The powder process - using ultrafine titanium in manufacturing
Bill Hopkins, Managing Director of Phoenix Scientific Industries, Hailsham, UK, describes the methods and advantages of using ultrafine titanium in manufacturing.
Production of, and demand for, titanium is at record levels driven by demand from aerospace and moves to reduce transport energy costs and corrosion. In aircraft such as the Airbus-XWB and Boeing Dreamliner 787, titanium fasteners and couplings are preferred over aluminium and steel because of titanium’s superior galvanic corrosion resistance in contact with the carbon-fibre wing and fuselage construction. Titanium sponge manufacture is expected to double in the coming years to meet this demand, and over a dozen new production processes, including the Fray, Armstrong and Materials and Electrochemical Research (MER) Corporation process, are being developed to reduce the cost of primary refinement.
There are a range of rapid solidification processing techniques, but Phoenix Scientific Industries (PSI) in Hailsham, UK, is best known for atomisation technology used to convert metallic melts to powder for use in powder metallurgy (PM) components and high performance engineering coatings. Gas atomisation is where a stream of liquid metal from a furnace, which can range from three to 10,000kg in capacity, is disintegrated by supersonic inert gas jets to form a spray of liquid droplets. These solidify in a large metal chamber to form solid, spherical powder particles. Production rates vary up to 50kg min-1.
Many applications outside the aerospace industry continue to develop, such as in medical prostheses. In new applications, the drive to use powder instead of machined or wrought forms is normal since it provides cost effectiveness by enhanced material use. This is particularly relevant in aerospace components where geometries are deliberately ‘airy’ for lightness and up to 90% of titanium can be lost to machining swarf compared to two to three per cent in PM processing. Other uses include – high strength compositions that can only be made via powder, better mechanical properties such as improved fatigue properties with reduced grain size, and quicker production lead times. However, the poor supply of low-cost, high quality titanium powder is preventing rapid market penetration.
Phoenix Scientific Industries is conducting a three-year programme that started in 2008 and is partly funded by the UK’s Technology Strategy Board. It will refine cold-wall induction melting techniques in conjunction with gas atomisation for powder manufacture. The objective is to reduce the cost of aerospace quality spherical powder to the extent that its application can be greatly extended. Two areas are being addressed to improve production efficiency –
- Production economics – Most titanium atomisation techniques are batch-based where equipment turnaround times eat into production efficiency. Melting methods are in development that are faster than conventional techniques, and, more importantly, are continuous. This produces benefits similar to those in continuous casting of steel compared to ingot production.
- Raw stock – Most spherical titanium powder is made by melting and atomising expensive bar stock or scrap of certified composition (melt stock), the cost of which is passed onto the cost of powder. The programme is developing the means to process sponge and other lowcost titanium precursors directly to spherical powder.
Metalysis Limited in the UK is working to scale-up the Frayprocess for titanium produced by the electrolytic direct reduction of titaniumdioxide. Phoenix Scientific Industries has processed a quantity of thismaterial directly to powder by cold wall melting and atomising. It has found aconsiderable reduction in the impurity levels associated with direct reductionelectrolysis process to levels comparable with those found in commerciallyavailable powder.
While PSI’s titanium development programme is directed atusing more powerful cold-wall furnaces, a laboratory-scale titanium atomiser hasbeen produced as a spin-off from the main research work. Using a miniaturecold-wall induction melting unit, it replaces the standard ceramic furnace inthe mini-atomiser and allows research establishments to produce novel alloycompositions at low cost, as well as standard alloys that are commercially available.
Conventional metal powder production by gas atomisinginvolves using jets of argon, nitrogen or air to disrupt the metal stream, withthe inert gas argon being chosen when the intention is to produce the purest powders free from oxide. Atomising nozzles are designed to produce the maximumdegree of turbulence and disruption in the flowing stream to create the finest metal powder. The atomising conditions can be harnessed to good effect when the intention is to deliberately cause a chemical reaction between the metal streamand the atomising gas. This is because the finely divided metal spray is intimately mixed with the atomising gas, allowing high specific surface areas available for reaction, approaching one square metre per gramme of metal.
The titanium market comprises a number of suppliers producing small batches of powder for exotic applications. Powder prices are high, meaning components using the metal only found acceptance in performance ‘at any cost’ sectors of the markets. Apart from a fragmented supply chain between powder producers and end-users causing premium prices to be demanded, the main reason behind high cost lies in the difficulty of melting titanium and producing a precise stream of liquid metal suitable for atomising into powder.
Titanium cannot be melted in conventional ceramic refractories because of rapid attack and contamination of the melt and is therefore melted using ‘cold-wall’ techniques where the melt is contained in water-cooled copper containers. If heat is added at a sufficiently high rate, a layer of solidified titanium exists in contact with the copper, but a liquid volume can still be contained within this solid ‘skull’. Induction melting of titanium is important because, in addition to the heating effect of the electromagnetic field, magnetic repulsion forces are generated that can be arranged to force the melt away from the copper. This eliminates the skull and reduces heat loss. With careful design, these magnetic forces can be arranged to direct the melt into a tight liquid stream, ideal for atomisation into contamination-free aerospace grade metal powder.
Further information: PSI