Machining of β-Titanium Under Cryogenic Conditions: Process Cooling by CO₂-Snow

Abstract

Titanium alloys of the-phase are currently growing share in the titanium market in comparison to well-established +-titanium alloys such as Ti-6Al-4V due to a deeper hardenability, the highest strength-to-weight-ratio among all titanium alloys and a good corrosion resistance. Several sequences of heat treatment lead to manifold mechanical properties which allow the application of titanium alloys as a construction material for highly stressed lightweight components in aggressive environments, i.e. as a hollow shaft in the aerospace sector. In contrast to the advantageous material properties, high manufacturing costs persist within the area of machining due to slow cutting speeds and feeds, and a high tool wear rate. An increase in productivity is often achieved by the adoption of the cooling and lubricating method applied in the cutting process. Beside emulsion-based high pressure cooling systems, a cryogenic cooling approach by technical gases like liquid nitrogen (LN₂) or carbon dioxide-based snow (CO₂-snow) often offers advantages while machining high strength titanium alloys. The impact of cooling with CO₂-snow is shown for the titanium alloy Ti-10 V-2Fe-3Al at a longitudinal turning operation in comparison to conventionally applied flood emulsion cooling. The analyses focus on the application of uncoated cemented carbide tools of different cutting edge roundness in contrast to a coated cemented carbide tool. In comparison to the flood emulsion approach, cooling with CO₂-snow reduced the tool wear rate and the tool life is increased. The quality of the workpiece was increased by the approach with CO₂-snow due to a reduced burr formation and the suppression of microstructural changes within the peripheral zone of the workpiece.