A novel 3D-printing process has opened up a new class of strong, ductile, tuneable titanium alloys that could potentially be made from waste products, without expensive additives like vanadium. It may also work for zirconium, niobium and molybdenum.
Titanium alloys are expensive, but highly useful materials frequently used in situations requiring high strength, low weight, and resistance to things like corrosion and high temperatures. They're often found in aerospace, high-end automotive, construction, sports, industrial and health applications.
A research team led by Australia's RMIT University, working with the University of Sydney, Hong Kong Polytechnic University and Hexagon Manufacturing intelligence in Melbourne, says it's developed a fundamentally different way of making new titanium alloys that are just as strong and workable as titanium/vanadium/aluminum alloys, but that use cheap, abundant oxygen and iron instead of the more expensive metals.
This is a huge departure from standard titanium alloy manufacturing. Oxygen, says the team, would be a great stabilizer and strengthener for the alpha phase of titanium, but it also makes it get brittle and crack – hence its nickname as the "kryptonite" of titanium. There are empirical design rules for industrial titanium alloys that limit the oxygen content to between 0.12% and 0.72%, depending on which alloy is being made, and aluminum is typically used for this purpose instead.
Likewise, iron is not just cheap and abundant, it's also the second-lightest candidate for beta-phase titanium stabilization. But it tends to cause the beta-titanium to clump together in large flecks, up to centimeters in size, causing structural defects in the final metal. So it's also tightly controlled, and kept below 2% in most industrial alloy manufacturing.
But the team found that it was able to eliminate these drawbacks by mixing the alloys as part of a 3D-printing process known as laser metal powder directed energy deposition, which allowed them to pay careful attention to the microstructure of the material as it was laid down.
They created and printed a series of alloys using oxygen and iron as stabilizers, and tested them in a number of ways, finding they were able to rival the strength and ductility of commercial titanium alloys. Being 3D-printed, these new alloys are created in the exact shapes required – but the metal's properties can also be tailored to what you're making – hence the nickname "designer" titanium alloys.
“This research delivers a new titanium alloy system capable of a wide and tunable range of mechanical properties, high manufacturability, enormous potential for emissions reduction and insights for materials design in kindred systems,” said co-lead researcher and University of Sydney Pro-Vice-Chancellor Professor Simon Ringer in a press release.
“The critical enabler is the unique distribution of oxygen and iron atoms within and between the alpha-titanium and beta-titanium phases," he explains. "We've engineered a nanoscale gradient of oxygen in the alpha-titanium phase, featuring high-oxygen segments that are strong, and low-oxygen segments that are ductile allowing us to exert control over the local atomic bonding and so mitigate the potential for embrittlement.”
Oxygen embrittlement is not just an issue for titanium – it's also a key factor stopping it being used in zirconium, niobium, molybdenum and other metals. The researchers believe the same process may be possible with these other metals, but further research is needed.
As well as limiting the use of expensive metals, this technique could also cut the cost of titanium alloys by making use of recycled industrial waste products and materials that are currently considered low-grade.
Lead author Dr Tingting Song, RMIT Vice-Chancellor’s Research Fellow, said the team is “at the start of a major journey, from the proof of our new concepts here, towards industrial applications. There are grounds to be excited – 3D printing offers a fundamentally different way of making novel alloys and has distinct advantages over traditional approaches. There’s a potential opportunity for industry to reuse waste sponge titanium-oxygen-iron alloy, ‘out-of-spec’ recycled high-oxygen titanium powders or titanium powders made from high-oxygen scrap titanium using our approach.”
The research is open access in the journal Nature.
Source: RMIT University