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RESEARCH PRODUCT

Dissimilar joining of copper to stainless steel and TA6V to stainless steel by high power beams : understanding and modeling of physicochemical phenomena

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The present study is dedicated to the comprehension of the mechanism of materials mixing during dissimilar welding by high power beam sources. We have been interested in joining of two couples of metallic materials which present different metallurgical problems: •copper- stainless steel (miscibility gap, important difference in physical properties);•TA6V- stainless steel (oxidation on air, formation of intermetallic phases which made the joint brittle).For the first couple of materials, continuous laser Nd:YAG welding and electron beam welding have been applied. The experimental study of morphology evolution, composition, microstructure and mechanical properties has allowed establishing the hypotheses on formation of heterogeneous mixture between the materials having limited solubility. To quantify the physical phenomena of continuous dissimilar welding, the numerical modeling has been carried out by means of FEM software package "Comsol Multiphysics". A number of models reproducing temperature field, convection movements and mixing (diffusion, level set method, phase field method) between the materials has been created. In case of continuous laser welding, the pseudo-stationary formulation of heat transfer based on simplified key-hole geometry and convection has been coupled with two-dimensional problems of diffusion and mixing in horizontal planes. The electron beam welding presenting the nonlinear development of the weld has needed employing of temporary formulation. Final model including complete coupling (simultaneous multiphysical solving) reproduces the process of development of periodic solidification structure during electron beam welding and allows explaining the mechanism of formation of altered structures between immiscible materials which have important difference in thermophysical properties.The second couple of materials presents weldability problems due to formation of brittle intermetallic phases making direct joining by fusion impossible. The local elementary composition becomes the key-aspect of successful joining: the introduction of the third material (pure copper) having better compatibility with titanium is necessary. To determine the ranges of optimal operational conditions, numerical models created previously have been adapted to the case of four joining techniques: electron beam and laser Nd:YAG (continuous and pulsed) welding and laser brazing with filler wire. Elementary analysis of microstructures of resistant welds has allowed developing the solidification scenario and understanding the influence of local composition of heterogeneous interfaces on tensile properties of the joints. The multiphysical models created during this study allow rapid access to high quantity of data on behavior of melted zone in function of welding parameters basing on relatively limited input data and several simplification hypotheses. The multiphysical approach to welding modeling allows recreating the shape of melted zone, to visualization the convection movements and providing the cartography of several elements in good correspondence with experimental results. A set of models allows determination of operational parameters respecting fixed criterions determined by metallurgy of dissimilar couple.