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.@en

Advanced high strength steels (AHSS) are increasingly used in automotive industry; thousands of resistance spot welds are applied to car body-in-white. High alloying levels of AHSS result in lower weldability. Residual stresses play an essential role on the formation of defects and the mechanical performance of the weld. An electrical-thermal-metallurgical-mechanical finite element model was constructed to simulate the temperature and stress distribution during single and double pulse resistance spot welding. The models are validated by ex-situ synchrotron X-ray diffraction stress measurements. In this paper, single pulse and double pulse resistance spot welds were made on 1.3 mm thin sheets of a 3rd generation AHSS. Depth resolved stress measurements in two orthogonal directions were carried out using high-resolution powder diffraction at beamline ID22 of the European Synchrotron Research Facility. A monochromic 70 keV X-ray was used to record the d-spacing of (200) bcc planes in transmission mode. The strains were calculated from the shift in the d-spacing of the planes. The stresses were calculated by the biaxial Hook’s law. The numerical and experimental results show that the residual stresses in the weld nugget zone and the heat affected zone of the welds are tensile in nature, whereas the base material experiences compressive stresses. Lower residual stresses at the weld nugget and HAZ were obtained by applying a second current pulse. The simulated results show a good agreement with the residual stresses measured. This study provides a better understanding of the stress distribution in resistance spot welds and allows prediction of stresses as a result of welding conditions applied. @en

In situ strain evolution during laser welding has been measured by means of digital image correlation to assess the susceptibility of an advanced high strength automotive steel to solidification cracking. A novel method realised using auxiliary illumination and optical narrow bandpass filter allowed strain measurements as close as 1.5 mm from the fusion boundary with good spatial and temporal resolution. A finite-element thermomechanical model of the welding process supports the experimentally measured transverse strain. The validated finite-element numerical model can be used to assess the local strain and associated stress conditions which influences weldability and in particular, solidification cracking.@en

Hot cracking during laser welding of advanced high-strength steels is reported to be a serious problem by automotive manufacturers. In this work, hot cracking susceptibilities of transformation-induced plasticity (TRIP) and dual-phase (DP) steels are studied based on a multi-scale modelling approach. Transient temperatures measured from welding experiments are used to validate a finite element (FE) model. The temperature, thermal gradient and cooling rate in the weld fusion zone are extracted from the FE model and pre-defined as boundary conditions to a phase field model. The welding-induced microstructural evolution is simulated considering thermodynamic and mobility data. Results show that, compared to the DP steel, the TRIP steel has a broader solidification range, a greater pressure drop at the inter-dendritic regions, and an increased phosphorus segregation at the grain boundaries; all these make this steel more susceptible for hot cracking.

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