Selective Laser Melting Of Ti Research Articles

Enhanced low cycle fatigue properties of selective laser melting Ti–6Al–4V with fine-tuned composition and optimized microstructure

Improving the low-cycle fatigue (LCF) properties of additively manufactured Ti–5.6Al–3.8V alloy is critical in ensuring its service safety and represents a significant research challenge. This work discusses a solution that optimizes the alloy's microstructure and ductility by precisely controlling the over-saturated strengthening elements and heat treatment. This was accomplished using selective laser melting (SLM), heat treatment at 800 °C for 2 h, and furnace cooling on a Ti–5.6Al–3.8V alloy with tightly controlled Al, V, and O concentrations in a lower range. The results showed that the SLM-fabricated Ti–5.6Al–3.8V alloy, post-heat treatment, exhibited α laths with a width of ∼1.4 μm and β columnar grains with a diameter of ∼126 μm, without experiencing coarsening or variant selection phenomena. The alloy balanced strength and ductility post-heat treatment with a UTS of 1015 MPa and an EL of 16.5% relative to the as-deposited state (UTS of 1199 MPa and EL of 11.9%). Notably, the LCF properties of the heat-treated SLM Ti–5.6Al–3.8V alloy are superior to those of other Ti–6Al–4V alloys produced by additive manufacturing and comparable to traditional forgings. At high strain amplitudes (1–1.5%), the fatigue life of this alloy was twice that of the Ti–6Al–4V forgings. Furthermore, we comprehensively analyzed the microstructure, strength, and ductility of the SLM Ti–5.6Al–3.8V alloy to elucidate the factors influencing its LCF properties. These findings provide a solid foundation for improving the LCF properties of additively manufactured Ti–6Al–4V alloy, thereby contributing to its safe and reliable use in critical applications.

Effect on microstructural and mechanical properties of selective laser melted pure Ti parts using stress relief heat-treatment process

This study investigated the correlation between the microstructure and the mechanical properties of pure Ti specimens manufactured via selective laser melting (SLM) using the stress relief heat-treatment process. Similar heat treatments were performed at a rate of 10 °C/min with a dwell time of 2 h to reach post-treatment temperatures ranging from of 490–890 °C in intervals of 100 °C. Phase change analysis using X-ray diffraction was conducted to verify the effects of the post-treatment process, while electron backscatter diffraction analysis was conducted to study the effects of microstructural changes induced by the post-treatment process on the mechanical properties of pure Ti specimens. In addition, a comprehensive analysis was performed to analyze residual stress following the stress relief heat treatment. The findings of this study establish a clear relationship between the mechanical properties and microstructural changes in SLM-manufactured pure Ti after heat treatment across a broad temperature range. Furthermore, the study confirms that heat treatment effectively relieves the residual stress in SLM Ti specimens.

Surface Modification of Porous Titanium and Titanium Alloy Implants Manufactured by Selective Laser Melting: A Review

Titanium (Ti) and its alloy implants with porous structures manufactured by selective laser melting (SLM) can match the elastic modulus of human bone to reduce the stress‐shielding effect and satisfy the personalized requirement in orthopedic surgery. Compared with conventional casting and forging Ti and its alloy implants, SLM implants possess unique microstructural features and excellent comprehensive mechanical properties. However, the unmelted powder particles inevitably adhere to the surfaces of SLM implants, which may result in excessive surface roughness and potential health risks. Moreover, there are significant issues encountered, such as bioactivity, toxicity, antibacterial activity, corrosion, and wear resistance. Consequently, surface modification methods are essential to remove the unmelted powder particles and improve biological and mechanical properties of SLM implants. Herein, the research efforts focus exclusively on chemical (acid treatment, alkali treatment, sol–gel, chemical vapor deposition, and atomic layer deposition) and electrochemical methods (anodization and microarc oxidation) for SLM Ti and its alloy implants, especially for porous structures. Particularly, the characteristics of these methods are summarized, and their commonly used pre‐ and post‐treatment methods are introduced. In addition, the development trends and challenges in surface modification of SLM Ti and its alloy implants are discussed.

Effect of hot isostatic pressing on the mechanical and corrosive properties of Ti–Ni alloy fabricated by selective laser melting

Hot isostatic pressing (HIP) is capable of mitigating the internal defects of materials, such as pores and cracks, resulting in improved mechanical properties and corrosion resistance, so it is particularly suitable as a post-treatment technology for the Ti–Ni alloys fabricated by selective laser melting (SLM), which display a broad application prospect in the biomedical field. Herein, the effect of HIP treatment on the mechanical and corrosion-resistant properties of SLM Ti–Ni alloys was systematically investigated. It was demonstrated that HIP treatment did not substantially affect the phase transformation behavior of the SLM Ti–Ni alloy, only showing a slight decrease in the values of As and Ms. However, HIP treatment improved the ultimate tensile strength and the elongation to fracture of the SLM Ti–Ni alloy by 67.3% and 23.2%, respectively. Meanwhile, the corrosion current density was reduced by two orders of magnitude after HIP treatment, indicating the significantly increased corrosion resistance. The enhanced mechanical properties and corrosion resistance of SLM Ti–Ni alloy were credited with the elevated temperature and pressure applied during the HIP process, which significantly eliminated the porosity defects and made the microstructure more uniform. These findings highlight the importance of performing HIP to enhance the comprehensive properties of SLM Ti–Ni alloys, which can expand the application prospects of SLM Ti–Ni alloys in the biomedical field.

Effects of laser power on the microstructural evolution of novel Ti–6Zr–5Fe alloy fabricated by selective laser melting

This work aims to study the influence of laser power on the microstructural evolution of novel Ti–6Zr–5Fe alloy fabricated by selective laser melting (SLM). The microstructural evolutions of all top surfaces in SLM Ti–6Zr–5Fe alloy samples fabricated from 80 W to 140 W were investigated in depth. The microstructure in SLM Ti–6Zr–5Fe alloy includes α phase and β phase, the size of the β phase and α phase increases with the increase of laser power due to the decrease of cooling rate. Meanwhile, the melt pool overlapping width increases with the increase of melt pool width due to the increase of laser power. The increase of melt pool overlapping causes the re-heating or re-melting of SLM Ti–6Zr–5Fe alloy. Moreover, as the laser power increases from 80 W to 140 W, the change of the melt pool greatly influenced the size, morphology, crystallographic orientation of the β phase, and their distributions. The increase of laser power causes adequate precipitation of the α phase, which improves the microhardness from 466 HV to 533 HV. The laser power can effectively regulate and control the microstructure and microhardness of SLM Ti alloys.

Surface morphology, roughness, and corrosion resistance of dental implants produced by additive manufacturing

Ti–6Al–4V alloy (ASTM F136) is the Ti alloy mainly used for machined dental and orthopedic implants. Additive manufacturing (AM) or additive layer manufacturing (ALM) is the industrial process name for 3D printing. The AM include some processes among them selective laser melting (SLM), and electron beam melting (EBM). The AM is used to make products with complex and customized geometries. However, these techniques have limitations in terms of surface finishing quality. Implants produced by AM have high roughness and the presence of partially unfused particles on the surface which decreases the reactions among the device and cells and proteins in the body environment. The objective of the present work is to compare the surface morphologies and corrosion resistance of machined and SLM Ti–6Al–4V samples before and after acid etching. The surface characterization was performed using scanning electron microscopy (SEM) and optical profilometry. The corrosion resistance was analyzed using open circuit potential (OCP), potentiodynamic polarization, chronoamperometry, and zero resistance amperometry. Galvanic corrosion of Ti–6Al–4V in contact with Co–Cr–Mo alloys was analyzed. The results showed that the presence of unfused particles on the Ti–6Al–4V SLM sample surface decreased the corrosion resistance. The hydrofluoric acid (HF) surface etching increased the corrosion resistance of the SLM samples. Ti–6Al–4V SLM treated with HF solution for 20 min showed the highest corrosion resistance and lowest roughness surface. After sanding, both machined and SLM Ti–6Al–4V samples showed similar corrosion resistance in the 0.9% NaCl electrolyte with and without fluoride.

Surface enhancement of Ti–6Al–4V fabricated by selective laser melting on bone-like apatite formation

Medical grade Ti–6Al–4V implants fabricated using selective laser melting (SLM) are recognized as a commercial biomaterial used for bone repairs and fracture fixation. However, there have been cases of failed bone remodeling and implant infections caused by deficient osteointegration. To improve osteogenesis, the Ti implants are treated by acid-etching. This study focused on the effect of surface treatment by using different percentages of sulfuric acid (H2SO4), hydrochloric acid (HCL), and hydrofluoric acid (HF) on the SLM Ti–6Al–4V. The microstructure and surface topography before and after treatment were evaluated. Then, the presence of a thicker anatase layer that was detectable on the surface was also observed by X-ray diffraction (XRD). The apatite-forming capabilities which indicated a sample bioactivity were assessed in simulated body fluid (SBF) for periods of 3 days and 7 days. The apatite formed on the surface of sample and XRD scanning revealed the deposition of Ca/P, suggesting successful bone-like apatite. The study discovered that these surface improvements were appropriate for the SLM Ti–6Al–4V prior to clinical applications and were likely to yield higher levels of osseointegration.

Micro/nano topography of selective laser melting titanium inhibits osteoclastogenesis via mediation of macrophage polarization

Selective laser melting (SLM) titanium (Ti) implants have shown good prospects for personalized clinical application, but further research is necessary to develop stabilized long-term properties. Since surface modification has been proven bioactive for osseointegration, conventional Ti surface treatment technologies, including sandblasting/acid-etching (SLA) and sandblasting/alkali-heating (SAH), were applied to construct micro and micro/nano surfaces. The SAH group with netlike nano-structure topography exhibited appropriate surface roughness and high hydrophilicity, and as expected, the osseointegration capacities in vivo of the three groups were in order of SAH > SLA > SLM. Besides, both in vivo and in vitro studies revealed that the SLA- and SAH-treated SLM Ti implants significantly inhibited osteoclast activity of peri-implants. Considering the close associations between osteoclasts and macrophages, the effects of Ti surface topography on macrophage polarization were detected. The results showed that the SLA- and SAH-treated SLM Ti implants, especially the latter, had the capacity to promote macrophage polarization to the M2 phenotype. Moreover, the cell culture supernatants of M2 macrophages and RAW264.7 cells seeded on SLA- and SAH-treated SLM Ti surfaces had an adverse effect on osteoclastogenesis. Collectively, this study demonstrated that micro/nano topographies of SLM Ti implants were effective for osseointegration promotion, and their inhibition of osteoclastogenesis might be attributed to macrophage polarization. Our findings shed some light on clinical application of SLM Ti implants and also prove a specific association between macrophage polarization and osteoclastogenesis.

Strengthening and deformation mechanism of selective laser-melted high-concentration nitrogen solute α-Ti materials with heterogeneous microstructures via heat treatment

Light elements such as oxygen (O) and nitrogen (N) significantly impact the microstructure and mechanical properties of Ti-based materials through solid solution strengthening. The microstructures of Ti-based materials processed via selective laser melting (SLM) have also been observed to contain a martensitic phase that improves the tensile strength. However, this improvement is achieved at the cost of reduced ductility. This study considered the use of post-heat treatment N dissolution to enhance the ductility of SLM-processed α-Ti materials. Tensile testing of the as-fabricated SLM Ti-(N) revealed a significantly increased strength of ~1200 MPa and a low ductility of 5% for N content of 0.5 wt%. However, the quenched samples exhibited increased ductility by up to 20%, with the microstructure, including primary α (αp) and transformed β structures. Further examination via electron probe micro-analysis (EPMA), transmission electron microscopy (TEM), in-situ high-temperature SEM observation and in-situ EBSD observation during tensile testing revealed that the enhancement in ductility of the quenched SLM-processed Ti-(N) samples was significantly due to alteration of the grain morphology, dislocations and N distribution. The findings of this study further clarify the microstructural evolution and deformation response of SLM-processed Ti-(N) materials under water quenching.

Hydrogen trapping in additive manufactured Ti–6Al–4V alloy

Environmental degradation of Ti–6Al–4V alloy in the presence of hydrogen strongly depends on the interactions between hydrogen atoms and various microstructural defects. Thus, the initial microstructure of Ti–6Al–4V influences its performance in the presence of hydrogen. In this study, we compared hydrogen interactions and desorption characteristics of Ti–6Al–4V alloy prepared by selective laser melting (SLM) and electron beam melting (EBM). It was found that the processing has a strong influence on the susceptibility to hydrogen embrittlement (HE). The degradation of SLM Ti–6Al–4V in hydrogen containing environment will probably occur through hydride formation and cleavage mechanism. On the other hand, Ti–6Al–4V prepared by EBM is more likely to degrade through hydrogen enhanced localized plasticity (HELP) mechanism.

Tensile property enhancement by oxygen solutes in selectively laser melted titanium materials fabricated from pre-mixed pure Ti and TiO2 powder

Ti-based materials with high oxygen solute contents were fabricated from mixtures of Ti powder and TiO2 particles by selective laser melting (SLM). Uniformly dissolved oxygen (O) from the TiO2 particles caused c lattice expansion in α-Ti crystals, which effectively increased the strengths of as-built SLM Ti–O materials. The as-built SLM Ti–O material using 1.5 wt% TiO2 showed a yield stress of 962 MPa and 15.3% elongation. The yield stress increases calculated by the Hall–Petch equation and Labusch model were equivalent to those observed experimentally, and O solid-solution strengthening was dominant in increasing yield stress.

Microstructure and mechanical properties of (TiB+TiC)/Ti composites fabricated in situ via selective laser melting of Ti and B4C powders

Titanium matrix composites reinforced with titanium boride (TiB) and titanium carbide (TiC) were fabricated in situ via selective laser melting (SLM). Varying contents of boron carbide (B4C) from 0 to 5 wt% were added to pure Ti to prepare blended powders for SLM. The influence of the B4C content on the printability, microstructure and mechanical properties of SLM-printed (TiB + TiC)/Ti composites was studied. The relative densities of all the fabricated composites were greater than 97.8 %; an increase in the B4C content resulted in a decrease in their relative density. The microstructure of the composites varied from a lath-shaped structure (0 wt%) to a dendritic structure (1 wt%) and to a cellular + dendritic structure (2–5 wt%), which was determined by the change of thermal convection within melt pools. Both the dendritic and cellular structures were arranged by coalescent clusters composed of TiB whiskers and TiC particles, while the clusters were formed through a self-joining behavior of the TiB whiskers that were mechanically locked with the TiC particles. The composites with the addition of 1 wt% B4C exhibited the highest ultimate tensile strength of 946 MPa, yield strength of 762 MPa and elastic modulus of 128 GPa, which were 62.4 %, 49.2 % and 15.3 % higher than those of the Ti matrix, respectively. The remarkable enhancement in the mechanical strengths was attributed to the synergistic effect of dispersion strengthening and grain refinement strengthening. The (TiB + TiC)/Ti composites fabricated in situ by SLM with an optimized material formulation not only ensured comparable mechanical properties to those of their counterparts manufactured by conventional processes but also demonstrated the potential in manufacturing complex and/or customized parts with design freedom for industrial applications.

Evaluation of Low Cycle Fatigue Performance of Selective Laser Melted Titanium Alloy Ti–6Al–4V

The material of Ti–6Al–4V has been widely applied in various industries, such as automobile, aerospace, and medical due to its high specific strength, superior thermal stability and strong corrosion resistance. In the recent decades, selective laser melting (SLM) has become an attractive method to fabricate Ti–6Al–4V parts, thanks to its significant advantages in low material consumption, the high degree of freedom in design, low carbon footprint, etc. Predictability of SLM material fatigue properties is especially important for the safety-critical structures under dynamic load cases. The present research is aimed at evaluating the low cycle fatigue (LCF) performance of SLM Ti–6Al–4V under high loading states. LCF tests were performed for as-built and annealed SLM Ti–6Al–4V. Comparison between LCF properties of SLM Ti–6Al–4V and the wrought Ti–6Al–4V was also made. It was found that as-built SLM Ti–6Al–4V demonstrated a comparable LCF performance with the wrought material. The LCF life of as-built SLM Ti–6Al–4V was longer than that of wrought Ti–6Al–4V at lower strain amplitudes. However, the wrought Ti–6Al–4V had better LCF performance at higher strain amplitudes. The results revealed that the porosity in the as-built SLM material exerted much more impact on the degradation of the material at high strain amplitudes. Annealing deteriorated the LCF performance of SLM Ti–6Al–4V material due to the formation of coarser grains. The cyclic Ramberg–Osgood and the Basquin–Coffin–Manson models were fitted to depict the cyclic stress–strain and the strain–life curves for the SLM Ti–6Al–4V, based on which the LCF performance parameters were determined. In addition, the fatigue fracture surfaces were observed by using scanning electron microscopy (SEM), and the results indicated that fatigue cracks originated from the surface or subsurface defects.

Titanium clasp fabricated by selective laser melting, CNC milling, and conventional casting: a comparative in vitro study

PurposeThis study aimed to investigate the effects of selective laser melting (SLM), milling methods, and casting on the behavior of titanium clasp. MethodsThe clasp and its die simulating the molar were designed using 3D software. Clasp specimens were fabricated using SLM approaches (SLM Ti) and computerized numerical control (CNC) milling technology (Milling CPTi). Cast clasps of the same forms were also prepared as controls using titanium alloy powder (Cast Ti) and commercial pure titanium (Cast CPTi), following the conventional casting methods. The surface roughness and accuracy of clasps were analyzed. The changes in retentive force and permanent deformation were measured up to 10,000 insertion/removal cycles. One-way analysis of variance and Tukey’s test or Kruskal–Wallis H test were performed for data analysis and comparisons. ResultsThe Milling CPTi clasps had a smoother inner surface than the other groups (p<0.05). The accuracy of the inner surface showed no significant difference among the groups, whereas that of the outer surface showed significant differences (p<0.05). The SLM Ti clasp had significantly higher retentive forces than the other groups (p<0.05), but it rapidly reduced after 2000 insertion/removal cycles until the fracture of all specimens was at 4000 cycles. The Milling CPTi clasps had more permanent deformation, but the rate of reduction of retentive force was only 9.5% (at 10,000 cycles). ConclusionsMilling has the potential to replace casting for fabricating removable partial denture (RPD) titanium clasps. However, SLM should be further improved for fabricating RPD titanium clasps before clinical application.

The positive effect of hot isostatic pressing on improving the anisotropies of bending and impact properties in selective laser melted Ti-6Al-4V alloy

Selective laser melting (SLM) is a versatile additive manufacturing process for fabricating solid or porous metallic materials with complicated three-dimensional shapes. SLM Ti alloys, particularly Ti-6Al-4V, and other alloys have been manufactured and analyzed in numerous studies. However, the high anisotropy of the microstructures and inconsistent mechanical properties of SLM materials have been extensively reported, and these disadvantages could prohibit its widespread use. To clarify how to alleviate the anisotropic behaviors of SLM materials, the main objective of this study was to evaluate the influences of hot isostatic pressing (HIP) on the microstructure, densification, bending strength, impact toughness, and fracture behavior of the as-built Ti-6Al-4V alloy.The results showed that the vertical and horizontal building directions obviously affect the bending and impact properties of as-built alloys. The transverse rupture strength (TRS) and impact energy of the horizontally-built alloy were respectively found to be 48% and 100% higher than those of the vertically-built one. In the vertically-built alloy, disc-shaped building defects, identified by X-ray computed tomography (CT) and microscopy, obviously reduce the effective load-bearing cross-section and deteriorate the bending and impact performances. After HIP at 1000°C/150MPa, the α′-martensite structure in the as-built alloy is transformed into an α+β lamellar one, and the disc-shaped building defects are evidently eliminated. As a result, the impact energies of as-built vertical and horizontal specimens are improved by 28J (560%) and 19J (190%), respectively, and the TRS of the as-built vertical alloy is raised by 550MPa (37%). Consequently, the discrepancies in TRS and impact energy between the HIPed vertical and horizontal specimens are merely 3% and 14%, respectively, and the anisotropic behaviors of the SLM Ti-6Al-4V alloy are thus substantially lessened.

Selective laser melting of Ti – 6 Al – 4 V for gas turbine components manufacturing

Selective laser melting of Ti – 6 Al – 4 V for gas turbine components manufacturing

Additive manufactured porous titanium structures: Through-process quantification of pore and strut networks

Titanium and its alloys are successfully used in aerospace through to marine applications. Selective laser melting (SLM) is an additive manufacturing technique, which promises to allow production of novel Ti structures. However, there is still a paucity of accepted methods for quantifying build quality. The viability of using X-ray microtomography (μCT) to quantify and track changes in morphology of SLM Ti porous structures at each stage of the post-laser melting production was tested, quantifying its quality through process. Quantification was achieved using an accessible volume tool to determine pore and strut sizes. Removal of partially sintered struts by cleaning was visualised and quantified. Eighty-eight percent of the struts broken by the cleaning process were found to have connecting neck diameters of less than 180μm with a mean of 109μm allowing build criteria to be set. Tracking particles removed during cleaning revealed other methods to improve build design, e.g. avoiding low angle struts that did not sinter well. Partially melted powder particles from strut surfaces were quantified by comparing surface roughness values at each cleaning step. The study demonstrates that μCT provides not only 3D quantification of structure quality, but also a feedback mechanism, such that improvements to the initial design can be made to create more stable and reliable titanium structures for a wide variety of applications.

Microstructure and properties of intermetallic composite coatings fabricated by selective laser melting of Ti–SiC powder mixtures

Transition metal silicides and carbides are attractive advanced materials possessing unique combinations of physical and mechanical properties. However, conventional synthesis of bulk intermetallics is a challenging task because of their high melting point. In the present research, titanium carbides and silicides composites were fabricated on the titanium substrate by a selective laser melting (SLM) of Ti–(20,30,40wt.%)SiC powder mixtures by an Ytterbium fiber laser with 1.075μm wavelength, operating at 50W power, with the laser scanning speed of 120mm/s. Phase analysis of the fabricated coatings showed that the initial powders remelted and new multiphase structures containing TiCx, Ti5Si3Cx, TiSi2 and SiC phases in situ formed. Investigation of the microstructure revealed two main types of inhomogeneities in the composites, (i) SiC particles at the interlayer interfaces and, (ii) chemical segregation of the elements in the central areas of the tracks. It was suggested and experimentally proven that an increase in laser power to 80W was an efficient way to improve the laser penetration depth and the mass transport in the liquid phase, and therefore, to fabricate more homogeneous composite. The SLM Ti–(20,30,40wt.%)SiC composites demonstrated high hardness (11–17GPa) and high abrasive wear resistance (3.99×10−7–9.51×10−7g/Nm) properties, promising for the applications involving abrasive wear.