Interface instability in the bulk processing of 2223 BSCCO powders

Interface instability (or 'sausaging') has been a major problem in the oxide powder-in-tube tape-rolling process. The fast Fourier transform of core instabilities are first performed to study the rolled tapes and to obtain quantitative information on the wavelengths and magnitudes of the nonuniform profiles. The existing experimental observations are also re-examined and summarized. Treating this problem as the bimaterial interface instability, both bifurcation analysis and finite-element modelling are applied to study the influences of roll-gap geometry, packing fill factor and clad material properties on the instability wavelength and magnitude. Good correlations between theoretical results and experimental observations are found. The critical wavelength/current core thickness ratio lambda */dc is found to be sensitive to the fill factor only, and insensitive to the reduction/pass ratio, and the core and clad material properties; that is, the relative tape geometry is the single dominating factor that affect the normalized critical wavenumber lambda */dc. Consistent with experimental observations, a smaller reduction/pass ratio, a higher initial core porosity, a higher hardening clad material and a larger core fill factor can reduce the normalized instability magnitude Delta v/dc (i.e. the instability magnitude/current core thickness ratio) at the same tape reduction strain level. The results suggest that the reason for the much smaller interface variation magnitude with a small reduction/pass ratio (i.e. 5% per pass) compared with a large reduction/pass ratio (i.e. 25% per pass) is not because the small reduction/ pass ratio can eliminate or delay the interface instability initiation; it is, however, most probably caused by the random disruption of the interface by the many rolling steps with critical wavelengths very close to each other between adjacent rolling steps.