Abstract: To study the filling behavior of transition corner of an aluminum alloy rectangular section tube in hydroforming process, a special test device is designed and manufactured. The quantitative relationship between the corner radius and internal pressure as well as the influence of lubrication condition on transition corner filling are tested, the mechanism of cracking at the transition point between straight side area and corner area is analyzed, and a comparison is made between the theoretical value of internal pressure and the tested value. The results show that the internal pressure needed in the process of filling increases along with the decrease of corner radius of the tube. When the relative corner radius is larger than 5, the change of corner radius tends to be fast; when the relative corner radius is smaller than 5, the change tends to be slow. Lubrication condition has significant influence on transition corner filling. Improving lubrication can reduce the internal pressure needed for forming the same corner and decrease the limiting corner radius. From the midpoint of a straight side to the transition point, the effective stress increases gradually. Therefore, excessive thinning of wall or even cracking is easier to occur at the transition region. For the strain-hardening materials, the theoretical value of internal pressure without considering the strain-hardening effect is significantly less than the experimental result, so the strain-hardening effect should be considered.

Key words: Aluminum alloys, Hydroforming, Lubrication, Rectangular cross sections, Transition corner filling