Finite element thermal/mechanical analysis for microscale laser joining of ultrathin coatings of titanium on glass/polyimide system

Abstract

Finite element (FE) thermal/mechanical analysis of microscale laser joining of biocompatible materials, polyimide (PI) and nanoscale coating of titanium (Ti) on glass (Gl), is important for the long-term application of bio-implants and the applications of nanoscale solid coatings. In this study, a comprehensive three-dimensional (3D) transient simulation for thermal/mechanical analysis of transmission laser microjoining of dissimilar materials has been performed using the FE code ABAQUS, along with a moving Gaussian laser heat source. The laser beam (wavelength of 1100 nm and diameter of 0.2 mm), moving at an optimized velocity (100 mm/min), passes through the transparent PI, gets absorbed by the absorbing Ti, and eventually melts the PI to form the bond. The laser bonded joint area is 6.5 mm long on three different Ti coating thicknesses of 400, 200, and 50 nm on Gl surface. Non-uniform mixed meshes have been used and optimized to formulate the 3D FE model and ensure very refined meshing around the bond area. During the microscale laser heating, FE modelling shows that the widths of PI surface experiencing temperatures above the glass transition temperature are similar to the widths of bonds observed in experiments for coating thicknesses of 400 and 200 nm of Ti on Gl. However, for the case of 50 nm coating, bond width using FE analysis cannot produce and is lower than the bond width observed experimentally. After these verifications, the residual stress profile of the laser microjoint (200 nm of Ti on Gl) has been calculated using the developed model with the system cooling down to room temperature. The transverse (to the laser travel direction) stress profiles on Ti surface have shown high tensile stress near the centre-line of laser travel, decreased to lower values from the centre-line symmetrically at both sides, and to the contrary, have shown compressive stresses near the centre-line on Gl surface. Maximum von-Mises residual stresses on PI and Gl surfaces are obtained at the start of laser travel, and on Ti surface they occur in between the ends. It has been explained that the patterned accumulation of residual stresses is due to the thermal expansion and contraction mismatches between the dissimilar materials at the opposite sides of the bond along with the melting and softening of PI during the joining process.