With the use of additive manufacturing (AM) on the increase in the aerospace industry, the lack of standards for testing such products is a source of growing concern.
AM offers total freedom in designing components but at the same time, it makes non-destructive testing (NDT) difficult. Firstly, the more complex the component is both internally and externally, the harder it is to inspect. Another issue is that AM creates a surface finish that is typically rougher than conventional manufacturing processes so inspection methods such as ultrasound, which require a finer surface, are less suitable.
Image courtesy of 3T RPD Ltd.
Internationally, a lot of effort is going into creating inspection standards and there is a real sense of urgency, as I am aware from my own work helping to develop best practice guidelines for BSI and ASTM.
X-ray computed tomography is emerging as the most promising inspection technology but it has its limitations and there is room for improvement to adapt it better to AM although research is taking place in this area. While computed/digital radiography is sometimes used, detecting flaws accurately depends on selecting the correct settings, such as enough projections to cover the part thoroughly.
The ideal solution would be to inspect products during the manufacturing process and 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 could spot the defects themselves and raise the alarm.
Another solution might be to inspect after machining. The smoother surface would potentially allow the use of other inspection methods, although this would add further to the manufacturing cost if the part is found to be defective after machining. Another post-process is hot isostatic pressing, which has shown to reduce the size of internal defects in the parts.
For now, companies should use only experienced practitioners, carry out visual inspection as an initial step, 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.
Dr Ben Dutton is a senior research engineer in the Metrology and NDT group at the Manufacturing Technology Centre (MTC), Coventry, UK. An instrumentation scientist, he has over 25 years’ experience in both academia and industry and 12 years’ experience in NDT. Dr Dutton will be speaking about inspection of AM components at 4pm on Tuesday 8 September at Materials Testing 2015 at Telford International Centre. For further information see www.materialstesting.org