Residual stress reduction of laser beam welds by use of low-transformation-temperature (LTT) filler materials in carbon manganese steels—In situ diagnostic: Image correlation

Abstract

Low-Transformation-Temperature (LTT) materials are used for residual stress reduction in weld seams and, subsequently, for the prevention of distortion. Typically, LTT materials are highly alloyed Fe-based materials with levels of chromium and nickel that ensure that austenite transforms to martensite at reduced temperatures. The dilatation associated with the transformation is prevented by the surrounding material. Consequently, compressive stresses develop within the transformed region, and these counteract the accumulation of stresses due to thermal contraction. The precise chemistry of the weld metal determines the transformation temperature as well as the magnitude of the volume expansion. The chemical composition of the weld metal can, in turn, be influenced by the energy input during welding. In recent years, LTT-filler materials have been applied successfully in arc welding processes for residual stress reduction. Digital image correlation provides one avenue for the detection of surface displacements or deformations, as they occur during the cooling down process, in the vicinity of a weld. Observation of the movement of surfaces using a stereo camera setup allows the recording of displacements in three dimensions. Technically, this is achieved by identifying recognizable points on the surface under observation and by tracking their movement via comparisons between several time-shifted images. Visualization of surface contractions, as well as the effects of solid-state phase transformations during the joining process, especially during cooling, is key to understanding and influencing the residual stress state in welds as well to reducing distortion. In this study, the used installation setup, and the methodology for sample preparation, shows the production of information relating to surface displacements in the direct vicinity of the laser weld during the cooling down stage. Different welding parameters were investigated, resulting in different dilution levels for the LTT-Filler material, thereby influencing the distances to the weld pool and temperatures at which transformations took place. In order to provide a point of reference, comparable welds, made with conventional filler wires, were also investigated. The displacements after welding are always lower when using an LTT filler wire when compared to a conventional wire, proving that LTT wires can be used to mitigate distortion during laser beam welding.