Papers by Author: Nigel A. Stone

Abstract: The processing of titanium powder has been presumed to have potential to reduce the cost of final parts. The non-melt routes to efficient powder processing require consistent flow, adequate tap density and minimal pick-up of interstitial contaminants. The powder produced by the continuous sodium reduction of titanium tetrachloride has a coral-like morphology and low tap density. In order to achieve the potential of low cost parts, the powder will need to be modified to match the optimum feed conditions of the particular processing technique. The approaches to control interstitial pick-up will be discussed in the context of manufacturing technology: contribution of raw materials (10%), passivation (60%) and densification (30%). A comparison of densification with a change of milling fluid media (argon to water will be made with the unexpected decrease in oxygen pick-up when using water). Powder spheroidization via gas atomization of Armstrong Process^® powder will be discussed. The product forms where the advantages of using powder can be exploited have to be carefully selected. Sheet is one such form: the results of using titanium powder to directly make titanium sheet will be presented.

Abstract: Australia is fortunate in that it has ~80,760 kt (Ti equivalent) representing 15% of the world’s titanium resources in the form of ilmenite, rutile, anatase and leucoxene. Although of late the annual quantity of mined material has reduced due to the global financial situation, the average amount of material mined was of the order of 700 kt (Ti eq). To date the most common process used for beneficiating the ilmenite in Australia has been the Becher process to produce synthetic rutile of the order of 92-96% TiO2. Due to the nature of the mineralogy of the deposits, a very small amount of naturally occurring radioactive materials (mainly thorium) is present which for some applications is considered unacceptable. Consequently a number of years ago the Synthetic Rutile Enhancement Process (SREP) was developed which removes the thorium contamination through the use of a boron containing mineral additive [1,2,3], which complexes the radioactive material during the reduction process. Unfortunately the consequences of this reaction is the build up of a flux-like material on the walls of the rotary kilns leading to significantly reduced throughput and even significant refractory loss. The aim of this paper is therefore to describe the development of a novel refractory castable system utilising the in-situ formation of aluminium titanate which in combination with matrix chemistry control and thermo-mechanical property design, greatly inhibited the adhesion of process flux on the refractory walls. The development strategy and subsequent history from simulated laboratory testing and early small scale trials up to the performance of the novel refractory in a fully lined kiln operating over a number of years will be highlighted.

Abstract: A significant research effort within the CSIRO Light Metal Flagship is aimed at developing new processes for the manufacture of (semi-finished) titanium products based on a powder metallurgy approach. The main driver for considering alternative processing and consolidation techniques to conventional ingot metallurgy is improved techno-economics associated with a reduction in processing steps and increased productivity via rapid consolidation of parts. In this respect, CSIRO has developed a process to manufacture sheet products utilising direct powder rolling; the process consists of cold rolling the powder feedstock to a green strip, which is then rapidly heated and hot rolled to consolidate the material completely. The work reported here was an investigation into the feasibility of fabricating Ti-6Al-4V strip by a blended elemental powder metallurgy route. The development of microstructures occurring during the processing and heat treatment steps has been studied. The generic roles of some process, material and heat treatment variables on the tensile properties and homogeneity of the final material have been assessed and are discussed in this paper.

Abstract: Major research and development efforts both in CSIRO and elsewhere aim to develop processes for commercial production of low cost titanium powder. These processes could reduce the current cost of titanium, but the major savings are expected to come by enabling powder metallurgical processes which transform these powders into products. Powder metallurgical (PM) processes (e.g. CIP/HIP) are well established for discrete components, but technologies for continuous production of “mill” product are immature. New processes will be needed for the economic manufacture of mill product (e.g. sheet and tube) from the large quantities of low-cost powder which will become available when the emerging powder production processes enter commercial production. The paper will present a process for the production of commercial purity (CP) sheet by direct rolling of powder. It is novel in that it avoids both the use of binders and densification via batch sintering. A roll compacted green sheet is rapidly heated under a controlled atmosphere before being consolidated to nominally 100% density by hot rolling. Following conventional batch annealing, strip samples exhibit properties approaching those of commercial wrought sheet of an equivalent grade. In order to achieve this, a number of key variables including powder chemistry, morphology and particle packing, the roll compaction and hot rolling parameters needed to be understood and closely controlled.

Abstract: Extrusion is a way to produce near net shape components from CP grade titanium powders of optimum density with minimum porosity and acceptable mechanical properties. Chemically pure, hydride/dehydride titanium powders were cold pre-compacted and extruded at 850oC under an argon atmosphere. The extrusion stress required was ~450MPa. To characterize the extrusions, the porosity distribution, qualitative microstructure and tensile properties were evaluated and compared with conventional extruded wrought titanium. Extrusion occurred after the green billets were upset to ~100% of theoretical density and adequate lubrication was applied to the die. The resultant product was 100% dense with a narrow band of surface porosity and exhibited an equiaxed microstructure of similar magnitude to the starting material. The tensile properties of the bars were observed to be significantly superior to conventionally extruded CP titanium bar products, a result associated with the much finer average grain size. Outcomes from this study have assisted in the identification of a number of key characteristics important to the extrusion of titanium from pre-compacted CP titanium powders, allowing the elimination of canning and hot isostatic pressing (HIPping) of billets prior to extrusion as per conventional PM processes.