Tom Eddershaw looks at the variety of laser processing methods used in aerospace production, including cleaning composites and laser marking aircraft parts
In order to reduce running costs, aircraft designers are using more exotic materials to reduce weight, improve aerodynamics, and improve fuel efficiency. Boeing’s 737 Max 200 aeroplane provides 20 per cent better fuel efficiency per seat than previous models, and at the start of December it was confirmed that Ryanair had ordered one hundred 737 MAX 200s in a deal worth $11 billion. Plane components are now made from more exotic materials and the laser is often the ideal tool to process these parts. Coherent’s excimer laser is used for the cleaning composites such as Carbon Fibre Reinforced Plastic (CFRP) in the aerospace industry. CFRP is lightweight while still being extremely strong, and is being used more frequently in aircraft manufacturing.
Dr Ralph Delmdahl, product marketing manager at Coherent, explained that the CFRP parts are made in moulds with sheets of the material soaked in epoxy resin to hold the sheets together. The material is pressed into shape and hardened in the mould.
To remove the combined sheets a mould release agent is applied to the part, which is then left on the material’s surface. The surface then needs to be cleaned of all traces of release agent, especially in order to glue two parts together to make a larger component, for instance – the surface has to be clean for the adhesive to form a reliable joint.
The CFRP could be cleaned mechanically, but this could damage the bulk material. ‘Once you start to break the fibres beneath, there is an increased risk of material failure. Even if you successfully remove the bonding agent, the material below it could be damaged and shear off afterwards below the bond,’ Delmdahl explained.
‘This is why the excimer laser is so exciting. You need UV light for the surface cleaning because it won’t dig too deep, but in order to do it quickly you have to have a lot of it, both of which the excimer laser provides.’
The laser source operates at 308nm wavelength, which creates a clean surface while only penetrating the material to around 0.2µm per shot. Coherent’s excimer laser can produce high pulse energies of up to 1J per pulse, which means the beam can be spread across a large area and at the same time be very homogeneous and effective. Typically for cleaning CFRP sheets, the beam is shaped as a long rectangle, roughly 30mm by 1mm.
Delmdahl said: ‘You effectively have a photonic stamp that can be moved over the surface of the component, and it only takes one or two shots to clean the surface. This large footprint and the fact that the systems can operate at up to 600Hz means that you can cover large areas extremely quickly, ranging from 1sqm per hour to 50sqm per hour.’
Tracking the components
One of the other applications of lasers within the aircraft production is part marking. Jim Leach, Electrox’s national sales manager for the UK, said: ‘Aerospace is one of our big markets. We sell a number of laser marking machines into that market and they tend to be high value machines. This is because even though they are used for relatively small batches, the parts tend to be quite awkwardly shaped. They normally require quite large workstations to house things as well.’
Electrox has had to adapt its larger workstations to fit some of the aircraft parts. One particular customer uses the company’s systems to mark propeller blades that are about 2m long, Leach recalled. The solution was a system that had a draw to insert the propeller. The machine also had to have a higher positioned laser in order to mark the bulky parts.
Within the aerospace industry there are two primary sub markets: defence and commercial. Leach said: ‘We mark a lot of parts for the defence market where there is a guideline standard MILSTD130N. Essentially, anything over a certain value that gets laser processed has to abide by this standard. It means that everything has to be marked for the lifetime of the component.’
The code states who it was made by and when it was made. It gives a traceability of an item back to its manufacturer and the batch that it’s from. Therefore, if a part from a batch fails, other aircraft using components from the same batch can be identified quickly so they can be replaced.
‘Every single component that goes into an aircraft is scanned as it is put on; this helps if there is a problem, but they can also check which parts are close to the end of their warranty lifespan,’ explained Leach.
Leach commented: ‘Certain companies have their own guidelines and we have to get individual acceptance and pass their stringent tests. You have to be careful; there are certain areas you can say you can mark, and you can mark very well. However there are areas that you can’t, it’s all down to what the customer wants.’
For defence applications, Leach explained that it’s a question of whether or not the mark will last the lifetime of the component, which is typically between 10 and 20 years. The manufacturers are assessing the lifespan of the mark, the depth of the mark and the minimal heat affected zone. However, certain materials such as titanium cannot be laser marked according to these standards, Leach noted.
Laser marking titanium can cause riverbed cracking, which occurs because of the heat put into the component. ‘Any cracking in titanium or any other material which is put into high stress or high wearing areas can cause a failure; in aerospace this is just not an option. In these instances they tend to chemically etch or use a stamp,’ Leach said.
There isn’t yet an easy route around this to create a laser-marking system for these materials, and the low volume production of titanium parts for different aircraft makes it economically unviable. ‘It’s not at the point where there are thousands of aircraft sold,’ Leach continued. ‘If you are marking a part that only features once on each plane, the numbers you require can be very small. It makes it very hard to justify spending the amount of money it would take to buy a laser system that can mark the titanium without creating the riverbed cracks.’
But with the materials that can be processed, using a laser marking system can be very beneficial. Leach said: ‘It’s the permanence of the mark. The majority of the cases we tend to sell YAG wavelength which is less expensive and can still produce a good mark.’
Aside from titanium, components made from materials like aluminium or steel can be marked with a YAG wavelength, with depth and contrast and which will last for the lifetime of the component. Chemical etching is messy and the chemicals have to be disposed of after use. ‘It’s [chemical etching] not a great technology and certainly not one that you want to use with high value items,’ Leach commented. ‘Manufacturers of these expensive parts often prefer a process that is much more aesthetic and much quicker to do; this is where laser marking comes into its own.’
Leach said that the size, shape, and complexity of components can be very different to more conventional markets: ‘They tend to be larger systems, something like a 1 x 2 metre enclosure. There are all sorts of things Electrox can do to assist with large and unwieldy components to be marked for aerospace applications.’ Electrox has provided marking systems where a crane has had to lift the part into a mounting bracket in order to get it in the system.
Flight safe mode
Safety is an important consideration, and for the larger laser enclosures quite a complex set of equipment has to be installed to protect the operator. Phil Jones, marketing manager at Lasermet, said that the company has provided laser safety equipment for the Manufacturing Technology Centre (MTC) in Coventry, UK, which can fit two double decker buses inside. Rolls-Royce, Airbus, and Aero Engine Controls are all members of the centre, which carries out manufacturing research. The prime contractor for the 20kW laser enclosure was Tec Systems; Lasermet built the enclosure using its active guarding system.
Lasermet provides laser safety enclosures, including the Laser Castle, a modular, rapid-build, certified laser safety enclosure, typically used to house a robot with an attached laser. For multi-kilowatt lasers, more than 5kW, an active laser guarding option is available, called Laser Jailer. This system was installed at the MTC and works by disabling the laser via the interlock if the walls, doors or roof are inadvertently struck by the beam
Jones commented that there are more fibre lasers now in use that require safety enclosures. Multi-kilowatt lasers are now commonplace and the safety devices need to keep up with the increasing power.
Lasermet is also UKAS accredited for laser testing. Jones said: ‘Now that a lot of the [aerospace] parts manufacture is subcontracted to smaller manufacturers, they will be making parts often using lasers and they need to be in a position where their laser environment is safe.’
Lasermet can carry out laser safety audits on facilities to make sure their laser safety equipment and systems are safe. Jones added: ‘If laser interlock control systems are ineffective, don’t exist, or are easily defeated, the environment cannot be signed off as being safe.’