Machining of Depleted Uranium Using Coated Cutting Tools

The machining of depleted uranium and its alloys are discussed in this article. Traditionally, these materials have been machined, with limited success, using uncoated cutting tools. New developments in titanium-based coatings such as cation-substituted Ti^sub 1-x-y-z^Al^sub x^Cr^sub y^Y^sub 2^N alloys, with y=0.03 and z=0.02, have been shown to offer enhanced high-temperature oxidation resistance compared with the presently used TiN and Ti^sub 1-x^Al^sub x^N films that are deposited to cutting tool surfaces. Layers (3 µm thickness) were deposited by unbalanced magnetron sputter (UBM) deposition to small-grain WC-Co unused cutting tools that had been ion-etched in situ using a steered Cr-metal-ion cathodic arc (CA) discharge at an Ar pressure of 6 × 10^sup -4^ mbar (0.45 m Torr). The metal ion etching promoted initial local epitaxy on individual substrate grains while the overall film texture evolved through competitive growth to a (111) plane in Ti^sub 0.44^Al^sub 0.53^Cr^sub 0.03^N alloys and (200) plane in Ti^sub 0.43^Al^sub 0.52^Cr^sub 0.03^Y^sub 0.02^N alloys. Although Ti^sub 0.04^Al^sub 0.53^Cr^sub 0.03^N layers exhibited a columnar microstructure that was similar to that previously observed in Ti^sub 1-x^Al^sub x^N alloys, the addition of 2 mol% YN resulted in significant grain refinement, giving rise to an equiaxed structure. The Knoop microhardness of Ti^sub 0.43^Al^sub 0.52^Cr^sub 0.03^Y^sub 0.02^N alloys was HK^sub 0.025^=2700 kg/mm compared with 2400 kg/mm for Ti^sub 0.44^Al^sub 0.53^Cr^sub 0.03^N alloys. The onset of rapid oxidation, as determined from thermogravimetric measurements, ranged from [asymptotically =]600 °C for TiN; to 870 °C for Ti^sub 0.466^Al^sub 0.54^N; to 920 °C for Ti^sub 0.44^Al^sub 0.53^Cr^sub 0.03^N; to 950 °C for Ti^sub 0.43^Al^sub 0.522^Cr^sub 0.03^Y^sub 0.02^N. Machining experiments indicated that cutting tool life is improved significantly using Y-doped Ti-based coatings when machining uranium alloys.[PUBLICATION ABSTRACT]