Search

This paper presents a comprehensive numerical investigation to simulate heat transfer and residual stress formation of Ti-6Al-4V alloy during the Laser Powder Bed Fusion process, using a finite element model (FEM). The FEM was developed with a focus on the effects of key process parameters, including laser scanning velocity, laser power, hatch space, and scanning pattern in single-layer scanning. The model was validated against experimental data, demonstrating good agreement in terms of temperature profiles and melt pool dimensions. The study elucidates the significant impact of process parameters on thermal gradients, melt pool characteristics, and residual stress distribution. An increase in laser velocity, from 600 mm/s to 1500 mm/s, resulted in a smaller melt pool area and faster cooling rate. Similarly, the magnitude of residual stress initially decreased and subsequently increased with increasing laser velocity. Higher laser power led to an increase in melt pool size, maximum temperature, and thermal residual stress. Hatch spacing also exhibited an inverse relationship with thermal gradient and residual stress, as maximum residual stress decreased by about 30% by increasing the hatch space from 25 µm to 75 µm. The laser scanning pattern also influenced the thermal gradient and residual stress distribution after the cooling stage. [ABSTRACT FROM AUTHOR]

The fatigue performance of laser powder bed fusion-fabricated Ti-6Al-4V alloy was investigated using four-point bending testing. Specifically, the effects of keyhole and lack-of-fusion porosities along with various surface roughness parameters, were evaluated in the context of pore circularity and size using 2D optical metallography. Surface roughness of Sa = 15 to 7 microns was examined by SEM, and the corresponding fatigue performance was found to vary by 102 cycles to failure. The S–N curves for the various defects were also correlated with process window examination in laser beam power–velocity (P–V) space. Basquin's stress-life relation was well fitted to the experimental S–N curves for various process parameters except keyhole porosity, indicating reduced importance for LPBF-fabricated Ti-6Al-4V alloy components. [ABSTRACT FROM AUTHOR]

In the present study, corrosion properties and biocompatibility of as-built and as-polished Ti–6Al–4V samples fabricated by Electron Beam Melting (EBM) and Selective Laser Melting (SLM) were investigated and compared with a conventional sample as a reference. Optical microscope, Scanning Electron Microscope equipped with Energy Dispersive Spectroscopy, and X-ray diffraction analysis were employed for studying the microstructure and composition of the samples. Polarization, electrochemical impedance, and immersion tests were carried out to investigate the corrosion behavior and bioactivity of the samples in the Simulated Body Fluid solution. The results revealed that the EBM samples exhibited a superior corrosion resistance compared to the SLM one, thanks to the absence of low corrosion resistant α′ martensitic phases and a higher fraction of β phase in the EBM samples. It was also observed that while the wrought Ti–6Al–4V samples had a higher corrosion current density than the additively manufactured ones, both EBM and SLM processes had a lower corrosion resistance in the as-built state than in the as-polished. The immersion tests in the SBF solution revealed a more significant bioactivity for the EBM samples than the SLM samples. Higher levels of the β phase in the EBM microstructure stimulated the nucleation and growth of the apatite on the sample surface. Also, higher surface roughness in the as-built samples improved the bioactivity by increasing the metal/electrolyte interface and thus forming more OH− groups on the Ti alloy surface. [ABSTRACT FROM AUTHOR]

Laser powder bed fusion (L‐PBF) technique is utilized in various industries to create intricate designs. The durability of L‐PBF components should be evaluated when subjected to different load cycles to widen their applicability in other fields. While previous studies have mostly concentrated on static and low cycle loads, high cycle and very high cycle loadings are crucial in many applications. This study examines the impact of stress‐relieved (SR) heat treatment on very high cycle fatigue (VHCF) performance of L‐PBF Ti‐6Al‐4V alloy. The SR heat treatment enhanced the VHCF response of the alloy by lowering defect levels and their size along with changing the microstructure, thus demonstrating their increased versatility for wider industrial applications. Highlights: Stress‐relieved heat treatment increases VHCF performance of L‐PBF Ti‐6Al‐4V alloy.Stress‐relieved heat treatment decreases defects number of L‐PBF Ti‐6Al‐4V alloy.Stress‐relieved heat treatment decreases the defects size of L‐PBF Ti‐6Al‐4V alloy.Stress‐relieved heat treatment changes the microstructure of L‐PBF Ti‐6Al‐4V alloy. [ABSTRACT FROM AUTHOR]

An unfavourable combination of metallurgical factors and high stresses may result in the fracture of additively manufactured parts, especially in the case of high-strength alloys. Frequently in direct laser deposition (DLD) fabrication of large-scale parts, it is impossible to obtain desirable structural-phase composition and required mechanical properties of the depositing metal due to the low inter-pass temperature. Thereby, the stress and strain levels must be reduced to prevent fracture of the build part. The present work seeks to analyse the effectiveness of the proposed approaches for reducing residual stresses and strains by (i) local adjustment of the buildup geometry and (ii) addition of a ductile transition layer. The process conditions were close to those used to produce large-scale parts by the DLD method, resulting in the buildup having the same thermal history. The following cases are analysed: a single-wall buildup of Ti-6Al-4V alloy with flat-faced and concave fillets, and a single-wall buildup with a ductile transition layer of commercially pure titanium between the rigid substrate and rest buildup of the Ti-6Al-4V. A simulation procedure based on the implicit finite element method was adopted for the theoretical study of the stress-strain field evolution. It was found that the addition of flat-faced fillets has little effect on the magnitude of the accumulated plastic strain. On the other hand, concave fillets result in considerable plastic strain reduction compared to buildup without fillets by a factor of 2.8 for 14-mm fillet radius and 5.8 for 28-mm fillet radius. The addition of a transition layer of a less strong and more ductile alloy than the rest buildup causes non-uniform deformation of the buildup. The more brittle buildup part of the Ti-6Al-4V practically does not deform plastically and has a considerably lower peak stress than the buildup entirely composed of the Ti-6Al-4V. [ABSTRACT FROM AUTHOR]

Our research aims to investigate the fabrication of additively manufactured (AMed) Ti-6Al-4V samples under reduced power with the addition of TiC heterogeneous nucleation site particles. For this aim, Ti-6Al-4V samples are fabricated with and without TiC heterogeneous nucleation site particles using an EOS M 290 machine under optimal parameters and reduced power conditions. The microstructure and tensile behavior of the produced samples were studied. In addition, a single-track test was performed to obtain a good understanding of the suppression of gas pores and balling formation with the addition of TiC heterogeneous nucleation site particles. It was found that the formation of gas pores and balling was suppressed with the addition of heterogeneous nucleation site particles within the metallic powder. [ABSTRACT FROM AUTHOR]

The surface quality of three-dimensional products manufactured via laser powder bed fusion (L-PBF) depends on solidification conditions, the staircase effect caused by layer-by-layer build-up processes, and the shape and size of un-melted particles. Although the surface quality can be slightly improved by optimizing process parameters such as layer thickness, laser power, hatch spacing and scan speed, the effect is quite limited, and thus additional post-treatment processes are required for dramatic improvement. In the paper, the effects of layer thickness (30, 60 µm) and build angle (0°, 15°, 30°, 45°, 60°, 75°, 90°) on the surface roughness of the Ti–6Al–4V alloy parts manufactured by L-PBF process were analyzed. In addition, in order to investigate the effect of blasting conditions on the surface roughness, the specimens were post-treated by various sizes of abrasives (#60, #80, #100, #120, #150), blasting angle (45°, 90°), and pressures (0.3, 0.4, 0.5 MPa), and the surface roughness of each specimen was measured and compared. The results indicate that the higher is the building angle and thinner is the layer thickness, the better is the surface quality; however, the difference tends to decrease via sandblasting, and the effect depends on the process conditions. Sandblasting process improved the surface roughness up to 62% and 71% compared as-built conditions for the layer thickness 30 µm and 60 µm for each. As a result, the surface roughness of minimum 5.0 µm and 3.8 µm for the layer thickness 60 µm and 30 µm was achieved in the case of building angle 90°. [ABSTRACT FROM AUTHOR]

The build plate location and post-processing effects on the properties of Ti-6Al-4V alloys fabricated via electron beam melting (EBM) are investigated in the context of spatial variability. Tensile test specimens are fabricated in four corner and center locations of the EBM machine build plate with three different dimensions. One specimen has standard dimensions of 6 mm gauge length diameter; the second one has 25% higher diameters, called the offset specimens, and the third is a cylinder with a diameter of 15 mm, called the block specimens. The standard specimen dimensions are obtained by applying machining to the block and offset specimens to evaluate the influence of the larger dimension and post-processing on the mechanical properties, including spatial variability. The yield strength, tensile strength, and ductility are obtained by performing tensile tests. The density and surface roughness of the specimens are measured before and after machining. Correlation plots and semivariograms are used to examine the effect of spatial variability on the properties of the fabricated specimens at different build plate locations. The densities of the block, offset, and standard specimens are 99.4%, 98.6%, and 98.8%, respectively. The surface roughness (Ra) values reduced from 29.5 to 0.7 µm after machining the block specimens, while the roughness is as high as 47.2 µm for the standard specimens. Following the machining process, the yield strength increases from 970 MPa for the standard specimens to 1039 MPa for the block specimens. The tensile strengths are approximately 1115 MPa and 1074 MPa for the block and offset specimens, respectively, whereas for the standard specimens, it is as low as 996 MPa. Similarly, the ductility increases to 13.8% when the block specimens are machined compared to the standard specimen result of 5.4%. For the standard specimens, approximately 5% change in the parameters could be observed at different build plate locations. The location dependency is lesser for the block and offset specimens after machining. There is higher correlation in material properties without any post-processing, whereas all the properties exhibit spatial variability according to the correlation plots and semivariogram analysis. [ABSTRACT FROM AUTHOR]

Ti–6Al–4V alloys undergo a multiple phase transformation sequence during electron beam powder bed fusion (EB-PBF) additive manufacturing, forming unique dislocation substructures. Thus, determining the dislocation density is crucial for comprehensively understanding the strengthening mechanisms and deformation behavior. This study performed time-of-flight neutron diffraction (TOF-ND) measurements of Ti–6Al–4V alloys prepared via EB-PBF and examined the dislocation density in the as-built and post-processed states using convolutional multiple whole profile (CMWP) fitting. The present TOF-ND/CMWP approach successfully determined the bulk-averaged dislocation density (6.8 × 1013 m−2) in the as-built state for the α-matrix, suggesting a non-negligible contribution of dislocation hardening. The obtained dislocation density values were comparable to those obtained by conventional and synchrotron X-ray diffraction (XRD) measurements, confirming the reliability of the analysis, and indicating that the dislocations in the α-matrix were homogeneously distributed throughout the as-built specimen. However, the negative and positive neutron scattering lengths of Ti and Al, respectively, lowered the diffraction intensity for the Ti–6Al–4V alloys, thereby decreasing the lower limit of the measurable dislocation density and making the analysis difficult. [ABSTRACT FROM AUTHOR]

In order to avoid the formation of defects in additive manufacturing (AM) by powder bed fusion (PBF) process, it is crucial to understand the relationship between the quality of powder bed and a powder spreading process. In this study, the influences of conditions of powder raking process on the densities and homogeneity of powder bed have been examined by computer simulation using Discrete Element Method (DEM) and experiment of powder bed formation using a blade-type spreader for Ti-6Al-4V powder by way of example. The results were analyzed with a special focus on the effects of the relative size of powder particles with respect to the gap between the blade and the build platform. It has been clearly shown that the gap needs to be larger than the upper bound of the distribution of powder particles diameter to obtain a high-density powder bed. The DEM simulation indicated that the blade can sweep even powder particles that are not in direct contact with the blade when the powder particles are in a close-packed tetragonal configuration. This is the probable reason for the experimental fact that powder particles smaller than half of the gap are suitable for PBF. [ABSTRACT FROM AUTHOR]

Currently, the interest in the application of metal additive manufacturing (AM), also known as 3D printing, is grown massively in the various fields of the industry and surgery. AM has significant multiple advantages compared to traditional subtractive technologies for making highly customized parts with complex geometries without causing noteworthy extra costs. Now, several powder-based AM technologies for metals’ 3D printing are in progress, in particular, selective laser sintering (SLS), selective laser melting (SLM), and electron-beam melting (EBM). In the past few decades, increasing research and developments are devoted to the wire-feeding-based 3D printing production of parts made of the Ti–6Al–4V alloy, which is widely investigated in different fields such as aerospace, automotive, energy, and marine industries as well as the prosthetics and the production of orthopaedic implants. Due to the feasibility of economical producing large-scale metal components with relatively high deposition rate, low machinery cost, high material efficiency, and shortened lead-time compared to powder-based AM, wire-feeding-based AM (WFAM) is attracting significant attention in the industry and academia owing to its ability for the production of the large components of the medium geometric complexity. In recent years, three options of WFAM are intensively researched, which differ by the wire-melting heating sources: wire + arc additive manufacturing (WAAM); wire-laser AM (WLAM), and wire electron-beam additive manufacturing (WEBAM). The purpose of the present review is systematic analysis of the mechanical properties of the Ti–6Al–4V alloy samples 3D-printed by WFAM with various heating melting sources, namely, arc, laser, and electron beam. Particularly, considering the literature data for the period of 2013–2020, such important properties as yield strength, tensile strength, elongation, and hardness are analysed for the samples in the as-printed and post-processed conditions. [ABSTRACT FROM AUTHOR]

In this work, the external longitudinal magnetic field hybrid metal inert gas welding (M-MIG) additive manufacturing method is employed to produce the Ti-6Al-4V alloy part. The effect of process parameters on the droplet transfer formation and microstructure of the part was studied by a high-speed camera, optical microscope, and electron backscattered diffraction. The results showed that a typical short-circuiting transfer was obtained with the wire feeding speed of 2 m/min–4 m/min. An external longitudinal magnetic field had an obvious effect on the arc shape. The uniform formation of the deposition layer was obtained with the wire feeding speed of 4 m/min. The width of M-MIG deposition layer was greater than that of the MIG, and the width of M-MIG deposition layer was increased with the increase of the magnetic excitation current. The microstructure of the deposition layer was mainly comprised of acicular martensite α' and massive martensite αm. In addition, the β grain size in the M-MIG was less than that of the MIG. The average microhardness of the MIG deposition layer was 281.6 HV, which was less than that of M-MIG. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Aeronautical Materials is the property of Editorial Board of Journal of Aeronautical Materials and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

3D printing (or more formally called additive manufacturing) has the potential to revolutionize the way objects are manufactured, ranging from critical applications such as aerospace components to medical devices, making the materials stronger, lighter and more durable than those manufactured via conventional methods. While the mechanical properties of Ti-6Al-4V parts manufactured with two major 3D printing techniques: selective laser melting (SLM) and electron beam melting (EBM), have been reported, it is unknown if the corrosion resistance of the 3D-printed parts is comparable to that of the alloy made with isothermal forging (ISF). The aim of this study was to identify the corrosion resistance and mechanisms of Ti-6Al-4V alloy manufactured by SLM, EBM and ISF via electrochemical corrosion tests in 3.5% NaCl solution, focusing on the effect of microstructures. It was observed that the equiaxed α + β microstructure in the ISF-manufactured Ti-6Al-4V alloy had a superior corrosion resistance to the acicular martensitic α′ + β and lamellar α + β microstructures of the 3D-printed samples via SLM and EBM, respectively. This was mainly due to the fact that (1) a higher amount of β phase was present in the ISF-manufactured sample, and (2) the fraction of phase interfaces was lower in the equiaxed α + β microstructure than in the acicular α′ + β and lamellar α + β microstructures, leading to fewer microgalvanic cells. The lower corrosion resistance of SLM-manufactured sample was also related to the higher strain energy and lower electrochemical potential induced by the presence of martensitic twins, resulting in faster anodic dissolution and higher corrosion rate. [ABSTRACT FROM AUTHOR]

Selective laser-melted (SLM) Ti-6Al-4V alloy was quasi-statically compressed in the transverse and longitudinal directions at a strain rate of 1 × 10−3 s−1 at room temperature. The twinning, in-grain misorientation axis (IGMA) distribution and active slip modes of individual grains in the deformed SLM Ti-6Al-4V alloy were studied in detail via transmission Kikuchi diffraction (TKD) and transmission electron microscopy (TEM). The α'/α phase was textured with the c-axis oriented either at ~45° or perpendicular to the building direction (BD). A combined analysis of the IGMA distribution and Schmid factor revealed that the prismatic slip or pyramidal slip was easily activated in the soft grains with their c-axes perpendicular to the BD (or the loading direction) in the longitudinal compressed sample, while slip was hardly activated in the transverse compressed sample due to the lack of soft grains. Prismatic slip with IGMA around <0001> Taylor axis also occurred in {10–11} twins. The observations revealed that the prismatic slip played a key role in accommodating the external strain and, thus, well explained the anisotropy of mechanical properties in the SLM Ti-6Al-4V alloy. [ABSTRACT FROM AUTHOR]

Direct energy deposition (DED) is a highly applicable additive manufacturing (AM) method and, therefore, widely employed in industrial repair-based applications to fabricate defect-free and high degree precision components. To obtain high-quality products by using DED, it is necessary to understand the influence of the process parameters on the product quality. The optimization of such processing parameters provides several advantages such as minimization of the loss of material and time. However, the optimization of the complex relationship between the process parameters-geometry-properties of the fabricated sample is difficult to realize and requires significant experimentation. Herein, a computational model based on artificial neural networks was developed to optimize process parameters for DED-processed Ti-6Al-4V alloy. The model was developed to estimate the density and build height (the actual build height realized after fabrication) of the sample as a function of power, scan speed, powder feed rate, and layer thickness. The optimum model with high-accuracy predictions was employed to construct the process maps for the DED-processed Ti-6Al-4V. The relative importance indices of process parameters on the build height and density were investigated. Further, the effect of power and scan speed on the microstructure of Ti-6Al-4V alloy was discussed. Finally, based on the obtained results, the optimum fabrication conditions for the DED-processed Ti-6Al-4V alloy were determined. [ABSTRACT FROM AUTHOR]

[Display omitted] In this work, we have studied the fracture mechanism and proposed a micromechanical method with a unit cell model to predict the ductile fracture of additively manufactured (AMed) Ti-6Al-4 V alloy. The tensile experiments with smooth round bars and notched round bars were conducted to investigate the mechanical properties and ductile fracture behavior of materials. The proposed unit-cell model was validated by quasi-static tensile fracture tests to predict the ductile failure of titanium alloy manufactured by laser power bed fusion (L-PBF) under different stress states, which include a wide range of stress triaxialities. An equivalent initial void fraction was introduced to evaluate the ductile behavior of the titanium alloy. Our investigation shows that the stress states significantly influence the void coalescence behavior of AMed Ti-6Al-4 V alloy, and the fracture locus of L-PBF fabricated Ti-6Al-4 V alloy under different stress states was successfully predicted by the proposed unit-cell model under proportional loading. Our study provides useful guidance for the future design and application of metal materials for additive manufacturing. [ABSTRACT FROM AUTHOR]

Additive manufacturing of Ti-6Al-4V alloy via laser powder-bed fusion leads to non-equilibrium α ′ martensitic microstructures, with high strength but poor ductility and toughness. These properties may be modified by heat treatments, whereby the α ′ phase decomposes into equilibrium α + β structures, while possibly conserving microstructural features and length scales of the α ′ lath structure. Here, we combine experimental and computational methods to explore the kinetics of martensite decomposition. Experiments rely on in-situ characterisation (electron microscopy and diffraction) during multi-step heat treatment from 400 ∘C up to the alloy β -transus temperature (995 ∘C). Computational simulations rely on an experimentally-informed computationally-efficient phase-field model. Experiments confirmed that as-built microstructures were fully composed of martensitic α ′ laths. During martensite decomposition, nucleation of the β phase occurs primarily along α ′ lath boundaries, with traces of β nucleation along crystalline defects. Phase-field results, using electron backscatter diffraction maps of as-built microstructures as initial conditions, are compared directly with in-situ characterisation data. Experiments and simulations confirmed that, while full decomposition into stable α + β phases may be complete at 650 ∘C provided sufficient annealing time, visible morphological evolution of the microstructure was only observed for T ≥ 700 ∘C, without modification of the prior- β grain structure. • We study martensite decomposition (α ′ → α + β) kinetics during heat treatments of additively manufactured Ti-6Al-4V alloy. • We combine in-situ microscopy and diffraction during heat treatment with phase-field modelling of phase transformation. • We confirm that full martensite decomposition to α + β is already complete at 650 °C if provided sufficient annealing time. • Notable coarsening of the microstructure is only observed for T ≥ 700 °C, without modification of the prior- β grain structure. [ABSTRACT FROM AUTHOR]

Despite the fact that the additive manufacturing (AM) technique has been established for almost two decades, its optimisation is still performed by trial and error experimentation. In the present work, a finite element modelling approach was used to study both the temperature distribution and heat flux vector characteristics during multi-layer deposition of a Ti–6Al–4V alloy that take place in the AM process. The results revealed the difference between different powder deposition time intervals on thermal cycles, heat flux vectors and the resulting microstructures. Good agreement between the numerical and experimental results was found. The results obtained can be used for process optimisation. [ABSTRACT FROM AUTHOR]

Abstract Laser direct melting deposited (LDMD) Ti-6Al-4V alloys are mainly used in aerospace fields. It is very important to study their deformation behavior as a function of the application conditions (i.e. temperature and stress). This work compared the dynamic microstructure evolution and mechanical properties of LDMD Ti-6Al-4V alloys at 23 ℃ and 650 ℃. A method based on in-situ scanning electron microscopy (SEM) tensile testing was used to identify the initiation of cracks and propagation mode of slip bands. The deformation mechanisms at low and elevated temperatures were discussed. The results showed that the strength decreased and the elongation increased with increasing temperature. These results were verified by the propagation modes of slip bands and opening of the active deformation systems. [ABSTRACT FROM AUTHOR]

Abstract Recent trend in additive manufacturing of Ti-6Al-4V alloy is to alter the microstructure by changing process parameters or by the addition of alloying elements to achieve the required performance. In the present investigation direct metal laser sintering (DMLS) processed Ti-6Al-4V alloy with addition of varying amount of TiB 2 has been successfully fabricated. Effect of addition of TiB 2 on microstructure, hardness and wear performance was studied using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), electron back scattered diffraction (EBSD), Vickers micro-hardness and pin-on-disk tribometer respectively. With the increase in addition of TiB 2, the microstructure changed from martensite (α′) lath to refined bimodal structure. Consequently, hardness and wear performance showed dramatic improvement. An elementary hypothesis is proposed for the formation of bimodal structure instead of the martensite structure which is generally observed in DMLS processed Ti-6Al-4V. Highlights • Effect of TiB 2 (0.2–2.0%) addition on Ti-6Al-4V microstructure is studied. • Effect of TiB 2 (0.2–2.0%) on wear performance is studied. • Development of bimodal structure due to increase in TiB 2 %. • Understanding the change of martensitic structure to bimodal structure. [ABSTRACT FROM AUTHOR]

Abstract Temperature distribution and thermal history have been demonstrated to greatly affect the microstructure of additive manufacturing (AM)-fabricated part, and further dedicate its mechanical properties. This paper, thus, develops an accurate and high-efficiency linear six-node "Tri-Prism" element for transient nonlinear heat transfer analyses of the selective electron beam melting (SEBM) of Ti-6Al-4V alloy. A stepwise activated approach is used to model the material layer-by-layer deposition process for the SEBM simulation. The physics of interaction between the electron beam and the material is modeled by a Goldak double-ellipsoid volumetric heat source. A backward difference technique featured with the unconditional stability is employed to address the transient heat conduction equations. In order to handle the material nonlinear and radiation issues, an accelerated Newton-Raphson method (ANRM) is utilized herein. Two examples of transient heat transfer are first given to demonstrate the accuracy and efficiency of "Tri-Prism" element. Finally, the thermal responses of an aircraft engine blade fabricated by SEBM are comprehensively investigated. Graphical abstract Image 1 Highlights • A linear six-node "Tri-Prism" element is presented for thermal response investigations. • An aircraft engine blade is manufactured by selective electron beam melting. • The "Tri-prism" element is more accurate and high-efficiency than isoparametric hexahedron element. • Increasing electron beam currents, decreasing travel speeds contribute to improve the temperature and melt pool dimensions. • The maximum thermal gradient declines gradually with the increment of building height during layer-by-layer fabrication. [ABSTRACT FROM AUTHOR]

Abstract Additive manufacturing has emerged in recent years as one of the most in-demand technology for manufacturing of prototypes and components with complex geometries. Its application in biomedicine shows a big productive potential concerning customized implants with optimized geometries, reducing risks of rejection and achieving better ergonomic results. However, Ti-6Al-4V ELI pieces produced by Direct Metal Laser Sintering (DMLS) need a post heat treatment in order to reach optimum mechanical properties as well as to release internal stresses resulted from the manufacturing process. This study aims at analyzing the change of microstructure in DMLS samples by effect of heat treatments and its influence in the corrosion behavior and in the anodizing process. The surfaces were analyzed by 3D confocal profilometry, scanning electron microscopy and energy dispersive X-ray spectroscopy. DMLS samples showed different microstructures after heat treatments in the range from 650 to 1050 °C. The main difference in the microstructures is the nucleation and growth of β phase, which could have a detrimental effect on its corrosion behavior. The corrosion behavior was studied by cyclic potentiodynamic polarization in phosphate buffered saline solution at 37 °C. After anodizing, an oxide film was grown on the material surface, composed of a barrier and a nanoporous layer doped with F ions. The passive current density was reduced by approximately two orders of magnitude for all conditions, despite morphological differences on the layers. This is not only an improvement of the corrosion resistance but also a decrease in the ion release into bloodstream. Graphical abstract Image 1 Highlights • Microstructures containing β phase displayed pitting potential. • Microstructures affects the morphologies of the anodized oxide layers. • Despite morphological differences, anodizing was successful for all conditions. • Anodic layers reduced current density by two orders of magnitude for all conditions. [ABSTRACT FROM AUTHOR]

• Innovative aspects of this work are the proposed effective SIF models that can be adapted for different modes of propagation. • The proposed SIF models can be used to evaluate the effective SIF depending on the instantaneous mode of propagation. • The μ-CT technique was used to understand crack propagation in an AM Ti6Al4V alloy stressed with in-phase multiaxial loading. • A large bulk of new experimental data have been deeply discussed, which can be useful for research in the same area. Selective laser melting (SLM) is one of the most promising additive manufacturing technologies for obtaining end-use components with excellent mechanical properties. However, the presence of manufacturing defects harms mechanical properties and fatigue performance. This work investigated the crack behaviour within an SLM Ti-6Al-4V alloy sample stressed with in-phase axial and torsional loading by X-ray computed μ-tomography (μ-CT). A sub-volume of material that contains the most significant defects was detected and analysed. The critical defect was identified by evaluating the effective strain intensity factor (SIF) using two different proposed models, that consider the effective area of a detected defect according to Murakami's method. The first model is based on the modified Smith Watson and Topper (MSWT) criterion. The second is based on Liu's virtual strain energy (VSE) method. The trend of the crack growth rate was obtained by measuring the effective area for a different number of cycles. The μ-CT data was also exploited to build a finite element model (FEM) of part of the gauge section containing surface defects. Finite element analysis (FEA) results have provided information on the stress state leading to high-stress concentrations at the crack tip influencing the propagation. [ABSTRACT FROM AUTHOR]

It is an often challenging but essential issue to develop a facile and cost-effective approach to additively manufacture crack-free and high-strength Al alloys for their wider commercial applications. However, efficient synergistic combination of advanced technology and material development is hardly likely to achieve without considering the extreme processing conditions during additive manufacturing. In this study, the addition of 4 wt% Ti-6Al-4V (TC4) alloy powders, the most commercially used Ti alloy powders, into the laser powder bed fusion (L -PBF) of 7075 Al alloy was performed and experimentally verified to be capable of eliminating the hot cracks while substantially refining the grains. The as-printed sample was crack-free and had a high relative density of up to 99.95%. Synergy effect of the in-situ formation of Al 3 (Ti x ,V 1−x) particles that act as the heterogeneous nucleation sites, and the solute effect of the elements Ti and V that possess a high grain growth restriction factor (Q value) in Al matrix can efficiently control the grain growth. After a direct ageing treatment, the resultant alloy displayed an excellent combination of the high ultimate tensile strength (441 ± 3 MPa) and large plasticity (10.1 ± 0.4%). Our work brings new insights between the grain refinement and crack elimination of additively manufactured Al alloys through addition of commercial Ti alloys, benefiting the tailoring and development of new crack-free and dense high-strength Al alloys for L -PBF. [Display omitted] • The addition of Ti-6Al-4 V alloy powders to laser powder bed fusion of 7075 Al alloy can eliminate the cracks. • The as-printed sample is crack-free and has a high relative density of up to 99.95%. • The grain refinement is due to the formation of Al 3 (Ti x ,V 1−x) particles and solute effect of the Ti and V. • The heat-treated sample shows a superior combination of high ultimate tensile strength and large plasticity. [ABSTRACT FROM AUTHOR]

In this study, the method to improve the bead quality and increase the wire-feed deposition efficiency propose in the CMT-GMA (cold metal transfer-gas metal arc) process of Ti-6Al-4 V alloy deposition. The proposed method uses He shielding gas to improve the Ti deposition quality by increasing the wettability through the effective arc energy transfer realized by suppressing the adverse effect of cathode spot on the molten pool. The cathode spot is dispersed by the high ionization energy of He shielding gas, which improves the Ti deposition quality owing to the stability, narrow tip, and broad tail of the arc, which enables a bead shape that improves the sound deposition quality. Increasing the detachment current increased the deposition rate, attributed to the electromagnetic force-induced separation in the molten bridge and increase in droplet repetition. Under a detachment current of 290 A and when using the He shielding gas, the deposition rate was 2.69 kg/h, which was 20.5 % higher than that under a detachment current of 50 A and when using Ar shielding gas. Through multilayer deposition, the deposition efficiency in single-bead multilayer deposition (1 ×5) was reduced to 30.1 % under Ar shielding gas and to 12.5 % under He shielding gas. In the case of multi-bead multilayer deposition (5 ×5), the Ar shielding gas induced cavities in the deposition structure. This result confirmed that the application of He shielding gas can help fabricate a high-density multilayer structure without interlayer defects. [ABSTRACT FROM AUTHOR]

• Ti-6Al-4V DED samples with different content of recycled powder demonstrated the stable chemical composition. • The tendency of decrease the α lath size with increase of recycled powder content in Ti-6Al-4V samples was determined. • The degradation of mechanical properties of annealed DED samples with increase of recycled powder content was not observed. • No difference in fracture surfaces with different contents of the used powder. [Display omitted] Direct energy deposition (DED) is a promising additive manufacturing technique in the area of aerospace, automotive, shipbuilding and medical industries. DED allows producing large-scale parts that imply the use of a significant amount of powders. Powder reuse is an important task that leads to a reduction in the cost of the final products. In this work, Ti-6Al-4V samples were fabricated by DED technique with a mixture of unused and recycled powders in the recycled powder content of 0%, 10%, 25%, and 50%. The results show that the morphology of the powder does not change significantly due to a low number of powder uses. Chemical analysis reveals a slight decrease of V in the recycled powder. The evolution of macrostructure with increasing recycled powder content was not revealed. In the as-built samples, the columnar prior β grains grow toward the build direction with a typical Widmanstätten structure consisting of the thin α' laths. The mix of Widmanstätten microstructure and basket weave microstructure appears after the annealing at 800 ℃. Furthermore, the horizontal and vertical annealed samples exhibit ductile fracture and inconspicuous anisotropy of tensile and impact mechanical properties. [ABSTRACT FROM AUTHOR]

In the present study, pre-alloyed Ti–6Al–4V powder is deposited on C P-Titanium substrate by laser engineered net shaping (LENS™) process using parameters optimized for best adhesion and densification. The optical montages from the three surfaces (front, side and top) show columnar β grains growing along the building direction due to conductive heat transfer through the substrate. The as-deposited microstructure contains thin lamellar (α+β)-colonies besides prior β grain boundaries and basket-weave (α+β)-structure inside prior β grains. Narrow band-like structure forms between consecutive layers due to re-melting of previously deposited layers, thereby creating additional interfaces in the microstructure. In addition, tiny isolated pores appears in negligible fraction throughout the LENSTM-processed specimen due to gas entrapment, shrinkage during cooling and un-melted or partially melted powder particles. Both the number and volume of the pores increase along the building direction. Hardness on different surfaces (front, side and top) differs considerably due to the presence of different heating/cooling zones, residual stresses and variations in the thermal cycles and consequent change in the α ' -martensite phase fraction. Larger variation in the hardness between these surfaces is observed in nano-indentation technique signifying for inhomogeneity in nano-scale structure. These microstructural variations also resulted in measurable changes in the coefficient in friction (COF) during scratch testing from the substrate along the building direction due to presence of different heating/cooling zones (diffused vs. reheating/re-melting zones). The variation in hardness and COF along different directions can ultimately lessen the in-service performance of the as-deposited parts. • Thin lamellar (α+β) colonies beside prior β grain boundary and acicular α′-martensites (basket-weave structure) inside. • Presence of primary α / α ' -phase along with small fraction of α"-martensite and remnant β phase. • Number of pores and sizes increase while sphericity decreases along the building direction. • Larger error in nano-indentation compared to macro- and micro-indentation indicates nano-scale microstructural inhomogeneity. [ABSTRACT FROM AUTHOR]

The particular microstructure of Ti-6Al-4V fabricated by additive manufacturing (AM) makes it exhibit a different corrosion resistance compared with that of the conventional forged Ti-6Al-4V. Here we utilize the electrochemical impedance (EIS) and potentiodynamic polarization (Tafel) to evaluate the electrochemical dissolution behavior in runway deicing fluids (RDF) of three kinds of Ti-6Al-4V alloy, which are respectively fabricated by forging, electron beam melting (EBM), and selective laser melting (SLM). The microstructure and electrochemical results suggest that EBM samples have a better corrosion resistance than others', which is attributed to an appropriate proportion of α-lath width and the volume fraction of β phase. A higher relative proportion of TiO 2 is formed on surface, resulting in the fine-lamellar structure with a higher density of grain boundaries. Besides, the corrosion resistance anisotropy of AMed Ti-6Al-4V samples is ascribed to the existence of β columnar gains. In addition, the acicular martensite α′ phase with metastable structure leads to the worst corrosion resistance of SLMed Ti-6Al-4V samples. This work enriches the corrosion resistance of Ti-6Al-4V alloy in strong electrolyte solutions, which helps to promote the service performance of aviation titanium alloys in harsh environments. [Display omitted] • EBM Ti-6Al-4V is more stable than forged one in runway deicing fluids (RDF). • There is anisotropy of corrosion in EBM and SLM Ti-6Al-4V alloys. • The passivation film formed on the SLM Ti-6Al-4V alloy is fragile due to α′ martensite. • The fine-lamellar α affects the proportion of Ti oxides formation on the surface. • Corrosion preferentially occurs at the grain/interfacial boundaries on the surface of Ti-6Al-4V. [ABSTRACT FROM AUTHOR]

Hot isostatic pressing (HIPing) treatment was generally carried out to eliminate possible defects and to homogenize the microstructure for Ti–6Al–4V alloy fabricated by the Electron beam rapid manufacturing (EBRM) process. But the HIPed alloy with relative coarse lamellar microstructure exhibited a low strength compared to the as-built one. Post solution-aging treatments were necessary to optimize the HIPed alloy's tensile properties. So far, the relationship between the heat-treated microstructure features and corresponding mechanical properties of EBRM-produced Ti–6Al–4V has not been completely understood. Therefore, in the present work, solution-aging treatments under various solution temperatures (930 °C–990 °C) were applied to the HIPed Ti–6Al–4V produced by the EBRM, and the corresponding microstructures and mechanical properties were investigated. The results showed that solution-aging treatments could increase the HIPed alloy's tensile strength but decrease its ductility. With increasing solution temperature, the tensile strength presented an upward tendency until to 980 °C, whereas a decrease after 990 °C solution treatment. The variation in tensile properties was caused by the microstructures evolved with solution temperatures. In-situ tensile results showed that the deformation and damage behaviors of the alloy were also highly affected by the solution temperatures. Owning to a decrease of content of the α p with increase of solution temperature, the alloy at high solution temperature presented superior resistance of crack initiation but inferior resistance of crack growth than that at low solution temperature. [ABSTRACT FROM AUTHOR]

Columnar grain morphology and typical α texture transformed from a strong β<100>//VD fiber texture are found in the Ti–6Al–4V alloy fabricated using electron beam rapid manufacturing (EBRM). This study mainly focused on the effect of the columnar grain morphology and the lamellar α orientation on the tensile damage behavior of the alloy. Samples in three different orientations were subjected to in-situ tensile tests. Results demonstrated that obvious anisotropic in-situ tensile behavior was presented in the alloy. Based on the columnar grain morphology and the β<100>//VD fiber texture, we analyzed the crystallographic and spatial orientation characteristics of α lamellae in the alloy, discussed the effect of the columnar grain morphology, the crystallographic and spatial orientations of α lamellae on anisotropic in-situ tensile behaviors, and proposed that the crystallographic texture of α phase was a main factor inducing anisotropic tensile properties. In addition, the anisotropic fracture displacement was also associated with the spatial orientation of α laths. • 1, The alloy presented concentrated distribution on both the crystallographic and spatial orientation of α lamellae. • 2, In-suit tensile behaviors were sensitive to the loading directions. • 3, The anisotropic in-suit tensile behavior was due to the crystallographic and spatial orientation of α laths. [ABSTRACT FROM AUTHOR]

The hot working behaviour of additively manufactured Ti–6Al–4V pre-forms by Electron Beam Melting (EBM) has been studied at temperatures of 1000–1200 °C and strain rates of 0.001–1 s−1. As a reference, a wrought Ti–6Al–4V alloy was also analyzed as same as the EBM one. In order to investigate the hot working behaviour of these samples, all the data evaluations were carried out step by step, and the stepwise procedure was discussed. No localized strain as a consequence of shear band formation was found in the samples after the hot compression. The flow stress curves of all the samples showed peak stress at low strains, followed by a regime of flow softening with a near-steady-state flow at large strains. Interestingly, it is found that the initial microstructure and porosity content as well as the chemistry of material (e.g. oxygen content) as being possible contributors to the lower level of flow stress that could be beneficial from the industrial point of view. The flow softening mechanism(s) were discussed in detail using the microstructure of the specimens before and after the hot deformation. Dynamic Recrystalization (DRX) could also explain the gentle oscillation in the appearance of the flow softening curves of the EBM samples. Moreover, the hot working analysis indicated that the activation energy for hot deformation of as-built EBM Ti–6Al–4V alloy was calculated as ~193.25 kJ/mol, which was much lower than the wrought alloy (229.34 kJ/mol). These findings can shed lights on a new integration of metal Additive Manufacturing (AM) and thermomechanical processing. It is very interesting to highlight that through this new integration, it would be possible to reduce the forging steps and save more energy and materials with respect to the conventional routes. • Hot working behaviour of as-built EBM is carried out for the first time. • EBM can be employed to fabricate Ti–6Al–4V pre-forms for hot forming. • This new integration of AM with conventional processes can reduce the number of forging passes. • No localized stain as a result of shear band formation is found. • EBM specimens have lower flow stress and higher grade of DRX. [ABSTRACT FROM AUTHOR]

The main purpose of this investigation was to identify microstructure and cyclic deformation behavior of a 3D-printed Ti–6Al–4V alloy via selective electron beam melting (SEBM). Abundant orientated α-lamellae were present in the prior columnar β grains along the building direction. Four distinct high-angle α/α grain boundaries were detected as stipulated by the Burgers orientation relationship in the β→α phase transformation at a relatively slow cooling rate during SEBM, including the unique low-energy prismatic/basal (PB) grain boundaries with a misorientation angle of 90° in the 3D-printed Ti–6Al–4V alloy. The alloy exhibited superior strength and adequate ductility, along with a certain extent of strain-rate sensitivity. Cyclic stabilization largely sustained, while cyclic softening occurred at higher strain amplitudes. Although fatigue life could be described by Smith, Watson and Topper (SWT) model, it would be better predicted via a total strain energy-based model. Crack initiation occurred from near-surface defects of entrapped gas pores or residual unmelted zones, and crack propagation was characterized by typical fatigue striations. The results obtained would help promote the safe and reliable applications of 3D-printed titanium components. Image 1 • A superior strength along with sufficient ductility is achieved via 3D printing. • Abundant α lamellae are present and positioned in the prior columnar β grains. • Distinct α/α grain boundaries are detected in the β.→α phase transformation. • Cyclic stabilization retains despite cyclic softening at higher strain amplitudes. • Fatigue life can be well predicted via a total strain energy-based approach. [ABSTRACT FROM AUTHOR]

In laser powder bed fusion (LPBF) processes, critical defect issues such as porosity limit the quality of the fabricated part. This work investigates the mechanism of pore evolution during the laser post-processing of an LPBF part. A laser beam was utilized to scan the top surface of a Ti–6Al–4V alloy part. The morphologies of the internal layers as well as the distribution of the porosity inside the Ti–6Al–4V sample were experimentally measured and compared using micro-computed tomography (micro-CT). It was found that the void fraction was significantly decreased, from 2.51% to 0.77%, by laser post-processing. A multi-physics coupled finite element model based on the Level-set method was built to analyze the evolution mechanism of the pore. This model simulated the effects of the energy density, Marangoni flow and mass transfer on the pore evolution. Typical processes were studied in detail, and a table summarizing the pore evolution under different post-processing parameters is given. Image 1 • Internal morphologies of Ti–6Al–4V sample measured by micro-CT. • A Level-set model is built to analyze the evolution mechanism of porosity. • Mass transfer on the bubble surface is the essence of pore evolution. • Main factor impacting gas bubble rising is from Marangoni effect rather than buoyancy. [ABSTRACT FROM AUTHOR]

Titanium alloy was extensively used in aerospace industrial field. It has a great significance to directly reveal the deformation process as a function of application conditions. This work investigates the real-time deformation behavior of laser direct metal deposited (LDMD) Ti–6Al–4V alloy at 200 °C, using self-developed high temperature in-situ scanning electron microscope (SEM) tensile stage with a controlled heating system. The results showed that the microstructure of the sample comprised of columnar β grains with zigzag grain boundaries decorated by tilted α plates along the building direction. During uniaxial tensile, slip bands develop along the grain boundaries between α plates with increasing stress. Grain boundary sliding contributes to the high tensile elongation. Compared the mechanical properties along transvers tensile with those at room temperature, the tensile stress decreased and displacement increased at 200 °C. Therefore, higher elongation and reduction in cross-sectional area were identified at 200 °C than those at room temperature. The fracture of specimen at 200 °C shows obvious necking character and deep dimples with thin rims. Coalescence of slip bands at α/α interface creates voids and the coupling of voids nucleates micro-crack. α/α grain boundary interface constitutes path to the crack propagation. The high misorientation angle at columnar β grain boundary hinders the cracks propagation. Finally, the cracks accumulate along the columnar β grain boundary leading to fracture. • The microstructure deformation of LDMD Ti–6Al–4V alloy is studied by high temperature in-situ SEM tensile stage at 200 °C. • Slip bands development, cracks propagation, and columnar β GB hindering at 200 °C are investigated. • Temperature effect on the deformation mode and corresponding mechanical properties have been revealed. [ABSTRACT FROM AUTHOR]