| Laser additive manufacturing(LAM)has the advantages of realizing the integrated forming of any complex large-scale structural parts,shortening the lead-time and improving material utilization,especially LAM-fabricated high-temperature Ti alloy has attracted widespread attention in the aviation industry because it can reduce the weight of components to improve aircraft engine fuel efficiency,cost-effectiveness and thrust-to-weight ratio.However,several key scientific problems need to be solved urgently,such as poor plasticity and low fatigue crack growth resistance caused by the unique microstructure of high-temperature Ti alloys fabricated by LAM,as well as its thermal stability under high-temperature service environment.In this thesis,a TA19 alloy fabricated by laser metal deposition(LMD)used in the gas turbine engines compressor blades was chosen.Systematic researches on the effects of heat treatment on its microstructure,tensile properties and fatigue crack growth properties at room temperature were carried out.The effects of matrix microstructure change,precipitated phase and surface oxidation on the room-temperature tensile properties and fatigue life during the long-term thermal exposure at high temperature were also studied.The mechanisms for phase interface strengthening,fatigue crack propagation and evolution of microstructure and mechanical performance of LMD-fabricated TA19 alloys were discussed.The research findings have important theoretical guiding role in microstructure design,forming process parameter optimization,and heat treatment process selection of high-temperature Ti alloys with excellent comprehensive mechanical properties fabricated by LMD.The main findings are summarized below.The effects of solution treatment temperature and cooling rate on the microstructure and room temperature tensile properties of LMD-fabricated TA19 alloy were investigated.A bi-lamellar structure LMD-fabricated TA19 alloy consisting of heterogeneous multiple phases was obtained.The results show that 1000℃/1 h solution treatment/oil cooling+595℃/8 h aging treatment/air cooling(1000SOA)is a heat treatment condition with the best synergy of strength and plasticity.Based on the mixed rule of the Hall-Patch relationship of lamellar a phase fine grain strengthening,a quantitative relationship model between the yield stress and the microstructure scale and area fraction of the LMD-fabricated TA19 alloy was established.Theoretical calculation models for the barrier strength and interface strengthening ability of dislocations across multiple heterogeneous interfaces were established based on the modulus and lattice mismatches of different phase interfaces in the lamellar Ti alloys fabricated by LMD.Theoretical calculation results show that the barrier strength of dislocations across the α/β interface is higher than that of the αp/βs interface.Thus,the reason why the strength of bi-lamellar Ti alloys is higher than that the full-lamellar Ti alloys is due to its large number of internal α/β interface.Based on the theoretical calculations of the strengthening ability of different phase interfaces,the prediction model of the overall phase interface strengthening ability of any lamellar structure Ti alloy was also established,which is directly related to the volume fraction of the α/βphase interface in the alloy.The essential factor affecting the fatigue crack propagation resistance of LMDfabricated TA19 alloy with columnar prior β grains and internal lamellar α phases has been determined to be the growth direction of columnar β grains,which controls the long axis orientation of lamellar α phase and the deflection angle of fatigue crack at the columnar β grain boundary,thereby significantly affecting the law of fatigue crack propagation resistance.The optimized design of microstructures for the LMDfabricated Ti alloys with high fatigue crack growth resistance was proposed,namely,the combination of several adjacent columnar grain growth directions deviating from the build direction should be 0°-15°-0°-15°,and the corresponding forming process strategy was also proposed.The surface stability and microstructure stability of the LMD-fabricated TA19 alloy and two heat-treated samples with the excellent synergy of strength and plasticity were investigated after thermal exposure at 593℃ for 0 h to 500 h.It is found that the surface oxide layer is mainly composed of TiO2(rutile type)and Al2O3,and the proportion of TiO2 is the highest.During long-term thermal exposure,the metastable α’martensite phase gradually decomposes into stable α+β phase,the content and size of the α2 phase precipitated in the lamellar αp phase and the silicide precipitated near theα/β phase interface increase gradually.The precipitation and growth of silicide make the initially continuous α/β phase interface gradually become discontinuous.The 970℃/1 h solution treatment/air cooling+595℃/8 h aging treatment/air cooling sample(970SAA)with the least number of α/β phase interfaces as the high-speed oxygen diffusion channel was determined to have the most excellent oxidation resistance at 593℃.The effect of the evolution of matrix microstructure and precipitates,as well as the surface oxidation in the LMD-fabricated TA19 alloy and two heat-treated samples with the excellent synergy of strength and plasticity during thermal exposure at 593℃ for 0 h to 500 h on the room temperature tensile and fatigue properties were investigated.It is found that the continuous decomposition and coarsening of α’ martensite phase contribute to improving the plastic and fatigue life of the alloy,but reduce the strength of the alloy.The gradual increase of the size and content for the α2 phase leads to more significant planar slip characteristics,thereby improving the strength of the alloy,but reducing the plasticity and fatigue life of the alloy.The gradual increase of silicide content reduces the length of the continuous α/β phase interface,that is,the length of dislocation pileup becomes shorter,thus improving the plasticity and fatigue life of the alloy.Based on the above three influencing factors,it was finally determined that the 1000SOA sample with heterogeneous multiple phases and the lowest overall content ofα2 phase,but the highest content of silicide,had the best thermal stability of tensile and fatigue properties at room temperature.The surface oxide layer and the oxygen diffusion zone preferentially crack during monotonic tension and cyclic fatigue loading,then the stress concentration at the crack tip leads to premature fracture of the sample,thereby reducing the tensile and fatigue properties of the alloy. |
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