The first part of this two-part review focused on the fundamental and diagnostics aspects of laser-induced plasmas, only touching briefly upon concepts such as sensitivity and detection limits and largely omitting any discussion of the vast panorama of the practical applications of the technique. Clearly a true LIBS community has emerged, which promises to quicken the pace of LIBS developments, applications, and implementations. With this second part, a more applied flavor is taken, and its intended goal is summarizing the current state-of-the-art of analytical LIBS, providing a contemporary snapshot of LIBS applications, and highlighting new directions in laser-induced breakdown spectroscopy, such as novel approaches, instrumental developments, and advanced use of chemometric tools. More specifically, we discuss instrumental and analytical approaches (e.g., double- and multi-pulse LIBS to improve the sensitivity), calibration-free approaches, hyphenated approaches in which techniques such as Raman and fluorescence are coupled with LIBS to increase sensitivity and information power, resonantly enhanced LIBS approaches, signal processing and optimization (e.g., signal-to-noise analysis), and finally applications. An attempt is made to provide an updated view of the role played by LIBS in the various fields, with emphasis on applications considered to be unique. We finally try to assess where LIBS is going as an analytical field, where in our opinion it should go, and what should still be done for consolidating the technique as a mature method of chemical analysis.

https://journals.sagepub.com/doi/pdf/10.1366/11-06574

Laser-induced breakdown spectroscopy (LIBS), part II: review of instrumental and methodological approaches to material analysis and applications to different fields.

Laser-based additive manufacturing (LBAM) processes can be utilized to generate functional parts (or prototypes) from the ground-up via layer-wise cladding – providing an opportunity to generate complex-shaped, functionally graded or custom-tailored parts that can be utilized for a variety of engineering applications. Directed Energy Deposition (DED), utilizes a concentrated heat source, which may be a laser or electron beam, with in situ delivery of powder- or wire-shaped material for subsequent melting to accomplish layer-by-layer part fabrication or single-to-multi layer cladding/repair. Direct Laser Deposition (DLD), a form of DED, has been investigated heavily in the last several years as it provides the potential to (i) rapidly prototype metallic parts, (ii) produce complex and customized parts, (iii) clad/repair precious metallic components and (iv) manufacture/repair in remote or logistically weak locations. DLD and Powder Bed Fusion-Laser (PBF-L) are two common LBAM processes for additive metal part fabrication and are currently demonstrating their ability to revolutionize the manufacturing industry; breaking barriers imposed via traditional, ‘subtractive’ metalworking processes. This article provides an overview of the major advancements, challenges and physical attributes related to DLD, and is one of two Parts focused specifically on DLD. Part I (this article) focuses on describing the thermal/fluidic phenomena during the powder-fed DLD process, while Part II focuses on the mechanical properties and microstructure of parts manufactured via DLD. In this current article, a selection of recent research efforts – including methodology, models and experimental results – will be provided in order to educate the reader of the thermal/fluidic processes that occur during DLD, as well as providing important background information relevant to DLD as a whole. The thermal/fluid phenomena inherent to DLD directly influence the solidification heat transfer which thus impacts the part's microstructure and associated thermo-mechanical properties. A thorough understanding of the thermal/fluid aspects inherent to DLD is vital for optimizing the DLD process and ensuring consistent, high-quality parts.

An overview of Direct Laser Deposition for additive manufacturing; Part I: Transport phenomena, modeling and diagnostics

A new procedure, based on the laser-induced plasma spectroscopy (LIPS) technique, is proposed for calibration-free quantitative elemental analysis of materials. The method here presented, based on an algorithm developed and patented by IFAM-CNR, allows the matrix effects to be overcome, yielding precise and accurate quantitative results on elemental composition of materials without use of calibration curves. Some applications of the method are illustrated, for quantitative analysis of the composition of metallic alloys and quantitative determination of the composition of the atmosphere.

New Procedure for Quantitative Elemental Analysis by Laser-Induced Plasma Spectroscopy

A method of fabricating a fully dense, three dimensional object by direct laser sintering is disclosed (flow chart). In a chamber (evacuated) with a partial pressure atmosphere, a degased powder layer is dispensed over a target surface and a beam of directed energy melts metallic powder in order to form a solid layer cross section. Another layer of powder is deposited and melted, along with a portion of the previous layer. The energy beam typically is in the form of a laser, scanning along a path resembling a parametric curve or another, arbitrary piecewise parametric curve (vector scan). In another embodiment, the previous layer is not remelted, thus creating an oxide film that acts as a clean stop to prevent unwanted downward growth.

Direct selective laser sintering of metals

Method and apparatus for embedding features and controlling material composition in a three-dimensional structure (130) are disclosed. The apparatus comprises a directed material deposition apparatus having a source (124) of a focused laser beam, a deposition head (11) being coordinated in a plurality of coordinate axes, a controlled atmosphere chamber (128), a control computer (129), a plurality of feedstock (126, 127), a design rendering a solid model including embedded features, and a positioning stage (16) with a substrate (19) disposed on it. Said deposition head (11) can be a multi-axis deposition head, with a volumetric powder feed unit, a rapid acting metering valve for powder feedstock control, a laser delivery system having an optical fiber with heat protection, a laser beam shutter dump assembly and a pluraltity of powder delivery nozzles comprising coaxial gas stream means for reduced powder dispersion. Material properties can be engineered by adjusting laser exposure, material deposition and material constants.

Forming structures from CAD solid models

A laser-generated spark was used to analyze liquids spectroscopically for elemental constituents. The spark was produced directly in liquids by a focused laser pulse of 15 ns duration and an energy of about 45 mJ/pulse. The size, temperature, and electron density of the spark are reported. Emissions from once-ionized and neutral atoms and simple molecules were observed. Limits of detection for Li, Na, K, Rb, Cs, Be, Mg, Ca, B, and Al in aqueous solutions were established with the use of a 10 Hz repetitive single spark (RSS). Most of these elements were only detectable at levels above 1 μg/mL, although the detection limit for Li was 0.006 μg/mL. The relative standard deviation for replicate sample analysis was 4-8%. The detectability of some species was enhanced with the use of a 10 Hz repetitive spark pair (RSP): a pair of sparks separated in time by about 18 μs. The detection limits for B/H2O with the RSP and the RSS were 80 and 1200 μg/mL, respectively. Species were also detected in organic solvents and in flowing samples. The laser spark method of liquid analysis would be useful in situations requiring noninvasive monitoring of species at high or moderate concentrations.

Spectrochemical Analysis of Liquids Using the Laser Spark

Method and apparatus for forming articles from materials in particulate form in which the materials are melted by a laser beam and deposited at points along a tool path to form an article of the desired shape and dimensions. Preferably the tool path and other parameters of the deposition process are established using computer-aided design and manufacturing techniques. A controller comprised of a digital computer directs movement of a deposition zone along the tool path and provides control signals to adjust apparatus functions, such as the speed at which a deposition head which delivers the laser beam and powder to the deposition zone moves along the tool path.

Laser production of articles from powders

Laser-induced breakdown spectroscopy (LIBS) is a novel method of elemental analysis based on a laser-generated plasma. Pulses from a laser are focused on a sample to atomize a small amount of material resulting in the formation of a microplasma. Because of the high plasma temperature, the resulting atoms are electronically excited to emit light. The plasma light is spectrally resolved and detected to determine the elemental composition of the sample based on the unique emission spectrum of each element. Because of the simplicity of the method, it is suited for analyses that cannot be carried out using conventional methods of atomic emission spectroscopy (AES). This is particularly true for measurements that must be conducted outside of an analytical laboratory. A particular advantage of LIBS is the ability to analyze most types of samples without any preparation. This allows the interrogation of samples in situ, providing rapid measurement capability, permitting the method to be used in the analysis of gases, liquids, and solids in a variety of different sampling configurations. Although LIBS provides sensitive detection for many elements, it is not an ultrasensitive detection technique. In addition, under field conditions, the method typically does not provide the high accuracy and precision offered by laboratory-based methods of AES. Although basically an element detection method, chemometric methods of analysis are being developed to analyze the LIBS spectrum to provide identification of chemical substances (e.g. explosives, pathogens, and chemical agents).

Laser-induced Breakdown Spectroscopy

Laser sustained discharge apparatus for the production of intense beams of high kinetic energy atomic species. A portion of the plasma resulting from a laser sustained continuous optical discharge which generates energetic atomic species from a gaseous source thereof is expanded through a nozzle into a region of low pressure. The expanded plasma contains a significant concentration of the high kinetic energy atomic species which may be used to investigate the interaction of surfaces therewith. In particular, O-atoms having velocities in excess of 3.5 km/s can be generated for the purpose of studying their interaction with materials in order to develop protective materials for spacecraft which are exposed to such energetic O-atoms during operation in low earth orbit.

Laser sustained discharge nozzle apparatus for the production of an intense beam of high kinetic energy atomic species

A method and apparatus for the qualitative and quantitative spectroscopic investigation of elements present in a liquid sample using the laser spark. A series of temporally closely spaced spark pairs is induced in the liquid sample utilizing pulsed electromagnetic radiation from a pair of lasers. The light pulses are not significantly absorbed by the sample so that the sparks occur inside of the liquid. The emitted light from the breakdown events is spectrally and temporally resolved, and the time period between the two laser pulses in each spark pair is adjusted to maximize the signal-to-noise ratio of the emitted signals. In comparison with the single pulse technique, a substantial reduction in the limits of detectability for many elements has been demonstrated. Narrowing of spectral features results in improved discrimination against interfering species.

David A. Cremers

Papers

Spectrochemical Analysis of Liquids Using the Laser Spark

Laser production of articles from powders

Laser-induced Breakdown Spectroscopy

Laser sustained discharge nozzle apparatus for the production of an intense beam of high kinetic energy atomic species

Apparatus and method for the spectrochemical analysis of liquids using the laser spark