Nd:YAG lasers, from very low to high power, with fiber-optic beam delivery and through-the-lens viewing capabilities, are often the preferred laser for welding applications because they are easy to integrate and control in the factory. It has long been thought that the better the beam quality of these lasers, the better the processing achieved. Consequently, 10 years of innovations have tended toward improved beam quality and electrical efficiency related to diode pumping of rods, disks and fibers. The author believes that with beam qualities better than about 15 mm-mrad (radius, half-angle), significant improvements in processing are elusive. How is it that disk lasers of about 1 kW average power with beam qualities of better than 5 mm-mrad show the same capabilities in welding aluminum as a lamp-pumped laser that has three times "worse" beam quality and electrical efficiency?1 The beam/material interaction appears to be the key.
Three aspects of the beam/material interaction, thermal diffusivity, beam scattering by the plume, and depth of focus tolerance, if improved, would mean a great deal to processing efficiency and repeatability. "Super Modulation," a modulation technique for CW lasers, has been shown to achieve improvements in these areas, with associated follow-on benefits.2
The technique
Super Modulation is a technique in which some energy is stored in the laser's power supply during the laser's off-time or during a low average power cycle, and then is delivered to the lasing medium during a later on-time or a high average power cycle. This produces a momentary output power that is up to 2.5X higher than that of the laser's mean power rating.
Simple examples of Super Modulation in use are square wave and sine wave output waveforms. Although the laser's peak power can be up to 2.5X that of the laser's rated mean power, the laser can still produce its rated average power at repetition rates of up to 1 kHz or higher, as well as run standard CW. Figure 1 shows some basic Super Modulated output waveforms compared to the CW output.
Figure 2. Weld penetration at 1 kW and 3 m/min.
The high peak power and modulated output achieved by Super Modulation, combined with "good" beam quality, produces some welding effects that are quite phenomenal. Refer to the stainless steel weld cross sections shown in Figure 2. CW, sine wave and square wave outputs having the same average power, same focus spot size and same travel speed were employed. The only variable changed between examples is the output waveform. Notice the rather large top bead of the CW waveform and its lower penetration. Sine wave output shows improvement with less melting width at the top of the weld and higher penetration, and square wave output is even better with a very narrow symmetrical weld.
Reduced thermal diffusion effects
It is believed that improvements achieved by Super Modulation are based on reducing the effects of thermal diffusivity and plume scattering. Thermal diffusivity is a measure of a material's thermodynamic "leaking" of laser-induced heat energy into the material. It is defined as thermal conductivity divided by both the density and heat capacity. Materials like aluminum have a high thermal diffusivity of about 0.91 cm2/sec, while easier-to-weld alloys like titanium have a thermal diffusivity of about 0.09 cm2/sec. Iron alloys have a thermal diffusivity of about 0.2 cm2/sec.
Figure 3. Material thickness versus welding speed for 6181 Al alloy (JK1002, mean power 1000 W, modulation frequency 500 Hz, peak power 2 kW).
By having high peak power "pulses" of Super Modulated energy the rate of applied laser heat can be more than doubled. This better overcomes the heat loss in the material-especially in aluminum and other high thermal diffusivity alloys. In Figure 3, aluminum welding rates can be increased by a factor of 600 percent when using square wave modulation at the 1mm penetration level. This laser operating in square wave modulation can process 80 percent higher penetrations than a CW laser under the same conditions.
Reduced plume scattering effects
Unlike the reverse-Bremsstrahlung absorption of CO2 laser beams by the weld plume, with Nd:YAG lasers small particles in the plume can interact with shorter wavelength light and scatter it away from the focus point, decreasing welding efficiency. In fact, researchers have seen more than 40 percent of the laser's power lost to scattering.3 Studies have shown that this is most likely the reason for the wider top bead in CW welds and is also the reason weld penetration can vary with cross jet control.
By delivering laser energy in a periodic fashion with high peak powers, time is allowed for the soot to diminish during the off-time or low-power cycle of the laser beam. When the next high peak power cycle arrives at the weld zone, the particulate density seems to have fallen below the significant scattering threshold. Because the laser is operating at full average power, the welding speed is not reduced.
Some alloys appear to have a longer particulate reduction time constant than others. In some tests, by changing the modulation frequency from 500 Hz to 250 Hz, the weld penetration increases by almost 100 percent with no change in average power. Overcoming the scattering phenomenon has been shown to allow welding with 25–40 percent less average power and to produce welds with much less distortion and more consistency.
Improved depth of focus tolerance
Super Modulation provides another benefit in improving the laser's depth of focus to produce good parts, especially with lower average power applications. Figure 4 shows some welding rates in stainless steel at about 500W. Note the significant benefit of using Super Modulation in welding penetrations and speeds. Now compare this to Figure 5, where the weld penetration versus focus position is shown between a 470W CW Nd:YAG and a 350W Super Modulated square wave Nd:YAG. The square wave Nd:YAG produces 10 percent higher penetration with 25 percent less average power, and, if the requirement is a 1mm-deep weld, then the increase in depth of focus is apparent from the values and the flat penetration curve for Super Modulation.
Improving laser efficiency and beam quality can be an academic exercise if it doesn't translate into better and more efficient processing. The results related to Super Modulation of the Nd:YAG beam produce a better process with less laser energy, or more capability for the same output power. In essence, the beam/material interaction improves. Also, a laser of lower mean power and lower cost can be as capable as a CW laser with twice the average power. Making the welding process more efficient might be better than making lasers more efficient.
Tom Kugler is applications manager with GSI Lumonics, Northville, MI. Contact him by e-mail at [email protected].
References
- C. Emmelmann and S. Lunding, High Power Disk Laser Technology-design and potential for laser macro applications, ICALEO 2002 proceedings, LIA Pub #594, ISBN 0-912035-72-2.
- T. Kugler, M. Naeem, Welding and Cutting Improvements with Super Modulated, High BQ, Nd:YAG Lasers, ICALEO 2002 proceedings, LIA Pub #594, ISBN 0-912035-72-2.
- J. Greses, P. Hilton, C. Barlow, W. Steen, Plume Attenuation under High Power Nd:YAG Laser Welding, ICALEO 2002 proceedings, LIA Pub #594, ISBN 0-912035-72-2