3D printing used to create hypersonic engine combustor

Materials World magazine
2 Mar 2016

3D printing meets aerospace

Additive manufactured components take off in aerospace tests in the USA.

Orbital ATK and Fabrisonic, USA, have celebrated success with 3D printing in supporting the aerospace industry, with Orbital ATK testing a 3D-printed hypersonic engine combustor, and Fabrisonic devising an ultrasonic additive manufacturing process that merges metal layers.

Hypersonic propulsion systems need to maintain stable combustion in extremely volatile conditions, and Orbital ATK’s combustor – produced through powder bed fusion – was able to withstand numerous high temperature flight conditions over a 20-day testing period, including one of the longest duration propulsion wind tunnel tests for a component of this kind.

Orbital Vice President Pat Nolan commented, ‘This successful test will encourage our engineers to continue to explore new designs and use these innovative tools to lower costs and decrease manufacturing time.’ The use of an additive manufacturing process has enabled Orbital to incorporate design features and integrated components that would not be possible through conventional manufacturing.

 A new ultrasonic additive manufacturing (UAM) process merging layers of metal foil including titanium, tantalum and aluminium can help protect aerospace components against radiation and reduce background noise for signal analysis, without changes in grain size, precipitation reactions or brittle inter-metallics experienced in fusion-based welding.

The solid-state bond of dissimilar metals achieved by ultrasonic welding, a method first devised in the 1940s, is achieved by ultrasonic vibrations that create relative motion between two surfaces held under pressure. This causes shearing and asperities, which disperses oxides and contaminants. As the asperities collaspe, metal-to-metal contact is increased through heat and pressure.

The Fabrisonic process is able to reach this state at relatively low temperatures. This makes it possible to embed electronics into metallic panels and combine dissimilar metals including tantalum and aluminium, which have long been used in satellites for radiation shielding, to create panels with a variation of Z values.

Mark Norfolk, President of Fabrisonic, said, ‘We are making parts that carry structural load and have shielding built-in, typically several inches thick. We use a solid-state process to build layer by layer – some of the parts are made of thousands of layers of foil – typically 0.005-0.010 inches thick.’

New coating, new danger

‘Polymers are susceptible to damage in the forms of small cracks that are often difficult to detect,’ said Nancy Sottos, Materials Science and Engineering Professor, at the University of Illinois, USA, whose team has developed a new coating that can detect damage to polymer materials. ‘Even at small scales, crack damage can significantly compromise the integrity and functionality of polymer materials. We developed a very simple but elegant material to autonomously indicate mechanical damage.’

Microcapsules of a pH-sensitive dye are embedded in an epoxy resin, and break open when the polymer suffers scratches, stress or fractures. The dye subsequently reacts with the resin, changing colour from yellow to a bright red, with the severity of the colour change indicating the depth of damage.

Cracks as small as 10nm – enough to indicate a loss of structural integrity – are sufficient to cause the colour shift. Subsequent tests from Sottos’ team suggest long-term stability of the coating – no leaking to produce false positives or colour fading. Sottos also commented on the coating’s low cost, noting, ‘A polymer needs only to be 5% microcapsules to exhibit excellent damage indication ability. It is cost effective to acquire this self-reporting ability.’ Although the coating has been positioned as a warning system for polymers, it can also be used to coat other substrates including glasses and metals. 

Aerospace is among the potential applications, as well as cars and petroleum pipelines, where small failures can lead to costly outcomes. Sottos’ team are exploring its use on other materials including fibre-reinforced composites, and compatibility with self-healing systems. ‘We envision this self-reporting ability can be seamlessly combined with other functions such as self-healing and corrosion protection to both report and repair damage,’ Sottos said.

Full details can be read in Autonomous Indication of Mechanical Damage in Polymeric Coatings, published in Advanced Materials.