Cutting metals with ns Pulsed Fiber Lasers
The success of pulsed nanosecond (ns) Fiber Lasers for marking applications is well known as over the past 10 years they have become the dominant Laser source for metallic marking, and as average powers are increasing their use for engraving is growing. Having typically less than a few millijoules (mJ) in pulse energy, with peak powers around >10kW and average powers now up to 100W, they pack an impressive punch.
Our Pulsed Fiber Lasers are based on a MOPA design using a semiconductor seed in combination with fiber amplifiers, giving unrivalled control over pulse characteristics providing more control than conventional q-switched designs. These Lasers are capable of operating over a broad range of high repetition rates (1kHz-1MHz), with variable pulse width control (3-500ns), CW operation as well as a modulated Quasi-CW mode. With such versatility and control these Laser sources are fast becoming the Laser of choice for a variety of micro-machining applications such as; engraving, ablation, scribing, texturing and micro cutting. These Lasers are most commonly used in conjunction with scanner based beam deliveries enabling very fast processing speeds.
Thin foil type metals can be cut using a simple single pass process where the cutting speed is the controlling parameter to achieve full penetration. The process is typically scanner based and so requires no process gas, where the material removal is through ablation and melt ejection resulting in a burr free cut with minimal heat affected zone.
One major application for such a process is in the cutting of copper and aluminium battery foils. These can be complex materials typically having a 20-40µm metallic core sandwiched between complex proprietary layers, which can result in a total material thickness of circa 100µm. These materials can be cut with 70-100W Laser at linear cutting speeds of >1m/s.
The versatility and speed is extremely attractive to end users who have until recently relied on mechanical cutting techniques that suffer from tool wear, periodic maintenance and a general lack of flexibility. Cut quality is extremely important in this application, where minimal burr and no damage to the thin film coating are essential. Cut quality can be controlled by optimisation of the pulse and process parameters, but it has been found that for some materials a 2 pass process can result in edge quality improvements (Image 1).
For cutting thicker materials multiple passes along a coincident path are required to achieve full penetration through the material. However, there are limitations to this technique in that the effects of beam attenuation due to the increasing aspect ratio of the groove results in a reduction of processing efficiency. The process is effectively self limiting; adjustment of the spot size and moving the focal position into the material can improve the situation, but only marginally.
As when cutting with CW Lasers, the beam quality can have an impact, but with Pulsed Fiber Lasers this is less about the M2, and more about the pulse energy and the peak power that can be achieved. In a direct comparison between three Lasers of different beam qualities (all operating at 20W though the same optical arrangement) it was found that the Laser with the highest beam quality M2<1.3 did not achieve the best cutting result. The Laser with an M2 <1.6 gave the best result, this can be explained in that the lower pulse energy and lower peak power of the high quality beam was not adequately compensated for by the smaller spot size. The Laser with M2<2 trailed a poor third because although it had higher pulse energy and peak power, the poor spot size generated a significantly lower energy density on the material.
To be able to cut thicker materials, novel cutting strategies need to be adopted to effectively widen the kerf and to allow more material to be removed. This can be achieved by increasing the spot size, but this has the adverse effect of reducing the incident energy density and hence material removal efficiency. A more effective method, particularly suited to X-Y table based systems, is to off-set the cut line between passes, typically <focal spot Ø 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. With scanner based beam delivery the “wobble” feature of the marking software (originally developed to widen the marking line) can be used to good effect.
Using this feature the beam is effectively spiralling at a predefined amplitude along the cutting line. Careful control of the wobble width, wobble frequency and the cut line speed is required to optimise the pulse overlap to maximise material removal. To cut thick materials >0.5mm more complex multi-pass machining strategies can be used where the parameters can be optimised for each pass.
These techniques can be used on all metallic materials but cutting speeds and achievable thickness can vary, for example with a 100W Laser 0.9m/min cutting speed can be achieved in 1mm aluminium, while in 1mm silver this could be just 0.2m/min. The high peak powers of the ns pulse mean that they can couple into highly reflective materials, which can be processed with Lasers of relatively low average power (Image 2).
The benefits of using scanner based Laser systems for micro cutting is the relatively low capital cost and the fact that no use of consumables are required with the Laser sources and systems, as compared with more conventional CW cutting. Plus, there is the added benefit that ns Pulsed Laser based systems can, mark, engrave and texture…as well as cut. This capability is well exploited in the jewellery industry, where ns Pulsed Fiber Lasers are the norm for processing fine filigree based silver and gold items, giving designers and manufacturers extreme flexibility. Many Fiber Lasers are in 24/7 manufacturing environments where the reliability of these Laser sources has ensured strong growth in the application area (Image 3).
The ability to change the pulse duration with Fiber Lasers offers an additional route to process optimisation. Material removal rates can be affected marginally by pulse duration and this typically needs to be optimised specifically by material type.
Cutting using scanner based techniques is typically limited to material thicknesses <1mm however thicker sections can be processed but require periodic adjustment of the focal position to maintain energy density. Fiber Lasers can also be used in conjunction with a standard cutting head and nozzle with assist gas just like conventional CW Lasers. However, even a 50W ns Pulsed Fiber Laser can cut silver, brass and copper that would normally require power levels >200W. When cutting with an assist gas Fiber Lasers can be used in either pulsed, CW or Quasi-CW mode, depending on the application.
In pulsed mode the high peak power can ensure coupling into the material but sometimes the associated cutting speeds can be low. In CW mode the intensity is limited to the average power of the Laser and so cannot be used on reflective materials as the coupling threshold is not achieved. The use of long ns pulses (>200ns) at high repetition rates (>200kHz) generates a modulated Quasi-CW pulse stream with peak powers >200W that are particularly well suited to this technique and able to cut a wide range of materials up to 1mm thick with <100W Lasers! (Image 4).
In some applications the process versatility of Fiber Lasers mean that the same Laser source can to used to cut, mark and even weld, complex micro components in the medical device industry. This can take out several manufacturing steps helping stream-line manufacturing processes.
It should be noted that the use of ns Pulsed Fiber Lasers for cutting is not restricted to just metals. These Lasers can be used effectively on a wide variety of non metallic materials that exhibit at least some level of absorption of 1µm wavelength. Other materials that can be cut include; silicon, CFRP, ceramics, rubber and even some plastics and polymers. In fact one application that benefits from the cutting of multilayer materials is in the electronics industry for component cross sectioning.
Our Pulsed ns Fiber Lasers with their MOPA design have created a valuable growing niche in the Laser cutting market for thin section materials, particularly for processing reflective materials.
As average powers increase they could provide a viable alternative to conventional CW cutting sources and design and manufacturing engineers should take note of the application versatility that Fiber Lasers provide for innovative manufacturing solutions.
Dr Jack Gabzdyl
VP Pulsed Business Line
SPI Lasers UK
Dr Jack Gabzdyl is VP of the Pulsed Business Line, with expert knowledge in Pulsed Fiber Lasers and more than 29 years of Laser materials processing experience. Jack obtained his PhD in Laser processing from Imperial College London in 1989 and has since had a number of technical and marketing positions at BOC Gases, Advanced Laser Solutions and TWI before joining SPI Lasers in 2007.
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