Superplastic deformation of Ti-6Al-4V extruded tube

Superplastic forming using conventional alloys and product forms may be a cost effective route for component manufacture. In this paper superplastic deformation of Ti-3Al-4V extruded tube is examined in terms of the strain anisotropy, microstructural changes, the texture and the post-formed tensile properties. Superplastic deformation with low flow stresses over a wide range of strain rates was observed. Strain anisotropy and surface roughening occurred during deformation and was associated with the two-phase, aligned and banded microstructure. A small reduction in the tensile properties was found and changes in the texture were noted after superplastic deformation.     

Superplastic deformation of strongly textured Ti-6Al-4V

Changes in microstructure and texture during superplastic deformation of strongly textured Ti-6Al-4V bar have been determined in order to establish the cause of stress and strain anisotropy. The effect of strain on the microstructure of the alloy was to cause a progressive break-up, due to grain-boundary sliding, of an initially directional microstructure containing contiguous-phase. The texture of the-phase, however, changed very little with superplastic strain but that of the-phase was randomized. Shape changes predicted by permitted deformation modes in the-phase did not correlate with the observed shape changes, whereas the observed anisotropy could be explained by the break-up of the contiguous-phase. A model to account for this anisotropy is described briefly, together with a typical microstructure which should exhibit isotropic superplastic deformation.     

Superplastic deformation of strongly textured Ti-6 Al-4 V

Stress and strain anisotropy of a strongly textured Ti-6 Al-4 V alloy bar during superplastic deformation at 880 and 928° C has been investigated. After 0.9 superplastic strain at 928° C the deformation tended to become isotropic. The anisotropic superplastic deformation was found to be dependent upon the aligned microstructure and not influenced by the original -phase crystallographic texture. The room-temperature anisotropy before and after superplastic plastic deformation was controlled by the original -phase texture, which was still present even after 1.48 strain (344% elongation) at 928° C.     

Biaxial deformation of Ti-6Al-4V and Ti-6Al-4V/TiC composites by transformation-mismatch superplasticity

Gas-pressure bulge forming of unreinforced Ti-6Al-4V and TiC-reinforced Ti-6Al-4V was performed while cycling the temperature around the allotropic transformation range of the alloy (880–1020 °C). The resulting domes exhibited very large strains to fracture without cavitation, demonstrating for the first time the use of transformation-mismatch superplasticity under a biaxial state of stress for both an alloy and a composite. Furthermore, much faster deformation rates were observed upon thermal cycling than for control experiments performed under the same gas pressure at a constant temperature of 1000°C, indicating that efficient superplastic forming of complex shapes can be achieved by transformation-mismatch superplasticity, especially for composites which are difficult to shape with other techniques. However, the deformation rate of the cycled composite was lower than for the alloy, most probably because the composite exhibits lower primary and secondary isothermal creep rates. For both cycled materials, the spatial distribution of principal strains is similar to that observed in domes deformed by isothermal microstructural superplasticity and the forming times can be predicted with existing models for materials with uniaxial strain rate sensitivity of unity. Thus, biaxial transformation-mismatch superplasticity can be modeled within the well-known frame of biaxial microstructural superplasticity, which allows accurate predictions of forming time and strain spatial distribution once the uniaxial constitutive equation of the material is known.     

On flow stress anisotropy in Ti-6Al-4V alloy sheet during superplastic deformation

The anisotropy of flow stress in a cold rolled sheet of Ti-6Al-4V alloy has been observed during superplastic deformation at 850 °C. At this temperature, the alloy has duplex microstructure with almost equiaxed grains of the alpha and beta phases. The maximum value of flow stress has been established for the rolling direction and minimum—for the transverse one. Also, the anisotropy of crystallographic texture weakening in the alpha phase has been observed. However, it has been demonstrated that texture in the alpha phase cannot be responsible for the observed anisotropic behavior. Texture in the beta phase is the suggested reason for the flow-stress anisotropy during superplastic deformation.     

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.     

Correlation of data on the hot deformation of Ti-6Al-4V

Published data on the hot deformation of Ti-6Al-4V alloy were correlated by the Zener-Hollomon relation. Within the temperature range of 700 to 1000? C, for which the alloy is two phase, two mechanisms are operable. The one associated with lower temperature-strain-rate combinations has an activation energy of about 85 000 cal mol^?1, which is considerably greater than any reported experimental activation energy for self diffusion in titanium. Thus, mechanisms other than dislocation climb must be considered. An apparent activation energy of 172 000 cal mol^?1 was found for higher temperature-strain-rate combinations and is felt to be related to the occurrence of dynamic recrystallization.     

Fatigue Behavior of Ti-6Al-4V

Low-cycle-fatigue texts in vacuum and air were performed. Under cyclic loading the Ti-6Al-4V showed both cyclic hardening and cyclic softening depending on heat treatment, stress amplitude, and microstructure. Plastic deformation of the β-phase in the unaged condition due to stress induced martensitic transformation caused cyclic hardening. Cyclic softening was observed if the α-phase hardened by coherent Ti₃Al particles was plastically deformed. Equiaxed microstructures exhibited a stronger cyclic softening than lamellar structures. This behavior could be explained by the pronounced texture of the equiaxed microstructures, whereas the lamellar structures were texture-free. The fatigue life was influenced by the cyclic softening process mainly in the low-cycle-fatigue regime. The fatigue life at normalized stress amplitude (σ_a/σ_y) was shorter for microstructures with strong cyclic softening as compared to microstructures with lower cyclic softening.     

Effect of dwell on the growth of fatigue cracks in Ti-6Al-4V alloy bar

The growth of cracks in Ti-6Al-4V alloy was studied in fatigue tests in which the maximum load each cycle was held constant for a chosen dwell-time, and the results were compared with those obtained using sinusoidal stressing.

A 57 mm thick forged and rolled bar of Ti-6Al-4V alloy was used. It had been annealed at 700°C for 2 hr and the microstructure was mainly hexagonal -phase with a strong preferred orientation (TD texture). Test pieces were cut from the bar to select crystal orientations of fracture planes and directions. When the stressing direction was parallel to the basal plane, the crack growth rate of dwell-cycling at room temperature was similar to that for sinusoidal stressing at 0.3 and 25 Hz. But when the stress was normal to the basal plane the rate of growth was increased considerably, depending on the dwell-time and the stress intensity factor range (ΔK).

The dependence of the dwell-effect on microstructure and temperature was investigated and the preferred plane of dwell-fracture determined. The failure mechanism is discussed, together with the implications of the results for aircraft components subjected to in-service cyclic loads such that each flight is broadly simulated by a stress pattern having a dwell time at maximum load.     

Ti及Ti-6Al-4V的水基凝胶注模成形研究

将凝胶注模工艺应用于金属Ti及Ti-6Al-4V合金粉末的坯体成形,研究了高固相含量的Ti粉和Ti-6Al-4V合金粉末的料浆的制备.结果表明,用凝胶注模工艺制备出了固相含量为54ψ%的钛合金粉末料浆和形状复杂的坯体;粉末的颗粒形状是影响料浆固相含量的重要因素,球形粉末配制成的浆料固相含量最高,近球形次之,片状最低;分散剂柠檬酸铵也可以显著提高浆料的固相含量.     

Effect of saline solution environment on the cyclic deformation of Ti-6Al-4V alloy

The present investigation studies the effect of physiological solution at 37°C on the cyclic deformation behaviour of a Ti-6Al-4V alloy, with a microstructure corresponding to that obtained in the substrate when a sintered metallic porous coating is produced. Cyclic deformation tests have been carried out up to fracture and the fatigue crack nucleation mechanisms have been analysed. Since fatigue is a phenomenon related with plastic deformation, which is enhanced at corrosion and/or at stress concentration sites, cyclic deformation tests conducted at a level of stress above the elastic limit can provide a clear picture of the crack nucleation mechanisms involved.     

Laser surface modification of Ti-6Al-4V alloy

Hardening of Ti-6Al-4V alloy with laser surface melting (LSM) and laser surface alloying (LSA) techniques was attempted. Both LSM and LSA were carried out in a nitrogeneous atmosphere. Niobium, molybdenum and zirconium were used as alloying elements in the LSA. A hardness increase was observed for both LSM and LSA. Maximum hardness was obtained for LSM and zirconium alloy addition. In LSM, hardness increased almost three-fold in comparison to the substrate, which has a Vickers hardness of 350, by the formation of TiN in the region of 100 m melt depth. Hardness then decreased slowly and reached a minimum of 580 VHN at the maximum melt depth of 750 m. However, hardness for the zirconium alloy addition was uniform throughout the melted zone. Ageing treatments were performed for all specimens at 450C and different ageing times. Hardness measurements and X-ray diffraction were utilized to delineate the features associated with the hardening of the melted zone.     

Laser surface melting of Ti-6Al-4V alloy

Surface hardening of Ti-6Al-4V alloy with laser surface melting (LSM) in a nitrogen atmosphere has been studied. In LSM, hardness increased almost three-fold in comparison to that of the substrate, the latter having a Vickers hardness of 350, by the formation of TiN in the range of 100m of melt depth. Hardness, then, decreased slowly and reached a minimum of 580 VHN at a maximum melt depth of 750m. -Ti was formed in the heat-affected zone (HAZ) with a VHN of 450. Ageing treatments were performed for all specimens at 450 °C and different ageing times (1–20h). Short ageing treatments increased the hardness in the melted zone as well as in the HAZ (1–3h). Long ageing treatments (7–20h) resulted in uniform hardness distribution in the melted zone.     

Anisotropic behavior and mechanical properties of Ti-6Al-4V alloy in high temperature deformation

Journal of Materials Science - Uniaxial hot tensile tests were carried out in the RD, 45°, and TD directions of cold-rolled Ti-6Al-4V alloy sheets. At 973 K-0.1 s?1, it...     

Effect of surface finish and sheet thickness on isostatic diffusion bonding of superplastic Ti-6Al-4V

Abstract

The aim of the present work was to determine the process conditions of pressure, temperature, and time required to produce high quality isostatic diffusion bonds in Ti-6Al-4Vfor two surface finishes. In addition, the conditions to produce a sound bond between two sheets or plates of Ti-6Al-4V having different thicknesses were investigated. An assessment of the quality of bonds produced was made on the basis of metallographic examination, lap shear strength measurements, and SEM examination of the shear fracture surfaces. Sound bonds were produced in the as received and pickled 1.6 mm thick sheet in under 1 h with an applied pressure of 2.1 MPa at both 940 and 920°C and for the P60 ground surface finish at 940°C, but the time increased to 1.5 h at 920°C for the coarser surface finish. Variations in sheet thickness from 1.6 to 10 mm were found to have no effect on the bonding time. These results were compared with the predictions of a model of isostatic diffusion bonding.     

Fabrication of highly ordered TiO2 nanotube arrays via anodization of Ti-6Al-4V alloy sheet

Uniform and highly ordered TiO2 nanotube arrays were fabricated by the electrochemical anodic oxidation on Ti-6Al-4V surface, using graphite plate as cathode and ethylene glycol (EG) with addition of a certain amount of H2O and NH4F as electrolyte, and the anodization voltage went up to a presetting voltage by stepwise increment. The morphology, structure and composition of TiO2 nanotube arrays were characterized by SEM, EDS, XRD and XPS. The formation process of TiO2 nanotubes was introduced in brief. The experiments were arranged by an orthogonal experiment method and the experimental results showed that the formation of TiO2 nanotube arrays was influenced by not only each factor (F- content, H2O content, external voltage and duration), but also cross correlation among the four factors. The optimal condition was F- content 0.2 wt%, H2O content 4 vol%, external voltage 40 V and duration 1 h in the studied electrochemical system, and the length of obtained nanotubes was 1.5 microm, the outer diameter was approximately 100 nm and the aspect ratio was 15. As-formed nanotube arrays were amorphous and changed to anatase TiO2 after annealed at 500 degrees C for 2 h in air ambience. XPS survey spectra revealed the surface of as-formed nanotube arrays containing Ti, O, C, F and N. The nanotube arrays on Ti-6Al-4V surface with better thermo-stability and crystallinity would have great potential in biomaterials.     

Microstructural assessment of laser nitrided Ti-6Al-4V alloy

A microstructural study of the phases developed during the laser nitriding of a Ti-6Al-4V alloy by, using a CL5 continuous CO₂ laser with a spinning beam and concentration of 80% nitrogen, was undertaken. The vertical sections, perpendicular to the melt track were examined by optical microscopy and scanning electron microscopy (SEM), while specimens for X-ray diffractometry (XRD), X-ray photospectroscopy (XPS) and transmission electron microscopy/selected area electron diffraction (TEM/SAED), were taken parallel to the melt track. In this way the variation in microstructure as a function of depth from the laser treated surface, was studied. This supplemented XRD and XPS investigations undertaken previously. Two zones were identified. Zone 1, within 50 m of the surface, contained well defined dendrites of fcc TiN_0.8, plus hcp TiN_0.3 and hcp Ti. Zone 2, below 50 m, consisted of needles of hcp Ti. From a consideration of the hardness profiles in Zone 2, it is suggested that at the top of the zone, the phase is, in fact, a solid solution containing 3–4% N, which decreased to <1% N at the bottom of the zone. The TEM/SAED study permitted the three phases fcc TiN_0.8, hcp TiN_0.3 and hcp Ti to be identified through a combination of morphology and SAED patterns. This also showed that the fccTiN_0.8 contained fringes, which were considered to be stacking fault fringes and allowed this phase to be readily recognized in the TEM. The presence of stacking faults may be associated with the high nitrogen concentration of 80% used for the laser nitriding in this work.