Ensuring Quality of Complex Metal AM Geometries in the Space Industry: How NDT Is About to Change
There is hardly anything more important in the process of component production in the (aero)space industry than destructive and non-destructive testing (NDT). Needless to say, conventional NDT methods are critical to ensure that all components are fit for purpose. As metal additive manufacturing (AM) parts have been introduced on a wide scale in the (aero)space industry, questions have been raised about whether the conventional methods of NDT are relevant for the often complex geometries designed with a 3D printer.
With the introduction of metal AM parts, several organizations (including America Makes & American National Standards Institute (ANSI) Additive Manufacturing Standardization Collaborative (AMSC) and Lloyd’s Register / TWI) identified gaps in the use and testing of those materials. Due to the nature of the (aero)space industry, the development of new testing methods happens at a relatively slow pace due to a number of factors, including the challenging testing of complex geometries and new alloys, the fact that metal AM is developing at an extremely rapid pace, and the ongoing development and adjustments of standards and certifications for such parts.
AM component manufacturers have started to bring quality control into their processes, by adding computed tomography (CT) scanning to the final stages of the production cycle. As with a CT scanner in, let’s say, a hospital, NDT CT scanning slices up the part, enabling testing organizations to establish whether their client’s component is crack and void free. Two of the first companies to introduce this into their production are the German company FIT AG and US company Sintavia. The advantages of including this type of testing into the (metal) AM process is to ensure that the component exactly matches the CAD model by analyzing the model without having to physically touch it.
Testing companies, such as Element Materials Technology, are receiving their NADCAP accreditations to start working with digital radiography, which is gaining more popularity but is known for its slow growth in the industry. “People are still using film because it has been approved, and, despite being time consuming, everyone knows how it works. Introducing digital will take some time because organizations will need a significant amount of time to trust it,” says Phil Ford, NDT Staff Engineer at the Performance Review Institute (PRI), a not-for-profit organization originally set up to administer NADCAP.
Digital radiography offers two types of testing: Computed Radiography (CR) and Digital Detection Array (DDA). CR uses a similar technology to conventional radiography – all the testing company needs to install is a scanning unit and replace the film with an image plate (IP). CR came off the ground in the 1990s and 2000s and was set to change radiography. “Unfortunately, it didn’t. The sensitivity and resolution required were too low, compared with film,” says Ford. Progress has been made on this front, however, and the technology is better and more accurate now.
DDA, a much more expensive technology than CR, is more specialized and more suited to automation than CR. Despite its considerable cost and specific automated technology, like a robotic arm or a turntable that can handle the part to manipulate it in the x-ray beam, DDA can be used for (metal) AM parts. “Plus, with a few add-ons such as software it is basically a CT scanning system this would probably be the best kind of DR for metal AM”, says Ford.
Another technology that doesn’t seem to be as widely adapted just yet is laser ultrasonic testing (LUT). Even though it has been around for quite a few decades, it wasn’t used much until Lockheed Martin introduced this testing technology (which is now owned by PaR Systems) during the Joint Strike Fighter program. Between then and now, the laser ultrasound technology has taken many steps forward: lasers are better and more accurate, although further testing into how this will impact metal AM components has yet to be undertaken.
LUT has advantages over conventional ultrasonic testing, since generation and detection lasers can get down to every part of (complex geometry) components, whereas conventional ultrasound loses its power and strength while in use. There are many developments needed to make this specific testing technology the norm for composites, and there is an even bigger need for development and innovation for the use of LUT for metal AM parts.
EWI, a leading innovator in manufacturing technology is currently conducting research into how to accelerate LUT. The research is led by Ultrasonics Engineer Lindsey Lindamood, who recently published a paper on LUT. “Laser ultrasonics has historically been an expensive and complicated technology; that’s why many organizations have shied away from adapting this technology into their testing processes,” she explains. Lindamood will spend her next few months interviewing OEMs to understand their requirements and how they would incorporate LUT into their production cycles. She is identifying commercial LUT systems EWI can customize and integrate into their clients manufacturing processes that would benefit or require LUT.
A company that has now integrated this technology into its testing facilities is Spain-based Tecnatom. The firm works closely with Airbus and is celebrating its 60th anniversary in 2017. In 2013, Tecnatom launched a collaboration agreement with Airbus to develop tecnaLUS, an industrial robotic system that uses two lasers to detect failures and porosities in parts with complex geometries. According to Esmeralda Cuevas, Technology Leader in Non-destructive Testing and Project manager of tecnaLUS, “It is important to mention that the LUT technology is very dependent on the materials inspected. TecnaLUS is specifically designed for the inspection of composites, solid laminate material. In the case of metal components, it will be necessary to perform an initial study of the combination of the lasers to be used, and some efforts in specification, design and industrialization will be required.”
Besides changing conventional NDT methods, in-process testing is gaining popularity and knowledge among metal AM components production companies. In-process testing may even become as important as NDT, as the manufacturing process of metal AM parts is key to the quality of the product. Examples of in-process testing include thermal imaging and meltpool spectroscopy. These technologies can detect irregularities while parts are being produced.
The future of NDT for metal AM materials is an open and challenging path. Not only does the industry need to keep up with the rapid development in the metal AM technology itself, it also needs to keep up with the requirements of end users and standards and qualification organizations.
Phil Ford mentions that, in order to take testing for additive manufacturing further, there needs to be more clarity on what is expected from OEMs when it comes to metal AM components. “The testing companies need to know from organizations what to look for specifically. Companies who bring in their components should say, ‘These are the testing criteria, can you please find X&Y,’ before we can test a component adequately.” Once this has been established, testing organizations will know whether or not the NDT methods are right for that material and specific component.
Cuevas says, “In my opinion there should be a study for the inspection strategies depending on the part being manufactured, so looking at its material, geometry, specification of the defects to be detected. An effort from a design and inspection point of view should be done in a very collaborative way. Currently, the AM process allows to generate geometries not possible before, such as pseudo-organic lattices and grids, by incorporating what is called ‘generative design’ at the macro but also at the micro level. The introduction of other, faster NDT methods is a must and a challenge.”
To see presentations from organizations like NTS and Element Materials Technology, register now for Space Tech Expo 2018 – May 22-24 at the Pasadena Convention Center in Pasadena, CA.
Register for the Space Tech Expo Webinar: ‘Bridging the Standards Gap in Metal AM Parts Used in Aerospace Applications – Defining Future Frameworks’. This webinar discusses the America Makes & ANSI AMSC road map, as well as in-process testing.