Numerical Study of Localized Necking in the Strain Path of Copper Hydroformed Tube: Effect of Friction

The effect of the friction coefficient in the strain path of hydroformed tube is discussed in this paper. A finite element simulation with the LS-DYNA/Explicit software has been performed and experiments have been carried out. The localized thinning can be related to necking and then, the use of the Forming Limit Curve (FLC) and the analysis of the strain path can give some indications on the risk of fracture in the hydroformed part. FE simulations have been performed with different friction coefficients to study their effect on the resulting strain path and to predict the localized thinning during tube hydroforming in a square section die. Hydroforming experiments have been performed on deoxidized copper (Cu-DHP) tubes to validate the finite element results. When the pressure increases, the strain increases firstly in the transition zone, that next leads to severe thinning in the corner zone and finally, in the straight wall. The comparison between the results obtained with the finite element and experiments confirms that the thickness reduction is more important in the transition zone between the straight-wall and the corner radius and the localized necking occurs in the transition zone. It is then possible to define the limits of the process and enhance the necking prediction.