Methodology for Quantification of the Roping Phenomena in 6xxx Automotive Car Body Sheet Alloys
An important aspect when Al sheets are to be used for visible outer car body panels is surface appearance. Some age-hardenable Al-Mg-Si alloys of the AA 6xxx series can suffer from a phenomenon called roping. Roping appears on a macroscopic scale and manifests itself as the development of ridges and valleys 90° to the original rolling direction with a wavelength ranging from 1 to 10 mm when the sheet is formed to typical parts like doors or hoods. In the past years Al industry has developed means to minimise roping by careful control of alloy composition and through appropriate thermo-mechanical processing schedule. The increasing use of aluminium for outer skin applications gives proof that these efforts were successful. However, the extent of roping, and further the judgment of the quality of the surface appearance has up to now been done on a qualitative basis with comparisons against reference samples, as there are no established experimental techniques and no established analysis procedures for the quantification of this phenomenon. The goal of such a methodology is to be able to quantify the extent of roping in 6xxx automotive car body sheet alloys and to correlate it to a visual appearance ranking of roping. In the present study, a novel combination of a characterisation technique with an evaluation method is used to characterize and evaluate the roping behavior in aluminum alloys. This approach combines these already individually existing methods to a powerful tool: the measurement of a 3D surface topography with a subsequent analysis by means of Fourier analysis. The analysis of various car body sheet grades by means of this technique for uniaxial and biaxial loading conditions is presented in this paper.
Knut Marthinsen, Bjørn Holmedal and Yanjun Li
C. Schäfer et al., "Methodology for Quantification of the Roping Phenomena in 6xxx Automotive Car Body Sheet Alloys", Materials Science Forum, Vols. 794-796, pp. 45-50, 2014