868 Machining

Previous sections of this chapter have discussed a number of approaches to reduce the cost of titanium components, particularly near-net-shape methods. However, most titanium parts are still produced by conventional techniques involving a significant amount of machining [12]. As a result the machining of titanium and its alloys has been extensively evaluated, and well-defined procedures for various types of machining operations have been defined including turning, end milling, drilling, reaming, tapping, sawing, and grinding [12].

In many instances considerable amounts of machining are required for the production of complex components from mill products such as forgings, plate and bar, that is, a high buy-to-fly ratio (BFR). Titanium is chemically reactive leading to a tendency for welding to the tool, chipping, and premature failure. Other problems involve the low heat conductivity of titanium, which adversely affects tool life, and the ease of damaging the titanium surface. The latter effect is of particular concern because surface integrity strongly influences crack-initiation-related properties such as fatigue.

The machining of unalloyed titanium is similar to \ - \ hard austenitic stainless steel. High-quality sharp tools, carbides for high productivity, and high-speed tool steels for more difficult operations are required for titanium. This, in combination with slow speeds, heavy feeds, and the correct cutting fluids, generally results in good machining behavior for titanium. The cutting fluids recommended are oil-water emulsions and water-soluble waxes at high cutting speeds, low viscosity sulfurized oils, and chlorinated oils at low speeds; in all cases the cutting fluids should be removed after machining, especially before heat treatment, to avoid potential stress-corrosion cracking problems.

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