Junjia Cui

Mechanical Properties and Microstructure of High-Strength Steel Controlled by Hot Stamping Process

A novel design and manufacturing method, dubbed “precast,” of the cooling system and tools for a hot forming process was proposed in this paper. The integrated structures of the punch and blank holder were determined by analyzing the bending and reverse-bending deformation of the forming parts. The desired crashworthiness performance of an automotive front bumper constructed with this process was obtained by a tailored phase transformation, which generated martensite-bainite in the middle and full martensite transformation in the corner areas. Varying cooling effects in the formed parts caused the highest temperature to be located in the bottom and the lowest on the end of the formed parts. Moreover, the microstructural distributions demonstrated that the bottom possessed a relatively lower content of martensite, while, conversely, the end possessed a higher content. This was precisely the most desired phase distributions for the hot formed parts. For the six-process cycle stamping, the temperatures reached a stable status after an initial rapid increase in the first three process cycles. The microstructural results verified the feasibility of the hot forming tools under multiprocess cycles.

Tensile behavior and microstructural evolution for AZ31 magnesium alloys sheet at high strain rate

Abstract Mechanical behavior and microstructural evolution of AZ31 sheet at high strain rates and elevated temperatures were investigated using a tensile Split-Hopkinson bar. Flow stress and elongation increased with increasing strain rate under high strain rate conditions, but temperature had little effect. Microstructural evolution was analyzed by optical and scanning electron microscopy. Twinning was observed at high strain rates, even at 200°C and 250°C, but not under quasi-static tension. Dynamic recrystallization was observed under quasi-static conditions with increasing temperature, but not at high strain rates. Dimples were observed over the entire temperature range at high strain rates. Increasing ductility was limited compared with that under quasi-static conditions. Dynamic recrystallization occurred at strain rates of 2 000 s−1 and 3 000 s−1 at 250 °C, but the recrystallized grains were very small.