Four lasers can be used for micro welding: pulsed neodymium-doped yttrium aluminum garnet (Nd:YAG), continuous wave (CW) fiber, quasi continuous wave (QCW) fiber, and nanosecond fiber. Each laser type offers unique features that work best for specific applications. Here is a comparison of the pulsed Nd:YAG laser with the three fiber laser options, and a discussion of why and when one might be chosen over the other. In some cases, several options may work; in that case, cost of ownership and serviceability can tip the scales.
With the Nd:YAG laser, the active gain medium is neodymium, which is doped into a host crystal of yttrium aluminum garnet. This solid rod of material is typically 0.1-0.2-inches in diameter and around 4 inches long. Micro welding Nd:YAG lasers are optically pumped using flashlamps and typically emit light with a wavelength of 1064 nm, but can be frequency doubled (532nm, green). The laser's optical design is relatively simple; its heart is the power supply that drives and controls the flashlamp voltage and allows precise control of peak power and pulse width during the laser pulse using internal optical feedback.
With excellent pulse control, the Nd:YAG laser also offers high peak powers in small laser sizes, which enables welding with large optical spot size. This translates to maximized part fit-up and laser to joint alignment accommodation. An example is a 25W laser that can provide 6kW of peak power, sufficient to weld steel and aluminum with a 600 micron spot size.
The pulsed Nd:YAG laser has been around for many decades and has by far the largest install base. In today's laser landscape, it is best suited to spot welding applications under 0.02-inch penetration and seam welding heat-sensitive packages. Figure 1 includes a few examples of Nd:YAG laser welding.
A fiber laser is generated within a flexible doped glass fiber that is typically 10 to 30 feet long and between 10 and 50 microns diameter. Ytterbium is used as the doping element because it provides good conversion efficiency and a near 1 micron output wavelength, which matches well with existing laser delivery components. The laser is generated wholly within a fiber, so there is no need to align the medium to cavity mirrors, nor is it necessary to maintain optics and alignment as with the pulsed Nd:YAG laser. The very efficient lasing process means the fiber laser can be small, air cooled, and provide high wall plug efficiencies.
The fiber laser's unique characteristics are its “focusability” and its range of beam qualities, which can be tuned for each welding application. The two ends of the beam quality spectrum are single mode and multi-mode – single mode is defined by a beam quality of M2 less than 1.2, while multi-mode is generally above M2 of 2. The mode defines how well the laser can be focused and the power density distribution across the laser. Figure 2 is a schematic of single and multi-mode fiber lasers.
The differences between single mode and multi-mode lasers are shown in Figure 3. All welds are shown in 0.06-inch thick stainless steel – (a) represents a 500W single mode fiber laser with an M2 value of 1.2 at 300 inches per minute (IPM) with a 30 micron spot size; (b) shows a 700W multi-mode laser with an M2 value of 15 at 100 IPM with a150 micron spot size; and (c) shows a 1kW multi-mode fiber laser with an M2 value of 15 at 80 IPM with a 250 micron spot size.
Continuous Wave (CW) Fiber Lasers
For high speed seam welding applications, this laser is operated in continuous wave mode, which means that the laser output remains on until being turned off. For spot welding either a single weld or a seam, the laser output can be pulsed or, more correctly, modulated – turning the laser on and off rapidly. The CW laser's peak power is the same as its maximum average power, so focused spot sizes are generally under 100 microns to attain sufficient power density for welding with power levels under 1kW. Due to the small optical spot size, lap and fillet weld geometries are preferred. Butt welding is possible if there is good part fit-up or if a seam location vision system is used. An alternate is using a scan head that can create motion lateral to the weld direction, known as wobble, which can effectively widen the weld to decrease joint alignment sensitivity.
CW fiber lasers are well suited to general seam welding up to 0.06-inch depth for a 500W laser, high speed seam welding of same and dissimilar materials, and producing spot welds under 100 microns in diameter. Figure 4 includes examples of CW fiber laser applications.
Quasi-Continuous Wave (QCW) Fiber Lasers
The QCW fiber laser's peak power and pulse width characteristics are similar to those of the Nd:YAG laser, though the parameter range is not quite as broad. Similar to CW fiber lasers, the QCW lasers offer single mode to multi-mode options with spot sizes from 0.001-inch to 0.04-inch as needed for the application. These lasers also shine in small spot size applications and penetration applications, although they do provide a fairly comprehensive coverage of many micro welding applications.
Figure 5 shows QCW fiber laser application examples.