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.     

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.     

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.     

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.     

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.     

Microstructure developed in the surface layer of Ti-6Al-4V alloy after sliding wear in vacuum

Microstructural changes in the surface layer of Ti-6Al-4V alloy after sliding wear in vacuum have been studied by means of scanning and transmission electron microscopy (SEM and TEM). The wear rates of Ti-6Al-4V alloy in vacuum were measured under different sliding velocities and loads. The experimental results showed that a severely deformed layer with a grain size of 50–100 nm and thickness about 70 μm was formed underneath the worn surface. Under the slower sliding velocities, the substructure of the layer had a high dislocation density, while under higher sliding velocities, twins were found to exist in the substructure. A process by which the deformed layer formed has been proposed and the deformation of materials at the contacting spots of the Ti-6Al-4V sample is discussed.     

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.     

Thermophysical Properties of Solid and Liquid Ti-6Al-4V (TA6V) Alloy

Ti-6Al-4V (TA6V) titanium alloy is widely used in industrial applications such as aeronautic and aerospace due to its good mechanical properties at high temperatures. Experiments on two different resistive pulse heating devices (CEA Valduc and TU-Graz) have been carried out in order to study thermophysical properties (such as electrical resistivity, volume expansion, heat of fusion, heat capacity, normal spectral emissivity, thermal diffusivity, and thermal conductivity) of both solid and liquid Ti-6Al-4V. Fast time-resolved measurements of current, voltage, and surface radiation and shadowgraphs of the volume have been undertaken. At TU-Graz, a fast laser polarimeter has been used for determining the emissivity of liquid Ti-6Al-4V at 684.5 nm and a differential scanning calorimeter (DSC) for measuring the heat capacity of solid Ti-6Al-4V. This study deals with the specific behavior of the different solid phase transitions (effect of heating rate) and the melting region, and emphasizes the liquid state (T > 2000 K).     

Ti-6Al-4V等离子弧焊对接板超塑胀形特性研究

通过自由胀形实验研究了等离子弧焊对接板的超塑胀形性能及影响因素．结果表明等离子弧焊对接板具有良好的超塑胀形性能,其极限胀形高度可超过凹模半径．在胀形过程中,焊缝组织发展成球状α 长条状α组织．焊缝和基体间存在变形不均匀性．在同样的胀形条件下,胀形气压有—最佳数值,气压过大或过小均降低极限胀形高度．最后给出了一个应用等离子弧焊对接板进行超塑胀形的实例．     

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.     

Effect of Si3N4 Addition on the Mechanical Properties, Microstructures, and Wear Resistance of Ti-6Al-4V Alloy

In order to improve the wear resistance of Ti-6Al-4V, different amounts of Si3N4 powder were added into the alloy powder and sintered at 1250℃. Porous titanium alloy with higher wear resistance was successfully fabricated. At sintering temperature, reaction took place and a new hard phase of Ti5Si3 formed. The mechanical properties of the fabricated alloys with different amounts of Si3N4 addition were investigated. The hardness of Ti-6Al-4V, which is the index of wear resistance, was increased by the addition of Si3N4. Amounts of Si3N4 addition have very significant influences on hardness and compressive strength. In present study,titanium alloy with 5 wt pct Si3N4 addition has 62% microhardness and 45% overall bulk hardness increase,respectively. In contrast, it has a 16.4% strength reduction. Wear resistance was evaluated by the weight loss during wear test. A new phase of Ti5Si3 was detected by electron probe microanalyzer （EPMA） and X-ray diffraction （XRD） method. The original Si3N4 decomposed during sintering and transformed into titanium silicide. Porous structure was achieved due to the sintering reaction.     

真空感应凝壳熔炼TC4合金的显微组织和力学性能

针对在航空航天等领域有重要应用背景的TC4合金 ,运用水冷铜坩埚真空感应熔炼炉制备了合金铸锭 ,研究了合金在铸态、热处理和热等静压条件下的显微组织和力学性能 .结果表明 ,铸态TC4合金的晶粒粗大 ,基体为大片状α+ β相组织 ,合金的强度较高 ,塑性低 ;合金经热处理 ,尤其经热等静压处理后 ,组织明显细化 ,塑性提高 .     

Fatigue behavior of Ti-6Al-4V cellular structures fabricated by additive manufacturing technique

Porous titanium and its alloys have been considered as promising replacement for dense implants, as they possess low elastic modulus comparable to that of compact human bones and are capable of providing space for in-growth of bony tissues to achieve a better fixation. Recently, the additive manufacturing (AM) method has been successfully applied to the fabrication of Ti-6Al-4V cellular meshes and foams. Comparing to traditional fabrication methods, the AM method offers advantages of accurate control of complex cell shapes and internal pore architectures, thus attracting extensive attention. Considering the long-term safety in the human body, the metallic cellular structures should possess high fatigue strength. In this paper, the recent progress on the fatigue properties of Ti-6Al-4V cellular structures fabricated by the AM technique is reviewed. The various design factors including cell shapes, surface properties, post treatments and graded porosity distribution affecting the fatigue properties of additive manufactured Ti-6Al-4V cellular structures were introduced and future development trends were also discussed.     

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.     

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.     

Fractographic and microstructural analysis of fatigue crack growth in a Ti-6Al-4V fan disc forging

The constant amplitude fatigue crack growth behaviour of a conventionally (+β) solution treated and aged Ti-6Al-4V fan disc forging was examined by fractographic and microstructural analysis. The crack growth process was complex with many interrelated fracture features. A transition in the fatigue crack growth curve correlated with a change from structure-sensitive to continuum-mode crack growth, primarily in the transformed and aged β grains, and a decrease in fracture surface roughness. The transition was probably caused by the cyclic plastic zone size becoming equal to and exceeding the average platelet packet size. The significance of such transitions for prediction of fatigue crack growth and service failure analysis is discussed.     

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.     

Nanoscale characterization of anodic oxide films on Ti-6Al-4V alloy

The paper analyses, at nanoscale levels, the chemical composition and mechanical properties of the anodic oxide films formed on Ti-6Al-4V alloy by galvanostatic polarization at maximum final voltages of 12-100 V. For the investigations Auger Electron Spectroscopy, Photoelectron Spectroscopy and nanoindentation measurements have been used. The results have shown that anodizing the Ti-6Al-4V alloy produces an oxide film whose thickness depends on the final voltage. The chemical composition is not significantly dependent on the thickness, the film consists of TiO₂ and Al₂O₃. However, the best insulating properties of the films, determined from the growth parameter nm/V, are achieved with a final voltage between 30 and 65 V. Nanohardness and Young's modulus measurements have shown that the anodic films formed by different voltages exhibit similar mechanical properties which is consistent with the results of the surface analysis.     

Improved properties of Ti-6Al-4V by combining isothermal equal-channel angular processing and hot-extrusion

The aim of this work was to study effects of hot extrusion on the microstructure of Ti-6Al-4V (wt-%) alloy processed by ECAP. Firstly, an isothermally Ti–6Al–4V alloy processed by Equal channel angular pressing(ECAP) was preheated at 950°C for 6?min and then hot extruded at 900°C. The hot extrusion minimised the grain size and maximised the mechanical strength. Therefore, it was demonstrated that hot extrusion of Ti-6Al-4V alloys that processed by ECAP could be performed without compromising any mechanical properties. Therefore, it is possible to use the ability to apply a reduced cross-section in hot extrusion for an Ti-6Al-4V processed by ECAP without concern about the reduction of properties.