Hot Forming of Titanium Alloys

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Titanium made its first appearance in the aerospace industry in the 1950's on military aircraft like the X-3 Stiletto, the F-4 Phantom and of course the SR-71 Blackbird. Its utilization has only increased over the years, and it's easy to see why. Titanium has an outstanding strength-to-weight ratio as well as thermal and corrosion resistance properties that are very well suited for airframe applications.

Unlike hot stamping, hot forming is an isothermal process, meaning that the tooling and the blank maintain a uniform temperature throughout the entire forming process. Springback can reduce the bend angle by 15 to 25 degrees. This springback effect is virtually eliminated at hot forming temperatures. Much tighter tolerances can be held with hot forming than with hot stamping. This is why hot forming is common in the aerospace industry, where engineering tolerances are very tight and production volumes are relatively low. Hot stamping is more common in industries with wider tolerances and higher production volumes.  

For geometries that are too complex for hot forming, superplastic forming may be necessary. Superplastic forming combines higher forming temperatures and pressurized argon gas to form complex shapes. 

• Sheet of metal is clamped between a die cavity and a plate
• Sheet and tooling are increased until they reach the superplastic forming temperature of the material (~1700° F for Ti-6Al-4V)
• Gas pressure is applied to deform the sheet by forcing it against the walls of the die cavity under suitable stress and deformation rate

The principal advantage of superplastic forming is that it enables the forming of complex, monolithic structures that can replace multi-piece assemblies and eliminate welding and fastening operations. The disadvantages include higher equipment and tooling costs, higher energy costs, localized thinning in the formed structure and the requirement for chemical milling to remove the alpha case layer.  

• PRO: SPF allows for more complex geometries and tighter radii compared to HF
• CON: Heterogenous thickness distribution throughout the part which typically leads to a thicker gauge blank
• CON: Long process times: typically ~3 hours to 5 hours
• CON: Higher equipment and tooling costs
• CON: Thicker alpha-case layer (oxygen-rich, brittle layer that typically must be removed with chemical processing)

OMADA has successfully converted a number of titanium components from SPF to HF resulting in substantial cost savings for the customer. 

OMADA has been successful in partnering with many aircraft OEMs over the years in hot form value engineering projects. In these projects, OMADA worked with OEM supply chain and engineering teams to identify part candidates to convert from machined plate or billet to hot formed sheet. The objective with these projects is to improve the buy-to-fly ratio and subsequently reduce component cost. The buy-to-fly ratio refers to the weight of raw material purchased relative to the weight that ends up on the aircraft. This ratio is critical with titanium components in particular because titanium has a much higher cost than aluminum and is also much more expensive to machine. 

In addition to alloyed titanium sheet metal, OMADA also has the capability to form other hard metals. For example, OMADA is very experienced in forming of Inconel 625 and Inconel 718. OMADA can also form unalloyed commercially pure (CP) titanium (Grades 1 - 4). With the recent push into hypersonic technologies, OMADA has fabricated sheet metal from more exotic alloys, including TZM (titanium-zirconium-molybdenum). 

• https://www.sciencedirect.com/topics/materials-science/superplastic-forming
• https://www.researchgate.net/publication/302479288_Evaluation_of_the_Bulk_and_Alpha-Case_Layer_Properties_in_Ti-6Al-4V_at_Micro-And_Nano-Metric_Length_Scale
• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6600757/