 (2012) 2, 233-236M. JOVANOVIĆ et al.: EXAMINATION OF WELD DEFECTS BY COMPUTED TOMOGRAPHYevant computer programs. VGI program served to cap-ture individual images which were processed by Image Tool. In this way, size and proﬁ  le of the LOF and distri-bution and size of pores in the vicinity of the LOF were measured. MAG welding was performed in forward direction. A ﬁ  llet weld (FW) was made at an angle of 21° between the weld pieces made of S235JR structural steel accord-ing to EN 10025-2, thickness 6 mm. The steel sheet was 20 mm wide and 100 mm long. Welding parameters were as follows: current intensity 240 A, voltage 24 V, welding speed 6,6 mm/s, ﬁ  ller wire feed rate 114 mm/s, wire stick-out length 20 mm, angle of blowpipe inclina-tion 76°. Filler wire was of G3Si2 type and 1,2 mm di-ameter. Shielding gas was a mixture of 82 %Ar+18 % CO2 with a ﬂ  ow of 15 l/min.Computed tomography is a new non-destructive testing method recently recognized especially in detec-tion of anomalies in metal industry products. Perform-ance of three-dimensional images by micro-focus com-puted tomography starts with two-dimensional images of a test specimen segment projections developed by gradual rotation of the test piece within the range of di-rected X-rays. The test piece rotation shall be performed gradually at a rotational angle of less than 1° as long as the specimen is radiated from all sides.Due to a limited radiation power several specimens with irregularities had to be cut out from the existing welded joint in form of a cube with a 5 mm side. The cutting position is given in Figure 1a, while the test specimen is given in Figure 1b. Afterwards, the speci-mens were examined by computed tomography – CT. For this purpose, the NANOMEX CT system of Phoe-nix X-ray Company was used. The system consists of a 160 kV nanofocus X-ray tube and a 16–bit detector with voxel accuracy of 16 μm. Radiation of test pieces was performed at the tube voltage of 140 kV and current intensity of 70 μA. Possibility of details resolution ranged within (200 – 300) nm. The X-ray tube power was 50 W. By use of my VGI 1,2 dedicated program for data visualization and documentation in form of voxels to-mograms representing one hundredth millimeter of a weld metal were obtained. Based on EN 12517 standard and by use of the Image Tool program a bright-dark contrast comparison was made which proved how the porosity amount in the molten pool changed with mov-ing away from the LOF surface, and how far pores were extending into the molten pool.RESULTS AND DISCUSSIONAngle and depth of a weld groove are important LOF parameters. In case of smaller angles, a narrow gap prevents a proper access of the weld gun into the root of the weld. So the arc starts burning on the nearest face of the groove which was proved by Sony DCR-TRV740E digital camera recording on an 8 mm tape with a frequency of 25 snapshots/s. The molten mixture of parent and ﬁ  ller materials pours to the other face of the groove which prevents melting of the parent mate-rial in this part of the joint and results in a LOF in the lower part of the weld metal (Figure 1b).The results of CT examinations are given in Figure 2, which shows a 3D image and three projections of a test specimen with a clearly evident LOF and pores. Frontal image (2a) gives a LOF well evidenced along the whole length as well as its width change. Figure 2b is a tomogram of the surface where the LOF passes into porosity. Numerous pores of different shapes and sizes are visible. This image represents the basis of a geo-metrical analysis of the LOF and porosity.Figure 2c shows the specimen from the top. The LOF and the accompanying pore are well evident. This view served for the proﬁ  le of the LOF along the weld metal length. Figure 2d is a 3D image of the specimen.Figure 3 gives tomograms of a 3D CT image projec-tion in different sections made from the direction per-pendicular to the LOF plane. Dark surfaces decrease with moving away from the LOF plane (from I to III).As it can be seen from the CT images, the LOF is well limited in space. So we can say that this is a plane Figure 1   Specimens preparation: a) specimen for macrographic examination, b) test specimen for CTFigure 2   Specimen image made by computed tomography: front (a), side (b), top view (c) and 3D image (d)235 METALURGIJA 51 (2012) 2, 233-236M. JOVANOVIĆ et al.: EXAMINATION OF WELD DEFECTS BY COMPUTED TOMOGRAPHYdefect. By VGI program it was established that the width of this LOF was 0,1 mm. In sector images it can be seen that a great number of pores of different sizes and shapes are arising from the LOF area. All these pores are directed from the LOF plane to the molten pool which is another indirect proof that the parent ma-terial is not melting there. The tomogram section (Fig-ure 3) gives an analysis of the pores diameter, cross sec-tion and length.The analysis was performed in a rectangle of 5,8 x 1,8 mm given in Figure 4 which represented approxi-mately one fourth of the weld surface. Total height of the weld cross-section was 7,2 mm. Inside the rectan-gle, the share of the image darkness was measured in each individual section whereby the darkness represent-ed an empty space (LOF or porosity). As it is evident from Figure 5, empty spaces (dark surfaces) were most-ly found in the area of LOF at a distance of 0 to 0,1 mm, where the darkness share reached more than 40 %.At a distance of 0,1 to 0,2 mm, the porosity or dark-ness share decreased from 20 % to 5 %. In this area the majority of pores disappeared. It is interesting that indi-vidual pores can reach into the molten pool even as far as 0,6 mm away from the parent material where the LOF occurred. Figure 5 gives a scheme of the LOF plane location with directions of individual axes which shall contribute to relevant understanding of LOF im-age analysis. In further tomograms analysis the top view given in Figure 3 was applied. For each section, the LOF height as per weld length has been measured. In this way the LOF proﬁ  le between the parent metal and the molten pool has been obtained (Figure 6). As it is evident, the LOF reaches up to 1,6 mm of the weld height at the most and up to 0,3 mm of the weld height at the least. Above the given line, full penetration with dilution of parent and ﬁ  ller materials occurred while under this line weak dilution resulted in a LOF. The size of pores accompanying the LOF, their reach into the weld and their distribution within a weld section were established as well. For this analysis, weld metal section images (paral-lel to the LOF plane) from the Figure 2 were used. Such tomograms are given also in Figure 3. Surface and di-ameters of all well distinguished pores in different sec-tions inside the weld metal have been measured by moving from inside of the weld metal toward LOF. Since pores are spatial defects, the most relevant data for the examination consist of their cross-sections at speciﬁ  c points. The diagram in Figure 7 gives the cross-section changes in relation to the distance from LOF. 13 well deﬁ  ned pores that reach more than 0,05 mm in length were examined. They are given in Figure 4. In some cases the pores’ cross-section is decreasing with Figure 3 Projection on LOF planeFigure 4   Dimensional analysis of porosity area (pores are numbered from 1 to 13)0204060801000 0,1 0,2 0,3 0,4 0,5 0,6distance from parent material / mmporosity/%Figure 5 Change in porosity as per weld section00,30,60,91,21,51,80246weld length / mmLOFheight/mmLOFFigure 6   LOF profi  le between the molten pool and parent material00,050,10,150,20,250,30,1 0,2 0,3 0,4 0,5 0,6 0,7distance from LOF / mmporecrossarea/mmpore 4 pore 1 pore 3 pore 5pore13 pore 2 pore 6 pore 10pore 9 pore 7 pore 11 pore 12pore 8Figure 7 Size of pores inside a welded joint236   METALURGIJA 51 (2012) 2, 233-236M. JOVANOVIĆ et al.: EXAMINATION OF WELD DEFECTS BY COMPUTED TOMOGRAPHYlength while in the others, the diameter or cross-section even increases. Moreover, the pores direction through the molten pool is changing as well. Some of them change their cylindrical shape and insert into the molten pool in form of a ring.Based on an average cross-section, the pores are classiﬁ  ed into three groups: a)   Pores with a cross-section greater than 0,2 mm2. They were 5 of them altogether. 3 of them reached more than 0,3 mm into the weld metal. The re-maining two were shorter than 0,2 mm.b)   Pores with a cross-section of 0,1 mm2. They were 3 altogether. One of them reached more than 0,3 mm into the weld metal. The remaining two were shorter than 0,2 mm. c)   Pores with a cross-section smaller than 0,05 mm2. They were 5 altogether and they were of different lengths; however, they were all shorter than 0,3 mm.As it is evident, only pores with an initial cross-sec-tion above 0,2 mm2 can reach a more explicit length (above 0,3 mm). Pores with a smaller initial cross-sec-tion are shorter than 0,25 mm.CONCLUSIONSIn MAG welding of low-carbon steel structures po-rosity and LOF occur quite often. It has been established that LOF and porosity are usually mutually connected. –   A very detailed 3D analysis of LOF and porosity in the weld metal can be made by computed tomog-raphy. –   Tomograms projections enable specimens’ analy-sis from different directions. It has been proved that incompletely fused parent material results in LOF which proceed in the direction of fused par-ent material to transform into pores.–   Further tomograms analysis by use of adequate computer software enabled to establish the LOF proﬁ  le in the weld metal section or the line sepa-rating the fused part of the weld from the infused one.–   The amount of incompletely fused material inside the weld metal has been established together with the percent of the empty space within a weld metal formed together by the LOF and porosity. The measured LOF thickness, without pores arising from the LOF, amounts to 0,1 mm.–   It has been concluded that regarding the cross-sec-tion size there occur three groups of pores; how-ever, only those with an initial cross-section above 0,2 mm2 reach signiﬁ  cantly into the weld metal depth.–   Computer tomography represents a very reliable and accurate non-destructive testing method for analysis of inner defects in metallic materials. For the time being, the only limitation lies in the size of test specimens. LITERATURE[1]   Causes for Weld Defects, IIW Doc. XII-B-046-83, Interna-tional Institute of Welding, 1983.[2]   Gas-Shielded Metal-Arc Welding of Steel, IIW Doc. XII-B-049-83, International Institute of Welding, 1983.[3]   Killing R., Hantsch H., Beitrag zur Frage der Bindefehle-rempﬁ  ndlichkeit beim Metall-Aktivgasschweißen mit Fül-ldrahtelektroden, Schweißen und Schneiden, 45 (1993) 12, 34 - 41.[4]   Jovanovic M., Rihar G., Grum J., Analysis of Ultrasonic Indications in LOF Occurring in Welds, Proceedings of the 9th ECNDT, Berlin, 2006.[5]   Yamauchi N., Inaba Y., Taka T., Formation Mechanisms of LOF in MAG Welding, IIW Doc. 212-529-82, Internatio-nal Institute of Welding, 1982[6]   Rihar G., Uran M., LOF - Characterisation of Indications, Welding in the World, 50 (2006) 1/2, 35-39.[7]   Buschke P., Roye W., Dahmen T., Multiple NDT methods in the automotive industry, GE Inspection Technologies, Huerth, 2000.[8]    Samardžić I., Mateša B., Kladarić I., Metalurgija, 50 (2011) 3, 145 – 216.[9]   EN 12517-1:2006 Non-destructive testing of welds – Part1: Evaluation of welded joints in steel, nickel, titanium and their alloys by radiography- Acceptance levels.[10]   Muhič M., Tušek J., Kosel F., Klobčar D., Pleterski M., Metalurgija, 49 (2010) 1, 9 – 12.[11]   Kosec B., Kosec L., Kopač J., Engineering Failure Analy-sis, 8 (2001) 4, 355-359.[12]   Kou S., Wang Y.H., Weld Pool Convection and Its Effect, Welding Journal, 65 (1986) 3, 63 -70.Note:   The responsible translator for English language is Lelja Vidan, prof., Welding Institute, Ljubljana.

Examination of weld defects by computed tomography

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