Cut above for thick plate processing

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Greg Blackman discovers that new beam shaping technology is speeding up and improving the quality of thick plate laser cutting

Laser cutting machines are now very powerful pieces of equipment. Those for cutting thicker metals might reach 20kW or higher, and even machines primarily for thin sheet metal cutting can border on the 10kW mark.

‘Power is available and it’s getting cheaper,’ commented Dr Andreas Wetzig, business unit manager for laser ablation and cutting at the Fraunhofer Institute for Material and Beam Technology IWS.

Power, however, isn’t everything, and now work is ongoing to increase processing throughput by other means, for cutting thicker metals by beam shaping, and by machine automation in thin sheet metal cutting. Fraunhofer IWS is working on R&D projects to develop beam shaping technology in order to improve the cutting efficiency, and therefore the speed, when cutting thick metal plates (‘thick’ is anything more than 4mm for the purposes of this article).

‘The goal is to use as much of the laser power available as possible for the cutting process,’ explained Wetzig. ‘Ideally you would use 100 per cent of the available laser power; in reality you’re using less than 50 per cent without beam shaping.’

Under ideal conditions, Fraunhofer IWS scientists have observed up to 50 per cent increase in speed by using beam shaping technology, according to Wetzig. ‘It doesn’t mean that will always be the case, but you can increase up to 50 per cent in the best case,’ he clarified.

‘In our opinion, it’s better to put effort into beam shaping instead of raising the power of the laser,’ he added.

The Fraunhofer IWS beam shaping technology uses two single galvo-driven mirrors to move the laser beam very fast within the kerf width in x and y directions. The beam can be manipulated at frequencies of up to 4kHz.

Laser systems manufacturer Amada has designed a commercial laser machine based on Fraunhofer IWS' beam shaping technology, which it first showed at Euroblech 2018. Amada’s Ventis machine is a 4kW solid-state laser cutter containing a cutting head that manipulates the beam in different patterns as it moves across the work piece, what Amada calls Locus Beam Control (LBC) technology. The optical unit can produce infinite beam patterns – which, depending on the pattern, improves cutting speed and quality for different materials and thicknesses.

Amada's Locus Beam Control can create different beam patterns, which improves cutting speed and quality for different materials and thickness.

The official launch of Ventis will be in May 2019 in Japan, and then in Europe during the second half of this year.

‘Its [Ventis’s] real strength is nitrogen cutting, especially stainless steel and aluminium, but mild steel as well,’ explained Matt Wood, senior product manager at Amada Europe. ‘It’s also very good for oxygen cutting mild steel, but our Ensis machines are good at that anyway.’

Compared to a standard 4kW solid-state laser, Ventis is 222 per cent faster at nitrogen cutting 8mm aluminium, according to tests made by Amada. ‘This puts it in the cutting speed range of what would be achieved by a laser machine equivalent to 6kW – but it’s only a 4kW machine,’ Wood emphasised. ‘On stainless steel and aluminium, it’s verging into 6kW and 8kW territory for cutting speed,’ he added.

The Ventis also improves upon the quality of the cut edge – by more than 50 per cent for 12mm stainless steel, according to Amada. This makes it close to the surface finish achieved with a CO2 laser, which still is superior to solid-state laser nitrogen cutting, particularly for stainless steel.

12mm stainless steel cut on Amada's Ventis machine, without Locus Beam Control (left) and with LBC turned on (right).

‘We had a lot of customers at Euroblech look at it [the cut surface], and quite a few comments were: “This is finally the machine that can replace my 4kW CO2”. That seemed to be a common theme; it’s particularly for people that want higher-quality cutting,’ Wood said.

The machine is also able to minimise dross underneath the cut, which reduces secondary processing. ‘If we can cut aluminium and stainless steel and it’s free from dross underneath, that can save a significant amount of overall production time,’ Wood added.

For thin plate cutting dross is reasonably easy to avoid, but for thick plate cutting dross is always an issue, according to Wetzig at Fraunhofer IWS. ‘If you can avoid dross, you can avoid expensive post processing,’ he said.

The Ventis fibre laser engine is a 4kW single-diode module. The engineers at Amada developed the 4kW diode module specifically for the Ventis to avoid losing beam quality when combining multiple diode modules. The high beam quality maximises the effect of the LBC technology on cutting.

Amada's Ventis machine was first shown at Euroblech 2018: the official launch was in May in Japan.

Wood explained that for the mid-thickness range – 4mm to 10mm – the Ventis would give an option to cut at high speed, or cut slightly slower but at higher quality. Moving to 12mm and 15mm stainless steel, however, the cut speed and the quality converge. Amada’s 12mm stainless steel test sample, which showed greater than 50 per cent improvement in cut quality, was also processed at 150 per cent of the standard 4kW cutting speed. In addition, the dross was reduced by 85 per cent on that sample, compared to a standard 4kW solid-state laser, according to the company. 

Gas powered

Higher laser power is good for the feed rate and for cutting quality, but the downside is that higher power lasers need more electricity to run; they require more cooling capacity, and the engineers have to take care of the optics. ‘Cutting uses transmissive optics and it’s not easy to keep the optics clean over time. It’s easy to buy a high-power laser and hook it up to the machine, but you have to have a processing head that is able to withstand the power over a long time without a lot of maintenance,’ explained Wetzig.

Focus shift in the laser beam is one thing that can lead to lower power density on the workpiece. It is caused by thermal effects on the laser’s optical components, and is something the German cutting machine supplier Messer Cutting Systems looked to minimise in its laser cutting machines with a software algorithm it developed using measurements from Ophir’s BeamWatch beam profiling device. BeamWatch is a non-contact device that measures Rayleigh radiation scattered from the beam to calculate the beam profile. The profiler can take measurements at video frame rates, which makes it possible to see any shift in focus.

The benefit of the abundance of laser power is that laser oxygen cutting, which is used to cut thick mild steel, is now being replaced by laser fusion cutting using nitrogen as an assist gas. Nitrogen is used to eject the molten material. In the past laser power was expensive, so the addition of oxygen was used alongside a lower power beam to create enough heat to make the cut. ‘Nowadays, because there is enough laser power available, even mild steel at thicknesses of 15mm can be cut by a fusion process using a 10kW laser, or higher power,’ Wetzig said.

Oxygen cutting is still used for very thick material of 50mm. But, compared to oxygen cutting, fusion cutting with nitrogen is faster and the cut edge has a better quality – it avoids oxidation of the cut edge.

One big challenge for laser cutting is gas consumption; nitrogen is delivered at a pressure of up to 20 bar in fusion cutting. ‘Gas consumption could account for up to 10 per cent of the running cost of the machine. It’s really significant,’ Wetzig said. ‘One goal for laser cutting machine manufacturers is to reduce the gas consumption, because you reduce costs.’

Machines for cutting thin metal sheets normally have enough laser power available. Machine manufacturers are therefore putting development efforts into automation, into ways of speeding up aspects like feeding the material into the machine or removing cut parts as fast as possible.

Wood at Amada said: ‘Automation is still very relevant to laser cutting in general. Because solid-state machines are cutting so much quicker compared to CO2 lasers – especially thin materials – the machine is finished before the operator has managed to get the parts out of the skeleton and put a new sheet on. It’s important to consider automation, because you’ll get more out of the laser machine,’ he continued. ‘Whatever laser technology you decide to buy you need to match that with some kind of automation to get the benefit.’

Process control

Fraunhofer IWS is working on new beam shaping optics, using one gimbal mirror instead of two galvo-driven mirrors, for instance, which is a more elegant way to manipulate the beam in a compact set-up. The scientists are also experimenting with a z-axis adaptable mirror to move the focal point of the laser from the top side to the bottom side of the material with a frequency of 2kHz – up and down like a saw.

‘We haven’t seen a positive effect on the speed and cutting quality yet, but we are expecting one,’ remarked Wetzig. 

‘The focal position of the laser within the material is a critical parameter for laser cutting. 

‘Even if there isn’t a direct advantage of moving the beam quickly in the z direction regarding the cut quality, we might have a tool for closed-loop control to change the focal position if it starts to shift over time. 

‘In addition, a quick adaption of the focal position will also help to optimise the so-called piercing process that is used to initiate the cutting process for thick plates.’

One of the parameters that should remain constant over time is the kerf width, the width between the two cut edges. ‘It’s important to have a robust process,’ Wetzig said. ‘This means having no breaks in machining and maintaining a constant cutting quality.’

Fraunhofer IWS is working on process monitoring for better cut quality, potentially with a view to have closed-loop control of the process at some point in the future.

If you have a way of detecting a decrease in cut quality then you can control the process – you can reduce or increase the power, adapt the position of the laser focus, adapt your feed rate, or change the laser beam parameters,’ Wetzig explained. ‘But first of all you need to monitor the process; you need correlation between what is observed and the cut quality.’

This might include information on whether there is dross left on the backside of the plate or not; information about how smooth the cut edge is; and some information on the width of the cutting kerf.

One monitoring method is to measure the intensity of radiation emitted during the process – from UV to infrared wavelengths – and compare these readings to the cut quality. There is some correlation between these variables, according to Wetzig. More promising is to use a thermal camera to image the molten material, which will provide localised information about the temperature field within the kerf width. Fraunhofer IWS is working on this.

There are other methods as well – a visible camera will provide some information about the kerf width, for instance.

There are many reasons why the cutting quality might degrade over time, such as dust on the optics or material restricting the gas flow. The aim is to have a machine that works more or less autonomously overnight or over the weekend.

‘One of our goals is to tie in process monitoring with beam shaping,’ Wetzig said. ‘We haven’t done it yet, but we will do it. We are currently working separately on both issues and then we will combine them."

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