The Microstructure And Damage Tolerance Properties Of Additive Manufactured Ti55531 Alloy

High strength&toughness titanium alloys have attracted extensive attention in serving as structural component in aerospace industry owing to their high specific strength,outstanding damage tolerance property and well corrosion resistance.With the pursuit of“less weight more efficiency”in designing structural components with high damage tolerance,the geometric configuration of titanium units is becoming much more complicated,leading to soaring demand for manufacturing technologies.Additive manufacturing（AM）is a rapid manufacturing technology featured by informatization,intellectualization and personalization.As the conventional limitations in the design and shaping of components are no longer problematic in AM,the integrated manufacturing of geometrically complicated titanium structural components is able to be realized.The powder bed fusion（PBF）technology with a high energy heating source is well-known for high manufacturing precision,short processing period and high utilization of raw materials.It is highly suitable for realizing the precise manufacturing of the structural components made from high strength&toughness Ti-alloys.However,the typical PBF technology shows a point-to-point sintering and layer-by-layer cumulative deposition building mode,the interaction time between high energy heat source and precursor powder is quite short.As a result,multiple unstable physical process and directional thermal cycle prompt complicated evolution of microstructure and ultimate mechanical properties in as-built high strength&toughness Ti-alloys.Therefore,the structure-function relationship of manufacturing process,microstructural evolution,and damage tolerance property remains unsolved in PBF prepared high strength&toughness Ti-alloys.Among a variety of the high strength&toughness Ti-alloys,Ti-5Al-5Mo-5V-3Cr-1Zr（Ti55531）alloy,which is one of the most widely used commercial Ti-alloys in aviation industry,has excellent comprehensive properties.As a typicalβTi-alloy,Ti55531 contains high concentration ofβstabilizers,resulting in a lowβ-transus temperature（T_β）.Consequently,the phase transformation of Ti55531 during a rapid cooling process is complex,and the microstructure evolution is sensitive to the thermal effect.However,the deep-rooted feature during PBF of Ti-alloys is the extremely high cooling rate,and steep thermal gradient in the molten metal pool.Therefore,the solidification process of PBF technology presents typical non-equilibrium solidification characteristics.It is thus easy to imagine that Ti55531 alloy also experiences a complex solidification in PBF process.Further investigation found that Ti55531 is heat treatable,because a variety of microstructures and mechanical properties can be designed through the manipulation of heat treatment parameters.Nevertheless,the feasibility of conventional heat treatment remains unclear in tailoring the non-equilibrium microstructure of PBF-fabricated Ti-alloys,and the structure-activity relationship between the resulting microstructure and damage tolerance property is still in the blank stage.Based on the above discussions,this investigation aims to explore the fabricability and microstructural evolution of high strength&toughness Ti-alloys in PBF process.Two typical PBF technologies,laser selective melting（SLM）and electron beam selective melting（EBM）,are selected to conduct fabrication of Ti55531 alloys.Primarily,the coupling relationship between processing parameters and fabricability is systematically studied.Furthermore,the microstructural evolution and phase precipitation mechanism of as-fabricated Ti55531 alloy during PBF process are elaborated.Finally,the effect of heat treatment on the microstructure and corresponding damage tolerance mechanical properties are investigated.Based on these experiments,the following progresses can be drawn:（1）Plasma rotating electrode process（PREP）and electrode induction melting-gas atomization（EIGA）are employed to prepare the Ti55531 spherical powder.Investigations by scanning electron microscope（SEM）,particle size distribution,chemical composition and flowability test demonstrate the good quality of the spherical powder.Subsequently,orthogonal experiments are designed to reveal the influence of the processing parameters of SLM and EBM on Ti55531 spherical powder.Therefore,the optimized parameter combination is obtained.The results show that the energy input of both SLM and EBM has a significantly influence on the surface morphology and density of as-fabricated Ti55531.In detail,insufficient energy input leads to a large surface roughness and incomplete melting defects.Excessive energy input induces a“Keyhole effect”during SLM process,which further causes circular/spherical pores.（2）The understanding of microstructure evolution and the mechanism of phase transformation are crucial for the design of post heat treatment.Therefore,the intragranular structure of Ti55531 fabricated by optimized processing parameters is systematically studied.Epitaxial growth is provoked during SLM and EBM process because of the steep thermal gradient,resulting in columnar priorβgrains with strong<100>_βorientation along the building direction.Based on the multiple physical field simulation,the phase transformation mechanism of SLM-fabricated Ti55531 is further revealed.It is found that the microstructure of SLM-built sample is mainly composed of metastableβphase with fewω,O?,nano-sizedαprecipitates and low angle grain boundaries dwelling inside.The complex phase transformation is induced by rapid cooling rate and various conditions including thermal stress and massive convection of melts.The SLM-fabricated Ti55531 exhibits a performance of low strength but excellent ductility.（3）The EBM-fabricated Ti55531 presents a typicalα+βlamellar structure due to the in-situ heat treatment of EBM process.Furthermore,the complex thermal effect of EBM causes increasingly heat accumulation along build height.In combination,the high preheating temperature in electron beam melting,the lowβtransus（850℃）in Ti-5Al-5Mo-5V-3Cr-1Zr,and the increasingly accumulated heat with build height result in an in-situ formed graded hierarchical microstructure from anα+βlamellar microstructure at the specimen bottom to anα_p+α_s bi-lamellar microstructure at the specimen top.No obviousαvariation selection is observed in EBM-fabricated Ti55531.Graded mechanical properties of Ti55531 with increased microhardness and strength are observed from bottom to top,i.e.increasing microhardness,yield strength and ultimate tensile strength while decreasing elongation as the build height increases.（4）After characterizing the microstructures of as-built samples prepared by SLM and EBM,a batch of post heat treatments are conducted to manipulate microstructures and enhance the mechanical properties.The post annealing above T_βwill induce Plateau-Rayleigh stability,prompting the transformation of priorβgrains from columnar to equiaxed so that the mechanical anisotropy is effectively eliminated.On the other hand,the post annealing below T_βpreserves the primitive morphology of columnarβgrains by the hindering effect ofαphase.The cyclic heat treatment promotes the globularization of rod-likeαphase to equiaxed,and the underlying mechanism correlates to a diffusion theory.Cooling rate determines the precipitation of lamellar structure,which could be realized by the heat treatment strategies such as triplex or hot isostatic pressing（HIP）.Based on the results of tensile tests,the yield strength and ductility of the post heat treated sample with bimodal structure is comparable to those of the forged one.Higher strength and lower ductility are found in lamellar structure.The best combination of strength and plasticity is obtained in the sample treated by HIP.（5）Damage tolerance test is conducted to investigate the damage and fracture mechanism of Ti55531 with different microstructures.Based on the results of K_IC and da/d N tests,the specimen with lamellar structure shows excellent resistance of crack propagation,because crack propagation is hindered byαlamella and the crack propagation paths become more tortuous.Theα_p phase in the bimodal structure is prone to yield,leading to crack propagation and fracture at this position.Therefore,the damage tolerance of the samples with bimodal structure is lower than that of the lamellar structure.Defects cause a negative effect on damage tolerance property,but hot isostatic pressing is an effective way to eliminate the defects so that mechanical properties can be improved.The major innovations of this study are as follows:（1）The effects of processing parameters of PBF technology on the printability are explored in the high strength&toughness Ti55531 alloy.The effects of processing parameter combination on the surface morphology and density of PBF-fabricated Ti55531 are explored through manipulating the energy input.（2）The microstructure evolution and phase transformation mechanism of Ti55531 alloy fabricated by PBF are revealed.The solidification structure and intragranular sub-structure of Ti55531 alloy fabricated by SLM and EBM are analyzed.These results unveil the complex solidification mechanism and phase transformation mechanism of Ti55531 alloy during SLM and EBM processes.The metastable mesophase caused by the solidification characteristics of SLM is identified.The relationship between thermal effect and microstructure evolution of high strength&toughness Ti-alloys during EBM is established.（3）The effect of heat treatment on the microstructure evolution of Ti55531alloy is evaluated.Conventional and optimized heat treatments are developed to evaluate the effect of heat treatment on the microstructure evolution of as-fabricated Ti55531 alloy,and the relationship between the microstructure evolution and the mechanical properties is also elaborated.（4）The structure-activity relationship between microstructure and damage tolerance mechanism is established.In this work,the damage tolerance of the lamellar and bimodal microstructure of the samples after heat treatment is studied.The fracture damage mechanism of Ti55531alloy fabricated by AM with different microstructures is revealed,and the relationship between crack propagation and microstructure is established.In this work,the solidification and phase transformation mechanisms of PBF manufactured high strength&toughness Ti-alloy are clarified.The results further demonstrate that PBF-fabricated metastableβTi-alloy has a strong heat treatability,which can provide different heat-treated microstructures and damage tolerance property.This work provides foundation for the processing parameter optimization and airworthiness certification of high strength&toughness Ti-alloys.