Equivalence-based and model-based certifications require reliable data set to validate complex multi-physics models. To move towards the certify-as-you-build scheme, industries call for sound in-situ process monitoring and quality control.
Process monitoring is typically used to identify the formation of defects as it occurs with the view to remediate it or induce the decision of terminate a build and limit the production costs of a flawed component. In general, the goal is to determine the presence of porosity, contaminant inclusions, swelling, warping and other defects. Temperature evolution in the material is also measured.
- Layer solidification homogeneity
- Temperature measurements and thermal build-up/history
- Powder layer uniformity
- Warping, curling, and other part deformations
- Melt pool characteristics (shape, temperature, dynamics, emission wavelenth)
If optical monitoring can pick up defect formation IR or near-IR imaging contribute to full layer thermal analysis . This data help better understand solidification and thermal history and general AM metallurgy . It is particularly useful for EBM, where it measures the elevated surface or powder-bed temperatures .
A feedback system using average temperature data acquired by in near-IR has already been used for on-the-fly process parameters correction .
Pyrometers have been used in Direct Energy Deposition  Electron Beam Melting  and Selective Laser Melting  The details of the sample location of the pyrometer are important to note, as the heat source may or may not pass in the measured area, depending on part geometry.
Post-build Non Destructive Evaluation
The Archimedes principle gives a qualitative indication of porosity in materials. It is a fast and economical technique for bulk measurements and seems to show congruent results with XRCT .
XRCT and neutron tomography are used to map out pores and determine the location of defects in the components. Ultrasonic transducers are capable of detecting smaller amounts of porosity (~0·5%) and should also be capable of porosity mapping.
Current implementation in commercial systems
The need remains to characterise 3D structures over relatively large areas to very high spatial resolution. Theses metrology tasks must be fast, compatible with the production environment and implemented in real time for closed-loop feedback.
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