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Aviation in an additive age

Weight, strength, and reliability represent the holy trinity of manufacturing challenges in the aerospace industry. While achieving these at reasonable cost is an ongoing endeavour, laser additive manufacturing technologies are changing what is possible for making aircraft components. The 3D printing techniques forge new alternatives to traditional milling methods.

Both the novel and the conventional have their place in an industry that still values Douglas Aircraft designer Ed Heinemann’s philosophy: ‘simplicate and add lightness’. Simplication means making a system more complex in order to improve ease of use.

Laser welding is one of those now conventional processes for building aircraft, particularly for manufacturing the engines, according to Richard Freeman, associate director at Cambridge, UK-based TWI, formerly known as The Welding Institute.

‘It’s interesting: you look at a contemporary airframe – there’s very little welding. In the main commercial aircraft, they are mostly riveted. But you could not make an engine without lots of kinds of welding,’ Freeman commented.

Airbus uses CO2 lasers to weld parts, noted Freeman, but added that ytterbium fibre lasers are now increasingly employed, especially for research and development. Fibre lasers can be attached to a robot arm, and they are more efficient than their CO2 counterparts.

A much newer laser technology now taking hold in the aerospace industry is additive manufacturing, which has found a captive audience in a market that requires a modest number of complex, lightweight parts. The geometrical freedom of 3D printing offers a big advantage relative to traditional manufacturing: in a complex part such as a fuel system, comprised of 10 nozzles, connectors, and tubes, it becomes possible to merge the pieces into a single part manufactured from a bed of metal powder, optimised for strength and weight.

‘3D printing of plastics has been around since the early 1980s. But if you look at what people refer to as 3D printing [in aerospace], they’re talking about a bed of metal powder which you melt with a laser or electron beam, and fuse into a structure,’ commented Freeman.

In powder bed manufacturing, a 3D CAD model is sliced into finite layers. Each layer is recreated by depositing metal powder layers, and melting their surfaces with a scanning laser beam. The melted particles fuse and solidify to form layers of the component with virtually no pores or voids.

In laser metal deposition – another additive method – a laser beam forms a melt pool on a metallic substrate, into which powder is directly deposited by a robot system. ‘You send powder down a nozzle, and as it exits the nozzle it’s melted by a 2kW CO2 laser and builds up a structure freeform,’ explained Freeman.

Using laser metal deposition, it took TWI only 7.5 hours to build thin-walled nickel-alloy engine casings as part of an EU project aimed at reducing the environmental impact of aerospace manufacturing. Traditional techniques would require weeks. Another advantage of this method is the ability to add material to existing structures for repairs, which TWI demonstrated as part of an automotive project with Rolls Royce. They deposited nickel-based alloy seal structures in criss-cross patterns onto the turbine surface, in a procedure that is now public domain and available for aircraft engines.

Lasers have a small heat affected zone and do not melt excess material, thus reducing waste. In general, powder bed fusion is used for metals: laser system fabrication of a component offers new levels of control and use of lightweight materials that cannot be traditionally machined.

Additive manufacturing isn’t replacing traditional manufacturing. Rather, it provides a new item in the manufacturing toolbox. The actual cost of additive frequently exceeds that of conventional. But extra value through reduced weight, enhanced design, and quick production time, along with a relatively small volume of parts, makes it likely to be aerospace’s production mechanism of choice in the future.

A titanium bracket

In September, Frank Herzog, founder of Concept Laser, along with Peter Sanders at Airbus and Professor Dr Claus Emmelmann of Laser Zentrum Nord, received a nomination for the German Future Prize 2015 for the project ‘3D printing in civil aircraft manufacturing – a production revolution is taking off’. This illustrates how important additive manufacturing is for the aerospace industry, with its potential to make individual parts and reduce the weight of the airplane.

The team used powder-bed technology to make a titanium connector for the Airbus A350 XWB outer shell.

‘What’s important is that there are many brackets in one plane – a high number of parts – and it needs to be lightweight; weight is money,’ explained Peter Appel, lead process development engineer at Concept Laser. Made conventionally, the bracket is milled out of an aluminium block. Laser additive manufacturing uses strong, lightweight, but difficult-to-mill titanium, and removes extra material while optimising topology.

As an engineering student in the 1990s, Herzog found that using metal powder in a laser sintering machine for polymers led to porous structures, because of the CO2 laser. He used a more powerful solid-state laser (Nd:YAG), which made it possible to completely fuse and melt the metal powder. This was used in Concept Laser’s first machine.

‘Herzog realised that you could totally fuse the metal and proceed with a stochastic, or randomly distributed, exposure strategy,’ explained Appel. In 2001, Concept Laser’s LaserCusing process was launched as an industrial additive laser system for metals. It has resolved technological issues in transitioning 3D printing from polymers to metals.

When a metal melts, its viscosity is near that of water. When it cools, thermal-induced stresses develop. This generates a problematic thermal gradient when melting from one side to the other across a large surface. But with Concept Laser’s stochastic melting, the whole surface is divided into small areas which are exposed in rapid succession, thus delocalising heat and reducing stresses within the component.

Polymers and metals

At present, there are two main applications of laser additive manufacturing. Titanium is currently a popular metal because of its light weight and strength, with aluminium also under investigation. For plastics, small series and spare parts are being designed additively.

Boeing started using additive manufacturing more than a decade ago, sintering powdered nylon materials for environmental control system ducts and non-structural, unloaded applications on military and commercial aircraft. ‘We’ve explored other materials that would fit our commercial grade machines. That’s transforming as we move to larger applications,’ said Mike Hayes, Boeing Research and Technology’s technical lead engineer for additive polymers.

Before laser sintering, there were rotomold ducts and aluminium weldment tubes. ‘All companies are exploring ways of making materials and processing better and more consistently,’ he added. ‘We have suppliers for our polymer parts, but we start internally so that we can understand the process and what we can or can’t do.’

Powder bed manufacturing is used for making small metal parts where extensive functionality is required in a tight space. ‘Powder bed systems can use electron beams or lasers. Laser systems tend to offer better surface finish, with finer features, while electron beam systems offer faster build time and less post processing. Lasers offer the advantage for most part candidates requiring complex features,’ said Dave Dietrich, Boeing Research and Technology’s technical lead engineer for additive metals, based at Oak Ridge National Laboratory.

Now, Boeing has several hundred types of 3D-printed parts flying on their products, and is sending an additively manufactured component into space aboard a 702MP satellite. The Receive Antenna Deployment Actuator cage holds thermal blankets in place during deflector deployment. Manufactured conventionally, many of the 21 parts had to be machined out of plate or bar stock and assembled together. The new additive method requires six parts integrated into the satellite assembly process, reducing assembly time and cost.

‘Structural performance can be increased through innovative design that can’t be fabricated through the original process. You can take advantage of lighter weight and structural soundness,’ said Dietrich. Another advantage is the savings on lead time. Replacing a casted part requires tooling and can take months to fabricate. With additive manufacturing, the part is effectively printed from a digital CAD file.

But, with all the complexity of laser systems, substantial training is required for the individuals who run these processes. Currently, nearly all 3D-printed parts used in production programmes are sourced from suppliers. ‘The technology is so new that there are places outside the company that specifically train their employees in these skill sets,’ added Dietrich.

Training the troops

The Lazer Zentrum Nord (LZN) technology transfer centre from the Hamburg University of Technology offers this kind of training. It worked with Airbus on titanium manufacture, using a bionic approach to create lightweight designs.

‘In 2008, we developed a bracket for Airbus based on a new idea for creating lightweight structures. The [German Future] Prize bracket design with Concept Laser also comes from us,’ said Eric Wycisk, key account manager for Airbus at LZN. But now, Airbus incorporates additive manufacturing into procedures for making small quantities of spare parts and tools with complex structures that are expensive to manufacture conventionally, as well as components that must be lightweight to meet aircraft weight goals. It has established groups that work on this technology, but still work closely with research counterparts.

‘We at LZN offer four days’ training for Airbus employees, which includes theoretical and practical training about design capabilities of the process and practical questions at the machines,’ said Wycisk. LZN also provides guidance and support for designing the structures. With four manufacturing machines on site, the institute makes prototypes for Airbus to test; however, the process must be streamlined before a major aerospace company can introduce it in-house.

‘We do research on higher laser power to improve the process,’ said Wycisk. ‘Together with our partners, we are working to establish serial production in the next years.’

Laser manufacturing is limited by the size of the powder beds in the machines. Now, LZN’s largest chamber is 500 x 280mm2. But a South African company is developing a 2m-long building chamber. Another limitation is build speed, which could be improved by using multiple lasers simultaneously and more power to melt more material.

Regulation and the future

Nadcap (National Aerospace and Defense Contractors Accreditation Program), the quality assurance industry body, is working to bring additive manufacturing into the civil aircraft world. ‘There are so many additive manufacturing machines out there in aircraft companies; it’s a bit of a free-for-all. You can build lots of parts, but you have to build to high quality and meet form, fit, and function,’ said TWI’s Freeman.

‘If you can’t ensure the quality of mechanical properties, nobody would build parts in an aeroplane,’ added Concept Laser’s Appel. Other steps to meet serial production challenges in aerospace include developing multi-laser systems to increase productivity, and new intelligent exposure strategies to improve part quality alongside productivity.

‘As the enterprise matures, and achieves full certification processes, the scales of machines will increase and we will collect data for confidence in new processes,’ added Boeing’s Dietrich. As machine manufacturers work with customers and gain better insight into how to design machines, higher throughput is expected, not just for small brackets, but also for larger applications.

Laser technologies already add strength and lightness in ways that early aviation designers never could have imagined. As their reliability becomes further proven, they will occupy an even stronger presence on Orville Wright’s ‘infinite highway of the air’.

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