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.     

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.     

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).     

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.     

Study on the microstructures and properties of the boride layers laser fabricated on Ti–6Al–4V alloy

Coatings containing stick borides were produced by laser surface alloying of Ti–6Al–4V with powder mixtures of boron and titanium. The test results indicated that the coatings have high microhardness, excellent wear resistance and are more resistant to oxidation than the original sample. The size and the morphology of the borides vary with laser scanning speed, which have an effect on the properties of the coatings.     

Influence of Island Scanning Strategy on Microstructures and Mechanical Properties of Direct Laser-Deposited Ti–6Al–4V Structures

To investigate the influence of island scanning on the microstructures and mechanical properties of direct laser-deposited Ti–6 Al–4 V structures, two samples are prepared using island scanning and orthogonal successive scanning, respectively.The microstructures, relative density, and mechanical properties of the samples prepared using these two scanning strategies are compared. Each sample exhibits columnar β-grain morphology and basket-weave microstructure characterization.The grains of the sample prepared using island scanning are significantly finer than that prepared by orthogonal successive scanning due to faster cooling during deposition. However, the relative density of the sample prepared using island scanning was slightly smaller due to the concentration of lack-of-fusion pores at the overlap zone of the island. Tensile testing at room temperature indicates that the ultimate tensile strength and yield strength of the sample prepared using island scanning is enhanced due to finer grains, while the ductility of the sample is weakened due to defects.     

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.     

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.     

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.     

Microstructure evolution during warm working of Ti–6Al–4V with a colony-α microstructure

A high-resolution electron backscatter diffraction technique was employed to investigate microstructure evolution during warm working of Ti–6Al–4V with a colony-α microstructure. Particular emphasis was paid to the specific mechanisms governing this process. Microstructure development was found to be driven mainly by the geometrical requirements of the imposed strain and by the kinking of α lamellae. For the most part, the lamellar microstructure was surprisingly stable during straining, with limited globularization observed only in kinked α colonies. The kinking process was shown to be closely linked with the development of shear bands within the colonies. These observations suggest that changes in strain path may be beneficial in promoting globularization during warm working.     

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.     

Some studies on high-pressure cooling in turning of Ti–6Al–4V

Titanium alloys are being increasingly employed in engineering and bio-medical applications, but their manufacturability via the cutting and grinding process is always a problem, which needs to be overcome. Inherent characteristics of Ti–6Al–4V alloy promote rapid tool wear, which needs to be curbed in order to attain high efficiency in metal cutting. Conventional or low-pressure cooling method fail to effectively conduct away the heat generated in the cutting zone, which is responsible for short tool life. High-pressure coolant jets were directed into the tool–chip interface to sufficiently penetrate and change the thermal, frictional and mechanical conditions in the cutting zone. High-pressure cooling using neat oil and water-soluble oil was undertaken and its effects on machining evaluation parameters such as chip form, chip breakability, cutting forces, coefficient of friction, contact length, tool life and surface finish of the finished workpiece were evaluated in comparison with those from the conventional cooling method. The results show that significant improvement in tool life and other evaluation parameters could be achieved utilising moderate range of coolant pressure.     

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.     

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.     

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.     

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.