Calibration of an FE code for numerical simulations of micro-wire drawing operations by integration of numerical and experimental techniques

The paper focuses the attention on the micro-wire drawing process; the research includes both experimental and numerical investigations and is mainly aimed to develop reological and tribological models suitable to be applied in bulk micro-forming processes. Two different materials have been investigated: i) a low-carbon steel and, ii) a nonferrous metal (copper) drawn in a 270 μm diamond die having a semi die angle of 7° and completely submerged in the lubricant (Chemetall Gardolube DO 338). MSC.Superform has been used for the numerical simulation of both drawing process and tensile test. The experimental procedure consists of i) annealing of the initial wire to remove any effect on material structure due to previous manufacturing process, ii) single pass wire drawing at very low drawing speed with a section reduction of 19%, iii) different reductions of drawn wire by electropolishing, iv) tensile test on section-reduced wires and micro-hardness measurements in the wire cross section, v) determination of the tensile strength of the wire at different radial positions. The results of the experimentation pointed out an increase of the tensile strength and the hardness of the external layer (20-30 μm) of the wire which was a) in agreement with the material properties for the copper wire and b) significantly higher than expected for the low-carbon steel wire. The numerical procedure consists of i) development of a numerical model of the drawing process calibrated on the drawing force measured in the experimentation, ii) numerical simulation of the tensile test on the drawn and electro-polished wires and comparison with experimental results, iii) development of a modified reological model to fit the numerical simulation to the experimentation. The numerical model applied to the copper wire resulted in a reasonable agreement with the experimentation without any modification of reological and tribological behaviour. On the other side the numerical simulation applied to the low carbon steel wire required some modifications to the constitutive material model in order to fit the experimental results.