Ultrasonic impact treatment (UIT) is a relatively novel technique applied to the toe of welded joints to improve the fatigue life by changing the weld geometry and the residual stress state. In this study, the stress relaxation due to ultrasonic impact treatment is investigated on a six pass welded high strength quenched and tempered steel section. Stress measurements in two orthogonal directions were conducted by energy dispersive synchrotron X-ray diffraction. Results show that the application of only ultrasound to a welded component re-distributes the residual stresses more uniformly, while mechanical impacts in combination with ultrasound is an effective way to release the residual stresses. After welding, diffraction peak broadening due to the lattice distortion, characterised by the full width at half maximum (FWHM), is observed in the region of the weld toes. Ultrasonic impact treatment reduces the FWHM at these locations.

Susceptibility to weld solidification cracking in transformation-induced plasticity steel sheets was studied using a modified standard hot cracking test used in the automotive industry. To vary the amount of self-restraint, bead-on-plate laser welding was carried out on a single-sided clamped specimen at increasing distances from the free edge. Solidification cracking was observed when welding was carried out close to the free edge. With increasing amount of restraint, the crack length showed a decreasing trend, and at a certain distance, no cracking was observed. With the aid of a finite element-based model, dynamic thermal and mechanical conditions that prevail along the transverse direction of the mushy zone are used to explain the cracking susceptibility obtained experimentally. The results indicate that the transverse strain close to the fusion boundary can be used as a criterion to predict the cracking behavior. The outcome of the study shows that optimum processing parameters can be used to weld steels closer to the free edge without solidification cracking.

In multipass welding, each successive thermal cycle will introduce local melting, solid state phase transformations, grain growth, grain refinement, recrystallisation and recovery, all of which lead to a complicated stress state. Most stress measurements performed on multipass welded components represent the final residual stress state. Information concerning stress evolution on a pass-by-pass basis is difficult to find. In this investigation, six pass welds were made on high strength quenched and tempered steel sections, and depth resolved strain measurements in two orthogonal directions were carried out after each weld pass using energy dispersive synchrotron X-ray diffraction. The residual stresses were calculated using biaxial Hooke's law. A thermalmetallurgical-mechanical welding model was constructed and validated with temperature and pass-by-pass stress measurements, which improves the reliability of the model. Cross-sectional stress distributions are presented after each pass, revealing the weld stress development in multipass welds.

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.

In situ high-temperature laser scanning confocal microscopy is applied to study solidification cracking in a TRIP steel. Solidification cracking was observed in the interdendritic region during the last stage of solidification. Atom probe tomography revealed notable enrichment of phosphorus in the last remaining liquid. Phase field simulations also confirm phosphorus enrichment leading to severe undercooling of more than 160 K 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 and high viscosity, resulting in solidification cracking.

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Six pass welds were made on a 16 mm thick high strength quenched and tempered structural steel plate (S690QL1, Fe-0.16C-0.2Si-0.87Mn-0.33Cr-0.21Mo (wt.%)). Depth resolved measurements in two orthogonal directions were carried out using energy dispersive synchrotron X-ray diffraction at the ID15 beamline of the European Synchrotron Radiation Facility. The strains were calculated from the shift in the local d-spacing for four bcc planes ({200}, {211}, {220}, {310}). The planar stresses were calculated from the biaxial Hooke’s law, using the diffraction elastic constants of the individual planes. A two dimensional cross-sectional residual stress map with a depth resolution of 2 mm was obtained. Transverse compressive stresses were found at the weld toes and root. Transverse tensile stresses were present in the middle of the plate. Longitudinal tensile stresses concentrated along the fusion line. This work describes the procedures to obtain the depth resolved residual stress map and the generated results provide necessary information to validate thermal mechanical finite element model of multi-pass welding.