Microstructural characteristics of TiC-reinforced composite coating produced by laser syntheses
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Microstructural characteristics of TiC-reinforced composite coating produced by laser syntheses Xiaolei Wua) Laboratory for Nonlinear Mechanics of Continuous Media, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China (Received 9 December 1998; accepted 16 April 1999)
An in situ method has been developed to produce a Ni alloy composite coating reinforced by in situ reacted and gradiently distributed TiC particles by one-step laser cladding. The dispersed, ultrafine TiC particles in the coating are observed. Most of TiC particles, evidently with a gradient distribution, are uniformly distributed within interdendritic regions due to the trapping effect of advanced solid–liquid interface. The TiC/␥–Ni interface is clean and free from deleterious surface reactions. The microhardness of the coating also has a gradient variation, with the highest value being 1250 Hv 0.2.
Ceramic particle-incorporation to provide a metal matrix composite (MMC) coating by laser cladding is a newly developed technique, in which the alloy powders of a desirable composition and a thin surface layer of the substrate materials are simultaneously melted and then rapidly solidified to form a dense coating that is metallurgically bonded to the base material. Some ceramicalloy systems, such as SiC/Co–Cr–W, WC/Ni–Cr–Si–B, and TiC (TiN, TiB)/Ni-based alloys, synthesized on traditional substrate materials, have been studied.1–3 The aim of this communication is to synthesize an in situ TiC dispersoid in a Ni alloy coating. A new method of in situ formation of TiC with a gradient distribution by one-step laser cladding is proposed. TiC particles are introduced by an in situ reaction of titanium and graphite during a laser clad process, instead of TiC particles being directly added into the laser molten pool. A power mixture of Ni alloy, titanium (99.7% purity), and crystalline graphite (99.5% purity) is used as the coating alloy. The volume fraction of titanium–graphite powders is 30%. The titanium–carbon powder ratio corresponds to that of stoichiometric TiC. The average sizes of titanium and graphite powders are 2 and 4 m, respectively. The Ni alloy powder possesses an average size of 28 m, and its chemical composition (wt%) is 16 Cr, 3.5 B, 4.5 Si, 0.8 C, and the balance is nickel. The substrate material is 5 CrMnMo steel in a quenched and tempered condition. Laser cladding is carried out with a 3-kW continuous wave CO2 laser to produce a series of single clad tracks without overlap. The processing parameters are 4–15 mm/s beam scanning speed, 3-mm beam diameter and 2-kW a)
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J. Mater. Res., Vol. 14, No. 7, July 1999 Downloaded: 13 Feb 2015
laser power. The mixed powders are painted on the substrate with a thickness of about 0.8 mm. Surface oxidation is prevented by argon. The morphology, microstructure, and interface structure are observed with a Neuphot21 optical microscope, a Hitachi S570 sc
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