Welding performance of a 2 kw continuous wave supermodulated ndyag laser increased weld speed, weld penetration and reduced porosity with supermodulated output power
1) Supermodulated lasers can pulse or modulate their output power with peak powers over 2 times their continuous wave (CW) rating, providing benefits for welding like increased weld penetration and speed.
2) Welding trials using a 2kW supermodulated laser showed increased welding speeds, greater weld penetration, and reduced porosity compared to CW output. The best results used lower modulation frequencies of 100-200Hz.
3) Modulated outputs produced narrower welds in stainless steel compared to CW, likely by disrupting plasma or vapor plumes that diffuse the laser beam in CW welding. Modulation also greatly reduced porosity in welds.
Similar to Welding performance of a 2 kw continuous wave supermodulated ndyag laser increased weld speed, weld penetration and reduced porosity with supermodulated output power
Similar to Welding performance of a 2 kw continuous wave supermodulated ndyag laser increased weld speed, weld penetration and reduced porosity with supermodulated output power (20)
Welding performance of a 2 kw continuous wave supermodulated ndyag laser increased weld speed, weld penetration and reduced porosity with supermodulated output power
1. rd
Proceedings of the 3 Pacific International Conference on Application of Lasers and Optics 2008
WELDING PERFORMANCE OF A 2KW CONTINUOUS WAVE SUPERMODULATED ND:
YAG LASER- INCREASED WELD SPEED, WELD PENETRATION AND REDUCED
POROSITY WITH SUPERMODULATED OUTPUT POWER
Mohammed Naeem
GSI Group, Laser Division
Cosford Lane, Swift Valley
Rugby, CV21 1QN, UK
mnaeem@gsig.com
processing [1-3]. Weld penetration improvements of
up to 30%, depth of focus improvements of 40%, and
Abstract
the ability to weld more reflective alloys are just some
of the gains made by employing super- modulation in
The development of disk and fiber laser with a high
welding. These lasers operate in CW mode but can
beam quality in the multikilowatt range has led to new
pulse or modulate the laser out power with peak power
industrial processes and enhancement of the standard
more than 2 times their CW rating (Table 1). This is
processes in terms of processing speeds, welding depth
usually accomplished by storing some energy in the
etc. However a laser with high average power and high
power supply during the beam off time. Extra energy
beam quality is not always the answer because:
sent to lasing medium during turn-on of the laser
results in a short duration of high peak power. These
o Laser with a high average power and beam
lasers are often very useful because they provide
quality is more expensive
thermal diffusivity yet they have high average power
o With high average power the processing
for fast processing.
speeds can be increased but there is no gain in
efficiency meaning that costs and/or distortion
Table 1: Laser Specifications
can be greatly increased.
o A better solution would be to get more from
the laser’s output- a more efficient beammaterial interaction.
Laser
Power
PP+
Frequency
Fiber Size
(kW)*
At GSI Group, we have undertaken a number of
initiatives to raise Nd: YAG laser power and its
processing performances. Work has centred on high
average power (400W-2000W) continuous wave (CW)
systems. Continuous wave (CW) Nd: YAG lasers with
beam quality of 25mm- mrad or better that can also
SuperModulateTM to peak powers of up to 2 times of
their average power ratings have been shown to
improve material processing. This paper documents
the basics of generating SuperModulated beams but
emphasizes the process improvements in welding of
standard alloys.
(kW)
(Hz)
(µm)
JK400
0.4
0.8
100-1000
400
JK500
0.5
1.0
100-1000
600
JK800
0.8
1.6
100-1000
400
JK1002
1.0
2.0
100-1000
600
JK2003
2.0
4.0
100-1000
600
* Average power @ workpiece end of lamp life
+
PP Peak power
Introduction to SuperModulation
Introduction
Continuous wave (CW) Nd; YAG lasers with beam
quality of 25mm- mrad or better that can also supermodulate to peak powers of up to 2 times of their
average power ratings have shown to improve material
TM
These lasers produce three- outputs i.e. CW, Sine
Wave and Square Wave (Figures 1-2). For CW
operation, the only parameter that is used is the
Demand Range. The laser begins to produce power at
about 7% Demand, approximately half its rated power
at about 50% Demand, and full rated power at
2. approximately 90-100% Demand. Pcal is used to set
100% demand to rated laser output. For Sine Wave
operation, the parameters used are Demand Range, and
Frequency. The Frequency is set between the valves of
100-1000Hz. The demand Range varies from 0% to
100%, with 7% being the threshold for laser operation,
50% produces approximately half the laser rated
average power, and 90- 100% produces full rated
power. The depth value ranges from 0-100% and if the
depth value is set to 0% than the laser will only operate
in CW mode with no sinusoidal output. If the Depth
value is set to 100% with a Demand Range of 100%
then the laser’s peak power will be 200% of the rated
average power and the minimum value will be 0W at
the trough of the sinusoidal waveform. If the Depth
valve is set to 50% with a Demand Range of 100%
then the peak power will be 150% of the average
power determined by the Demand Range and the
minimum power at trough of the sinusoidal would be
50% of the rated average power. Adjusting the
Demand Range changes the peak and minimum
powers of the sinusoidal output but the waveform
shrinks in proportion to the mean power that the
Demand Range calls for so there is no “clipping” of
the sinusoidal waveform at 0% or 200%. Adjusting the
frequency does not affect the depth values of the laser,
simply the frequency of the sinusoidal output.
100% than the peak power will be 150% of the average
power determined by the demand range and the
minimum power at trough of the sinusoidal would be
50% of the rated average power. Adjusting the demand
range changes the peak and minimum powers of the
sinusoidal output but the waveform shrinks in
proportion to the mean power that the demand range
calls for so there is no “clipping” of the sinusoidal
waveform at 0% or 200%. Adjusting the frequency
does not affect the depth values of the laser, simply the
frequency of the sinusoidal output.
For Square Wave operation, the parameters used are
demand range peak power, and frequency. The
frequency is set between the values of 100-1000Hz.
The percent on- time or duty cycle of the laser in
square wave is determined by the ratio of the demand
range to the peak power. The actual on time or pulse
width is determined by the duty cycle and the
frequency.
200Hz, 100%
SqWave-200Hz
50%Demand
Peak 50%
100% Peak
Demand
100Hz, 100% Depth
100Hz,100% Demand
100% Depth
100% Demand
200Hz, 50% Depth
200Hz, 50% Depth
100% Demand
100% Demand
5msec
5msec
10msec
10msec
CW, 100% Demand
CW 100%
Demand
CW 50% Demand
CW 50%
Demand
100Hz, 100Hz,
Sq Wave- 200%
100% Demand,
Peak 100%
200% Peak
Demand
Figure 2: Square wave modulation up
to 200% peak power
200Hz, 50% Depth
200Hz, 50% Depth
50% Demand
50% Demand
CW 100%
CW, 100% Demand
Demand
5msec
5msec
10msec
10msec
CW 50% Demand
CW 50%
Demand
Figure 1: Sine wave modulation up to 200% peak power
If the depth value is set to 100% with a demand range
of 100% then the laser’s peak power will be 200% of
the rated average power and the minimum value will
be 0 W at the trough of the sinusoidal waveform. If the
depth valve is set to 50% with a demand range of
For example, if the laser parameters on JK2003 are set
to 200% peak power and 100% demand range and
500Hz, the duty cycle is 100/200 or 50% on-time.
Because the laser is operating at 500Hz, the pulse
period is 1/500 or 2mses. 50% of 2msec is 1msec so
that laser these parameters will be operating at 500Hz
with pulse – width or on time of 1msec and a peak
power of 200% of the rated power or 4kW. These
parameters are being produced at the full 2000W rated
average power of laser since the demand range is set at
100%. Pcal must be used to set 100% demand to rated
laser output power to celibate modulation range.
3. In order to understand SuperModulationTM, welding
trials were carried out in low carbon steel, stainless
steel, zinc coated steels, titanium and aluminium
alloys. Some of the results achieved with
SuperModulationTM i.e. welding speeds, penetration
depth and porosity formation are presented.
Experimental Work
Laser and Welding Trials
The processing trials were carried out with JK2003
laser (Figure 3) and the laser specification is
highlighted in Table 2. The beam from the laser was
transmitted through 600µm fiber, which terminated in
200mm-output housing fitted with focusing optics. The
output housing was fitted with a 200mm focal length
recollimating lens and a 200 -focusing lens. This
arrangement gave a calculated spot size of 600µm. In
each of the materials, welding trials were carried out
with CW, square and sine wave outputs to develop
laser parameters and welding speeds for the production
of full penetration melt runs. Examples of the square
and sine wave outputs for different modulation
frequencies are shown in Figure 4. Parameters and
welding speeds were adjusted to produce welds with
consistent topbead and underbead with minimal
spatter. Gas shielding for the weld topbead was
supplied via a 10mm diameter pipe. In all cases, argon
(10l/min) was used for shielding.
Table 2: Laser Specifications
Maximum Average power
(W)
1
Figure 3: JK2003SM CW Laser
Sq. wave 200% peak, 100Hz
Sq. wave 200% peak, 1000Hz
2000
Sq. wave 200% peak, 500Hz
Sine wave, 1000Hz
Figure 4: Some Examples of various modulated outputs
Maximum Modulated Peak
Power1 (W)
4000
Beam Quality2 (mrads)
24
Low carbon steel
Fiber Diameter (µm)
600
Output Modes
CW, Square, sine
The welding results show a significant increase in
welding with supermodulated compare to CW output
for the same average power (Figure 5). The results
show that the welding speed was greatest at the lowest
modulation frequency, 200Hz, because at lower
frequencies the pulse width or on time and the
corresponding pulse energy is greater than the higher
frequencies (Table 3). Figure 5 shows micrographs of
the welds made with square wave modulation at
different modulation frequencies.
Results and Discussion
Modulation Frequency (Hz)
Delivery Options
1
Up to 4 way time
share
Rated @ workpiece at end of lamp life
2
Halfangle radius
4. Table 3 Pulse widths and pulse energies for different modulation frequencies
(4mm thick LCS, Sq. wave, 100% mean and 170% peak)
%
Demand
%
Peak
% Ontime
Pulse
period
(ms)
Pulse
width
(ms)
Pulse
energy (J)
Weld
penetration
(mm)
200
100
170
58.8
5.0
2.94
10.0
4.0
400
100
170
58.8
2.5
1.47
5.0
3.6
600
100
170
58.8
1.67
0.98
3.33
3.0
800
100
170
58.8
1.25
0.74
2.5
2.75
1000
100
170
58.8
1.0
0.59
2.0
2.50
Welding speed (m/min)
Modulation
Frequency
(Hz)
8
7
6
5
4
3
2
1
0
Sq.wave (170% Peak, 200Hz)
Sine wave (70% Depth, 200Hz)
CW
0
2
4
6
Thickness (mm)
8
10
Figure 4: Material thickness Vs. Welding speed for LCS
(Average power 2000W, 600um spot)
Stainless Steel
200Hz
400Hz
800Hz
1000Hz
Figure 5: 4.13mm LCS, 1.9 m/min,
Sq. wave, 100% Mean, 170% Peak
Figure 6 highlight welding data for three outputs. The
modulation frequency for both square wave and sine
wave was 200Hz with 170% peak for square wave and
70% depth for sine wave. The results show that both
sine and square wave modes produced narrower weld
beads than CW mode, as shown in Figure 7. A possible
explanation for change of the weld shape is that the
CW welds produced in stainless steel exhibits a wine
glass shape (Figure 7). This is commonly associated in
CO2 laser welding due the formation of plasma above
the surface of the weld. The formation of plasma
causes the high-density beam to become more diffuse
and lose its characteristic narrow shape. In Nd: YAG
laser welding, the formation of plasma is thought to be
less prevalent, but it appears that a similar mechanism
is occurring with the CW mode. This could be related
to the formation of a “cloud” of vaporized material,
5. which has a similar effect in diffusing the laser beam.
For the pulsed laser welds, the effect is reduced, as the
modulation of the beam will disrupt the plasma (or
vaporized), which allows the beam to reach the
stainless steel with less diffusion of the energy.
Welding speed (m/min)
CW
Sine wave
Square wave
Figure 8: Weld Cross Sections- (304SS,
argon shield gas, spot size 600µm)
12
10
8
6
4
Sq. wave
Sine wave
CW
Zinc coated Steels
2
0
0
2
4
6
8
10
Thickness (mm)
Figure 6: Material thickness vs. welding speed for 304SS
(Average power 2000W, 600um spot, argon shield)
CW
Sine wave
Square wave
Figure 7: 2mm thick 304SS, 600µm spot, argon shield
Formation of weld porosity is problematic when
welding with high power CW output. Although its
presence is not necessarily catastrophic, its remains
undesirable and poses not- easily – quantified risks;
weld strength being main concern. The extent to which
the attendant strength reduction is problematic depends
upon the weldment’s intended characteristics: size,
frequency, and location. Common remedies to this
phenomenon involve optimizing laser and processing
parameters i.e. power density, weld speed, gas
shielding etc, but these adjustments are generally
applied unsystematically in reaction to observed weld
behaviour. The present study show that with
modulated output the formation of porosity is
drastically reduced compare to CW out put (Figure 8).
The modulated output produces very stable keyhole
during welding and the power reduction between peaks
in super- modulation greatly reduces plume or soot
shading and allows higher weld penetration and hence
reduced porosity compared to CW operation.
The majority of steel used in the automotive industry is
zinc coated. In comparison to uncoated steel, zinc
coated steels need extra care during overlap welding.
During the welding process, the heat will vaporize the
zinc at approx. 900 °C, which is significantly lower
than the melting point of the steel. The low boiling
point of zinc causes a vapour to form during the
keyhole process, which needs to escape from the weld
pool. In most cases, the zinc vapour can become
trapped in the solidifying weld pool resulting in
excessive undercut and weld porosity. For lap joints
(two or three layers), this effect is particularly critical
as two layers of zinc are present at the interface
between the sheets. However, producing a gap of 0.10.2mm at the sheet interface can circumvent these
problems; such systems have already been installed in
car production for the welding of double or triple layer
sheets for roof welding. Various techniques are
currently used to produce a controlled gap between the
sheets i.e.
• Joint design
• Dimples, (during stamping)
• Metal shims
• Fixture design, (controlled clamp pressure)
• Different types of zinc coatings
• Twin spot; (dual laser spots)
• Knurling [4]
The welds with modulated show that, it is possible to
weld lap joints in tightly clamped specimens of zinccoated steel sheet with a square wave modulated laser
output. With optimised laser and processing
parameters welds were produced resulting in
acceptable visually sound appearance, no internal
cracks, and no zinc gas blow- holes or pitting on the
top surface of the material. The reason for the success
of welding is due mainly to the venting of the zinc
vapour through the keyhole [5].
6. The use of a continuous wave (CW) laser on joint with
no clearance can lead to spattering and potential
porosity formation in the welds (Figure 9). During
welding the only route for exhausting the zinc vapour
is through the weld pool along with the iron vapour
formed in the keyhole. The high pressure of zinc at the
leading edge of the weld will distort the location of the
keyhole forward. Where as, the modulated laser beam
produces more stable keyhole that helps to produce
defect free welds (Figure 10). With modulated laser
beam it was confirmed through experimentation that %
peak power, modulation frequency are two major
factors governing the laser welding process. The welds
produced with high peak power (170% and 200%) had
excessive undercut top and bottom bead due to high
peak power intensity at the workpiece (9.44kW/mm2
and 11.11kW/mm2).
Summary
The work carried out has shown that super- modulation
is not just an incremental feature blip and it is a
significant new processing technique that can produce
real benefits during welding a range of materials i.e.
•
By using high peak power modulation, a laser
of lower average power can weld to greater
penetration than a similar CW unit, but with
reduced heat input.
•
High reflective materials and materials with
high conductivity (aluminium alloys)
•
Increased depth
modulation
•
The power reduction between peaks in supermodulation greatly reduces plume or soot
shading and allows higher weld penetration
compared to CW operation.
•
Greatly reduce porosity- much better than
CW welding
of
focus
with
super-
References
Top bead
Transverse cross section
Figure 9: Typical defects found in laser lap welding
Zinc coated steels without gap at the interface
[1] Naeem M, SuperModulation Cost of Ownership
Proceedings of the Fourth International WLTConference on Lasers in Manufacturing 2003, Munich,
June 2003
[2] Naeem M, Material Processing with Super
Modulation; Proceedings of the 21st International
Congress on Applications of Lasers and ElectroOptics (ICALEO 2002), Scottsdale, Arizona, USA,
October 14-17 2002
[3] Graham Helen, Throwing new light on materials
processing... an addition to the laser family; TWI
Bulletin, May - June 2006
Top bead
Transverse cross section
Figure 10: Cross section of welds made with modulated
laser beam %peak 150, modulation frequency 600Hz
Aluminium and titanium alloys
Super- modulation also offers advantages when
welding aluminium and titanium alloys [2-3]. The high
peak power modulation increases weld penetration by
developing a stable keyhole.. The stable keyhole also
improves weld quality in terms of porosity and
cracking in 6000 series aluminum alloys and porosity
levels in Ti-6Al-4V alloys.
[4] Forrest, M.G. (1996) Laser Knurling Seam
Preparation for Laser Welding of Zinc Coated Sheet
Metal – Process Development Preliminary Results,
Technical Digest of the 15th International Congress on
Applications in Lasers and Electro Optics (ICALEO
’96), Southfield, MI, pp. 133
[5] Naeem M, Lap Joint Welding of Zinc Coated
Steels without the Gap with Super Modulated
Continuous Wave Laser Beam, Patent No
WO2007060479