THE INFLUENCE OF CHEMISTRY INHOMOGENEITY ON MICROSTRUCTURE DEVELOPMENT AND RESIDUAL STRESS

Advanced high-strength steels (AHSS), which are increasingly used in the automotive industry, meet many functional requirements such as high strength and crash resistance. Some of these steels contain high amounts of alloying elements, which are required to achieve the necessary mechanical properties, but render these steels susceptible to weld solidification cracking. Weld solidification cracking results from the complex interplay between mechanical and metallurgical factors. Our recent work is focused on studying solidification cracking in dual phase (DP) and transformation induced plasticity (TRIP) steels using the following modeling and experimental strategies: 1. A finite element (FE) based model was constructed to simulate the dynamic thermal and mechanical conditions that prevail during bead-on-plate laser welding. To vary the restraint, laser welding was carried out on single sided clamped specimens at increasing distances from the free edge. In TRIP steel sheets, solidification cracking was observed when welding was carried out close to the free edge and at a certain minimum distance, no cracking was observed. For the no cracking condition, in situ strain evolution during laser welding was measured by means of digital image correlation to validate the strain from the Fe-model. Subsequently, a phase field model was constructed using the validated thermal cycles from the FE-model to simulate the microstructural evolution at the tail of a weld pool, where primary dendrites coalesce at the weld centerline. From the phase field model, elemental segregation and stress concentration are used to explain the cracking susceptibility in TRIP and DP steels. For DP steel, both the experimental and modeling results indicate a higher resistance to solidification cracking. 2. A phase field model was constructed to simulate the directional solidification in TRIP and DP steels. The thermal cycle and temperature gradient were derived from the in-situ solidification experiments conducted using high temperature laser scanning confocal microscopy (HTLSCM). The model showed that longer and narrower interdendritic liquid channels exist in the case of TRIP steel. For the TRIP steel, both the phase field model and atom probe tomography revealed notable enrichment of phosphorus, which leads to a severe undercooling in the interdendritic region. In the presence of tensile stress, an opening at the interdendritic region is difficult to fill with the remaining liquid due to low permeability, resulting in solidification cracking. The overall study shows that a combination of factors is responsible for the susceptibility of a material to solidification cracking. These include particularly mechanical restraint, solidification temperature range, solidification morphology, solute segregation and liquid feeding capability.