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.     

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.     

Effect of micro-textured tools on machining of Ti–6Al–4V alloy: An experimental and numerical approach

In this work, an attempt is made to reduce the detrimental effects that occurred during machining of Ti–6Al–4V by employing surface textures on the rake faces of the cutting tools. Numerical simulation of machining of Ti–6Al–4V alloy with surface textured tools was employed, taking the work piece as elasto-plastic material and the tool as rigid body. Deform 3D software with updated Lagrangian formulation was used for numerical simulation of machining process. Coupled thermo-mechanical analysis was carried out using Johnson-cook material model to predict the temperature distribution, machining forces, tool wear and chip morphology during machining. Turning experiments on Ti–6Al–4V alloy were carried out using surface textured tungsten carbide tools with micro-scaled grooves in preferred orientation such as, parallel, perpendicular and cross pattern to that of chip flow. A mixture of molybdenum disulfide with SAE 40 oil (80:20) was used as semi-solid lubricant during machining process. Temperature distribution at tool–chip interface was measured using an infrared thermal imager camera. Feed, thrust and cutting forces were measured by a three component-dynamometer. Tool wear and chip morphology were captured and analyzed using optical microscopic images. Experimental results such as cutting temperature, machining forces and chip morphology were used for validating numerical simulation results. Cutting tools with surface textures produced in a direction perpendicular to that of chip flow exhibit a larger reduction in cutting force, temperature generation and reduced tool wear.     

Preparation of titanium oxide and hydroxyapatite on Ti–6Al–4V alloy surface and electrochemical behaviour in bio-simulated fluid solution

In order to improve the bonding strength between hydroxyapatite (HA) coating and Ti–6Al–4V substrate, a uniform titanium oxide film was obtained by controlled anodic oxidation. After that an alkaline treatment with NaOH solution was used to make them more bioactive. Finally hydroxyapatite coating has been prepared on Ti–6Al–4V substrate through electrochemical deposition. Comparative electrochemical behaviour of untreated and surface modified Ti–6Al–4V alloy, in bio-simulated fluid solution was investigated by electrochemical techniques. SEM was used to observe the morphology of modified surfaces and the thicknesses of the oxide films prepared were evaluated on the cross-sections of the samples using SEM–FIB.     

Effect of hot-dip aluminizing on the oxidation resistance of Ti–6Al–4V alloy at high temperatures

Hot-dip aluminizing and interdiffusion treatment were used to develop a TiAl₃-rich coating on Ti–6Al–4V alloy. Interrupted oxidation at temperatures from 600 to 900 °C and isothermal oxidation at 700 and 800 °C of the coating were conducted. The coating markedly decreases the oxidation rate in comparison with the alloy at temperatures below 800 °C during the interrupted oxidation. The oxidation kinetics follows parabolic relations at 700 and 800 °C during the isothermal oxidation. A layered structure of Al₂O₃/TiAl₃/TiAl₂/TiAl/alloy from the outside to the inside forms after oxidation at 700 °C without changing the main body of the coating.     

Influence of the surface etching on the corrosion behaviour of a three-dimensional printed Ti–6Al–4V alloy

Recently, there has been an extension of three-dimensional (3D) printing technology of metal materials in the medical field. Additive technology has made it possible to manufacture customized implants. However, 3D printing products often require surface treatment. The possible treatments include acid etching. This study investigated the effect of surface etching on the corrosion resistance of Ti–6Al–4V alloy concerning biological applications. The samples were etched in a mixture of hydrofluoric acid and nitric acid. The corrosion behaviour was described by measuring the time dependence of polarization resistance in a saline solution and surface analysis. The results showed that etching creates a fluoride-rich layer on the surface, which negatively affects the corrosion behaviour of the material for up to 24 hr. Cytocompatibility tests showed that the resulting layer does not affect the biocompatibility of the material.     

Influence of various aging treatments on microstructure,strength and corrosion behaviour of high Zn content Al–Zn–Mg–Cu alloy

In this paper, the influence of T6, T74 and RRA aging treatments on microstructure, strength and corrosion behaviour of high Zn content Al–Zn–Mg–Cu alloy was investigated by tensile properties tests, inter-granular corrosion (IGC) tests, exfoliation corrosion (EXCO) tests, polarisation tests, metallographic microscope and transmission electron microscopy (TEM) analysis. The results show that the T74 and RRA temper can increase the size and the distribution discontinuity of the grain boundaries precipitates (GBPs), thus leading to improvement of the corrosion resistance. However, with the coarser matrix precipitates (MPs) relative to T6 treatment, RRA and T74 temper both have a decrease in strength. Besides, all the performances (including mechanical properties and corrosion properties) of the RRA treatment show an intermediate level relative to T6 and T74. Therefore, we can select the appropriate heat treatment process according to the different performance requirements in the industrial production.     

Effects of temperature and initial microstructure on the equal channel angular pressing of Ti–6Al–4V alloy

Equal channel angular pressing of Ti–6Al–4V alloy was successfully carried out isothermally above 600 °C. The equiaxed microstructure presented more uniform material flow than the Widmanstätten microstructure, which was discussed in relation to flow softening behavior of the two microstructures.     

Effect of Base Material on Microstructure and Texture Evolution of a Ti–6Al–4V Electron-Beam Welded Joint

The effect of base material(BM) on microstructure and crystallographic orientation evolution of a Ti–6Al–4V electron-beam welded joint was investigated. Meanwhile, the crystallographic orientation of prior b grains was studied by advanced electron backscattered diffraction data processing. The inhomogeneity of microstructure within welded joint was formed due to the different microstructures of BM. By comparing microstructure details of the welded joint, including microstructure morphology and crystallographic orientation with those of the base material, it can be found that both the microstructure morphology and crystallographic orientation of the EBW joint would be controlled by BM.     

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.     

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.     

Effect of Surface Polishing Treatment on the Fatigue Performance of Shot-Peened Ti–6Al–4V Alloy

In this study, shot peening is applied to the titanium alloy Ti–6Al–4V, and the surface treatment effect on fatigue life of shot-peened specimens under high cycle loading is investigated. The induced residual stress is measured by using the orbital hole-drilling method. Surface profilometer and optical microscopy are employed to characterize the surface roughness and morphology. The deformed microstructure layers of the shot-peened specimens are investigated by using scanning electron microscopy. Experiments reveal that the fatigue life of Ti–6Al–4V is improved by the shot peening process, and the surface pre-peening polishing. The combination of pre-and post-peening polishing treatments further improves fatigue life of Ti–6Al–4V specimens. The present work provides useful guidelines for developing more efficient shot peening strategies.     

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.     

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.     

Large gap joining of Ti–6Al–4V alloy with mixed powder interlayers

Abstract

Titanium based brazing alloys containing chromium, iron, copper, and nickel as β stabilisers have been studied for joining the titanium alloy Ti–6Al–4V. Two of these alloys were selected for use in producing large gap joints. The first brazing alloy, Ti–12Zr–14Cr–12Cu–12Ni (type 1), can be used to braze Ti–6Al–4V below its β transus temperature. Joints of thickness up to 150 μm can be made in a normal brazing cycle without prolonged holding. The interlayer consists of a β titanium alloy with no precipitation of intermetallic compounds. The second brazing alloy, Ti–12Zr–14Cr–6Fe–5Cu–5Ni (type 2), has to be brazed above the β transus temperature of Ti–6Al–4V. Its powders were mixed with pure titanium and Ti–6Al–4V powders and the mixture was used as the joining interlayer. Interlayers 5 mm in thickness were used to produce joints for microstructural examination and mechanical testing. It was found that residual pores in the interlayers were related to the amount of the brazing alloy in the interlayer. A fully dense interlayer could be obtained with 60 wt-% brazing alloy in the interlayer. The as bonded joints revealed tensile strength equal to 50% of that of the base metal. Diffusional treatment of the joints improved the joint efficiency to about 70%, compared with the base metal.     

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.     

Effect of heating temperature on the formation of structure and phase composition of a biocompatible alloy Ti–6Al–4V–ELI subjected to equal-channel angular pressing

The changes in the structure, phase composition, and in the microhardness after heating in the temperature range of 600–1000°C and subsequent quenching in a Ti–6Al–4V–ELI alloy preliminarily subjected to equal-channel angular pressing at 600°C have been studied by methods of structural analysis, X-ray diffraction analysis, and microdurometry. The presence of the α₂ phase in the deformed alloy has been detected. The diagram of the change in the phase composition of the alloy upon quenching was constructed. The interrelation between the character of structural and phase transformations, and the level of the microhardness of the alloy after quenching from different temperatures has been found.     

Formation of adiabatic shear band and deformation mechanisms during warm compression of Ti–6Al–4V alloy

Adiabatic shear band(ASB) was narrow region where softening occurred and concentrated plastic deformation took place. In present study, the effects of height reduction and deformation temperature on ASB were investigated by means of optical microscopy(OM) and scanning electron microscopy(SEM). And the deformation mechanisms within the shear band were discussed thoroughly with the help of transmission electron microscopy(TEM). There is a critical strain for the formation of ASB during warm compression of Ti–6Al–4V alloy. The width of ASB increases with height reduction increasing. Elongated alpha grains within shear band grow up with deformation temperature increasing. Some ultrafine grains that confirm the occurrence of dynamic recrystallization are observed within shear band during warm compression of Ti–6Al–4V alloy.