| For aerospace components,in order to meet the different requirements of different parts of the use of temperature,it is necessary to manufacture titanium alloy thin-walled heterogeneous material structure,low temperature service area for TA15 titanium alloy,high temperature service area for Ti2 Al Nb titanium alloy,the two titanium alloy in the mechanical properties,especially the elongation of great difference.Direct welding or direct laser melting deposition can easily cause cracks on the side of the material with poor shape,and can not guarantee the mechanical properties of the parts at high temperature.Based on the actual demand and the application demand of the key parts of the aerospace engine,the research on the laser melting and deposition characteristics of Ta15 /Ti2 Al Nb gradient materials was carried out to realize the gentle gradient transition of the two materials,providing important theoretical support and experimental verification for the rapid manufacturing of dissimilar titanium alloy thin-walled components.Feeding with three beam coaxial nozzle is presented in this paper two phase powder flow model of the finite volume method,system research the feeding amount of air flow,powder,powder particle size and protection gas flow on the influence law of bipolar powder flow spot features: with the same density bipolar powder particle size uniformity,powder flow uniformity good convergent and elements;The convergence and composition uniformity of powder flow can be improved by increasing the powder feeding rate and carrying gas flow rate.Because the three-beam nozzle used for coaxial powder feeding has better robustness,the convergence of the protective gas flow on the powder flow is not obvious.The concentration control method of dual-phase powder flow was obtained.The concentration control of dual-phase powder flow was determined by the particle size and the flow rate of the powder-carrying gas.In order to ensure the same convergence of the dual-phase powder flow,the powders with different particle sizes needed to adapt to different flow rates of the powder-carrying gas.Through the study of the feeding parameters,the optimized feeding parameters of the two-phase powder flow were obtained: the feeding rate of 6g/min,the carrying gas flow rate of 12L/min,and the particle size of the powder 65-85?m.At the same time established the TA15 / Ti2 Al Nb mixed powder laser melting deposition process of molten pool heat coupling model of molten pool of different ratio of powder had been used to characterize the temperature field,flow field characteristics,molten pool flow driver is given priority to with metal steam recoil pressure,with high melting point of Ti2 Al Nb material than lower,peak temperature,molten pool temperature gradient increases,The Marangoni effect is more obvious,and the depth and length of the molten pool increase obviously,which is conducive to the transition of Ta15 to Ti2 Al Nb from columnar grain to equiaxed grain.Laser melting deposition was used to continuously prepare Ta15 /Ti2 Al Nb gradient materials.The size of Ti2 Al Nb powder should be controlled within100?m,otherwise the white spheres of unmelted Ti2 Al Nb would easily appear in the deposited materials.When Ti2 Al Nb is increased at the ratio of 20% by mass fraction,the change process of microstructure is as follows: Alpha + beta and alpha,beta,plus a small amount of alpha 2 and B2-alpha + + type B2 + gather alpha beta 2-> B2 + alpha + beta,alpha 2 + a small amount of O-> B2 + O +acicular alpha 2 + a small amount of alpha,beta-> B2 + O + alpha 2,obvious when Ti2 Al Nb content more than 60% O phase segregation;The EBSD results show that with the increase of Ti2 Al Nb content,the texture type changes to the fiber texture,and the texture strength first increases and then decreases,and the texture strength reaches the maximum when the Ti2 Al Nb content is 60%.Intragrain orientation difference distribution shows that the orientation,size and content of substructure play a major role in mechanical properties.Combined with the microstructure properties and fracture position before and after heat treatment,the failure modes of gradient materials were expounded.The fracture position of the deposited gradient materials all occurred on the TA15 side,and the fracture form was ductile fracture with the tensile strength of 979 MPa and the elongation of 10.5%.In a reasonable process range,the elongation of gradient materials can be effectively improved by reducing the scanning speed when the heat input is constant.The microstructure homogenization is obvious after stressfree annealing.The deformation incompatibility between the coarse columnar grain and the coarse equiaxed grain will reduce the mechanical properties of the part with higher Ti2 Al Nb content.The maximum tensile strength is 817 MPa,which is 16.5% lower than that of the deposited state,and the elongation 1.5% is85.7% lower than that of the deposited state.The fracture position is close to the position with 100% Ti2 Al Nb content,and the fracture form is cleavage fracture or quasi-cleavage fracture.The dissolving,precipitation and growth of O phase and ?2 phase after solution and aging treatment promote the dislocation and plug accumulation of phase near grain boundary.Compared with annealing state,the tensile strength of O phase and ?2 phase is significantly increased,and the tensile strength is 956 MPa,which is close to deposition state.The fracture location is still mostly in the part with higher Ti2 Al Nb content,but compared with the annealed state,it is generally closer to the Ta15 side,which is cleavage fracture or quasi-cleavage fracture.Therefore,the heat treatment process of Ta15/Ti2 Al Nb gradient material needs to be further explored... |
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