By Mike Davey
Richland, Washington -- August 23, 2016 -- The quest for strong, lightweight materials never ends. The collision repair industry is on top of aluminum and high-strength steels of various grades. Carbon fibre is widely anticipated to be next revolution, but recent reports indicate that titanium may start showing up in production vehicles before too long.
It won’t actually be pure titanium, of course, but an alloy. Researchers at the US-based Department of Energy's Pacific Northwest National Laboratory (PNNL) have developed an improved titanium alloy that they say is stronger than any commercial titanium alloy currently on the market.
The material gets its strength from the way the atoms are arranged to form a special nanostructure. For the first time, researchers have been able to see this alignment and then manipulate it to make the strongest titanium alloy ever developed. The new alloy also has a lower cost process than previous titanium alloy processes.
High-strength, low weight and a reduced manufacturing process make this new titanium alloy an excellent candidate for producing vehicle parts, something the researchers themselves noted in a paper published this year by Nature Communications.
Raw titanium is 45 percent of the weight of low carbon steel, but it’s not very strong. Metallurgists tried blending it with iron, combined with vanadium and aluminum. The resulting alloy, Ti185, was first produced about 50 years ago. It was light and also very strong, but only in certain places. The mixture tended to clump, with the iron clustering and creating defects. This made it a difficult proposition to produce the alloy for commercial purposes.
Researchers at PNNL and their colleagues at other institutions found a way around that problem about six years ago. They also developed a lower cost process to produce the material on industrial scales. One key was to use titanium hydride powder instead of molten titanium. The Advance Materials Group, also known as ADMA, co-developed the process with PNNL metallurgist Curt Lavender and now sells the titanium hydride powder and other advanced materials to the aerospace industry and others.
However, researchers kept working on the material in an attempt to make it even stronger. Using powerful electron microscopes and a unique atom probe imaging approach they were able to peer deep inside the alloy's nanostructure to see what was happening. Once they understood the nanostructure, they were able to create the strongest titanium alloy ever made.
The answer relies on heat-treating the material. Heating the alloy in a furnace at different temperatures and then plunging it into cold water essentially rearranges the elements at the atomic level in different ways. The resulting material is even stronger than before.
"We found that if you heat treat it first with a higher temperature before a low temperature heat treatment step, you could create a titanium alloy 10 to 15 percent stronger than any commercial titanium alloy currently on the market and that it has roughly double the strength of steel," said Arun Devaraj, a material scientist at PNNL.
The new titanium alloy still has a relatively high price compared to steel, but it’s much stronger.
"This alloy is still more expensive than steel but with its strength-to-cost ratio, it becomes much more affordable with greater potential for lightweight automotive applications," said Vineet Joshi, a metallurgist at PNNL.
The team collaborated with Ankit Srivastava, an assistant professor at Texas A&M's material science and engineering department, to develop a simple mathematical model for explaining how the hierarchical nanostructure can result in the exceptionally high strength. The model, when compared with the microscopy results and processing, led to the discovery of this strongest titanium alloy ever made.
"This pushes the boundary of what we can do with titanium alloys," said Devaraj. "Now that we understand what's happening and why this alloy has such high strength, researchers believe they may be able to modify other alloys by intentionally creating microstructures that look like the ones in Ti185."
In other words, the process the team invented to make stronger low-cost titanium alloys may also prove to be a way of producing aluminum alloys that are both stronger and cheaper. Raw aluminum is more expensive than the iron used to produce mild steels, but still less expensive than titanium. This means the process invented to produce the new titanium alloys may lead to even cheaper aluminum alloys, which in turns means that titanium may never see its day in the sun as a material of choice for auto manufacturers.
That’s not to say that titanium hasn’t already made its way into automobiles. Repairer Driven News reports that the Icona Vulcano, billed as the first full titanium bodied car, is priced at $2.8 million and is expected to be sold in September. Note that this is a unique vehicle, rather than a mass-produced car.
“Automobile reported that the titanium quarter panels are 0.5 mm thick, which is about the thickness of five pieces of printer paper,” writes John Huetter for Repairer Driven News. “The metal also apparently explodes fairly easily, based on the magazine’s report that the Vulcano ‘got its name partly because the titanium welding process required a special vacuum chamber so it wouldn’t combust, like a volcano.’”
You can see more on the new titanium alloy process in the video below.