Selective Laser Melted Ti6Al4V Alloy Research Articles

Correction to [Development of Heat Treatments for Selective Laser Melting Ti6Al4V Alloy: Effect on Microstructure, Mechanical Properties, and Corrosion Resistance

Correction to [Development of Heat Treatments for Selective Laser Melting Ti6Al4V Alloy: Effect on Microstructure, Mechanical Properties, and Corrosion Resistance

Effect of heat treatments on mechanical properties and wear resistance of laser-powder bed-fused Ti6Al4V alloy

The effects of heat treatment on the microstructure, mechanical properties and wear resistance of Selective Laser Melting Ti6Al4V alloy were systematically studied. The results show that the as-built part has an acicular [Formula: see text] martensite structure. The increase in heat treatment temperature causes the decomposition of acicular [Formula: see text] martensite, which leads to the decrease of the hardness of Ti6Al4V alloy. When the heat treatment temperature is 800∘C, the alloy forms an [Formula: see text]/[Formula: see text] structure, which increases the tensile strength and elongation of the alloy by 12% and 23%, respectively. When the heat treatment reaches above 900∘C, the mechanical properties of the alloy decrease sharply due to the disappearance of the [Formula: see text]/[Formula: see text] structure, the coarse grains of the [Formula: see text] phase, and the increase of the content of the [Formula: see text] phase. The tensile strength decreased from 935[Formula: see text]MPa to 815[Formula: see text]MPa, and the elongation decreased from 8.7% to 3.1%. With the increase of heat treatment temperature, the friction coefficient and wear rate increase linearly, which is opposite to the changing trend of hardness, and the wear mechanism is changed from single abrasive wear to abrasive wear and oxidative wear. This is attributed to the increase in the grain size of the alloy and the increase in the content of the [Formula: see text] phase, resulting in a decrease in the overall hardness of the alloy.

Study on high cycle fatigue performance at elevated temperature for a selective laser melted Ti6Al4V alloy

Selective laser melted Ti6Al4V alloy has broad application prospects in aeroengine field. In this study, high cycle fatigue tests were carried out systematically using hourglass shaped specimen (K t=1) at 400 °C. The S-N curves were acquired and compared with data of the casting and the forging. The anisotropy and the defect effects of the fatigue performance were analysed. The LOF defects on the surface and subsurface preferentially act as the fatigue crack initiation site for almost all the tested HCF specimens. The position and irregularity of defect have greater influence on elevated temperature fatigue life than the defect size. The fatigue strength of horizontal orientation is lower than that of vertical orientation, because of the characteristics of larger size, more irregular shape and higher density for the crack source defects of horizontal specimens. The fatigue performance of the SLM Ti6Al4V alloy in this study is better than that of the casting and even the forging, but the dispersion of fatigue data of the SLM alloy is much greater than that of the both traditional process alloys.

Mechanism of impact toughness enhancement obtained by globularization of αGB phase for selective laser melted Ti–6Al–4V alloy

The low impact toughness of selective laser melted (SLM) Ti–6Al–4V (Ti64) is a main factor limiting its widespread application in engineering. In this work, we've greatly improved its toughness by modifying the morphology of the α phase via annealing. Specifically, the as-built sample had a toughness of 14.87J/cm2. Annealing at 800 °C for 4 h boosted the toughness to 25J/cm2. Notably, annealing at 950 °C for 4 h increased the toughness by a remarkable 90 %, reaching an impressive 36.88J/cm2, which is comparable to its wrought counterpart. Particularly, 950 °C annealing induces the globularization of αGB phase (α phase along prior β grain boundary) while maintains the lamellar morphology of the α phase within the prior β grain. This mixed structure significantly outperforms the full lamellar structure in the 800 °C annealed sample in terms of toughness. The mechanism of toughness enhancement differs from the traditional one where thick lamellar α phase improves the toughness by increasing the crack path's tortuosity. Our study indicates that more deformation twins are activated in 950°C-annealed samples in impact test, which effectively absorb additional impact energy and improve toughness. In the 800°C-annealed sample, only a limited number of grains with specific orientations can activate deformation twins. Whereas in the 950°C-annealed sample, a broader range of grain orientations demonstrated the capability to activate deformation twins. The mechanism for twin activation in the 950°C-annealed sample is proposed. It is associated with equiaxed αGB, inducing significant interface deformation accompanied by stress concentration. This ultimately leads to the formation of the deformation twins.

Quantitative study of geometric characteristics and formation mechanism of porosity defects in selective laser melted Ti6Al4V alloy by micro-computed tomography

The defects formed inside the additive manufactured (AM) part have a detrimental effect on the mechanical performance of the printed component. The effect of defects on the performance of parts varies with their geometry features. The characteristics of porosity defects in 36 bulk Ti6Al4V specimens with different printing parameters were investigated via X-ray computed tomography (XCT). The porosity defects are classified into four types: spherical- shaped, elliptical-shaped, elongated-shaped and irregularly-shaped pores, based on their geometric characteristics. Elongated-shaped defects account for the largest proportion of defects with all printing parameters, and the proportion of irregularly-shaped defects enhances with the increase of scanning speed. The characteristics of gradual accumulation of defects in single-track and multi-track samples were also analyzed. Porosity may gradually accumulate with the overlay of layers, while the appropriate overlapping rate may melt the pores between the tracks. The formation of defects is greatly influenced by surface roughness and pool wetting.

Effects of Wire Electrical Discharge Finishing Cuts on the Surface Integrity of Additively Manufactured Ti6Al4V Alloy.

The Selective laser melting (SLM) technology of recent years allows for building complex-shaped parts with difficult-to-cut materials such as Ti6Al4V alloy. Nevertheless, the surface integrity after SLM is characterized by surface roughness and defects in the microstructure. The use of additional finishing technology, such as machining, laser polishing, or mechanical polishing, is used to achieve desired surface properties. In this study, improving SLM Ti6Al4V alloy surface integrity using wire electrical discharge machining (WEDM) is proposed. The influence of finishing WEDM cuts and the discharge energy on the surface roughness parameters Sa, Svk, Spk, and Sk and the composition of the recast layer were investigated. The proposed finishing technology allows for significant improvement of the surface roughness by up to 88% (from Sa = 6.74 µm to Sa = 0.8 µm). Furthermore, the SEM analyses of surface morphology indicate improving surface integrity properties by removing the balling effect, unmelted particles, and the presence of microcracks. EDS analysis of the recast layer indicated a significant influence of discharge energy and the polarization of the electrode on its composition and thickness. Depending on the used discharge energy and the number of finishing cuts, changes in the composition of the material in the range of 2 to 10 µm were observed.

Effects of soft sparking on micro/nano structure and bioactive components of microarc oxidation coatings on selective laser melted Ti6Al4V alloy

We used the soft sparking mode to fabricate microarc oxidation coatings containing micro/nano-scale pores and bioactive Ca/P components on Ti6Al4V alloy samples formed by selective laser melting. The effect of soft sparking on the microstructure and corrosion resistance of the coatings and the codeposition mechanism of the electrolyte anions in the soft sparking layer were investigated using different negative/positive charge ratios. The results showed that the negative charge significantly decreased the intensity of microarc discharge, resuling in the soft sparking phenomenon, which promotes the formation of a thin amorphous TiO2 layer at the barrier layer/substrate interface. In turn, this interface facilitated the electron tunneling and aggregation of the electrolyte anions, which leads to soft sparking at voltages significantly lower than the breakdown voltage of the anodic oxide film on the selective laser melted Ti alloys. Soft sparking promoted the codeposition of the electrolyte anions, including PO43− and Ca2+ ions, in the form of bulk amorphous phases in the coating formed by soft sparking. The soft sparking layer contained amorphous phases, which were corroded more readily than that the outer ceramic layer. However, the submicro/nano-scale pores in the soft sparking layer improved its corrosion resistance. With an increase in negative/positive charge ratio, the effect of soft sparking on the surface morphology of the MAO coating weakened, and the amounts of Ca and P codeposited in the soft sparking layer increased. Finally, the corrosion resistance of the coating was found to first increase and then decrease, reaching the maximum at the 1.1 ratio.

Research on the Cutting Force and Serrated Chips in Ultra-Precision Micro-Grooving of SLM Ti6Al4V Alloy.

Selective laser melting (SLM) has significant advantages in the near net shape manufacturing of metal parts with complex geometries. However, SLM parts usually have problems such as poor surface quality and low dimensional accuracy, which require post-processing. This paper focuses on the research around the influence of ultra-precision micro-grooving the SLM Ti6Al4V alloy on the cutting force and serrated chips. The influence of the processing parameters on the cutting force and surface processing quality was analyzed in detail, and the cutting simulation model of the SLM Ti6Al4V alloy was established. The formation process of the serrated chip was successfully simulated, and the experiments verified the reliability of the established model. The research results show that the dynamic cutting force and surface processing quality are mainly related to the depth of cut, and the two trends are consistent. It is also shown that the serrated chip begins on the free surface of the workpiece and propagates deeply in the shear zone, forming a shear band, and its serrated nodules move upward and forward to form periodic serrated chips.

Study on Surface Roughness Improvement of Selective Laser Melted Ti6Al4V Alloy

To improve the surface quality of Ti6Al4V parts formed by selective laser melting (SLM), this paper systematically studies the effects of laser power, scanning speed and inclination angle on the different surface morphology and roughness of parts. On this basis, the effect of surface remelting and multi-layer profile scanning process strategies on improving the surface quality of parts is explored. The upper surface roughness varies parabolically with increasing line energy density, the line energy density value that minimizes the upper surface roughness is around 0.22 J/mm, and the minimum Ra value is 4.41 μm. The roughness of upper and lower sides increases significantly with the increase in scanning speed. As the inclination angle increases, the roughness of the upper and lower sides gradually decreases, which is caused by the combined influence of powder adhesion and step effect. The surface remelting process strategy can reduce the upper surface roughness by 35.68% and reduce its Ra value to 2.65 μm. The multi-layer profile scanning process strategy can reduce the upper side and vertical side roughness by more than 50%, down to Ra 5.10 μm and Ra 4.61 μm, respectively.

Corrosion Performances of Selective Laser Melting Ti6Al4V Alloy in Different Solutions

Selective laser melting (SLM) can fabricate titanium and its alloy components with both elaborate internal architectures and complex shapes without geometric constrictions. The corrosion resistance of SLM-produced Ti and its alloy is crucial in some applications such as marine and biomedical environments. Here, potentiodynamic polarization and electrochemical impedance spectroscopy were used to evaluate the corrosion behaviors of SLM-produced Ti-6Al-4V in the four corrosive media (simulated body fluid (SBF), phosphate buffered saline solutions (PBS), 3.5 wt.% NaCl aqueous solution, 15 wt.% NaCl aqueous solution). The relevant results demonstrate the inferior corrosion resistance of the SLM-produced Ti-6Al-4V sheet compared with the commercial casting Ti-6Al-4V sheet in the four solutions. The corrosive current density of SLM-produced Ti-6Al-4V in PBS solution is 1.78 μA cm−2 and 7.065 μA cm−2 in 15 wt.% NaCl solution, and the values of charge transfer resistance for SLM-produced Ti-6Al-4V in the four solutions are in the order: 17.9 kΩ cm−2 (in 15 wt.% NaCl) < 25.2 kΩ cm−2 (in 3.5 wt.% NaCl) < 28.1 kΩ cm−2 (in SBF) < 39.8 kΩ cm−2 (in PBS), demonstrating the best protective performance of the passivation film on the SLM-produced Ti-6Al-4V sheet in PBS.

The effect of process parameters on mechanical behavior of selective laser melted Ti6Al4V alloy

The effect of process parameters on mechanical behavior of selective laser melted Ti6Al4V alloy

On study of process induced defects-based fatigue performance of additively manufactured Ti6Al4V alloy

Additively manufactured Ti6Al4V are high strength alloys but associated with inferior ductility and inherent defects. These process-induced defects act as a stress raiser and deteriorates the mechanical and fatigue properties of the alloy. Post-processing is generally done to enhance the ductility and fatigue properties but at the expense of strength of the additively manufactured Ti6Al4V alloy. In this study, the effect of defects on mechanical properties and fatigue performance of additively manufactured (selective laser melting) Ti6Al4V alloy has been investigated based on experimentally calibrated defects-based model. The as-built specimens were subjected to different heat treatments in order to optimize their mechanical properties and the most optimum heat treatment was then selected for investigation of changes in fatigue properties. The beneficial effect of heat treatment on mechanical properties was mainly associated with the reduction in fraction of α ’-martensitic microstructure and increase in α -lamellae thickness. Most of the fatigue failures occurred from surface and sub-surface defects, owing to the high stress concentration factor associated with them, and improved fatigue lives as a result of optimum heat treatment. A process-structure–property (PSP) linkage has been established to predict the mechanical properties. Experimentally calibrated defects’ based fatigue model has been formulated to predict the fatigue performance using the experimentally observed data i.e. the process induced defects’ size and their locations. • Different post-processing of SLM-based AM Ti6Al4V was investigated. • Heat treatment was carried out, which improved both mechanical & fatigue properties. • A Process-Structure-Property (PSP) linkage was established. • A new defects-based model was proposed to predict the fatigue properties.

High cycle fatigue behavior of a selective laser melted Ti6Al4V alloy: Anisotropy, defects effect and life prediction

Selective laser melting (SLM) exhibits more and more utilization potentialities in today’s aviation industry. In order to ensure safe operation of SLM produced aero parts, it is important to have the fatigue performance well-characterized, and fatigue life prediction methods developed by considering the initial feature of this material. In this study, high cycle fatigue tests under stress ratios of −1, 0.1 and 0.5 at room temperature for the SLM Ti6Al4V alloy were carried out, and the fracture morphologies of fatigue specimens were observed. The results show that the subsurface pore is the main source of crack initiation for fatigue failure. By quantitative and statistical analysis of the pore characteristics in the crack source zone of all fractured specimens, it is revealed that the anisotropy and dispersion of fatigue performance are closely related to the difference of the shape, size, location and density of the pores in the crack source zone of specimens. The relationship between pore size and location, such as the ratio of radius to the distance to the specimen surface, plays an important role in estimating the fatigue life of the specimen. Aiming at the nature of defect-induced fatigue damage, the fatigue life of the SLM Ti6Al4V alloy was reasonably predicted using the fracture mechanics method based on the small-crack theory and the long crack growth data.

Prior β grain evolution and phase transformation of selective laser melted Ti6Al4V alloy during heat treatment

Prior β grain evolution and phase transformation of selective laser melted (SLM) Ti6Al4V alloy after subtransus and supertransus solution heat treatments are investigated. A method based on the special angle grain boundaries (in the range of 15–55° and 70–85°) is proposed for a clear and straightforward description of the prior β mesostructure. Post solution treatments below β transus retain the prior β grains and alter the fully martensitic microstructure into a mixture of α and α’/β phases. A non-traditional “bimodal structure” consisting of α P and α S ’ phases is produced by subtransus treatments at a relatively high temperature. While treatments above β transus lead to the growth of prior β grains and transform the martensitic α’ phases into β phases first, and then back into α’ martensites again during quenching, leading to a new martensitic microstructure. The microhardness is determined by the α lath thickness and the amount of α’ martensite, and the latter is more predominant. • Prior β grain can be described by grain boundaries in the range 15–55° and 70–85°. • Subtransus solution have no influence on the prior β mesostructure. • Solution at 900 ℃, α’ martensites between the primary α laths are produced. • Microhardness is determined by: amount of α’ martensite> α lath thickness.

Fatigue performance differences between rolled and selective laser melted Ti6Al4V alloys

In this work, the performance differences between rolled and selective laser melted (SLM) Ti6Al4V alloys used commonly for orthopedic implants were investigated. By rotation bending fatigue (RBF) and in-situ fatigue tests, the SN curves, crack propagation rate (d a /d N ) curves and cracking behavior of the SLM samples (in 0°, 45° and 90° directions) and rolled samples (fabricated along the rolling and transverse directions (RD and TD)) were compared. Slight fatigue performance distinction was identified for the rolled TD and RD Ti6Al4V samples. However, the building directions exhibited an obvious impact on the SN and d a /d N curves of the SLM Ti6Al4V alloy. The SLM 45° samples revealed excellent crack propagation resistance and the fatigue limit of the SLM 0° samples reached to that of the rolled Ti6Al4V alloy. The fatigue performance difference of the SLM components in different SLM directions can be expected. Moreover, the crack aspect ratios ( a / b ) of the SLM Ti6Al4V alloy were obtained from sample fracture surfaces. The effect of SLM building directions on the a / b values was revealed. Based on the results, the residual fatigue lives of the SLM and rolled Ti6Al4V hip joint prostheses were compared by finite element simulation. The SLM prostheses exhibited excellent fatigue performance and could replace the existing components fabricated by traditional alloys in some fields. • The various material anisotropies were identified for the rolled (RD and TD) and SLM Ti6Al4V (0°, 45° and 90°) samples. • The SLM 0° and 45° samples exhibited excellent fatigue limit and crack propagation resistance, respectively. • The effects of AM directions, sample types and crack depths on the crack aspect ratios of the SLM samples were revealed. • A K IC estimation method was applied for the rolled and SLM Ti6Al4V alloys. The validity of the method was proved. • The residual fatigue lives of SLM Ti6Al4V prostheses were calculated, which exhibited excellent fatigue performance.

Surface roughness effect on multiaxial fatigue behavior of additively manufactured Ti6Al4V alloy

Additive manufacturing (AM) is a special technology that offers several advantages compared to the conventional methodologies. For instance, it allows to build components with complex geometry, difficult to make in a different way, build integrated parts deleting joining issues, etc. However, its application in engineering fields is still limited because the dependency between the mechanical properties of the obtained components and the manufacturing parameters is not in depth understood. Improve the performances of AM components, under static and dynamic loadings, is crucial to ensure their durability and reliability and in recent years it became a challenging research field. In this work, the effect of the surface roughness on the multiaxial fatigue resistance of Ti6Al4V thin-walled tubular samples, made by the Selective Laser Melting (SLM) process, was investigated. In particular, experiments under combined axial–torsional loadings were carried out on two batches of samples, made by different surface roughness (machined and as built samples). Maximum valley depth Rv was used as a representative parameter of the surface roughness as it geometrically represents a stress concentration zone where crack initiates and propagates. An effective strain intensity factor range, based on the modified Smith Watson and Topper (MSWT) model, that includes the roughness parameter Rv, is proposed and used for a better correlation of the fatigue data. To prove the reliability of the proposed model, the Socie and the Reddy & Fatemi effective strain-based intensity factor range were also used but a bigger scattering of results was observed. The MSWT model was also applied to predict the failure plane and the obtained results were compared with the predictions of the Fatemi-Socie model. Results revealed that, for SLM Ti6Al4V alloy components, the MSWT model is more accurate.

Bioactive and antimicrobial macro-/micro-nanoporous selective laser melted Ti–6Al–4V alloy for biomedical applications

Metal Additive Manufacturing (AM) technology is an emerging technology in biomedical field due to its unique ability to manufacture customized implants [Patients-specific Implants (PSIs)] replicating the complex bone structure from the relevant metal powders. PSIs could be developed through any AM technology, but the ultimate challenge lies in integrating the metallic implant with the living bone. Considering this aspect, in the present study, Ti alloy (Ti–6Al–4V) powder has been used to fabricate scaffolds of channel type macropores with 0–60% porosity using selective laser melting (SLM) and subsequent post-treatments paving way for surface microporosities. Surface chemical and subsequent heat treatments were carried out on thus developed Ti alloy scaffolds to improve its bioactivity, antibacterial activity and osteoblastic cell compatibility. NaOH and subsequent Ca(NO3)2/AgNO3 treatment induced the formation of a nanoporous network structure decorated with Ca–Ag ions. Ag nanoparticles covering the entire scaffold provided antibacterial activity and the presence of Ca2+ ions with anatase TiO2 layer further improved the bioactivity and osteoblastic cell compatibility of the scaffold. Therefore, SLM technology combined with heat treatment and surface modification could be effectively utilized to create macro-micro-nano structure scaffolds of Ti alloy that are bioactive, antibacterial, and cytocompatible.

Densification, Tailored Microstructure, and Mechanical Properties of Selective Laser Melted Ti-6Al-4V Alloy via Annealing Heat Treatment.

This work investigated the influence of process parameters on the densification, microstructure, and mechanical properties of a Ti–6Al–4V alloy printed by selective laser melting (SLM), followed by annealing heat treatment. In particular, the evolution mechanisms of the microstructure and mechanical properties of the printed alloy with respect to the annealing temperature near the β phase transition temperature were investigated. The process parameter optimization of SLM can lead to the densification of the printed Ti–6Al–4V alloy with a relative density of 99.51%, accompanied by an ultimate tensile strength of 1204 MPa and elongation of 7.8%. The results show that the microstructure can be tailored by altering the scanning speed and annealing temperature. The SLM-printed Ti–6Al–4V alloy contains epitaxial growth β columnar grains and internal acicular martensitic α′ grains, and the width of the β columnar grain decreases with an increase in the scanning speed. Comparatively, the printed alloy after annealing in the range of 750–1050 °C obtains the microstructure consisting of α + β dual phases. In particular, network and Widmanstätten structures are formed at the annealing temperatures of 850 °C and 1050 °C, respectively. The maximum elongation of 14% can be achieved at the annealing temperature of 950 °C, which was 79% higher than that of as-printed samples. Meanwhile, an ultimate tensile strength larger than 1000 MPa can be maintained, which still meets the application requirements of the forged Ti–6Al–4V alloy.

A study on defect-induced fatigue failures in SLM Ti6Al4V Alloy

Additive Manufacturing has been developed to fulfill the current growing demand for design freedom structures with near-net complex shapes. The additive manufacturing process is associated with lots of defects such as porosities, incomplete fusion, hot cracking, delamination, etc. These defects act as stress raiser and are known to deteriorate the mechanical properties of the alloy. In this present study, the defect-based fatigue performance of selective laser melted Ti6Al4V alloy has been investigated. The Ti6Al4V alloy specimens were vertically built with optimized process parameters and layer thickness of 60 µm. These as-built specimens were subjected to heat treatment in order to reduce the residual stress generated due to higher thermal gradient. The as-built alloy mainly consisting of α’ martensitic microstructure decomposes into α-β microstructure with the heat treatment. The mechanical properties and fatigue performance were then evaluated. Most of the fatigue failure occurred from the surface and sub-surface regions. The endurance limit then predicted using the defects’-based model using the defects’ area and its location.

Role of laser remelting and heat treatment in mechanical and tribological properties of selective laser melted Ti6Al4V alloy

The reliability and quality of additively manufactured parts are questionable. The present study aimed to improve the surface quality and mechanical and tribological properties of a Ti6Al4V alloy manufactured using selective laser melting (SLM). The effects of the laser remelting on its surface topology, mechanical properties, and sliding wear in a ball-on-plate configuration with a ball either above or below the plate were investigated systematically. The influence of the laser remelting approach on the anisotropy of the mechanical and tribological properties was compared with that of the heat treatment approach. The surface quality and high cycle fatigue strength improved with an increase in the number of melting steps. The compressive and impact strengths also increased with an increase in the number of melting steps. The wear resistance in both configurations was higher in the remelted samples than in the samples after SLM. The results confirmed that laser remelting, as an affordable approach, could significantly improve the reliability of parts fabricated by the SLM process.