Surface Treatment of Titanium Using Lasers
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SURFACE TREATMENT OF TITANIUM USING LASERS B.L. MORDIKE AND H.W. BERGMANN Institut fUr Werkstoffkunde und Werkstofftechnik, Technische Universitit, Agricolastr. 2, 3392 Clausthal-Zellerfeld, FRG ABSTRACT Thermochemical treatments of titanium surfaces by laser are presented and the beneficial effects attainable eg wear resistance, corrosion resistance etc., discussed. It is shown that it is possible to produce titanium surfaces with load bearing capability. INTRODUCTION Titanium is renowned for its high strength/weight ratio and has been used, therefore, for high duty aircraft applications. It exhibits good corrosion resistance and is non-magnetic and thus suitable for marine and submarine applications. There is an ever increasing amount of recycled, secondary titanium available which cannot be employed in the aircraft industry because of the stringent and conservative specifications but which nevertheless offers considerable advantages in other fields over current materials - ferrous and nickel base for example. The present work shows how the disadvantages of titanium, namely high friction and poor wear resistance can be overcome by laser thermal chemical treatment. PRINCIPLES Titanium can be hardened by solid solution hardening, or precipitation/dispersion hardening. It is not possible to harden titanium by martensitic hardening and interstitial hardening usually causes embrittlement. Solid solutions can be produced in the surface layer by melting-in a pre-coated layer (e.g. D-gun or electroplated Ni, Fe, Cr, etc.). Precipitation hardened surfaces can be produced by either injecting particles into the melt [1] or by alloying the laser heated surface to react with the atmosphere to produce precipitates. The reaction with the atmosphere can be described by the law of mass action [2]. It is thus possible by fixing the activity (fugacity) of the reactant gas to determine the degree of reaction and direction of the reaction. For instance,
2Ti + N2 + 2TiN
If the surface is melted the volume of material precipitating in the reaction is much larger, as convection ensures a homogenization of the molten layer. Convection has the advantage that the enrichment takes place over the whole melted layer, typically a few millimeters compared with microns in CVD or other techniques [3,4,5,6].
Mat. Res. Soc. Symp. Proc. Vol. 58
1986 Materials Research Society
336
EXPERIMENTAL DETAILS Various titanium alloys were produced in the surface by laser surface treatment. In thermochemical treatment an appropriate gas atmosphere is produced which permits the heated surface to react in a controlled manner. The essential parameter is the partial pressure of the reactant gas. Smooth crack free surfaces could be produced by ensuring lamellar flow of the reactant gas across the heated surface and the inclination and azimuthal angles. The following properties were determined: mechanical properties (tensile, hardness and fatigue tests), wear, corrosion and component behaviour. The surfaces were investigated for crystallographic structure, mi
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