Structure-Process-Property Relations in Excimer Laser Surface Processed Ti-6Al-4V Alloy
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STRUCTURE-PROCESS-PROPERTY RELATIONS IN EXCIMER LASER SURFACE PROCESSED Ti-6AI-4V ALLOY T. R. JERVIS*, T. G. ZOCCO**, AND J. H. STEELE, JR*** *Center for Materials Science **Materials Science and Technology Division ***Nuclear Materials Technology Division Los Alamos National Laboratory, Los Alamos, NM 87545 Abstract Excimer laser processing results in very rapid solidification of metal surfaces. In addition to mixing or segregation processes, rapid heat treatment can result in phase transformations which yield beneficial surface properties. We have investigated the effect of pulsed excimer laser radiation on the microstructure and surface hardness of Ti-6AI-4V. This material undergoes a well defined martensite transformation during rapid quenching from temperatures in the P phase field. The depth of the transformed layer is thus a marker for the temperature profile during processing. We find that the depth of the transformed layer agrees well with a simple 1-D calculation of heat flow following the laser pulse. As measured by the nanoindenter, we find that the surface martensite is softer than the mechanically polished alloy. Multiple pulse processing at high fluences results in an increase in surface hardness, but at a depth much less than that of the martensite, suggesting an independent mechanism. Introduction Titanium alloys, in particular Ti-6AI-4V, are useful engineering alloys in a number of applications. However, their wear and corrosion properties restrict their use in extreme environments such as those encountered in space or surgical implant applications. Surface processing by ion implantation and mixing has been successful in improving the mechanical properties of the alloy. Laser surface processing also has the potential for improving the properties of the alloy through microstructural changes or surface alloying. Previous work has concentrated on laser nitriding I11, production of dispersoids 121, and surface finish 131 using lasers operating in the infra red (IR) portion of the spectrum. Excimer laser processing, using light in the ultra-violct (UV) (248 nm wavelength), offers a rapid and efficient alternative to IR techniques for surface processing. We have demonstrated the formation of an amorphous B-Ti alloy layer by excimer alloyingl41. In order to more fully understand the effect of excimer laser processing we have begun to study the effects of processing parameters on the properties and microstructure of Ti-6AI4V. In this paper we report preliminary results of this study. Thermal Calculations Ultra-violet excimer laser radiation is absorbed within a very short distance in virtually all materials and is of the order of 3 nm in Ti. Absorption, typically in the electron cloud, is rapidly thermalized to a heat pulse which is conducted away from the surface. There is also radiation from the hot surface and convection at the free 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 inelt front
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