Lightening the load

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Matthew Dale reports on ICALEO 2019, where experts highlighted the emerging challenges associated with processing lightweight materials for aerospace

Challenges in beam delivery, quality control, safety, and education are currently limiting the adoption of lasers for the cutting and joining of lightweight materials in aerospace. 

Such was heard by attendees of a panel discussion held at the LIA’s International Congress on Applications of Lasers & Electro-Optics (ICALEO), in Orlando, Florida, in October.

Overcoming these challenges will be vital in addressing the growing demand for such materials, including thermoplastic composites and aluminium-based alloys, which aerospace firms are increasingly turning to in order to reduce the weight of their aircraft, and ultimately lower fuel consumption.

Verena Wippo, head of the composites group at the Laser Zentrum Hannover, highlighted in the discussion that firms including Stelia Aerospace and GKN Aerospace are already turning to carbon fibre-reinforced polymers (CFRPs) – a thermoplastic composite – to produce large aircraft parts, such as winglets and torsion boxes, with the eventual goal of producing entire fuselages using them.

A 39ft-span carbon fibre-PEKK torsion box demonstrator. (Image: GKN Aerospace).

‘Lasers can be used to cut, trim and weld these materials,’ she said. ‘This is a topic we have to address in the next couple of years.’

Wippo continued by highlighting that across each of these applications there are currently limitations preventing lasers from being used to their full capacity.

Faster scanning technology needed

Take cutting and trimming, for example. These processes can be used to bring thermoplastic parts into their final shape after their fabrication in a thermoforming process. When using a laser to do this, it’s important that a high power be used in order to obtain short processing times and a high cutting quality, according to Wippo. However, in order to reduce any thermal damage to the material, a very high processing speed is also required. 

‘Here is where we run into limits on the optics side,’ said Wippo. ‘While we do have access to high-power laser systems, we don’t have an option to move the laser beam really fast over the workpiece – we need higher scanning speed and mirrors allowing higher laser powers. That’s really important for us.’

Expanding on this, she told Laser Systems Europe that her team is yet to see scanner technology on the market capable of addressing the requirements of such cutting applications.

Moving the beam at higher speeds is not the only challenge currently being faced by Wippo and her colleagues when cutting thermoplastics: ‘Bore hole preparation, for example for riveting, is also an issue,’ she said. ‘If you have a thick CFRP part it’s really hard to get a bore hole with a 90-degree cutting angle into this part, because of the aspect ratio between the laser beam diameter and the material thickness. That’s a really big topic as there is currently a lot of demand for technologies that can provide a solution.’

Challenges when welding fibre-reinforced parts

Welding thermoplastics is also not without its challenges. 

According to Wippo, it is not currently possible to join fibre-reinforced parts with a low transparency more than 2mm-thick using lasers, meaning that the technique is really only currently limited to joining thin parts for an aircraft’s interior, such as lining.

These welds often involve large weld seams, which in itself creates certain challenges: ‘In order to be able to perform these welds, we need to be able to not just have a homogenous welding spot, but also the ability to adapt the energy distributions within this welding spot,’ Wippo explained. ‘We are currently addressing this as part of a project that has now been running for over a year. Within it we are developing a new welding head consisting of multiple single spots, with each spot’s power being individually controllable. This has required the development of a new special optic that will allow us to introduce new temperature fields within the weld seam.’

A thermoplastic fuselage demonstrator. (Credit: Stelia Aerospace)

Wippo continued by explaining that having this capability will be particularly important when a large weld seam must go around a small curve, as it could be used to prevent too much energy being introduced to the inside of the curve, which would damage the thermoplastics.

Another challenge when performing large welds is ensuring the quality of the seam. Wippo explained that while for small parts it is possible to measure the melt displacement to determine seam quality, for large parts the online monitoring of weld seams is currently a very big challenge. 

Challenges when welding lightweight metal alloys

Welding challenges do not only exist for CFRP and GFRP (glass-fibre reinforced polymer) parts in aerospace, however, as was highlighted by Dr Mohammed Naeem, director of business development and special projects at Prima Power Laserdyne, who also took part in the panel discussion on joining lightweight materials.

‘We are seeing an increasing use of aluminium-based alloys in lightweight aerospace designs. However, the challenge for our customers here is that aluminium is very hard to weld,’ he said. ‘Most of these alloys tend to crack, so the challenge for us is to come up with a solution to weld these without destroying the parts – some of which can be quite large.’ 

He explained that even after optimising a laser’s peak/average power, pulse duration, pulse energy, pulse repetition rate and power density to such alloys, the majority of welds made still do not meet the stringent quality requirements of the aerospace industry. However, a technique often advocated to improve the quality of such welds, known as temporal pulse shaping, can be used to reduce and even prevent cracking and porosity in welds. This can be seen in Figure 1, which shows micrographs of 6xxx series aluminium alloy welds – prone to solidification cracking and porosity – made with and without temporal pulse shaping.

Figure 1: Using a conventional pulse shape exhibits cracks in 6xxx aluminium alloy (left), while using a ramp-down pulse shape results in no solidification cracking or porosity (right)

Reducing oxidation when welding such materials is also currently proving to be a challenge for Prima Power Laserdyne. While this can be prevented to a degree using a shielding gas, which protects the weld area from water vapour and oxygen, shielding large aerospace parts is no simple task. ‘Some of the components being processed are non-linear and have very complex shapes, and so trying to shield that in such a way that you get a perfect weld seam is the biggest challenge for us right now,’ said Naeem. ‘While it’s ok for linear welds, when you start doing 3D welds and are welding around corners, shielding becomes a nightmare.’

Will welding be enough?

Naeem noted that currently, in Europe, there is a growing aerospace market that is seeing more and more companies turning to lasers to weld engine components – especially in France and the UK.

‘We are getting a lot of inquiries for subcontracts for companies such as GE and Rolls Royce, who are increasingly getting into laser welding,’ he said. ‘In Korea as well there is some growth in laser welding. This is coming from the companies that already have laser drilling expertise and are realising that they can also use lasers for welding.’

Wippo remarked that for the joining of structural parts, however, she doesn’t believe the aerospace sector is ready to depend solely on laser welding. ‘At the moment most parts are joined by riveting, and while laser welding could be used to join anything up to large parts with long weld seams, I’m not so sure whether the aerospace industry would move completely away from riveting,’ she commented. 

Challenges in safety and education

One of the final challenges highlighted by Wippo was not attached to any laser application in particular, but more the general usage of laser processing in aerospace.

‘Due to the scale of the parts involved, much of an aircraft’s fabrication takes place in large factory halls, which can also introduce additional challenges when working with lasers,’ she said. ‘When you want to introduce a laser system into this production line, you have to find new ways to ensure laser safety, because you have to make very large areas laser-safe without interrupting the work in other areas,’ said Wippo. ‘This is one of the biggest challenges we are facing right now.’

Both panellists agreed however that one of the main issues facing the uptake of lasers for materials processing in aerospace is the education of the sector on the capabilities of the technology.

‘Laser technology is still quite new to the aerospace industry,’ said Wippo. ‘If you talk to people from this industry, they still don’t know much about laser technology – what is possible and where the limits are. It would therefore help if they had, for example, some courses or workshops available where they could learn what is possible with lasers, and also what are the dangers. I think this would really help us to introduce the laser into the production lines of companies such as Airbus and Boeing.’ 

‘I definitely agree,’ concluded Naeem. ‘I think the biggest problem for us when we started performing welding in aerospace was the education side. There is indeed a need for education that informs aerospace personnel on the structural integrity that welding can offer, and the benefits of adopting the laser technology capable of performing these welds into their production line.’