The potential for weight reduction, ease of manufacturing and improved crashworthiness makes advanced and ultra high strength steels attractive for automotive applications. Resistance spot welding is by far the most widely used joining method in the automotive industry due to the high operating speeds, the reliability of the process and the suitability for automation. Safe microstructures in resistance spot welds in AHSS and UHSS have to be assured to promote acceptance of these steels in the automotive industry. However, the higher alloying contents of AHSS/UHSS steels limit their weldability and unfavourable modes of weld failure are frequently observed. The main aim of this research is to identify and understand the unfavourable failure of the AHSS welds and to modify the microstructure and thus the mechanical response of the welds. In this PhD thesis the results of alternative welding schedules to modify the microstructure and mechanical performance of the AHSS resistance spot welds are reported. The effects of a paint bake cycle on the microstructure of the welds have also been investigated and the predominant mechanisms involved were studied. The residual stress within these welds were measured and simulated to facilitate the residual stress prediction before welding. Double pulse resistance spot welding with different second pulse current levels was applied to improve the microstructure of the weld edge. The second current pulse equal to the first pulse anneals the weld edge and modifies the weld edge microstructure. Microstructural analysis was performed using optical microscopy, scanning electron microscope, electron probe microanalysis (EPMA) and electron back scattered diffraction (EBSD). The double pulse weld showed a reduction in segregation of alloying elements such as phosphorous and a change in grain morphology from dendritic to a more equi-axed shape and smaller grain size. The results obtained from the mechanical testing i.e. cross tension strength test (CTS) and tensile shear strength test (TSS) showed enhanced cross-tension strength and energy absorption capability of the weld for the double pulse welds. @en

In this paper, we describe the effects of mechanical loading on bcc-to-bcc phase transformations of an Advanced High Strength Steel during cooling. In-situ synchrotron diffraction was employed to measure time–temperature–load diffraction patterns. Calculations were made of the volume fractions of the phases, the transformation kinetics, and the austenite lattice parameter during cooling and simultaneous loading. In addition, volume fractions and lattice parameters of retained austenite at room temperature under different loading conditions were obtained. The results show that applying a load during cooling of the fcc phase significantly increases the volume fraction of a bcc phase before the start of the martensitic transformation. The kinetics of phase transformations were affected by the applied loads. The volume fraction and lattice parameter of retained austenite at room temperature vary in different samples and the highest retained austenite and the largest lattice parameter were obtained in the sample subjected to the highest load.Correction to article: https://doi.org/10.1007/s11661-017-4453-7@en

The effect of an automotive paint bake (PB) thermal cycle on the microstructural evolution and the mechanical properties of resistance spot welded advanced high strength steel is presented in this work. Mechanical behavior of the heat-treated welds reveals an increase in maximum cross-tension strength, displacement and subsequently energy absorption capability when 453 K (180 °C)-20 minutes a bake thermal cycle is applied after welding. The microstructures of resistance spot welds with and without a PB heat treatment were characterized using scanning and transmission electron microscopy (TEM). TEM analysis revealed that the weld nugget and HAZ of the resistance spot welds consist of a martensitic microstructure. The microstructural analysis of the post-weld heat-treated samples shows the presence of ε carbides in a martensitic matrix within the weld nugget and the HAZ. It is shown that the improved mechanical response of the paint-baked welds is associated with carbide precipitation during heat treatment.

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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

This paper presents the effects of double pulse resistance spot welding (RSW) on the microstructural evolution, elemental distribution and mechanical properties of a 3rd generation 1 GPa advanced high strength steel (AHSS). In order to investigate the effect of double pulsing, the steel was exposed to single and various double pulse RSW schedules. The first current pulse was applied to create the weld nugget, while the second current pulse generated a secondary weld nugget and annealed or (partial) re-melted the primary weld nugget, depending on the magnitude of the current. The effect of the second current pulse on the weld nugget and heat-affected zone characteristics was investigated using optical microscopy and electron probe microanalysis (EPMA). Optical and electron microscopy revealed that the secondary weld nugget is fully martensitic, showing a typical solidification microstructure, while the annealed zone reveals an equi-axed martensitic structure. EPMA results showed that elemental segregation has been considerably reduced in the annealed zone. Mechanical properties of the welds show that the AHSS studied is prone to weld metal failure for single pulse RSW. However, the double pulse RSW method can lead to significantly improved mechanical performance and favourable failure modes.@en

The effects of single and double pulse resistance spot welding on the microstructures of an advanced high strength automotive steel are presented in this work. The double pulse welding schemes partially remelt the primary weld nugget and anneal the area at the fusion boundary of the nugget. The effects of the annealing treatment on the segregation and the microstructure have been studied by electron probe microanalysis (EPMA) in combination with electron backscatter diffraction (EBSD). Results show that phosphorus has been redistributed at the primary weld nugget edge of the double pulse welds, while the mean block width and ellipticity of the prior austenite grains were smaller in welds subjected to double pulsing compared with single pulse weld. A favourable failure mode was obtained for the double pulse welds although behaviour did not correlate with the measured grain size.

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