Introduction and accuracy improvement of laser speckle photometry for in-situ monitoring material densities in laser powder bed fusion of metals

Laser-based powder bed fusion of metals is increasingly used for the fabrication of industrial components and products. However, the complex physical mechanisms during melting and solidification process-specific defects like voids and pores within the components are difficult to predict and avoid. These defects eventually deteriorate the mechanical properties and challenge current quality assurance. Especially in medical and aerospace applications, the components must meet high safety requirements and thus the quality of each component must be guaranteed. Despite the present challenges, the layer-wise principle of PBF-LB/M potentially allows a comprehensive quality assessment throughout every layer of the respective part. This work presents the adaptation of the optical laser speckle photometry to the laser-based powder bed fusion of metals process as a new method for an in-situ characterization of the material quality. The laser speckle photometry system consists of a high-speed camera and an extinction laser, which are both integrated in an off-axis approach. With the actual set-up it is possible to analyze an area with the dimensions of 2 × 2 mm2 with 6000 frames per second and a resolution of 3.9 μm/pixel. Within the presented experimental work, suitable settings for the specific hardware and illumination settings in dependence of the signal quality are determined. In this matter, the repeatability of the measurements with different settings of the laser speckle photometry is analyzed and an approach to reduce measurement errors and deviations is presented. In further investigations, samples with provoked lack of fusion defects are fabricated and analyzed. Further, a method that derives specific material properties such as material density from the LSP data is elaborated.