Study On Process,Microstructure Evolution And Mechanical Properties Of TA15 Titanium Alloy By Selective Laser Melting

Titanium alloy is widely used in aerospace industry because of its super high specific strength and excellent corrosion resistance.However,due to its high activity,low thermal conductivity and high deformation resistance,it is very difficult to process it by traditional manufacturing methods.In addition,aerospace parts tend to be functional,lightweight and structurally integrated,making it difficult for traditional manufacturing technology to meet those demands.Based on the concept of"discrete-accumulation"additive manufacturing,selective laser melting(SLM)technology can directly form complex structural parts such as lattice sandwich,special curved surface and inner cavity runner,which provides a reliable solution for the manufacturing of aerospace titanium alloy parts with complex structure.At present,the research on titanium alloy manufactured by SLM focuses on Ti6Al4V titanium alloy.Compared with Ti6Al4V,the near?titanium alloy TA15(Ti-6.5Al-2Zr-1Mo-1V)shows better performance in strength(under both room temperature and high temperature),fracture toughness,fatigue limit,stress resistance,corrosion resistance and welding performance,and is widely used to manufacture various load-bearing structural parts and welded structural parts in the aerospace field.However,there is still a lack of research on TA15 alloy formed by SLM,which leads to the development and application of TA15 lagging behind.The research on the development of SLM preparation technology of TA15 alloy is conducive to broaden its application in aerospace complex structural parts.Therefore,the densification mechanism,microstructure and mechanical properties,heat treatment control method and surface quality optimization of TA15 formed by SLM are systematically studied in this paper.The main research contents and achievements are as follows:(1)Study on densification mechanism of TA15 alloy formed by SLM.The effect of SLM process parameters on the single track characteristics and density of TA15 alloy was analyzed.The mechanism of single track forming and densification was revealed.Combining with response surface methodology,the SLM forming process interval of TA15 alloy with high density was obtained.The results show that the relationship between the single track width(W)and the laser linear energy density(E _L)is W=406.85?E_L^{0.6 311}.With the increase of laser linear energy density,the depth of single track molten pool deepens rapidly and changes into"keyhole mode".In the"conduction mode",the inadequate of scanning distance will produce approximately circular pores in the edge area of the sample.TA15 alloy samples with high density can be formed in the process range of laser line energy density of 0.165-0.195 J/mm and overlap rate of15-40%.Considering the forming efficiency and forming density,the optimal forming process was determined as follows:laser power 190 W,scanning speed 1050 mm/s,scanning spacing 100?m,sample density was 99.98%.(2)Study on the microstructure and mechanical properties of TA15 alloy formed by SLM.Under the optimal process,the phase distribution,morphology,size distribution,grain orientation and substructure of the microstructure of TA15 alloy formed by SLM were systematically studied.The mechanical properties of TA15samples with different forming directions were tested,and the microstructure evolvement regularity,strengthening mechanism and anisotropy were explored.In addition,the effect of SLM parameters on the microstructure and mechanical properties of TA15 alloy was studied in the range of high density forming process.The results show that the microstructure of TA15 alloy formed by SLM is composed of size-graded??martensite and a small amount of?phase.The martensite contains a large number of sub-micron size{1011}twins and ultra-high density dislocations.Martensite structure has a variety of orientations and presents a feature of weak texture.The specimens have high strength(1289-1296 MPa),hardness(HV 395-398)and relatively low plasticity(6.8-7.6%).No obvious mechanical anisotropy is detected.Under similar linear energy density,the decrease of scanning speed will cause the in-situ decomposition of??martensite in TA15 alloy,which leads to the increase of plasticity but the decrease of strength and hardness.(3)Study on heat treatment process of TA15 alloy formed by SLM.The microstructure and mechanical properties of TA15 alloy formed by SLM under different heat treatment processes were studied.The microstructure evolution of TA15 alloy during heat treatment was analyzed,and the optimized heat treatment process was obtained.The results show that during annealing,the fine acicular??martensite of TA15alloy is gradually decomposed into?+?phase.The?phase is lamellar and transforms into equiaxed?grain at higher annealing temperature,and the?phase mainly precipitates between??martensite and the enrichment of internal lattice defects.After annealing,the microstructure of the sample presents a hierarchical structure and poor grain orientation distribution similar to that of the SLMed sample,which has strong tissue heredity.With the increase of annealing temperature,the strength and hardness of the samples decrease,and the elongation increases.The tensile strength and elongation of the samples annealed at 900?are 1117 MPa and 11.2%respectively due to the dispersion strengthening of?phase,showing the most excellent comprehensive mechanical properties.Double annealing will not obviously change the microstructure and grain size of the annealed samples,but will precipitate more?phase and show better plasticity.Heat treatment above the?phase transition temperature will lead to the transformation of columnar primary?-grains into widmanstatten structure with coarse equiaxed?grains,which leads to the lowest plasticity of the samples.(4)The optimization of surface quality of TA15 alloy formed by SLM.According to the process characteristics of SLM,the surface of formed parts is classified.The effects of process parameters and scanning strategy on the forming quality of different types of surface of TA15 formed by SLM are studied comprehensively,and the surface forming mechanism is proved.The results show that the remelting strategy can effectively improve the surface quality of SLM forming top surface.The influence of scanning speed and scanning distance on the forming quality of the top surface is highly significant.By reducing the scanning speed of the contour,the phenomenon of sticking powder on the side surface of TA15 formed by SLM can be effectively reduced,and the forming quality of the sample side surface can be improved.With the decrease of energy densityE_F,the forming quality of the lower surface improves at first and then deteriorates.Heat accumulation caused by laser heating on the powder bed is the main reason for the poor forming quality of the lower surface.The quality of the lower surface can be effectively judged by recording the cooling time of the molten pool.The cross-remelting strategy and the non-remelting strategy can obtain relatively better quality of the lower surface under the condition of lower and higher laser energy density respectively.