Development of electric resistance sintering process for the fabrication of hard metals: Processing, microstructure and mechanical properties

This work presents the development of the Electrical Resistance Sintering (ERS) process for the fabrication of hard metals. The compositions of the materials produced were WC with 6 and 10 wt% of Co. In addition to the specific characteristics of the technology, the characterization of the produced parts is presented and compared to materials obtained by conventional processes. The parts produced by ERS present densities comparable to the ones obtained by conventional methods. The microstructural comparison shows a considerable grain size reduction in the ERS materials which consequently brings a hardness increase. ERS materials show similar fracture toughness to conventional ones. The very fast sintering allows performing the process without any protective atmosphere, therefore making this process very attractive for the production of materials that need to be sintered under non-oxidising environments. The total duration of the cycle, including heating, holding time and cooling is few seconds. Finally, some considerations about the scale up and possible industrialization of the technology are explained.This work is financially supported by the Seventh Framework Program of the Commission of the European Communities under project EFFIPRO contract no. NMP2-SL-2013-608729

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.The authors want to acknowledge the financial support from the Spanish Ministry of Science through the R&D Projects MAT2009-14448-C02-02 and MAT2009-14547-C02-02, and from the Regional Government of Madrid through the ESTRUMAT (S2009/MAT-1585) projectPublicad

Magnetic and structural properties of spark plasma sintered nanocrystalline NdFeB-powders

Near-stoichiometric NdFeB melt-spun ribbons have been subjected to spark plasma sintering varying the process temperature TSPS and pressure pSPS between 600 and 800 °C and 50–300 MPa, respectively. Produced bulk magnets were analyzed regarding microstructure and magnetic properties. For all samples the intrinsic coercivity Hc,J gradually decreases with increasing sintering temperature and pressure, while residual induction Br increases simultaneously with sample density. Densities close to the theoretical limit were achieved for pSPS≥90 MPa and TSPS≥650 °C. With increasing TSPS precipitations of Nd-rich and Fe-rich phases have been observed as a result of a decomposition of the hard magnetic Nd2Fe14B phase. Under optimum sintering conditions of pSPS=300 MPa and TSPS=650 °C high-density bulk magnets with Hc,J=652 kA/m, Br=0.86 T and (BH)max=106 kJ/m3 have been produced

The structural changes of Y2O3 in ferritic ODS alloys during milling

Oxide dispersion strengthened (ODS) ferritic steels are usually fabricated via mechanical alloying and subsequent consolidation via hot extrusion or hot isostatic pressing. During the individual process steps, a complex evolution of the nanoparticle structure is taking place. Powders with different Y 2O3 contents were milled and examined by means of X-ray diffraction (XRD) and atom probe tomography (APT). It has been observed that the Y2O3 is fragmented and becomes partially amorphous upon milling due to the grain refinement of Y2O3 during the milling process. There was no compelling evidence for Y2O3 dissociation and dissolution into the steel matrix. © 2014 Elsevier B.V. All rights reserved

Application of spark plasma sintering for manufacturing of thermoelectric materials

V-Vl thermoelectric compounds like Bi2Te3-based alloys are well known for room temperature applications like Peltier coolers or thermogenerators. Their anisotropic physical properties and mechanical weakness are a problem for the manufacturing. To overcome the mechanical problem Spark Plasma Sintering (SPS) was used. 2 inch Wafers of polycrystalline bismuth telluride based n-type and p-type thermoelectric materials were successfully fabricated through SPS-technique, starting from commercially available zone melted ingots. This paper will report on the milling technique and the influence of the SPS process on the microstructure, the thermoelectric and mechanical properties of the polycrystalline materials. A preferentially oriented microstructure was formed keeping the high quality of the the starting material. Through the polycrystalline structure, the bending strength of the sintered material was increased about three times compared with coarse crystalline ingots. Entnommen aus TEMA</a

Microstructural and mechanical characterisation of ODS ferritic alloys produced by mechanical alloying and spark plasma sintering

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Microstructural and mechanical characterisation of ODS ferritic alloys produced by mechanical alloying and spark plasma sintering

Powders with nominal composition Fe-14Cr-2W-0·4Ti were mechanically alloyed (MA) with Y2O3 in a planetary ball mill at two different rotational speeds. Consolidation of the as milled powders was performed by spark plasma sintering (SPS). As milled powders showed a highly deformed microstructure with elongated nanometre grains and, depending upon the rotational speed, different stages of the nanocluster evolution were observed to be produced. In the case of consolidated materials, grain growth occurred during the SPS process and it was possible to observe the influence of the MA parameters, with larger and more homogeneously distributed grains at the higher rotational speed. Additionally, Ti was observed to be incorporated to the nanoclusters after SPS, indicating a further step in their evolution during consolidation. The mechanical behaviour of the SPS compacts was evaluated by tensile and small punch testing also showing the influence of the MA parameters in the material behaviour