Study on hot formability of high-strength steel laser tailor welded blanks based on microscopic damage modeling

Microscopic damage mechanics suggest that the primary cause of sheet metal forming fracture is attributed to inherent microscopic defects within the material. This study applies the GTN damage model to evaluate the damage progression and formability of high-strength steel laser-welded blanks (TWBs), made from Usibor1500P and Ductibor500, during hot forming. The thermo-rheological behavior and failure characteristics at fracture locations of the two base materials were assessed through uniaxial tensile tests and SEM across varying temperatures. Furthermore, a retrograde calibration of the GTN model's damage parameters for the TWBs was executed. Significantly, variations in the nucleation void volume fraction (fN) and the critical void volume fraction (fC) exert a substantial influence on the fracture strain value (R3) of the specimens. Subsequently, the calibrated GTN damage parameters were further validated through hot Nakajima-type bulging experiments. Finally, a comprehensive analysis is conducted to elucidate the impact of critical process parameters on the failure modes of TWBs during thermoforming at 700 degrees C. The results indicate that the maximum thinning rate of the sheet is directly proportional to the coefficient of friction (COF) and the blank holder force, while the limiting dome height (LDH) is inversely proportional to its effect. The proposed GTN damage model proves to be an effective tool for accurately predicting TWBs' fracture behavior and analyzing sheet metal forming processes.