Interface instability in the bulk processing of 2223 BSCCO powders

Interface instability (or s̀ausaging’) has been a major problem in the oxide powder-in-tube tape-rolling process. The fast Fourier transform of core instabilities are ® rst performed to study the rolled tapes and to obtain quantitative information on the wavelengths and magnitudes of the nonuniform pro® les. The existing experimental observations are also re-examined and summarized. Treating this problem as the bimaterial interface instability, both bifurcation analysis and ® nite-element modelling are applied to study the in ̄ uences of roll-gap geometry, packing ® ll 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 ̧*/dc is found to be sensitive to the ® ll 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 a€ ect the normalized critical wavenumber ̧*/dc. Consistent with experimental observations, a smaller reduction/pass ratio, a higher initial core porosity, a higher hardening clad material and a larger core ® ll factor can reduce the normalized instability magnitude D 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. §