Experiment And Numerical Simulation Of Laser Welding For Titanium Alloy Lap-joint

In this dissertation, Experiments and Simulations of the laser welding process for lap-joints using finite element analyses were presented, The material is titanium alloy TC4.In the experiments of the lap-joint laser welding, regularity between laser welding parameters and the geometry size of the weld-pool depth and bead width was investigated. Microstructure and microhardness of the joints were analyzed, and longitudinal residual stress in the joint was measured with the method of stress release.The laser welding process were simulated using commercial Finite Element Method (FEM) program ANSYS. The simulations were carried out using a two-step process: first the dynamic temperature distribution throughout the weld seam and the plates was obtained by a non-linear transient heat transfer analysis, then followed by a separate quasi-steady state mechanical analysis based on the thermal history, where the mechanical process was considered as thermo-elasto-plastic.Deep laser penetration welding was characterized by the heat transfer of the keyhole. the heat transfers of keyhole was simulated by a new combined heat source model, which consists of a Goldak double ellipsoid heat source which in the above and a Gauss distribute column heat source in the below. In the finite element model, the moving heat source, the convection and radiation effect, the latent heat of phase transformation and the temperature dependence of thermal-physical properties were considered. Under the penetrate welding condition and set the heat source parameters as that the two heat source's effective length is equal,the ration of powers is 3:2 and radius of keyhole is 0.8mm, the temperature fields in different welding parameters were calculated. Comparing with the surface width of weld-pool and bead width, the discrepancy between of the simulation results and the experimental results is 10%, it validates that this model is adaptive to the simulation of laser welding of titanium alloy.On the basis of the simulation of temperature field,the 3-D stress distribution on the lap-joint were calculated. The simulation results shows:(a)The residual stress distribution has an obviously boundary effect:a lower stress section exists in front of the end of weld seam,and where residual stress varies significantly; (b)The residual stress in penetrate welding joint is larger than that in un-penetrate welding joint even through the linear energy of laser is equal;(C)As the laser power is equal and the under penetrate welding condition, residual stress is growing as the weld velocity increases, which means the residual stress is sensitive to variety of the weld velocity and insensitive to variety of laser power. When the laser power is 1500W and weld velocity is 2.0m/min, comparing with longitudinal residual stress distribution in the middle of joint,the simulation results showed a good agreement with the experimental results.