Investigation of the formability limit of aluminium tubes drawn with variable wall thickness

Structural aluminium tubes have very important industrial applications, particularly in automobile industry. Tube drawing process is widely used to reduce the outer and inner diameters of tubes. An important issue in the tube drawing process to obtain variable wall thickness is how to determinate and predict its formability limits. Previously published works generally deal with the formability limit of conventional tube drawing based on experimental analysis, analytical method and finite element method. However, in the case of variable wall thickness tubes, there is a lack of knowledge and data in order to predict their limit of formability. In the present study, both theoretical and experimental methods are proposed for estimating the formability limit of the variable wall thickness aluminium tubes used for the transportation purposes. A modification of a conical mandrel was proposed and a special control system for mandrel displacement during the process was used to carry out the drawing tests. During the drawing process, the tube pulling axis was controlled at constant speed while the mandrel was moved to achieve the continuously variable wall thickness. The formability limit in term of minimum wall thickness and maximum area reduction was obtained before tube rupture. These values are useful data for the determination of the extent of deformation during a drawing process that a material can experience without failure. The maximum drawing stress ratio was also determined experimentally. Further, an extension of an upper bound solution developed in previous publications is proposed to predict the drawing stress field. The maximum drawing stress ratio was used as a criterion for fracture analysis. It was shown that the analytical model with its new extension combined to the fracture criterion predicts quite well the thickness and area reduction limit. The experimental studies were completed by examining the microstructure and strain field at the limit state.