August 2024 Volume 6

MATERIALS

properties, it is important that sound heat treating practices be followed, with spacing of forgings to allow quick and uniform cooling. Often it is necessary to solution treat in a single layer to insure a fast enough quench. 6-2-4-2 titanium forgings typically include a creep requirement that can only be met if the forgings are cooled rapidly, and the solution temperature is close enough to the beta transus to generate a microstructure with less than 30% alpha content. 6-4 titanium forgings are occasionally required in the beta solution treated an over aged (BSTOA) condition. This requires a very time sensitive solution treatment above the beta transus followed by an age. It is important to recognize that forgings that are required to be supplied in the BSTOA condition cannot be forged above the beta transus – doing so will produce excessive grain growth that will not be recoverable during the deformation and subsequent heat treatment. Following heat treatment, titanium forg ings are cleaned to remove coating and alpha case, followed by grinding/polishing to remove any surface imperfections. Alpha case (oxygen enriched surface due to expo sure to oxygen at high temperatures) needs to be removed to ensure a good surface for

Figure 2: Typical 6-4 titanium microstructure in the annealed condition.

subsequent machining, as alpha case is very brittle. Alpha case depth is commonly to a depth of .005-.010”, so the finish forging die design needs to accommodate the removal of at least .005” per surface following forging. Destructive testing of titanium forgings typically includes room temperature tensile and both micro and macrostructure exami nation. Elevated tensile, stress rupture and/ or creep may be required for some alloys. Hydrogen content is always checked, with common requirements of 150 ppm maximum. Coating and good furnace

controls should keep hydrogen levels well below 75 ppm. Titanium forgings are typically subjected to non-destructive testing. Fluorescent pene trant inspection (FPI) for surface defects, sonic inspection for internal defects and/or blue etch for macrostructure anomalies are the three most common. Sonic inspection is typically utilized for very demanding appli cations like hollow fan blades or discs. A key challenge of closed-die forging of tita nium is the need to maintain a consistent working temperature, especially for smaller parts that are more subject to dramatic swings in temperature due to the forging practice and environment. This need for consistency is not just the typical desire to maintain heat in a part to aid in formability. Titanium forged at too low a temperature can exhibit adiabatic heating. This phenom enon happens when the deformation occur ring at a low temperature results in the localized overheating due to the increased friction forces required to move the metal. If the temperature spike happens rapidly and the heat cannot be dissipated quickly enough the microstructure can be impacted. The temperature spike occurs at a 45-degree angle to deformation, resulting in a “shear band”. The microstructure will exhibit bands of low alpha structure where the material has seen temperature close to, or above, the beta transus. There is no remedy for parts that have been overheated in this manner. Shear banding can often be detected by a blue etch inspection following forging.

Figure 3: Titanium processed at or above the beta transus.

FIA MAGAZINE | AUGUST 2024 44