Residual Stresses in Laser Micro-Joints of Dissimilar Materials

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Detailed analyses of thermal residual stress profiles due to laser micro-joining of several dissimilar material combinations, titanium (Ti) and polyimide (PI) [1], titanium coated glass (Gl) and polyimide [2], and glass and silicon (Si), are vital for the long-term applications of bio-implants and opto-electronics. In these studies, comprehensive three dimensional (3D) transient simulations for thermal and thermal stress analyses of transmission laser micro-joining of dissimilar materials have been performed by using the finite element (FE) code ABAQUS, along with moving Gaussian or super Gaussian laser heat source. The laser beam moving at a certain velocity, passes through the transparent material (e.g. PI or Gl), gets absorbed by the absorbing material (e.g. Ti or Si), and eventually softens/melts the interfacial materials to form the bond. Computational results have been compared with experiments in terms of temperatures and bond widths, and good comparisons have been observed in all combinations. Then the computational model has been used to optimize the process parameters and after this residual stress profiles of the laser-micro-joints have been calculated using the temperature profiles obtained from the thermal analyses as the systems cool down to room temperature. According to the various stress profiles, none of the stresses cross the tensile or the compressive strengths in cases of all the dissimilar material combinations at room temperature. A critical discussion has been included comparing the thermal residual stresses generated in the three combinations of dissimilar materials being used in these studies. A conclusion has been made on the formation processes of these stresses in laser micro-joining processes of dissimilar materials. This report will help to estimate the expected residual stresses in the laser micro-joining processes of dissimilar materials and will shed insight on how to select a better dissimilar material combination for a laser precision joining process.