August 2024 Volume 6

MATERIALS

FORGING TITANIUM 101 By Pat Burke

T itanium remains a workhorse of the aerospace industry due to its high strength to weight ratio. Roughly 15% of the weight of a Boeing 787 is 6-4 titanium. Titanium forgings can be found throughout the aircraft, including the landing gear and engine. The engine applica tions are primarily in the front of the engine in the compressor section due to the lower operating temperatures. The higher tempera tures in the back of the engine necessitate nickel rather than titanium. Common alloys found for engine appli cations include Ti - 6Al-4V, Ti-6Al-2Sn 4Zr-2Mo and Ti- 8Al -1Mo-1V (commonly referred to as 6-4, 6-2-4-2 or 8-1-1). Applica tions are blades, vanes, discs, and impellers. All three alloys are very forgeable. Titanium can be readily forged for both closed-die and open-die applications. Hydraulic, mechanical and screw presses, along with hammers, can be utilized. Tita nium can be forward and back extruded, upset and ring rolled. Heating of stock for

forging can be done with both electric and gas fired furnaces, along with induction heating. Forging applications for making parts (vs the forging of ingot to billet) are typically conducted below the beta transus to provide alpha-beta work. The transus for both 6-4 and 6-2-4-2 are both typically in the 1810-1830F range, and closed-die forging is conducted in the 1720 – 1775F range. 8-1-1 titanium has higher transus, around 1900F, and forging is conducted at 1825F. Glass coating is used to provide protection from air at high temperatures and improved forgeability. Graphite spray is typically used on the dies as a lubricant. Reheating preforms for additional work is common. Titanium forged below the beta transus is not susceptible to many of the problems encountered with steels (surface condition) or nickel (grain size growth) forg ings that must be reheated multiple times to complete the forging process. Preforms are usually cleaned and conditioned between

operation using a molten salt bath to remove coating followed by an acid dip (HF-nitric) to remove alpha case. Grinding can be used to remove any surface indications prior to recoating for additional forging. Occa sionally forgings can be reforged with the remnant coating, but this is dependent on the condition of the coating following the original forging operation and the need to remove any surface imperfections. Some titanium alloys require very specific forging practices to develop the desired properties. 10-2-3 titanium is an example of this – 80% of the deformation needs to be done above the beta transus and 20% below the transus. This requires fairly exten sive deformation modeling and strict control of processing variables – making forging of alloys like this a significant challenge for all forgers, especially those using hammers. Following forging, titanium alloys need to be heat treated to develop the desired microstructure and properties. The two most common heat treatments are either annealed or solution treated followed by age. An anneal cycle is commonly used for compressor blades and vanes where primary alpha morphology and percentage is not crit ical to the application. Discs, impellers, and structural forgings are commonly provided in the solution treated and aged condition, allowing the forger to more closely control the percentage alpha and resultant mechan ical properties. This is done by selecting a solution temperature relative to the beta transus that provides the desired alpha content on the final microstructure. This also aids in the generation of mechanical properties. Different titanium alloys have different cooling rate requirements from the solution temperature. 6-4 titanium usually requires a fast, aggressive water quench to generate strength. 6-2-4-2 titanium must be fan cooled rapidly. The solution temperature of both alloys is typically 50-75F below the transus. To generate the required mechanical

Figure 1: Typical 6-2-4-2 titanium microstructure following solution treat and age.

FIA MAGAZINE | AUGUST 2024 43