Project B3: Ti powder processing
•	Project Chief Investigators: Graham Schaffer (Project Leader), Kenong Xia, Michael Ferry
•	Senior Researchers/Research Associates: Monash: Colleen Bettles, Rimma Lapovok; UNSW: Sammy Chan; UniMelb: Singhai Bian, Xiaolin Wu, UQ: Ian Robertson
•	Research Students: UQ: Ray Low; UNSW: Gregory Guo
•	Research Assistants Visiting Scholars, Hons Students etc: UQ: Cheryl Berquist
New, potentially low-cost technologies for the reduction of titanium ore generate titanium metal powder directly. Efficient processing of such powder would allow major cost reductions in the production of titanium components and, consequently, wider application of titanium. There are two main components of the project: designing alloy compositions and sintering conditions for efficient and inexpensive fabrication of components using the press-and-sinter approach, and obtaining fully dense titanium parts by hot forging either loose powder or sintered pre-forms.
The titanium-nickel binary system has been selected in order to explore a number of approaches to achieving rapid densification of blended elemental powders. Nickel is a “fast-diffuser” in titanium and alloying with nickel also allows a liquid phase to be formed at high temperatures. Both effects were expected to increase the rate of densification during sintering.
Nickel has proven effective in accelerating diffusion and densification during solid-state sintering. This allows a reduction in sintering temperature or time compared with unalloyed titanium. However, it also accelerates coarsening of the pore structure (Ostwald ripening) and grain growth, so the maximum density is not greatly improved. Future work will explore methods for restricting grain growth so that more complete shrinkage of pores may take place by means of grain boundary diffusion.
Work reported earlier showed that swelling rather than densification often occurs during liquid-phase sintering of titanium alloys. A more complete understanding of the conditions under which swelling can be avoided has been developed so that the advantages of liquid phase sintering can be retained. Compacts with low green density as a result of low compaction pressure do not swell until after about two hours of liquid-phase sintering. However, they suffer from the disadvantage of large dimensional change and the possibility of distortion. For compacts subjected to high compaction pressure, swelling can be avoided by sintering for times of less than about ten minutes. Dilatometry data indicate that densities of between 90% and 95% are achieved.
Better densification was achieved under both solid-state and liquid-phase sintering when compacts were sintered in a tube furnace with a better quality vacuum than attainable in the dilatometer used for the work described above. The susceptibility to swelling was also reduced. The benefit of the improved vacuum was small in the case of commercial purity titanium but increased with increasing nickel content. Densities of over 95% were achieved at 1200°C using liquid-phase sintering and up to about 92% in the solid state.
A detailed review of the press-and-sinter processing of titanium alloys has been completed. Attempts to develop an alloy suitable for liquid-phase sintering using additions of rare earth elements were unsuccessful due to oxidation problems, as were attempts to discover an additive that would serve as both a solid lubricant during pressing and a sintering aid. More recent work on identifying alloys and sintering conditions that give rise to the closure of surface-connected porosity has shown considerable promise. A dense rim is often found at the surface of sintered compacts.
Closure of the porosity would allow containerless hot isostatic pressing (HIP) to full density without the high cost of manufacturing containers. Some sintered compacts have been sent to the University of Birmingham for HIP evaluation. Work on clarifying the effect of powder particle size on sintering rate and densification has also begun. Part of this work consists of attempting to separate the effect of alloy composition from the effect of the particle size of the alloying addition.
In the hot forging section of the project, sintered compacts have been forged between flat dies. Cylindrical preforms of 99.5% titanium were compacted at 400 MPa and sintered for two hours at 1200°C. Forging tests were carried out at different temperatures, strains and strain rates. The density initially increased linearly with forging strain but saturated at about 97% density. The shape and distribution of pores, and the process of deformation and closure of pores, were examined using scanning electron microscopy.
Activity plan for 2008
The possibility of restricting grain growth to achieve higher density in Ti-Ni alloys sintered in the solid state is to be explored by making additions of TiB2 to the alloys. Boron is almost insoluble in titanium and Ti-B intermetallics have high melting points. Some other alloying additions will also be tried, including the slowly-diffusing element molybdenum and silicon, which forms stable intermetallic compounds with titanium.
Optimisation of the liquid-phase sintering of Ti-Ni alloys will be attempted. We intend to seek out the best combination of composition, sintering temperature, sintering time, compaction pressure and residual gas pressure (vacuum). In the process of doing this we also hope to come to a better understanding of the mechanism of swelling.
Research on containerless HIP processing will concentrate on understanding the process of pore closure. This includes the effect of conditions during pressing and during sintering, such as inhomogeneity in the green compact and the formation of transient and persistent liquid phases.
The effects of particle size will be examined by sieving commercial powders into different size fractions and designing experiments to determine interactions with compaction pressure and sintering temperature. Data of this kind allow better assessment of published results of sintering experiments and improved modelling of the sintering of titanium.
Forging dies are to be re-designed in an attempt to induce more shear deformation and hence greater densification. A new forging press is to be designed and constructed.