Applications of Laser-Induced Breakdown Spectroscopy (LIBS) in Molten Metal Processing
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I.
INTRODUCTION
THE
ability to analyze liquid metals has direct applications for real-time process control. In the case of liquid metal processing, it is critical that operating parameters be adjusted accordingly so that the chemistry and quality of the melt be within predetermined limits. Current analytical approaches for determining chemical composition of the melt include spark optical emission spectroscopy, atomic absorption spectroscopy (AAS), X-ray florescence (XRF), and inductively coupled plasma spectroscopy (ICP). These methods are limited because they are offline in nature, based on the analysis of solid metal at ambient temperature, and require laborious manual sampling. Because of the potential in saving time, energy, and materials, as well as improved quality assurance, the use of laser-induced breakdown spectroscopy (LIBS) in liquid metal for real-time analysis has generated significant interest in metal processing. In situ and rapid chemistry analysis, as well as inclusion assessment (or melt cleanliness) is a significant enabling tool that can transform the manner in which cast components are manufactured. This paper reviews the work done using LIBS for such applications and also highlights future opportunities for this technology.
SHAYMUS W. HUDSON is with the Advanced Casting Research Center, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609. Contact e-mail: [email protected] JOSEPH CRAPARO and ROBERT DE SARO are with the Energy Research Company, 1250 South Avenue, Plainfield, NJ 07062. DIRAN APELIAN is with Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609. Manuscript submitted March 1, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS B
II.
FUNDAMENTALS OF LIBS
A. Physics In laser-induced breakdown spectroscopy (LIBS), a pulsed laser is repeatedly fired and focused onto a target metal to create a plasma which has the same chemical makeup as the target. Measuring the plasma’s chemistry then provides a complete description of the target metal’s chemistry. A typical flash lamp pumped solid-state laser used in LIBS will have a pulse energy of about 100 mJ with a pulse duration of about 10 nanoseconds, and a firing frequency of one to several Hz. While the laser pulse energy is modest, its power is significant; 10 MW since its pulse duration is so brief (power is energy divided by time). Additionally, the laser light can be focused to a tight spot (about one millimeter) at the focal point, so the laser light irradiance can be significant at 109 W/cm2. Since plasmas are created for irradiances as low as 106 W/cm2 small, compact lasers are sufficient for LIBS measurements.[1] The practical application of LIBS requires measurement of the spectral peaks of the electron transitions of each element of interest, which can then be used to determine which elements are present and their individual concentrations. However, it is not possible to achieve this by simply measuring all the plasma light emitted since there will be a significant continuum background, line broadening,
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