Abstract
Defects produced during selective laser sintering (SLS) are difficult to non-destructively detect after build completion without the use of X-ray-based methods. Overcoming this issue by assessing integrity on a layer-by-layer basis has become an area of significant interest for users of SLS apparatus. Optical coherence tomography (OCT) is used in this study to detect surface texture and sub-surface powder, which is un-melted/insufficiently sintered, is known to be a common cause of poor part integrity and would prevent the use of SLS where applications dictate assurance of defect-free parts. To demonstrate the capability of the instrument and associated data-processing algorithms, samples were built with graduated porosities which were embedded in fully dense regions in order to simulate defective regions. Simulated in situ measurements were then correlated with the process parameters used to generate variable density regions. Using this method, it is possible to detect loose powder and differentiate between densities of ±5% at a sub-surface depth of approximately 300 μm. In order to demonstrate the value of OCT as a surface-profiling technique, surface texture datasets are compared with focus variation microscopy. Comparable results are achieved after a spatial bandwidth- matching procedure.
Highlights
Process monitoring and process control methodologies are commonplace for established machining processes, but are not currently applied in additive manufacturing (AM) methods [1]
To increase the level of confidence in the surface texture measurement results obtained by focus variation microscopy (FVM), they have been compared with the results obtained by a stylus instrument
The algorithm is based on exponential fitting of a selected window of pixel numbers, which limited the resolution of the functional image
Summary
Process monitoring and process control methodologies are commonplace for established machining processes, but are not currently applied in additive manufacturing (AM) methods [1]. There has been significant effort to control feedstock, indirectly monitor sintering and control machine processes, which is reported in [3,4,5] and in Everton et al.’s review on the subject [6] Despite this effort, assurance cannot be provided through current technologies for the success of an operation. OCT performs cross-sectional imaging by measuring the magnitude and echo time delay of backscattered light. Cross-sectional images are generated by performing multiple axial measurements of echo time delay (axial scans or A-scans) and scanning the incident optical beam transversely. The specimens were produced with a melting laser scan speed of 2500 mm s−1, a hatch spacing of 250 μm and a layer thickness of 100 μm They were designed to mimic a range of densities of loose powder caused by un-melted/insufficiently sintered volumes in SLS-produced parts. The variation in sub-surface powder density was created by modulating the laser power from switch-on to 11 W in equal steps of 1 W
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