Laser peen forming titanium alloy sheets for the aerospace sector
For two years now, researchers at the Institute of Materials Mechanics at Helmholtz-Zentrum Hereon, Germany, have experimented with laser peen forming to investigate its potential for the processing of titanium alloy sheets in aerospace.
Their work was published in the Journal of Materials Processing Technology in July, and continues to take place with partners Formtech, Zentrum für Angewandte Luftfahrtforschung (ZAL) and the Institute of Product and Process Innovation at Leuphana University Lüneburg.
Laser peen forming could be advantageous to the forming of certain aircraft components that must be shaped into a curve, such as the front part of wings, parts of the tail and structures of the fuselage.
“These parts are usually made from the lightweight, high-strength alloy Ti-6Al-4V in many aircrafts currently in service,” said Siva Teja Sala, a PhD student in the Department of Laser Material Processing and Structural Assessment at the Institute of Materials Mechanics. “The challenge with this alloy however is that it isn’t very easy to work with. If you try to form it, for example with traditional hot stamping or incremental sheet forming processes, you experience significant spring back – a geometrical change observed in a formed component when it’s released from the loads and constraints of the applied forming process. Engineers therefore have to design tools that account for this spring back and accommodate the tolerances involved, which is incredibly challenging to do.”
Industries often experience problems when traditionally forming titanium alloys sheets due to material spring back. This is particularly troublesome in the aerospace industry, where parts often have to be scrapped due to the very strict tolerance requirements involved.
“This isn’t ideal at all for large scale production, and so aerospace manufacturers are currently looking for a flexible, cost-effective correction process that allows these tolerances to be controlled,” said Sala. “Laser peen forming is perfect for serving this kind of application. It's very flexible – only limited fixtures or clamping systems are required (no huge machinery) – and all that is needed is a pulsed laser and a robot that can handle the part. It’s highly automatable and the energy requirement is quite low compared to traditional forming processes.”
Sala and his colleagues therefore began investigating laser peen forming within the framework of project ‘PEENCOR’ which aims to correct inevitable deviations – with spring back being just one example – that occur during the production of curved titanium aircraft parts. The project seeks to develop an automated laser peen forming process to measure these complex deviations and correct them according to specified tolerances, which reduces scrap, part rejection and enhances overall productivity. Helmholtz Zentrum Hereon itself is responsible for the sub-project ‘Development of optimal process parameters and identification of an artificial intelligence algorithm for autonomous forming and straightening with laser peen forming.’
Bending in the peened region of an LPF-treated 2mm thick Ti-6Al4V specimen using stainless steel as a protective overlay. (Credit: Helmholtz Zentrum hereon GmbH)
Another reason for needing to conduct this work, according to Sala, was that most of the pre-existing publications previously exploring laser peen forming had done so in the context of processing aluminium alloys, which are far simpler to form compared to titanium. “So with there only being a few publications out there actually studying the laser peen forming of titanium alloys, we wanted to conduct a comprehensive qualitative assessment of the process parameters and their influence on bending on this interesting and challenging material,” he said.
Controlled, miniature explosions
The researchers’ experiments involved using a pulsed laser to irradiate the surface of a sample titanium alloy part – which itself is covered with a protective steel foil – to create a plasma that causes a miniature explosion to occur at the surface.
“We confine this explosion using a laminated flow of water over the surface of the materials, which causes a pressure wave that penetrates down into the material,” explained Sala. “The pressure wave is so intense that it can literally bend the material. Therefore, by adjusting the parameters of the laser, we can control how much bending occurs in the titanium alloy sheet.”
Without the water, the efficiency of the process would be reduced by around 50 per cent, as half of the energy would then be directed away from the titanium alloy surface. Water would therefore still be required should this technique be scaled up to industrial levels.
Bending in the peened region of an LPF-treated 1mm thick Ti-6Al4V specimen without a protective overlay. Oxide layer formation in the peened region was observed due to surface ablation during the process. (Credit: Helmholtz Zentrum hereon GmbH)
The laser system used was a bespokely designed Nd:YAG 1,064nm laser from Quantel, capable of producing 5J pulses at a frequency of 10Hz with a 10-20ns duration, a homogenised beam profile and a 1 x 1mm2 cross section.
“This corresponds to an intensity of up to 25GW/cm2,” said Sala. “However, we’re planning to add even more power into this system – using 10J pulses rather than 5J – because for titanium what we are currently using is still suboptimal. From this work we identified that the laser power density is one of the most important factors that influence the bending of the sheet. And so the more we're able to control this parameter, the more we can control the deformation taking place. If we wanted, for example, only half a millimetre of deformation, with the right power density we can make this happen, and with more accuracy than traditional forming.”
Asked whether this technique could face similar throughput issues faced by ultrafast laser texturing applications – which have recently seen increasing average powers and complex multi-beam delivery systems to achieve faster, parallel processing – Sala explained that it “currently takes about a minute to treat a 20 x 20mm square, so doing a square metre would certainly take a considerable amount of time. There is potential to use multiple lasers, or lasers directed by scanning technology, but we would need to do further experimentation here, as this would influence how the pulses interact with the surface compared to the setup we are currently using.”
The particular challenge in conducting this work for the aerospace industry, according to Sala, are the stringent requirements – which dictate that absolutely no surface defects can be present on any parts.
“We can’t afford to have any cracks on the surface, so it's extremely important – and also quite challenging – to achieve critical process control,” said Sala. “This is why we require the protective steel foil, as with it we can achieve a very nice surface finish, indistinguishable from the un-treated regions of the titanium-alloy surface. Without it we observed a 1.5-micron oxidised surface being formed over the treated region, containing undesirable microcracks that increase risk of part failure under fatigue loading. In addition, the residual stresses left in the material following peening penetrate relatively deeper along the thickness of the material when the protective foil is used, unlike those left when it isn’t, meaning they will perform better compared to the sheets that are treated without a protective layer.”
The researchers' partner ZAL possesses the biggest commercially available laser shock peening facility in the Europe – capable of treating components up to 5m x 1m x 1m and 180kg within minutes (Image: ZAL/DReinhardt)
While laser peening is still in its early stages in terms of being applied on an industrial scale, the application has been catching the eye of numerous industrial parties, including those in the maritime and aerospace sectors.
Sala remarked that one of the commercial firms making decent headway in these areas is LSP Technologies, which has been experimenting with different applications such as correcting deformations on gear shafts, extending the life of jet engine blades, and even treating aluminium plates on naval combat ships. The firm has even done work with companies such as Airbus, with which it developed a portable laser peening system to carry out maintenance on the more hard-to-reach areas of an aircraft.
“While big industry players such as Airbus and Lufthansa Technik are now experimenting with this technology, until they achieve consistent, reliable results that can be certified via the proper authorities, they’ll likely want to keep it all under wraps, and so we can’t expect to be hearing the results of their trials any time soon,” he added.
However, while laser peen forming does show great promise for industry, the process does pose certain challenges concerning its applicability, as the process needs to be tailored for each use case. “Therefore, traditional process optimisation strategies should be replaced with state of the art artificial intelligence or machine learning methods for maximum automation,” said Sala. “This work we are doing has enough quantitative data to develop process planning algorithms that can significantly improve the automatability of laser peening, which is why we are working on this at the moment within the scope of the PEENCOR project.’’
Overall, Sala believes that while laser peening won’t be able to completely replace traditional forming techniques – especially as so many companies have already invested tens of millions into traditional forming equipment – it does indeed show great promise for being applied as an incredibly precise correction process. “The technique really does have huge economic industry potential,” he confirmed.
Work funded by