Edm

Chem Mill

Fatigue Endurance Limit (ksi)

Fig. 4.33. Effects of Machining Conditions onMillAnnealedTi-6Al-4V4

Fatigue Endurance Limit (ksi)

Fig. 4.33. Effects of Machining Conditions onMillAnnealedTi-6Al-4V4

improper grinding procedures. Post-machining processes, such as grit blasting or shot peening, can also be used to induce residual compressive stresses at the surface and improve fatigue life as shown in Fig. 4.34.

Titanium is an extremely reactive metal and fine particles of titanium can ignite and burn; however, the use of flood coolant in most machining operations eliminates this danger to a large extent. Chips should be collected, placed in covered steel containers for recycling, and preferably stored outside of the building. In the event of a titanium fire, water should never be applied directly to the fire; it will immediately turn to steam and possibly cause an explosion. Instead, special extinguishers containing dry salt powders developed specifically for metal fires should be used.

Chemical milling is often used to machine pockets in skins in lower stressed areas to save weight. The use of maskants allows multiple step-cuts and tolerances as tight as 0.001 in. are possible. Chemical milling is conducted by masking the areas in which no milling is desired and then etching in a solution of nitric-hydrofluoric acid. The hydrofluoric acid removes the titanium by etching, while the nitric acid limits the pick-up of hydrogen. After milling, the maskant is stripped.

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