Excimer laser surface processing of Ti-6Al-4V
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I.
INTRODUCTION
THE alloy Ti-6A1-4V is the most widely used Ti alloy. It is used in many applications which require a high strength to weight ratio at relatively high temperatures (up to 300 ~ as well as in surgical prostheses on account of its biocompatibility, m However, Ti alloys are also known for their poor tribomechanical properties, and as a result, numerous surface modification techniques have been applied in order to increase wear resistance of the alloy surface. These have generally relied on forming a hard N-rich surface layer by ion f2-9J or laser t1~ techniques, although some work on C + and B + implantation has also been carried o u t . [7"14-16] All of the laser techniques mentioned above have relied on infrared (IR) laser energy, generally from either CO2 lasers operating at 10.6 /xm or Nd-YAG lasers operating at 1.06 /xm, as these are standard industrial lasers used for a number of surface modification processes, tJT] We have initiated a program to alter the surface properties of Ti-6AI-4V using excimer laser radiation at 0.248 /zm, a wavelength at which coupling between the laser light and the metal surface is quite strong. We have processed the surface in air and found extensive oxygen incorporation into the surface and precipitation of T i t particles. As is well known, interstitial oxygen incorporation results in increased hardness [~81and precipitation hardening is an efficient process. [191 We observe substantial surface hardening of Ti-6A1-4V from dissolved oxygen and T i t precipitates. This article will report on the effect of excimer laser melting and solidification in air on the microstructure, phase composition, and surface hardness of Ti-6AI-4V. T.R. JERVIS, Center for Materials Science, T.G. ZOCCO, Nuclear Materials T e c h n o l o g y D i v i s i o n , and K . M . H U B B A R D and M. NASTASI, Materials Science and Technology Division, are with the Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted February 28, 1992.
METALLURGICAL TRANSACTIONS A
II. E X C I M E R L A S E R MATERIAL INTERACTIONS Although the energy per photon of excimer laser radiation is high ( - 5 eV for 248 nm), the effect of laser radiation on the surface of metals is essentially purely thermal below the threshold for ablation. Thus, at the fluences used in these experiments ( - 1 . J-cm-2), processing can be thought of in terms of a short duration ( - 2 5 ns) energy pulse which is thermalized essentially instantaneously. Further, the short absorption depth of ultraviolet light in metals ( - 3 nm in Ti) means that this energy is deposited at the surface of the material with no appreciable distribution. The interaction can therefore be modeled in terms of one-dimensional heat flow into the bulk of the sample from a surface source, t2~ There is also radiation from the hot surface and some convection at the free liquid surface, but these effects are relatively minor at the time scales of concern here. With sufficient energy deposition, the surface of the material melts and the melt front p
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