Selective Laser Melted Ti6Al4V Alloy Research Articles

Effect of heat treatment on the corrosion resistance and mechanical properties of selective laser melting Ti6Al4V alloy

AbstractThe present work shows that the effect of several heat treatments on the corrosion resistance and mechanical properties of Ti6Al4V processed by selective laser melting (SLM). The microstructure of Ti6Al4V alloy produced by selective laser melting exhibited bulky prior β columnar grains, and a large amount of fine acicular martensites α′ were observed inside the prior β columnar grains. The acicular martensitic α′ were transformed to a mixture of α and β after heat treatment, and the grain size increases with the increase of heat‐treated temperature. The results of 3.5 wt% NaCl solution electrochemical corrosion test showed that the heat‐treated samples possess a higher corrosion resistance than the as‐received sample. Among of them, the sample after heat‐treated at 730 °C exhibited best corrosion resistance and excellent fracture strain. The sample heat treated at 1015 °C showed worst mechanical properties due to the formation of Widmanstätten structure.

Microstructure and anisotropic mechanical properties of selective laser melted Ti6Al4V alloy under different scanning strategies

Apparent anisotropies have been found in the microstructure and mechanical properties of selective laser melted (SLM) Ti–6Al–4V alloy, which will significantly influence the comprehensive performance of the SLM parts. In this study, the effect of scanning strategies on material anisotropy, including surface morphology, microstructure, microhardness, quasi-static and dynamic mechanical properties, were comprehensively investigated with 0°, 67.5°, and 90° scanning strategies. The SLM Ti6Al4V alloy prepared by the 0° scanning strategy exhibited the most pronounced anisotropy. The size of the primary columnar crystals on the front surface (158–173 μm) is approximately 1.8–3.2 times larger than that of the top surface (54–88 μm), and the microhardness at the top surface is ∼30% higher than that of the front surface. In addition, the modified Johnson-Cook constitutive model for the SLM Ti6Al4V alloy is developed based on the results of quasi-static compression and dynamic compression with the average absolute relative error controlled within 10%. This provides a reliable solution and an effective material constitutive model for modelling and simulation of the machining SLM parts with higher accuracy and enhanced physical meaning.

Effects of Post Heat Treatments on Microstructures and Mechanical Properties of Selective Laser Melted Ti6Al4V Alloy

The unique thermal history of selective laser melting (SLM) can lead to high residual stress and a non-equilibrium state in as-fabricated titanium alloy components and hinders their extensive use. Post heat treatment, as a classical and effective way, could transform non-equilibrium α’ martensite and achieves desirable mechanical performance in SLMed Ti alloys. In this study, we aimed to establish the correlation between the microstructure and mechanical performances of SLMed Ti6Al4V (Ti-64) by using different heat treatment processes. The columnar prior β grain morphology and grain boundary α phase (GB-α) after different heat treatment processes were characterized, with their influences on the tensile property anisotropy fully investigated. Scanning electron microscope (SEM) observation of the fracture surface and its cross-sectional analysis found that the tensile properties, especially the ductility, were affected by the GB-α along the β grain boundary. Furthermore, the discontinuous ratio of GB-α was firstly proposed to quantitatively predict the anisotropic ductility in SLMed Ti-64. This study provides a step forward for achieving the mechanical property manipulation of SLMed Ti-64 parts.

Effect of Heat Treatment on the Wear Properties of Selective Laser Melted Ti–6Al–4V Alloy Under Different Loads

Effect of Heat Treatment on the Wear Properties of Selective Laser Melted Ti–6Al–4V Alloy Under Different Loads

Prediction of Yield Strength of Selective Laser Melted Ti–6Al–4V Alloy Using Melt Pool Geometry

Abstract Additive manufacturing (AM) method has attracted huge interest in the past decade due to its ability in building complicated geometries with a much lower cost than conventionally produced parts. In AM, the final mechanical properties can be controlled by the AM process parameters. In other words, the AM process parameters control the amount of energy that is transferred into the powder and consequently the resulting microstructure. In this study, the correlation between melt pool geometry and mechanical properties of selective laser melted (SLM) Ti–6Al–4V samples is investigated.

Vacuum Thermal Treatments for Surface Engineering of Selective Laser Melted Ti6Al4V Alloy

In the present work, surface engineering of Ti6Al4V, produced via selective laser melting parts, was carried out with the aim of investigating how surface features of substrate may improve the coupling with AlTiN coatings deposited by physical vapor deposition reactive high-power impulse magnetron sputtering. In particular, the work highlighted how vacuum thermal treatments at 800 °C induced peculiar mesoscale morphology and surface chemical modifications of the Ti6Al4V, which contributed to improve the adhesion of the deposited AlTiN thin films. Chemical composition, crystallographic structure, and surface properties of both substrates and coatings were analyzed by field emission scanning electron microscopy (equipped with energy dispersive spectroscopy), atomic force microscopy, x-ray photoelectron spectroscopy, x-ray diffraction, nanoindentation, and scratch test measurements.

Multiaxial fatigue behavior of SLM Ti6Al4V alloy under different loading conditions

AbstractThe multiaxial fatigue behavior of Ti6Al4V samples, made by selective laser melting (SLM) process, was investigated in this work. Fatigue tests were carried out under three different loading conditions: (i) tensile, (ii) torsion, and (iii) in‐phase axial–torsional. A modified damage model, based on the Sines criterion, is proposed and used to analyze proportional multiaxial fatigue data. Preliminary results showed that the proposed version provides a better prediction with respect to the Sines model. Infrared thermography and digital image correlation were also used to capture both temperature and strain evolutions during fatigue tests. Fatigue results highlighted different failure modes due to surface crack initiation and internal crack initiation, as also demonstrated by scanning electron microscopy of the fracture surface.

Effects of embedded spherical pore on the tensile properties of a selective laser melted Ti6Al4V alloy

Selective laser melting (SLM) is one of the most promising additive manufacturing (AM) technologies in recent years. However, some sub-millimeter defects often appear accidently in SLM specimens, resulting in deterioration of mechanical properties. Therefore, the effects of these defects on the mechanical properties of the SLM materials are significantly important. In this study, the effect of a single spherical defect on the tensile properties was investigated by embedding a spherical pore inside the SLM Ti6Al4V specimens. Different combinations of pore diameters (Ф100 μm, Ф300 μm, Ф500 μm, Ф1000 μm) and locations (center, sub-surface, near-surface) were designed to mimic the sub-millimeter printing defects. The results demonstrated that these embedded pores had negligible influence on the elastic modulus and tensile strength but had a noticeable influence on the tensile elongation. With the increase of pore size, the elongation to failure of the specimens tended to be stochastic, stably high and stably low in sequence, which could be related to different fracture modes. The tensile specimens fractured at the embedded pore when the embedded pore size was increased to a critical diameter (approximately Ф1000 μm). Concurrently, the location of the embedded pore started to have an obvious influence on ductility, namely, the elongation to failure decreased as the embedded pore became closer to free surface.

Effects of electropulsing on the microstructure and microhardness of a selective laser melted Ti6Al4V alloy

Optimizing the microstructure and mechanical properties of the selective laser melted (SLM) Ti6Al4V alloy is an important issue both in the scientific and engineering interests. In this study, the rapid hardening and softening of the SLM-Ti6Al4V alloy is realized due to the obvious evolution of microstructures during the electropulsing treatment (EPT) with different discharge voltages. The microhardness of the EPT-7.5 kV sample decreased about 7% while the EPT-8 kV sample increased about 10% compared with the as-received sample. Scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) analysis were used to characterize the evolutions of α and β grains and dislocation structures which are corresponding with the microhardness changes induced by EPT. Analysis shows that EPT could reduce the thermodynamic barrier for α → β solid-state phase transformation, further arise microstructure refinement in the SLM-Ti6Al4V alloy. This research shed some new light on rapidly and efficiently improving the microstructure and mechanical properties of the additive manufactured metal components.

Effect of heat treatment on microstructure and tribological behavior of Ti–6Al–4V alloys fabricated by selective laser melting

In this paper, the effect of heat treatment (760–1060 °C) on tribological behavior of selective laser melted Ti6Al4V alloy against Si3N4 counter-ball under different applied loads was investigated. Results show the wear rate of as-built Ti6Al4V alloy increased firstly and then gradually decreased with increasing the heat-treatment temperature, which obtained the lowest value when the heat treatment temperature was 1060 °C, exhibiting the best wear resistance. The higher applied load led to larger wear rate and lower coefficient of friction. The wear mechanism was strongly dependent on the microstructure transformation, micro-hardness and applied load, which was discussed in detail.

Reverse effect of hot isostatic pressing on high-speed selective laser melted Ti–6Al–4V alloy

Abstract Despite recent progress in achieving high mechanical properties of 3D printed metal products, the low productivity still remains a major limitation for their cost-effective feasibility in practical applications. To achieve high-speed printing with affordable mechanical properties, we increased the scanning speed of selective laser melting process with Ti–6Al–4V up to 1800 mm/s and applied a hot isostatic pressing (HIP) process to compensate for the porosity. In these high-speed printed specimens, the HIP process led to a microstructural change from αʹ-lath martensite to a Widmanstӓtten α-lamellar structure, which deteriorated their tensile properties due to the segregation of β-stabilizing atoms and caused inter-lamellar fracture. The deterioration phenomenon of high-speed printed Ti–6Al–4V specimens after the HIP process was found to be critically affected by the surface roughness of as-built state, which can be efficiently controlled with a build angle set-up.

Effect of ultrasonic shot peening on microstructure evolution and corrosion resistance of selective laser melted Ti–6Al–4V alloy

The ultrasonic shot peening (USP) treatment was adopted to improve the compressive residual stress, microhardness and corrosion resistance of Ti–6Al–4V (TC4) titanium alloy fabricated by selective laser melting (SLM). The optical microscope (OM), scanning electron microscope (SEM), X-ray diffraction (XRD) and hardness test were used to investigate the microstructure evolution. The effect of the USP on electrochemical behaviour of the SLM TC4 alloy in a 3.5 wt. % NaCl solution was studied using potentiodynamic polarization. After USP treatment, compressive residual stress was induced and microhardness significantly increased. Proper USP treatment can effectively improve corrosion resistance of the SLM TC4 alloy, which is manifested by the increase in corrosion potential and decrease in corrosion current density.

Investigation into the microstructure and dynamic compressive properties of selective laser melted Ti–6Al–4V alloy with different heating treatments

As a commonly used engineering material, the mechanical properties of titanium alloy under dynamic loads are closely related to their microstructure. In this work, the effects of solution treatment (ST) and solution and aging treatment (SAT) on the microstructure and dynamic compressive properties of Ti–6Al–4V alloy manufactured by selective laser melting were studied. The results showed that the microstructure of selective laser melted Ti–6Al–4V consisted of nearly full acicular α′ martensite, then the acicular α′ martensite was decomposed into α+β phase with basket-weave morphology with solution treatment. Clusters of α2 particles with size of several hundred nanometers were precipitated in the α plates further with solution and aging treatment. The ultimate compressive strength (UCS) of selective laser melted TC4 alloy was increased with the increasing strain rate, showing strong strain rate hardening effect. Stress collapse happened once the strain exceeded 1500/s, which is the dominant failure model of selective laser melted TC4 under impacting load. As expected, the UCS of the ST sample decreased, but the ductility increased compared with the as-built sample; however, both the UCS and ductility of the SAT samples were enhanced synergistically due to the widely distributed α2 precipitates. Besides, the SAT samples had the highest energy absorption compared with the as-built and ST counterparts under the same conditions, indicating that the SAT samples had better load-bearing capacities.

Controlling the tensile and fatigue properties of selective laser melted Ti–6Al–4V alloy by post treatment

The microstructure, phase composition, tensile and fatigue properties of Ti–6Al–4V samples produced by selective laser melting (SLM), SLM followed by heat treatment (HT), and SLM followed by hot isostatic pressing (HIP) were investigated to explore a simpler approach to improve the mechanical performances by post treatment at minimum cost in our work. Results showed that the tensile and fatigue properties of the as-SLMed samples were far below the level of wrought counterpart. Heat treatment and HIP can more obviously enhance tensile properties and fatigue performances, respectively. The microstructure consisted of the lamellar or acicular α and grain boundary α within prior β columnar grain boundaries in the SLMed samples after solution treatment at 850 °C followed by water quenching, whose tensile properties were improved to be 1085 MPa, 859 MPa, 14.0%. A tri-modal microstructure including lamellar α, polygonal α and acicular nanoscale α+β phase formed in the samples after 850 °C solution treatment and 550 °C aging treatment. Their corresponding tensile properties were enhanced to be 1192 MPa, 1054 MPa, 11.4%. The microstructure was composed of lamellar α+β phases for the SLMed sample after HIP treatment, which improved the tensile (>1000 MPa, >850 MPa, and >10%) and fatigue (105∼6 at the stress amplitude of 500 MPa) properties. Therefore, solution treatment at 850 °C followed by water quenching or HIP treatment is a simpler method to control the performances of the SLMed Ti–6Al–4V alloy.

Corrosion resistance of selective laser melted Ti–6Al–4V alloy in salt fog environment

In this study the corrosion resistance of an additively manufactured (AM) titanium alloy Ti–6Al–4V (Ti64) in a simulated marine environment was investigated. Selective laser melting (SLM) process was used to fabricate Ti64 coupons that were tested in a simulated marine environment using a salt fog chamber. The corroded specimens were characterized by scanning electron microscopy, X-ray diffraction, and energy dispersive spectroscopy. The AM printed Ti64 was determined to form a protective oxide layer that develops after approximately 400 ​h of exposure in a salt fog chamber, after which mass change plateaued. The protective oxide formed was determined to be a mixed oxide primarily consisting of TiO2and Al2O3.

Dynamic evolution of oxide film on selective laser melted Ti–6Al–4V alloy

The oxide film growth on Ti–6Al–4V alloy is a dynamic process due to the coexisting and competing dissolution of oxides, which significantly affects the corrosion and applications of Ti–6Al–4V alloys. In this paper, the components and semiconductor characteristics of the oxide films on the surface of the selective laser melted (SLM) Ti–6Al–4V alloys were analyzed. The growth and evolution mechanism of the oxide films were proposed. From electrochemical analysis, the SLM-produced Ti–6Al–4V was more resistant to corrosion in course of the seawater immersion than the wrought Ti–6Al–4V, and the two tested alloys showed difference in performance with immersion time increasing. A film resistance increase was observed in the SLM-produced alloy, indicating the formation of a TiO2 barrier layer.

Study of pore defect and mechanical properties in selective laser melted Ti6Al4V alloy based on X-ray computed tomography

The selective laser melting (SLM) technology is widely used for the manufacturing of Ti6Al4V alloy components in the biomedical and aerospace industries, and the residual pores are one of the significant defects obstructing its further application. X-ray computed tomography (XCT) technique was applied for the examination and analysis of the spatial and morphological features of pore defects, which will influence the tensile properties of the SLM-manufactured Ti6Al4V samples. Hence, the effects of processing parameters on pore defects and their formation mechanism are discussed thoroughly. The results showed that two major types of pore defects are observed: the lack-of-fusion caused by low volumetric energy density (VED), and the keyhole mode resulting from excessive energy densities. The highest relative density of 99.995% was obtained at a VED value of 58.8 J/mm3. Besides, the quantitative information of the porosity defects, such as size distribution, morphology and formation direction, was evaluated using XCT to reveal their formation mechanisms. Furthermore, the tensile properties of SLM-manufactured Ti6Al4V alloy under different VED values were evaluated to correlate with the porosity variation. Overall, the tensile strength and ductility were notably degraded with the increase of porosity. The fractography study based on XCT and SEM showed that lack-of-fusion and porosities are responsible for the early fracture. In conclusion, the XCT technique can provide direct and precise inspection of the process of laser-based additive manufacturing to control the overall quality.

Effects of machining surface and laser beam scanning strategy on machinability of selective laser melted Ti6Al4V alloy in milling

Overlapping melting trajectories and partially-melted powders result in poor surface morphology and high surface roughness values (Ra = ~13.34 μm) of selective laser melted (SLMed) Ti6Al4V alloy. Secondary processing of SLMed components is thus an essential finishing operation to produce functional SLMed parts in precision engineering. This paper investigates the effects of laser scanning strategies (0°, 67.5° and 90° laser scanning schemes) and machining surfaces (top and front surfaces) on the machining performance of SLMed Ti6Al4V alloy in milling, compared with that of annealed ASTM B265 Ti6Al4V alloy. High degree of anisotropy of SLMed Ti6Al4V alloy is reflected in cutting force, surface morphology and surface roughness on different machining surfaces. The machining anisotropy is dominated by the anisotropy of microstructure and mechanical properties of SLMed Ti6Al4V alloy, where the anisotropy weakens following the sequence of 0°, 90° and 67.5° SLMed samples. It is verified that high cutting speed can improve machining anisotropy features of SLMed titanium alloy. The chip shape of SLMed Ti6Al4V alloy is a typical conical spiral chip, and the bending degree and length of chips produced by SLMed Ti6Al4V alloy are larger than those produced by annealed Ti6Al4V alloy.

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

To evaluate the introduction of selective laser melting (SLM) for serial application, some features directly related to the components safety and performances are to be deeply analyzed. To this purpose, the present article applies three different types of heat treatment on Ti6Al4V as‐received samples produced by SLM: 1) annealing stress relieving, 2) solubilization super‐β transus, and 3) sub‐β transus followed by a tempering. Then, the related microstructures, mechanical resistance, residual stresses, and corrosion resistance are studied. The results identify a set of heat treatment parameters (super‐β transus followed by tempering), which guarantee the required properties. These samples show equiaxial grains composed of α + β phase, good mechanical properties, a high resistance to corrosion, and residual stresses almost null. Finally, this study confirms that Ti6Al4V made by SLM is applied in the transport field.

Dimensional Deviation Management for Selective Laser Melted Ti6Al4V Alloy Blade

Selective laser melting (SLM) is prone to generate large thermal stresses, which could induce a heavy geometric change in the parts. In this study, Ti6Al4V blade was prepared by Selective laser melting (SLM). Microstructure, dimensional deviation and residual stress were investigated. Microstructure is acicular α' martensite in prior columnar β grain. The smaller the grain size, the larger the dimensional deviation. The directions of dimensional deviation at the leading and trailing edges of blade are opposite to those in the middle. The distribution of dimensional deviation exhibits a parabolic law at the trailing edge of blade. The thermal stress is big at the edges of the blade body. The large tensile stress along the edges of the blade body caused the edges to deform toward the suction side of the blade. The residual stress distribution of the pressure surface by the XRD measured has a good agreement with thermal stress distribution by calculated.