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

Abstract

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.Les structures tubulaires en aluminium ont de tr\ue8s importantes applications industrielles, particuli\ue8rement dans l\u2019industrie automobile. Le proc\ue9d\ue9 d\u2019\ue9tirage est couramment utilis\ue9 pour r\ue9duire les diam\ue8tres externes et internes des tubes. Un probl\ue8me majeur li\ue9 au proc\ue9d\ue9 d\u2019\ue9tirage de tube afin d\u2019obtenir une \ue9paisseur de paroi variable est de pouvoir d\ue9terminer et pr\ue9dire ses limites de formabilit\ue9. Des travaux d\ue9j\ue0 publi\ue9s traitent g\ue9n\ue9ralement de la limite de formabilit\ue9 de l\u2019\ue9tirage de tube classique fond\ue9e sur l\u2019analyse exp\ue9rimentale, la m\ue9thode analytique et la m\ue9thode des \ue9l\ue9ments finis. Toutefois, dans le cas des tubes d\u2019\ue9paisseur variable, peu de connaissances et de donn\ue9es existent qui permettent de pr\ue9dire leur limite de formabilit\ue9. Dans la pr\ue9sente \ue9tude, on propose des m\ue9thodes th\ue9oriques et exp\ue9rimentales pour estimer la limite de formabilit\ue9 de tubes d\u2019aluminium \ue0 paroi d\u2019\ue9paisseur variable, utilis\ue9s \ue0 des fins de transport. On propose la modification d\u2019un mandrin conique et l\u2019utilisation d\u2019un syst\ue8me de contr\uf4le particulier pour le d\ue9placement du mandrin au cours du processus, lors des essais d\u2019\ue9tirage. Au cours du proc\ue9d\ue9 d\u2019\ue9tirage, l\u2019axe d\u2019\ue9tirement du tube est r\ue9gl\ue9 \ue0 une vitesse constante pendant que le mandrin se d\ue9place pour obtenir l\u2019\ue9paisseur de paroi variable de fa\ue7on constante. On obtient la limite de formabilit\ue9 par l\u2019\ue9paisseur minimale de la paroi et la r\ue9duction maximale de la surface avant la rupture du tube. Ces valeurs constituent des donn\ue9es utiles pour d\ue9terminer le degr\ue9 de d\ue9formation que peut subir un mat\ue9riau avant qu\u2019il ne se rompe au cours d\u2019un proc\ue9d\ue9 d\u2019\ue9tirage. Le rapport de contrainte d\u2019\ue9tirage maximal est \ue9galement d\ue9termin\ue9 exp\ue9rimentalement. De plus, on propose une extension d\u2019une solution de limite sup\ue9rieure mise au point dans des publications ant\ue9rieures pour pr\ue9dire le champ de contrainte d\u2019\ue9tirage. Le rapport de contrainte d\u2019\ue9tirage maximal a servi de crit\ue8re dans l\u2019analyse des fractures. On montre que la combinaison du mod\ue8le analytique comportant sa nouvelle extension avec le crit\ue8re de fracture permet de pr\ue9dire particuli\ue8rement bien la limite d\u2019\ue9paisseur et de r\ue9duction de surface. Ces \ue9tudes exp\ue9rimentales sont r\ue9alis\ue9es en examinant la microstructure et le champ de d\ue9formation \ue0 l\u2019\ue9tat limite.Peer reviewed: YesNRC publication: Ye