A study into effects of CO 2 laser melting of nitrided Ti-6Al-4V alloy
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A Study into Effects of C02 Laser Melting of Nitrided Ti-6AI-4V Alloy M.A. Mohammed, M.S.J. Hashmi, and B.S. Yilbas Multiple treatment of engineering surfaces can provide improved surface properties that cannot be obtained by a single surface treatment. Consequently, this study investigates the effects of laser melting on the microstructures of plasma nitrided Ti-6AI-4V alloy. The study consists of two parts. In the first part, governing equations pertinent to the laser melting process are developed, and temperature variation across the melted zone is predicted. In the second, an experiment is conducted to nitride the surface of the alloy through plasma nitriding process and to melt the plasma nitrided and the untreated alloy surfaces with a CO 2 laser beam. The resulting metallurgical changes are examined using x-ray diffraction (XRD), energy-dispersive spectrometry (EDS), and scanning electron microscopy (SEM) techniques. It is shown that three distinct nitride layers are formed in the vicinity of the alloy surface prior to the laser melting process, and that after the melting process nitrided species are depleted while cellular and dendritic structures are formed. In addition, the structure consisting of transformed I~containing coarse and fine acicular ot is observed in the melted regions.
Keywords plasma nitriding, laser melting, titanium alloy
Introduction Alloy Ti-6AI-4V is used widely in industry because of its low density and high toughness/mass ratio. Due to low wear properties at the surface, several techniques have been considered to improve tribological properties of the alloy. These include: plasma nitriding, coating, and carbonizing (Ref 1-3). The basic concept in using plasma nitriding to improve the surface properties of a titanium material is the possibility of forming nitride species at and below the surface. Titanium nitrides are hard materials that can increase the wear resistance of the alloy. It is documented that when nitrogen forms a solid solution as a consequence of plasma nitriding in titanium alloys, it results in hardening through a dislocation pinning mechanism (Ref 4). The main advantages of the plasma nitriding include: thick layer of nitride formation in the vicinity of the surface and high reproducibility. On the other hand, because of the brittleness of the resulting compound layer formed at substance surface, a secondary surface treatment becomes fruitful for plasma nitrided materials. Increasing demand is evident in laser processing to improve the wear resistance of the substance (Ref 5). In surface processing with a laser beam, a surface area of the substance is scanned through a moving laser source. Because of the high power absorbed by the substance surface, melting of the surface takes place. Moreover, solidification occurs during the cooling cycle, which in turn results in metallurgical changes in the resolidified region that differs from the original surface. This process may provide improved mechanical and wear properties (Ref 6). Moreover, duplex sur
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