Iryna Tomashchuk

The modeling of dissimilar welding of immiscible materials by using a phase field method

A multiphysical model of high power beam welding of immiscible materials is developed to explain the influence of operational parameters and materials properties on resulting morphology by simultaneous solving of heat transfer, fluid flow and mass transfer problems. The introduction of phase field description of the interface motion between two immiscible liquids allows obtaining the cartography of melted zone in function of two key-parameters: the position of heat source relatively to joint line and the welding speed. Due to the short thermal cycle limiting mass transfer, high power beam welding techniques may result in very inhomogeneous melted zones. In this study, the interest is paid to copper to stainless steel Nd:YAG laser and electron beam joints, which often present the repetitive heterogeneous patterns. The modeling results are validated by the comparison with experimental data on weld shape, composition and morphology.

Dissimilar steels laser welding: Experimental and numerical assessment of weld mixing

Upcoming strict CO2 regulations lead car manufacturers to look for mass saving solutions. The use of advanced high strength steel (AHSS) solutions enable optimizing both crash performances and mass saving. Particularly, the use of laser welded blanks made of dissimilar high strength steels is an efficient weight optimization solution. To support the joining of AHSS in car body design, a 3D model of heat transfer, turbulent flow and transport of species in the laser weld pool has been developed. It aims at providing a better understanding of diffusive-convective mixing in the weld and its influence on the weld mechanical properties. The presented model allows predicting the weld geometry and the element distribution. To validate the model, experimental tests were carried out. Welding of two dissimilar steels with different laser beam offset from the joint line was performed. Numerical and experimental investigations of dissimilar butt laser welding between high Mn and dual phase steels were carried out. The cross sections of the welds were characterized by scanning electron microscope (SEM) with energy-dispersive X-ray spectroscopy (EDX) elemental analysis. Quantitative mappings of Mn distribution in the melted zone offer an overview of mixing intensity. The results of the simulation have been found in good agreement with the experimental data. To go further and to assess the effect of weld mixing on mechanical performances, tensile tests were done. It was found that tensile behavior of the welds is determined by level of Mn and C dilutions. For attaining maximal joint performances, it is necessary to comprehend the elements distribution in the melted zone and to be able to control it through an accurate choice of operational parameters.Upcoming strict CO2 regulations lead car manufacturers to look for mass saving solutions. The use of advanced high strength steel (AHSS) solutions enable optimizing both crash performances and mass saving. Particularly, the use of laser welded blanks made of dissimilar high strength steels is an efficient weight optimization solution. To support the joining of AHSS in car body design, a 3D model of heat transfer, turbulent flow and transport of species in the laser weld pool has been developed. It aims at providing a better understanding of diffusive-convective mixing in the weld and its influence on the weld mechanical properties. The presented model allows predicting the weld geometry and the element distribution. To validate the model, experimental tests were carried out. Welding of two dissimilar steels with different laser beam offset from the joint line was performed. Numerical and experimental investigations of dissimilar butt laser welding between high Mn and dual phase steels were carried out. Th...

Characterization of Fusion Lines Obtained with Laser Welding on Ductile Iron Plates

This paper studies the ductile iron (DI) weldability using laser welding. For performing an Yb:YAG continuous laser was used, with a maximum power of 6 kW. The parametrical window power (P) - welding speed (S) was explored by carrying out the fusion lines on ductile iron plates without preheating, to determinate areas of weldability (complete penetration, correct geometry) to allow further characterization. The criteria for selection of focus areas were the geometry of the fusion lines and the absence of the welding defects. The unsatisfactory domains were characterized by: collapse of the melted metal, incomplete penetration, low fusion lines quality (geometry, compactness). In present study, several values of power and welding speed have been tested to identify their influence on geometry, compactness of the joints and mechanical properties. As result, the power-welding speed diagram for feasible domains of laser welding was generated.

The numerical simulation of heat transfer during a hybrid laser-MIG welding of duplex steel

The present study is dedicated to the numerical simulation of an industrial case of hybrid laser-MIG welding of high thickness duplex steel UR2507Cu with Y-shaped gap geometry. It consists on stimulating the heat transfer during the welding by COMSOL Multiphysics software using heat equivalent source approach. A numerical exploratory designs method is used to identify and to optimize the heat sources parameters in order to obtain a small relative error between the numerical results and the experiment.

Comparison of keyhole characteristics obtained by two experimental methods: The “direct observation of drilled hole” method and the “sandwich” method

The present article is dedicated to the study of the evolution of the keyhole obtained during the drilling of samples of Zinc by YAG laser beam. Two experimental methods are used to observe the shape of the keyhole. The first one is the Direct observation of Drilled hOle, that lets to obtain the shape of the keyhole for different laser powers and different pulse durations. The observations are carried out after drilling. The second one needs to clamp the zinc sheet with a quartz glass. This glass is chosen because of its excellent thermal shock resistance and its high light transmittance. The observation of the keyhole during the drilling shape is then realized through the quartz with a high-speed CCD camera. The evolution of the shapes of the keyhole obtained by both methods lets to highlight the role of the multiple reflections on the shape of the keyhole. When multiple reflections occur, the absorption of the energy by the wall of the keyhole increases and consequently the depth of the keyhole increase...