This paper investigates the manipulation of chemical composition of a laser weld by dissimilar filler material and its effect on residual stress. The aim is to minimize residual stresses in the weld seam. In order to negate residual stresses, dissimilar combinations of low-carbon manganese steel (S235JR) base material with high-alloyed solid filler wires (G19 9 and G25 20), as well as similar combinations with low-alloyed solid filler wire G3Si1 are analyzed. The goal of the paper is to show that the so-called low-transformation-temperature effect can be used to induce residual compressive stresses in a weld without the use of specially manufactured filler wires. Chemical compositions are generated within a laser-beam-welding process by means of dilution, proving that the concept of in situ alloying is usable in order to affect the martensite formation on a weld. Dilatometry measurements show that a varying Cr and Ni content in a weld reduces the phase-transformation temperature and increases dilatation. EBSD analysis indicates that a fully martensitic weld with a negligible amount of retained austenite is created while the base material preserves its ferritic-pearlitic microstructure. Residual stress measurements with the hole-drilling method demonstrate a reduction in longitudinal tensile residual stresses, whereby the magnitude of the induced residual compressive stresses depend on the M_s temperature. As a result of this research, it was proven that a reduction in tensile residual stress by means of targeted alloying with conventional materials in low-carbon manganese steel is possible. Under the experimental conditions, residual stress in the weld seam could be reduced to 0 MPa. In some cases, even compressive residual stress in the weld could be achieved.

Residual stress and distortion of welded specimens are issues when it comes to geometrical requirements. The surrounding material prevents the dilatation associated with transformation in the area of heat input resulting in residual stress and distortion due to thermal contraction. In the past few years, low transformation temperature (LTT) material was successfully used as filler wire to reduce residual stress as well as distortion in the weld seam in arc welding processes. High alloy Fe-based filler materials with levels of chromium and nickel ensure a martensitic transformation at reduced temperatures in a low alloy base material. The LTT properties counteract the accumulation of stresses due to thermal contraction with compressive stresses that develop within the transformed region. This work used a high alloy base material in combination with a low alloy filler wire resulting in a microstructure that shows the same properties as LTT weld metals. This in situ alloying allows for an alloy composition tailored to the process. In order to provide a point of reference, comparable welds were made using conventional high alloy filler wire. As a result, the distortion and longitudinal residual stress was significantly reduced compared to welding with conventional filler wire.

Laser hybrid welding presents several challenges when used to weld thick steels. A typical weld is divided into the arc-dominated and laser-dominated zone. These zones lead to variations in the mechanical properties of the weld. The laser-dominated zone is of particular importance regarding mechanical properties, notably Charpy impact toughness, due to the high cooling rates and the absence of filler wire. The low heat input of the laser can lead to martensitic microstructure causing hardening and deterioration of impact toughness. The high heat input of the arc can lead to grain coarsening and even loss of impact toughness. This study examines the influence of heat input on the cooling rates, microstructure and mechanical properties of single-pass laser hybrid welded steels of S355J2 and EH36 with thicknesses up to 30 mm. The experiments were performed with a 20-kW fibre laser and a contactless electromagnetic weld backing in the butt-joint configuration in 1G welding position. The cooling time was measured in three different locations near the fusion lines corresponding to different heights of the seam using a special configuration with pyrometers, collimators and optical fibres. The test specimens for the Charpy impact testing and tensile testing were extracted in three different depths. The experiments indicated that a heat input of 1.6 kJ/mm–2 kJ/mm, 2 kJ/mm–2.4 kJ/mm and 3.7 kJ/mm were recommended when single-pass laser hybrid welding of 20-, 25-, and 30-mm-thick structural steels regarding the minimum requirements of the mechanical properties, respectively.

Ultra-high strength steel 22MnB5 with 1.5 mm thickness was laser beam butt welded under varied welding parameters such as focal diameter, welding speed and beam power. The ultra-high strength material is softened by the laser beam welding process due to tempering in the heat-affected zone, which causes a loss in tensile strength and hardness compared to the base material. The influence of softening on the mechanical-technological properties was investigated. Hardness profiles of welded specimens were examined and tensile tests with digital image correlation were carried out. The digital image correlation showed the area in which the strain was concentrated as well as the location of fraction initiation. The microstructure was examined using micrographs and scanning electron microscopy images; the composition of the structure was subjected to X-ray diffraction. Dilatometry was used to map the individual areas of the heat-affected zone with respect to temperature and hardness. An increase in the energy per unit length influenced the width of the heat-affected zone and thus, the width of the softened zone. It was not possible to exert influence on the depth of hardness drop. The tensile strength, however, increased due to the narrower width of the softened zone.