Influence of die semi-angle on mechanical properties of single and multiple pass drawn copper

Drawing is a metalworking process widely used in the manufacturing of cold worked bars, rod and wire. An important characteristic of the process is the die semi-angle (alpha), which influences the drawing forces, the lubrication in the process and also the mechanical properties of the final product. In this investigation, an experimental program was carried out in order to evaluate the influence of die semi-angle on the mechanical properties of round section annealed copper bars. Single passes involved drawings through dies with semi-angle of 1,23; 5; 10 and 24,5 deg. Molykote paste was used as lubricant and reductions in area were in the range of 2 to 31%. In multiple passes total reduction of area was kept at a constant value of 30%, produced by two different drawing sequences: a first pass of 5% and a second pass of 25%; a first pass of 25% and a second pass of 5%. In both cases, the die semi-angle in the first pass was 4 deg. In the second pass the die semi-angle varied between 6 and 12 deg (first sequence), and 2 and 10 deg (second sequence). After drawing the samples were submitted to tensile testing in order to evaluate mechanical properties. The values of yield strength and ultimate tensile strength increase with increasing die semi-angle, while the values of percentage elongation decrease as alpha increases. For multiple passes, it was observed that die semi-angle in the second pass does not influence the values of yield strength, ultimate tensile strength and percentage elongation as markedly as it was observed in single pass drawing.