1 Department of Management Engineering, Technical University of Denmark2 unknown3 Department of Mechanical Engineering, Technical University of Denmark
Superconducting tapes based on the ceramic high temperature superconductor (HTS) is a new promising product for high current applications such as electro-magnets and current transmission cables. The tapes are made by the oxide powder in tube (OPIT) method implying drawing and rolling of silver tubes containing ceramic powder. The final product is a composite tape, where ceramic superconducting fibres are embedded in a silver matrix. The critical current density Je [kA/cm 2 ] is the primary quality parameter of the product. The quality of the superconducting tape depends very much on the mechanical and thermal processes applied. One of the most crucial processes is probably the flat rolling process, where the round or square wire is rolled to form a thin tape (about 3 mm x 0.2 mm), while the density of the powder fibres increase and the fibres obtain their final geometry. For instance rolling a tape to a thickness of 250 µm may give a very high Je, whereas further reduction to 200 µm may be fatal. In the present work the flat rolling process is analysed systematically from a mechanical forming point of view. This work implies · Mechanical characterisation of the plastic parameters of the superconducting ceramic powder Bi2Sr2CaCu2O by a three-point test procedure implying die compaction, diametrical compression and uniaxial compression. · Application of plastic parameters to the constitutive model Drucker/Prager-cap. This model features mean pressure dependent flow stress and volumetric strains. · Numerical modelling of the density evolution in the flat rolling process by the finite element method (FEM). The model applies the Drucker-Prager/cap model and the presently obtained plastic parameters for the powder. Comparing to experimental values FEM gives a very good prediction of the density in the individual fibres. · The stresses and strains in the deformation zone are analysed. It is concluded that more detailed mechanical tests and a more detailed constitutive plasticity model is desirable in order to improve the precision of the numerical modelling. New test equipment is designed implying the new powder in flexible die (PIFD) test. · Experimental investigation of the strains in flat rolling and their influence on the critical current density. It is shown that it is important to control the ratio between length strain and width strain during the thickness reduction in order to obtain a good quality. A new parameter, the logarithmic strain ratio (LSR), describing the ratio between length and width strain is invented. A new rolling strategy, called flexible rolling, implying multistep rolling with different roll diameters is invented as a technique for controlling the stresses and strains in the flat rolling process. A flexible rolling mill with easy exchangeable rolls ranging from Ø50 mm to Ø270 mm is designed, and the technique shows promising results regarding critical current density. The present work contributes to a better understanding of the flat rolling process and the behaviour of the implied materials.