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Laser metal deposition is a rapidly evolving method for additive manufacturing that combines high performance and simplified production routine. Quality of production depends on an instrumental design and operational parameters, which require constant control during the process. In this work, feasibility of using optical spectroscopy as a control method is studied via modeling and experimentally. A simplified thermal model is developed based on the time-dependent diffusion-conduction heat equation and geometrical light collection into detection optics. Intense light emitted by a laser-heated spot moving across a sample surface is collected and processed to yield the temperature and other temperature-related parameters. In the presence of surface defects, the temperature field is distorted in a specific manner that depends on shape and size of the defect. Optical signals produced by such the distorted temperature fields are simulated and verified experimentally using a 3D metal printer and a sample with artificially carved defects. Three quantities are tested as possible metrics for monitoring the process: temperature, integral intensity, and correlation coefficient. The shapes of the simulated signals qualitatively agree with the experimental signals; this allows for a cautious inference that optical spectroscopy can detect surface defects and, possibly, predict their characters, e.g., inner or protruding.
Equilibrium model of titanium laser induced plasma in air with reverse deposition of titanium oxides
(2022)
A chemical-hydrodynamic model of laser induced plasma is developed to study a process of deposition of titanium oxides from titanium laser induced plasma to the titanium target surface. The model is relevant to texturing and coating of titanium bone implants that is done by scanning the ablation laser across implant surfaces. Such the procedure improves the biocompatibility and durability of the implants. The model considers plasma chemical reactions, formation of condensed species inside the plasma plume, and deposition and accumulation of these species on the ablation surface. A chemical part of the model is based on minimization of Gibbs free energy of the chemical system; it is used to calculate the chemical composition of the plasma. A hydrodynamic part uses the 2D fluid-dynamic equations that model a 3D axisymmetric plasma plume and assumes the mass and energy exchange between the plasma and the surface. The initial parameters for the model are inferred from experiment.
The model shows that condensed titanium oxides, mostly TiO2, form in a peripheral plasma zone and gradually adhere to the surface during the plasma plume evolution. The model predicts the major component and thickness of the deposit and can be applied for the optimization of experiments aimed at surface modification.
The deposition of titanium oxides during titanium laser ablation in air has been experimentally and numerically investigated. A titanium sample was irradiated by nanosecond pulses from an Yb-fber laser with a beam scanned across the sample surface for its texturing. As a result, the hierarchical structure was observed consisting of a microrelief formed by the laser ablation and a nanoporous coating formed by the reverse deposition from the laser induced plasma plume. The chemical and phase composition of the nanoporous coating, as well as the morphology and structure of the surface, were studied using scanning electron microscopy, atomic force microscopy, and X-ray microanalysis. It was found that the deposit consists mostly of porous TiO2 with 26% porosity and inclusions of TiO, Ti2O3, and Ti2O3N. Optical emission spectroscopy was used to control the plasma composition and estimate the effective temperature of plasma plume. The chemical-hydrodynamic model of laser induced plasma was developed to get a deeper insight into the deposition process. The model predicts that condensed titanium oxides, formed in peripheral plasma
zones, gradually accumulate on the surface during the plasma plume evolution. A satisfactory agreement between the experimental and calculated chemical composition of the plasma plume as well as between the experimental and calculated composition and thickness of the deposited film was demonstrated. This allows a cautious conclusion that the formation of condensed oxides in the plasma and their consequent deposition onto the ablation surface are among the key mechanisms of formation of porous surface films.
Additive manufacturing by laser metal deposition (LMD) requires continuous online monitoring to ensure quality of printed parts. Optical emission spectroscopy (OES) is proposed for the online detection of printing defects by monitoring minute variations in the temperature of a printed spot during laser scan. A two-lens optical system is attached to a moving laser head and focused on a molten pool created on a substrate during LMD. The light emitted by the pool is collected by an ultraviolet–visible (UV–vis) spectrometer and processed.
Two metrics are used to monitor variations in the surface temperature: the spectrally integrated emission intensity and correlation coefficient. The variations in the temperature are introduced by artificial defects, shallow grooves, and holes of various widths and diameters carved on a substrate surface. The metrics show sufficient sensitivity for revealing the surface defects, except for the smallest
holes with an under-millimeter diameter. Additionally, numeric simulations are carried out for the detection of emission in the UV–vis and near-infrared (NIR) spectral ranges at various surface temperatures. It is concluded that both the metrics perform better in the NIR range. In general, this work demonstrates that spectrally resolved OES suits well for monitoring surface defects during 3D metal
printing.
A brief introduction will be given on modeling chemical reactions in laser induced plasmas using stoichiometric and non-stoichiometric approaches. Several applications will be considered, which can benefit from such modeling. Those include plasma enhanced chemical vapor deposition (PECVD), surface modification and surface coating, and molecular analysis by LIBS. Each application will be illustrated by simulations of relevant chemical systems. For PECVD, chemical systems are BCl3/H2/Ar, BF3/H2/Ar, BCl3/BF3, Mo/BF3/H2; for surface modification/coating it is Ti/air; for molecular LIBS they are CaCO3/Ar, Ca(OH)2/Ar, and CaCl2/Ar. Advantages and shortcomings of equilibrium chemical hydrodynamic models of laser induced plasmas will be discussed.
Laser induced dielectric breakdown (LIDB) on a surface of solid Mo in H2/BF3 atmosphere at 30-760 Torr and in a gaseous mixture MoF6/H2/BF3 + at 760 Torr pressure is tested for synthesis and deposition of superhard molybdenum borides that are needed in many areas of industry and technology. The emission spectra of the plasma and the dynamics of the gas discharge near the substrate are investigated. A comparative analysis of the gas mixture before and after exposure to LIDB plasma is carried out using IR spectroscopy. The conditions for the formation of molybdenum borides are determined. A thermodynamic analysis of the MoF6/H2/BF3 and Mo/H2/BF3 systems is carried out to determine the temperature range for the formation of molybdenum borides and establish the main chemical reactions responsible for their formation. Deposits containing MoB and MoB2 phases are obtained. For the mixture MoF6/H2/BF3, the deposit exhibits an amorphous layered structure, which contains 19.15 wt% F, 30.45% O, and 0.8% Si. For the Mo/H2/BF3 system at the pressures 30 and 160 Torr, nanopowder of molybdenum boride is produced with a characteristic grain size of 100 nm. At pressures above 160 Torr, Mo nanopowder with a grain size <30 nm is obtained.
Surface modification of titanium by laser ablation is investigated theoretically and experimentally. The modification consists in texturing the surface and redeposition of chemically transformed material from the ablation plasma. The redeposition is driven by the hydrodynamic flow in the plasma. Such surface modification improves the biocompatibility of titanium implants.
This course will provide an introduction to plasma diagnostic techniques. The major focus of the course will be on the discussions of the practical procedures as well as the underlying physical principles for the measurements of plasma fundamental characteristics (e.g., temperatures and electron number density). Particular emphasis will be placed on laser induced plasma–atomic emission spectrometry, but other analytical plasmas will also be used as examples when appropriate. Selected examples on how one can manipulate the operating conditions of the plasma source, based on the results of plasma diagnostic measurements, to improve its performance used for spectrochemical analysis will also be covered. Topics to be covered include thermal equilibrium, line profiles, temperatures, electron densities, excitation processes, temporal and spatial resolution.
Vor allem in den letzten Jahren ist das Interesse der Industrie an der additiven Fertigung deutlich gestiegen. Die Vorteile dieser Verfahren sind zahlreich und ermöglichen eine ressourcenschonende, kundenorientierte Fertigung von Bauteilen, welche zur stetigen Entwicklung neue Anwendungsbereiche und Werkstoffe führen. Aufgrund der steigenden Anwendungsfälle, nimmt auch der Wunsch nach Betriebssicherheit unabhängig von anschließenden kostenintensiven zerstörenden und zerstörungsfreien Prüfverfahren zu. Zu diesem Zweck werden im Rahmen des von der BAM durchgeführten Themenfeldprojektes „Prozessmonitoring in Additive Manufacturing“ verschiedenste Verfahren auf ihre Tauglichkeit für den in-situ Einsatz bei der Prozessüberwachung in der additiven Fertigung untersucht. Hier werden drei dieser in-situ Verfahren, die Thermografie, die optische Emissionsspektroskopie und die Schallmissionsanalyse für den Einsatz beim Laser-Pulver-Auftragschweißen betrachtet.
Superhard materials with a Vickers hardness in the range of 30-40 GPa are of great interest, both from a fundamental and a practical viewpoints, since they have outstanding mechanical, thermal and chemical properties. Molybdenum borides belong to this group of materials. A review of publications on synthesis of molybdenum boride indicates a) great interest in this superhard material and b) the need for new effective methods of its synthesis, especially in a nanocrystalline form.
Very promising are the plasma-chemical methods based on laser induced breakdown. The breakdown can be created either in reactive gases containing volatile compounds of boron and molybdenum or on solid samples. For gas breakdown, molybdenum hexafluoride MoF6 and boron trifluoride BF3 were used in the mixture with hydrogen and argon; for solid breakdown, the pure molybdenum sample was ablated into the mixture of H2 and BF3.
The plasma-chemical synthesis of MoxBy structures was carried out in the reactor shown in Figure. Laser breakdown was created by a pulsed Nd: YAG laser operating at 1064 nm with a 15 ns pulse duration, 5 Hz repetition rate, and 800 mJ pulse energy. The laser was focused by a 5 cm focal length lens to produce 26 J/cm3 energy density in the focal point. The ratios H2:BF3: MoF6 = 5:2:1 and H2:BF3 = 3:1 were used in a pressure range 30 - 760 Torr.
After ablation in the mixture H2 + BF3 + MoF6, the deposit contained an amorphous phase with a small impurity of crystalline molybdenum and no boride phase. After ablation of metallic Mo into H2 + BF3, the main phase was MoB2 in the form of nano dispersed powder with an average grain size of 100 nm. The degree of conversion of boron trifluoride and the yield of molybdenum boride were studied as a function of pressure. It was established that 30 Torr is optimal for the formation of MoB2. This work was supported by the Russian Science Foundation grant No. 20-13-00035.