Titanium advances into new markets

Tom Shelley reports on why more designers should take a serious look at using titanium alloys.

Titanium alloys – as strong as the strongest steels yet half the weight – should no longer be considered as exotics of interest only to aerospace. Prices are tending to come down, and may drop quite sharply in the next few years, while technical developments continue that enable them to perform better and be manufactured into products in novel ways. Part of the reason for downward pressure on price is that Chinese companies have gone into production in a big way, mostly based in Baoji, China's 'Titanium Valley', but we should caution that many of them are small, and quality control is very variable. In the next few years, assuming that the new UK FFC process scales up as its developers believe it will, costs of basic titanium sponge should drop by a further 25% to 50%. The FFC process, unlike the present day Kroll process, in which molten metallic magnesium is used to reduce titanium tetrachloride gas to titanium sponge in stainless steel retorts, makes titanium oxide the cathode of an electrolytic cell with a molten calcium chloride electrolyte and a graphite anode. The oxygen is then taken away from the oxide, leaving titanium sponge behind. The process, which was the brainchild of Professor Derek Fray, Dr Tom Farthing and Dr George Chen, at the University of Cambridge, is now well beyond laboratory scale, and a pilot production cell is to be commissioned at the premises of developer Metalysis, about the same time that this article appears in print. Demand for titanium and its alloys is driven by their extreme corrosion resistance, their strength to weight ratios (which are about double those of the best steels), its suitability for use in the human body, and its springiness. Titanium alloys have half the Young's Modulus of high strength steels, which means that they can bend more before deforming. This makes them the ideal material with which to make springs, according to United Springs in Rochdale. Because they bend more elastically than steel, they can be made with half the number of coils, as well as being half the weight per unit length of metal, making a titanium spring one quarter the weight of its steel equivalent. The only problem is that heat treatment is critical, and they are expensive to manufacture in small volumes because processing is special. They have been used in rally sport for years – United Springs makes titanium coil springs for Citroen rally car suspension units that cost £600 each. However, according to Dr Alan Catchpole of the Titanium Information Group at Namtec: "One of the new production Volkswagens is starting to use titanium springs." He also pointed out that its springiness makes titanium risers a favourite with the offshore oil and gas industry because the titanium pipe can be reeled off a ship without risk of its becoming permanently deformed. In the medical field, many people are being kept alive by nickel titanium stents holding open their arteries, but for skeletal repairs and implants, Dr Catchpole said that there is now a move away from '6-4', the most common titanium alloy with 6% aluminium and 4% vanadium to '6-7', where the vanadium is replaced by 7% niobium, which is thought to be slightly better for human health. At the same time, he said, manufacturers are now looking at manufacturing porous titanium alloy implants, so that bone can grow into them. He also pointed out:?"Headway is being made with making them using powder metallurgy." While traditional powder metallurgy manufacture of titanium parts remains expensive, EOS has brought out a new version of its M 270 selective laser sintering (SLS) machine suitable for making parts out of either pure titanium or 6-4. The Eosint M 270 can either use nitrogen in its build chamber or argon. The latter gas is ideal for laser sintering titanium and a new filtering system has been incorporated that removes titanium condensate during the build process, which was a problem with earlier models. Titanium powder is processed in 30µm thick layers. Users include the UK division of Airbus, 3T RPD in Newbury, and the Department of Engineering and Technology at the University of Wolverhampton. Dr Mark Stanford, responsible for R&D, using what was originally an older M 270, upgraded to the new specification, reports that surface finish is now two to three times better: 4 to 5 Ra instead of 10 to 12 Ra and scanning speed is up to three to four times faster: 350 to around 1200 mm/s. In addition, the components produced using the new machine are homogeneous and fully dense throughout, regardless of the length of the build cycle. Research into titanium's value for armour plate has also produced cost savings. Indeed, it was work on this by defence researcher QinetiQ that got the FFC process under way. ATI Allegheny in the US has also developed several interesting new products in its quest to develop better and more cost-effective armour plate. ATI's 425-MIL was originally conceived of as a lower-cost alternative to 6-4, with iron replacing some of the more costly vanadium as a beta phase stabiliser. Among its advantages is that, unlike 6-4, it can be cold worked relatively easily. It can also be welded and is super plastic formable at 830ºC to 900ºC at a strain rate of 30,000/s. Its corrosion resistance is similar to that of 6-4 and 3-2.5 in marine environments and to that of many media used in the chemical process industry. One type of titanium alloy that has until now only found use in some automotive and aerospace applications is titanium aluminide. This has very good oxidation resistance and good mechanical properties over 600ºC, while being half the weight and much less costly than nickel superalloys. The only problem with such alloys is their low ductility and the low reproducibility of their microstructures. Research undertaken by G. Angella and colleagues at the IENI-CNR Institute in Milan as part of the EU IMPRESS (Intermetallic Materials Processing in Relation to Earth and Space Solidification) project showed that the microstructures can be greatly improved by the presence of 8% niobium. The main aim of the project is to produce a 40cm long titanium aluminide intermetallic, low-pressure gas turbine blade. General Electric has already announced that gamma titanium aluminide low-pressure turbine blades are to be used in its GEnx engine.