Inclusion Detection in Aluminum Alloys Via Laser-Induced Breakdown Spectroscopy
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INTRODUCTION
IN the context of aluminum processing, metal cleanliness refers to the concentration of inclusions, dissolved hydrogen, and residual elements in the solid or molten metal. Inclusions are defined as unwanted solid particles, usually oxides, which act as nucleation sites for hydrogen pores and cracks within an aluminum casting.[1,2] Their quantity depends on a number of factors including initial melt composition, solidification rate, and pouring atmosphere. It has been shown that cleaner metal, free of oxide inclusions, results in greater metal fluidity, improved mechanical properties, improved machinability, better surface finish, and overall reduction in reject castings.[3–5] To properly control melt cleanliness, it must first be measurable. Many laboratory and shop floor techniques exist to assess inclusion content in aluminum and its alloys. Methods range from optical microscopy, filtration (PoDFA, Prefil), Coulter counters (LiMCA), to X-ray radiography.[6–11] However, very few techniques exist that can detect the presence of inclusions in situ.[12]
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 Energy Research Company, 1250 South Avenue, Plainfield NJ, 07062. DIRAN APELIAN is with Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609. Manuscript submitted September 28, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Laser-induced breakdown spectroscopy (LIBS) has been of interest as an industrial tool for probing liquid metal. Similar to conventional spark optical emission spectroscopy (OES), LIBS uses a short laser pulse to form plasma on the metal surface. The elements present in the plasma emit characteristic electromagnetic radiation, which is collected and processed by a spectrograph. The melt composition can be quickly determined from the data gathered. Because LIBS vaporizes a small part of the metal during a laser pulse, it is technically a destructive test. However, the volume sampled is on the order of 108 to 105 cm3, which results in very small losses even If thousands of measurements are taken.[13] Other relevant advantages of LIBS over other atomic emission techniques include: (1) LIBS can be applied to both conducting and non-conducting materials, (2) sample preparation is not necessary, (3) only an optical line of sight is required for measurement, and (4) measurements are performed in seconds.[14,15] As illustrated in Figure 1, Energy Research Company (ERCo, Plainfield, NJ) developed and constructed a LIBS immersion probe for molten aluminum.[16–18] An inert gas streams through a lance to form a bubble at the end. The bubble allows for a constant, fresh view of the metal for the laser. Fiber optic cables transmit the incoming laser pulse and outgoing light from the spark. It was hypothesized that in addition to solute elements, LIBS could also be used to detect discrete inclusions in molten metal. Inclusio
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