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Improving sheet metal cutting using a high-power fibre laser with variable beam quality

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Paul Harrison, chief engineer for product applications at SPI Lasers, describes a new mode of operation for fibre lasers that enables beam quality to be varied on demand

The laser cutting of sheet metals is a well-established production method, with many thousands of cutting systems in operation worldwide. In the last 10 years solid-state fibre-coupled lasers such as fibre and disk systems have become the laser of choice, replacing the CO2 lasers used in many older systems. 

Machines using either fibre or disk lasers operate in a similar manner – a cutting head is used to focus the laser beam onto the sheet metal through a conical nozzle with co-axial high-pressure assist gas. Until now, such laser sources have used a fixed beam quality, and the cutting performance has been characterised over a wide range of output powers, identifying the range of suitable focal spot sizes which produce good cuts. The selection of a particular focal spot size for a given laser cutting system can be made using a ‘One Spot Size Fits All’ approach, where one lens combination is used to process all metal types and thicknesses. For a low-cost, straightforward machine this can work acceptably well, and the cutting performance for a given laser power can be well-defined for a range of sheet metals.

However, this approach always involves compromise because of the different metals being processed. Fusion cutting, typically using nitrogen assist gas, is used for cutting stainless steel and requires a smaller focal spot with low divergence. On the other hand, reactive gas cutting using oxygen assist gas is used for mild steel cutting, and requires a large focal spot with high divergence. 

Figure 1: Focal spot size vs divergence for a laser with 4.5mm.mread beam quality delivered through a Ø100µm fibre.

It is difficult to optimise for both fusion and reactive cutting when using the ‘One Spot Size Fits All’ approach. One option available to cutting machine builders is to use a more capable cutting head with a motorised zoom collimator and variable beam expander, which enables the focal spot size to be modified. However, as shown in figure 1, this does not provide the right combination of spot size and divergence as long as the laser beam quality is fixed, since a small focal spot will always have a higher divergence compared to a larger focal spot. In order to improve the cutting process for both fusion and reactive cutting, the laser beam quality needs to be variable.

SPI Lasers has therefore developed a new option for its high-power lasers enabling variable beam quality to be added, known as variMODE. This is achieved by controlling the output beam divergence and thus beam quality. This is all done inside the laser, with no need for any external optics. Initially the laser has been fitted with a Ø100µm delivery fibre, and the beam quality can be switched from 3.2mm.mrad (M2 = 9.5) to 5.8mm.mrad (M2 = 17), as shown in figure 2. The lower beam quality has been set to match the numerical apeture of standard commercially available cutting heads, to ensure that all of the laser power is delivered to the sheet metal and that beam clipping does not occur within the cutting head. The switching time from low to high beam quality is typically 30ms, which is fast enough to be changed between piercing and cutting.

Figure 2: Switchable beam characteristics from the variMODE laser

Stainless steel cutting

Cutting trials were made with a 3kW laser, comparing the variMODE performance to the 4.5mm.mrad beam quality of a standard laser. The variMODE laser was set to high beam quality (3.2mm.mrad) and lower divergence, and the focal spot size was identical in both cases since the same cutting head optics were used. As shown in figure 3, the results using higher beam quality are significantly faster than the standard – for 2mm stainless steel the cutting speed is 45 per cent faster, and for 4mm the speed is 25 per cent faster. This is using the same laser output power for all trials. Note that all of these cut speeds are production speeds, having process bandwidth with tolerance to changes in focus position and nozzle stand-off height.

Figure 3: Stainless steel cutting results

Thick mild steel cutting

When cutting mild steel using oxygen assist gas there is not a great difference in cut speed within the range of available beam qualities, however there is a significant change in cut-edge quality. When using the lower beam quality (5.8mm.mrad) the cut-edge quality improves and the process window increases, particularly with thick (20mm) samples, where cuts at this beam quality show an average roughness (Rz) less than 30µm. Cuts at higher beam quality instead show a rougher, unstable cut-edge. Experimental trials have shown that cuts made with lower beam quality are much cooler compared to using higher beam quality. For example, when cutting 20mm mild steel with a 3kW laser the sample reached 190°C using high beam quality (3.2mm.mrad) but the same cut profile only reached 110°C using lower beam quality (5.8mm.mrad) with all other parameters kept the same. It’s likely that the mechanism that produces a cooler cut is also producing the lower cut-edge roughness.

As well as improved edge quality, finer higher aspect ratio features can be cut out of thick mild steel. An example of this is shown in figure 4, where a 2.5mm-wide link shape was cut in 20mm mild steel using a 3kW variMODE laser operating with 5.8mm.mrad beam quality at a cutting speed of 0.7m/min.

Figure 4: Narrow cut feature in 20mm mild steel using 3kW variMODE laser set to 5.8mm.mrad beam quality.

An added benefit of using a variMODE laser is that different beam qualities can be used for piercing and cutting. Piercing involves drilling a hole through the sheet metal prior to starting a cut, which must be smooth walled, particularly for oxygen cutting. Piercing thick mild steel can take a long time as it’s common to pulse the laser to improve the pierce quality, but if higher beam qualities are used the piercing time is significantly reduced. With the variMODE laser it’s possible to pierce with high beam quality to obtain a faster, higher quality hole and then switch to lower beam quality for cutting. This process also benefits from the fast beam quality switching speed of the variMODE laser to prevent pauses in the cutting process. 

Summary 

There are clear advantages to using a variable beam quality laser for sheet metal processing compared to a laser with a fixed beam quality. For the same laser power, the variable beam quality laser can out-perform the fixed beam quality system. When set to high beam quality, fusion cutting speeds with stainless steel are significantly higher. When set to lower beam quality, the cutting speeds with thick mild steel are the same – but the cut-edge quality is improved and finer cuts can be made since the work-piece temperature is lower.

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