Weld integrity of the superconducting cable aluminium jackets of W7-X

Abstract

The Wendelstein 7-X (W7-X) modular stellarator is under construction at the Max-Planck-Institut für Plasmaphysik in Greifswald, Germany. The W7-X magnet system contains 70 coils made up from helium cooled superconducting cables in aluminium alloy (EN AW-6063) jackets. Several hundred connections of the jacket to the cable joints are made by aluminium-to-aluminium welds. Due to geometrical and thermal boundary conditions these welds cannot be accomplished free from defects. Microscopic analyses of the welds show that a variety of small flaws such as cracks and pores develop during welding. The welds have thus to be dimensioned accordingly, and appropriate weld qualification, investigation and testing has to be done in order not to jeopardise the structural integrity and leak tightness. The weld is mechanically loaded during cool-down due to the difference in thermal contractions between the GRP insulation and the aluminium, and during operation by bending moments from electromagnetic forces. An extensive number of different pressure, bending, and pulling tests were performed over the years in order to verify sufficient quality of the welds. This paper is concerned with loading of the weld during cool-down in cable axis direction. It is shown that a series of simple pulling tests on the conductor jacket, welded to the cable joint, indicates that the failure starts in the heat affected zone of the jacket rather than at the weld tips where the maximal influence of welding irregularities and the notch effect is expected. In order to investigate the mechanical load distribution in the weld region, a 3D FE model was created for help in judging the criticality of weld flaws. In the course of these investigations a simple load configuration for a fast approval test of the weld was developed that can be used for the routine qualification of the welding process before real welding in W7-X takes place. Currently, acceptance criteria for the test are under development.