Detailed Analysis of Residual Stress Profile at Micro-Scale in Transmission Laser Bonded Titanium/Polyimide System

Detailed analysis of residual stress profile due to laser micro-joining of two dissimilar biocompatible materials, polyimide (PI) and titanium (Ti), is vital for the long-term application of bio-implants. In this work, a comprehensive three dimensional (3D) transient model for sequentially coupled thermo-mechanical analysis of transmission laser micro-joining of two dissimilar materials has been developed by using the finite element (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, 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 by 0.3 mm wide. First the transient heat transfer analysis is performed and the nodal temperature profile has been achieved, and then used as an input for the residual stress analysis. Non-uniform mixed meshes have been used and optimized to formulate the 3D FE model and ensure very refined meshing around the bond area. Heat resistance between the two materials has been modeled by using the thermal surface interaction technique, and melting and solidification issues have been approximated in the residual stress analysis by using the appropriate material properties at corresponding temperature. First the model has been used to observe a good bonding condition with the laser parameters like laser traveling speed, power, and beam diameter (burnout temperature of PI > maximum temperature of PI achieved during heating > melting temperature of PI) and a good combination has been found to be 100 mm/min, 3.14 W and 0.2 mm respectively. Using this combination of parameters in heating, the residual stress profile of the laser-micro-joint has been calculated using FE model after cooling the system down to room temperature of 27 °C and analyzed in detail by plotting the stress profiles on the Ti and PI surfaces. Typically the residual stress profiles on the PI surface show low value in the middle, increase to higher values at about 160 μm from the centerline of the laser travel symmetrically at both sides, and to the contrary, on Ti surface show higher values near the centerline of traveling laser beam. The residual stresses have slowly dropped away on both the surfaces as the distance from the bond region increased further. Maximum residual stresses on both the Ti and PI surfaces are at the end of the laser travel, and are in the orders of the yield stresses of respective materials.