Deformation behaviors of as-built and hot isostatically pressed Ti-6Al-4V alloys fabricated via electron beam rapid manufacturing

The deformation behavior of as-built and hot isostatically pressed(HIP) Ti-6Al-4V alloys fabricated using electron beam rapid manufacturing(EBRM) were investigated in this work.The deformation characteristics were characterized using a laser scanning confocal microscope and electron back-scattered diffraction(EBSD).In the as-built sample,prismatic slip was the main mode of deformation,as well as a small amount of basal slip and cross-slip.Some planar slip lines with large length scales were observed across severalα lamellae.After hot isostatical pressing,prismatic and basal slip were the main mode of deformation,accompanied by abundant cross-slip and multiple slip,and most of the slip lines were blocked within an a lamellae.These differences in deformation behavior were associated with the coarsening of a laths and the more retained p phase after HIP compared to the as-built alloy.More cross-slip and multiple slip can lead to superior elongation-to-failure and a greater strain hardening effect in the HIP alloy compared to the as-built sample.     

Ti3Al基合金与Ti-6Al-4V合金TLP连接接头的组织转变

采用TiZrNiCu合金作为中间层材料研究了Ti3Al基合金与Ti-6Al-4V合金的瞬间液相(TLP)扩散连接接头成分、组织转变及显微硬度.研究结果表明,连接温度和连接时间对接头成分和组织有较大的影响.随着连接温度的提高和连接时间的延长,接头中元素分布趋于均匀,连接区宽度增大.连接温度为850℃和900℃时,液相的残留使得接头中存在Ti-Cu金属间化合物.当连接温度为950℃,连接时间为30min时,等温凝固的完成使Ti-Cu金属间化合物从接头中消失.随着连接温度的提高和连接时间的延长,接头连接区硬度降低.当连接温度为950℃,连接时间为30min时,接头硬度分布较均匀.     

Additive manufacturing of Ti-6Al-4V lattice structures with high structural integrity under large compressive deformation

Additively manufactured Ti-6 Al-4 V lattice structures have found important niche applications. However, they often show insufficient compressive ductility or insufficient structural integrity. In this study,a batch of 45 octahedral Ti-6 Al-4 V lattice structures was manufactured in three different strut diameters(0.5, 1.0, 1.5 mm) by selective electron beam melting(SEBM). The influence of post-SEBM annealing on the compressive deformation characteristics of the lattice structure was investigated. The as-built Ti-6 Al-4 V lattices fragmented when the compressive strain reached 13%–23% depending on strut diameter.Annealing at 950?C(β transus temperature: 995?C) only slightly improved the compressive ductility of the lattice structures. However, annealing at 1050?C(β-annealing) fundamentally changed the compressive deformation mode of the lattice structures. The resultant compressive stress-strain curve was featured by a long smooth plateau and no facture occurred even after significant densification of the lattice structure had taken place(50% of compressive strain).     

Fracture mechanics based fretting fatigue life predictions in Ti-6Al-4V

A fracture mechanics based crack propagation analysis is developed to work directly with the output of a contact mechanics stress analysis for fretting fatigue. A series of remote load fatigue tests were conducted on specimens that had previously been subjected to fretting fatigue loading conditions. The growth of these prior fretting induced cracks were monitored and compared to results from the crack propagation analysis. A combined fatigue crack formation and propagation analysis was then applied to other fretting fatigue experiments with good success. The creation of fretting fatigue stress-life curves is also demonstrated.     

A study on the hot corrosion behavior of Ti-6Al-4V alloy

The titanium alloys are potential materials for high temperature applications in turbine components due to their very high temperature strength and lightweight properties. However, hot corrosion is a life-limiting factor when Ti alloys are exposed to different chemical environments at high temperature. In the present paper, hot corrosion behavior of Ti-6Al-4V (Ti-31) alloy in different salt environments viz. air, Na₂SO₄-60% V₂O₅ and Na₂SO₄-50% NaCl at 750 °C was studied. The parabolic rate constants were calculated for different environments from the thermo-gravimetric data obtained for the samples and they show that corrosion rate is minimum in air when compared to chemical environment. The scale formed on the samples upon hot corrosion was characterized by using X-ray diffraction (XRD), SEM, and EDAX analysis to understand the degradation mechanisms.     

Selective electron beam manufactured Ti-6Al-4V lattice structures for orthopedic implant applications: Current status and outstanding challenges

Additively manufactured Ti-6Al-4V lattices display unique mechanical and biological properties by virtue of their engineered structure. These attributes enable the innovative design of patient-specific medical implants that (i) are conformal to the intended surgical geometry, (ii) mimic the mechanical properties of natural bone, and (iii) provide superior biological interaction to traditional implants. Selective electron beam melting (SEBM) is an established metal additive manufacturing (AM) process that has enabled the design and fabrication of a variety of novel intricate lattices for implant applications over the last 15?years. This article reviews the technical and clinical characteristics of SEBM Ti-6Al-4V lattices, including (i) the SEBM process and its capabilities, (ii) the structures of human bones with an exhaustive list of corresponding mechanical properties from literature, (iii) the mechanical properties of SEBM Ti-6Al-4V lattices of various designs and their shortcomings when compared to human bones, (iv) microstructural control of SEBM Ti-6Al-4V lattices for improved performance, (v) the lattice manufacturability and associated geometric errors, and (vi) clinical cases. Existing literature on the mechanical response of SEBM Ti-6Al-4V lattice structures is exhaustively evaluated for documentation quality using established theoretical models. This extensive data-set allows novel insights into the effect of lattice design on mechanical response that is not possible with the individual data; and provides a comprehensive database for those who are actively involved in patient-specific SEBM implant design. On this basis, outstanding challenges and research opportunities for SEBM Ti-6Al-4V lattices in the biomedical domain are identified and discussed.     

Effect of mean stress on fretting fatigue of Ti-6Al-4V on Ti-6Al-4V

Fretting fatigue tests of Ti‐6Al‐4V on Ti‐6Al‐4V have been conducted to determine the influence of stress amplitude and mean stress on life. The stress ratio was varied from R=−1 to 0.8. Both flat and cylindrical contacts were studied using a bridge‐type fretting fatigue test apparatus operating either in the partial slip or mixed fretting regimes. The fretting fatigue lives were correlated to a Walker equivalent stress relation. The influence of mean stress on fretting fatigue crack initiation, characterized by the value of the Walker exponent, is smaller compared with plain fatigue. The fretting fatigue knockdown factor based on the Walker equivalent stress is 4. Formation of fretting cracks is primarily associated with the tangential force amplitude at the contact interface. A simple fretting fatigue crack initiation metric that is based on the strength of the singular stress field at the edge of contact is evaluated. The metric has the advantage in that it is neither dependent on the coefficient of friction nor the location of the stick/slip boundary, both of which are often difficult to define with certainty a priori.     

Targeted heat treatment of additively manufactured Ti-6Al-4V for controlled formation of Bi-lamellar microstructures

Laser powder bed fusion (L-PBF) was utilized to produce specimens in Ti-6Al-4V,which were subjected to a bi-lamellar heat treatment,which produces microstructures consisting of primary α-lamellae and a fine secondary α-phase inside the inter-lamellar β-regions.The bi-lamellar microstructure was obtained as (i)a direct bi-lamellar heat treatment from the asbuilt condition or (ii) a bi-lamellar heat treatment preceded by a β-homogenization.For the bi-lamellar treatment with β-homogenization,cooling rates in the range 1-500 K/min were applied after homogenization in β-region followed by inter-critical annealing in the α + β region at various temperatures in the range 850-950 ℃.The microstructures were characterized using various microscopical techniques.Mechanical testing with Vickers hardness indentation and tensile testing was performed.The bi-lamellar microstructure was harder when compared to a soft fully lamellar microstructure,because of the presence of fine α-platelets inside the β-lamellae.Final low temperature ageing provided an additional hardness increase by precipitation hardening of the primary α-regions.The age hardened bi-lamellar microstructure shows a similar hardness as the very fine,as-built martensitic microstructure.The bi-lamellar microstructure has more favorable mechanical properties than the as-built condition,which has high strength,but poor ductility.After the bi-lamellar heat treatment,the elongation was improved by more than 250 ％.Due to the very high strength of the as-built condition,loss of tensile strength is unavoidable,resulting in a reduction of tensile strength of～18 ％.     

Strain Accumulation in Ti-6Al-4V during Fatigue at High Mean Stress

The effect of mean stress and frequency on the high cycle fatigue behavior of Ti-6Al-4V has been investigated. It has been shown that a transition in the fatigue behavior occurs at a stress ratio of approximately 0.7. Above this value, the material exhibits measurable strain accumulation and necking. Since Ti-6Al-4V is susceptible to room temperature creep, an empirical model was developed using static creep data in an attempt to predict the cyclic behavior of the material. The model was unable to account for the large amounts of strain seen experimentally. In addition, closer examination of the data revealed that the deformation was more closely related to the number of cycles than to time.     

Preparation method of low-oxygen Ti-6Al-4V alloy by solid state re-deoxidation using calcium

The oxygen concentration in commercial Ti-6Al-4?V alloys was reduced to less than 400?ppm in this study by the method of solid state re-deoxidation, using calcium as a reductant. The concentration of oxygen in the deoxidised Ti-6Al-4?V alloy was 630?ppm at the optimum deoxidation temperature of 1000°C. When the degree of vacuum was increased and re-deoxidation was carried out, the oxygen concentration decreased to 355?ppm. Therefore, it is possible to prepare a Ti-6Al-4?V alloy with an oxygen concentration of less than 400?ppm by using the solid state re-deoxidation method at a high degree of vacuum of 1.5?×?10-6 Torr.     

Microstructure and properties of Ti-6Al-4V fabricated by low-power pulsed laser directed energy deposition

Thin-wall structures of Ti-6Al-4V were fabricated by low-power pulsed laser directed energy deposition. During deposition, consistent with prior reports, columnar grains were observed which grew from the bottom toward the top of melt pool tail. This resulted in a microstructure mainly composed of long and thin prior epitaxial β columnar grains (average width ≈200 μm). A periodic pattern in epitaxial growth of grains was observed, which was shown to depend upon laser traverse direction. Utilizing this, a novel means was proposed to determine accurately the fusion boundary of each deposited layer by inspection of the periodic wave patterns. As a result it was applied to investigate the influence of thermal cycling on microstructure evolution. Results showed that acicular martensite, α' phase, and a small amount of Widmanstätten, α laths, gradually converted to elongated acicular α and a large fraction of Widmanstätten α laths under layer-wise thermal cycling. Tensile tests showed that the yield strength, ultimate tensile strength and elongation of Ti-6Al-4V thin wall in the build direction were 9.1%, 17.3% and 42% higher respectively than those typically observed in forged solids of the same alloy. It also showed the yield strength and ultimate tensile strength of the transverse tensile samples both were ˜13.3% higher than those from the build direction due to the strengthening effect of a large number of vertical β grain boundaries, but the elongation was 69.7% lower than that of the build direction due to the uneven grain deformation of β grains.     

Study on the flow properties of Ti-6Al-4V powders prepared by radio-frequency plasma spheroidization

Spherical Ti-6Al-4V powders were prepared using radio-frequency plasma spheroidization. A laser particle size analyser, a scanning electron microscope, an X-ray diffractometer and a Freeman FT4 powder rheometer were used to analyse the granulometric parameters, micro-morphologies, phase constitutions and flow properties of the raw and the spheroidized powders, respectively. The spheroidized powders exhibited an almost 100% degree of sphericity, smooth surfaces, favourable dispersion and narrow particle size distribution under appropriate plasma technological parameters. The average particle size of the spheroidized powders increased slightly as compared with that of the raw powders. In addition, the spheroidized powders exhibited higher conditioned bulk density and improved flow properties (including the dynamic flow properties, aeration, compressibility, permeability and shear properties) as compared with those of the raw powders.     

Laser Surface Modification of Ti-6Al-4V with Addition of Pr

The behaviour of oxidation at elevated tem-peratures and fretting in the laser-alloyed layer ofTi-6Al-4V with the addition of Pr was studied.The results show that the addition of Pr changes thestructure of oxide scale of Ti-6Al-4V,controls theshort-range diffusion of oxygen to thescale/substrate interface and increases the adhe-sion and ductility of the scale,thus changing theoxidation kinetics and considerably reducingoxidation rate.The analysis of fretting test showsthat the existence of high hardness layer in the al-loyed zone,fine dendrites perpendicular to the sur-face of the high hardness layer and the oxide scaleproduced during fretting at elevated temperaturesare all beneficial to the improvement of wear resist-ance.     

The effect of angle on dovetail fretting experiments in Ti-6Al-4V

The objective of this work was to compare the fretting fatigue performance of Ti‐6Al‐4V dovetail specimens on Ti‐6Al‐4V pads having various contact angles typical of engine hardware; 35°, 45° and 55° dovetail angles were considered. The dovetail fixtures were instrumented with strain gages so that the local normal and shear contact forces could be calculated. The contact force hysteresis loops were recorded showing the stick‐slip history. At R= 0.1, gross slip was observed for several thousand cycles followed by partial slip after the average coefficient of friction increased. At R= 0.5, gross slip was present only during the first half cycle. During partial slip, the slope of the shear versus normal force was a function of the dovetail angle. The local contact loads, therefore, differed for the same remotely applied force. Despite this, the fretting fatigue life depended primarily on the remotely applied load not dovetail angle.     

Hydrogen Treatment of Cast Ti-6Al-4V Alloy

This paper presents the results of an investiga-tion of the effect of hydrogen treatment onmicrostructures and tensile and low cycle fatigueproperties of a Ti-6Al-4V cast alloy.The phasetransformation and the refining mechanism of thecast microstructure during the process of hydrogentreatment were studied.It was found that afterhydrogen treatment,the coarse Widmanstttenstructure of the as-cast Ti alloy was transformedinto a very fine and equiaxed α+β microstructurewithout any GBα phase.The tensile strength andductility and the low cycle fatigue life of thehydrogen treated specimens were significantly im-proved.     

TI-6Al-4V合金置氢-除氢组织与力学性能

为研究除氢处理对置氢钛合金组织与性能的影响,对Ti-6Al-4V合金在不同参数条件下进行了置氢与除氢处理,采用光学显微镜分析了置氢-除氢处理过程中Ti-6Al-4V合金微观组织的演化规律,通过室温拉伸试验研究了置氢-除氢处理后Ti-6Al-4V合金的力学性能,探讨了Ti-6Al-4V合金置氢-除氢组织与力学性能之间的相...     

基于气体捕捉法的泡沫Ti-6Al-4V等温发泡规律研究

为了确定气体捕捉法制备泡沫Ti-6Al-4V等温发泡过程中孔隙率和微观孔洞的变化规律,在不同发泡温度及发泡时间下制备了泡沫Ti-6Al-4V.运用阿基米德原理对泡沫Ti-6Al-4V的孔隙率进行测量,通过OM和SEM对其微观特征进行观察.研究表明:泡沫Ti-6Al-4V的孔隙率及孔径均随等温发泡温度升高而增加;但当发泡温度大于950℃时,孔隙率和孔径均减小,且孔洞形态由球形变成多边形,这是由于基体内生成大尺寸β相造成的.增加发泡时间能以促进孔洞长大的方式提高泡沫Ti-6Al-4V的孔隙率,球形孔洞数量随着发泡时间的增加逐渐增多.经950℃/10 h发泡得到了孔隙率34.2%、孔径平均值156μm、孔洞为球形且分布弥散的泡沫Ti-6Al-4V.     

Mechanical properties of Ti-6Al-4V specimens produced by shaped metal deposition

Abstract

Shaped metal deposition is a novel technique to build near net-shape components layer by layer by tungsten inert gas welding. Especially for complex shapes and small quantities, this technique can significantly lower the production cost of components by reducing the buy-to-fly ratio and lead time for production, diminishing final machining and preventing scrap. Tensile testing of Ti-6Al-4V components fabricated by shaped metal deposition shows that the mechanical properties are competitive to material fabricated by conventional techniques. The ultimate tensile strength is between 936 and 1014 MPa, depending on the orientation and location. Tensile testing vertical to the deposition layers reveals ductility between 14 and 21%, whereas testing parallel to the layers gives a ductility between 6 and 11%. Ultimate tensile strength and ductility are inversely related. Heat treatment within the α+β phase field does not change the mechanical properties, but heat treatment within the β phase field increases the ultimate tensile strength and decreases the ductility. The differences in ultimate tensile strength and ductility can be related to the α lath size and orientation of the elongated, prior β grains. The micro-hardness and Young’s modulus are similar to conventional Ti-6Al-4V with low oxygen content.     

Failure analysis of a Ti-6Al-4V rotating main rotor component from an army helicopter

This paper discusses the failure analysis of a Ti-6Al-4V rotating main rotor component and contrasts the perspectives of the design/mechanical engineer and the manufacturing/materials engineer. Cracking initiated at mechanical marks located on the surface of the outer diameter of a planetary post at the transition radius and was propagated by high-cycle fatigue in service. These crack initiation defects were most likely produced by a machining or a surface finishing tool. Fractographic evidence suggests that high stresses were also encountered in service and played a significant role in the premature cracking of these components. The debate centers on whether the components would have failed in the absence of the surface defects. There were several manufacturers of this component, which are compared in this study. The workmanship on the outer diameter of the planetary post at the transition radius of a carrier that had not failed, manufactured by Company B, was superior to that of the two cracked carriers produced by Company A. However, analysis of the service conditions indicates that the components may have been loaded near the yield strength of the material.     

Mechanical properties of Ti-6Al-4V specimens produced by shaped metal deposition

Shaped metal deposition is a novel technique to build near net-shape components layer by layer by tungsten inert gas welding. Especially for complex shapes and small quantities, this technique can significantly lower the production cost of components by reducing the buy-to-fly ratio and lead time for production, diminishing final machining and preventing scrap. Tensile testing of Ti-6Al-4V components fabricated by shaped metal deposition shows that the mechanical properties are competitive to material fabricated by conventional techniques. The ultimate tensile strength is between 936 and 1014 MPa, depending on the orientation and location. Tensile testing vertical to the deposition layers reveals ductility between 14 and 21%, whereas testing parallel to the layers gives a ductility between 6 and 11%. Ultimate tensile strength and ductility are inversely related. Heat treatment within the α+β phase field does not change the mechanical properties, but heat treatment within the β phase field increases the ultimate tensile strength and decreases the ductility. The differences in ultimate tensile strength and ductility can be related to the α lath size and orientation of the elongated, prior β grains. The micro-hardness and Young’s modulus are similar to conventional Ti-6Al-4V with low oxygen content.