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ANALYSIS: FIBRE LASER


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.


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


Figure 3: Stainless steel cutting results


variMODE 3.2mm.mrad


Standard laser 4.5mm.mrad


2mm SS: 16m/min


2mm SS: 11m/min


4mm SS: 5m/min


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.


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


Figure 2: Switchable beam characteristics from the variMODE laser


4mm SS: 4m/min


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. l


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


M2


9.5 (3.2 BPP) Divergence 130mrads WWW.LASERSYSTEMSEUROPE.COM | @LASERSYSTEMSMAG


M2


17 (5.8 BPP) Divergence 230mrads SUMMER 2019 LASER SYSTEMS EUROPE 17


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