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Proceedings of the 3 Pacific International Conference on Application of Lasers and Optics 2008

WELDING PERFORMANCE OF...
approximately 90-100% Demand. Pcal is used to set
100% demand to rated laser output. For Sine Wave
operation, the paramete...
In order to understand SuperModulationTM, welding
trials were carried out in low carbon steel, stainless
steel, zinc coate...
Table 3 Pulse widths and pulse energies for different modulation frequencies
(4mm thick LCS, Sq. wave, 100% mean and 170% ...
which has a similar effect in diffusing the laser beam.
For the pulsed laser welds, the effect is reduced, as the
modulati...
The use of a continuous wave (CW) laser on joint with
no clearance can lead to spattering and potential
porosity formation...
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Welding performance of a 2 kw continuous wave supermodulated ndyag laser increased weld speed, weld penetration and reduced porosity with supermodulated output power

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Welding performance of a 2 kw continuous wave supermodulated ndyag laser increased weld speed, weld penetration and reduced porosity with supermodulated output power white paper from JK Lasers.

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Welding performance of a 2 kw continuous wave supermodulated ndyag laser increased weld speed, weld penetration and reduced porosity with supermodulated output power

  1. 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. 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. 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. 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. 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. 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

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