Variation of Young's modulus of high-strength TA18 tubes and its effects on forming quality of tubes by numerical control bending

Young's modulus is one of the most important material parameters that influence the plastic-forming quality of bent components, especially spring-back. Young's modulus actually changes with plastic strain, and its accurate characterisation is necessary to improve simulation accuracy for the tube bending and obtain stable, high-accuracy bent parts by practical bending. Using cyclic loading-unloading tensile tests, the variation of Young's modulus of high-strength TA18 titanium alloy tubes (TA18-HS tubes) with respect to plastic strain has been obtained. Next, the effects of the variation of Young's modulus on the spring-back, cross-section flattening, and wall thinning of TA18-HS tubes by numerical control bending are described. The results show the following. (1) With an increase in plastic strain, the value of Young's modulus reduces rapidly at first, then reduces more slowly, and finally settles to a stable value. This behaviour can be expressed as an exponential function. (2) The variation of Young's modulus has no effect on the trends of spring-back angles or spring-back radius with the bending angle, however it makes the values increase. (3) The cross-section flattening before spring-back is larger with considering the variation of Young's modulus than without considering the variation, and the cross-section flattening after spring-back is less with considering the variation of Young's modulus than without considering the variation. Thus, the recovery of cross-section flattening with considering the variation of Young's modulus is larger than that without considering the variation. (4) The variation of Young's modulus has a limited effect on the wall thinning of tubes.