Powder metallurgy processing for low-cost titanium
Non-melt solid-state consolidation, explored by scientists at Oak Ridge National Laboratory (ORNL), USA, could pave the way for low-cost titanium components made using powder metallurgy for brake rotors, joint replacements and armour for military vehicles.
Despite the high strength-to-weight ratio, lightweight, and corrosion and ballistic resistant properties of the metal compared to steel, the expensive technique for extracting and processing titanium has limited its use.
Researchers at Oak Ridge, however, have made use of low-cost titanium alloy powders produced using the Armstrong Process, developed by International Titanium Powder, based in Woodridge, USA. The process involves reducing titanium tetrachloride by injecting its vapour into molten sodium.
Non-melt solid-state consolidation of these powders – such as by vacuum hot pressing for net shape components, hot pressing extrusion for rods, and pneumatic isostatic forging (patented by Ametek in Paoli, USA), or roll compaction, for plates and sheets – could overcome the bottleneck.
Bill Peter, a researcher at ORNL’s Materials Science and Technology Division, explains that powder metallurgy has traditionally not been used to fabricate titanium products due to the high cost of conventional powders.
Instead, 95% of titanium used today is produced using the extraction process developed by Wilhelm Kroll 60 years ago – magnesium reduction of titanium tetrachloride into a sponge – and subsequent melting, milling and machining of the sponge into the end product. Double or triple vacuum arc re-melting to ingot, prior to thermomechanical processing, is energy and cost intensive.
‘Production of titanium today takes 16 times more energy per tonne than the production of steel,’ says Peter. ‘Instead of conventional melting, milling and machining, with the new method, the powders remain in solid form during the entire procedure. This saves a tremendous amount of processing energy.’ The team foresees cost reductions of over 50%.
Peter adds, ‘Solid-state consolidation [also] allows for net shape consolidation with minimum scrap, so less titanium is required for making parts’.
Preliminary mechanical tests have revealed that the components can compete with wrought titanium products.
Dr Martin Jackson, a Royal Academy of Engineering/EPSRC Fellow at the University of Sheffield, UK, who is dedicated to providing a step change in the economics of titanium alloys, comments, ‘The work at Oak Ridge is leading the way with respect to titanium from low-cost powder. Many applications, such as in heavy vehicles and tractors do not require aerospace-grade triple-melt quality titanium. Once adequate mechanical properties have been demonstrated, new market sectors will be confident of employing [such] “non-aerospace” titanium’.
However, Jackson says, ‘the [concern] is the potential for remnant salt contamination on top of titanium’s strong affinity for picking up oxygen during high temperature downstream processing. This is why Oak Ridge “can” and “degas” the powder before extruding. Preliminary work by the Australian Commonwealth Scientific and Research Organisation and Imperial College London, UK, has demonstrated canless extrusion with little or no oxygen pick up [providing] a substantial cost saving’.
The team at Oak Ridge is working on commercialisation with powder producers and consolidators, and equipment manufacturers.