| As a high strength and high toughness alloy,TA15 titanium alloy has advantages such as high specific strength,excellent oxidation resistance,good thermal strength and weldability.TA15 titanium alloy is promising for the preparation of aerospace structural parts and bearing parts that serve at 500℃,such as metal plate,large wall panels and load-bearing frames.However,the characteristics of titanium alloys are easy to cause the damage of cutting tools during the processing of complex components,resulting in the poor machining processability and low material utilization rate.Additive manufacturing(AM)process that based on powder bed can overcome the shortcomings in processing titanium alloys by traditional techniques,and enable the rapid,high-quality and flexible manufacturing of parts.Electron beam selective melting(EBSM),as one of the powder bed fusion AM processes,adopt electron beam as the heat source,and cyclically melt the pre-set powders over many passes in a vacuum surrounding to fabricate components,which can reduce the residual stress in titanium alloy components effectively and avoid the formation of cracks and oxidation of components.In this thesis,TA15(Ti-6.5Al-2Zr-1Mo-1V)pre-alloyed powders were used as raw material,and the control of metallurgical quality was systematically studied by optimizing the process parameters.The formation and in-situ decomposition mechanism of martensite phase of EBSM-built TA15 titanium alloy samples were investigated,and the globularization mechanism of lamellar phases was clarified.Moreover,the deformation behaviors of EBSM-built parts at high temperature was studied.The metallurgical quality control of EBSM-built TA15 titanium alloy samples was carried out,and the energy input was adjusted by the change of beam current I and scanning speed v,where the value of I/v represented the magnitude of the energy density.As other parameters fixed,samples with a flat and dense upper surface can be obtained when the value of I/v fluctuated between 5.0 and 7.5,and there were no pores or cracks within the sample.The upper surface of the sample was porous as the value of I/v was less than 5.0,and there were a lot of pores and un-melted powders within the sample with poor metallurgical quality.At this time,the energy input was low and powders were partially melted.When the value of I/v was higher than 7.5,the upper surface of the sample was wavy,and there were a large number of circular droplets on the surface and pores within the sample.The area and life of molten pool increased due to the high energy input,and the molten pool fluctuated violently under the action of Marangoni effect and recoil force.Combined with selective evaporation of low melting point elements,the molten pool flow was aggravated,resulting in a significant increase of the thickness in local positions.The microstructure evolution under different energy densities was studied,and the formation mechanism and in-situ decomposition of martensite phase were investigated.Martensite phase existed near the free surface of the samples under the conditions of low beam current and high scanning speed.There were a large number of dislocations inaphases andbphases,and twins existed in some acicular martensite phases.These martensite phases will undergo cyclic solid-state phase transformation during the subsequent layer-by-layer accumulation,and the difference in the printing sequence leads to the different thermal cycles that the pre-solidified layer has undergone.During the layer-by-layer accumulation,the peak temperature in each thermal cycle fluctuated near the martensite transformation point(M_s),resulting in the in-situ formation and decomposition of martensite phase.As the peak temperature was greater than M_s,the pre-solidified layers will undergo a cyclic β(?)α′transformation,anda′will decompose into(a+β)phases when the peak temperature drop below M_s.The strength of samples with martensite phases was significantly increased,and the strength will decrease as the decomposition of martensite phases.Meanwhile,the tensile strength of samples without the formation of martensite phases during the solidification process were significantly reduced.The relationship between the process parameters,surface quality,microstructure,and mechanical properties of EBSM-built TA15 titanium alloy samples was established according to the characterization of microstructure and mechanical properties of samples that undergone different thermal cycles.The microstructure evolution of EBSM-built TA15 samples under long-term thermal cycle conditions was studied,and the formation mechanism of gradient structure was clarified.The solidification rate on the surface of the molten pool was higher than that inside,resulting in the formation of an ultra-fine phase region near the free surface with a nano-hardness of 4.7 GPa.Lamellar structure obtained by EBSM did not reach thermodynamic equilibrium,and these lamellar phases will be fused and globurized under high temperature during thermal cycles.Dislocations generated in the rapid solidification process were in a thermodynamically unstable state and will form sub-grain boundaries through slip,climb and recombination under the action of thermal cycling and powder bed preheating.Lamellar phases were divided into segments,leading to the globularization of lamellar phases.The high temperature mechanical properties of EBSM-built TA15 samples under500℃～750℃ were studied,and the mechanism of high-temperature work hardening and softening was revealed.The tensile strength of the sample at 500℃ was 636 MPa,which can satisfy the service requirements under medium temperature conditions.The strength decreased and the plasticity increased gradually with the temperature.The yield ratio increased with the temperature as the temperature range from 500℃ to 650℃,which was decreased with the temperature above 650℃.There was an obvious work hardening phenomenon at 500℃ and 550℃,and dynamic recovery occurred during the tensile process as the temperature increased to 600℃,while dynamic recrystallization played a dominant role in 650℃～750℃. |
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