Titanium aluminides have a high potential to be used as high-temperature structural materials due to their outstanding mechanical properties combined with low density. In particular, the specific strength and elasticity remain attractive up to temperatures of about 950°C and so is a material of great interest for intermediate-temperature—up to 850°C—aerospace and power-generation applications such as low-pressure turbine blade applications, exhaust nozzle components and compressor cases in advanced subsonic and supersonic engines. And until last year it could be found inside a Formula One automobile engine.
As part of the Plansee Mitsubishi Materials Global Sinter Group, a joint venture that was formed in December, 2005, between the Plansee Group and the Mitsubishi Materials Corp., mainly to service the automotive industries, PMG is relatively immune to the whims and ways of the Federation Interna-tionale de l’Automobile (FIA). However, one of its eight business units that does have to sit up and pay attention to the governing body’s regulation changes is the Racing Components business unit. Starting with just 13 employ-ees in March, 2002, Racing Components was formed to capitalize on its knowledge in the production of high-performance combustion engine valves. “We at Sinterstahl, as it was known then, were developing titanium alumi-nide with Plansee, which was supplying the aerospace industry,” says Wolfgang Lausecker, Racing Components business unit manager. “However, as aerospace has a 15 to 20-year timeframe, we looked around to see where we might have some more applications, naturally looking at motorsport and the possibility of getting this material into race engines.”
They struck gold. The material’s inherent high stiffness, low density and high strength—up to high temperatures—were of immediate interest to many of the engine manufacturers, leading to PMG supplying the entire Formula One grid in one shape or another. “Formula One is the most demanding in terms of material data, and they appreciate the knowledge of materials that we can give them,” says Lausecker. “So when we talk to the team people, we not only talk to the valve train guys but also to the metallurgical people which makes it very easy for us because we know what we are talking about in terms of fatigue and strength and grain structures.”
The advantage of titanium aluminide is its weight being around 15% lower than titanium 6242, for example. “Our density is in the area of 4.01 grams per cubic centimeters compared to 4.5-4.6 grams per cubic centimeter for titanium, so that gives you 15% less weight,” says Lausecker. Stiffness is another issue. “Titanium aluminide retains its strength up to very high temperatures, so it is very popular as an exhaust valve due to this property and on the inlet side due to its weight. The general trend in the past was that the material was changed but the design retained. However, engine designers soon found that when they changed the material they could even reduce their design. For example, they could make the neck and shaft a little bit thinner, further reducing mass and weight. This in turn led to a reduction of material in the surrounding parts.”
However, it is the production process that is the key to PMG’s technology. Lausecker explains, “Following our participation in a German-funded program for titanium aluminide valves, we developed a totally different way of producing them. Instead of going from a very thin rod and doing the head forging, we started with plenty of diameter being extruded to shaft thereby giving us a one-piece construction with a gradient transition area between the head and the shaft. You do not see this in a forged valve where you have the grain structure of a hot-rolled shaft and a forged grain structure on the head and a very sharp transition between the two grain structures. This leaves an area that might be weaker in an area where you do not want it.”
The process starts with aerospace-quality 200 x 400-mm billets with the first layer machined and quality inspections carried out. “The first extrusion results in 6 to 8 m rods with a 40- to 60-mm diameter, so the extrusion ratio is about 20:1,” says Lausecker. “We then slice them into individual parts with a little bit of machining being necessary before the second extrusion. After our key process—the second extrusion—is the standard machining process with turning and single point grinding.”
In 2003, though, PMG had to adjust its sights slightly as a new regulation stipulated a specific Young’s modulus of 40 giga-pascal per density. “This made us develop what we call internally the ‘F1 alloy’ which had a lower aluminum content allowing us to make the material worse than it could be.” Then the really bad news arrived two years later: despite the excitement that this material is causing in the aerospace industry, the FIA banned it from 2006 on the grounds of cost. The company quickly had to re-focus its efforts on titanium applications, particularly valves, but found that it was now off many teams’ radar as it had been so successful as the sole commercial supplier of the titanium aluminide components. However, Lausecker maintains that where PMG scores over its rivals is in the extrusion process that it has carried over.
“Supplying just titanium valves means that the challenge is now higher and that is because there are many other suppliers of titanium valves, which was not the position with titanium aluminide. So our challenge is to get it into an engine because we have not been the number-one supplier of titanium valves to Formula One. Where we score, though, is in our extrusion process—the high cycle fatigue of an extruded shaft as we make it in titanium 834, our number-one high-temperature titanium material that gives at least a 30% gain in high-cycle fatigue strength compared to the same material with the hot-roll process. We also still have the gradient transition, the one-piece construction and the fine grain that we are looking for in the shaft area. Titanium 834 is also so highly used in aerospace that it is difficult to acquire it for other applications, but with our supplier we do have access to it.”
However, titanium aluminide is still widely used in Moto GP, not just as valves but also as pins, retainers and lifters. “There is a very big interest in all the materials in NASCAR,” Lausecker says. “My impression is that they are looking for European technology. They are evaluating titanium aluminide because it is a solution for them to lower costs because they can run the valves two or three times while also having a higher performance. They are also interested in titanium and our processes.
“Titanium valves are exchanged after every race, and coming back to Formula One, the challenge was to run engines for two or three weekends, including testing and qualifying as well as the races, which we judged to be fairly easy with titanium aluminide. So this same theory applies to NASCAR where a valve can be used in a Nextel car for two races and even put into a car running in the Busch series afterwards, effectively saving a great deal of money while gaining power. This is the message we are trying to get out.”