Berlin 2008 – scientific programme

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MM: Fachverband Metall- und Materialphysik

MM 41: Poster SYM Hydrogen in Materials: New Developments

MM 41.12: Poster

Thursday, February 28, 2008, 18:00–20:00, Poster B

The role of hydrogen in the development of free-machining titanium alloys — Carsten Siemers, Martin Bäker, •Hans-Rainer Sinning, and Joachim Rösler — Institut für Werkstoffe, TU Braunschweig, Langer Kamp 8, 38106 Braunschweig

Recently considerable effort has been made to improve the poor machinability of titanium and titanium alloys like Ti6Al4V. One of different possibilities is the use of hydrogen as a temporary alloying element: specimens are etched to remove surface layers, and heat treated in hydrogen atmosphere at 600-850^∘C to charge the Ti matrix with hydrogen (ca. 10-20 at%). Cutting operations are performed in the hydrogen-loaded state, which reduces the cutting force progressively with increasing cutting speed, up to 50% under high-speed cutting (up to 100 m/s) conditions. Finally, the hydrogen is removed from the samples by a heat treatment in vacuum, to restore the mechanical properties of the original alloy.

Such H-induced reduction of the cutting force can be attributed to two effects: the stabilisation of the high-temperature bcc β phase of titanium, which is softer and more ductile than the hcp α phase, and the so-called HELP (hydrogen-enhanced local plasticity) mechanism. The latter results from the shielding of the elastic interaction between dislocations and obstacles due to hydrogen moving along with the dislocations, which eases the shear deformation in the cutting zone where the temperatures exceed 200^∘C. The effect becomes more pronounced at higher cutting speed, with higher local temperatures and therefore faster hydrogen diffusion in the cutting zone.