th
8 Brazilian Congress of Manufacturing Engineering
May, 18th to 22th 2015, Salvador, Bahia, Brazil
Copyright © 2015 ABCM
INFLUENCE OF MOLTEN POOL GEOMETRY AND PROCESS
PARAMETERS ON SOLIDIFICATION CRACKS FORMATION IN HYBRID
LASER - GMA WELDING OF THICK API 5L X70 STEEL PLATES
Luiz Delagnelo Barbetta, [email protected], 2
Walter Lindolfo Weingaertner, [email protected]
Oliver Seffer, [email protected]
Rabi Lahdo, [email protected]
André Springer, [email protected]
Stefan Kaierle, [email protected]
1
Universidade Federal de Santa Catarina Campus Reitor João David Ferreira Lima, Florianópolis - Santa Catarina - Brasil
Laser Zentrum Hannover e.V. Hollerithallee 8, 30419 - Hannover - Germany
2
Abstract. Formation of solidification cracks in deep penetration hybrid laser - GMA welding of API 5L X70 pipeline
steel plates were studied through metallographic and X-ray images. Experiments were operated with varying laser
beam width, laser beam power, filler metal and welding speed and with transverse laser beam oscillation. The
resulting molten pool geometry, especially bulges in the laser dominated zone, were observed to have a high influence
on the occurrence of solidification cracks. To measure the severity of a bulge, the bulge widening angle was introduced
as an indication of how steep the widening of the molten pool from a contraction to a bulge along its depth is. During
the experiments a high correlation between the bulge widening angle and the occurence of solidification cracks was
observed.
Keywords: Hybrid welding, Laser, GMAW, Solidification Crack
1. INTRODUCTION
Under the interest of increasing productivity in thick wall pipeline welding, the oil and gas industry is trying to
replace more traditional processes, like Gas Metal Arc Welding (GMAW) and Submerged Arc Welding (SAW), which
might require multiple passes to achieve the desired welding depth, for other processes that can weld such thick walls in
a single pass, like Laser Beam Welding (LBW), Electron Beam Welding (EBW) and Hybrid Laser-Arc Welding.
When those techniques, which produce high depth/width ratio welds, started to be developed, particularly a special
kind of discontinuity was observed (Jüptner, 1998; Weise, 1998 and Keller, 2003). Jüptner (1998) has found that those
discontinuities show matching opposing side, suggesting it was formed as an intergranular crack and, therefore, as a
solidification crack. Those solidification cracks are short embedded discontinuities that occur with certain regularity
along the weld seam centerline (Webster, 2009). The formation and the prevention of this defect are still not well
understood (Jüptner, 1998; Weise, 1998; SLV, 2009 and Gebhardt, 2013). If there is enough tensile strain and a liquid
film during solidification, a solidification crack might occur. The welding seam bulge would be a preferable location for
solidification crack nucleation, due to the higher amount of molten metal (Gebhardt, 2013). Figure 1 shows two
examples of solidification cracks in deep laser welding.
Figure 1. Examples of solidification cracks. Sources: a) (Rethmeier, 2007) and b) (BAM, 2005)
th
8 Brazilian Congress of Manufacturing Engineering
May, 18th to 22th 2015, Salvador, Bahia, Brazil
Copyright © 2015 ABCM
There are many different approaches and results in the prevention of those defects. Some works suggest faster
welding speed should result in more parallel keyhole walls, reducing the propensity for solidification crack formation
(SLV, 2009 and Kristensen, 1997). On the other hand, others found slower welding speed and excess laser beam energy
input should produce more stable keyholes and less strain, resulting in a reduction of solidification cracks (BAM, 2005
and Kristensen, 1999). What is well accepted by most is that the molten pool geometry is an important factor, especially
the depth/width ratio and the presence of a bulge, like the one presented in Fig. 2 (Gebhardt, 2013; Lloyds Register of
Shipping, 1997 and Russel, 2001).
Figure 2. Cross-sectional view of a welding seam with a bulge (highlighted).
The objective of this work is to create a better understanding of this defect. It was carried under the scope of the
publically funded HYBRILAS project, which was developed by the Laser Zentrum Hannover e.V. along with other
partners, with the objective of developing the hybrid laser-GMA welding for the joining of API 5L grade X70 pipeline
steel and for the joining of the aluminum alloy EN AW-6082-T6 (Seffer, 2014).
2. EXPERIMENTAL PROCEDURE
The welding process developed for the HYBRILAS project uses a 16 kW disk laser and two GMAW torches. The
first torch is positioned in a leading direction to the laser beam, and works together with it in the same molten pool,
resulting in a hybrid laser-GMA welding process. The second torch follows the hybrid system and works in a separate
molten pool. Figure 3 shows the geometry of that process.
Figure 3. HYBRILAS welding process geometry.
th
8 Brazilian Congress of Manufacturing Engineering
May, 18th to 22th 2015, Salvador, Bahia, Brazil
Copyright © 2015 ABCM
As a laser source, a TruDisk 16002 equipment was used and its main characteristics are found in Tab. 1. The
delivery of the laser beam was done through optical fibers. Those were available with diameters of 200 µm and 300 µm,
resulting in beam waist diameters of 400 µm and 600 µm respectively. The GMAW sources are two Phoenix 522 RC
Puls that work in the focused spray metal transfer mode.
Table 1. TruDisk 16002 disk laser source main characteristics.
Beam power (W)
Lowest BPP
Wavelength
Minimum fiber diameter
Energy efficiency
320 ~ 16,000
8 mm∙mrad
1,030 nm
200 µm
> 30%
The plates composition attends to the grade X70 from API 5L and can be found in Tab. 2. The plates dimensions
and edge preparation are presented in Fig. 4. To observe if the filler metal has any significant influences on the
solidification crack formation, two different filler metals were used: the AWS A5.28-05: ER90S-G and the AWS A5.18
ER70S-6. The mixture ISO 14175 M21, composed of 82% argon and 18% carbon dioxide was used as shielding gas for
the GMAW processes with a flow of 20 l/min.
Table 2. Steel plates composition.
C
0.094
Si
0.362
Mn
1.761
P
0.015
S
0.0024
Al
0.031
Cu
0.008
Cr
0.008
Ni
0.007
Mo
0.001
V
0.003
Sn
0.001
Ti
0.020
Nb
0.056
Co
0.002
Ca
0.0013
B
0.0001
Pb
0.001
W
0.003
Zr
0.001
Mg
0.0001
N
0.0005
As
0.002
Figure 4. Plates dimensions (a) and edge preparation (b).
The plates were fixed without a gap using vertical clamps with a torsional moment of 75 Nm. The plate position
and the gap were verified using a coaxial camera implemented into the laser beam welding head.
A total of seven parameter combinations were used for the experiments, as shown in Tab. 3. For each parameter
combination, between one and four welding seams were joined and consequently analyzed. The filler metal was fed as a
1.2 mm diameter wire with a contact tip to work distance of 12 mm and the mean values of electric current and voltage
as measured during the process were:
 For 14.0 m/min feed rate and AWS ER 90 S-G electrode: 405.2 A and 23.4 V.
 For 14.0 m/min feed rate and AWS ER 70 S-6 electrode: 414.2 A and 23.2 V.
 For 12.0 m/min feed rate and AWS ER 90 S-G electrode: 372.9 A and 22.5 V.
th
8 Brazilian Congress of Manufacturing Engineering
May, 18th to 22th 2015, Salvador, Bahia, Brazil
Copyright © 2015 ABCM
Table 3. Parameter combinations.
Laser beam
power (kW)
A
B
C
D
E
F
G
16.0
16.0
16.0
16.0
14.0
16.0
16.0
Welding
speed
(m/s)
1.8
1.8
1.8
1.6
1.2
1.4
1.2
Beam
width
(µm)
400
400
600
400
400
400
400
Filler
metal
90S-G
70S-6
90S-G
90S-G
90S-G
90S-G
90S-G
Feed rate
leading
(m/min)
14.0
14.0
14.0
14.0
12.0
12.0
12.0
Feed rate
trailing
(m/min)
14.0
14.0
14.0
14.0
14.0
-
Scanner
amplitude
(mm)
0
0
0
0
0
1.1
1.1
Scanner
frequency
(Hz)
0
0
0
0
0
100
100
X-ray inspection and metallographic analyses were used for the parameters A, B and C. However, for parameters
D, E, F and G, only metallographic analyses were performed. The results of the X-ray inspections show the shortest
crack, the longest crack and the total length of cracks. Two cross-sectional cuts were made on each welding seam for
the metallographic analyses, one was taken 50 mm after the beginning of the seam and the other 50 mm before the end,
as to avoid any process instabilities that might occur at the start and at the end of the process.
The depth of the seam bulge and the depth of the solidification crack were measured within the scope of the
metallographic analyses. The solidification crack depth was defined as the depth of its midpoint to the plate upper
surface. The seam bulge, though, has no clear borders, so its depth was defined as the point where the bulge first
reaches its maximal width.
To evaluate the influence of a seam bulge, the bulge widening angle (β) was defined. It is a measurement of how
steeply the seam widens from a narrowing above the bulge to the bulge itself. The bulge widening angle (β) employs the
geometry of an isosceles trapezoid, where the shorter base length (b) corresponds to the width of the narrowing, the
longer base length (B) to the maximum width of the bulge and the height (h) is the distance between them. Figure 5
depicts this definition.
Figure 5. Definition of the bulge widening angle (β).
3. RELATION BETWEEN BULGE AND SOLIDIFICATION CRACK
The metallographic analyses showed that the solidification cracks are always accompanied by seam bulge. Figure 6
presents several examples. The mean values obtained for the depths of the solidification cracks and the bulges confirm
this observation, as they are very close, b = 11.0 mm and B = 11.5 mm respectively. It should be noted that the bulge
depth definition in this work tends to underestimate its real depth, so it can be even closer. The metallographic images
were also used to measure the bulge widening angle (β), and the results are shown in Tab. 4.
th
8 Brazilian Congress of Manufacturing Engineering
May, 18th to 22th 2015, Salvador, Bahia, Brazil
Copyright © 2015 ABCM
Figure 6. Examples of weld seams with solidification cracks.
Table 4. Bulge widening angles (β) measured in the metallographic analyses.
Parameters
Seam
number
β (°)
A
B
C
D
E
F
G
1
4
1
1
2
1
2
1
1
1
2.8
0.8
1.4
4.6
2.5
1.7
2.2
3.7
2.8
1.6
Table 5 presents the results from the X-ray inspection in terms of the total crack length per welding seam length
(C/W), the shortest crack, and the longest crack, for the first half (1/2) and for the second half (2/2) of the welding
seam.
Table 5. Results of X-ray inspection.
Parameters
Seam number
C/W 1/2
C/W 2/2
Shortest crack 1/2 (mm)
Shortest crack 2/2 (mm)
Longest crack 1/2 (mm)
Longest crack 2/2 (mm)
A
1
0.05
0.43
1
1
3
6
2
0.03
0.17
1
1
3
8
3
0.01
0.31
1
1
1
10
4
0.00
0.21
2
3
B
1
0.12
0.35
2
1
4
6
C
1
0.63
0.72
1
4
19
19
2
0.40
0.61
2
2
7
19
The X-ray inspection cannot distinguish solidification cracks from several other seam imperfections. However, with
the exception of one welding seam, only solidification cracks were observed in the metallographic analyses.
Furthermore, all X-ray inspection detections show short discontinuities, with a maximum length of 19 mm, which is
characteristically for solidification cracks. Therefore, crack detections from the X-ray inspection are most probably
solidification cracks, and will be considered so in this work.
The second halves of the welding seams show many more solidification cracks than the first halves. The gap
increased during the welding process, in spite of clamped and tacked plates, which could be the reason for the higher
occurrence of solidification cracks in the second halves. To reduce this effect, only the first halves of the welding seams
were considered for further analyses in this work.
Figure 7 shows a graph of the total crack length per welding seam length (C/W) as a function of the bulge widening
angle (β) of seams that were subjected to metallographic analyses and X-ray inspection. It shows a good correlation
between β and C/W. The same trend can be observed between two welding seams joined with the same parameter
combination: C1 exhibits a higher bulge widening angle (β) and also a higher number of solidification cracks than C2
and the same occurs for A1 and A4. The bulge widening angle (β) was observed to vary greatly along the seam length,
so that a higher amount of metallographic images would be necessary for more reliable results.
th
8 Brazilian Congress of Manufacturing Engineering
May, 18th to 22th 2015, Salvador, Bahia, Brazil
Copyright © 2015 ABCM
Figure 7. Total crack length per welding seam length as a function of the bulge widening angle (β).
4. PARAMETERS INFLUENCE ON SOLIDIFICATION CRACK FORMATION
Parameter combinations A, B, and C were analyzed using X-ray inspection, and the results are shown in table 5.
These investigations show that the use of AWS A5.18 ER70S-6 (B) as filler wire material instead of AWS A5.28-05:
ER90S-G (A) increased the total length of solidification cracks. Between the welding tests of parameter combination A
and B, a significant difference in the voltage or the current of the electric arc was not detected, so the higher quantity of
solidification cracks should be related to the composition of the weld metal.
The welding seams joined using parameter combination C with a beam width of 600 µm (C) exhibit a considerably
higher amount of solidification cracks (C/W) than the seams joined with a beam width of 400 µm (A and B), while
retaining the other parameters. The modification of the keyhole and molten pool geometries should be the main cause
for this high increase in solidification crack formation. In this case, the mean bulge widening angle (β) for parameter
combination C amounts to 3.5°, and distinctly exceeds the mean bulge widening angle (β) for parameter combination A
of 1.5°.
The welding seams using the parameter combinations D, E, F and G were not tested using X-ray inspection, and
therefore the investigations are based on the seam geometry and the occurrence of solidification cracks shown in the
metallographic analyses.
The seams joined with a reduced welding speed (D), in comparison to parameter combination A, exhibit a
somewhat higher mean bulge widening angle (β) of 2.0°, but solidification cracks were not observed in the
metallographic analyses, as partially shown in Fig. 8. Due to a higher heat input and a reduced welding speed,
solidification of the molten pool is delayed, facilitating material flow for thermal and solidification shrinkage
compensation.
Figure 8. Cross-sectional metallographic images of welding seams (parameter combination D).
th
8 Brazilian Congress of Manufacturing Engineering
May, 18th to 22th 2015, Salvador, Bahia, Brazil
Copyright © 2015 ABCM
For the seams welded with parameter combination E, a reduced laser beam power and welding speed were used.
The seams exhibit severe bulging, with a mean bulge widening angle (β) of 3.7°, and the detection of a solidification
crack in one cross-section of the metallographic analyses, as shown in Fig. 9a). In comparison to parameter combination
A, the decrease in welding speed (1.8 m/min to 1.2 m/min) and the increase in laser beam energy per section
(533 kJ/m to 700 kJ/m) resulted in a less desirable molten pool geometry and a higher tendency to solidification crack
formation. This agrees with observations of other authors (SLV, 2009 and Kristensen, 1997), and contradicts the usual
recommendation of reducing the welding speed to avoid solidification cracks (BAM, 2005 and Weise, 1998).
Figure 9. Cross-sectional metallographic images of a welding seam (parameter combination E).
Welding with parameter combination F and G included transversal beam oscillation. Parameter combination F used
a welding speed of 1.4 m/min, which resulted in a laser beam energy per section of 686 kJ/m, and parameter
combination G used a welding speed of 1.2 m/min, with an accordingly laser beam energy per section of 800 kJ/m.
Despite the relatively small difference in the welding speed, the results were quite diverse. The mean bulge widening
angle (β) for parameter combination F amounts to 2.8°, and solidification cracks were detected in both cross-sections of
the metallographic analyses, as seen in Fig. 10. On the other hand, in the metallographic analyses of parameter
combination G, Fig. 11, solidification cracks were not detected. In addition, the seam exhibits a much gentler geometry
with a mean bulge widening angle (β) of 1.6°. In this case, a decrease in the welding speed accompanied by an increase
in laser beam energy per section had a beneficial effect on the welding seams.
Figure 10. Cross-sectional metallographic images of a welding seam (parameter combination F).
th
8 Brazilian Congress of Manufacturing Engineering
May, 18th to 22th 2015, Salvador, Bahia, Brazil
Copyright © 2015 ABCM
Figure 11. Cross-sectional metallographic images of a welding seam (parameter combination G).
The two cross-sectional metallographic images from the welding seam done with parameter combination G showed
however another discontinuity in the arc zone of the hybrid process. This discontinuity is probably caused by the
insufficient fill, but it is out of the scope from this work.
5. CONCLUSIONS
In this work, the bulge widening angle (β) was proposed as a means of evaluating the severity of a bulge in the
molten pool geometry for the formation of solidification cracks. The initial results found in this work support its
usability, but more tests are still required.
A high relation between the welding seam bulge and the occurrence of solidification cracks was observed, as the
solidification crack is always associated with a bulge, and the seams with more severe bulges exhibit a higher amount of
solidification cracks.
The composition of the filler wire material can influence solidification crack formation, even if the defect occurs
deep in the laser dominated area, with a long distance to the arc dominated zone.
The laser beam width has a great influence on the molten pool geometry and accordingly on solidification crack
formation.
As has been found in the literature, the variation of welding speed and of laser beam energy per section produced
conflicting results. It seems that their effect on the solidification crack formation depends mostly in the resulting
keyhole and molten pool geometries, and, for this reason, it is hard to anticipate if the increase or reduction of welding
speed and of laser beam energy per section will yield better or worse results.
6. ACKNOWLEDGEMENTS
The authors would like to thank the German Federal Ministry of Education and Research (BMBF), supported by the
Association of German Engineers Technology Centre (VDITZ) within the research funding initiative Materials
Processing with Brilliant Laser Beam Sources (MABRILAS).
7. REFERENCES
BAM, 2005, “Qualifizierung des Nd:YAG- und CO2-Laser-Plasma-Pulver Hybridschweißens” Berlin.
Gebhardt, M.O., 2013 “Einfluss von Konstruktion und Schweißparametern auf die Erstarrungsrissentstehung beim
Laser-MSG-Hybridscweissen dickwandiger Bauteile” Thesis (Dr.), Technischen Universität Berlin, Berlin.
Jüptner W., Sepold, G., Schubert, E., Weise, S., Kurgusow-link, A., 1998 “Modellierung der Heißrißneigung unlegierter
Stähle beim Laserstrahlschweißen”, Strahltechnik Band., Vol. 6, Bremen, pp. 143-150.
Keller, H., 2003, “CO2-Laserstrahl-MSG Hybridschweißen von Baustählen im Blechdickenbereich von 12 bis 25 mm”,
Aachen: Shaker, 157 p.
Kristensen, J. K., Borggreen, K. and Knudsen, S., 1997, “Large scale testing of laser welded structural steels”
International Conference on the Joining of Materials, Vol. 8, Elsinore, Denmark, pp. 1-13.
Kristensen, J. K., Hansen, L. E., Nielsen, S.E. and Borgreen, K., 1999 “Laser welding of C-Mn steels and duplex
stainless steels” European symposium on assessment of power beam welds, Vol. 3, Geesthacht.
Lloyds Register of Shipping, 1997 “Guidelines for the approval of CO2 laser welding”
Rethmeier, M. and Gumenyuk, A., 2007, “Schweißen mit dem Faserlaser” IFS Kolloquium.
th
8 Brazilian Congress of Manufacturing Engineering
May, 18th to 22th 2015, Salvador, Bahia, Brazil
Copyright © 2015 ABCM
Russel, J.D., 2001, “Laser weldability of C-Mn steels” Weld. Res. Abroad, Vol. 47, pp. 23–28.
Seffer, O., Lahdo, R.; Springer, A., Kaierle, S., Kracht, D., 2014 “Schweißen von Dickblechen mit brillanten
Laserstrahlquellen (HYBRILAS) – Grundlagenuntersuchungen für Stahl und Aluminium” Schlussbericht. FKZ
13N10549. Laser Zentrum Hannover e.V. Hannover.
SLV, 2009, “Erstarrungsbedingte Gefügetrennungen beim Hochenergieschweißen” Halle.
Webster, S.E., Tkach, Y., Langenberg, P., Nonn, A., Kucharcyzk, P., Kristensen, J. K., Courtade, P., Debin, L., and
Petring, D. 2009, “Hyblas: economical and safe laser hybrid welding of structural steel” final report. Luxembourg:
Off. for Official Publ. of the European Communities.
Weise, S., 1998, “Heißrißbildung beim Laserstrahlschweißen von Baustählen” Bremen: BIAS, 122 p.
8. RESPONSIBILITY
The authors are the only responsible for the printed material included in this paper.