W/steel joint fabrication using the pulse plasma sintering (PPS) method

doi:10.1016/j.fusengdes.2011.01.083

Fusion Engineering and Design

Volume 86, Issues 9–11, October 2011, Pages 2573-2576

https://doi.org/10.1016/j.fusengdes.2011.01.083 Get rights and content

Abstract

The paper presents application of pulse plasma sintering method (PPS), developed at the Faculty of the Materials Science and Engineering of Warsaw University of Technology. Unlike other electric-field assisted sintering methods, the PPS method employs pulse high-current electric discharges for heating and activating the material to be sintered. The phenomena, taking place during the high-current pulses, which heat the powder during the PPS treatment and activate the sintering process, are similar to those occurring in SPS technique. However, in PPS, thanks to much higher energy the pulse discharge, these phenomena run much more intensively.

The aim of the present study was to fabricate by the PPS a joint between tungsten and Eurofer 97 steel. Because of the large difference in thermal expansion coefficients of the joined materials, stresses are induced at the joint interfaces. To reduce these stresses a thin interlayer was incorporated between the joined materials. Four different materials were tested.

The experiments allowed to establish the optimal PPS sintering parameters. It was shown that the interlayers between W and Eurofer 97 steel fabricated at 1000 °C for 10 min were highly dense and no delamination at joint interfaces occurred. The results of the thermocycle tests proved a high strength of the joints produced by PPS.

Introduction

The objective of the European Long Term technology R&D programme is to develop and qualify the materials and technologies intended for the in-vessel components (Breeding Blankets and Divertor) of DEMO. In the modular He-cooled divertor concept the tungsten parts will be joined with steel (ODS EUROFER). Because of the great difference in the thermal expansion coefficients between W (4–6 × 10⁻⁶ K⁻¹) and ODS steel (10–14 × 10⁻⁶ K⁻¹) the fabrication of reliable W/steel joints is a serious challenge. This difference could result in large stresses being induced at the interfaces during the manufacturing process and/or operation which may lead to cracking, delamination and reduce the lifetime of the components. A potential solution to the problem may be an interlayer introduced between the two materials, whose thermal expansion coefficient lies between those of the joined materials. So far, braze techniques have been developed as an extension of the bonding technique for the plasma facing components of the ITER [1]. However, the major concern in brazing the joint is the degradation of the brazed material during neutron irradiation [2]. The bonding tests of W and F28H have been also carried out using the SPS technique [3], [4], [5] with a layer of the ferrite phase formed by decarburization [2].

The PPS method has been used for sintering a wide variety of materials, such as WC/diamond [6] and Cu/diamond [7] composites, nanocrystalline sinters [8], [9], [10], [11], combined with SHS reaction, for fabricating high-melting ceramics [12], [13], [14]. The technique used in powder metallurgy has been applied in this study to joining metallic sheets. Recent results [15] concerning the fabrication of W/Eurofer 97 steel joints by the pulse plasma sintering has proved its suitability and an attempt was made to fabricate tungsten/steel joints, utilizing various interlayers built of low activation elements only, for the helium-cooled component of the thermonuclear reactor shown in Fig. 1.

Section snippets

Experimental methods

Eurofer 97 steel and tungsten rods, were supplied by the FZK Co and Plansee Co, respectively. The interlayers were fabricated of an Fe (ABCR – 99%, 200 mesh – ABCR) and a Ti powder (99.5%, 200 mesh – Alfa Aesar GMBH&CoKG). Two powder mixtures were also prepared, 25 wt.% Fe–Ti and 86 wt.% Fe–Ti, in a turbular mixer. The parameters and mixing time were chosen to assure homogeneity of the mixtures.

An attempt was made to find the optimum PPS parameters for the powders and their mixtures. To this end,

Sintering the powder mixtures

The Fe–Ti powder mixtures were subjected to PPS sintering. By conducting the investigations at three different temperatures (900, 950, and 1000 °C) the influence of the sintering temperature on the microstructure was studied and the sintering process was optimized. SEM observations were conducted on sample cross-sections. An example of the results is given in Fig. 2.

The slight density decrease observed in this sample compared with the density of the sample sintered at 950 °C may be explained by

Conclusions

The investigations have shown that the PPS method is suitable for the fabrication of tungsten/steel joints with various interlayers. Three different materials, containing low activation elements only, were tested as the interlayers between steel and the tungsten plates: (a) Fe, (b) 86FeTi and (c) Ti. The microstructure and basic properties of the joints were characterised.

The most promising joints, namely W/Fe/Eurofer 97 and W/86FeTi/Eurofer 97, were subjected to thermal shock tests. The joints

Acknowledgments

This work has been a part of the EURATOM-IPPLM Physics Program funded by European Communities and Polish Ministry for Science and Higher Education under the contracts FU07-CT-2007-00061 and 1170/7PR-EURATOM/2009/7, respectively.

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W/steel joint fabrication using the pulse plasma sintering (PPS) method

Abstract

Introduction

Section snippets

Experimental methods

Sintering the powder mixtures

Conclusions

Acknowledgments

Fusion Eng. Des.

Fusion Eng. Des.

Mater. Sci. Eng. A

J. Mater. Process. Technol.

Scripta Mater.

Int. J. Refract. Met. Hard Mater.

Intermetallics