QMT Features: April 2016
NDT standards for additive manufacturing
Experts at the Manufacturing Technology Centre in Coventry are addressing the lack of standards for testing 3D printed parts

Additive manufacture gives designers and engineers an extraordinary degree of freedom, allowing complex shapes and components to be produced in a single operation. However, it is that geometric complexity which makes NDT difficult.

The problem is compounded by the surface finish; additive processes for metal components cannot currently achieve the same finish as conventional machining. This means that inspection methods such as conventional contact ultrasonic testing, which require a smooth surface for efficient coupling, are ineffective.

This is a global issue, and international organisations have been working to develop NDT standards. The MTC team is feeding its work and findings into the development efforts of ISO/TC 261 - ASTM F42 JG59 and ASTM E07 WK47031 through BSI links. The outcome will be a best practice guide under the JG59 effort, bringing together existing standards with new methodologies for detecting additive manufacturing defects.

The MTC team started by classifying additive manufacturing defects, then grouped the defects based on whether they would be detected by dimensional inspection (e.g. distortion), materials characterisation (e.g. undesirable microstructure) or non-destructive testing. The team reviewed existing NDT standards for casting and welding, which are related processes with some defects similar to those observed in additive manufacturing. Some defects unique to additive manufacturing have been identified, and it is for these that new NDT methods are required.
The work is being led by Dr Ben Dutton who is a senior research engineer in the Metrology and NDT group at the Manufacturing Technology Centre. The participants include Rolls-Royce, BAE, GKN Aerospace, Alstom Power, 3TRPD, ARCAM, CDS, Renishaw, TWI, the University of Birmingham and the University of Nottingham.

Dr Dutton says new standards and more advanced NDT methods will be required. This may include phased array ultrasonic testing, thermography or X-ray computed tomography. Dr Dutton said, “X-ray computed tomography is emerging as the most promising inspection technology but it has its limitations. It is currently used predominantly during process development rather than manufacture. Affordable, rate capable test procedures for manufacture are needed.

“An ideal solution might be to inspect products during the manufacturing process and to take a snapshot of every layer. Some machines already have the capability to do this, but the next step would be to develop systems which automatically spot defects, raise the alarm and ultimately adapt or correct the process.”

He added, “As for conventional manufacturing routes, the timing or sequence of inspections has an impact. To overcome the issue of surface roughness, inspection could be carried out after a finishing process, although it is preferable to avoid wasted effort by identifying scrap before carrying out additional manufacturing steps.
“Whilst the manufacturing processes and related testing is immature, companies should use only experienced inspectors, and when in doubt use double testing – where the product is inspected twice by different inspectors. Ultimately, manufacturers need to consider how critical the part is in safety terms and the requirements for inspection when making the decision to use additive manufacturing.”

The team at the MTC is now moving towards detailed testing of potential advanced NDT methods on defects unique to additive manufacturing, establishing limitations and new standards. The results will be contained in a new additive manufacturing guide, covering all current applicable standards and new ones as they are developed.
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