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Surface modification of titanium subjected to hydrostatic extrusion

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The basic technology for producing large volumes of bulk nanocrystalline metals is based on the Severe Plastic Deformation (SPD) methods, which enable refining the structure from the micrometric to the nanometric level. The microstructural changes are accompanied by a significant improvement of the mechanical strength and hardness [1÷7]. One of the SPD methods is hydrostatic extrusion, employed in the present experiments, which has been proven to efficiently refine the grains in aluminum, copper, nickel and steel [8÷12]. Our earlier studies have shown that hydroextrusion can also be used for producing nanostructured titanium [13÷15] with an equivalent diameter of grains about 60 nm and substantially increased strength and hardness. The chief difficulty in producing nanocrystalline titanium by HE is that the reduction of the rod cross-section must be realized gradually in many passes. Other disadvantageous effects that occur during HE of Ti are: the high extrusion pressure, the recurrent elastic deformation, the stick slip effect (stepwise displacement of the material) and a considerable wear of the die resulting from the tribological conditions. The aim of the study was to reduce or eliminate these adverse effects, and thereby to optimize the hydroextrusion process of titanium. To this end the surface of titanium was subjected to various modifications. The beneficial effect of Ti-Al coatings was shown in our earlier publications [15, 16]. Even though the extrusion pressure was reduced, the Ti-Al layers underwent wear in a short time. In the present study, the Ti billets were covered with Al coatings of various thicknesses. The effect of these coatings on hydroextrusion process, the surface topography of the Ti products and their microstructure after the extrusion were investigated. The maximum strain was also determined that could be applied in a single and two extrusion passes. MATERIALs AND PROCESSING The material examine[...]

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