Formable Range in Three-Dimensional Hot Bending and Direct Quench Process

Abstract

A three-dimensional hot bending and direct quench (3DQ) process that enables the fabrication of automotive parts having hollow tubular structures with an ultrahigh tensile strength and a three-dimensional complex shape has been developed. This 3DQ process is a tube bending process that uses an induction heater, a cooling device, and a robot. In this research, a formable range in 3DQ was investigated. The main results are as follows: (1) When the width of the heating area is large, the ratio between the circumferential and axial stresses of the tube during deformation is almost constant irrespective of the bending radius, yielding a notable change in cross-sectional height. On the other hand, when the width of the heating area is narrow, the stress ratio approaches 0.5 (plane strain) with decreasing bending radius, reducing the change in cross-sectional height. (2) Twist forming with a small cross-sectional shape change is possible in 3DQ. (3) Wrinkling limit strain is almost proportional to (t/Lp) 2 for a rectangular tube and to (t/D) 2 for a circular tube, where t, Lp, and D, are the thickness, the length of the plane in the inner side of bending, and the diameter of the tube, respectively. (4) The localization of a high-temperature area increases wrinkling limit strain.