WELDING OF THICK WALLED STEEL
COMPONENTS WITH 32 KW LASER POWER
C. HØJERSLEV1
1
Lindoe Welding technology A/S, Munkebo, Denmark.
Abstract
Since the commissioning of a 32 kW disc laser system ultimo 2012, the newly established
company Lindoe Welding Technology, has performed weld tests utilizing up to 32 kW laser
power; with welding of thick walled steel components the main focus.
So far it has been demonstrated that it is possible to yield weld penetrations exceeding 40 mm
in depth by one-sided welding.
Also, it has been demonstrated that, even adopting laser only welding, hardness values can be
controlled to conform to standardized acceptance levels.
The main challenge with respect to welding quality seems to be solidification shrinkage and
cracking. Preliminary test results obtained, suggest that this challenge can be mitigated.
Keywords: 32 kW laser power, thick walled welding, steel
1
Introduction
Lindoe Welding Technology (LWT) is a private R&D intensive company established medio
2012 with a main emphasis on bringing changes to the market for thick walled welding by
means of high-power laser welding.
A main focus for LWT is the 200 M€ market for wind turbine towers – as cost out is the
mantra in the highly competitive wind turbine industry. The main argument promoting laser
welding for the stakeholders in this industry is that up to 80 % cost savings per meter welded
can be reached; together with an increased productivity by a factor 10 in comparison to submerged arc welding (SAW), which is currently the dominating technology used for welding
of wind turbine towers.
Despite that laser systems delivering 50 kW have existed since 20081, a 32 kW laser system
still ranks among the most powerful in the world medio 2013. This illustrates, that it is not
enough to have a substantial amount of laser power available – it must be utilized
meaningfully to.
The Pipeline Research Council International (PRCI) recommends utilization of GMAW-laser
hybrid welding for joining the root side pipeline steels demonstrate that laser welding
technologies is industrially matured even for thick walled steel components. The fact that
PRCI merely recommends the technology for root side welding, and recommends classical
GMAW techniques for filling and capping demonstrates that there is still work to undertake
before laser welding technology is applicable for real thick walled welding – tentatively
defined as thicknesses exceeding 20 mm. Recent publications2,3 supports this suggestion, and
demonstrates that internal weld imperfections forming during solidification of the weld metal
as a main challenge to address.
This paper will focus on technical findings at LWT experienced through ultra-high power
laser welding of thick walled components, and will show examples of welds virtually free of
internal defects.
2
Experimental
LWT is equipped with a 32 kW laser system comprising two individual 16 kW disk lasers.
Via a so-called twin laser light cable (Twin-LLK), laser power from each laser can be led to
the laser optics via two parallel and closely spaced cores – one from each laser. Each core has
a diameter of 0.20 mm and the center-to-center distance between the cores is 0.36 mm for
details see4.
The two individual and parallel laser beams is led to conventional laser optics, thereby
yielding two spots on the work piece during welding. For most welding applications,
particularly in the high-power range, the volume of the melt pool will have such a magnitude,
that it is probed by laser intensity from both lasers simultaneously i.e. up to 16+16 kW (32
kW) laser power can be delivered in a single effective volume. Combined with the welding
heads at LWT’s disposition, the described dual-spot arrangement gives a large degree of
freedom of how the system can be operated. This flexibility is a big advantage in the dual spot
concept.
Table 1. The dual beam concept of LWT’s 32 kW laser system
Focus test performed with the RFO C f300 mm welding head. The superimposed lines illustrate the
flexibility in defacto spot size depending on the selected spot orientation relative to the welding
direction.
Fiber
Diameter
Distance between fibers
LLK (single fiber diameter)
0.2 mm
NA
Twin-LLK (twinned fiber)
0.2 mm
0.36 mm
CFO f200 (Mag. 1:1)
RFO C f300 (Mag. 1.5:1)
RFO C f600 (Mag. 3:1)
LLK
0.20 mm LLK
Twin-LLK
0.36 mm
Twin-LLK
0.30 mm LLK
0.54 mm
Twin-LLK
0.60 mm
1.08 mm
The main disadvantage of the dual beam concept is that x+x  2x. Meaning that all other
parameters are fixed, the weld penetration and the weld quality differs significantly depending
on whether the “2x” kW laser power is supplied by a single source laser or by two laser
sources each supplying “x” kW laser power via a Twin-LLK. In figure 1 a comparison is
shown between BOP welding with, respectively, 16 kW single source laser power and 8+8
kW laser power from two laser sources. In this particular experiment, it was found, that dual
beam laser welding was significantly more sensitive to the surface condition of the test
sample and that the yielded weld penetration was significantly less as compared to single
source laser welding.
24
16 kW; oxide scaled surface
8+8 kW; oxide scaled surface
8+8 kW; milled surface
22
Weld penetration [mm]
20
18
16
14
12
10
8
6
0
2
4
6
8
10
12
14
16
18
Weld speed [m/min]
Fig. 1 BOP weld test, RFO C f300, spot orientation: Leading trailing (Twin-LLK),
FP= -3mm, 8+8 kW (dual spot) and 16 kW (single source).
3
Experimental
3.1
Bead on plate (BOP) welding
Test results obtained by bead on plat (BOP) welding differ significantly from those obtained
on actual joining by welding. The fact that BOP welding only allows vapors and plasma to
exit towards the incident laser beam is found to be a contributing factor. Accordingly, LWT
primarily uses BOP weld testing to get a first impression of where to establish weld
parameters for operation; or as a first approximation to investigate specific weld topics.
Still BOP welding has yielded interesting results. The “x+x vs 2x” comparison shown above
is one, and below is presented an example of, respectively, hardness values as function of
welding parameters and achievable weld penetration
Hardness values
The BOP weld test presented in table 2, was made with the purpose to see if the weld
parameters could be controlled to yield hardness values that conforms with industrial
acceptance levels; which it clearly did (typically applied acceptance levels for the hardness
value is that it must not exceed HV 360).
The large solidification crack observed in the center of the melt metal is partly explained
simply by volumetric effects: Molten steel takes up a volume approximately 3% than the solid
steel. Further, the heated base material elongates linearly approximately 3 % in the
temperature range from 20- 1550 °C. Both effects causing some of the weld material top over
the sample surface. During cooling some of this overtopping material will solidify as excess
weld material. As no material is added, this material is missing further down below causing
porosities and or solidification cracking. As already stated in the introduction, solving of this
challenge is, in LWT’s opinion, required before thick walled welding by high-power laser
technology reaches real industrial maturity. From previous experiments (details omitted) it
was found that these solidification imperfections form more or less on a straight line, why
their presence make the weld fail to comply with standard quality requirements.
Table 2. BOP welding aiming to yield hardness values that conform with standardized
acceptance levels.
Hardness measurement
Position
n
A
Laser power:
16+16 kW
1
3
2
x x
4
x
5
x x
Spot orientation:
leading trailing
Welding speed:
B
50 cm/minute
Focal position:
xx
0 mm
x
x x
Welding position:
PA flat
Position
1
2
3
4
5
A
B
245 HV
264 HV
264 HV
264 HV
245 HV
260 HV
270 HV
274 HV
285 HV
264 HV
Weld Penetration
Another BOP weld test was performed with the aim to reach maximum weld penetration; in
this respect LWT has managed to obtain weld penetrations exceeding 40 mm (see figure 2).
Again the weld quality does not conform to basic industrial requirements due to weld
imperfections formed during solidification.
Fig. 2 BOP weld test; RFO C f600mm, spot orientation: Side by side, FP=0 mm, 16+16 kW
3.2
Joining by laser welding
It has been surprising to experience how sensitive the quality of welds made using laser
technology is to seemingly small variations in one of many parameters, such as focal position,
variations of the chemical composition of base materials, groove preparation and the
alignment of the components to be joined by welding.
Accordingly, all developments of welding procedures at LWT are based on test specimens
with welding set-up’s designed to resemble that of the final component closely. Further, we
have adopted classical welding experience from the shipyard industry as backbone in the
development of laser welding procedures. This vaguely formulated approach has among
others resulted in the two examples, virtually free of solidification imperfections, shown
below. It can be mentioned, that one of the important factors contributing in reaching these
results is the flexibility, of the fiber bourn laser system, which allows utilization of the
benefits different welding positions offer.
Fig. 3 Weld test; RFO C f600mm, spot orientation: Side by side, FP=0 mm, 16+16 kW
Figure 3 shows an example of double-sided fillet weld joining a 40 mm thick steel plate
(S355) to a larger steel plate (S355) using 16+16 kW laser power.
By visual inspection the weld passes visual requirements for internal solidification
imperfections. The substantial incompletely filled groove observed to the right is merely a
consequence of the welding position: PB horizontal vertical. Hardness measurements
indentations can be discerned as discrete bright spots.
In this set-up, the weld penetration obtained from each side is approximately 22 mm (the
vertical carbide / sulfide stringer in the 40 mm thick steel plate marks its center).
As an example of real joining by welding a single-sided but weld joining two 50 mm thick
steel plates (S355) is presented below. Welding was made using 16+16 kW laser power, and
the weld position was PF upwards. In the cross section in figure 4 ack of fusion can be
observed where indicated by an arrow. Otherwise, the visually assessed weld quality
conforms to industrially applied standards.
Fig. 4 Weld test; RFO C f600mm, spot orientation: Side by side, FP=0 mm, 16+16 kW
The superimposed arrow point of a small weld imperfection: Lack of fusion.
4
Summary
LWT has during the past six months gained experience of thick walled laser welding with up
to 32 kW laser power.
It has so far been demonstrated that it is possible to control welding parameters to achieve
hardness values that conform to standardized acceptance levels.
Substantial weld penetrations, exceeding 40 mm in depth, has been reached by single-sided
laser welding - BOP as well as real joining.
It is experienced that it is crucial to have and deploy the flexibility of a fiber bourn laser
system, in order to be able to join (thick walled) components successfully by laser welding.
The sensitivity to the weld quality is highly influenced by even small changes to the set-up,
why development of welding procedures cannot be based (solely) on bead on plate weld
testing.
5
References
[1]
Grupp, M., Et al.
Welding of high thicknesses using a fibre optic laser up to 30 kW Welding
international Vol. 27, No. 2, pp. 109- 112 (2013)
[2]
Zheonglong, L. et. al.:
Microstructure and Mechanical Properties of Fiber Laser-Metal Active Gas
Hybrid Weld of X80 Pipeline Steel, Journal of Pressure Vessel Technology, Vol. 135,
p9. 011403-1- 7 (2013),
[3]
Vollertsen, V, Et. al.:
Welding in thick steel plates with fibre lasers and GMAW,Welding in the World,
Vol. 54 no 374, pp. 1- 10 (2010)
[4]
Honoré, M.; Højerslev, C.:
Study of the Welding Characteristics of a 32 kW disc-laser system delivering up to
2*16 kW in a common melt pool, Proceedings NOLAMP 14, August 2013, Luleå,
Sweden (2013)