Laser cutting is often accepted as the most widely adopted laser materials processing application, second only perhaps to laser marking. While laser cutting is synonymous with high-power continuous-wave (CW) lasers (both CO2 and fiber), marking is truly the preserve of the nanosecond pulsed fiber laser. What is not generally appreciated is that these nanosecond lasers are impressive when it comes to cutting.
Over the last decade, the versatility and control offered by nanosecond pulsed fiber lasers has made them the laser of choice for a variety of micromachining applications such as engraving, ablation, scribing, drilling, and texturing, as well as cutting. Even though these lasers typically have less than a few millijoules (mJ) of pulse energy, they can deliver peak powers >10 kW at high beam quality and with average powers increasing in recent years (now up to 300 W), they pack an impressive punch. Available as a simple Q-switched design with limited pulse control or with a more sophisticated master-oscillator power amplifier (MOPA) architecture, as epitomized by TRUMPF’s TruPulse nano range, they offer greater parametric control of the pulses in terms of pulse duration and repetition frequencies.
With such low pulse energy and modest powers, it’s perhaps no surprise that nanosecond pulsed fiber lasers are generally not considered for cutting and indeed they cannot compete with the sheet cutting prowess of multikilowatt systems. However, these lasers have carved out a growing number of niche applications in which they have demonstrated exceptional capabilities.
Nanosecond pulsed fiber lasers are most commonly used in conjunction with scanner-based beam deliveries, enabling very fast processing speeds. Small spot sizes are required to ensure that there is sufficient power density to enable melting and vaporization of the substrate material. By making a single line scan, sufficient pulse overlap is required to enable material to be removed by a combination of vaporization and melt ejection. Reducing scanning speed to increase the pulse overlap will change basic surface marking to light material removal and eventually to deeper engraving. If the material is thin enough, the process results in a cut; otherwise, the result is a scribe. This single-pass cutting regime is limited to relatively thin materials, but has found a very specific application in the manufacture of today’s batteries.
The use of nanosecond pulsed fiber lasers is quickly replacing traditional mechanical slitting and stamping processes, offering a noncontact, wear-free process with unrivaled flexibility and control. Lithium-ion battery cells are made up of layers of coated anodes/cathodes made from thin aluminum/copper foils that are used in all cell designs. The foils are typically very thin—6 to10 µm for copper anodes and 10 to 15 µm for aluminum cathodes. Both sides of these electrodes are coated with proprietary mixes, including lithium metal oxides and graphite and can be up to 100 µm thick, making some of these coated foils over 0.2 mm in total thickness. Single-mode CW fiber lasers can be used very effectively for the cutting of bare metal foils, capable of achieving extremely high cutting speeds with exceptional edge quality—but they are not the best choice for the cutting of coated electrodes.
Although some applications are just basic slitting where a single material is being processed, the majority—such as notching where a profiled tab is cut—require the processing of both bare foil and coated materials in the same pass, with an insulating ceramic layer sometimes adding a third material to further increase complexity (see Fig. 1). In these applications, nanosecond pulsed fiber lasers excel, as they are able to cut these coated electrodes at speeds >1 m/s with just 200 W of average power. The MOPA design is key to allow users to tune-in the pulse characteristics to optimize the process performance, balancing the cut quality and processing speed requirements of the various materials (see Fig. 2).
Scribing with these lasers is also extensively used, particularly in the solar industry where single-mode nanosecond pulsed lasers are used in a “score and break” application in the manufacture of silicon solar cells. The current trend is to cut the larger cells into smaller strips to increase performance by reducing resistive losses. These half-cell and shingle cell designs rely on the high-quality, low thermal-damage scoring achievable with these lasers (see Fig. 3).
In metallic materials, multipass processing (that is, going over the same line again) can increase the achievable depth, but this process is self-limiting. As the depth and therefore the aspect ratio to the cut increases, the effective energy density decreases to a point where further passes have no impact to depth or material removal.
To increase the depth that can be cut, the effective cut (kerf) width needs to be increased, which is governed by the focal spot. This cannot be achieved by simply increasing the focal spot size, as this would reduce the incident energy density below the processing threshold. However, this can be achieved in a number of ways. The first is to off-set the cut line between passes, which is particularly suited to X-Y table-based systems, typically focal spot diameter Ø and thus effectively doubling the cut width. Care needs to be taken in programming to ensure that the dimensions of the finished part are acceptable and as the thickness of the material cut increases, a slight taper on the cut edge is created. Secondly and more effectively with scanner-based beam delivery, the “wobble” feature of the marking software (originally developed to widen the marking line width) can be used to increase cut depth. This feature effectively generated a spiraling of the beam at a predefined amplitude along the cutting line. Control of the wobble width, wobble frequency, and cut line speed can optimize the pulse overlap to maximize material removal.
This capability is well exploited in the jewelry industry, where the high peak power of the nanosecond pulses couples effectively into reflective metals such as silver and gold. Fiber lasers are now the norm for processing fine filigree silver and gold items, where they can be used for cutting, marking, engraving, and texturing, giving designers and manufacturers enhanced flexibility (see Fig. 4). Many fiber lasers are in 24/7 manufacturing environments where the reliability of these laser sources has ensured strong growth in this application area.
Another interesting niche application is the cutting of wires, which are mechanically cut. This is particularly relevant for soft metals that deform or very hard/brittle material that can shatter. A good example is the cutting of tungsten wires, where mechanical cutting can lead to cracking and chipping and can leave an uneven finish. Using a simple 20 W nanosecond pulsed laser with a multipass wobble technique can give an accurate and repeatable cut finish (see Fig. 5).Cutting using scanner-based techniques is typically limited to material thicknesses <1 mm; however, thicker sections can be processed by periodic adjustment of the focal position into the material to maintain energy density. This does give rise to slightly more taper to the cut edges than conventional fixed-head laser cutting. However, nanosecond fiber lasers can also be used in conjunction with a standard cutting head and nozzle with assist gas just like conventional CW lasers. A 50 W nanosecond pulsed fiber laser can cut silver, brass, and copper that would normally require CW power levels >200 W.
In some applications, the process versatility of fiber lasers mean that the same laser source can be used to cut, mark, and even weld complex micro-components in, for example, the medical device industry. This can take out several manufacturing steps, helping to streamline manufacturing processes.
It should be noted that the use of nanosecond pulsed fiber lasers for cutting is not restricted to just metals. These lasers can be used effectively on a wide variety of nonmetallic materials that exhibit at least some level of absorption of the 1 µm wavelength. Other materials that can be cut include silicon, carbon fiber-reinforced plastics, ceramics, rubbers, and even some plastics and polymers (see Fig. 6).
Pulsed nanosecond fiber lasers, particularly those with a MOPA design, have created a valuable growing niche in the laser cutting market for thin-section hard and reflective materials. As average powers increase, they could provide a viable alternative to conventional CW cutting sources for some applications. Design and manufacturing engineers should take note of the application versatility that these nanosecond fiber lasers provide for innovative manufacturing solutions for the products of tomorrow.