Formability and microstructure evolution during hydroforming of drawing quality welded steel tube

Abstract

Tube hydroforming is an advanced manufacturing process in which a tube is placed inside a die cavity and plastically deformed under hydraulic pressure, so that it takes the shape of the die. This process is commonly applied in the automotive and aerospace sectors. Welded tubes are commonly used in the hydroforming process. In this work, therefore, the entire study is carried out on resistance-welded tubes. Forming limit diagram is a measure of formability of any forming process; and in the case of the tube hydroforming process, it was obtained by deforming the tubes under different strain paths. A simulation of the hydroforming process was carried out using the finite element–based commercial software, PAM-STAMP 2G. The forming limit diagram obtained through experiments was predicted using this simulation. A fractography study revealed that in the case of axial feed, the nature of fracture was ductile; whereas in the fixed condition, some brittle characteristics were seen. In these experiments, it was observed that the neck and crack were parallel to the weld for all strain paths. A simulation incorporating the weld was able to capture accurately the neck and crack orientation in all the strain paths. The electron backscattered diffraction images show grain refinement of the base metal as well as a high percentage of low-angle grain boundaries, which is an indication of higher plasticity. The weld metal microstructure does not show significant change during deformation, as the extent of deformation is much lower in the weld.