Prediction of Earing in Deep Drawing of Roll-cast Aluminium Alloy Sheets

摘要

Commercial aluminium alloy sheets are presently sem ic ontinuously, direct chill casting billets that are hot and cold rolled to the fi nal gauge. Interest has been shown in continuous methods which eliminate the ho t rolling step through rapid solidification of the molten metal to the final sla b. Accordingly, sheets are produced by homogenization, cold rolling, intermedia te and final annealing of these roll-cast slabs. The problem of earing is of gr eat concern as it causes frequent interruption of production runs and leads to m aterial wastage. Therefore, it is quite desirable that earing can be predic ted and consequently necessary measures be taken to minimize or eliminate this u nwanted phenomenon. It is accepted generally that, the principal source of earing is the crystallogr aphic anisotropy arising from non-random distribution of crystal orientations i n the material. Accordingly, several attempts have been made to correlate the m echanical and crystallographic properties of the materials to the earing behavio ur for predictive purposes. Some of these are based on continuum concepts which concentrate on the macroscopic rather than the microscopic aspects of the mater ials. To accommodate the microstructural features of the material, some models have been developed. A more recent approach which provides a connection between texture and plastic anisotropy parameters of the material is the Continuum Mech anics of Textured Polycrystals (CMTP) method proposed by Lin et al. A simplifie d version of this method has been suggested by Chan with promising accuracy for aluminium and copper sheets. AA3105 and AA8011 aluminium alloy sheets were used in this investigation. The a s-cast slabs were cold rolled to the final thickness of 1.0 mm. Different anne aling temperatures in the range of 420 ℃ to 540 ℃ produced a range of R-value s. Circular blanks of 60 mm diameter were machined and deep drawn using a cylind rical flat-bottom punch of 33 mm diameter. The heights of the drawn cups were measured at 0, 45 and 90° to the rolling direction, with the aid of a microme ter accurate to 10 -2 mm. The earing percentage was then calculated usin g the following formula: % earing=h p-h v1/2(h p+h v)(1) where h p is the distance between the bottom of the cup and the peak of ear , and h v is the distance between the bottom of the cup and the valley of t he ear. For the measurement of plastic strain ratios (R-values), tensile specimens cut at 0, 45 and 90° to the rolling direction were photogridded with 1mm square s. These specimens were then stretched in the range of uniform deformation and the dimensional changes were measured with the aid of a travelling microscope. The strain ratios, whether R 0, R 45 or R 90 were determined from the following equation: R θ=dε wdε t=dε wdε l+dε w(2) where Θ refers to the specimen orientation and dε w and dεl refer to the transverse and longitudinal strains of the gauge section, respectively. The av erage strain ratio, R, and the parameter ΔR were then calculated from: R=14(R 0+2R 45+R 90)(3) ΔR=12(R 0-2R 45+R 90)(4) where R 0, R 45 and R 90 values are determined using specimen s cut at 0, 45 and 90° to the rolling direction, respectively. Finally, a continuum mechanics approach using different yield criteria is employ ed for the prediction of earing behaviour under different conditions of the mate rials. Instead of using texture data, the yield stress values are obtained by d ifferent anisotropic yield criteria such as; Hosford, Hill, and Zhou. The predicted earing profiles are compared to the experimental data and the suit ability of different yield criteria is discussed.