BRAZING TITANIUM
Several brazing techniques are applicable to titanium. These include induction brazing, resistance brazing and furnace brazing in an argon atmosphere or in vacuum. Torch brazing is not applicable to titanium.

Since brazing techniques have the potential for contaminating titanium surfaces, cleanliness is important and consideration should be given to argon or helium gas shielding.

Alloys for brazing titanium to itself or other metals are titanium-base (70Ti-15Cu-15Ni), silver-base (various), or aluminum-base (various). The titanium-base alloy requires temperatures in the vicinity of 1700 degrees F., whereas the silver and aluminum-base alloys require 1650 degrees F. and 1100- 1250 degrees F. respectively. If corrosion resistance is important, tests should be run on brazed joints in the intended environment prior to use. TIMET's research lab is available for consultation on titanium's corrosion resistance in your environment. The titanium-base alloy reportedly offers superior resistance to atmospheric corrosion and saline environments.

HEAT TREATING TITANIUM
Heat treatment of titanium fabrications is not normally necessary. Annealing may be necessary following severe cold work if restoration of ductility or improved machinability are desired. A stress relief treatment is sometimes employed following severe forming or welding to avoid cracking or distortion due to high residual stresses, or to improve fatigue resistance.

Cleanliness of titanium parts to be heat treated is important because of the sensitivity of titanium to contamination at elevated temperatures. Titanium fabrications should be cleaned carefully prior to heating, using nonchlorinated solvents or a detergent wash, followed by a thorough water rinse.

Handling following cleaning should be minimized to avoid potential surface contamination.

As indicated in Table 27, unalloyed titanium, TIMETAL 50A, 35A, 50A Pd, 35A Pd, etc. and TIMETAL Code-12 grades, are typically stress-relieved at 900-1000 degrees F. for 45 minutes and annealed at 1300 degrees F. for two hours. A slightly higher stress relief temperature (1100 degrees F., 2 hrs.) and annealing temperature (1350 degrees F., 4 hrs.) are appropriate for the TIMETAL 6-4 alloy. Air cooling is generally acceptable.

Although no special furnace equipment or protective atmosphere is required for titanium, a slightly oxidizing atmosphere is recommended to prevent pickup of hydrogen. Direct flame impingement for extended periods, leading to temperatures in excess of 1200 degrees F, should be avoided because of the potential for contamination and embrittlement. Hydrogen or cracked ammonia atmospheres, also, should never be used, because their use would lead to excessive hydrogen pick-up, and embrittlement.

If a scale removal treatment, following a high temperature (1200 degrees F) anneal is not feasible, a vacuum or inert gas (dry argon or helium) atmosphere is recommended.

Surface discoloration, caused by annealing below 1200 degrees F, may be removed by acid pickling in a 35% nitric acid--5% hydrofluoric acid bath at 125 degrees F. A molten caustic bath or mechanical descaling treatment, followed by nitric- hydrofluoric acid pickling, is necessary to remove scale formed by heat treatments above 1200 degrees F.