Study On Laser Melting Deposition Process And Mechanism Of TA15/Ti₂AlNb Gradient Titanium Alloy

The design and manufacture of key components for high performance aerospace engines is increasingly demanding in terms of material systems to meet the higher performance requirements of the aircraft.The new air intake tract design is fabricated using TA15/Ti₂Al Nb heterogeneous materials,with TA15 titanium alloy used in the low and medium temperature regions in service below 500°C and Ti₂Al Nb based alloy used in the high temperature region in service between 650°C and 750°C.Laser melting deposition technology is the most effective method to achieve TA15/Ti₂Al Nb composite component forming,which can flexibly control the ratio of the two materials during the manufacturing process,effectively avoid cracks and microstructure defects with the optimal transition,and realise the integrated manufacturing of high-performance materials and structures.In this thesis,the laser melting deposition TA15/Ti₂Al Nb gradient titanium alloy is manufactured by using coaxial powder feeding nozzles,and the research is carried out systematically on the proportional stability and convergence control of the mixed powder flow transfer process,the microstructure characteristics and mechanical properties of the deposited material with different composition of TA15-Ti₂Al Nb alloy,the design of transition form and high performance preparation of TA15/Ti₂Al Nb gradient titanium alloy.The influence of nozzle structure,powder feeding parameters and powder particle size range on the proportional stability and convergence of the mixed powder flow during the transport of TA15/Ti₂Al Nb was investigated.The results show that the multiple collisions of powder particles in the nozzle feeding line are the root cause of powder flow dispersion,and the convergence of powder flow can be effectively improved by extending the length of the feeding line,increasing the flow rate of powder carrier gas and reducing the median particle size of powder.The range of powder particle size needs to be reasonably determined in combination with the density of the two materials.Based on systematic experimental research and theoretical analysis,the proportional stability and convergence performance of the mixed powder transfer process are well controlled.Six single-wall samples of TA15-Ti₂Al Nb alloy of different compositions were deposited by coaxial 3-jet nozzle,with Ti₂Al Nb mass ratios of 0,20%,40%,60%,80%and 100%in each part,and the problem of residual unfused Ti₂Al Nb powder particles in the deposited material was successfully solved through powder particle size control,resulting in a 16.0%increase in tensile strength and 246.2%increase in elongation after break.The tensile strength and elongation after break of the prepared TA15-Ti₂Al Nb alloys ranged from 942 MPa to 1117 MPa and 15.9%to 4.3%,respectively,with the highest strength of TA15-80Ti₂Al Nb and the lowest strength of TA15-60Ti2Al Nb.When the proportion of Ti₂Al Nb reaches above 40%,the elongation after break decreases rapidly,all around 4.5%.With the increase of Ti₂Al Nb content,the histomorphology of the alloy material shows obvious differences.In the TA15 to TA15-40Ti₂Al Nb alloy stage,the histomorphology is a net basket structure with the distribution of elongatedαphase,short rod-likeα+α₂phase and needle-likeα+α₂ phase within the columnar crystal of the coarseβphase matrix,respectively.In the TA15-60Ti₂Al Nb to Ti₂Al Nb alloy phase,the tissue morphology is a granularα+α₂ phase,a lamellarα+α₂+O phase,and a fine lamellar O+α2 phase structure distributed within isometric crystals of theβ/B2 phase matrix,respectively.A comparative study of the microstructure and mechanical properties of titanium alloys with different compositions reveals the mechanism of the evolution of the microstructure of TA15-Ti₂Al Nb alloys with different ratios deposited by laser melting on the mechanical properties,in which theα-phase reinforcement dominates with higher plasticity when the ratio of TA15 is larger,and the O orα₂ phase reinforcement dominates with higher strength when the ratio of Ti₂Al Nb is larger.and Ti₂Al Nb in similar proportions form a near Ti₃Al-based alloy,which is characterised by low overall performance.Based on the analysis of the fracture mechanism of gradient materials with different combinations of mechanical properties,various transition forms of TA15/Ti₂Al Nb gradient materials were designed and prepared,and their properties and failure mechanisms were comparatively studied.The transition design of TA15/TA15-20Ti₂Al Nb/TA15-80Ti₂Al Nb/Ti₂Al Nb was used to successfully prepare thin-walled components of high-performance gradient materials with a tensile strength of 1058 MPa and an elongation after break of 8%,and crack-free and well-formed large-size components were prepared based on this design.Further,the stress relieving annealing,recrystallisation annealing and solution ageing heat treatment processes of the gradient material were explored and the effects of different heat treatment processes on the material properties were analysed to provide an experimental reference for subsequent studies.