Proceedings of the 3rd

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

Abstract

The development of disk and fiber laser with a high

beam quality in the multikilowatt range has led to new

industrial processes and enhancement of the standard

processes in terms of processing speeds, welding depth

etc. However a laser with high average power and high

beam quality is not always the answer because:

o Laser with a high average power and beam

quality is more expensive

o With high average power the processing

speeds can be increased but there is no gain in

efficiency meaning that costs and/or distortion

can be greatly increased.

o A better solution would be to get more from

the laser’s output- a more efficient beam-

material interaction.

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.

Introduction

Continuous wave (CW) Nd; YAG lasers with beam

quality of 25mm- mrad or better that can also super-

modulate to peak powers of up to 2 times of their

average power ratings have shown to improve material

processing [1-3]. Weld penetration improvements of

up to 30%, depth of focus improvements of 40%, and

the ability to weld more reflective alloys are just some

of the gains made by employing super- modulation in

welding. These lasers operate in CW mode but can

pulse or modulate the laser out power with peak power

more than 2 times their CW rating (Table 1). This is

usually accomplished by storing some energy in the

power supply during the beam off time. Extra energy

sent to lasing medium during turn-on of the laser

results in a short duration of high peak power. These

lasers are often very useful because they provide

thermal diffusivity yet they have high average power

for fast processing.

Table 1: Laser Specifications

Laser

Power

(kW)*

PP+

(kW)

Frequency

(Hz)

Fiber Size

(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 SuperModulationTM

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

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.

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

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.

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.

100Hz, 100% Depth100% Demand

10msec

200Hz, 50% Depth 100% Demand

200Hz, 50% Depth

50% Demand

CW, 100% Demand

CW 50% Demand

5msec

100Hz, 100% Depth100% Demand

10msec

200Hz, 50% Depth 100% Demand

200Hz, 50% Depth

50% Demand

CW, 100% Demand

CW 50% Demand

5msec

100Hz, 100% Depth100% Demand

10msec

200Hz, 50% Depth 100% Demand

200Hz, 50% Depth

50% Demand

CW, 100% Demand

CW 50% Demand

5msec

100Hz, 100% Depth 100% Demand

200Hz, 50% Depth 100% Demand

200Hz, 50% Depth 50% Demand

CW 50% Demand

CW 100% Demand

10msec 5msec

Figure 1: Sine wave modulation up to 200% peak power

10msec5msec

CW, 100% Demand

CW 50% Demand

SqWave-200Hz

50%Demand 100% Peak

Sq Wave- 100Hz,

100% Demand, 200% Peak

10msec5msec

CW, 100% Demand

CW 50% Demand

SqWave-200Hz

50%Demand 100% Peak

Sq Wave- 100Hz,

100% Demand, 200% Peak

200Hz, 100% Peak 50% Demand

100Hz, 200% Peak 100%

Demand CW 50% Demand

CW 100% Demand

10msec 5msec

Figure 2: Square wave modulation up

to 200% peak power

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 power1

(W)

2000

Maximum Modulated Peak

Power1 (W)

4000

Beam Quality2 (mrads) 24

Fiber Diameter (µm) 600

Output Modes CW, Square, sine

Modulation Frequency (Hz)

Delivery Options Up to 4 way time

share

1 Rated @ workpiece at end of lamp life

2 Halfangle radius

Figure 3: JK2003SM CW Laser

Results and Discussion

Low carbon steel

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.

Sq. wave 200% peak, 100Hz Sq. wave 200% peak, 500Hz

Sq. wave 200% peak, 1000Hz Sine wave, 1000Hz

Figure 4: Some Examples of various modulated outputs

Stainless Steel

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,

Modulation

Frequency

(Hz)

%

Demand

%

Peak

% On-

time

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

Figure 4: Material thickness Vs. Welding speed for LCS

(Average power 2000W, 600um spot)

0

1

2

3

4

5

6

7

8

0 2 4 6 8 10

Thickness (mm)

Wel

din

g s

pee

d (

m/m

in)

Sq.wave (170% Peak, 200Hz)

Sine wave (70% Depth, 200Hz)

CW

Table 3 Pulse widths and pulse energies for different modulation frequencies

(4mm thick LCS, Sq. wave, 100% mean and 170% peak)

Figure 5: 4.13mm LCS, 1.9 m/min,

Sq. wave, 100% Mean, 170% Peak

200Hz 400Hz

800Hz

1000Hz

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.

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.

Zinc coated Steels

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 0.1-

0.2mm 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 zinc-

coated 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].

Figure 6: Material thickness vs. welding speed for 304SS

(Average power 2000W, 600um spot, argon shield)

0

2

4

6

8

10

12

0 2 4 6 8 10

Thickness (mm)

Weld

ing s

peed

(m/m

in)

Sq. wave

Sine wave

CW

CW Square wave Sine wave

Figure 7: 2mm thick 304SS, 600µm spot, argon shield

CW Sine wave Square wave

Figure 8: Weld Cross Sections- (304SS,

argon shield gas, spot size 600µm)

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).

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.

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 of focus with super-

modulation

• The power reduction between peaks in super-

modulation greatly reduces plume or soot

shading and allows higher weld penetration

compared to CW operation.

• Greatly reduce porosity- much better than

CW welding

References

[1] Naeem M, SuperModulation Cost of Ownership

Proceedings of the Fourth International WLT-

Conference 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 Electro-

Optics (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

[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

Top bead Transverse cross section

Figure 9: Typical defects found in laser lap welding

Zinc coated steels without gap at the interface

Figure 10: Cross section of welds made with modulated

laser beam %peak 150, modulation frequency 600Hz

Top bead Transverse cross section