Research On Laser Welding Of TA15 Titanium Alloy Structural Parts Based On Thermo-fluid Dynamics

Laser deep penetration welding has become a key technological tool in achieving high-quality joint in lightweight alloys.This welding technique offers superior efficiency,high energy density,fast processing speed and,good joint quality.TA15 titanium alloy is one of the common lightweight alloys applied in aerospace industry owing to its ideal properties such as good creep performance under heavy load working conditions and,good machinability.However,the application of this alloy on spacecraft was limited due to some joining difficulties resulting from laser keyhole instability and unfortunate processing parameters.With the aim of seeking solutions to these problems,this paper explores the stability of keyhole in laser welding under variable processing conditions and further examines the resulting welding morphology.The study involves experiment,numerical simulation and,optimization of welding process parameters.The study incorporates analysis of temperature distribution,flow field and,keyhole formation process with various welding styles including;pulsed laser,non-swing laser,pulsed swing laser and,swing with Gaussian power distribution.Effect of process parameters on weld bead width and penetration was explored to predict the optimal range of welding speed,peak power,base power and,pulse interval.The numerical simulation utilizes skin-skeleton structure involving establishment of unified physical model between solid,liquid and,gas phases.Dynamic changes on the surface of the free flow field in the welding pool were calculated using Volume of fluid(VOF)method.The thermo-fluid coupled dynamics was used to explain the evolution of the velocity and thermal fields in the weld.Different laser heat source models were combined to develop a new type of heat source model.Cellular automaton(CA)method was applied to simulate the microstructure evolving in the weld pool of laser welded TA15 using algorithm written with Fortran programming language.The out put of the thermal simulation,temperature field under single beam laser welding,was imported as an input to simulate the gradual micro-grain growth phenomenon occurring throughout the entire solidification process starting from nucleation to final solidified morphology.