E. Gordo

Wear mechanisms in high speed steel reinforced with (NbC) p and (TaC)p MMCs

Abstract Powder metallurgy techniques were used to manufacture metal matrix composites. M3/2 high-speed steel was used as the matrix, and NbC and TaC, in different percentages, as reinforcements. Graphite was added to the M3/2 powders, to compensate carbon losses during sintering, and copper–phosphorous (Cu–P) to promote liquid phase sintering. The conventional powder metallurgy (P/M) route consists of dry mixing, uniaxially compacting (at 700 MPa) and vacuum sintering (at 1190°C and 1230°C) for 30 min. Their characterisation included a broad microstructural study by optical and scanning electron microscopy (SEM). The wear behaviour of all the sintered materials was studied. Wear tests were carried out on polished samples through a pin-on-disc test, using alumina as countermaterial. Wear tracks were analysed by SEM to clarify wear mechanisms. Wear is abrasive in all the materials and both NbC and TaC withstand wear. Only some TaC particles have been detached and spread across the wear track. NbC composites show a wear track clean of carbide particles.

Development and characterisation of high-speed steel matrix composites gradient materials

Abstract Powder metallurgy (P/M) offers the advantage to obtain a composite material with a high content of reinforcement and, consequently, with a big hardness as well as the possibility of changing the composition as a function of the requirements. The materials developed in this work are constituted by a high-speed steel (HSS) matrix core and composite materials with this matrix and gradient concentration of NbC reinforcement, from the core to the surface, through different steps. Composite powders of different content in NbC were produced through high energy milling in order to obtain the gradient composition. These powders were characterised in their morphology and microhardness. The materials were processed through conventional P/M techniques, pressing and sintering. The behaviour of the final materials was studied by means of the microhardness profile and bending strength, showing a high increment in this one by means of the gradient composition.

Mechanical properties and microstructural evolution of vacuum hot-pressed titanium and Ti-6Al-7Nb alloy.

Hot-pressing is a powder metallurgy process where loose powder is loaded into a mould, usually of graphite, and sintered by the simultaneous application of high temperature and pressure. In this study elemental titanium and Ti-6Al-7Nb alloy powders are hot-pressed under different conditions in order to study the influence of the processing parameters on the microstructure and mechanical properties. The samples are characterised in terms of relative density, microstructure, XRD, percentage of interstitials, three-point bending test and hardness. Relative densities as high as 99% are obtained, the oxygen and carbon content remains almost constant but nitrogen percentage increases. This is due to the interaction with the BN coated mould and leads to the formation of a reacted layer in the surface, composed by different titanium compounds, which greatly affect the mechanical properties. Nevertheless, the removal of this reacted layer leads to an important improvement of the ductility, especially for elemental titanium.

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

Abstract 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.

Influence of powder characteristics on sintering behaviour and properties of PM Ti alloys produced from prealloyed powder and master alloy

Abstract The use and development of titanium and titanium alloys have been strongly correlated to high technology industries where costs are not the most important aspect. Titanium could see its market grow by the application of lower cost and more efficient processing methods such as powder metallurgy. This work deals with the characterisation of two types of powders: commercial prealloyed powder and powder produced from master alloy combining mechanical milling and conventional blending to adjust the particle size. The characteristics of the powders, sintering behaviour and final properties of the parts indicate that the master alloy approach leads to better compressibility than the prealloyed powders and, therefore, to lower dimensional change during sintering. The most important result is that it is possible to obtain Ti alloys with properties similar to or better than alloys from prealloyed powders and to obtain homogeneous microstructures, which allows the composition to be adjusted to requirements.

Mechanical behaviour of pressed and sintered CP Ti and Ti–6Al–7Nb alloy obtained from master alloy addition powder

The Ti-6Al-7Nb alloy was obtained using the blending elemental approach with a master alloy and elemental titanium powders. Both the elemental titanium and the Ti-6Al-7Nb powders were characterised using X-ray diffraction, differential thermal analysis and dilatometry. The powders were processed using the conventional powder metallurgy route that includes uniaxial pressing and sintering. The trend of the relative density with the sintering temperature and the microstructural evolution of the materials sintered at different temperatures were analysed using scanning electron microscopy and X-ray diffraction. A minimum sintering temperature of 1200°C has to be used to ensure the homogenisation of the alloying elements and to obtain a pore structure composed of spherical pores. The sintered samples achieve relative density values that are typical for powder metallurgy titanium and no intermetallic phases were detected. Mechanical properties comparable to those specified for wrought Ti-6Al-7Nb medical devices are normally obtained. Therefore, the produced materials are promising candidates for load bearing applications as implant materials.

Mechanical behaviour of pressed and sintered titanium alloys obtained from master alloy addition powders.

The fabrication of the workhorse Ti-6Al-4V alloy and of the Ti-3Al-2.5V alloy was studied considering the master alloy addition variant of the blending elemental approach conventionally used for titanium powder metallurgy. The powders were characterised by means thermal analysis and X-ray diffraction and shaped by means of uniaxial pressing. The microstructural evolution with the sintering temperature (900-1400 °C) was evaluated by SEM and EDS was used to study the composition. XRD patterns as well as the density by Archimedes method were also obtained. The results indicate that master alloy addition is a suitable way to fabricate well developed titanium alloy but also to produce alloy with the desired composition, not available commercially. Density of 4.3 g/cm³ can be obtained where a temperature higher than 1200 °C is needed for the complete diffusion of the alloying elements. Flexural properties comparable to those specified for wrought Ti-6Al-4V medical devices are, generally, obtained.

Low-cost titanium alloys? Iron may hold the answers

Spanish researchers have been looking at means of using alloying elements such as iron and chromium to make low-cost titanium alloys. They are not commonly used in conventional titanium processing, but PM techniques can avoid such problems as segregation and provide a way to develop new beta or alpha-beta alloys. They describe work on blending titanium powder with different iron-based powders to obtain alpha-beta alloys…

Mechanical behaviour of pressed and sintered titanium alloys obtained from prealloyed and blended elemental powders

The applicability of irregular prealloyed Ti-6Al-4V powder for the fabrication of titanium products by pressing and sintering and its employment as a master alloy to obtain the Ti-3Al-2.5V alloy was studied. To this end, the starting powders were characterised by dilatometry, differential thermal analysis and XRD. Green samples were obtained by cold uniaxial pressing, and the evolution of the microstructure over the sintering temperature range 900-1400°C was studied. The variation of the final density and mechanical properties with the sintering temperature was considered. Based on the study carried out, it can be stated that more reliable powders are needed to open the titanium market to new applications. A relative density of 95% and diverse microstructural features and mechanical properties equivalent to those of biomedical devices can be obtained by the pressing and sintering route.

Mechanical and wear behaviour of high-speed steels reinforced with TiCN particles

Metal matrix composites (MMCs), based on M3/2 high-speed steel (HSS) and reinforced with two different percentages of TiCN (2.5% and 5% by wt), were manufactured following a conventional powder metallurgy (P/M) route: mixing, compacting and sintering. The carbide and base material powders were dry mixed and uniaxially compacted at 700 MPa. After this, vacuum sintering was carried out at 1275 °C, determined as optimal sintering temperature in a previous sinterability study. Sintered materials were characterised by measuring hardness, transverse rupture strength (TRS) and wear behaviour. The study is completed with a microstructural analysis by scanning electron microscopy (SEM) along with energy dispersive X-ray analysis (EDXA).