FEATURE

Sky’s the limit for laser processing

Keely Portway discovers some of the different ways advances in laser processing have been used to help improve aircraft efficiency

The aerospace industry has been taking advantage of laser processing for some time, most traditionally for applications such as cutting, drilling and welding. As the technology has advanced, it has improved the capabilities of these core uses in aerospace markets. 

That’s the view held by Mark Barry, vice president of sales and marketing at Prima Power, whose machines are used in the aerospace industry across different manufacturing applications, for example drilling cooling holes on hot turbine engine parts and in the fabrication of complex parts that may require cutting and welding.

‘Fibre lasers have become the laser source of choice for laser material processing across many industries,’ explained Barry, ‘and they have dramatically expanded the possibilities to use laser processing for a multitude of applications. They are more energy efficient than CO2 and Nd:YAG systems, have a lower total cost of ownership, and are available in a wide range of power levels. Additionally, with fibre lasers available in CW and QCW modes, the laser pulse (energy) can now be very precisely controlled. This improves the laser cutting, drilling, and welding capabilities and allows process engineers to investigate and put into practice techniques that were impossible with older style lasers.’  

Barry sees fibre laser and machine tool manufacturers continually working to expand the capabilities for cutting, drilling and welding in complex applications such as those in aerospace, by controlling the key elements of pulse frequency, pulse shaping, beam profiles and power levels.  

Ifredo Aguilar, a scientist with the surface funtionalisation team at Fraunhofer IWS, operates the world's largerst 3D DLIP system based at TU Dresden. (Image: Fraunhofer IWS)

‘This is a good news/bad news story,’ he said. ‘The good news is, the additional parameters are expanding the laser processing capabilities, enabling fibre lasers to be used for a wider range of applications, material types and for improving the workpiece quality. The bad news is, with each additional parameter, effort is required to find the optimal processing solution. Having a simple material table with a power level and cut speed is no longer sufficient for optimal processing.’

Manufacturer support

When it comes to partnering with manufacturers for aerospace, for Prima, the biggest challenge is the qualification of the processes. These customers need more intensive support and know-how from the machine suppliers, whilst the machines used for drilling, as well as cutting and welding applications, have to be particularly accurate, stable and reliable, in order to maintain the certificated parameters. This also ensures the quality of parts and reduces scrap and rework, which are very expensive.

‘The focus is on optimising the laser process by leveraging all the available parameters,’ said Barry. ‘This improves the workpiece quality and reduces the need for secondary operations, for example, by improving the cut quality, the dross and slag on the back side is minimal, the material properties at the cut edge remain the same, and recast layer is minimal.’

Looking at the ‘primary’ laser processing functions – cutting, drilling and welding – Barry explained how he believes fibre lasers have influenced the process: ‘For cutting applications, fibre lasers have provided the product development engineers with increased design flexibility to reduce the number of parts and improve the quality of the final product. For example, many product designers are familiar with traditional 2D part cutting from a flat sheet stock, but are not familiar with 3D laser processing. Designers can use tubes, metal spinning, hydroforming, deep draw die parts, or other 3D stamping to create the part shape. Then, using a multi-axis [laser] system to trim the part, cut in the basic or complex shapes. Rather than a complex assembly, they can use the forming and trimming to provide a single more cost-effective part and reduce secondary operations.

Advances in laser processsing have improved cutting, drilling and welding for aerospace. (Image: Prima Power)

‘Drilling applications have been influenced with the delivery of laser energy through a fibre having as small as a 100µm core diameter, meaning a precise laser beam diameter is delivered to the workpiece. This enables the fibre lasers to very quickly drill holes in the workpiece through percussion drilling or trepanning. Similar to cutting, by delivering the laser energy to the workpiece through multi-axis machines, the holes can be drilled into the workpiece at varying angles and shapes. Many EDM processes have been converted to fibre laser drilling.’

Under control

Barry continued: 'Fibre lasers have changed the welding industry. They deliver a very controlled beam, and therefore energy, to the workpiece and have enabled complex geometries to be welded both with and without filler material. This translates to repeatable welding processes and high-quality parts. With the growing use of speciality alloys, high-strength nickel alloys,and titanium alloys, new challenges arose because the processing parameter window for these materials is narrow.  With optimal fibre laser parameters, the welding process can be very tightly controlled and previously difficult-to-weld materials are now routinely welded together.’

Barry explained that process engineers have experienced the industry migration from CO2, to Nd:YAG, to fibre lasers, which he believes have been proven to be versatile, reliable, and adaptable laser sources for many different materials and processes. ‘More recently, fibre lasers are opening the door to variable beam control technology and the possibility of multicore fibre delivery,’ he said. ‘These technologies will further expand the use of fibre lasers and provide product development engineers with more manufacturing flexibility, quality and performance. Going forward, it is guaranteed that fibre lasers will be used in more manufacturing applications both difficult, like aerospace, and more common.’

But while laser processing has become more widely used over its manual processing counterparts for applications such as welding, cutting and drilling, as the technology advances, other use cases have been presenting themselves. 

Clever coating

One example is in laser coating, and the Fraunhofer Institute for Material and Beam Technology IWS, which as part of a collaborative project with Airbus and the Technische Universität Dresden, has been working towards the development of a laser process that can be used as an alternative ‘lotus effect’ provider to existing coatings, producing textured surfaces on aircraft to help prevent surface contamination.

The Laser4Fun project began its journey several years ago, as Dr Tim Kunze, group manager of surface functionalisation at Fraunhofer IWS explained: ‘The group was founded in 2009, and while the project is predominantly between Fraunhofer IWS and Airbus, Professor Andres Lasagni, now of Technische Universität Dresden, prepared the proposal and made the basic things for the technology nearly 10 years ago at Fraunhofer IWS. He is very much still involved through his role at Technische Universität Dresden. In 2016, we started the project with, amongst others, Airbus.’ 

Within the project, the group created the functionality by restructuring a sample of airbus material using its direct laser interference pattern (DLIP) technology. Special optics split one laser beam into several partial beams, which are later combined on the surface being structured. If the interference pattern is focused onto a titanium sheet, the high-energy laser light melts and ablates the material in the bright areas, while it leaves the material unaffected in the dark areas.

With DLIP technology the distances between the pillars can be set between 150nm and 30μm, so water droplets no longer wet the surface and stick to it. (Image: Fraunhofer IWS)

Looking to nature

As a result, the scientists produced microstructures over the titanium surface. Viewed under a microscope these resemble microscopic halls of pillars or corrugated iron roofs. The distances between the pillars can be set between 150nm and 30µm. This means that water droplets no longer wet the surface and stick to it as they do not have enough grip on the surface. Kunze continued: ‘It was a combination of a micro- and a nano-texture. It’s a really tiny, complex structure like those found in nature. What we do is try to mimic natural examples in order to bring these functions to surfaces with laser technology.’

The advantages of this for aircraft include increased sweat-ability and water repellence, with less risk of contamination. ‘Think about an aeroplane flying in the air,’ emphasised Kunze. ‘You have some environmental factors like insects, rain water, or ice. These can all go on the surface and Airbus is highly interested in advanced surfaces with different functionalities to increase efficiency. So, if you make a trip in winter, chemical treatments are applied on the surface of the wing in order to avoid icing. If you can reduce the icing of a surface and also the contamination, then you need less time for surface maintenance and also less chemicals. In this sense, we are ecological because we modified a laser surface and with that you have less chemical treatment.’

DLIP technology can reduce the icing of a plane's surface (such as this grounded model) whislt in the air, without the need for chemicals. (Image: Shutterstock)

To achieve this effect until now, water repellent, or ‘superhydrophobic’ coatings were traditionally used. Kunze believes the DLIP method offers several advantages. ‘If you think of the coating as a chemical treatment,’ he said; ‘if the plane is in the air, you have this environment with UV radiation and a lot of processes which could destroy the functionality of the coating. Sometimes coatings can contain toxic components inside or maybe require toxic disposal. The micro-texture can offer a lot of advantages because you are not putting the chemical force on the product or material, it is just contactless processing. This is an advantage compared to other technologies. What’s more, coatings may age over time or damage easily. The structures produced by the DLIP method may last years and do not raise environmental concerns.’

Even split

Elaborating on the technology, Kunze explained: ‘You need a pulsed laser in the nanosecond, picosecond or femtosecond range – these are really short pulses. What we do to use the technology is split these beams into two or three sub-beams. 

‘Then, using some optical concepts and some technological solutions, we bring these sub-beams on to the surface and energy is shifted into a certain shape, which means a certain pattern can be, for instance, a line-like pattern or a dot-like pattern, and this can exist in the beam profile. You bring this beam profile on the material and you imprint this laser profile into the material, like a laser stamping process. When a sensitive material becomes clearer, we imprint the laser profile and the energy profile into the material and with that we can imprint some structures onto the material. If you don’t scratch or damage the surface then the structure remains very, very stable.’ 

This may sound complex, but Kunze emphasises that it is, in fact, quite simple, and the technology is developing all the time. ‘The technology can be really nicely controlled so, in principle, it’s not overly complex. The interesting thing about this technology is it’s scalable, which means that the lasers are developing very fast. They are becoming more powerful and energetic, and so I think in the last 10 years there has been a lot of development with the lasers becoming more powerful, and the processing time reduced. Because our technology is DLIP, the more powerful the lasers are, the faster this technology is. It directly depends on the power of the laser. 

‘We can reach processing speeds up to 1m2 per minute, which is very fast. There is no other technology which can be so fast. It is only due to the laser. We use a laser system close to 200W, which is not so high, so we can be even faster. The reason why industry is looking forward to the development, is because it is interesting for them to bring the textures on to the surface as fast as possible.’

Road testing

When Airbus was happy with the results, the surface went for functionality testing so it could interact with the environment. ‘In this case,’ said Kunze, ‘we optimised the water repellence of the surface. We processed a certain part of the surface and this was placed on the wing tip of a special testing Airbus liner, which is currently flying through the air.’ The first results are promising. 

‘The structures are stable, so they can resist the environment in the air in terms of contamination, as well as any particles around that could destroy the structure. Our technology was the only one selected to be tested on this plane,’ concluded Kunze. ‘We know we can be fast enough and accurate enough, now we need to see if real road testing is successful. This is currently ongoing.’

Related article 

Powering up the Pulse - After the international laser technology congress AKL, in Aachen in May, Matthew Dale learned that dramatic increases in power are on the horizon for ultrafast lasers

Latest issue

Analysis and opinion
Analysis and opinion
Analysis and opinion
Analysis and opinion
Feature

Gemma Church investigates the explosion of cross-industry plastic welding techniques