Additive Manufactured Ti 6Al 4V Research Articles

Tribological investigation of additive manufacturing medical Ti6Al4V alloys against Al2O3 ceramic balls in artificial saliva

Additive manufacturing Ti6Al4V alloys have found their potential applications in artificial teeth and hip joints. In this work, the relationships between wear resistance, hardness and microstructure of Ti6Al4V alloys fabricated using various routes were investigated in artificial saliva. The results indicate that comparing with wrought and wrought + heat treated (HT) samples, the as-SLMed samples with hcp-α′ martensite and few bcc-β phase exhibit higher hardness (∼410 HV) and better wear resistance. The as-SLMed samples, however, exhibit the worst wear resistance when wear direction is parallel to the molten pool line on XOZ-plane due to containing the softer fusion zone. Finally, the wear mechanism is discussed in detail, mainly including the abrasive and adhesive wear mechanism. The high hardness of matrix as well as the strong adhesion between the hardened layer, oxide layer and matrix are indispensable conditions for maintaining the optimum wear resistance.

A study on obtaining equiaxed prior-β grains of wire and arc additive manufactured Ti–6Al–4V

A Ti–6Al–4V alloy wall with 80 vol.% equiaxed prior-β grains was fabricated via wire and arc additive manufacturing. However, the equiaxed prior-β grains were characterized by strong texture. Formation mechanisms of the equiaxed prior-β grains were discussed. The tensile results presented a much lower anisotropy than that in other studies.

Evaluating Corrosion Resistance of Additive-Manufactured Ti–6Al–4V Using Electrochemical Critical Localized Corrosion Temperature

A new concept to evaluate the localized corrosion resistance of stacked Ti alloys such as additive-manufactured (AM) Ti–6Al–4V alloy is proposed. Crevice corrosion of subtractive-manufactured (SM) Ti–6Al–4V occurred on the surface beneath the crevice former, whereas localized corrosion of the AM alloy occurred in any vulnerable site irrespective of the crevice assembly. The electrochemical critical localized corrosion temperature (E-CLCT) was measured to evaluate the resistance of AM Ti–6Al–4V alloys to localized corrosion. The results showed that the localized corrosion of AM Ti–6Al–4V was attributable to the anisotropy and microstructure that resulted from rapid cooling, which were completely different from the anisotropy and microstructure of SM Ti–6Al–4V. The optimum applied potential of AM Ti–6Al–4V in 25 wt% NaCl aqueous solution was deduced. The E-CLCT provides a useful criterion for determining the resistance of AM Ti alloys to localized corrosion and for comparing their resistances. The shape of localized corrosion on additive-manufactured Ti–6Al–4V alloy.

Study on machinability of additively manufactured and conventional titanium alloys in micro-milling process

Capability of Additive Manufacturing (AM) technology in the production of complex parts with high flexibility has led to the growing interest in their application as an alternative for conventional manufacturing processes. Despite the outstanding benefits of the AM process, due to their poor surface quality, the precision parts produced by this method generally need to be machined, ground, or polished. This paper addresses the machinability of AM Ti6Al4V titanium alloy parts in the micro-milling process with a specific focus on cutting forces, specific cutting energy, burr formation, and surface quality. Additive parts were produced by Electron Beam Melting (EBM) technique and were compared with the extruded Ti6Al4V parts in the micro-milling process. No significant difference could be observed in the cutting forces of both materials at chip thicknesses between 7.4 and 37.3 μm, despite the higher hardness of the EBM Ti6Al4V compared to the extruded Ti6Al4V. However, micro-milling of the EBM parts produced finer surfaces. Cutting forces and specific cutting energies of EBM parts were less than those of extruded parts at minimal chip thicknesses (lower than 7.4 μm). Continuous wavy-type burrs were formed in micro-milling of the EBM Ti6Al4V and were larger than those of extruded Ti6Al4V.

Effects of build direction on tensile and fatigue performance of selective laser melting Ti6Al4V titanium alloy

Additive manufacturing Ti6Al4V titanium alloy has a great potential to be applied in aviation structural components. During additive manufacturing process, the deposition layers present because of its intrinsic features, which may affect mechanical properties of additive manufactured parts and components. In order to study the effects of build direction on tensile and fatigue performance of Ti6Al4V manufactured by the selective laser melting technique, three different build direction Ti6Al4V samples (0° sample, 45° sample, 90° sample) were designed and manufactured in this work. Tensile and fatigue tests were performed. Stress versus strain curves, S-N curves and crack growth rate curves were measured and studied. Fracture surfaces were investigated and compared with each other. It is shown that build direction can affect the fatigue performance, but has little effect on fatigue crack growth rate. The tensile and fatigue properties of the 45° sample are the best of the three build directions in this study.

Mechanical properties of AM Ti6Al4V porous scaffolds with various cell structures

Porous scaffolds as succedaneum of natural bone were investigated and applied in medical field. In this work, we carried out studies on mechanical properties of solid parts and porous scaffolds obtained by additive manufacturing (AM) technique. It is found that productions of AM process have a higher yield strength and higher microhardness compared to commercial Ti6Al4V. Roughened surface was observed for layer-by-layer process of AM and sticking of powder particles. The machining accuracy is affected by both dimensions and angles. Meanwhile, mechanical properties of porous scaffolds are influenced by machining accuracy and microdefects. In addition, the unit cell structures also impact the mechanical properties of porous scaffolds in terms of elastic modulus, yield strength and failure mode. Overall, considering the mechanical properties and biological properties, scaffolds with cube (CB) crystal cells are the best choice in our study.

Multi-scale mapping for collagen-regulated mineralization in bone remodeling of additive manufacturing porous implants

Abstract Long term success of metallic fusion cages depends on mechanobiological processes through the bone incorporation and rich osseointegration. An optimal configuration of porous titanium-aluminum-vanadium (Ti 6Al 4V) implant fabricated via the additive manufacturing was evaluated in complimentary structure examinations to investigate the growth of autologous osseous at multi-length scales. X-ray microcomputed tomography (micro-CT) and transmission X-ray microscopy (TXM) using newly-built analysis method indicate the porous Ti 6Al 4V is much better for bone ingrowth compared to commercially non-porous titanium (Ti) and porous tantalum (Ta) implants at the ultramicrostructural level. The evolution of bone formation and remodeling acquired by nano X-ray Laue diffraction mapping exhibits the isotropic orientation and low crystallinity of all newly formed bone whereas mature bone in Ti 6Al 4V discloses the preferential alignment and higher crystallinity volumes of constituent hydroxyapatite (HA) crystallites. The high degree in mineral crystallinity of the fully mature bone suggests additive manufactured Ti 6Al 4V pores enhance the collagen-regulated mineralization.

Microstructure and mechanical behaviour of additive manufactured Ti–6Al–4V parts under tension

Metal-based additive manufacturing technologies using electron or laser beams as a heat source for melting a metal powder or wire have been the subject of keen interest in recent years. At present paper a comparative analysis of the microstructure, strain response during tensile test and mechanical properties of Ti–6Al–4V samples produced by selective laser melting, electron beam melting or electron beam free-form fabrication were performed. A microstructural study using transmission electron microscopy revealed columnar prior β grains transformed into a lamellar α-morphology in the samples. According to X-ray diffraction study, the volume fractions of the β-Ti phase in the samples were equal to 2, 4 and 6 % respectively. It has been shown that the Vickers microhardness of SLM and EBM Ti–6Al–4V samples was similar (~5.4 GPa) while the hardness of EBF3 parts was significantly lower (4.5 GPa). The uniaxial stress-strain response of the Ti–6Al–4V samples fabricated by different additive manufacturing technologies were compared. Crystallographic (dislocation motion) and non-crystallographic (shear banding) deformation mechanisms of the loaded samples were studied by scanning electron microscopy and optical profilometry.

Additive manufacturing of Ti6Al4V alloy: A review

In this paper, the recent progress on Ti6Al4V fabricated by three mostly developed additive manufacturing (AM) techniques-directed energy deposition (DED), selective laser melting (SLM) and electron beam melting (EBM)-is thoroughly investigated and compared. Fundamental knowledge is provided for the creation of links between processing parameters, resultant microstructures and associated mechanical properties. Room temperature tensile and fatigue properties are also reviewed and compared to traditionally manufactured Ti6Al4V parts. The presence of defects in as-built AM Ti6Al4V components and the influences of these defects on mechanical performances are also critically discussed.

EBW and LBW of Additive Manufactured Ti6Al4V Products

EBW and LBW of Additive Manufactured Ti6Al4V Products

Strain rate effect on the tension and compression stress-state asymmetry for electron beam additive manufactured Ti6Al4V

The present experimental investigation lays the groundwork for coupling the plastic flow stresses of an Additive Manufactured (AM) Ti6Al4V with the corresponding strain hardening rate responses and fracture morphologies for this material at varying strain rates and stress states for the first time. The macroscopic response was obtained under different stress states (uniaxial tension and compression) and deformation rates (ranging from quasi-static at 0.001s−1 to the high rate domain at 1500s−1) to elucidate the tension-compression asymmetry and strain rate dependence of the material. The results identified that, (a) compressive yield strengths are higher than their tensile counterparts and (b) the strength differential effect is more prominent under high strain rates. The mechanical behavior is explained in terms of deformation mechanisms reasoned from close inspection of strain hardening rate curves. Dislocation glide and mechanical twinning contribution as dominant plastic units were identified in stages and their relation with deformation mode and rate was established. Based on fractography analyses of the tensile samples, a rate dependence showed a shift from classic cup-cone fracture at quasi-static rates to fracture along the shear plane at high strain rates. The high strain rate compression samples also show an interesting fracture morphology with high speed video capturing adiabatic shear localization.

Effects of heat treatment on microstructure and mechanical behaviour of additive manufactured porous Ti6Al4V

Titanium and its alloys such as Ti6Al4V play a major role in the medical industry as bone implants. Nowadays, by the aid of additive manufacturing (AM), it is possible to manufacture porous complex structures which mimic human bone. However, AM parts are near net shape and post processing may be needed to improve their mechanical properties. For instance, AM Ti6Al4V samples may be brittle and incapable of withstanding dynamic mechanical loads due to their martensitic microstructure. The aim of this study was to apply two different heat treatment regimes (below and above β-transus) to investigate their effects on the microstructure and mechanical properties of porous Ti6Al4V specimens. After heat treatment, fine acicular α′ martensitic microstructure was transformed to a mixture of α and β phases. The ductility of the heat-treated specimens, as well as some mechanical properties such as hardness, plateau stress, and first maximum stress changed while the density and elastic gradient of the porous structure remained unchanged.

Laser Additive Manufactured Ti–6Al–4V Alloy: Heat Treatment Studies

The effect of heat treatment on microstructure and mechanical behaviours of direct metal laser sintered Ti–6Al–4V samples have been studied. Rectangular parts were built in two different directions; vertical and horizontal and subjected to two different heat treatment cycles: above β transus and below β transus with air cooling. Surface characteristics, microstructural examination and mechanical properties have been investigated. Below β transus treatment creates a modification in the surface morphology with a fine dimple network. Above β transus treatment leads to extensive grain growth at the middle section of the vertically build component thereby increasing its microhardness. Both the selected heat treatment cycles significantly reduces the tensile strength and improves the elongation when compared to as-sintered material. However, below transus temperature treated vertical built specimen results in optimum combination of tensile strength (1124 MPa) and elongation (20%). Higher coefficient of friction has been recorded for specimens after heat treatment.

Coupling machining and heat treatment to enhance the wear behaviour of an Additive Manufactured Ti6Al4V titanium alloy

The effect of coupling machining with a subsequent heat treatment on the wear behaviour of the Ti6Al4V alloy produced by Electron Beam Melting (EBM) was investigated. Reciprocating sliding wear tests in saline solution and temperature-controlled environment were performed. Results showed that the heat-treated cylinders presented a lower coefficient of friction, less wear rate and higher degree of adhesive wear compared to the not heat-treated ones. It was then demonstrated that coupling machining and heat treatment had a synergistic effect that can be used as an efficient strategy in order to improve the EBM Ti6Al4V wear resistance.

Laser additive manufactured Ti–6Al–4V alloy: tribology and corrosion studies

The effect of heat treatments (HT) on wear and corrosion of direct metal laser-sintered Ti–6Al–4V specimens have been studied. Rectangular parts were built in vertical (VB) and horizontal (HB) directions and heat treated above β transus and below β transus with different cooling rate. Rotary wear tests have been carried out under varying loads of 5 N, 15 N and 25 N at 25 m/s. Corrosion behaviour were analysed in 1 M H2SO4,1 M HCl and 3.5% NaCl solutions. Wear volume loss of both HB and VB are less in specimens subjected to HT 2 than in HT1 which is attributed to more grain refinement. Presence of compact oxide debris on the surface of HT 2-VB specimen could have contributed to the low wear volume. A sharp positive difference could be observed in OCP immersed in 1 M H2SO4solution from −0.6 V (as-sintered) to 0.069 V (HT 1-HB) and 0.089 V (HT 2-HB). Icorr value in 1 M H2SO4 improved after both the heat treatments from 17.450 μA/cm2 to 1.470 and 0.152 11 μA/cm2, respectively. Generally, heat-treated specimens (both horizontal and vertical) show better corrosion resistance than the as-sintered specimens in all the three media.

<i>In Vitro</i> Assessment of Hydroxyapatite Coating on the Surface of Additive Manufactured Ti6Al4V Scaffolds

Custom orthopedic and dental implants may be fabricated by additive manufacturing (AM), for example using electron beam melting technology. This study is focused on the modification of the surface of Ti6Al4V alloy coin-like scaffolds fabricated via AM technology (EBM®) by radio frequency (RF) magnetron sputter deposition of hydroxyapatite (HA) coating. The scaffolds with HA coating were characterized by Scanning Electron microscopy, X-ray diffraction. HA coating showed a nanocrystalline structure with the crystallites of an average size of 32±9 nm. The ability of the surface to support adhesion and the proliferation of human mesenchymal stem cells was studied using biological short-term tests in vitro. In according to in vitro assessment, thin HA coating stimulated the attachment and proliferation of cells. Human mesenchymal stem cells cultured on the HA-coated scaffold also formed mineralized nodules.

Residual stress of as-deposited and rolled wire+arc additive manufacturing Ti–6Al–4V components

Wire + arc additive manufacturing components contain significant residual stresses, which manifest in distortion. High-pressure rolling was applied to each layer of a linear Ti–6Al–4V wire + arc additive manufacturing component in between deposition passes. In rolled specimens, out-of-plane distortion was more than halved; a change in the deposits' geometry due to plastic deformation was observed and process repeatability was increased. The Contour method of residual stresses measurements showed that although the specimens still exhibited tensile stresses (up to 500 MPa), their magnitude was reduced by 60%, particularly at the interface between deposit and substrate. The results were validated with neutron diffraction measurements, which were in good agreement away from the baseplate.This paper is part of a Themed Issue on Measurement, modelling and mitigation of residual stress.

Influence of the machining parameters and cooling strategies on the wear behavior of wrought and additive manufactured Ti6Al4V for biomedical applications

The paper presents the effect of the machining parameters and cooling strategies on the wear behavior of the wrought and Additive Manufactured Ti6Al4V used for biomedical applications. Wear tests were performed using a cylinder-on-plate configuration in a wet and temperature-controlled environment in order to investigate the reciprocating sliding wear behavior under human body conditions. The obtained results showed that the adoption of the cryogenic cooling during machining significantly affected the Ti6Al4V surface properties improving its wear performances, in terms of lower friction coefficient and less release of metal debris due to abrasive wear compared to dry cutting conditions, regardless the alloy as-delivered condition.

Crack path selection at the interface of wrought and wire + arc additive manufactured Ti–6Al–4V

Crack propagation deviation tendency in specimens containing an interface between wrought alloy substrate and Wire+Arc Additive Manufacture (WAAM) built Ti–6Al–4V is investigated from the viewpoints of microstructure, residual stress and bi-material system. It is found that a crack initiated at the interface tends to grow into the substrate that has equiaxed microstructure and lower resistance to fatigue crack propagation. Experimental observations are interpreted by finite element modelling of the effects of residual stress and mechanical property mismatch between the WAAM and wrought alloy. Residual stresses retained in the compact tension specimens are evaluated based on measured residual stress in the initial WAAM built wall. Cracks perpendicular to the interface kept a straight path owing to the symmetrical residual stress distribution. In this case the tangential stress in bi-material model is also symmetric and has the maximum value at the initial crack plane. In contrast, cracks parallel to the interface are inclined to grow towards the substrate due to the mode II (or sliding mode) stress intensity factor caused by the asymmetric residual stress field. Asymmetric tangential stress in the bi-material model also contributes to the observed crack deviation trend according to the maximum tangential stress criterion.

Effect of surface roughness on fatigue performance of additive manufactured Ti–6Al–4V

Additive manufacturing is increasingly considered for production of high quality, metallic, aerospace parts. Despite the high potential of this manufacturing process to reduce weight and lead time, the fundamental understanding of additive manufactured Ti–6Al–4V material is still at an early stage, especially in the area of fatigue and damage tolerance. This paper covers the effects of inherent surface roughness on the fatigue life. In the as built condition, metallic parts have a poor surface texture, which is generally removed in fatigue critical areas. It is shown that the fatigue properties of Ti–6Al–4V samples, produced by direct metal laser sintering and electron beam melting, are dominated by surface roughness effects. A simple model based on an equivalent initial flaw size is formulated.