During laser melting of metals, localized metal evaporation resulting in the formation of a keyhole shaped cavity can occur if processing conditions are chosen with high power density. An unstable keyhole can have deleterious effects in certain applications (e.g., laser powder bed fusion) as it increases the likelihood of producing defects such as porosity. In this work, we propose a pipeline that enables complete segmentation and extraction of various geometric features in keyholing conditions. In situ synchrotron high-speed X-ray visualization at the Advanced Photon Source provides large datasets of experimental images with a high spatio-temporal resolution across a range of laser parameters for Ti-6Al-4V. Computer vision image processing techniques were used to extract time-resolved quantitative geometric features (e.g., depth, width, front wall angle) throughout keyhole evolution which were subsequently analyzed to understand the relationship between the variation of local keyhole geometry and processing conditions. This analysis is the first to employ a data-driven approach to further our understanding of the keyholing process regime.
The competition between epitaxial vs. equiaxed solidification has been investigated in CMSX-4 single crystal superalloy during laser melting as practiced in additive manufacturing. Single-track laser scans were performed on a powder-free surface of directionally solidified CMSX-4 alloy with several combinations of laser power and scanning velocity. Electron backscattered diffraction (EBSD) mapping facilitated identification of new orientations, i.e., “stray grains” that nucleated within the fusion zone along with their area fraction and spatial distribution. Using high-fidelity computational fluid dynamics simulations, both the temperature and fluid velocity fields within the melt pool were estimated. This information was combined with a nucleation model to determine locations where nucleation has the highest probability to occur in melt pools. In conformance with general experience in metals additive manufacturing, the as-solidified microstructure of the laser-melted tracks is dominated by epitaxial grain growth; nevertheless, stray grains were evident in elongated melt pools. It was found that, though a higher laser scanning velocity and lower power are generally helpful in the reduction of stray grains, the combination of a stable keyhole and minimal fluid velocity further mitigates stray grains in laser single tracks.
The recent development of the Enriched Analytical Solution Method (EASM) for evaluating the spatio-temporal distribution of the temperature fields generated during the Laser Powder Bed Fusion (LPBF) Additive Manufacturing (AM) processes is provides an opportunity to study the sensitivity of the morphological parameters characterizing the associated melt-pools as a function of process parameters. The present work exercises the EASM for the case of a single-path trace over a 316L base plate under LPBF heat deposition conditions. To assist in the evaluation of solidification parameters, the spatial derivatives of the EASM are also derived. A process parameter subspace spanned by the scan velocity and the laser power is considered and the EASM is utilized for deriving a number of geometrical morphological characteristics of the melt pool as well as the quantities controlling the evolution of the solidification front. Finally, comparisons with initial experimental results obtained by in-situ high speed synchrotron X-ray imaging, capturing the spatio-temporal evolution of the melt pool profile are also presented.
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