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.     

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.     

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.     

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.     

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.     

Influence of the residual stress state of coatings on the wear behavior in external turning of AISI 4140 and Ti–6Al–4V

The residual stress state of coatings influences tool life and performance in machining significantly. Due to thermo-mechanical loads during the cutting process the coatings require tailored properties. Beside the coating structure the two most important properties are the residual stress state and the chemical composition of the coating. Therefore, the influence of these two properties is investigated in this study, comparing the cutting performance in continuous and interrupted cutting of 42CrMo4 (AISI 4140) and Ti–6Al–4V. It is shown that the residual stress states of the coating close to the surface and close to the substrate are important for the wear behavior. High compressive residual stresses near the substrate combined with a material-optimized composition increase the resistance against chipping. Flank wear resistance increases with high compressive residual stresses near the surface and decreasing stresses towards the substrate.     

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.     

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.     

Fatigue in Ti–6Al–4V–B alloys

The addition of small amounts of B to Ti–6Al–4V alloy reduces the as-cast grain size by an order of magnitude and introduces TiB phase into the microstructure. The effects of these microstructural modifications on both the high cycle fatigue and cyclic stress–strain response were investigated. Experimental results show that B addition markedly enhances the fatigue strength of the alloy; however, the influence of prior-β grain size was found to be only marginal. The presence of TiB particles in the matrix appears to be beneficial with the addition of 0.55 wt.% B to Ti–6Al–4V enhancing the fatigue strength by more than 50%. Strain-controlled fatigue experiments reveal softening in the cyclic stress–strain response, which increases with the B content in the alloy. Transmission electron microscopy of the fatigued specimens indicates that generation of dislocations during cyclic loading and creation of twins due to strain incompatibility between the matrix and the TiB phase are possible reasons for the observed softening.     

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.     

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.     

Simulation of slip band evolution in duplex Ti–6Al–4V

Formation of slip bands plays an important role in deformation and fatigue processes of duplex Ti–6Al–4V. In this study, shear-enhanced crystal plasticity constitutive relations are proposed to account for the slip softening due to breakdown of the short-range order between titanium and aluminum atoms. A hybrid strategy is developed which allows the softening to occur in slip bands only within the primary α phase, with the degree of localization depending on the specific polycrystalline initial-boundary-value problem and the requirements for compatibility of each grain or phase with its neighbors. The proposed model is calibrated by performing finite-element (FE) simulations on an experimentally studied Ti–6Al–4V alloy. The slip behavior of a Ti–6Al–4V sample subjected to an in situ (scanning electron microscopy (SEM)) tensile test is investigated. A two-dimensional (2-D) FE with 3-D crystal plasticity relations is constructed to represent the microstructure of the Ti–6Al–4V sample. Due to the lack of access to fully 3-D microstructure, a generalized plane-strain condition is used in the FE model which assumes columnar grains that are free of net traction in the direction normal to the surface. The assumption of columnar grains significantly reduces the computational cost. The contours of effective plastic strain are compared with the surface SEM micrographs from experiments at various strain levels. It is shown that the proposed approach for modeling slip bands qualitatively captures experimentally observed slip band behavior.     

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.