This article summarizes the findings of a paper entitled, “Hot cracking investigation during laser welding of high-strength steels with multi-scale modelling approach”, by H. Gao, G. Agarwal, M. Amirthalingam, M. J. M. Hermans.G-4
Researchers at Delft University of Technology (TU Delft) in The Netherlands and Indian Institute of Technology Madras in India attempted to model Hot Cracking susceptibility in TRIP and DP steels. For this experiment, TRIP and DP steels are laser welded and the temperatures experienced are recorded with thermocouples at three positions. Temperatures experienced during welding are measured and used to validate a finite element model which is then used to extract the thermal gradient and cooling rate to be used as boundary conditions in a phase field model. The phase field model is used to simulate microstructural evolution during welding and specifically during solidification. The simulation and experimental data had good agreement with max temperature deviation below 4%.
Summary
Referencing Figure 1 (Figure 6 in the original paper) which shows the microstructure where the dendritic tips meet the wel, centerline, it is observed that TRIP steels reach a solid fraction of 93.7% and DP steels reach a solid fraction of 96.3% meaning that TRIP steels have a larger solidification range than the DP steels. Figure 8 shows the phosphorus distribution where the dendritic tips reach the weld centerline. TRIP steels show a concentration of up to 0.55 wt-% where segregation occurs compared to the original composition of 0.089 wt-%. DP steels show a max of 0.06 wt-% which is significantly lower than the TRIP steels. In addition to phosphorus, Al is seen in high concentration in TRIP steels which contributes to the broder solidification range. A pressure drop is the last factor contributing to the Hot Cracking observed in TRIP steels(figure 2). The pressure drop is due to a lack of extra liquid feeding in the channels and forms a pressure difference from the dendrite tip to root. The pressure drop in TRIP steels is calculates to 941.2 kPa and 10.2 kPa in DP steels. The combination of element segregation, pressure drop, and thermal tensile stresses induced during laser welding results in a higher Hot Cracking susceptibility in TRIP steels as compared to DP steels.
Figure 1: Phase distributions in the TRIP and the DP steel when the dendritic tips reach the weld centreline.G-4