Influence Of Y And Ce Elements On Microstructure Control And Laser Welding Performance Of Ti6Al4V Alloy

Ti6Al4V alloy has the advantages of high specific strength,good corrosion resistance and good biocompatibility.With the continuous expansion of the application range of titanium alloy,for the selection of some key aeronautical parts,dual-phase（α+β）type Ti6Al4V alloy is still one of the preferred materials.But Ti6Al4V alloy still needs to improve the tensile strength and elongation on the basis of maintaining the existing properties.Moreover,Ti6Al4V alloy has poor wear resistance,in the casting process at high temperature for the impurity element oxygen element has a great affinity leading to the mechanical properties of the alloy decline and poor welding performance and other problems.In this paper,the addition of Y and Ceto Ti6Al4V alloy was studied in order to further improve its tensile strength,elongation and wear resistance.The influence of Y and Ceon the microstructure and weld microstructure of the alloy was systematically studied,and the influence of Y and Ceon the mechanical properties,wear resistance and welding properties of the alloy was analyzed.The strengthening mech-anism of Y and Ceon Ti6Al4V alloy was systematically studied,which provided a certain experimental and theoretical basis for the regulation of microstructure and properties of Ti6Al4V alloy.Firstly,the as-cast Ti6Al4V-x Y（x=0,0.1,0.3,0.5,0.7）alloy was used to analyze theα-Ti phase with dense cubic（HCP）structure andβ-Ti phase with body centered cubic（BCC）structure.When Y was added,the impurity element O was absorbed and equiaxed crystalline Y₂O₃ precipitated phase was formed.The precipitated phase of Y₂O₃ is mainly through the formation of nucleating particles to make theαphase andβphase microstructure becomes small,and through the obstruction of grain boundaries to refine the grain,the dispersion distribution makes the presence of dislocation pin effect,the dispersion strengthening makes the mechanical properties and wear resistance of the alloy enhanced.With the increase of Y content,the tensile strength first increases and then decreases.When Y content is 0.3%,the tensile strength is the highest,and the highest tensile strength is 998.5 MPa.When the mass fraction of Y is 0.1%,the highest elongation is 9.0%.The friction coefficient decreases first and then increase with the addition of Y.The mass fraction of Y element is 0.3%,and the friction coefficient is the smallest（0.4359）,the wear volume is the smallest（0.1009mm³）,and the wear resistance is the best,and the wear mechanism is abrasive wear.For the laser welding experiment of Ti6Al4V-x Y（x=0,0.1,0.3,0.5,0.7）alloy as the filler layer,the addition of Y can effectively refine the originalβcolumn grain of the weld,through the testing of mechanical properties and friction properties of the weld,the comprehensive properties are enhanced.But Y content can not be added too much,otherwise Y₂O₃ precipitated phase in the grain boundary aggregation or segregation into large particles,affecting the performance of the alloy.Secondly,the addition of Ceelement can refine the microstructure morphology,not only can generate CeO₂ nano-precipitated phase,but also can strengthen the matrix through solid solution in Ti6Al4V alloy matrix.The two-phase coexistence structure of the alloy matrix did not change.However,Ceelement is solitvely dissolved in Ti6Al4V alloyα-phase andβ-phase matrix,resulting in the microstructure of titanium alloy from dendritic structure to two-phase cross and fine two-phase structure.It was found that the precipitate phase of nano-CeO₂ was precipitated fromβ-Ti phase by TEM analysis of the sample with Cecontent of 0.3%（mass fraction）.During the casting process,the harmful element O element in the lattice was absorbed by Ceto form the precipitate phase of nano-CeO₂,and there was orientation relationship withβ-Ti.It not only introduces more nucleation,but also refines theβphase wafer.When the mass fraction of Ceelement is 0.1%～0.5%,the comprehensive properties of titanium alloy are improved.When the mass fraction of Ceelement is 0.3%,the elongation of titanium alloy increases from 6.5%to 13.2%,and the strength does not decrease.When the mass fraction of Ceis 0.5%,The maximum tensile strength is 978.1 MPa,the elongation is also enhanced,the strength and plasticity are enhanced at the same time;The friction coefficient decreases first and then increases with the addition of Ceelement.The mass fraction of Ceis 0.3%,the friction coefficient is the smallest（0.5575）,the wear volume is the smallest（0.1247 mm³）,and the wear resistance is the best,and the wear mechanism is abrasive wear.For Ti6Al4V-x Ce（x=0,0.1,0.3,0.5,0.7）alloy as the filling layer of laser welding experiment of Ti6Al4V alloy.The results showed that the addition of Cechanged the micro-structure of the weld and effectively refined the originalβcolumnar grain of the weld.The tensile strength of the weld was increased by 7.9%and the wear resistance of the joint was increased by 41%by testing the mechanical properties and friction properties of the weld.Finally,according to the calculation of the mismatch formula,the precipitated phase Y₂O₃is semi-coherent with the matrixα-Ti.The semi-coherent interface can transfer the load and improve the properties of the alloy,especially for the strength improvement.There is a coherent relationship between the precipitated phase of CeO₂ andβ-Ti matrix.The coherent interface can transfer the load and improve the properties of the alloy,especially for the improvement of plasticity.By adding Y and Ceelements in Ti6Al4V alloy,the structure and properties of the alloy can be effectively improved,and the strong plasticity of the alloy can be improved at the same time,but also effectively solve the problem that adding Y is easy to form the agglomeration grain boundary precipitated Y₂O₃ large particles caused by the alloy performance decline;Laser welding of Ti6Al4V alloy using Ti6Al4V alloy filling layer with Y and Ceelements can effectively improve the welding performance of Ti6Al4V alloy.