Laser Speckle Photometry for Monitoring of Porosity and Failures during Additive Manufacturing

Bendjus, Beatrice; Fraunhofer IKTS; Germany

Bendjus, B.; Fraunhofer Institute for Ceramic Technologies and Systems IKTS; Germany
Chen, L.; Fraunhofer Institute for Ceramic Technologies and Systems IKTS; Germany
Cikalova, U.; Fraunhofer Institute for Ceramic Technologies and Systems IKTS; Germany

ID: ECNDT-0499-2018
Session: In-process monitoring for Additive Manufacturing
Room: H1
Date: 2018-06-13
Time: 13:50 - 14:10

Laser Speckle Photometry (LSP) is an innovative non-destructive monitoring technique based on the detection and analysis of thermally or mechanically activated characteristic speckle dynamics in the non-stationary optical field. With the development of speckle theories, it is found that speckle pattern contain information of the object’s surface.
Concrete material parameters such as the porosity are obtained by LSP based on the evaluation of the temperature line. The implementation of this evaluation concept for control of additive processes shows a great potential. Moreover, the defects such as pores and microcracks occurring during the laser-melting process is possible to detect immediately after their formation by appropriate countermeasures. So the defects can be eliminated by re-melting of the affected area. As result time and cost of intensive material testing and rework will be significantly reduced. The designed sensor system supports the minimization of the dropout rate of process conclusively so that reduces energy, material and inert gas consumption significantly. LSP sensor is integrated directly in the manufactured machine. The communication between the two processes is realized depending on the development of the electronics and algorithms for a regulative approach.
First investigation for LSP test was carried out on Selective Laser Melting (SLM) and Micro-Laser Cladding (LMC) test specimens. The paper shows that the defects and inhomogeneities in the samples that were produced by reduced and greatly increased input energies are detectable by using LSP technique. At the same time, based on the calculation of the thermal diffusivity of the optical speckle temperature, the so-called speckle diffusivity decreases with increasing porosity of the samples. This LSP-Parameter was used for the detection of the mixing degree of contact with the substrate by LMC manufacturing. The accuracy of the results by the measuring speed of 100 contacts per minute is 6.7% verified by X-ray spectroscopy.