Analysis of Hot Press Forming with Tailored Strength Properties by Differential Cooling

Abstract

In this study, tailor-welded blank (TWB-HPF) and partial quenching hot press forming (PQ -HPF) properties were investigated and analyzed to secure tailored properties. Al-Si coated boron steel sheets with different thicknesses were used and welded with solid-state ND-YAG laser in TWB-HPF process. Welded zone was evaluated by energy dispersive X-ray spectroscopy (EDX) to establish the cause of defect. And tailored properties in hot press forming can be achieved by a local reduction of the cooling rate below the critical value of 30 ◦C/s using the partial quenching HPF (PQ-HPF) technology. A U-draw stamping die was designed and PQ-HPF tests were performed to evaluate the tailored properties. Dies heated-up to 120, 220, 320 and 400 °C were employed to reduce the cooling rate. The strengths obtained after PQ-HPF with dies at 320 and 400 °C were significantly lower than those obtained with cooler dies. In addition, a transition zone was observed at the boundary between the heated and cooled die areas. Finally, finite element (FE) simulations of partial quenching were carried out using LS-Dyna for validation with experimental results. The strength of the final part in the PQ-HPF process was effectively predicted by the coupled thermo-mechanical-metallurgical FE simulation.