Laser Processing of Fe-Based Bulk Amorphous Alloy Coatings on Titanium
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INTRODUCTION
BULK amorphous alloys (BAA) or bulk metallic glasses (BMG) show interesting mechanical and electrochemical properties that result from the absence of grain boundaries in the amorphous structures.[1–7] To utilize these properties in structural and functional parts, more emphasis has been placed in recent years on various amorphous alloy systems in the form of coatings.[8–10] Major challenges in processing of BAA coatings on different metallic substrates range from maintaining the amorphous nature of the coating, achieving good bonding between the substrate and the coating, to improving mechanical properties of the coating without compromising the substrate quality. Various techniques have been investigated in the last three to four decades to solve these processing issues and produce amorphous alloys with better properties. Techniques of sputtering,[10] copper mold casting,[11–14] aluminothermic reactions,[15] the compression shearing technique,[16] very rapid quenching from the melt,[17,18] plasma-based processes,[19] spraying-based pro[20,21] spin casting or melt spinning,[22–24] microcesses, wave processing,[25] and the most modern techniques of using high powered lasers[26–29] are being extensively researched upon. The traditional approaches using laser
HIMANSHU SAHASRABUDHE and STANLEY A. DITTRICK, Ph.D. Students, and AMIT BANDYOPADHYAY, Professor, are with the W.M. Keck Biomedical Materials Research Center, School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164. Contact e-mail: [email protected] Manuscript submitted January 31, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
involve a process of first spreading a dense layer of the amorphous powder and then using a high power laser to remelt and coat the substrate. Avoiding the deleterious effects of oxygen from the environment remains a challenge, often requiring high vacuum for processing. Another drawback of these methods is the size and shape limitation of the material that can be processed. Samples of small size, best suited for laboratory studies, can be readily processed and investigated. The freeform deposition techniques can mitigate some of these processing challenges. In a laser freeform deposition method, a stream of amorphous powder is melted and deposited simultaneously using a continuous high power laser beam or by remelting and solidifying a previously formed coating or ingot. The remelting method can produce amorphous materials depending upon the process parameters used. The precursor need not be an amorphous coating layer or alloy. However, the simultaneous deposition and melting method can create amorphous coatings in just one step. It offers high cooling rates that are the best for preventing crystallization of the amorphous precursor powder to the maximum extent and gives the ability to create large components with small morphological variations.[30] For coatings of various thicknesses and relatively complex shapes of different metallic substrates, the freeform laser deposition technique
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