Metal or Machine?

27 May, 2016

Quality assurance is critical in additive manufacturing (AM) so when an unexpected result occurs during a routine build it is important to identify whether the source is the metal powder or the AM machine and most importantly how to correct it. LPW has an enviable array of analytical techniques in its world-leading lab. Coupled with an experienced team of applications engineers and metallurgists, LPW is ideally placed to determine which route to pursue – metal or machine.

When an application which was in routine production began to show changes in the surface roughness of the finished part, a natural starting point was to question the consistency of the metal powder. The alloy powder, a long-established product supplied to an exact specification, was examined alongside a reference sample from a successful, previous production batch. The LPW in-house laboratory tested both powders for flow, size distribution and morphology.

The lab is well-trained in industry standard tests, so when the powder flow for both samples measured using Hall Flow (ASTM B213) and Carney Flow (ASTM B964) each gave results within specification, this indicated the powders did not have fundamentally different flow characteristics. But there was more to be understood. To establish the likely root cause LPW began by developing the design of experiment, DoE.This began by looking at the powder more closely, the particle size distribution was explored using Laser Size Diffraction (ASTM B822) and morphology inspected by powerful SEM imaging.

Combining all these techniques the flow, size distribution and morphology showed that both the perfectly performing retained sample and the current powder sample under investigation conformed to product specification. Although the powder tests suggest that the powder is not at fault, the fact remains that the parts were not building satisfactorily so a more in depth root cause analysis was needed. LPW moved on to examine the microstructure of the actual deposited material to evaluate where the inconsistency in processing may be occurring. The samples were mounted, ground and polished to reveal the cross section. The SEM image revealed large pores in the deposited layer and does indeed show a poor surface finish.

 SEM micro-graph of deposited layer

So is this batch of powder more porous than the process can tolerate?
The powder particles are between 20-53 microns in diameter, the pores average 30-50 microns in size, so are highly unlikely to be contained stably within a powder particle. Since powder porosity would also not explain poor surface finish, the machine processing conditions were carefully investigated. After replacement of a key component of the deposition system, the process was restored to output products that passed the stringent quality controls.

Initial flow measurements had indicated that the root cause of poor build was unlikely to be the powder, but it was only by applying in-house industry expertise in AM processing and thorough analysis capabilities of both powder and part, that LPW was able to deliver confidence and a holistic processing solution.

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