Hydride-dehydride Powders Research Articles

Microstructure and mechanical properties of hot isostatically pressed Ti–6Al–4V alloy

We report the microstructural analysis and mechanical characterization of Ti–6Al–4V alloy parts fabricated using hot isostatic pressing (HIP). Prealloyed powders were manufactured via gas atomization (G/A), plasma atomization (P/A), and the hydride/dehydride (HDH) processes, and were characterized and pressed at temperatures in the range 880–980°C. The microstructure of the specimens was found to be strongly dependent on the beta-transus temperatures. The G/A and P/A powders were pressed at temperatures above and below the transus temperature to enable comparison of the alpha–beta and beta microstructures. An alpha–beta mixed phase in the specimens that underwent HIP from HDH powders was observed, regardless of the pressing temperature because the temperature was always below the beta-transus temperature. The billets formed from the atomized powders exhibited higher tensile strength and ductility than the billets produced from the HDH powder. The specimens consolidated from the HDH powder showed brittle behavior because of the increased oxygen content and microstructural inhomogeneities.

Flexural properties, thermal conductivity and electrical resistivity of prealloyed and master alloy addition powder metallurgy Ti–6Al–4V

A comparison between the properties achievable by processing the Ti–6Al–4V alloys by means of two powder metallurgy approaches, precisely prealloyed and master alloy addition, was carried out. Prealloyed and master alloy addition hydride–dehydride powders characterised by an irregular morphology were shaped by means of cold uniaxial pressing and high vacuum sintered considering the effect of the variation of the sintering temperature and of the dwell time. Generally, the higher the temperature and the longer the dwell time, the higher the relative density and, consequently, the better the mechanical performances. Nevertheless, a higher processing temperature or a longer time leads also to some interstitials pick-up, especially oxygen, which affects the mechanical behaviour and, in particular, lowers the ductility. Although some residual porosity is left by the pressing and sintering route, mechanical properties, thermal conductivity and electrical resistivity values comparable to those of the wrought alloy are obtained.

Tensile Properties of Ti-6Al-4V Discs Produced by Open Die Powder Compact Forging of Pre-Alloyed HDH Powders

Open die powder compact forging was employed for consolidation of Ti-6Al-4V(wt.%) powders which were produced by a hydride-dehydride (HDH) process. The powders were initially warm-compacted into cylindrical shapes. Induction heating was used for preheating the compacts prior to forging into plates. The effect of the forging parameters such as temperature and pre-sintering time on the microstructure and tensile properties of the forged plates were studied. Porosity and the alpha-beta phase distribution were investigated to enable a better understanding of their effect on the fracture behaviour of the tensile tested specimens. The fracture surfaces of the broken tensile specimens were analysed using Scanning Electron Microscopy (SEM) to find the correlation between microstructure, residual porosity and fracture behaviour. A combination of high ultimate tensile strength of over 1200MPa and a good ductility reflected by an elongation to fracture of around 10% was achieved in some of the forged discs.

Titanium Sheet Fabricated from Powder for Industrial Applications

In collaboration with Ametek and Commonwealth Scientific and Industrial Research Organization (CSIRO), Oak Ridge National Laboratory has evaluated three different methods for converting titanium hydride–dehydride (HDH) powder into a thin-gauge titanium sheet from a roll-compacted preform. Methodologies include sintering, followed by cold rolling and annealing; direct hot rolling of the roll-compacted sheet; and hot rolling of multiple layers of roll-compacted sheet that are encapsulated in a steel can. Fabrication of fully consolidated sheet has been demonstrated using all three methods, and each processing route has the ability to produce a sheet that meets ASTM B265 specifications. However, not every method currently provides a sheet that can be highly formed without tearing. The degree of sintering between powder particles, postprocessing density, and the particle-to-particle boundary layer where compositional variations may exist have a significant effect on the ability to form the sheet into useful components. Uniaxial tensile test results, compositional analysis, bend testing, and biaxial testing of the titanium sheet produced from hydride–dehydride powder will be discussed. Multiple methods of fabrication and the resulting properties can then be assessed to determine the most effective and economical means of making components for industrial applications.

PM processing and characterisation of Ti–7Fe low cost titanium alloys

This work studies a set of low cost beta alloys with the composition Ti–7Fe, processed by conventional powder metallurgy (PM). The materials were prepared by conventional blending of elemental Ti hydride–dehydride powder with three different Fe powder additions: water atomised Fe, Fe carbonyl and master alloy Fe–25Ti. The optimal sintering behaviour and the best mechanical properties were attained with the use of Fe carbonyl powder, which reached a sintered density of up to 93% of the theoretical density, with UTS values of 800 MPa in the ‘as sintered’ condition. Coarse water atomised powder particles promoted reactive sintering, and coarse porosity was found due to the coalescence of Kirkendall porosity and by the pores generated during the exothermic reaction between Ti and Fe. The addition of Fe–25Ti produced brittle materials, as its low purity (91·5%) was found to be unsuitable for formulating Ti alloys.

Rheological and Thermal Debinding Behaviors in Titanium Powder Injection Molding

Powder injection molding (PIM) is suitable for the fabrication of complex shape titanium and its alloys and has a great potential in many applications. This article deals with the injection molding of hydride-dehydride (HDH) titanium powder, spheroidized HDH titanium powder, and gas-atomized (GA) titanium powder. Rheological and thermal debinding behaviors are compared between feedstocks made from the three powders. Torque and capillary rheometers are used to investigate rheological behavior as it relates to the power-law model of viscosity and moldability index. Thermogravimetric analysis (TGA) was employed to analyze debinding behavior given a master decomposition curve consisting of two sigmoids. Spheroidized HDH Ti powder behaves in a manner similar to GA Ti powder. The analysis of the results in this study indicates the possibility of using a combination of HDH and GA titanium powders for PIM.

The powder sintering and isothermal forging of Ti-10V-2Fe-3Al

The synthetic technology of powder sintering and isothermal forging was explored to prepare a powder Ti-10V-2Fe-3Al (Ti-1023) alloy. Hydride-dehydride powder was provided from unqualified Ti-1023 ingot alloy because of “beta fleck” defect. After cold isostatic pressing and sintering, the compact presented uniform chemical composition, but lamella alpha microstructure. Isothermal forging technology was used to further densify the sintered compact and improve the microstructure and properties. The final forged compact exhibited equiaxed, recrystallized, and fined alpha phase microstructure, and excellent tensile properties at room temperature.