Hot Press Forming of Tailor Welded Blank: Experiments and FE Modeling

Abstract

Hot Press Forming (HPF) of Tailor Welded Blank (TWB) was investigated with particular focus on the formability and phase transformation of the constituent base materials. Dilatometric analyses were conducted to study strength differentials for various cooling rates after heat treatment. U-draw bending tests were performed to evaluate the influence of a number of process parameters, namely, blank and tool temperatures, and cooling time, on the mechanical properties of the product. Formability at elevated temperature was assessed using forming limit diagrams for each monolithic material under realistic hot press forming conditions. Lab-scaled forming tests of a newly designed B-pillar were carried out to evaluate the formability of TWB under an actual hot press forming process. Finally, coupled thermomechanical finite element simulations, which utilize flow stresses and forming limit diagrams determined under realistic HPF conditions, were carried out. In particular, finite element simulation results for the weld-line displacement and thinning of the TWB were compared to experimental data for validation purpose.

Experimental and Numerical Analyses of Hot Stamped Parts with Tailored Properties

“…2. 26) This figure indicates that the calculated stress-strain curves for various strain rates are in reasonable agreement with those measured in an average sense for both materials, except in the strain range below 0.1. The stress-strain curves calculated by JMatPro were employed for the other phases because of difficulty in measuring them.…”

“…Figure 18 shows the microstructure of the U-channel for HPF1470 with various initial tool temperatures. 26) The heat-treated samples consist of (almost) full martensite at the initial tool temperatures of 200°C or less. In particular, as shown in Fig.…”

Stress development and shape change during press-hardening process using phase-transformation-based finite element analysis

“…Then, this additive strain decomposition strategy has reasonably been applied to commercial codes such as LS-DYNA (Åkerström et al, 2007;Choi et al, 2012), DEFORM3D , ABAQUS (Bok et al, 2014). In particular, Choi et al (2012) accurately predicted thickness distributions, formability, weld-line movement in a real hot press formed automobile TWB part using the same additive decomposition employed in the current study.…”

“…On the other hand, for the forming process, the contact-heat-transfer coefficient h c between the blank and tools was expressed as a function of the contact distance l gap (Choi et al, 2012). The magnitude of h c was 1500 W/m 2 K when l gap was within a critical distance l gap_c , which was 0.067 mm in this study.…”

“…(24) are listed in Table 4. In this table, specific heat c p , and conductivity k were experimentally measured with H-PHS and L-PHS sheets using the TC-700 calorimeter by ULVAC Technology Inc. (Choi et al, 2012) and fitted to be functions of temperature.…”