TORE SUPRA experience of copper chromium zirconium electron beam welding

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Abstract

The specification of TORE SUPRA to perform quasi steady state plasma operation has induced several R&D studies on the actively cooled structures of the plasma facing components and the associated assembling processes of the materials. Various industrial copper alloys have been characterized and tested to select the most optimized grade for EB welding. Welding samples of representative geometry were also analysed.

Introduction

Copper alloy such as copper chromium zirconium (CuCrZr) has been extensively used as a structural material for the heat sink of TORE SUPRA (TS) components: toroidal pumped limiter (TPL), antenna protection, guard limiters (bumper) and endoscopy systems.

The precipitation hardened copper material has been selected for its good thermomechanical properties at high temperature associated to the possibility of industrial assembling by electron beam (EB) welding. As difficulties appeared to perform these welding joints many investigations have been performed [1]. Conclusions on EB welding study carried out with PLANSEE AG highlight the effects of chemical composition, mechanical properties, heat treatment, geometry of assembly and welding parameters are presented in detail in this paper.

Section snippets

Motivation of investigation

The CuCrZr is intensively used in TS for the heat sink of actively cooled plasma facing components (PFCs) which are designed for steady state power extraction through the pressurized water loop (up to 200 °C, 4 MPa). The tightness of the cooled structure is imperative. The main motivation to lead this investigation was the repeated appearance of cracks in the EB welded joint of the PFCs. These cracks, as shown in Fig. 1, could propagate towards the water feed pipe and lead to water leaks.

The

Supplying examination

As EB welding of copper alloys in the soft state does not reduce the sensitivity to crack formation, the study has been done on age hardened CuCrZr alloy. Various suppliers are presented in Table 1.

The age hardened treatment at 450 °C for more than 3 h after quenching is applied. A preliminary study showed that the annealing treatment at 450 °C up to 25 h does not lead to a drop in hardness [2]. A metallographic sample was picked from one room temperature tensile test specimen of each material

Mechanical properties and chemical composition effect

Tensile tests on material in as delivered condition were carried out at room temperature and at 400 °C. All these tests have been carried out in the air atmosphere with the strain rate 10−3 s−1.

Each value was determined from three separate samples picked from the same batch of material giving a very small data scattering. The different materials do not differ significantly in their mechanical properties in the age-hardened state at room temperature, except with regard to their ductility at 400

Mechanical characterization

A tensile test campaign at 250 °C has been performed by PLANSEE AG. All EB welded samples CuCrZr–CuCrZr received a thermal treatment 450 °C for 5 h before the mechanical tests to restore their initial mechanical properties. The samples failed directly in the welded seam with 3.6% and 4.5% fracture strain, Fig. 4.

The examination of fracture showed a ductile behaviour in the fusion zone (FZ) due probably to the lower alloy content and/or a coarsening of the grains. To determine the distribution

Conclusion

EB welded joints of CuCrZr/CuCrZr were characterized. A weaker behaviour at operating temperature was demonstrated. Detailed investigations showed that the welded joint properties seem to be strongly influenced by chromium content. An increase in chromium weight percent decreases significantly the ductility of this copper alloy. Consequently, to enhance the weldability a low chromium content should be specified less than 0.6 wt%, an high zirconium content could be recommended more than 0.14

References (3)

  • M. Lipa, in: Proceedings of the 20th Symposium On Fusion Technology, Marseille, France, 7–1 September...
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