High

May 21, 2024

The new HPWF process was used to form this aerospace part from Ti6Al4V at 520 degrees F (270 degrees C) and at 20,000 PSI (1,400 bar).

Market data indicates a significant growth of titanium usage in new aircraft. Volumes are expected to grow threefold in a five-year period (see Global Aerospace Growth Propels Titanium Expansion Trajectory sidebar).

A new, faster, and more effective way to form aerospace-grade titanium is needed.

There is good reason for the ascent of titanium usage in aircraft manufacture. Titanium alloys are lightweight, possess extraordinary corrosion resistance, and can withstand extreme temperatures. However, the high cost of raw materials and current forming methods have limited the commercial use of titanium alloys to narrowly specialized applications in aircraft, spacecraft, turbines, medical devices, and other highly stressed components.

Titanium grades 1 to 4, also referred to as commercial pure, are formable at room temperature. However, grade 5, titanium/6 percent aluminum/4 percent vanadium (Ti6Al4V), is the grade now more commonly preferred in aircraft designs. Currently, Ti6Al4V requires fabrication methods such as milling or hot forming processes, which are conducted at temperatures of 1,300 to 1,650 degrees F (700 to 900 degrees C).

The drawback inherent in each of these methods is high cost. The high scrap rate (50 to 70 percent) in milling, combined with the high price of titanium itself, has severely limited its widespread use. Similarly, hot forming processes can be time-consuming and require costly tooling. Hence, the aerospace industry’s adoption of titanium has been slower than first anticipated, preventing manufacturers from fully realizing its benefits.

A newly introduced technology, high-pressure warm forming (HPWF), has been developed to form aerospace-grade sheet titanium at temperatures lower than hot forming, hot stamping, and superplastic forming.

High-pressure fluid cell pressing technology has been used commercially to fabricate aerospace components for decades all over the globe. Advancements in pressure capacity, combined with modernized tool design, have allowed the airframe industry to keep pace with rising demand by using this cold forming process. The increased pressure provided the capability to form parts into their final shape, eliminating both the dependency on hand correction and the need for intermediate heat treatments.

In keeping with continuous improvement, the high-pressure fluid cell process has now advanced even further by applying the high-pressure process at elevated temperatures. This combination of high pressure and heat increases forming speed, decreases cost, and enhances the precision of forming Ti6Al4V.

This new approach introduces an induction heating system to warm the blank and tool set to approximately 520 degrees F (270 degrees C) just before it enters the press. The HPWF temperatures required are markedly lower than the range required for hot forming. Operating at a pressure of 20,000 pounds per square inch (PSI) or 140 megapascal (MPa), the fluid cell press is equipped with measurement, control, and traceability features to meet the parameters critical to the HPWF process.

Third-party analysis of parts produced with the HPWF process indicates that forming parameters are within required tolerances.

Figure 1 A springback analysis of parts formed in Ti6Al4V, t = 2.0 mm, showed a decrease with HPWF. Image courtesy of the Advanced Forming Research Centre, Glasgow, Scotland.

Studies completed by the Advanced Forming Research Centre (AFRC) at the University of Strathclyde in Glasgow, Scotland, in late 2017 and early 2018 confirm that parts that have undergone HPWF have a postforming springback deviation of less than 0.5 millimeters (see Figure 1). It should be noted that the flexibility of the process allows for springback control in the die design, hence compensation for material springback can be incorporated into the process. This makes final-shape parts as a direct result. The consistent degree of springback is related to the part shape, the material thickness, and the process parameters that are followed. The pressure level used seems to have a vital impact.

The HPWF process may usher in some cost-reduction benefits.

Protection gas may be eliminated in the HPWF process because of the relatively low temperature required. For titanium alloys exposed to temperatures above 800 degrees F (425 degrees C), the alloy generally oxidizes and forms a hard, brittle layer enriched with oxygen, called the alpha case. To prevent the creation of the hard and brittle alpha case, hot forming and superplastic forming fabrication requires an oxygen-free process atmosphere to prevent the pickup of oxygen or nitrogen. Because HPWF operates below the alpha case temperature limit, the process has no need for protection gas.

The time required for part cleaning after HPWF can be reduced as well. The need for tool and die maintenance is also decreased due to the absence of cladding on the titanium blanks at the relatively low process temperatures.

Finally, energy consumption is reduced substantially compared to traditional hot forming methods.

Current hot forming processing times are measured in hours, typically. The long duration required to complete hot forming significantly limits fabrication capacity. The HPWF process, in contrast, can produce parts in a matter of minutes.

Further, the fluid cell pressing technology, with its flexible rubber diaphragm, allows for fabrication of several parts in the same forming operation, reducing the forming step per part to seconds. These efficiencies give an HPWF system the theoretical capacity to produce as many as 140,000 parts per year in a two-shift operation.

HPWF has been found effective in the fabrication of double-curved, relatively shallow parts, making it well-suited for a number of typical airframe and jet engine components (see Figure 2).

Before manufacturers can understand how HPWF works, it’s important that they know how high-pressure fluid cell pressing technology works.

Complex sheet metal parts are formed over a single shape-defining tool half, similar to a bottom die (see Figure 3). A flexible rubber diaphragm substitutes the upper die half. High hydraulic pressure is applied to the diaphragm from above.

Figure 2 The HPWF process is well-suited to form shallow shapes such as C-shaped frames with both stretch and shrink flanges; angles with both flanges curved; small curved single-bend angles; twisted or irregular parts; and panels with flanges, circular, or irregular shapes.

The flexible rubber diaphragm forms scratch-free parts with complex shapes, including undercuts, with different sheet thicknesses in all materials. High and ultrahigh uniform forming pressures ensure high-quality parts with close tolerances directly from the press. Low tool costs and short lead times make the technology ideal for low-volume production of sheet metal parts for a range of applications.

When combined with heating, this process is now also viable for Ti6Al4V.

The fluid cell press technology allows several parts to be formed in the same forming operation. A rubber diaphragm acts as a flexible upper die upon which hydraulic fluid pressure is applied.

The HPWF process bears similarity to hot stamping in that it is done in stages and the forming is executed when the metal temperature is elevated to depress springback (see Figure 4).

References

Matthew J. Donachie Jr., Heat Treatment Process, June/July 2001.

Matthew J. Donachie Jr., ed., “Heat Treating,” Chapter 8, Titanium: A Technical Guide, 2nd edition, ASM International, 2000.

R. Gaddam et al., 2013 IOP Conference Series: Materials Science and Engineering, 48 012002.

Olivier Jarrault, Alcoa Inc., American Metal Market, Special Section on Titanium, October 2015.

Sture Olsson heads business development, metal forming for Quintus Technologies, [email protected], 46-705-327-241, www.quintustechnologies.com.

Major airplane manufacturers predict that demand for more than 30,000 new passenger and freighter aircraft will occur within the next 20 years. This projection is driven not only by steadily rising traffic volumes, but also by the need to replace the existing fleet with models that are more fuel-efficient to have a lighter environmental impact.

Meeting the new delivery and lowered fuel consumption performance targets requires the production of more efficient engines and improved aerodynamics. Weight reduction is a key factor in the success of this progress, sparking the quest for lighter-weight materials than used before and new designs. As a result, new composite materials are emerging to challenge the traditional choice of aluminum for airframe design and construction. The shift to composites will have a significant impact on the role of titanium with its light weight, great strength, and corrosion resistance that make it an attractive alternative to aluminum alloys.

The growing preference for titanium in the aerospace sector has been clearly documented. In 2015 the industry accounted for 45 to 60 percent of global titanium consumption. Compared to past rates, the Boeing 787, which was commissioned in 2009, uses 5.3 times more titanium than the Boeing 767, commissioned 27 years prior. The Airbus 350, which enters service this year, uses 4.5 times more than the A330, which first flew in 1992.

The difference represents an increase from approximately 15 tons of titanium in older aircraft to 100 tons of titanium in new aircraft designs.