Lattice parameters of Ti–4Al–2V alloy with thermal oxidation

In this study, the effect of thermal oxidation on the lattice parameters of Ti–4Al–2V alloy was studied.Samples were oxidized at 450, 600 and 650 °C for 1–7 h in electric furnace under air atmosphere. The lattice parameters were determined using the Cohen method as a function of oxidation time at each temperature. The lattice parameters of as-received alloy are calculated as a = 0.29289 nm and c = 0.46652 nm. The thermal oxidation at 450 °C results in a gradual increase in a-parameter, whereas it goes through a maximum at higher temperatures(600 and 700 °C). The results show that these maximums are reduced to an approximately constant value after a long-time oxidation. The c-parameter generally increases over the whole treatment condition. It is believed that these variations could be due to the dissolution of oxygen atoms in octahedral sites of hcp lattice of titanium.     

Effect of welding processes on joint characteristics of Ti–6Al–4V alloy

Abstract

Although Ti–6Al–4V alloys show reasonable weldability characteristics, the joint properties are greatly influenced by the welding processes. Microstructures and tensile and impact properties of welded Ti–6Al–4V alloy were evaluated for high vacuum electron beam welding, CO₂ laser beam welding and gas tungsten arc welding. The resultant tensile and impact properties of the welded joints are correlated with the weld metal microstructure and hardness. The results indicate that the electron beam welding is more suitable for Ti–6Al–4V sheet welding and the welding seam without defects can be obtained. The full penetration butt welds are obtained by gas tungsten arc welding process, but they have many drawbacks such as wide weld seam, big deformation and coarse grains. Laser beam welding has many advantages such as the narrowest weld seam, the least deformation and the finest grains, but it should be studied again for the reasons of unstable welding technologies and strict condition.     

Stitch welding of Ti–6Al–4V titanium alloy by fiber laser

Stitch welding of plate covered skeleton structure of Ti–6Al–4V titanium alloys has a variety of applications in aerospace vehicle manufacture. The laser stitch welding of Ti–6Al–4V titanium alloys was carried out by a 4 kW ROFIN fiber laser. Influences of laser welding parameters on the macroscopic geometry, porosity, microstructure and mechanical properties of the stitch welded seams were investigated by digital microscope, optical microscope, scanning electron microscope and universal tensile testing machine. The results showed that the three-pipe nozzle with gas flow rate larger than 5 L/min could avoid oxidization, presenting better shielding effect in comparison with the single-pipe nozzle. Porosity formation could be suppressed with the gap between plate and skeleton less than 0.1 mm, while the existing porosity can be reduced with remelting. The maximum shear strength of stitch welding joint with minimal porosity was obtained by employing laser power of 1700 W, welding speed of 1.5 m/min and defocusing distance of +8 mm.     

Corrosion resistance of Ti–6Al–4V alloy with nitride coatings in Ringer’s solution

The corrosion behaviour of Ti–6Al–4V alloy with nitride coatings was investigated in Ringer’s solution at 36 and 40 °С. Nitride coatings of different composition, thickness and surface quality were formed because of changing nitrogen partial pressure from 1 to 10⁵ Ра and nitriding temperature from 850 to 900 °С. Results shown that nitride coatings improve anticorrosion properties of alloy at both solution temperatures. Corrosion resistance of alloy increases with the content increase of TiN phase in nitride coating. With increase of temperature from 36 to 40 °С the corrosion resistance of alloy is determined significantly by quality of nitride coating.     

Probing into diffusion bonding of laser surface treated γ-Ti–Al alloy/Ti–6Al–4V alloy

Abstract

In this work, a novel diffusion bonding technique combining the laser surface treatment (LST) with the diffusion bonding is used to join a γ-Ti–Al alloy with a Ti–6Al–4V alloy. By using the LST and subsequent heat treatment, a layer with a fine grain structure can be obtained on their surface of the two alloys. The diffusion bonding behaviour between γ-Ti–Al alloy and Ti–6Al–4V alloy with or without LST under the different bonding conditions is investigated. The result reveals that LST can improve the diffusion bonding behaviour of the two alloys, and the three point bending strength of the joints can be promoted. The sound bonding between the two alloys with the LST is achieved at 1173 K under 80 MPa in 2 h.     

Deformation mechanisms of Ti–6Al–4V alloy with a martensitic microstructure subjected to oligocyclic fatigue

The objective of this work is to characterise the deformation modes when Ti–6Al–4 V alloy with α′ martensitic structure is subjected to oligocyclic fatigue at various strain amplitudes. It is believed that, unlike pure titanium single crystals at room temperature, {11̄01} twinning occurs in the α′ martensitic microstructure, because of the factors, such as the prior existence of dislocations or α′/α′ interfaces, which may reduce the activation energy required for the nucleation of {101̄1} twins.     

An enhanced constitutive material model for machining of Ti–6Al–4V alloy

Material failure due to adiabatic shear banding is a characteristic feature of chip formation in machining of Ti–6Al–4V material. In this paper, an enhanced Zerilli–Armstrong (Z-A) based material flow stress model is developed by accounting for the effects of material failure mechanisms such as voids and micro-cracks on the material flow strength during shear band formation. These effects are captured via a multiplicative failure function in the constitutive material flow stress model. The strain and strain rate dependence of the material failure mechanism are explicitly modeled via the failure function. The five unknown constants of the failure function are calibrated using cutting force data and the entire model is verified using separate force, chip segmentation frequency and tool–chip contact length data from orthogonal cutting experiments reported by 0035 and 0040. Model predictions of these quantities based on the enhanced material model are shown to be in good agreement with experiments over a wide range of cutting conditions.     

Improving the tribological properties of Ti–6Al–4V alloy by nitrogen-ion implantation

The tribological properties of N⁺₂-ion-implanted Ti alloy (Ti–6Al–4V) were studied by performing lubricated ball-on-disk tests against steel balls. The friction coefficients of N⁺₂-ion-implanted disks ranged from 0.05 to 0.2, which were lower than that of the unimplanted disk. N⁺₂-ion implantation reduced the volumetric wear rate of the disks as well as that of the steel balls. Moreover, the seizure limit of N⁺₂-ion-implanted disk was increased. These improvements were remarkable for doses above 2.5×10¹⁷ ions cm⁻². However, N⁺₂-ion implantation did not monotonously improve the tribological properties with increasing ion dose. The results were not simply attributed to an increase in the surface microhardness by N⁺₂-ion implantation. Surface analysis revealed that the structure consisted of titanium oxide on titanium, and titanium nitrides were formed by N⁺₂-ion implantation. The observed transition in the tribological properties of Ti–6Al–4V alloy was discussed in terms of surface structure produced by N⁺₂-ion implantation.     

Optimization of Ti6Al4V titanium alloy laser–arc hybrid weld parameters

This article reports the results of a study aimed at using statistical methods to optimize the parameters for laser–arc hybrid butt welding of Ti6Al4V titanium alloy sheets with a thickness of 3.0 mm. The study has examined the effects of the hybrid welding process parameters, such as laser beam power, arc pulse frequency, arc length, arc current, wire speed, laser and arc relative positions, and weld speed. Microstructure has been studied using light microscopy and morphological analysis of weld bead cross sections. This article reports the results of energy and morphological tests.     

Thermal relaxation of residual stress in laser shock peened Ti–6Al–4V alloy

Laser shock peening (LSP) induced residual stresses in Ti–6Al–4V, and their thermal relaxation due to short-term exposure at elevated temperatures are investigated by an integrated modeling/simulation and experimental approach. A rate and temperature-dependent plasticity model in the form of Johnson–Cook (JC) has been employed to represent the nonlinear constitutive behavior under both LSP and thermal loads. By comparing the simulation results with experimental data, model parameters for Ti–6Al–4V are first calibrated and subsequently applied in analyzing the thermal stability of the residual stress in LSP-treated Ti–6Al–4V. The analysis shows that the magnitude of stress relaxation increases with the increase of applied temperature due to material softening. Most of stress relaxation occurs before 10 min to 20 min exposure in this study, and stress distribution becomes more uniform after thermal exposure. An analytical model based on the Zener–Wert–Avrami formula is then developed based on the simulation results. The activation enthalpy of the relaxation process for laser shock peened Ti–6Al–4V is determined to be in the range of 0.71 eV to 1.37 eV.     

Early stage consolidation mechanisms during hot isostatic pressing of Ti–6Al–4V powder compacts

Mechanisms contributing to early stage compaction of metal powder compacts were identified in a series of Ti–6Al–4V powder compacts hot isostatically pressed to relative densities ranging from 71% to 100%. The partially dense compacts, consolidated from loose powder in thin-walled containers, were examined using optical microscopy of polished sections and stereo pair scanning electron microscopy of fracture surfaces. Relative particle motion, characterized by small relative movements of particles and clusters of particles, was found to contribute significantly to compaction over a broad range of relative densities (from 63% to 90%). A mechanism was identified by which the preferential deformation of small particles at large–small particle contacts enables rigid body motion of larger particles which, in turn, increases the relative density of the compact. Tensile fractures within the compact occurred between 82% and 90% relative density providing direct evidence of cooperative movement among clusters of particles. In comparisons with mechanistic powder compaction models, measurements of particle deformation, contact areas, and coordination numbers were found to substantiate the importance of the experimentally identified mechanisms.     

Mechanical properties of friction welded joint between Ti–6Al–4V alloy and Al–Mg alloy (AA5052)

Abstract

The present paper describes the mechanical properties of a friction welded joint between Ti–6Al–4V alloy and Al–Mg alloy (AA5052). The Ti–6Al–4V/AA5052–H112 joint, made at a friction speed of 27.5 rev s^?1, friction pressure of 30 MPa, friction time of 3.0 s, and forge pressure of 60 MPa, had 100% joint efficiency and fractured in the AA5052–H112 base metal. The Ti–6Al–4V/AA5052–H34 joint, made under the same friction welding conditions, did not achieve 100% joint efficiency and it fractured in the AA5052–H34 base metal because the AA5052–H34 base metal had softened under friction heating. The joints made at low friction speed or using short friction time showed fracture at the welded interface because a sufficient quantity of heat for welding could not be produced. However, the joints made at high friction speed or using long friction time were also fractured at the welded interface: in this instance, the welded interface also had an intermetallic compound layer consisting of Ti₂Mg₃Al₁₈. The Ti–6Al–4V/AA5052–H34 joint made at a friction speed of 27.5 rev s^?1 with friction pressure of 150 MPa, friction time of 0.5 s, and forge pressure of 275 MPa had 100% joint efficiency and fractured in the AA5052–H34 base metal, although the AA5052–H34 side softened slightly. In conclusion, the Ti–6Al–4V/AA5052–H112 joint and Ti–6Al–4V/AA5052–H34 joint had 100% joint efficiency and fractured in the AA5052 base metal when made under the friction welding conditions described above.     

Diffusion bonding of Ti—6Al—4V titanium alloy powder and solid by hot isostatic pressing

The Ti—6Al—4V (TC4) alloy powder and forged solid were diffusion bonded by hot isostatic pressing (HIP) to fabricate a powder—solid part. The microstructure of the powder—solid part was observed by scanning electron microscope (SEM). The microhardness and tensile tests were conducted to investigate the mechanical properties. The results showed that the powder compact was near-fully dense, and the powder/solid interface was tight and complete. The microhardness of the interface was higher than that of the powder compact and solid. The fractures of all powder—solid tensile specimens were on the solid side rather than at the interface, which indicated that a good interfacial strength was obtained. The tensile strength and elongation of the powder compact were higher than those of the solid. It is concluded that the HIP process can successfully fabricate high-quality Ti—6Al—4V powder—solid parts, which provides a novel near net shape technology for titanium alloys.     

Physical simulation of microstructural evolution in linear friction welded joints of Ti–6Al–4V alloy

Abstract

The novel shear compression specimen was used to simulate the microstructural evolution in linear friction welding joints of Ti–6Al–4V alloy. Similar formation mechanisms of microstructures and microtextures were found in the linear friction welding joints and shear compression specimen. Accordingly, the shear compression test was proved to simulate the microstructural evolution and the thermomechanical conditions that occurred in linear friction welding joint. Furthermore, the strain rate in linear friction welding was estimated to exceed the value of 70?s^??1.     

Effect of cryogenic and aging treatments on low-energy impact behaviour of Ti–6Al–4V alloy

The objective of this study is to examine the effects of cryogenic and aging treatments on the impact strength and mechanical properties of Ti–6Al–4V alloy. To accomplish that objective, cryogenic treatment (CT), aging treatment (AT) and cryogenic treatment followed by aging treatment (CAT) were conducted on Ti–6Al–4V alloy. Impact tests were performed on heat-treated and untreated samples using different impactor nose geometries (hemispherical, 60° and 90° conical) to determine the effect of impactor nose geometry on the damage characteristic. The findings showed that energy absorption increased and areas of damage decreased as a result of heat treatment in all treated samples. The highest energy absorption was observed in the CAT samples, due to the increase in energy absorption, the smallest damaged area occurred in the CAT sample, and the largest deformation was seen in the untreated samples. Additionally, it was seen that the damaged area and deflection were strongly dependent on impactor nose geometry. The maximum deflection and narrowest deformation area were seen with 60° conical nose geometry. The deformation area increased with increasing impactor nose angle.     

Causes of surface stripe cracks of Ti–4Al–2V alloy cold-rolled sheet

The stripe cracks which formed along rolling direction on the surface of Ti–4Al–2V alloy cold-rolled sheet were observed by stereoscope, scanning electron microscope（SEM）, and energy-dispersive spectrometer（EDS）. Morphology analysis indicates that cracks are dominantly in zigzag shape, with 10–30 mm in length and less than 10 lm in depth, and there is no evidence of crack tips. Chemical composition analysis shows that crack regions feather high oxygen concentration while smooth surface is at normal oxygen level. It is obvious that the occurrence of strip cracks is mainly related to residual oxide on the surface of Ti–4Al–2V alloy cold-rolled sheet.     

A coupled thermal and metallurgical model for welding simulation of Ti–6Al–4V alloy

The accurate prediction of the mechanical behavior of welded components made of Ti–6Al–4V requires a particular material model considering the significant effects of the material behavior during welding. Especially, phase transformations are assumed to have an influence on the temperature distribution. The flow curves of the material are changed during welding by complex mechanisms which might be describable using time-temperature history dependent flow curves. The following paper shows (as a step forward), how the influence of phase transformations on the transient heat conduction in components made of Ti–6Al–4V during tungsten inert gas (TIG) arc welding is investigated using a coupled thermal and metallurgical model. Kinetics of α+β→β phase transformation during heating and β→α phase transformation during cooling are studied using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation. A numerical heat transfer model is used to calculate the transient temperature field during welding. The thermal properties are calculated by a linear mixing rule based on the phase fractions and the thermal properties of each pure phase. Using these obtained thermal properties, the welding process of Ti–6Al–4V alloy is modeled using finite-elements for the spatial discretization and finite-differences to predict the transient temperature field. Additional calculations neglecting the phase changes are carried out to compare the temperatures and visualize the effects of phase transformations on the cooling behavior. The comparison of these models with measurements shows that the model considering the influence of solid phase transformations describes the temperature profile during cooling accurately.     

Corrosion behaviour of Ti DLC deposition on prenitrided 316L stainless steel and Ti–6Al–4V alloy

Abstract

316L and Ti–6Al–4V are widely used as biomaterials and materials of various mechanical components. In biomedical applications, they are used to manufacture coronary and pulmonary stents, hip prosthesis, screws and external fixations. However, Cr, Al and V are released from the alloys to the body environment and these ions mix into the blood stream. Release of even small amounts of these ions may cause local irritation of the tissues surrounding the implant. This situation may be prevented by applying suitable surface treatments to the biomaterials. The overall objective of the present paper is to examine the corrosion properties of duplex treated (nitrided and with a diamond-like carbon coating) 316L stainless steel and Ti–6Al–4V alloy. Diamond-like carbon films were deposited on nitrided samples using closed field unbalanced magnetron sputtering system. The corrosion behaviour of duplex treated samples was tested using the potentiodynamic method in ringer’s solution at 37°C. The corrosion resistance of duplex treated samples was significantly improved in comparison with the uncoated and single treated samples. In addition, the corroded surfaces were investigated by SEM where small pits were observed on all samples.     

High speed laser surface modification of Ti–6Al–4V

Titanium and its alloys have been commonly used for biomedical implant applications for many years; however, associated high coefficient of friction, wear characteristics and low hardness have limited their long term performance. This article investigates the effects of the high speed laser surface modification of Ti–6Al–4V on the microstructure, surface roughness, meltpool depth, phase transformation, residual strain, microhardness, and chemical composition. Laser treatment was carried out using a 1.5 kW CO₂ laser in an argon gas environment. Irradiance and residence time were varied between 15.7 to 26.7 kW/mm² and 1.08 to 2.16 ms respectively. Laser treatment resulted in a 20 to 50 μm thick surface modified layer without cracks. An increase in residence time and irradiance resulted in higher depth of processing. Surface roughness was found to decrease with increase in both irradiance and residence time. Metallography showed that a martensite structure formed on the laser treated region producing acicular α-Ti nested within the aged β matrix. The laser treatment reduced volume percentage of β-Ti as compared to the non-treated surface. Lattice stains in the range of 0.81% to 0.91% were observed after laser surface modification. A significant increase in microhardness was recorded for all laser treated samples. Microhardness increased up to 760 HV_0.05 which represented a 67% increase compared to the bulk material. Energy Dispersive X-ray Spectroscopy (EDS) analysis showed that laser surface modification produced a more homogenous chemical composition of the alloying elements compared to the untreated bulk metal.