Titanium: some light relief

Paul Boughton

New ways of working with titanium, making it easier to process, could see it making inroads into the automotive industry. Lou Reade reports.

Titanium has many advantages as an industrial material: as well as being 40 per cent lighter than steel, it is available in abundance, highly corrosion- and temperature-resistant and very malleable.

However, it is difficult to process by metal forming techniques such as deep drawing or hydroforming. It is also very difficult to weld. When processed at high temperature, it has a tendency to react with oxygen and nitrogen in the atmosphere. For this reason, shielding gases such as argon are used to prevent oxidisation. This pushes processing costs sky high.

But several European research teams are working around these disadvantages. At the Fraunhofer Institute in Germany, for example, researchers have developed a cost-effective way of processing titanium that could expand its use in the automotive industry.

In this sector, titanium has only been used for high-end vehicles and motor sport applications, because it is very expensive to process. The new technique could lead to titanium being used in parts like manifolds, exhaust pipes, catalytic converters and mufflers - which are usually manufactured from high-alloy stainless steel.

"Titanium tends to adhere to the forming tools," says André Albert, group leader for media-based forming technologies at the Fraunhofer Institute for Machine Tools and Forming Technology (IWU) in Chemnitz, Germany. This, he says, leads to major damage which can cause component failure. The effect is amplified by the fact that titanium must be formed at temperatures of up to 800°C.

In collaboration with colleagues at the Fraunhofer Institute for Surface Engineering and Thin Films (IST) in Braunschweig, Albert has developed a way to hydroform titanium car exhaust systems at high temperatures. It enables forming to be undertaken in a single process stage.

Previously, a minimum of three stages was necessary, using intermediate heat treatments which required processing in different locations. The researchers have now developed a process - and a custom tool - that can withstand temperatures above 800°C.

This is because forming titanium at room temperature leads to severe cold work hardening of the processed pipe. To prevent cracking, the metal requires frequent treatment by means of recrystallisation - a complex, multi-stage forming process that is not economically viable for large-volume production.

"This microstructural change can be avoided at extremely high temperatures," says Albert.

The forming tool, which measures around 1.4x1.2m, is made from materials such as nickel-base alloys which remain stable above 800°C without oxidising. A special coating, just a few microns thick, prevents titanium from adhering to the tool, which can lead to component cracking and severe damage to the surface.

Above 500°C, titanium reacts readily with oxygen and nitrogen in the atmosphere. For this reason, shielding gases such as argon are used at very high temperatures to prevent oxidisation.

Martin Weber, an expert in tribological coatings at Fraunhofer IST, says: "After extensive testing with various materials, we developed the ideal coating for the special conditions encountered within the various temperature ranges."

Oxygen is the enemy

At the same time, a collaborative team in the UK has devised a way to make a welded titanium car chassis. Sports car manufacturer Ariel is planning to incorporate titanium tubing into the chassis of its Atom series of sports cars, with help from Caged Laser Engineering and Reynolds Technology.

Reynolds makes high strength metal tubing (usually in steel, but also in titanium) for applications ranging from cycles to the Bloodhound SSC. Caged Laser specialises in steel roll cages for cars. It says that making the Ariel Atom 3's roll cage from titanium tubing could make it 40 per cent lighter - shaving up to 10 per cent off the weight of the car. (With the wind in the right direction, this could allow Ariel to bring the Atom in at 500kg or less.)

"Titanium is lightweight and strong - a perfect material - but it's hard to machine, hard to cut and weld, and hard to do anything with," says Phil Squance, technical director at Caged Laser.

The three companies are investigating the feasibility of producing a titanium roll cage for lightweight niche vehicles. Because titanium must be welded under oxygen-free conditions, this means using a barometric chamber to contain the parts under argon gas.

The main challenge was building the chamber itself: for a complete chassis, the chamber has to be 3.5x2x2m - which is larger than any chamber currently in existence, says Squance.

"We were laughed out of the building by three chamber manufacturers," he says. "Nobody has ever made a chamber this large for titanium."

The challenge was to evacuate the chamber of all traces of oxygen (and nitrogen). This is not easy when you consider that many materials - rubber, steel, ceramics - actually have oxygen embedded within them.

"We were battling with items in the chamber," says Squance. "They outgas what you want to get rid of."

This meant that items like hosing had to be redesigned. Out went rubber, in came high spec 216 stainless steel. The whole chamber was shrouded in special glass that contained no oxygen.

Even after all this, the tolerance for oxygen was very low. Anything over 100ppm was too much: it would lead to brittleness, and the likelihood of a catastrophic failure of the part. The aim was to weld in an atmosphere of 20-70ppm. To visualise this, Squance likens it to a 3mm layer of water in an Olympic-sized swimming pool.

"That's how much oxygen we're allowed in the chamber," he says. "Oxygen is our big enemy."

A titanium-based Atom car has now been built, and is being tested. A limited edition will be launched in the near future. If successful, it will be added to the options list, says Squance.

He says that the project was instigated because of the ongoing need to reduce the weight of cars in the face of emissions legislation. And, he says, learning how to work with titanium could eventually see it competing with the industry's lightweight material of choice: aluminium.

"So many large companies are into aluminium: it's lightweight, and quite strong, but it corrodes badly unless it's pre-treated correctly," he says.

A simple scratch can destroy this corrosion protection, he says. At the same time aluminium is expensive to process, and not as strong as steel - meaning that parts must be designed thicker.

The titanium project was part-funded by the Technology Strategy Board, through its Niche Vehicle Network programme.

"These projects are about innovation, and thinking outside the box," he says. "They're ideal for small companies, who have the will and the know-how to do things that larger companies don't want to consider."

And Caged Laser is already involved in another titanium-related NVN project. In partnership with Lotus Engineering and S&D Metals, it is investigating whether titanium sheets might be bonded together with adhesives.

"Working on the earlier project showed us just how difficult it is to weld titanium," he says. "So we thought that bonding it might be a good idea - as there's no need for a chamber."

Inevitably, a new set of challenges are already presenting themselves.

"Titanium may be hard to weld, but it's also difficult to bond," says Squance.

Make it quick

Researchers at the University of Rostock in Germany have developed a technique to boost the strength of metallic alloys, which claims to be more energy efficient - and less costly - than existing methods.

It relies on a modified Spark Plasma Sintering (SPS) system, which has an integrated gas quenching mechanism that can alternate the phase compositions and retain the smallest grain features inside a structured metallic alloy. SPS is used to fuse fine powders into a solid material, by applying electric current and mechanical pressure.

By varying SPS cooling rates, it is possible to tune its mechanical properties by controlling phase and grain sizes.

The team showed that rapidly cooling a material directly after SPS fabrication can enhance hardness, strength and ductility.

After sintering, most SPS systems are left to cool naturally, or are flooded with argon gas. The new system pumps nitrogen gas into the chamber at high speed, which rapidly cools the material.

To demonstrate the system, the team produced Grade 5 Titanium (Ti-6Al-4V) - the most commonly used titanium alloy, with applications in the aerospace, biomedical and marine industries - at different cooling rates.

The most-rapidly cooled alloy was up to 12 per cent harder than the naturally-cooled alloy, with more than 30 per cent higher ductility.