High beam quality and efficiency with wide variety of materials present opportunities for welding
Fiber lasers have unique properties of high brightness, selectable beam quality, fine focusability, application flexibility, and a low cost of ownership. This makes the fiber laser a credible option as a welding source for a range of applications, especially at power levels from 100 to 1000 Watts (W).
The high power densities available from fiber lasers are ideal for use in high-speed seam and penetration welding of steels, and also welding of more reflective materials, including copper. The fiber laser also offers a number of applications for conduction welding, which occurs at much lower power densities and therefore with larger optical spot sizes. In addition, with fine control over pulse widths and pulse frequency, welding of thin materials and very small components are also possible.
Furthermore, fiber lasers have the lowest cost of ownership among today’s laser technologies. They are inexpensive to run, with extremely high wall plug efficiency, low electrical energy use, and very long life. Maintenance costs are low and there are no internal consumables within the laser. Their use in a wide range of core welding applications, together with their high speed and application flexibility, makes fiber lasers an excellent choice for automotive, medical, electronic, and aerospace industries.
Fiber laser basics — A fiber laser is produced within a small core silicon fiber, typically between 9 and 50 microns in diameter, doped with Ytterbium. (See Figure 1.)
Because the laser is generated wholly within a fiber, there is no need to align the medium to cavity mirrors, nor to maintain optics and alignment as with other lasers.
A unique feature of the fiber laser is its “focusibility.” For example, a 500-W laser can be focused to a 10-micron spot size. For many processes this would not a practical, but with effectively no lower limit on spot size, the fiber laser provides unique process parameter capabilities.
The fiber core medium can be pumped either by single emitter diodes that are spliced into the cladding surrounding the core, or by diode arrays that are launched into the cladding.
The fiber laser operating at 1,070 nanometers (nm) can be delivered to the workpiece using a flexible fiber optic cable. This lasing-fiber-to-focus-head-delivery-fiber connection can be made either by direct splicing within the laser or by using an external mechanical coupler. The advantage of using a mechanical coupler is that if the delivery fiber is damaged it can be replaced easily. By comparison, if damage occurs with a typical spliced connection, the manufacturer usually must repair the laser. Also, an external coupler may be useful to avoid damage to the laser due to back-reflections from the workpiece, most likely only an issue when welding such reflective materials as aluminum and copper.
Low cost of ownership — Fiber lasers have a very low cost of ownership. The single-emitter option offers estimated lifetimes of 100,000 hours and the diode arrays offer an estimated 50,000-100,000 hours of lifetime. To put this into perspective, 100,000 hours represents a “continually on” operational lifetime of about 11 years.
In addition, the fiber laser has an excellent wall-plug efficiency of around 30 percent, which minimized the need for the chiller (or the size of the chiller, if one is required), and also results in low electrical energy consumption. Also, there are no consumables or replaceable parts within the laser. It is worth noting, however, that fiber lasers tend to be non-repairable in the field, so if failure occurs the unit would need to be repaired by the manufacturer.
Beam quality — The fiber laser offers the highest beam quality of any laser source, so essentially it can be tuned to whatever is needed for the application. Increased weld penetration and speed are directly related to better beam quality. However, weld stability and accommodation of manufacturing variances, for example, part placement and joint fit-up requirements, tend to favor lower beam quality. There is always the option to reduce the quality of the beam to match the application, but it is impossible to increase the quality of the laser once it has exited the laser generator.
So, the key differentiator for the fiber laser is that it offers the maximum “tunable” range for beam quality. Thus, a particular process may use the optimal beam quality rather than a compromise selection made because of limitations in laser technology. In addition, the high beam quality offers unique laser parameter combinations, such as very small spot sizes of less than 0.0010-inch; or very high power densities that can access new welding applications. One excellent example of this is high speed welding of thin copper sheets for battery applications.
The fiber laser is provided in two brightness configurations: single mode, the highest brightness used primarily for cutting and multi-mode, primarily used for welding. Figure 2 indicates the correlation of beam quality to penetration/speed performance and weld width for fit-up accommodation.
It is clear from Figure 2 that a large range in welding performance exists according to beam quality, and that the selection must be made from a standpoint of overall process reliability and not simply best penetration. The center picture shows the result of a compromise made between the two extremes of beam quality and brightness.
Micro welding — The fiber laser can be routinely focused to a 0.001-inch spot size, so it can be used to produce sub 0.004-inch weld widths for very fine spot welds or high speed lap geometry seam welds. Typically, the laser power for these types of applications is around 200 to 500 W, according to penetration and welding speed. With a high degree of control over the pulse width, the weld aspect ratio (weld width/weld depth) can be tailored according to the application. Two examples of spot and seam welding are shown in Figure 3.
As well as being able to lap weld thin steels at very high speeds (in the range of 50-100 inches per second), the high power density that results from high beam quality enables the fiber laser to weld copper, which has traditionally posed a challenge for lasers because of the high surface reflectivity and reflectivity variation of copper to 1,064- and 1,070-nm wavelengths. This meant that laser welding was unstable and unrealistic for production.
The fiber laser’s high brightness overcomes this limitation because the high power density enables consistent absorption of the laser power. In addition, the relatively high welding speed that can be used avoids overheating the weld, so it stabilizes the process. The welding of copper in the sub-kilowatt range can be effectively achieved only by using a single-mode laser.
Conduction welding — In addition to keyhole welding, the fiber laser offers a number of applications for high-speed conduction welding, which occurs at much lower power densities and therefore larger optical spot sizes. Conduction welding is extremely stable, because it has no keyhole and no welding plume.
In addition, the larger spot size tends to be less sensitive to part fit-up variance. This method can be very effective for barrier sealing that requires a penetration of 0.01-inch. The resulting weld has a very smooth, highly aesthetic appearance. Figure 4 shows a step down circumferential weld in a pressure sensor.
Penetration welding — Penetration welding generally describes welding depths beyond 0.04-inch and in most cases up to 0.25-inch thickness. The beam quality of fiber laser sources offers the most penetration depth per watt of laser power of any laser, with even a 500-W laser capable of welding up to 0.08-inch thick. This welding performance is achieved by using a relatively small spot size, around 0.003-inch. To take advantage of this, the joint geometry must be either lap weld or a butt weld, with very high control of fit-up tolerances. In most cases this is not practical for manufacturing, so typical laser powers range from 1 to 6 kW for most penetration welding applications.
Even if the particular process cannot take advantage of the fiber laser’s welding performance due to part fit and joint configuration, the fiber laser offers other benefits that result in increased process stability and implementation flexibility. The 1-micron wavelength of the fiber laser has much less effect from the weld plume compared to a CO2 laser, and so the gas shielding used to suppress the welding plume requires a minimal flow rate, with a low sensitivity to positioning relative to the weld.
The fiber laser’s high beam quality and the single- and multi-mode options make it an effective and efficient option for welding many materials and applications. The flexibility of implementation along with the low cost of ownership contributes to the technology’s appeal.
Geoff Shannon, Ph.D., is the laser technology manager for Miyachi Unitek Corp. Contact him at Geoff@muc.miyachi.com.