An improved billet on billet extrusion process of continuous aluminium alloy shapes for cryogenic applications in the Compact Muon Solenoid experiment

Abstract

The Compact Muon Solenoid (CMS) is one of the experiments being designed in the framework of the Large Hadron Collider accelerator at CERN. CMS will contain the largest and the most powerful superconducting solenoid magnet ever built in terms of stored energy. It will work at 4.2 K, will have a magnetic length of 12.5 m, with a free bore of 6 m and will be manufactured as a layered and modular structure of NbTi cables embedded in a high purity (99.998%) Al-stabiliser. Each layer consists of a wound continuous length of 2.55 km. In order to withstand the high electromagnetic forces, two external aluminium alloy reinforcing sections are foreseen. These reinforcements, of 24 mm×18 mm cross-section, will be continuously electron beam (EB) welded to the pure Al-stabiliser. The alloy EN AW-6082 has been selected for the reinforcements due to its excellent extrudability, high strength in the precipitation hardened state, high toughness and strength at cryogenic temperatures and ready EB weldability. Each one of the 2.55 km continuous sections of reinforcement is extruded billet on billet and press quenched on-line from the extrusion temperature in an industrial extrusion plant. The region where billet on billet welds develop is usually defective and represents a discontinuity in the extruded product, where improper quenching conditions associated to the stop of the line induce poorer mechanical properties. In the present case, thanks to an optimisation of the process, almost isothermal extrusion could be achieved, independently of the discontinuous process. This, associated to a proper conditioning of the mating surface of the billet, allowed the local degradation of the yield strength to be reduced from the usual value of 30 to less than 10%. The extent of the transition region was also decreased from 8 to 2 m. The achievement of homogeneous strength along continuously extruded long shapes, which is important for the CMS coil winding, might also be crucial for structural applications where homogeneity of properties has to be maintained over long lengths.