Microstructural gradients in the superplastic forming of Ti-6Al-4V

The effect of microstructural gradients, introduced by local induction heating, on the behavior of a Ti-6Al-4V sheet in superplastic forming has been investigated. Heat treatment led to a change in the morphology of the α phase present at the start of superplastic deformation, which caused an increase in initial flow stress. This has a significant effect on the strain distribution. Trials using two axially symmetric shapes showed that the effect of microstructural gradients can be adequately predicted—via numerical modeling—and controlled. The technique has the potential to control the thickness distribution in formed parts.     

Ti-6Al-4V在步进轧制中的显微组织变化

对步进轧制在940℃α β区轧制Ti 6Al 4V合金棒材的锥形体取变形量不同的部位,利用光学显微镜、透射电镜观察其组织形貌及位错组态,分析了其变形机制,探讨了材料显微组织随变形量变化的规律及其形变硬化、回复、再结晶的演变过程.实验表明利用步进轧机加工,所得Ti 6Al 4V合金棒材芯部和边部的组织较均匀.     

High-temperature mechanical behavior of Ti-6Al-4V alloy and TiC p /Ti-6Al-4V composite

Mechanical behaviors at 538 °C, including tensile and creep properties, were investigated for both the Ti-6Al-4V alloy and the Ti-6Al-4V composite reinforced with 10 wt pct TiC particulates fabricated by cold and hot isostatic pressing (CHIP). It was shown that the yield strength (YS) and ultimate tensile strength (UTS) of the composite were greater than those of the matrix alloy at the strain rates ranging from approximately 10⁻⁵ to 10⁻³ s⁻¹. However, the elongation of the composite material was substantially lower than that of the matrix alloy. The creep resistance of the composite was superior to that of the matrix alloy. The data of minimum creep strain rate vs applied stress for the composite can be fit to a power-law equation, and the stress exponent values of 5 and 8 were obtained for applied stress ranges of 103 to 232 MPa and 232 to 379 MPa, respectively. The damage mechanisms were different for the matrix alloy and the composite, as demonstrated by the scanning electron microscopy (SEM) observation of fracture surfaces and the optical microscopy examination of the regions adjacent to the fracture surface. The tensile-tested matrix alloy showed dimpled fracture, while the creep-tested matrix alloy exhibited preferentially interlath and intercolony cracking. The failure of the tensile-tested and creep-tested composite material was controlled by the cleavage failure of the particulates, which was followed by the ductile fracture of the matrix.     

Mechanisms of globularization of Ti-6Al-4V during static heat treatment

The mechanisms controlling static globularization of Ti-6Al-4V after deformation and annealing at 900 °C and 955 °C were established. Microstructural observations suggested that the process of globularization can be divided into two stages. The first includes microstructural changes during deformation and the initial stages of static heat treatment; the second occurs during prolonged static annealing. The initial stage consists of segmentation of the lamellae via boundary splitting, whereas microstructural coarsening characterizes the latter stage. Thus, the process of static globularization is only moderately dependent on the formation and evolution of dislocation substructure; the additional driving force is provided by the reduction in interface energy. The duration of the initial stage of static globularization was calculated by estimating the time required for the completion of the boundary-splitting process. The calculations were in excellent agreement with microstructural observations and showed that the duration of the initial stage at 900 °C and 955 °C lasted approximately 10 hours and 1 hour, respectively.     

Microstructural evolution in laser-deposited multilayer Ti-6Al-4V builds: Part I. Microstructural characterization

The macro and microstructure of laser-deposited Ti-6Al-4V has been investigated to determine the evolution of unique microstructural features in mutilayer builds. The macro and microstructures exhibited in the build include large, columnar prior-beta grains, a gradient in the individual alpha-lath thickness between the deposited layers, and the presence of layer bands within each layer, except for the last three layers deposited. The layer band consists of a colony Widmanstätten alpha morphology, while the nominal microstructure between layer bands exhibits a basketweave morphology. Optical microscopy, hardness, and composition measurements were used to determine that the layer-band and gradient morphologies are resultant from the complex thermal history the build experiences and not a result of segregation or oxidation. The gradient alpha and layer-band morphologies form in layer n after the deposition of layer n+3.     

Cavitation-Induced erosion of Ti-6Al-4V

Cavitation-induced erosion has been examined in Ti-6A1-4V in the mill annealed, solution-treat and aged, and beta annealed conditions. Weight loss data show only small differences between heat treatments with the solution-treat and aged microstructure exhibiting the lowest weight loss rates. Sequential micrographs of the same specimen area as a function of erosion time show that initial fracture occurs along the α-β interfaces and along crystallographic slip bands in the α-phase. The early stages of erosion are also dependent on the orientation of the Widmanstatten colonies in the beta annealed condition. These observations strongly suggest that fatigue fracture is important, at least in the early stages of the cavitation erosion process. Depression of the softer α- phase also occurs at short exposure times, and this facilitates fracture and removal of the exposed material;i.e., β-phase or tempered martensite. Examination of the eroded surfaces in the later stages where considerable material has been removed shows little evidence of the underlying microstructure, despite the distinct differences in the micro-structures of the samples tested. Formaly Undergraduate Students at Michigan Technological University     

Diffusion Bonding of Ti-6Al-4V Sheet with Ti-6Al-4V Foam for Biomedical Implant Applications

Advanced metallic bone implants are designed to have a porous surface to improve osseointegration and reduce risks of loosening. An alternative approach to existing surface treatments to create a porous surface is to bond separately produced metallic foams onto the implant. To assess the feasibility of this approach, a Ti-6Al-4V foam was diffusion bonded onto bulk Ti-6Al-4V in an argon atmosphere at temperatures between 1173 K and 1223 K (900 °C and 950 °C) for times between 45 and 75 minutes. These specimens were tested in tension to determine bond quality: failures occurred in the foam, indicating a strong diffusion-bonded interface. The quality of the bond was confirmed by metallographic studies, indicating that this approach, which can also be applied to creating of sandwich with porous cores, is successful.     

Fretting wear behaviour of plasma nitrided Ti-6Al-4V fretted against unnitrided Ti-6Al-4V and alumina counterbodies

Ti-6Al-4V samples were plasma nitrided at 520°C in two environments (nitrogen and a mixture of nitrogen and hydrogen in the ratio of 3:1) for two different time periods (4 h and 18 h). Fretting wear tests were conducted on unnitrided and nitrided samples for 50,000 cycles using two counterbody materials (unnitrided Ti-6Al-4V and alumina). Gross slip prevailed at a normal load of 4.9 N while mixed stick-slip prevailed at 9.8 N. Tangential force coefficient values of plasma nitrided samples were lower than those of unnitrided samples. The tangential force coefficient nearly stabilised after thousand cycles in case of samples tested against Ti-6Al-4V counterbody. On the other hand, it showed a continuously increasing trend in case of specimens tested against alumina counterbody. The samples nitrided for 4 h exhibited higher hardness and lower tangential force coefficient compared to the specimens nitrided for 18 h. The samples nitrided in nitrogen-hydrogen mixture environment exhibited higher hardness and lower tangential force coefficient compared to the specimens nitrided in nitrogen. The samples plasma nitrided in nitrogen-hydrogen mixture for 4 h exhibited the highest hardness and the lowest tangential force coefficient. The wear volume of the plasma nitrided samples was lower than that of the unnitrided samples. Owing to tribochemical reactions, the wear volume of unnitrided and nitrided samples fretted against alumina ball was higher than that of the samples fretted against Ti-6Al-4V. A consistent trend was not observed regarding which nitriding condition would result in lower wear volume at different loads.     

Superplastic deformation of Ti-6Al-4V alloy

The alloy Ti-6-Al-4V deforms superplastically in the temperature range 750 to 950° The most important factor which is responsible for superplastic behavior was found to be the very fine grain size. Strain rate has no direct effect on superplasticity, however when the strain rate is very low (approximately 2 × 10 s), prolonged exposure to high temperature causes grain growth and early failure. The strain rate sensitivity factorm = 0.5 and the apparent activation energyAH = 45,000 cal/mole, which is approximately the same as the activation energy for grain boundary self diffusion of titanium. Both values are independent of strain rate within the range 10 - 2.5 × 10 s. All the experimental points fall in a straight line when plotted as log (εkTd* ²/D_gbGb³) vs log (σ/G) with a slopen = l/m = 2. This is in excellent agreement with the theory of grain boundary sliding accommodated by dislocation motion.     

Low temperature creep of Ti-6 Al-4 V

Creep tests were conducted at 295 K on Ti-6 Al-4 V in the solution treated and aged (4 h at 815 K) condition, and in the as-welded condition. Some aged specimens were tested after pre-straining. Creep stresses ranged from 40 to 90 pct of the aged material yield strength. Results showed that creep was of the primary or transient kind in all cases, and was much greater in welded than in aged material. In general, pre-strains reduced creep, although a strain larger than 10^-3 was needed to do this at the highest creep stress. Activation areas A* were between 10 and 20 b², and thus were similar to tensile results on titanium and its alloys. The microstructural rationale applied to Ti-5 Al-2.5 Sn in earlier work, based on the character of dislocation sources, proved successful in understanding the effects of prestrain in this work. Formerly with Sandia Laboratories, Livermore, Calif.     

Microstructural Investigations of the White and Deformed Layers Close to the Turned Surface of Ti-6Al-4V

In the aircraft industry, along with geometrical and dimensional integrity, the surface integrity of manufactured parts is a necessity. In fact, severe anomalies generated during machining may have a substantial impact on the lifetime of the parts. Nevertheless, these anomalies are not well known in terms of microstructures such as the white layer in titanium alloys. Based on this observation, the present paper deals with microstructural investigations performed on Ti-6Al-4V white and deformed layers generated during turning with a round uncoated carbide insert. The aim of this study is to characterize these anomalies in terms of microstructure and phases. In particular, this study provides a better understanding of metallurgical transformations in the sublayer of machined surfaces through qualitative models.

     

Producing basal textured Ti-6Al-4V sheet

This study determined the conditions under which Ti-6Al-4V sheet with a strong basal plane texture can be produced on a commercial basis. Texture development during all stages of processing starting with the forged sheet bar was followed using pole figures. The influences of sheet bar forging temperature, rolling temperature, rolling procedures and prior texture were examined. The deformation mechanisms by which a basal plane texture can form are discussed and it is shown that (1120) slip principally on {1010} prism planes can account for it. To demonstrate that texture can be controlled on a production basis, two 1.2 m×2.4 m×1.5 mm sheets from each of six heats of Ti-6Al-4V were rolled with a uniform basal plane texture.     

Hydrogen in β transformed Ti-6Al-4V

The internal reactions associated with hydrogen absorbed at ambient temperature by Ti-6A1-4V having a transformedβ microstructure were determined by using X-ray diffraction analysis. Below 650 ppm, the absorbed hydrogen was concentrated primarily in theβ phase causing an increase in the d(200) spacing and considerable X-ray line broadening. The a phase, however, was not significantly affected by the absorbed hydrogen showing no change in the d(1120) and d(1012) spacings and only a finite amount of line broadening. At approximately 650 ppm H, hydride precipitation began at thea-β interface. With increasing hydrogen content, theβ phase d(200) spacing continued to increase, the (200) X-ray line broadening reached a limiting value, and massive hydrides were formed. The data indicates that low concentrations of hydrogen absorbed by a-β titanium alloys can be detected by examining the X-ray line profile of theβ phase. W. D. HANNA, formerly with the McDonnell Douglas Astronautics Co.     

两种3D打印用雾化Ti-6Al-4V粉末的对比研究

本研究分别利用水冷铜坩埚真空感应熔炼气雾化(VIGA-CC)和等离子旋转电极(PREP)两种技术制备出球形Ti-6Al-4V合金粉末,作者利用SEM、同步辐射CT扫描-三维重建和氩气含量测试等分析手段对不同粒径的Ti-6Al-4V合金粉末的孔洞缺陷和氩气含量、硬度值进行了表征。实验结果表明, VIGA-CC粉末粒度分布宽,细粉收得率较多,粉末粒度分布在40~180 μm之间, PREP粉末的粒度分布较窄,主要集中在110~180 μm之间;金属粉末内部的孔隙率、气体含量和孔尺寸随着粉末粒度的增大而增大,且同一粒径范围内VIGA-CC粉末的气孔概率多于PREP粉末;随着粉末粒径减小,粉末截面组织逐渐细化,其硬度值逐渐升高,整体上VIGA-CC粉末硬度值高于PREP粉末。     

Microstructural Characterization of Ti-6Al-4V Metal Chips by Focused Ion Beam and Transmission Electron Microscopy

During machining, the cutting surface of a metal is subjected to high strain rates and temperatures. Due to the small mass of the formed chip, the metal is rapidly quenched, preserving the as-machined microstructure. These extreme conditions are reported to be favorable to form nanograin or ultrafine-grain microstructures. However, detailed investigation of this region is problematic due to the size of the chips and the difficulty in preserving the cutting surface microstructure during traditional transmission electron microscopy (TEM) preparation. This study investigates the use of focused ion beam (FIB) specimen preparation to preserve and TEM to image the microstructure of the secondary deformation zone (SDZ) at the cutting surface in chips of Ti-6Al-4V formed during machining. Use of the FIB allowed precise extraction of a side or transverse view specimen, which preserved the cutting surface to reveal an inhomogeneous microstructure resulting from the nonuniform distribution of strain, strain rate, and temperature. Initial imaging of a conventional TEM foil prepared from the plan view of the cutting surface revealed microstructures ranging from heavily textured to regions of fine grains. Using FIB preparation of a transverse foil, a layered microstructure was observed revealing a variation of fine grains near the cutting surface, which transitioned to coarse grains toward the free surface. At the cutting surface, a 10-nm-thick recrystallized layer was observed capping a 20-nm-thick amorphous layer.     

Excimer laser surface processing of Ti-6Al-4V

We have examined the effect of surface processing in air, using excimer laser light at 248 nm wavelength, on the oxygen content, microstructure, and surface hardness of Ti-6Al-4V. Processing with a single pulse results in the transformation of theα +β material toα′ martensite. Multiple pulse processing results in rapid incorporation of oxygen in the material. Oxygen initially dissolves in the material in the liquid phase. As the concentration exceeds the solid solubility limit during solidification, TiO particles precipitate. In contrast to equilibrium oxidation processes in Ti, only TiO is observed as an oxidation product; further processing results in increased oxygen incorporation and an increased volume fraction of TiO but no other oxides of Ti. The TiO particle size is a function of the oxygen concentration and the number of pulses, with some grain growth occurring after many pulses. The effects of solution hardening by dissolved oxygen and precipitation hardening by the TiO are identifiable as functions of oxygen concentration and mean free path between particles, respectively. A maximum surface hardness almost twice that of electropolished Ti-6Al-4V is observed.     

Deformation characteristics of age hardened Ti-6Al-4V

A commercial Ti−6Al−4V alloy with an equiaxed grain shape was investigated after solution annealing at 810°C and after aging at 550 and 350°C. Age hardening at both temperatures produced significant increases in Young's modulus and yield strength. Finely dispersed α₂(Ti₃Al) precipitates formed within the α phase upon aging at 550°C, but not when aging at 350°C. However, there is evidence of order, probably of oxygen, in the α grains of specimens which were aged at 350°C. The formation of the ordered Ti₃Al precipitates at 550°C and the occurrence of oxygen ordering at 350°C can account for the increases in Young's modulus and yield strength. since January 1977 with General Electric Co., Lighting Research Division, Nela Park, Cleveland, OH. KANAY GAZIOGLU, formerly with DFVLR, is deceased.     

Ti-6A14V合金表面改性技术

Ti-6Al4V合金作为一种重要的钛合金,其使用量占到了钛合金总使用量的75％～85％,但其耐磨性差、阻燃性差、疏水疏冰性能差、生物相容性不理想等性能缺陷在一定程度上限制了其在某些领域中的应用。首先对Ti-6Al4V合金在各个领域应用时,其性能缺陷的表现形式及危害进行了概述,然后介绍了目前改善Ti-6Al4V合金性能缺陷所普遍采用的以及具有创新性的表面改性技术,评述了部分表面改性技术的优缺点,最后提出了需对Ti-6Al4V合金表面改性技术进一步研究的方向。