Strain gradient and deformation localization at the thickness in tube hydro-bulging process

Hydroforming is an advanced technology that enables integrated forming for complex components and promotes lightweight construction and high reliability. However, the hydroforming process can result in a strain gradient at the wall thickness of tubes, which directly determines the deformation order in the thickness and is closely linked to the occurrence of defects like springback and wrinkling of tubular components. In this study, a geometric model of tube hydro-bulging that considers wall thickness was established, and the effects of length-diameter and diameter-thickness ratios on the radial strain gradient were studied through theoretical analysis and numerical simulations. Higher strains are experienced on the inside and lower on the outside during tube bulging. The strain disparity increases with greater length-diameter ratios and decreasing diameter-thickness ratios. In the case of a tube with an outer diameter of 78 mm and a wall thickness of 4 mm, the maximum equivalent strain difference observed was 0.03. Additionally, a tube hydro-bulging test was carried out to confirm the microstructural gradient, with high-density dislocations concentrated near the inner surface, resulting in noticeable strain localization. This study reveals the radial deformation mechanism of hydroformed tubular components, essentially providing a reliable scientific basis for controlling defects in tubular parts.