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.     

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.     

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.     

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.     

Modulated Nd : YAG laser welding of Ti–6Al–4V

Abstract

Modulating the output of Nd : YAG laser sources has been evaluated as a technique for producing high quality welds in titanium alloys. Welds with high internal quality were produced when a square wave form was used with a modulation frequency ≥125 Hz and a duty cycle of 50%. Undercut present in the weld profile can be reduced if the correct combination of modulation amplitude and laser beam focal plane position are used. High speed observation and subsequent Fourier analysis of the vapour plume and keyhole behaviour have shown that they both exhibit the same periodic tendencies. With the correct parameters, an oscillating wave can be set up in the weld pool, which appears to manipulate the vapour plume behaviour and hence reduce porosity formation.     

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.     

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.     

Mechanical properties of strengthened surface layer in Ti–6Al–4V alloy induced by wet peening treatment

A modified surface layer was formed on Ti–6Al–4V alloy by wet peening treatment. The variations of the residual stress, nano-hardness and microstructure of the modified layer with depth from surface were studied using X-ray diffraction analysis, nano-indentation analysis, scanning electron microscopy and transmission electron microscopy observations. The results show that both the compressive residual stress and hardness decrease with increasing depth, and the termination depths are 160 and 80 μm, respectively. The microstructure observation indicates that within 80 μm, the compressive residual stress and the hardness are enhanced by the co-action of the grain refinement strengthening and dislocation strengthening. Within 80–160 μm, the compressive residual stress mainly derives from the dislocation strengthening. The strengthened layer in Ti–6Al–4V alloy after wet peening treatment was quantitatively analyzed by a revised equation with respect to a relation between hardness and yield strength.     

TLP bonding of Ti−6Al−4V to Al 2024 using thermal spray Babbitt alloy interlayer

Thermal spray assisted transient liquid phase (TLP) bonding of Ti−6Al−4V to Al2024 alloys was investigated, where the interlayer was 80 µm Babbitt thermal spray coat on Al substrate. Thermal spray creates a rough and clean surface which leads to establishing a joint with higher strength. The optimized parameters were bonding temperature of 580 °C and bonding time of 30 and 60 min. Microstructural observation together with XRD patterns confirmed the existence of Al₂Cu, Al₂CuMg, Cu₃Ti, TiAl₃, TiAl and Mg₂Sn intermetallic compounds formed in Al weld side. On the other hand, Ti₃Al, Sn₃Ti₅ and Ti₃Sn intermetallic compounds formed in Ti side. With increasing bonding time from 30 to 60 min, although the interlayer was not completely consumed, the thickness of remained Babbitt interlayer decreased to approximately 15 µm. The study showed that shear strength of the joint reaches the high value of 57 MPa obtained at larger bonding time of 60 min.     

Diffusion bonding of γ-TiAl alloy to Ti-6Al-4V alloy under hot pressure

1 Introduction Alloys based on the ?-TiAl intermetallic compounds have been of much interest in recent years as light-mass structural materials for elevated temperature aerospace applications[1- 5]. Successful joining and cost effective fabrication method…     

Influence of various process conditions on surface finishes induced by the direct metal deposition laser technique on a Ti–6Al–4V alloy

The direct metal deposition (DMD) with laser is a free-form metal deposition process for manufacturing dense pieces, which allows generating a prototype or small series of near net-shape structures. One of the most critical issues is that produced pieces have a deleterious surface finish which systematically requires post machining steps. This problem has never been fully addressed before.The present work describes investigations on the DMD process, using an Yb-YAG disk laser, and a widely used titanium alloy (Ti–6Al–4V) to understand the influence of the main process parameters on the surface finish quality. The focus of our work was: (1) to understand the physical mechanisms responsible for deleterious surface finishes, (2) to propose different experimental solutions for improving surface finish.In order to understand the physical mechanisms responsible for deleterious surface finishes, we have carried out: (1) a precise characterization of the laser beam and the powder stream; (2) a large number of multi-layered walls using different process parameters (P(W), V(m/min), D_m (g/min), Gaussian or uniform beam distribution); (3) a real time fast camera analysis of melt pool dynamics and melt-pool – powder stream coupling; (4) a characterization of wall morphologies versus process parameters using 2D and 3D profilometry.The results confirm that surface degradation depends on two distinct aspects: the sticking of non-melted or partially melted particles on the free surfaces, and the formation of menisci with more or less pronounced curvature radii. Among other aspects, a reduction of layer thickness and an increase of melt-pool volumes to favor re-melting processes are shown to have a beneficial effect on roughness parameters. Last, a simple analytical model was proposed to correlate melt-pool geometries to resulting surface finishes.     

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.     

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.     

Machining Ti–6Al–4V alloy with cryogenic compressed air cooling

A new cooling approach with cryogenic compressed air has been developed in order to cool the cutting tool edge during turning of Ti–6Al–4V alloy. The cutting forces, chip morphology and chip temperature were measured and compared with those measured during machining with compressed air cooling and dry cutting conditions. The chip temperature is lower with cryogenic compressed air cooling than those with compressed air cooling and dry machining. The combined effects of reduced friction and chip bending away from the cutting zone as a result of the high-speed air produce a thinner chip with cryogenic compressed air cooling and a thicker chip with compressed air cooling compared to dry machining alone. The marginally higher cutting force associated with the application of cryogenic compressed air compared with dry machining is the result of lower chip temperatures and a higher shear plane angle. The tendency to form a segmented chip is higher when machining with cryogenic compressed air than that with compressed air and dry machining only within the ranges of cutting speed and feed when chip transitions from continuous to the segmented. The effect of cryogenic compressed air on the cutting force and chip formation diminishes with increase in cutting speed and feed rate. The application of both compressed air and cryogenic compressed air reduced flank wear and the tendency to form the chip built-up edge. This resulted in a smaller increase in cutting forces (more significantly in the feed force) after cutting long distance compared with that observed in dry machining.     

A study of the microstructural evolution during selective laser melting of Ti–6Al–4V

Selective laser melting (SLM) is an additive manufacturing technique in which functional, complex parts can be created directly by selectively melting layers of powder. This process is characterized by highly localized high heat inputs during very short interaction times and will therefore significantly affect the microstructure. In this research, the development of the microstructure of the Ti–6Al–4V alloy processed by SLM and the influence of the scanning parameters and scanning strategy on this microstructure are studied by light optical microscopy. The martensitic phase is present, and due to the occurrence of epitaxial growth, elongated grains emerge. The direction of these grains is directly related to the process parameters. At high heat inputs it was also found that the intermetallic phase Ti₃Al is precipitated during the process.     

Process optimization of Ti–Nb alloy coatings on a Ti–6Al–4V plate using a fiber laser and blended elemental powders

A fiber laser was used to modify the surface composition of a Ti–6Al–4V plate through deposition of the blown powder mixture of Ti–45 wt.%Nb. Scanning electron microscopy and energy dispersive spectroscopy (EDS) were employed to examine the clad sections microstructure and chemical composition. The optimized set of laser processing parameters, including the laser power of 1100 W, the laser scan speed of 350 mm/min (or ∼5.83 mm/s), the laser spot diameter of 2 mm and the powder feed rate of 0.1 g/s was found with the identification of combined parameters, the laser specific energy, the powder density and the newly defined laser supplied energy (i.e. representing the amount of energy given to the unit mass of the blown powder). It is shown that, with these parameters, continuous beads can be formed with pore-free sections and a homogeneous composition corresponding to that of β (Ti, Nb) solid solution phase. Furthermore, Al and V elements are thoroughly replaced with a more biocompatible element, Nb, in the second layer of a Ti–Nb cladding build-up on the surface of the Ti–6Al–4V plate (i.e. after ∼1 mm in clad thickness from the clad/substrate interface).