QMT Features: October 2017
Phased array ultrasonics adds up
Danny McMahon, senior manufacturing engineer and team lead for metrology and digital manufacturing at The University of Strathclyde’s Advanced Forming Research Centre (AFRC) points the way ahead for additive metrology

The potential benefits of additive manufacturing for many industries has been well documented – but it also comes with a host of unknowns. The ability to print products and components where they are actually needed, could be transformative for manufacturing.

To take an extreme example, the International Space Station announced in December 2014 that it had printed a ratchet wrench with a design file transmitted from the ground – other products have been printed in space since then.
Here on earth though, many sectors are still grappling with what additive manufacturing means for them. Across the board, it throws up one very large problem: validating the components produced by 3-D printers for specific applications. In highly regulated industries it’s a serious challenge and, if it’s not tackled, additive manufacturing’s potential could be hamstrung from the get-go.
There are many steps required to get to the point where we have a tried-and-tested system for proving the suitability of additive manufactured parts. Looking specifically at the measurement and validation of their internal characteristics and integrity, there are a couple of ways currently being considered for how this can be achieved.
The first, under examination by the High Value Manufacturing Catapult network, is X-ray computed tomography (XCT). XCT is a non-destructive technique for visualising the internal properties of a component.

This approach is being analysed at the higher end of the value curve – on aerospace components, among others, largely because of the limitations inherent to XCT. Although it offers great accuracy, XCT is also expensive, requires a great deal of power to look inside metallic parts, and needs a lot of time to perform scans – normally four hours for a reasonable-sized component. All of these factors rule it out for a range of industries and applications.

Ultrasound approach
The alternative technique, which the AFRC is tackling, is ultrasonics – specifically, phased array ultrasonics. This approach is similarly non-invasive, but even cutting-edge systems are about a fifth of the cost of XCT, both in terms of set up and running. It is also scalable, meaning it can be used on small and large parts, with very few restrictions in terms of the volumes it can handle. Likewise, the maintenance requirements are much higher for XCT systems, which often need replacement parts. 

Phased array ultrasonics can provide a more accurate picture of the internal dimensions of a part, compared to more basic, single-element ultrasonic inspections. This is achieved through the ability to pulse individual ultrasonic elements independently within the array, delivering greater access to different regions within the part, without the need to physically move the probe. With a wide variety of data gathering and processing options available, the latest techniques offer significantly higher levels of resolution and accuracy than was achievable with traditional phased-array methods.

We’ve been working with Cranfield University on this technology for about a year, using it to inspect wire arc additive manufacturing (WAAM) parts. Like other additive manufacturing parts, they’re produced by adding layer upon layer of material, with the addition of a welding power source. This approach to manufacturing can create small gaps, or cavities, within the part and, therefore, defects which need to be understood and managed. Additive manufacturing techniques also allow the creation of internal features where there is no physical access for inspection or measurement. 

Previously, these defects and internal features would not have existed, as traditional manufacturing techniques prohibit the creation of internal features. The closest technique that could be used is sand casting. This process sees molten metal poured into a mould, created by compressing sand. Inserts can then be placed inside the mould to create internal features or cavities.

Once the mould is made, the sand is broken up and can simply be ‘shaken’ out of the part.  Using this, or other similar techniques, the quality control would be applied to the mould, instead of the part, and a manufacturer could have confidence that a good mould would result in a conforming component. This cannot be done for additive manufacture, as there are no moulds and near unlimited possibilities for printing internal features.

Using the latest technology and techniques, the AFRC has been conducting research in phased array ultrasonics for a variety of applications. We have been able to reconstruct the bony surfaces of the knee joint using robotically manipulated probes, aiding robotic knee arthroplasties. This has pushed forward the ability to reconstruct complex three--dimensional surfaces using ultrasound – a capability particularly suited to components made using additive manufacturing.

Internal structures have also been inspected on wing components, combining optical geometric surface reconstruction techniques, robotic positioning, and phased array ultrasonics. With this application-based research, we are looking to push the technique up the Technology Readiness Level (TRL) scale so that these advancements can be fully exploited.

3D reconstruction of AM parts

We have begun trials in the inspection of additively manufactured components, with full 3D reconstruction of outer and internal features achieved on simple parts, as well as the identification of defect locations. Scaling up this capability to larger and more complex parts would allow the next course of action in the production process to be defined.

For example, rather than scrapping a component at great cost, the defect could simply be repaired, cut out, or left in there and treated in a particular way further down the production line. In a real manufacturing context, this could lead to significant cost savings, shorter lead times, and, ultimately, assurance over the integrity of 3-D printed products.

It’s still early days and many of the biggest challenges are yet to come. The real litmus test will eventually be phased array ultrasonic testing’s adoption by industries like aerospace. Rightly, the approval processes for getting components into these sectors are very stringent and there will need to be many procedures in place to prove the integrity of additive manufactured parts is just as solid as those created using traditional means.

Nevertheless, this could be an important step forward in defining a proven metrological approach, for more and more products are going to be manufactured in the future. There are hundreds more that will need to be taken concurrently. But initial indications suggest that phased array ultrasonics will be an important part of that journey.
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