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Text | Xiaoxin
Editor|Xiaoxin
Selective laser melting is an additive manufacturing technique that enables the production of almost completely dense 3D metal parts in a layer-by-layer manufacturing process, followed by layers of powder 50 to 100 μm thick that are melted and solidified by a focused laser beam to obtain a given computer-aided design model.
This machining allows direct fabrication of metal parts with complex geometries with little to no further machining, which greatly reduces material waste and investment costs for machining tools.
For the above reasons, the aircraft manufacturing industry is increasingly interested in the manufacture of nickel-based components by SLM, and as a result, Inconel 718 processed by SLM has been extensively studied in recent years.
IN718 is a common nickel-based austenitic alloy with high strength and excellent corrosion resistance at high temperatures up to 700°C, and due to these properties, it is widely used in turbine engines or high-speed airframe components in the aviation industry, as well as valves, packers or fasteners in the oil and gas industry.
●○Materials and methods○●
The powder mainly consists of spherical particles with good flowability, an average diameter of 3512 and 2012 μ m determination of IN718 and Re powders respectively using HoribaLA-950 laser diffraction particle size analyzer, grinding containers with a capacity of 500 ml at a speed of 200 rpm in a single ball mill with ZrO containing 25 balls with a diameter of 25 mm.
A cylindrical sample with a diameter of 8 mm is fabricated in two construction directions, with a cylindrical axis perpendicular to and parallel to the build direction, and the XY scanning strategy is used for sample preparation with the following process parameters: laser power 400W, scanning speed 500mms−1 layer thickness 50μm, and distance between scan lines 160μm Chemical composition of the resulting SLM-treated IN718-Re alloy measured by an X-ray fluorescence analyzer.
Examine the microstructure with a NikonEclipse MA200 light microscope and a HitachiSU8000 scanning electron microscope equipped with a backscattering electron detector, grind, polish, and etch sections for metallographic observation using 20 ml HCl, 10 g FeCl solution for a few seconds 3 and 30 ml ethanol.
Energy dispersive X-ray spectroscopy and detailed precipitate characterization of samples were performed on the high-resolution scanning transmission electron microscope Hitachi HD-2700, and thin slices for STEM research were prepared by focused ion beam extraction technology using the FIB-SEM Hitachi NB5000 system.
Corrosion resistance at room temperature was studied by potentiodynamic polarization and electrochemical impedance spectroscopy methods, both of which were 0.1 mNa2 Therefore 4 and 0.1MNaCl solutions, both of which were cut into 1mm thick discs 0° and 90° by adding H acidification to pH42 and HCl, and samples for corrosion testing were obtained, these ~0.5 cm exposed area 2 discs correspond to XZ and XY planes, respectively.
Prior to exposure, ground all surfaces to a particle size of 1200 with SiC sandpaper and ultrasonically cleaned in acetone, using the AutoLabPGSTAT100 potentiostat to perform potentiodynamic and impedance tests in a three-electrode device and auxiliary platinum electrodes.
A potential curve is recorded from a potential of about 150 mV below the corrosion potential to 1500 mV with a potential scan rate of 0.2 mVs−1. The corroded sample was observed with SEM, impedance tested at 105 to 10−3 Hz, sinusoidal signal amplitude of 5 mV, impedance spectrum recorded in potentiostatic mode at open potential, and subsequently analyzed by Boukamp's Equivcrt software.
●○The microstructure of IN718 ○●
The microstructure of the 718 sample is shown in the figure in the plane XY and XZ perpendicular to each other, the typical microstructure of SLM processed materials with layered and columnar structures is observed, considering the XY plane, the width of the laser scanning trajectory is about 150 ° μm clearly visible. The parallel scan trajectories are oriented in a zigzag pattern generated by the construction strategy, and their widths correspond to the applied 160° shadow spacing μ meters.
The individual orbitals consist of elongated particles parallel to each other, and similarly, equiaxed grain μm in size 10 to 30 can be identified between adjacent scanning orbitals.
This may be caused by the remelting process of overlapping regions, and SEM observations in the XZ plane also revealed that the microstructure of the as-built sample was dominated by columnar grains with a width of 10 to 30 μm and a length of several hundred microns, which elongated in the direction of construction.
In addition, the arc of the molten pool can be seen in the XZ plane, showing the development of the layers during the laser beam fabrication process, the size of the melt pool is determined by the applied layer thickness and hatch spacing.
The observed unidirectional columnar grains longer than the thickness of the scanning layer are the result of epitaxial grain growth occurring during SLM treatment in the direction determined by the heat flux, as the heat is dissipated mainly in the building direction and the highest temperature gradient and solidification rate occur along the Z-axis, a processing condition that results in the formation of fine columnar/honeycomb dendritic substructures in IN718.
Each grain consists of columnar/cytoid dendrites, the equiaxed cell structure and columnar/honeycomb dendrites growing almost parallel to the construction direction can be seen in the XY plane, the typical width of the honeycomb and columnar structures is approximately the same, and in the range between 700 and 1100 nm, it should be mentioned that in the case of SLM-treated IN718 alloy, such fine dendritic substructures are standard.
The formation of columnar, honeycomb dendritic substructures appears to be accompanied by microsegregation, and SLM treatment, characterized by the high solidification rate of the molten pool, prevents macrosegregation of alloying elements that are highly sensitive to segregation, i.e. Nb or Mo, however, it is not possible to completely avoid microsegregation of these elements.
The interdendrite region is highly enriched with Nb and Mo, which leads to the formation of irregularly shaped MC-type carbides between Laves phase and dendritic cells, in addition, spherical MC-type carbides rich in Nb and Ti, which are less than 100 nm in size, and a small amount of fine aloxides can be seen in the matrix, according to the phase transition order of IN718 alloy during cooling, the appearance of Laves phase and MC-type carbides in the interdendrite space can be explained.
The solidification process of IN718 alloy begins with the reaction of liquid phase (L) to the austenite matrix γ phase (L→γ), and then, the enrichment of interdendrite liquid of Nb, Mo, Ti and C and a series of eutectic reactions, as the temperature decreases, the eutectic reaction L→ (γ + NbC) occurs, resulting in the depletion of the remaining liquid in c, after which, the concentration of Nb and Mo in the remaining liquid is high enough, and the second eutectic reaction L→ (γ + Laves) leads to the formation of the Laves phase.
●○Microstructure of IN718-Re alloy ○●
The addition of rhenium does not cause any drastic changes in the microstructure of the SLM-treated IN718 alloy, in the case of IN718-Re alloy, a columnar structure with a fine dendrite substructure is still maintained, as shown in the figure, spherical MC type carbides are also present in the IN718-Re alloy and the Laves phase in the interdendrite gap rich in Nb and Mo, rhenium is mostly dissolved in the γ phase matrix, however, some undissolved rare earth particles ranging in size from a few microns to 20 microns μ m can also be seen.
Due to the presence of Re atoms in the γ phase matrix, the main microstructural change is the size of columnar/cytodendrite and the width of the columnar/honeycomb structure increases with the increase of Re content from 893±223 to 1936±515 nm, IN718 and IN718-6Re, respectively, El-Bagoury also observed similar effects of rare earth elements and others for casting IN718 alloys.
Higher Re content leads to an increase in the volume fraction of the γ phase and an increase in the dendrite spacing, this change in microstructure can be explained by the redistribution of alloying elements, rhenium is considered a solid solution enhancer in nickel-based alloys and tends to segregate into γ dendrites, as a result, the volume fraction of columnar/cyto dendrite in the γ phase increases with the increase of Re content, sacrificing the interdendrite region rich in Nb and Mo.
●○ Kinetic potential polarization ○●
In order to observe the effect of the addition of rhenium on the corrosion resistance of IN718 alloy, a potentiodynamic polarization measurement was carried out in 0.1MNa, so and 0.1MNaCl solution (both acidified to pH4), all the electrochemical parameters of IN718-Re alloy manufactured in both manufacturing directions, the figure shows the polarization curve of IN718-Re alloy manufactured in 0 degrees, 0.1M sodium 2 so 4 environment, according to the current response can be noted passive region and transpassive region.
IN718 alloy is passivated over a wide potential range, a passivation platform is observed between 0-700mV, and when the current density rapidly increases to about 1000mV, the passivation region can be seen, which corresponds to the dissolution of chromium compounds, further anodic polarization above 1000mV leads to the degradation of the passivation film and results in thick and cracked chromium-based oxide layers.
Electrochemical behavior and corrosion current density icorr and corrosion likelihood EcorrIN718-Re alloy is slightly different from IN718 without Re, one of the differences is that the cathode branch and active-passive region of IN718-Re alloy have a lower current density, which indicates that the passivation film formed by air on IN718 alloy is easier to decrease.
On the other hand, the addition of rhenium reduces the durability of the passivation layer, as evidenced by the shortening of the passivation platform and the shift of the polarization curve to a lower potential value in the range of 500-900mV, it should also be noted that for higher re content, the curve moves further. However, the main difference between IN718 and IN718-Re alloys is the icorr and Ecorr values, and the addition of rare earths improves the corrosion resistance of IN718 alloys.
The corrosion current density icorr significantly decreased from 0.20 to 0.07-0.09μ one centimeter−2 in IN718-Re alloys, respectively, in addition, the corrosion potential Ecorr shifted to a positive value with the increase of Re content, and the total added value of IN718-6Re alloy was about 60mV.
The building orientation of 0° and 90° produced similar electrochemical behavior and corrosion resistance, the corrosion current density icorr 90 degree sample had a slight decrease in value, while the 0 degree sample showed a greater positive Ecorr, nevertheless, the positive effect of the addition of rhenium on the corrosion resistance of IN718 alloy was observed as a corrosion potential Ecorr increased with increasing Re content.
Potentiodynamic measurements in chloride-containing environments also show that compared to IN718-Re alloys without Re, the corrosion resistance of IN718-Re alloys is enhanced, the corrosion current density icorr greatly reduces the possibility of corrosion Ecorr For IN718-Re alloys to move towards the corrected value, samples manufactured at 0 and 90 degrees show similar corrosion resistance.
In addition, the addition of Re stabilizes the passivation layer in Cl−containing the environment, as evidenced by the presence of sharp current density spikes in the potential range of 300 to 600 mV for IN718 and IN718-2Re alloys. This current density spike corresponds to metastable pitting growth and is not present in IN718-4Re and IN718-6Re alloys.
The SEM image of IN718 sample fabricated in two building orientations, 0° and 90°, after potentiodynamic polarization to about 1000 mV in 0.1MNaCl solution, is shown, in the case of 0 degree sample, the entire exposed surface has been etched, showing the initial microstructural unit, arc-shaped molten pool and fine columnar/honeycomb dendritic substructure, in contrast, SEM observation of 90° of the sample shows local corrosion on the sample surface.
The thicker oxide layer forms in bands parallel to each other, and these bands are approximately separated. 150 μThis corresponds to the overlapping regions between adjacent laser scanning orbitals, at the same time, there is a thin oxide layer between the overlapping regions, grinding scratches are still visible, it should be noted that the current density of the 1000mV potential at 0° of the sample is almost an order of magnitude higher than at 90° of the sample.
Therefore, considering the potentiodynamic polarization measurements and SEM observations, the overlapping region appears to be the preferred location for SLM to treat IN718-Re alloy corrosion, possibly due to the fact that the overlapping region is characterized by a higher proportion of grain boundaries and higher cumulative energy, further electrochemical corrosion leads to the formation of a uniform oxide layer on the sample surface and exposes the initial microstructural units produced by laser beam fabrication, such as arc-shaped molten pools or laser scan trajectories.
In this study, the effect of rhenium addition on the microstructure and corrosion resistance of IN718 alloy treated with SLM was studied, and the main conclusions can be summarized as follows:
The cast microstructure of IN718-Re alloy is mainly composed of columnar grains growing in multiple layers parallel to the construction direction, each columnar grain is characterized by a fine columnar, honeycomb dendritic substructure, and the interdendrite space is highly enriched with Nb and Mo, which leads to the formation of Laves phase and MC-type carbide between dendrites.