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

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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.

Introduction

The magnet system of the Wendelstein 7-X (W7-X) modular stellarator has fivefold symmetry and incorporates five types of non-planar coils and two types of planar coils, attached to the support structure. The winding pack of the coils and the bus system are produced from cable-in-conduit conductors (CICCs) whose jackets are extruded from aluminium EN AW 6063 [1]. The conductors of each coil are mutually interconnected, and the outgoing ends are also connected to the conductors of the busbar system by a specially developed busbar joint. In order to avoid an on-site junction between the stainless steel of this joint and the aluminium of the jacket, a friction welded transition part from an external supplier is employed that consists at one side of stainless steel and at the other side of aluminium alloy EN AW 6061 T6 [2]. The aluminium cable jackets are welded to the aluminium side of the transition part, see Fig. 1.

A TIG weld process is applied using an AlSi5 filler of 2 mm diameter. The welding parameters were optimised at IPP. In addition, the transition part and filler are dried for 24 h under vacuum conditions at 70 °C in order to minimise the flaws in the weld.

The welded ends of the jackets are electrically insulated by glass fibre reinforced epoxy (GRP) wrapping. The GRP insulation of the busbar jacket is additionally interleaved with two Kapton foils. Around the weld region, a prefabricated GRP cap is included in the wrapping with the aim of increasing the strength of the connection, see Fig. 2.

Obviously, the jackets and all joints must be helium tight during all modes of operation. The leakage rate limit for each welded connection has been set to 10−9 mbar l/s.

The weld is a critical zone as the welding process introduces, due to given boundary conditions, unavoidable irregularities such as pores and cracks, and the heat input reduces the mechanical properties of the fused zone and especially the heat affected zone (HAZ) of the heat treatable Al-alloy. The stresses that arise within the weld region during cool-down are assessed analytically and confirmed by numerical calculations showing that the weld and the HAZ are highly loaded, not only due to electromagnetic forces but due to cool-down as well.

Obviously, the weld resistance plays a decisive role in the risk of overloading. Therefore, an extensive experimental program was carried out to determine the lower bound of the weld resistance.

Finally, a fast approval test procedure is being developed using various preliminary test setups and 3D finite element (FE) simulations. From the extensive experimental program it was confirmed that the weakest cross-section with respect to axial pulling is within the HAZ rather than in the weld itself. In order to focus the fast test procedure on the weld itself, where the flaws are expected, another loading configuration has been developed and investigated. This permits routine checks of the welding process before real welding within the W7-X machine.

Section snippets

Criticality of the weld

The weld is a critical part with respect to high vacuum cold helium tightness. Cable jackets belonging to the coil are artificially aged and thus hardened by heat treatment for 14 h at 165 °C, and those belonging to the bus system are naturally aged for several months up to several years. The heat input during welding has a detrimental effect on the strength gained during aging of the aluminium alloy.

Microscopic inspections of the weld show that flaws such as cracks and pores develop during

Experimental assessment of lower bound resistance of the HAZ

In order to determine the lower bound resistance of the HAZ a series of 40 axial tensile tests have been carried out on jackets welded onto dummy transition parts. The test set up has been developed in a number of trial tests. No GRP was applied in these tests. Half of the specimens were subjected to five thermal cycles with cool-down times of 20 min to 77 K. The other half was not submitted to any thermal cycle.

In order to apply the load at the transition part, a bolt was screwed into it. At the

Development of fast approval test procedure

In order to ensure a constant weld quality during assembly of W7-X, a fast approval test procedure is being developed with the help of FEA. At least once for every welding application, trial welds have to be made on a dummy sample and tested destructively to check quasi continuously the welding parameters and the welding capability of the craftsman. Primary goal of the test procedure is to prove in a fast and easy way the weld rather than the HAZ of the jacket. Principle idea is to carry out an

Conclusions

The welds between the aluminium alloy jacket of the superconductor and the aluminium alloy transition part need to remain helium tight throughout all operation conditions where they are highly loaded due to cool-down and electro magnetic forces. During cool-down, the differential thermal contraction between the aluminium and the GRP insulation causes thermal stresses in the weld and particularly in the HAZ of the jacket. The GRP cap reduces the thermal stresses significantly due to its higher

References (4)

  • Measured by FZK, Karlsruhe,...
  • EN 573-1: 2004

    Aluminium and Aluminium Alloys—Chemical Composition and Form of Wrought Products. Part 1. Numerical Designation System

    (2004)
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