Selective Laser Melted Ti6Al4V Alloy Research Articles

Effect of Heat Treatment on Electrochemical Corrosion Behavior of Selective Laser Melted Ti6Al4V Alloy

Effect of Heat Treatment on Electrochemical Corrosion Behavior of Selective Laser Melted Ti6Al4V Alloy

Investigation of microstructural evolution in a selective laser melted Ti6Al4V alloy induced by an ultrasonic surface rolling process

Surface modification treatment was carried out on a selective laser melted Ti6Al4V alloy using an ultrasonic surface rolling process (USRP). The results showed that a deformed layer with a gradient microstructure was produced near the surface, in which the microstructure changed from a coarse lamellar α structure to ultrafine lamellar grains, ultrafine equiaxed grains and nanograins. Moreover, there was a decrease in the intensity of the texture from the matrix to the surface. Furthermore, the dislocation density in the deformed layer first increased and then decreased from the matrix to the surface. The deformed layer exhibited a much higher hardness than the matrix, which was ascribed to grain boundary strengthening and dislocation strengthening.

Modified wear behavior of selective laser melted Ti6Al4V alloy by direct current assisted ultrasonic surface rolling process

Ti6Al4V alloy samples prepared by selective laser melting were treated by three different post-treatments: namely heat-treatment, ultrasonic surface rolling process (USRP) and direct current assisted ultrasonic surface rolling process (DC-USRP). Wear behaviors were evaluated using a block-on-disc rig under lubrication conditions. Results showed that the DC-USRP treated sample exhibited the lowest wear rate of 1 × 10−6 mm3 N−1 m−1, compared to that of the as-built (4 × 10−5 mm3 N−1 m−1), heat-treated (2 × 10−5 mm3 N−1 m−1) and USRP-treated samples (5 × 10−6 mm3 N−1 m−1). Consistent with changes in the wear rate, the friction coefficient was also gradually reduced. The as-built and heat-treated samples showed severe abrasive wear and delamination, while the USRP and DC-USRP treated samples displayed only mild abrasive wear. The improved wear resistance of the USRP and DC-USRP treated samples was attributed to the modified lubrication mechanism, increased hardness and compressive residual stress. Moreover, the wear resistance of the DC-USRP treated samples decreased with the increasing depth from the surface due to the effect of gradient work hardening.

Improvement of tribological performance by micro-arc oxidation treatment on selective laser melting Ti6Al4V alloy

The technique of Selective Laser Melting (SLM) has been widely applied as an additive manufacturing method in fabricating the Ti6Al4V parts. However, its relatively low wear resistance restricts the application of the SLM titanium alloy. In this paper, a promising way (Micro-arc Oxidation, MAO) was proposed to improve its wear resistance by forming a dense oxidized coating onto the SLM Ti6Al4V alloy surface. The dry sliding wear test and microhardness measurement were used to characterize the mechanical performance. It is found that with increasing treatment duration, coating growth was slowing down accompanied gradually roughening appearance. The oxidized layer was composed of γ-alumina and TiAl2O5. Meanwhile, a significant improvement of tribology performance was found on the Ti6Al4V alloy by employing the MAO treatment. It can be believed that the MAO treatment can improve the reliability of the bearing surfaces on SLM Ti6Al4V parts.

Ductilization of selective laser melted Ti6Al4V alloy by friction stir processing

The low ductility of Ti6Al4V alloy manufactured by Selective Laser Melting (SLM) adversely impacts the component performance in practical applications. A local post-treatment by Friction Stir Processing (FSP) significantly reduces the porosity and homogenizes the microstructure. This results in an increase in fracture strain from 0.21 after SLM to 0.65 following the FSP post-treatment. The porosity reduction was evidenced by 3D X-ray micro-computed tomography. A fully transformed β microstructure is formed after FSP. This microstructure involves α plates, α colonies, as well as equiaxed dynamically recrystallized α phases inside equiaxed prior-β grains. The deformed microstructure was observed during in-situ tensile test, using scanning electron microscopy, with the aim to unravel the damage mechanisms. In addition to the beneficial effect of initial porosity reduction, the transformed microstructure after FSP bears more damage before failure than the typical α’ martensite laths in the as-built SLM samples.

Microstructure and fatigue performance of SLM-fabricated Ti6Al4V alloy after different stress-relief heat treatments

Abstract The main interest in Additive Manufacturing (AM) technology relates to its ability to produce complex components with relatively reduced weight that are difficult to produce or cannot be produced by other conventional technologies. Selective laser melting (SLM) is extensively used, as one of the AM technologies to fabricate metallic parts. This advanced method allows to produce various parts with complex geometries with high three-dimensional (3D) accuracy from fusion powders in a layer-by-layer style. Ti6Al4V alloy is a widely used material for structural applications in aerospace and biomedical due to high specific fatigue strength. SLM processing makes this alloy attractive when weight reduction is a design objective. The SLM Ti6Al4V microstructure is influenced by process parameters and build orientation. The localized high energy input during very short interaction times leads to the formation of very fine structures and to the generation of internal stresses. Therefore, the SLM parts are heat treated to decrease or completely remove residual stresses. The present study aims at evaluating the effect of stress-relief heat treatments on the microstructure, the mechanical properties and the fatigue performance of SLM Ti6Al4V alloy. Ti6Al4V alloy specimens were manufactured according to the SLM process with an EOS M290 system. Post fabrications heat treatments at different temperatures (i.e. 740˚C vs. 900˚C) resulted in different structure and mechanical properties that were identified and measured. Fatigue testing of specimens with as-built surfaces was performed at room temperature on modified Schenk-type fatigue testing machine applying a pulsating plane bending (load cycle ratio R = 0) to the specimens at a frequency f = 15 Hz.

VHCF response of heat-treated SLM Ti6Al4V Gaussian specimens with large loaded volume

Among the materials used for the production of components through Additive Manufacturing (AM) processes, the Selective-Laser-Melting (SLM) Ti6Al4V alloy is widely employed in aerospace applications for its high specific strength and in biomedical applications for its good biocompatibility. Actual structural applications are generally limited to static loading conditions where the large defects originating during the SLM process do not play a significant role for the static failure. On the contrary, the same defects strongly affect the fatigue response of the parts since they act as crack initiation sites that rapidly lead to fatigue failure. In the literature, a lot of research has been carried out to investigate the quasi-static and the High-Cycle Fatigue properties of the SLM Ti6Al4V alloy but there are still few studies on its Very-High-Cycle Fatigue (VHCF) response.In the paper, the VHCF response of Ti6Al4V specimens, which are vertically orientated during the SLM building and then subjected to a conventional heat treatment (2 hours heating in vacuum at 850°C), is experimentally assessed. Ultrasonic VHCF tests are carried out on Gaussian specimens with a large risk-volume (2300 mm3). Fracture surfaces are investigated for revealing the defect originating the fatigue failure. The Stress Intensity Factor Threshold associated to the experimental failures is finally estimated.

The effect of heat treatment on microstructure evolution and tensile properties of selective laser melted Ti6Al4V alloy

The phase transition, microstructure, detailed structures and tensile properties of selective laser melted Ti6Al4V alloy were been examined and analyzed. The results reveal that the as-fabricated Ti6Al4V samples consist of lamellar α' and acicular α'. Much twins in the lamellar α' and small α2 precipitation in the acicular α' were found. The effects of different heat treatment methods on the as-fabricated samples were also investigated. As a result of the microstructure evolution, samples heat-treated at 800 °C and 850 °C for 4 hours and cooled in air contain the most β and the finest lamellar α, exhibiting good comprehensive properties.

Optimization of processing parameters and establishment of a relationship between microstructure and mechanical properties of SLM titanium alloy

The processing parameters of a selective laser-melted (SLM) Ti6Al4V alloy were systematically optimized for optimal surface roughness and relative density. The relationship between microstructure and mechanical properties were established in the optimal parameter range. The results showed that the optimal laser power and scan speed was 200–250 W and 850–1150 mm/s, respectively. The laser power and scan speed played contradicting roles in the resultant roughness and porosity. An appropriate increase in laser power and decrease in scan speed could reduce the surface roughness and simultaneously improve the density and dimensional accuracy. When the laser power was 200 W, the microstructure varied from equiaxed grains to mixed equiaxed-columnar grains, and then it varied to columnar grains as the scan speed changed from 250 mm/s to 850–1750 mm/s. The acicular structure inside the grains transformed from an α (α′) + β phase to an α′ (α) phase with gradual grain refinement. Some nano-β phase precipitated near the acicular phase. Micro-hardness measurements revealed different values at the top, middle and bottom of the samples with increasing laser power and showed a reduced hardness gap with increasing scan speed. However, the elongation continuously increased with increasing scan speed and reached a maximum value of 7.8% at a scanning speed of 1150 mm/s. Decrease in elongation was observed when the laser power increased to 275 W. These variations in the hardness and mechanical properties were attributed to the combined effects of nano-β phase and grain refinement of acicular α (α′) martensites.

Microstructure and mechanical properties of selective laser melted Ti6Al4V alloy

The present work was focused on the properties of porous Ti6Al4V specimens processed by selective laser melting (SLM) and tested in tension and compression before and after heat treatment. The SLM samples were annealed at 955 °C, water quenched and aged at 600 °C with following air cooling. The values of the mechanical tests showed that the samples exhibited high mechanical properties. The anisotropy of tensile and compressive strength was observed, which was related to the occurrence of voids. The plastic properties of specimens were improved by means of the heat treatment that led to the transformation of martensitic to lamellar structure composed of α + β phases. The microstructure of SLM samples were evaluated before and after the heat treatment. The brittle nature of failures of non-heat treated samples can be explained by synergy of martensite presence, microcracks and residual stresses produced by SLM.

Microstructural evolution and microhardness of a selective-laser-melted Ti–6Al–4V alloy after post heat treatments

Microstructure and hardness of a powder-bed-type selective laser melted Ti–6Al–4V alloy after post heat treatments at from 300 °C to 1020 °C were systematically investigated by using optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM) and Vickers hardness (HV) tester. Long columnar original β grains together with the inside dominated parallel acicular martensite in the side view, and chessboard pattern in the top view, were found in the as-received specimen. The subtransus heat treatment does not enable modification of the morphology of the original columnar β grain, only leading to the acicular α′ martensite decomposition into the α platelet and whether surrounded β phase or transformed α′ phase depending on the heating temperature; while the supertransus heat treatment would thoroughly break up the original long columnar β grain, leaving only big original equiaxial β grain filled with the new forming weave-type acicular α′ martensite like the supertransus heat treated wrought specimen. Vickers hardness evolution strictly follows the trend of the microstructural change as the heating temperature increasing, and the double peak phenomenon of the Hardness–Temperature plot should be attributed to substructural refinement effect at around 500 °C, martensitic refinement effect at around 1000 °C, and softening effect resulting from the completely decomposition of the martensite at around 875 °C.

Anisotropy in the impact toughness of selective laser melted Ti–6Al–4V alloy

Apparent anisotropies have been found in the mechanical properties of selective laser melted Ti–6Al–4V alloy. This study investigated the influences of the vertical and horizontal building directions on the impact toughness of selective laser melted Ti–6Al–4V, and a mechanism for the anisotropy in toughness is proposed. Disc-shaped building defects were clearly observed, even though the relative density was as high as 99.5%. The directionality of these defects reduces the load-bearing cross-section of a vertical specimen relative to that of a horizontal specimen; consequently, the impact energy of a horizontal specimen is 96% higher than that of a vertical specimen.

Residual Stress in SLM Ti6Al4V Alloy Specimens

Selective Laser Melting (SLM) presents a modern manufacturing process with an innovative technology which allows the production of full-density objects or fine-structured parts with complex geometry and inner structures. Stability and certification of the properties of SLM parts are important tasks for all producers and end-users. One of the drawbacks of this technology is high residual stress in as-made SLM objects. In this study X-ray diffraction technique was used for investigating the residual stress induced into SLM Ti6Al4V alloy samples. Principal stresses were estimated for the cut rectangular specimen. Two types of the cantilevers were produced and numerical simulation of the stress was performed. The bending of cut cantilevers was measured before and after heat treatment. Next series of the samples had rectangular shapes and different thicknesses from 1 to 46 layers. All as-manufactured specimens attached to the substrate showed the presence of tensile residual stresses near the top surface. Residual stress along the laser scanning direction had magnitudes twice that of the stress in the perpendicular direction. Conclusions regarding directions and values of stresses in SLM objects from Ti6Al4V powder are given.

Cracking Behavior and Formation Mechanism of TC4 Alloy Formed by Selective Laser Melting

Crack is the most common and great destructive defect in selective laser melting(SLM) Ti6Al4V(TC4) alloy process. A TC4 alloy test specimen is fabricated by progressive alternative scan strategy in SLM process. The cracking behavior and formation mechanism is discussed in TC4 alloy SLM processing using scanning electron microscopy and X-ray energy spectrum analysis. The experimental results show that the crack is considered as cold cracking in SLM TC4 alloy part, which has typical characteristic of the transgranular fracture. By using the progressive alternative scan strategy, the basket-like martensitic structure is formed in SLM process. The high residual stress is caused by high temperature in forming process of SLM part. Under the residual stress, the crackle will occur in the martensitic structure because it has poor cracking strength. Finally, the coarse crack decomposes into smaller cracks in the process of expansion and the extension is terminated. Further research shows that by improving the process parameters, the microstructure of TC4 alloy will be changed and the residual stress will be weakened. At the same time, achieving to the purpose of weakening or eliminating cracks.

Relation between pore structure and fatigue behaviour in sintered iron-copper-carbon : K.D.Christian, R.M.German. (Pennsylvania State University, Pennsylvania, USA), Int. J. Powder Metallurgy, vol 31, no 1, 1995, 51–61

Apparent anisotropies have been found in the mechanical properties of selective laser melted Ti–6Al–4V alloy. This study investigated the influences of the vertical and horizontal building directions on the impact toughness of selective laser melted Ti–6Al–4V, and a mechanism for the anisotropy in toughness is proposed. Disc-shaped building defects were clearly observed, even though the relative density was as high as 99.5%. The directionality of these defects reduces the load-bearing cross-section of a vertical specimen relative to that of a horizontal specimen; consequently, the impact energy of a horizontal specimen is 96% higher than that of a vertical specimen.