I. M. Robertson

Comparison of sintering of titanium and titanium hydride powders

Abstract The cold compaction and vacuum sintering behaviour of a Ti powder and a Ti hydride powder were compared. Master sintering curve models were developed for both powders. Die ejection force, green strength and green porosity were lower for hydride powder than for Ti powder, all probably resulting from reduced cold welding and friction during compaction. For sintering temperatures above ∼1000°C, most of the difference in the sintered density of Ti and hydride is explained by assuming equal densification, while taking into account the lower green porosity of compacts made from hydride powder. However, there is evidence that particle fracture during compaction also contributes to increased sintered density for hydride powder. The Ti powder conformed to a master sintering curve model with apparent activation energy of 160 kJ mol−. The activation energy for Ti hydride also appeared to be about 160 kJ mol−, but the model did not fit the experimental data well.

Review of densification of titanium based powder systems in press and sinter processing

Abstract The development of novel extractive metallurgy techniques for titanium offers the prospect of lower cost Ti powder and therefore wider application of Ti. This review is largely confined to coverage of the low cost press and sinter methods of powder metallurgy, consisting of cold pressing of mixed elemental powders followed by sintering without the application of external pressure. Cold die compaction, sintering behaviour and densification are reviewed in detail. Some information on powders and cold isostatic pressing is included. Microstructure, mechanical properties and applications are considered in less detail. The review deals mostly with the sintering of alloys, but there is some reference to synthesis of intermetallic compounds, such as the shape memory alloy NiTi and titanium aluminides for high temperature applications. Densification is discussed in terms of the four fundamental processing variables: compaction pressure, particle size, sintering temperature and sintering time. Other factors such as alloy composition, the form of alloying addition, type and impurity content of powders and heating rate are also considered.

Swelling during sintering of titanium alloys based on titanium hydride powder

Abstract Compacts were prepared by pressing titanium and titanium hydride powders mixed with nickel powder and sintering under vacuum. Severe swelling was observed only for compacts based on TiH2 powder. Pressure changes in the vacuum furnace, dilatometry results and mass loss data all indicate that dehydrogenation of TiH2 powder compacts occurs at lower temperature than any significant sintering. Swelling appears to have been caused by a contaminant in the TiH2 powder rather than hydrogen. The onset of severe swelling during heating was associated with the formation of liquid phase as the solidus was crossed. However, some swelling appears to take place under solid state sintering conditions. Various results indicate that the mechanism of swelling is high gas pressure within closed pores. Large pores appear to form by breakage of ligaments between small pores followed by opening of the pore. It appears that the use of (uncontaminated) TiH2 powder in place of Ti powder would allow the benefit of lower green porosity to be retained during sintering to achieve low sintered porosity.

Suitability of nickel as alloying element in titanium sintered in solid state

Abstract The suitability of nickel as an alloying element in titanium alloys produced using the blended elemental powder metallurgy approach has been explored. Nickel initially accelerates sintering, providing greater densification at lower temperature than observed for unalloyed titanium. However, it provides only a minor improvement in the density achieved after long sintering times or at high solid state sintering temperatures. Swelling is observed under liquid phase sintering conditions. The highest density was achieved by sintering at just below the solidus temperature. Nickel also accelerates the Ostwald ripening of the pore structure and the conversion of open porosity into closed porosity.

Swelling during liquid phase sintering of Ti–Ni alloys

Abstract The compaction and sintering conditions required for avoidance of swelling and development of very coarse pores during liquid phase sintering of mixed elemental titanium powder metallurgy alloys are described with reference to the titanium–nickel binary system. Swelling is minimised by using low compaction pressure or short sintering time. Swelling appears to require the presence of persistent liquid and closed pores. Gas pressure within closed pores is the probable mechanism of swelling.

Design of titanium alloy for efficient sintering to low porosity

Abstract Few Ti alloys have been designed for ease of sintering. This paper considers the design of alloys for processing using the mixed elemental technique, in which powders are mixed, cold pressed in a die to near net shape and sintered under vacuum at high temperature. The authors describe steps in the process of developing a Ti–Ni–Sn alloy, able to be sintered to near full density at a sintering temperature as low as 1100°C without requiring unusually fine powder or high compaction pressure. Higher sintering temperature allows the Ni content of the alloy to be reduced, but swelling of the alloy probably imposes an upper limit on practical sintering temperature. The increase in green density, and hence sintered density, conferred by Sn in Ti–Sn alloys, and the increase in sintered density due to the high diffusivity of Ni in Ti–Ni alloys, are combined in Ti–Ni–Sn alloys.

Effect of residual pressure on vacuum sintering of Ti-Ni alloys

Abstract In earlier work the authors examined the sintering of Ti–Ni alloys by means of dilatometry of mixed elemental powders. Some notable differences were observed when heat treatments were carried out using a vacuum tube furnace rather than the dilatometer: higher sintered density was achieved due to a combination of lower heating rate and lower residual pressure, and swelling during liquid phase sintering was greatly reduced. This observation is consistent with the idea that gas pressure within closed pores causes swelling during liquid phase sintering and retardation of shrinkage in solid state sintering. In addition to the results of measurements of density and open and closed porosity as a function of Ni content and sintering temperature, macrographs and optical micrographs of the sintered compacts are presented, and the effects of heating rate and compaction pressure are described.

On sintering of Ti–Ni (–TiB2) alloys to near full density

Abstract Using a combination of mixed elemental powders and TiB2, a series of Ti–Ni and Ti–Ni–B alloys were optimised for sintering by varying the nickel and boron contents, the particle size of the elemental powders and the compaction pressure. The sintering temperature was maintained at 1200°C to limit the costs of a potential commercial sintering operation. For Ti–Ni alloys, a density of 99% was attained in Ti–7Ni made using fine Ti and Ni powders sintered in the solid state, and from liquid phase sintering of Ti–8Ni made using coarser powders. Porosity was almost eliminated from Ti–7Ni–xB alloys made by adding 1–3%TiB2 to the coarser Ti and Ni powders. The action of TiB2 as a sintering aid is possibly owing to a combination of the formation of a small amount of liquid at the sintering temperature and the restriction of grain growth owing to the presence of TiB particles.

Sintering of Titanium Powder Compacts for Containerless Hot Isostatic Pressing

The liquid-phase sintering of Ti-Si binary alloys using mixed elemental powders has been explored. Sintering at low liquid contents avoids excessive porosity and bulging of compacts, and leads to development of a densified surface. Compacts made in this way can be HIPped without a container to full or near full densities. Exploiting the surface densification phenomenon may enable cost-effective net shape manufacturing of full density titanium components.

A Microstructural Study of the Effect of Particle Aging on Dynamic Continuous Recrystallization in Al-4Mg-0.3Sc

Tensile specimens of an Al-4Mg-0.3Sc alloy in the peak-aged (8 hours at 553 K) and the over-aged (96 hours at 623 K) conditions, cold-rolled to a 70% reduction, exhibited dynamic continuous recrystallization during superplastic testing at 733 K and a strain rate of 10 -3 s -1 . Although the removal of subboundaries and the development of an equiaxed grain structure occurred more rapidly in the over-aged alloy there was no discernible difference in the microstructures as observed in the transmission electron microscope (TEM). The fundamental mechanisms controlling dynamic continuous recrystallization have been studied by a combination of post-mortem examinations of the developing microstructure frozen at several points during the forming process and dynamic, high-temperature, deformation experiments performed in the TEM. The latter experiments provide direct observation at high spatial resolution of the operating mechanisms and have shown migration, pinning, disintegration and annihilation of subboundaries as well as the incorporation of dislocations into grain boundaries as they occur in real time.