Simulation Research On Dendritic Growth Of Ti-Al Alloy Welded Pool Under Different Al Contents

In the fields of machinery manufacturing,aerospace,petrochemical and other industrial fields,titanium alloys are widely used.This is due to the advantages of titanium alloy,such as specific strength,high specific modulus and good corrosion resistance.If titanium alloy is used in the manufacture of welded structures,its high melting point and poor thermal conductivity may cause high temperature oxidation.Therefore,it is extremely easy to improperly select the welding process parameters,resulting in poor plasticity and toughness of the joint and even welding cracks.Currently,many researchers use test methods to study the welding process,welding materials,joint microstructure and properties of titanium alloys.It has the disadvantages of long time and high cost.Computer numerical simulation technology is improved rapidly in recent years,which undoubtedly provides a powerful auxiliary means for the study of the microstructure and properties of titanium alloy welded joints.Based on nonlinear thermo-mechanical coupling analysis theory in this paper,the convection and radiation boundary conditions are considered and the heating and cooling analysis steps are set up.Then a three-dimensional model of the welding temperature field is established by ABAQUS.The meshing uses non-uniform mesh and the welding heat source adopts a double ellipsoid heat source.From the above model,the macroscopic temperatur,e field of TIG welded joints of titanium aluminum alloy plate is calculated,and the calculated results are verified by tests.The results show with the increasing of welding current,the welding cross-section is widened while the shape of the molten pool is unchanged and it is ovate-shaped when the welding reaches quasi-steady state.These welding thermal cycle curves along the weld direction and perpendicular to th,e weld direction present a tendency to rise rapidly and then decline slowly,which is consistent with the actual welding process.Comparing the weld cross-section geometry,the simulation results are consistent with the experimental ones.From the calculated welding temperature field and the solute diffusion theory of solidification process,CA-FE(cellular automaton-finite element)technique combined with cellular automaton method and finite element method is used.Then the microscopic temperature field of the selected region is obtained by interpolation method,and the mathematical physical model of dendrite growth is built.From the above model,the dendrite growth process of TC4(Ti-6A1-4V)and Ti-45%Al alloy at the edge and center of molten pool is simulated under uniform temperature field.It is shown that with the aluminum content increased from 6%to 45%,the growing speed of the equiaxed crystals becomes faster,the primary and secondary dendrite arms are more developed and its forms become more complicated under the greater degree of undercooling,the stronger interface energy anisotropy and the greater preferred orientation angle.Furthermore,when the number of initial nucleation is larger,the temperature gradient is larger and the disturbance is stronger,the competition among the columnar dendrites grows more intensely and the dendrite spacing becomes smaller with the increasing of the degree of solute segregation.In order to more accurately reflect the crystallization process in the molten pool,a coupled model combining macro with micro grain evolution under a non-uniform temperature field is constructed.For two kinds of titanium aluminum,the competitive growth process and the distribution of solute concentration of dendrites in the molten pool are simulated by using the coupled model.In addition,the confirmatory tests are carried out.It can be found that with the aluminum content increased from 6%to 45%,the rate of columnar crystals transformed to equiaxed crystals is increased,the number of nucleation grains is raised and the grains become finer.Moreover,the solute concentration also is correspondingly increased,a larger nucleation rate and thermal conductivity will facilitate the formation of more fine equiaxed crystals.The microstructure morphology of the molten pool after welding is mainly columnar and equiaxed,which is agreement with the simulated results.As the aluminum content in the weld increases,the number of phases is increased and the weld hardness becomes higher.In summary,the visualization of microstructure evolution during solidification of weld pool can be accomplished by numerical simulation.It can be used to visually characterize the dependent relationship between temperature and composition change and microstructure.It can also provide guidance for the selection and the parameters optimization of titanium alloy welding process.These will facilitate to achieve the ultimate goal of reducing costs,reducing structural weight,and obtaining excellent welded joints and improved welding efficiency.