Theta Technologies on the development of nonlinear resonance NDT technology for additive manufacturing

While advancements in post-processing technologies have undoubtedly made the adoption of additive manufacturing (AM) in production environments much smoother, in some instances, post-processing (the steps that come after the printing part such surface smoothing to create injection mould-like quality or removal of excess powder from a complex part) has sustained its place as the industry’s ‘dirty little secret’, one rarely revealed when being dazzled by perfectly polished pieces on perfectly lit trade show displays. The time-savings promised by AM’s rapid production rates can oftentimes be lost to hours spent manually chiselling, tumbling, or blasting to achieve that single finished component. Worse still, what if you invest precious time and resources into a part, only to reach your final inspection step and discover it’s not up to spec?

But what if you could test earlier and catch any flaws before a part even arrives at the post-processing station? Theta Technologies, a UK-based engineering company specialising in non-destructive testing (NDT), says you can.

"If we can detect a flaw when it happens, then that opens up the opportunity to pull [the part] out of production and see why," Prof James Watts, Chief Technical Officer at Theta Technologies, told TCT in a recent conversation about how rapid inspection performed throughout the manufacturing process, can save time, costs, and headaches, further down the line.

A spinoff from the University of Exeter, Theta Technologies has developed what is thought to be the world’s only nonlinear resonance NDT solution capable of performing a complete NDT in under one minute. The RD1-TT made its debut at TCT 3Sixty last June and is said to be able to detect flaws in complex metal 3D printed parts, including those that are not visible with other NDT technologies. Theta Technologies says the process can be widely applied to a range of manufacturing techniques but, for the last six months, has focused its attention on the metal AM space where the demand for NDT to further the technology’s adoption in critical applications was clear.

“If you look at pretty much any review paper on the options for non-destructive tests for metal AM, it is pretty much X-ray CT or visual inspection,” Watts elaborated. “Many of the machine manufacturers will tell you that you can qualify the process and once you've qualified the process, you can just trust it. But talk to anyone who actually uses the parts and they know that's not true."

Like post-processing, the measurement and inspection of AM parts can also be a challenge, particularly for those end-use applications where nothing less than full NDT will suffice to ensure safety and reliability in critical environments. For industries like aerospace and medical, where parts are being sent into the sky or implanted into the human body, stringent testing must be adhered to, and for additive parts in those industries, that expectation is no different. It’s a “cultural shift”, Watts argues, and while there are already thousands of AM parts out there in such scenarios, safety critical parts are often constrained by the need for this form of meticulous, expensive and time-consuming testing, and as a result, forego the fundamental benefits of design for AM.

“Whereas it would have been possible to make that part using a really beautiful organic structure where there's material only where the stress analysis says that it needs to be, they've gone for a solid block because they can use ultrasound to see if there are any flaws,” Watts explained. “If it's made using AM how AM should be used, which is to put material where it's absolutely required, then it's just impossible to test.”

The benefits of Theta Technologies’ rapid RD1-TT system mean AM parts can be put through non-destructive testing at multiple stages of the production process. Using Theta Technologies' unique method, manufacturers can quickly test complex parts with various surface finishes even while they're still on the build platform and, when compared to X-ray CT scanning, Theta Technologies estimates the RD1-TT can detect a greater range of cracks, delamination and lack of fusion, and penetrate thicker walls in under 60 seconds, versus two hours with X-ray CT. This not only relieves manufacturers from sinking unnecessary time and costs into non-viable parts but also means potential negative impacts of rigorous post-processing techniques on a part can be identified and eliminated.

“Printing the part is only the first step,” Watts explained. “The steps that follow the printing are really brutal and if we can test the part shortly after printing, before some of those processes, and we can show that the part is a good part at that stage, we can identify if any of those process steps are breaking a part. We know, for example, that some of the de-powdering processes can put quite a lot of stress and fatigue into a metal part. [...] Support removal and de- powdering are all potentially risky steps. If we can test the part before support removal, before it's been removed from the base plate, that's really important because, if we can detect a part is flawed, then we don't waste any more time on it.”

Though laser powder bed processes are more common, Watts suggests early inspection could hold even more value for processes like wire arc additive manufacturing where abrasive machining steps, used to take a part from near net to finished, are a central part of the process. It could also offer potential sustainability benefits too.

“Anything you do to a material makes it harder to recycle,” Watts added. “Nobody wants to produce scrap. But if you have produced a part that's defective, then I think the sooner you recognise that, the easier it is to put that back into a recycling workflow.”

While testing at various intervals throughout end-to-end manufacturing is key, the real step change, Watts claims, is the ability to test every single printed part in a production run. 

“Don't sample test, 100% test,” Watts said. “Then every single part you ship to your customer has been through a quality assurance process. And that's a big contrast to pulling out a sample from a build plate. We know that the properties and the printing characteristics in different parts of the build plate are different for all sorts of reasons. But if you can test every single part, then you don't need to rely on assuming that every part of the batch is identical.”

Adopting AM into an established industry or new business requires confidence, and Theta Technologies aims to provide that to manufacturers for parts that may have been limited by traditional testing, and can now take advantage of AM’s design freedoms.

“It's not the solution for everything but it allows us to test parts that are completely untestable using other methods,” Watts cautioned. “Some of the denser materials and larger parts just can't be put into an X-ray CT machine and there's no other option. [...] It means that we can allow AM to be used in those critical industries where NDT is essential, when it wouldn't be otherwise.

"That's really exciting," Watts concluded, "because we should be able to allow AM to be used how it should be."