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CFRP MACHINING TAXIS FOR TAKE-OFF: Tim Gillett investigates the process of laser machining composite materials for the aerospace industry.

The day after I was asked to investigate laser machining in the aerospace industry, my father gave me a collection of animal skulls he'd been amassing over the years.

Among them was the skull of a sparrow hawk, which had met its maker after flying into a glass conservatory while chasing a pigeon. Picking up the skull, it struck me immediately that it weighed almost nothing--which, of course, is a quality that is essential when it comes to the concept of flight. Whether you are considering a raptor weighing 200g, or an aircraft weighing 200 tonnes, the ratio between weight and strength is crucial.

In two three-year research projects, the Laser Zentrum Hannover (LZH) --sponsored by the German Federal Ministry of Education and Research and the German Federal Ministry for Economic Affairs and Energy--is aiming to further develop the laser machining of lightweight composites for series production in the aircraft industry. Their focus is on designing an efficient system and a process technique that meets the demands of aviation.

The use of composites in aerospace is not a new thing--it's already decades old, in fact--but these days as much as 50 per cent of a modern aircraft can be made from carbon-fibre reinforced plastic (CFRP). It's extremely light, durable, and strong--but there have long been difficulties around the process of machining the material.

Material challenges

Hagen Dittmar, who works in the production and systems department at LZH, explained that there are specific issues in machining a non-homogenous material such as CFRP: 'Unlike a metal, for example, CFRP is made up of two chief components--fibres and plastic --with completely different properties (carbon fibre is very strong, whereas the plastic, which acts as the matrix material and forms the actual shape of the specific product, is very weak). Carbon fibre produces a large amount of wear on traditional tooling, which is one of the drivers towards laser machining.'

The LZH project ReWork, set to end in June, is looking at the process of reworking CFRP components that have either been damaged in use or those that suffered faults in the original production process--the procedure is the same in either case.

Dittmar said: 'Using near-infrared lasers, the machining process effectively removes material layer by layer to eliminate the damage or fault. If we are working on parts of an aircraft shell the material might be 2mm thick, and made up of around 10 layers. In the case of damage, all 10 layers may have to be removed, whereas in other cases it may be only three or four layers. A replacement patch can then be fitted--and while this doesn't 100 per cent replicate the strength of the non-machined CFRP, it will generally retain 90 to 95 per cent of the strength of the original component.

An innovative system technology --consisting of a laser, a scanner, an interferometry system and a control software--determines the individual process parameters according to the shape of the component. The interferometry system measures the depth spatially with high resolution, and thus ensures a precise layer-by-layer removal. In that way, surface deformations due to local increases in thickness, which occur frequently during mechanical processing, can be avoided.

Dittmar continued: 'It's a very easy industrial application. The laser can be on the opposite side of the room to the component, and can just be guided by cable rather than by a system of mirrors. It also incorporates a measurement technique; the laser process can be monitored as it progresses, so we can measure how much material has been removed at any time. There is no need to check the process in steps, which can obviously represent a saving in terms of time and cost.'

Drilling down on efficiency

Another project ongoing at LZH, LaBoKomp, involves the development of laser drilling of composites, also for use in the aerospace industry.

For aviation in particular, the drilling of CFRP has an enormous market potential.

Aircraft manufacturers produce increasing quantities of components with a high number of drilled holes for riveted and screwed joints.

This requires reliable, fast and cost-efficient processes to withstand international competition--and, for this purpose, laser beam drilling is an ideal alternative to conventional processes.

Richard Staehr, who is working on the project, explained that the aim was to achieve processing times similar to that of conventional drilling--that is, around 10 seconds to drill a hole through material that is 2mm thick.

Staehr told Laser Systems Europe: 'The process is carried out at very high speed so the material is not damaged by the heat, and we adopt a very particular strategy according to the thickness of the material. If we move the laser too slowly there is the risk of damage to the plastic. It's like putting your fingers in a flame--if you do it too fast there is no problem, but if you leave your fingers there they will get burned. So in fact we repeat the process many times, with a short break in between each time to allow the material to cool down. As with many industrial processes, we have had to find a compromise between speeding up the process time and ensuring that the material is not damaged.'

He explained that one particular advantage to laser drilling derives from the fact that there is no physical contact between the tool and the material: 'Because we are not putting physical pressure on the CFRP, the material does not need to be secured in the same way as if it was machined with conventional tools. But you have to weigh everything up. There are some issues around emissions using laser machining--CFRP dust is not the sort of thing that should be inhaled. The process also causes the vaporisation of small quantities of plastic, so we have to use an exhaust system to remove the emissions.'

Long road to commercialisation

The three-year LaBoKomp project is coming to an end in July. Demonstration samples of support struts for aircraft holds have been produced--and one of the project partners, Premium Aerotec, will soon begin to look at the lengthy process of commercialisation.

But, Staehr warned, progress is slow in an industry where safety and reliability are paramount. Before any consideration of commercial use, the process must be certified by the relevant authorities--not just the parts that are produced, but also the machines that are used for the process --and this is likely to take a couple of years.

Dittmar concluded: 'Industry will only consider a new technique on a new product --they generally will not even think about introducing a new process on a part that is already made in another way. But when the time comes to introduce a part or component, aerospace companies are increasingly looking at the use of lasers in the manufacturing process.'

There is clearly a lot of promise for machining CFRP in aerospace--and, with increasing use of the material in high-end automotive manufacture and in the windenergy industry, many in the laser industry will be watching developments like hawks.
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Author:Gillett, Tim
Publication:Laser Systems Europe
Date:Jun 22, 2019
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