Scientists investigate the influence of defects in metal AM parts

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Laser powder bed fusion. (Image: Johns Hopkins APL/Ed Whitman)

Researchers at the John Hopkins Applied Physics Laboratory (APL) in Maryland, USA, have published work investigating the impact of defects on the structural performance of metal additively manufactured (AM) parts.

The work could be a critical step in enabling the qualification of AM parts in the future.

In the Journal of Materials Processing Technology, the researchers explored two types of defect that commonly occur in laser powder bed fusion, ‘lack of fusion’ and ‘keyhole’ defects.

The former occur when there is not enough energy to completely melt the metal powder bed, whereas the latter occur when excessive energy density forms a fluid dynamic instability in the molten powder bed. As the energy density deviates above or below the optimal levels, the quantity and size of the defects increase.

‘Laser powder bed fusion is a dominant additive manufacturing technology that has yet to reach its potential,’ said corresponding author Steven Storck, a mechanical engineer in APL’s Research and Exploratory Development Department (REDD). ‘The problem is that tiny bubbles or pores sometimes form during the printing process, and these create uncertainty in strength or performance in areas of the finished products.’

According to the researchers, the engineering impact of these defects is not yet well understood; and, in a field where certifications and standards reign supreme, it is difficult to field these parts because of the lack of processing data and standard protocols.

They therefore set out to better understand the influence of the two types of defect on the mechanical performance of AM parts and provide data that would help inform future decision making. 

‘We modified the la​ser processing conditions to simulate natural faults in the process and generated three similar amounts of defects in the keyhole and lack of fusion domains,’ Storck explained. ‘Then, we scanned and quantified material from each processing condition using X-ray computed tomography to map the defect size and distribution, and compared samples containing these resultant defects in monotonic tension testing to determine the preferred defect domain for a given amount of defects.’

Results showed that while high amounts of each type of defect are of course unfavourable, it is more favourable to have keyhole defects than lack of fusion defects at similar concentrations. The team also discovered that microstructural refinement around a keyhole defect can counteract its weakening effect. Even up to 4-5 per cent porosity in the keyhole domain results in the same yield strength as a part with negligible porosity, a target metric many mechanical engineers use to design parts.

This research was part of APL’s ongoing efforts with the Naval Air Systems Command to understand the effects of defects in additive manufacturing. ‘Our current research is now using this finding combined with machine learning to rewrite the way we process materials with laser melting,’ Storck said. 

‘This work is a critical step in laying the foundation to enable qualification of AM parts in the future,’ added Morgan Trexler, who manages REDD’s Science of Extreme and Multifunctional Materials programme. ‘A general understanding of the influence of the effects of processing conditions on the resulting microstructure and properties of a material and component will provide the scientific basis to enable protocols for safe implementation of additively manufactured parts.’

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