Additive Manufactured Ti 6Al 4V Research Articles

The Experimental Assessment of Deformation and Springback Behavior for Additive Manufactured Ti6Al4V

The Experimental Assessment of Deformation and Springback Behavior for Additive Manufactured Ti6Al4V

Tensile Response Characterization and Constitutive Modeling of LPBF Ti6Al4V Thin Struts

Additive manufacturing enables the fabrication of orthopedic implants with lattice materials whose topology is tailored to mimic the mechanical response of native bone. Detailed analysis of such lattice structures for optimizing their mechanical biocompatibility requires an understanding of the mechanical response of their constituent components, that is, “struts”. The present study conducts a series of miniature‐specimen mechanical experiments to analyze the apparent stress–strain response of additive manufacturing Ti6Al4V struts. The “true” geometries of the struts are then derived based on observations from microcomputed tomography to discuss the size and orientation dependence of the “geometrical mismatch” as well as the “true” stress–strain response of the struts. It is however argued that considering the “true” mechanical response of struts in the design and topology optimization of lattice‐based implants is not practically feasible, since it requires information regarding the “true” geometry of each strut within the implant that is not accessible at the design stage. As an alternative, consideration of a representative “apparent” constitutive model in finite‐element simulations representing the “nominal” geometry of the lattices provides acceptable approximations of their experimentally observed mechanical response and, therefore might be employed for design analysis and topology optimization of lattice‐based implants.

Characterization of hydroxyapatite coatings produced by pulsed-laser deposition on additive manufacturing Ti6Al4V ELI

Additive manufacturing is a disruptive technology that has changed the design of bone implants for clinical applications. In this work, the effect of the deposition energy parameter on the mechanical properties and surface microscopy of Ti6Al4V alloy substrates manufactured by electron beam melting and coated with hydroxyapatite deposited by the pulsed laser technique was investigated. The average hardness and microhardness values were obtained under microindentation and indentation tests. The morphology and chemistry of the substrate coating were evaluated using scanning electron microscopy and X-ray photoelectron spectroscopy, obtaining average values of the size of the coating particles. Results indicate that high incident energy values yield a more homogeneous morphology, with smaller grain size and higher hardness values. The chemical composition indicates a hydroxyapatite coating with good stoichiometry.

Discovering the role of the defect morphology and microstructure on the deformation behavior of additive manufactured Ti–6Al–4V

The defects remain a significant issue for additive manufactured titanium alloys, which hinders their further market penetration. In this work, we proposed a new understanding of the inhomogeneous microstructure formation affected by defects with detailed EBSD characterization, and how the defects and inhomogeneous microstructures determined the strain accumulation behavior by in-situ SEM tensile testing. Smaller prior β grains and coarser α laths were identified around the defects. In-situ tensile behavior investigations revealed that irregularly-shaped and elongated defects with high stress intensity factor (ΔKI) could induce severe strain accumulation, showing that the defect morphology was the primary factor that could determine the strain accumulation behavior. In the meantime, the inhomogeneous microstructures around the defects could further contribute to the strain accumulation, with the coarser α laths more favorable for deformation and stronger microtexture for easier slip transmission. In addition, microcracks were found initiated from a spherical pore with small loading, and arrested due to the negligible strain accumulation. Further characterization showed that oxidation, which was found more probable in spherical pores, was the cause of this microcrack initiation.

Mechanism of ultrahigh ductility obtained by globularization of αGB for additive manufacturing Ti–6Al–4V

In the present work, two post-processes, the 800 °C and 950 °C for 4 h annealing below β transus, were designed to improve the performance of selective laser melted (SLM) Ti–6Al–4V (Ti64) alloy. The results show that 800 °C annealing raises the ductility to 18%, while 950 °C annealing raises the ductility high up to 26%. Although the residual stress relief and martensite decomposition fully accomplished in both annealed samples, there is still a noticeable difference in ductility. The difference is due to that 950 °C annealing induce noticeable globularization which further alters the fracture mode. Further study indicates that with globularization of αGB phase (α phase along prior β grain boundary), the transgranular fracture replaces the detrimental intergranular fracture. Particularly, the high angle grain boundary (HAGB) between equiaxed αGB phase effectively inhibits the initiation and propagation of crack, when comparing to the low angle grain boundary (LAGB) in α lamella. The reason for globularization in 950 °C annealing could be concluded as the boundary splitting model. The β phase grow along the LAGB in α lamella and form thermal grooving. With the groove deepening, the lamella was separated into equiaxed α phase. In this process, high temperature annealing promotes sufficient bulk diffusion, which is a necessary condition for globularization of lamellar α phase in SLM Ti64 alloy.

Experimental study on laser polishing additive manufacturing Ti6Al4V

As a new precision processing technology, laser polishing is widely used in metal and non-metal. Laser polishing technology is especially suitable for parts with high hardness and small size. In this paper, a full factor experimental were carried out to investigate the effect of continues wave laser with a wavelength of 1064nm on the surface quality of additive manufacturing Ti6Al4V. The experimental result shows that a smooth surface can be obtained and the average roughness is reduced from 14.95μm to 1.756μm with the reduction rate is as high as 88.3% when laser power is 170W and scan speed is 60mm/s. And the height of the peaks and valleys on the sample surface has also been greatly reduced in condition of the optimized laser processing parameters.

Effects of the morphology of grain boundary α-phase on the anisotropic deformation behaviors of additive manufactured Ti–6Al–4V

Grain boundary α-phase (GB-α) in titanium alloys is the vulnerable site for failure, while the correlation between deformation behavior and local dislocation motion is still ambiguous. In this study, we made a detailed investigation into the deformation behavior of GB-α with different morphologies in a laser powder bed fusion (LPBF) fabricated Ti-6Al-4V (Ti-64) by using in-situ and interrupted tensile testing, and discussed the underlying mechanisms of dislocation motion. The deformation-induced dislocations were found severely accumulated within the continuous GB-α with the transverse loading direction, which corresponds to the strain localization, while uniformly distributed between the continuous GB-α and surrounding grains with the perpendicular tensile loading. In contrast, discontinuous GB-α could fully accommodate uniform deformation with the matrix microstructure under both loading directions. The new understanding of dislocation motions in GB-α provides a novel perspective to effectively eliminate the tensile property anisotropy with the columnar microstructure present in LPBF Ti-64.

Hybrid Ti6Al4V/Silk Fibroin Composite for Load-Bearing Implants: A Hierarchical Multifunctional Cellular Scaffold.

Despite the tremendous technological advances that metal additive manufacturing (AM) has made in the last decades, there are still some major concerns guaranteeing its massive industrial application in the biomedical field. Indeed, some main limitations arise in dealing with their biological properties, specifically in terms of osseointegration. Morphological accuracy of sub-unital elements along with the printing resolution are major constraints in the design workspace of a lattice, hindering the possibility of manufacturing structures optimized for proper osteointegration. To overcome these issues, the authors developed a new hybrid multifunctional composite scaffold consisting of an AM Ti6Al4V lattice structure and a silk fibroin/gelatin foam. The composite was realized by combining laser powder bed fusion (L-PBF) of simple cubic lattice structures with foaming techniques. A combined process of foaming and electrodeposition has been also evaluated. The multifunctional scaffolds were characterized to evaluate their pore size, morphology, and distribution as well as their adhesion and behavior at the metal–polymer interface. Pull-out tests in dry and hydrated conditions were employed for the mechanical characterization. Additionally, a cytotoxicity assessment was performed to preliminarily evaluate their potential application in the biomedical field as load-bearing next-generation medical devices.

The small-scale limits of electron beam melt additive manufactured Ti–6Al–4V octet-truss lattices

The emergence of powder-based additive manufacturing (AM) processes, such as electron beam melting (EBM), enables the one step manufacture of microarchitected metamaterials from topology optimized models. However, many applications are optimized by low relative density lattices with slender trusses whose diameter approaches small multiples of largest powder particles, potentially resulting in surface roughness. The thermal history experienced by alloy powders also modifies the alloy microstructure, and thus mechanical behavior, posing a significant challenge to metallic metamaterial designs and fabrication. We therefore build and characterize the multiscale structure and mechanical properties of EBM manufactured Ti–6Al–4V octet truss lattices with strut diameters approaching the particle diameter-imposed fabrication limit. We measure the dependence of their relative density, elastic modulus, and compressive strength on the fabrication process-controlled truss topology and microstructure, and compare them to identical smooth surface structures made from an annealed, wrought version of the same alloy built using a snap-fit assembly method. Micro-x-ray tomography confirmed that the lattice strut surfaces were covered with partially melted powder particles, resulting in about 29% of the lattice mass that inefficiently supported the applied loads. The use of a powder bed held at a temperature of 600–700 °C also resulted in a lamellar α/β phase microstructure with an elastic modulus, yield strength, and a ductility that were less than the equiaxed α/β microstructure of snap-fit assembled structures. However, the higher tangent modulus of the lamellar AM processed alloy resulted in significant strengthening of EBM lattices that failed by inelastic buckling during compression. The ability to increase the alloy tangent modulus during an EBM build process therefore provides a promising approach for increasing lattice compressive strength and therefore compensates for surface roughness induced losses.

Role of Volumetric Energy Density and Post-Heat Treatments to Achieve Stable Microstructure on Additive Manufactured Ti6Al4V Alloy

Role of Volumetric Energy Density and Post-Heat Treatments to Achieve Stable Microstructure on Additive Manufactured Ti6Al4V Alloy

Effect of HIP Defects on the Mechanical Properties of Additive Manufactured Ti6Al4V Alloy

The expanding use of Additive Manufacturing (AM) technology enables engineers and designers to plan and manufacture highly complex geometries that are impossible to manufacture with any other conventional technology. When comparing this with building parts using powder bed technology, the main differences found in the quality of the products concern fracture toughness, fatigue, and inferiority in tensile tests. To overcome these issues, the Hot Isostatic Press (HIP) procedure may be used to improve the material quality by reducing product porosity. Regarding fatigue, the standard procedure consists of HIP and the machining of specimens to their final geometry. However, in many AM parts, geometrical complexity does not enable complementary machining. Recently, some AM vendors integrated in-process milling capabilities into their machines, in an attempt to address this challenge. In this study, the authors examine the effect of the HIP procedure on representative samples in order to demonstrate its effect on the final products of TI-6Al-4V parts. The results indicate that the fatigue limit of HIPed parts can increase by 12%; however, a dramatic decrease in the fatigue limit was observed if any failure in the HIP process occurred. The authors suggest an optional procedure to improve performance in such cases.

Experimental investigation of hole quality in drilling of additive manufacturing Ti6Al4V parts produced by hole features

Although additive manufacturing (AM) applications are on the rise, machining processes such as drilling are required to complete the manufacturing process successfully. This study envisaged that after the Ti6Al4V alloy designed as a pre-hole feature is produced by the Selective Laser Melting (SLM) method, there will be significant advantages to drilling the holes the desired diameter size will bring to the production costs. Hole accuracy, hole surface roughness, cutting forces, cutting tool wear, and chip form were determined as evaluation criteria. Their relations with drilling parameters were evaluated. After drilling, the deviation values from the average diameter, circularity, and cylindricity in the AM holes improved about 98%. The increases in cutting forces in uncoated twist drills negatively affected the hole quality. The increased cutting forces increased deviation values from diameter, cylindricity, and circularity. Better values were obtained in coated twist drills. The lowest surface roughness (0.58 μm) was recorded at 50 m/min cutting speed and 0.15 mm/rev feed in coated drills. Minimum deviation from cylindricality and circularity were obtained in uncoated drills. It was determined that the diameter deviation of the coated tool was 76% less than the uncoated tool. With the study, it has been revealed that it may be advantageous to drill AM parts produced with unsupported front holes instead of drilling a full hole. Considering the smaller chip cross-section in the drilling process, the cutting speed and feed values could be increased to increase the drilling efficiency. In addition, in the study, it was determined that the proper drilling of the pre-drilled hole has a sensitive relationship with the fact that the pre-drilled hole centre and the drill centre come to the same point during drilling.

Extended mechanics of structural genome for predicting mechanical properties of additively manufactured Ti6Al4V considering porosity and microstructure

In this paper, a novel computationally efficient multiscale framework based on the extended mechanics of structure genome (XMSG) is presented for predicting mechanical properties based on microstructure for Ti6Al4V parts fabricated by additive manufacturing (AM). The XMSG offers its novel capability to account for the effects of microstructure heterogeneity, porosity growth, and crack propagation for the multiscale calculations of the elastic/plastic/damage behavior of the AM parts in a computationally efficient manner. It is shown that XMSG can provide several orders of magnitude improvement in computational efficiency with the same level of accuracy compared to other conventional methods such as representative volume elements. The XMSG framework was applied to predict the tensile and compression behavior of AM Ti6Al4V, which showed very good agreement with the experimental results. In addition, the XMSG framework was able to predict the asymmetry in Young's modulus of the AM Ti6Al4V under tensile and compression loading as well as the anisotropy in the mechanical behavior under tension.

Measurement of the Anisotropic Dynamic Elastic Constants of Additive Manufactured and Wrought Ti6Al4V Alloys.

Additively manufactured (AM) materials and hot rolled materials are typically orthotropic, and exhibit anisotropic elastic properties. This paper elucidates the anisotropic elastic properties (Young’s modulus, shear modulus, and Poisson’s ratio) of Ti6Al4V alloy in four different conditions: three AM (by selective laser melting, SLM, electron beam melting, EBM, and directed energy deposition, DED, processes) and one wrought alloy (for comparison). A specially designed polygon sample allowed measurement of 12 sound wave velocities (SWVs), employing the dynamic pulse-echo ultrasonic technique. In conjunction with the measured density values, these SWVs enabled deriving of the tensor of elastic constants (Cij) and the three-dimensional (3D) Young’s moduli maps. Electron backscatter diffraction (EBSD) and micro-computed tomography (μCT) were employed to characterize the grain size and orientation as well as porosity and other defects which could explain the difference in the measured elastic constants of the four materials. All three types of AM materials showed only minor anisotropy. The wrought (hot rolled) alloy exhibited the highest density, virtually pore-free μCT images, and the highest ultrasonic anisotropy and polarity behavior. EBSD analysis revealed that a thin β-phase layer that formed along the elongated grain boundaries caused the ultrasonic polarity behavior. The finding that the elastic properties depend on the manufacturing process and on the angle relative to either the rolling direction or the AM build direction should be taken into account in the design of products. The data reported herein is valuable for materials selection and finite element analyses in mechanical design. The pulse-echo measurement procedure employed in this study may be further adapted and used for quality control of AM materials and parts.

Effect of recycling powder on the fatigue properties of AM Ti6Al4V

Additive manufacturing (AM) techniques offer significant advantages with respect to conventional manufacturing (CM) ones such as the realization of highly customized components both dealing with their geometry and their mechanical properties. An important advantage of AM is the significant reduction of wasted material with respect to CM. However, AM techniques, such as the powder bed fusion, involve during the production an amount of powder higher than that needed to realize the final component even if the excess of powder in not interested by the melting process and can be recovered and used again. However, it is obvious to expect that the new powder feedstock that include this reused powder has different morphology characteristics since it has been subjected to a thermal history in the building chamber. These changes in the material feedstock can results in a different morphology of defects and consequently in different fatigue properties even for AM components realized with the same design geometry and same process parameters. In the present study, the effect of the use of recycled powders on the fatigue properties of AM Ti6Al4V has been investigated by considering specimens realized from virgin powder and recycled powder. The specimens fracture surfaces are investigated to correlate the effect of the powder on fatigue properties with the effect on the defects’ morphology.

Numerical and Experimental Analysis of Dual-Beam Laser Polishing Additive Manufacturing Ti6al4v

Numerical and Experimental Analysis of Dual-Beam Laser Polishing Additive Manufacturing Ti6al4v

Indentation strain rate sensitivity of laser-powder bed fused and electron beam melted Ti–6Al–4V

Indentation hardness (Hin) and strain rate sensitivity (SRS) of additive manufactured Ti–6Al–4V (Ti64) alloys fabricated through both laser powder bed fusion (L-PBF) and electron beam melting (EBM) are investigated. One major difference between these processes is the higher cooling rates associated with the L-PBF. The indentation tests were performed at loading rates of 1, 10, and 50 mN/s. The L-PBF alloy showed a higher hardness value (3.45 GPa at 1 mN/s, 3.57 GPa at 10 mN/s, and 3.66 GPa at 50 mN/s) being correlated with its finer grain structure as compared with the EBM counter material (3.12 GPa at 1 mN/s, 3.37 GPa at 10 mN/s, and 3.55 GPa at 50 mN/s). The indentation SRS was explained based on the print orientation and dislocation density. EBM-Ti64 shows a higher SRS (0.0335 compared with 0.0147 in the L-PBF alloy) due to the rotation of α-prismatic planes along the building direction.

Effect of microstructure on fatigue crack growth of wire arc additive manufactured Ti–6Al–4V

In the present paper, the effect of microstructure on fatigue crack growth (FCG) behavior of wire arc additive manufactured (WAAMed) Ti–6Al–4V was investigated. The fatigue crack growth rate (FCGR) in WAAMed sample with heat-treated was lower than that in the forged sample. The 1050-AA (1050 °C/2h/air cooling (AA)+550 °C/4h/air cooling (AC)) sample showed the lowest FCGR because of its highest ΔKT (18.01 MPa m1/2), while the forged sample exhibited the highest FCGR because of its lowest ΔKT (13.3 MPa m1/2). The larger the plastic zone (rp) at the crack tip, the higher values of the stress intensity factor transition factor (ΔKT), the lower FCGR in the sample. In addition, the sample with continuous grain boundary α (αGB) or discontinuous αGB showed different FCG behavior and its mechanisms were discussed. For the sample with continuous grain boundary α (αGB), the crack in the horizontal sample (H-sample) passed through the continuous αGB and grew along with basket-weave microstructure and colony microstructure, while the crack in the vertical sample (V-sample) grew along with basket-weave microstructure. Therefore, the FCGR in these samples showed anisotropy. For the sample with discontinuous αGB, the crack in the H-sample and the V-sample passed through the discontinuous αGB and grew along with colony microstructure or basket-weave microstructure. The two samples showed the same FCG behavior, which indicated that there was no anisotropy.

Coupling effects of high magnetic field and annealing on the microstructure evolution and mechanical properties of additive manufactured Ti–6Al–4V

Additive manufacturing (AM) is playing a critical role in the areas of in-space intelligent manufacturing and on-orbital serving technologies. One of challenges is to reveal the composition-processing-microstructure-property relationship during post-treatment process, which would promote the AMed-product with excellent quality. In present work, the integration of heat and magnetic fields provides an approach to optimize microstructures and to enhance the mechanical properties of the selective laser melting (SLM) fabricated Ti–6Al–4V alloy. The conditions for post-treatment processes are set up to 400 °C for 30 min with 2 T, 4 T, 6 T, 8 T and 10 T, respectively. This coupling effects not only promote the phase transformation of α'→α+β, but also modify the width of the α'/α phases. The α'/α morphology after post-treatment becomes finer than the as-built ones, attributing to grain refinement strengthening. Moreover, the yield strength and the ductility of the annealed specimen in 8 T high magnetic field are 1092.1 MPa and 15.1% respectively, which could beat the classical reported SLM-fabricated ones and be comparable to the wrought ones. This work provides the connections between magnetic heating treatments and mechanical properties, paving a path to accelerate the development of space technology and exploration.

Investigating the microstructural evolution during deformation of laser additive manufactured Ti–6Al–4V at 400 °C using in-situ EBSD

A high-temperature in-situ tensile stage setup developed by the authors was combined with electron backscatter diffraction (EBSD) to study the microstructural evolution behavior of Ti–6Al–4V alloy produced by laser direct melting deposition (LDMD) at 400 °C. This technique provided direct observation of the microstructure evolution and deformation behavior of materials under dynamic tensile loading at operating temperature. The evolution of grain morphology, misorientation angles of grain boundaries, and Schmid factor (SF) were systematically investigated. The results indicated that the grains no longer sustained the initial lamellar morphology and underwent a severe microstructural evolution during plastic deformation. Various grain morphologies were observed at different tensile strain levels. The statistical analysis revealed the reduction of high-angle grain boundaries’ (HAGBs) fraction among α/α grains, with the opposite trend for low-angle grain boundaries (LAGBs). The increased Schmid factor values (SF > 0.4) were observed for prismatic (1–100) <11–20> and pyramidal (1–101) <11–20> slip systems, while those of the basal (0001) <11–20> slip system exhibited a sharp drop to SF > 0.4 at higher strain levels. Moreover, it was observed that high-SF grains changed their orientations, while low-SF ones split into smaller grains under tensile load. The kernel average misorientation (KAM) maps showed that deformation promoted dislocation motion, generating many LAGBs. The increasing number of LAGBs and slip openings reduced the boundary resistance and provided an easy path to detour the main crack propagation in response to the applied stress, while the LDMD Ti–6Al–4V specimen showed ductile fracture behavior.