| The need to reduce vehicle weight has increased the interest in using light weight materials, such as aluminum alloys, in automotive structures. One process with great potential for fabricating structural components is hydroforming; hence, the current research focuses on the formability of aluminum alloys in tube hydroforming. The prediction of the formability of aluminum alloys in tubular hydroformed parts, in particular the interaction between the pre-bending and subsequent hydroforming of AlMg3.5Mn aluminum alloy tubes, is examined. Pre-bending induces large strains and strain variations in the tube, which reduces the formability in the subsequent hydroforming process. The associated large change in strain path between pre-bending and hydroforming also invalidates use of the forming limit diagram (FLD) approach to predict formability. As an alternative approach, the use of the Gurson-Tvergaard-Needleman (GTN) damage-based constitutive model to predict damage and formability was investigated in the current research.; Numerical simulations were developed for three sets of hydroforming and/or tube bending experiments: (i) free expansion (with three levels of end feed); (ii) straight tube corner fill (with three lubricants); and (iii) pre-bending and subsequent hydroforming (with three levels of boost and two bend radii), in which the damage history from pre-bending was carried through to the hydroforming stage, to allow prediction of the overall damage history.; The effect of pre-bending on the subsequent tube hydroformability was shown to be significant, with a 42% reduction in burst pressure and a 51% reduction in corner fill expansion, for example, due to the introduction of pre-bending at R/D = 2.0, relative to initially straight tube.; It was also found that higher boost levels and larger bend radii reduced the severity of the pre-bending operation. The GTN constitutive model adopted in this research contains several material parameters that have been determined for AlMg3.5Mn tube stock. This damage model was used to simulate all of the experimental cases in this project and to predict the formability of the material at critical sections of the parts. In addition, the microstructural damage after deformation was predicted and compared with the measured values. Based upon comparison of the measured and predicted data, the Gurson-based predictions appear promising to predict the formability of aluminum alloy tubes. Material parameters for damage nucleation and coalescence (cracking) of AlMg3.5Mn are now available for industry to apply to bending and hydroforming applications. |
