Selective Laser Melting Inconel Research Articles

Optimizing Selective Laser Melting of Inconel 625 Superalloy Through Statistical Analysis of Surface and Volumetric Defects

This article delves into optimizing and modeling the input parameters for the selective laser melting (SLM) process on Inconel 625. The primary aim is to investigate the microstructure within the interlayer regions post-process optimization. For this study, 100 layers with a thickness of 40 µm each were produced. Utilizing the design of experiments (DOE) methodology and employing the Response Surface Method (RSM), the SLM process was optimized. Input parameters such as laser power (LP) and hatch distance (HD) were considered, while changes in microhardness and roughness, Ra, were taken as the responses. Sample microstructure and surface alterations were assessed via scanning electron microscopy (SEM) analysis to ascertain how many defects and properties of Inconel 625 can be controlled using DOE. Porosity and lack of fusion, which were due to rapid post-powder melting solidification, prompted detailed analysis of the flaws both on the surfaces of and in terms of the internal aspects of the samples. An understanding of the formation of these imperfections can help refine the process for enhanced integrity and performance of Inconel 625 printed material. Even slight directional changes in the columnar dendrite structures are discernible within the layers. The microstructural characteristics observed in these samples are directly related to the parameters of the SLM process. In this study, the bulk samples achieved a microhardness of 452 HV, with the minimum surface roughness recorded at 9.9 µm. The objective of this research was to use the Response Surface Method (RSM) to optimize the parameters to result in the minimum surface roughness and maximum microhardness of the samples.

Fatigue analysis of selective laser melting Inconel 718 and the mechanism of ductile–brittle continuous alternating steps during fatigue process

In this paper, Inconel 718 powber fabricated by selective laser melting were studied to explore the mechanical characteristics and microstructures. The results of microstructure show that the material exhibits the columnar, honeycomb, and mixed crystal structures. The microhardness is very uniform, with a average value of 317 HV. Tensile test shows the ultimate tensile strength of 785Mpa, elongation of 4.93%, which is high stress brittle fracture. The fatigue strength of 180 MPa was obtained from the fatigue test. The fatigue life was sensitive to the effect of stress. In the scanning electron microscope observation, small crack propagation was found in the fracture, and a large number of step-like fracture characteristics of repeated transformation of toughness and brittleness were found. The mechanism analysis of the step-like ductile–brittle fracture is carried out, which is divided into four stages. Mathematically analyzing the tough-brittle fracture of the step type, where the crack is subjected to both the stress field around the pore and the stress field at the crack tip on the way to extension, a stress field model for crack propagation around the pore and a fracture toughness formula for transverse and longitudinal propagation of tough-brittle steps applicable to this paper are proposed.

Room Temperature Corrosion Behavior of Selective Laser Melting (SLM)-Processed Ni-Fe Superalloy (Inconel 718) in 3.5% NaCl Solution at Different pH Conditions: Role of Microstructures

Inconel 718 (UNS N07718) is a nickel-base superalloy containing iron that is used at cryogenic temperatures (arctic pipe components) and at high temperatures (gas turbines). This alloy is also used in off-shore oil drilling due to its high overall strength and resistance to corrosion. Inconel 718 components are created by a selective laser melting (SLM) additive manufacturing route and result in isotropic fine-grained microstructures with metastable phases (such as Laves phases) that are not usually present in conventional manufacturing processes. In this work, SLM Inconel 718 alloy specimens were investigated in four different conditions: (1) As-manufactured (AS-AM), (2) Additively manufactured and hot isostatically pressed (AM-HIP), (3) As-manufactured and heat-treated (solution annealing followed by two-step aging), and 4) AM-HIP and heat-treated. Localized corrosion behavior was evaluated at room temperature in a 3.5% NaCl solution at three different pH conditions (pH 1.25, 6.25, and 12.25). Electrochemical tests, including linear polarization, cyclic polarization, potentiostatic conditioning, electrochemical impedance spectroscopy, and Mott–Schottky analyses, were used to compare the corrosion behaviors of the SLM specimens with that of the conventionally wrought IN718 samples. The results showed that the additively manufactured specimens showed better corrosion resistance than the wrought material in the acidic chloride solution, and the AM-HIP specimens exhibited superior corrosion resistance to the as-manufactured ones. Hot isostatic pressing resulted in the visible elimination of the dendritic structure, indicating compositional homogeneity as well as a significant decrease in porosity. In addition, the deleterious secondary phases, such as Laves and δ phases, were not observed in the microstructure of the HIPed samples. The AM-HIP material showed the highest corrosion resistance in all the pH conditions. The two-step aging treatment, in general, resulted in the deterioration of corrosion resistance, which could be attributed to the formation of γ′ and γ″ precipitates that increased the cathodic reaction catalytic activities. In the additively manufactured samples, the presence of the Laves phase was more detrimental to corrosion resistance than any other phases and MC carbide and grain boundary δ phase increased the susceptibility to corrosion in wrought materials.

Comparative Analysis of Melt Pool Evolution in Selective Laser Melting of Inconel 625 and Inconel 718 Nickel-Based Superalloys

One of the key advantages of Additive Manufacturing is the versatility in working with a wide range of materials. Among these materials, Nickel-based superalloys have drawn great attention of specialists. This study investigates the behavior of Inconel 625 and Inconel 718 during selective laser melting. While these alloys have many similarities, thus their distinct chemical compositions determine different responses to this new process, which the authors aimed to elucidate in this study. Numerical simulations using ANSYS Additive® software were conducted to compare the melt pool dimensions (depth and width) of Inconel 625 and Inconel 718. The results reveal that the material's thermal properties play a significant role in determining the melt pool geometry. The Inconel 718 consistently exhibited larger melt pool dimensions than Inconel 625. The findings highlight the importance of understanding the connection between the material properties and process parameters.

Selective laser melting of Inconel 718: Effect of thermal treatment on mechanical properties

Selective laser melting of Inconel 718: Effect of thermal treatment on mechanical properties

Achievement of grain boundary engineering by transforming residual stress in selective laser-melted Inconel 718 superalloy

The complicated geometric parts produced by selective laser melting (SLM) are not amenable to grain boundary engineering (GBE), which is primarily implemented via thermomechanical processing. Herein, the high internal residual stress produced by SLM processing is utilized as stored activation energy for direct GBE in a SLM-fabricated Inconel 718 superalloy. The grain boundary distribution characteristics, grain size, texture, and corresponding mechanical properties of the as-built specimens subjected to different heat treatments were investigated. As the annealing temperature increased, the recrystallized fraction increased, accompanied by an initial increase and subsequent decrease in the number of special grain boundaries. The numerous annealing twin boundaries effectively divided the grains, despite grain coarsening induced by grain boundary migration at a higher temperature. Compared to the as-deposited sample, the yield strength and ultimate tensile strength increased from 707 to 1071 MPa and from 947 MPa to 1271 MPa, respectively, and the ductility increased from 17% to 21% in the sample annealed at 1200 °C for 1 h followed by double aging treatment because this sample had the highest fraction of twin boundaries (64.89%). The “Additive Manufacturing + GBE” strategy based on utilizing the residual stress offers a feasible pathway for broadening the application scope of additive manufactured alloys.

Influence of heat treatment on the tool life while machining SLM Inconel 718 with reference to C&W Inconel 718

In this study tool life experiments were performed on Inconel 718 (IN718) alloy. The Inconel 718 samples were produced by conventional Cast and Wrought (C&W) and by Laser Powder Bed Fusion (L-PBF) techniques like Selective Laser Melting (SLM). The as-built SLM samples which were subjected to only Hot Isostatic Pressing (HIP) are labeled as SLM HIP and both Hot Isostatic Pressing and Aeronautic Heat Treatment are labeled as SLM HIP-AHT. The tool life machining tests were performed using PVD coated carbide insert under emulsion conditions. The surface integrity parameters such as surface roughness, residual stresses, and microhardness were evaluated. The wear mechanisms on the rake and flank faces of the PVD insert were characterized. It's deduced that notch and flank wear near the depth of the cutting zone were the predominant failure mode affecting the tool life. The age-hardened microstructural features of C&W and HIP heat-treated SLM samples have a negative impact on the tool life. Whereas the microstructure obtained by the SLM HIP-AHT sample indeed played an important role by exhibiting 26 % more tool life than SLM HIP samples and 79 % more tool life than the C&W sample.

Heat treatment for selective laser melting of Inconel 718 alloy with simultaneously enhanced tensile strength and fatigue properties

Inconel 718 alloy fabricated by selective laser melting (SLM) was used to investigate the evolution of precipitated phase from as-SLM to heat-treated (HT) samples. A heat treatment, including homogenization, solution and aging heat treatments, was proposed to obtain SLMed Inconel 718 alloy components for desirable mechanical performance. Post heat treatment process can reduce the number of original print defects due to grain boundary migration and grain growth. The solid solution plus double aging treatment (HT-1) results in the precipitation of large amounts of δ phase within the grain and at grain boundaries, as well as a small amount of massive MC. Homogenization, solid solution plus double aging (HT-2) eliminated a large amount of internal δ phases in the grain as well as needle-like δ phases at grain boundaries. However, the single aging (HT-3) still retains some of the needle-like δ phase. Heat-treated specimens increased tensile strength by 20–24% at room temperature and maintained high strength at 650 °C compared to the as-SLM. Further fatigue results also confirmed that the HT-2 treatment increases the fatigue life by an order of magnitude compared to the as-SLM to 10 8 cycles at a stress amplitude of 300 MPa. This work is anticipated to provide a potential route to obtaining attractive mechanical properties for SLM superalloy components. • The evolution of precipitated phase in SLMed Inconel 718 alloy under different heat treatment systems was studied. • Post heat treatment significantly improves strength of SLMed Inconel 718 alloy and maintains the high strength at 650 °C. • HT-2 heat treatment significantly increases the ultra-high cycle symmetric bending fatigue strength and life of the specimens .

Simultaneous consideration of relative density, energy consumption, and build time for selective laser melting of Inconel 718: A multi-objective optimization study on process parameter selection

Additive manufacturing (AM) is regarded as sustainability friendly thanks to its significantly reduced production steps and simplified supply chain at the system level, but it is often not energy efficient at the process level. In this study, we propose that the optimization of AM process parameters should not only maximize build quality, but also minimize environmental impact and build time. A multi-objective optimization problem of maximizing relative density (RD), minimizing energy consumption and build time per unit volume (ev and tv respectively) is formulated and solved for selective laser melting (SLM) of Inconel 718 alloy. A high accuracy RD prediction model with R2 of 93.23% is obtained by fitting SLM experiment data from wide ranges of laser power (P), scanning speed (SS), hatch spacing (HS), and layer thickness (LT). The ev and tv models are created considering printing area utilization rate (U) based on laser scanning and powder coating process. The two approaches of Pareto front and global criterion method are adopted for solving the multi-objective optimization problem. It is found that the highest RD of close to 100% can be obtained at the expense of high ev and tv of close to 500 J/mm3 and 0.3 s/mm3, respectively, and about three times ev and tv are needed to increase RD from about 95% to 100% than from 85% to 95%. At the same U level, the solutions obtained from prioritizing ev and tv are consistent, while those from prioritizing RD show smaller process parameter values. The multi-objective optimization formulation provides an enabling tool for industry to obtain high quality SLM-ed parts with improved sustainability and efficiency at the process level. The methodology developed is applicable to selective laser melting of other materials, and such consideration can be readily expanded to other AM processes as well.

Multiaxial fatigue of Inconel 718 produced by Selective Laser Melting at room and high temperature

This work investigates the fatigue and cyclic plasticity of Inconel 718 produced by Selective Laser Melting (SLM) at room and elevated temperature. Strain-controlled axial, torsional and proportional tests with Rɛ=0 were carried out at 20 °C and 450 °C using thin-walled specimens, resulting in fatigue lives ranging from 103 to 105 cycles. SLM Inconel 718 exhibits a columnar microstructure along the build direction, and the dominant defect related to the manufacturing process was gas pores. Fatigue crack initiation occurred a carbides and/or metallic inclusions for all tests and, therefore, intrinsic defects related to the manufacturing process did not induce premature initiation of fatigue cracks. The orientation of macroscopic fatigue cracks was consistent with the plane of maximum shear stress for all loading conditions. The fatigue crack propagation mechanism of SLM Inconel 718, on the other hand, depends on the loading path and temperature. Both the equivalent strain amplitude and the Fatemi–Socie criterion could correlate fatigue life of all loading conditions at 20 °C and 450 °C within factor-of-two boundaries. As a general trend, the fatigue endurance of SLM Inconel 718 is inferior to its forged counterpart, especially at 450 °C. The cyclic plasticity behaviour of SLM and forged Inconel 718 is similar at both 20 °C and 450 °C and can be simulated by the Chaboche model.

Effect of welding speed on microstructure and mechanical properties of selective laser melting Inconel 625 alloy laser welded joint

The Inconel 625 manufactured by SLM is limited in size and practical application requires advanced welding techniques to join them. In this paper, Inconel 625 fabricated by SLM was laser welded and the effect of welding speed on the microstructure of the joint was investigated. The microstructures were analyzed by SEM, EBSD and TEM. The fusion zone of the joint was mainly columnar dendrites, a large number of equiaxed crystals formed along with the fusion line, and nanoscale carbides and Nb-rich phases precipitated at the grain boundaries can be observed. With increasing welding speed, the grain size and the ratio of high angle grain boundary (HAGB) were decreased, and the tensile strength and elongation are improved. The tensile strength of joints exceeds 875 MPa, joint efficiency reaching 97.8%. Moreover, the elongation of joints is near 39.8%, reaching 94.8% of the base metal. In addition, the cause of softening of the fusion zone was explored， and The fracture method of the joint shows that it is ductile fracture. The material properties for selective laser melting fabrication meet the requirements of the application.

Dissimilar linear friction welding of selective laser melted Inconel 718 to forged Ni-based superalloy AD730™: Evolution of strengthening phases

The continuous growth in the manufacture of aerospace components such as blisks has led to an increase in the application of different hybrid materials fabricating methods, and thus the requirements for joining and strengthening of dissimilar welds. According to this goal, selective laser melted (SLM) Inconel 718 was joined with forged AD730™ Nickel-based superalloy through linear friction welding (LFW) in this study. Microstructure variation, specifically with respect to secondary phases precipitation was investigated. The microhardness and strengthening mechanisms of the weldment were also studied. The precipitation (volume fraction and size of particles) at different regions of both sides of the weld line was characterized. Close to the weld line, the dissolution of γ'/γ" and Laves phases and grain refinement occurred which reveals the effects of both compression strain and high temperature on recrystallization and high degree of elemental diffusion in the weld zone (WZ). It is shown that the size, volume fraction, and shape of secondary phases increased and changed (from spherical to long-striped for Laves particles) as we went from the WZ toward the base metal. However, the measured microhardness indicated that the strength of AD730™ alloy depends significantly on the grain size, while strength in SLM Inconel 718 was dominated by shape (or size) and the presence of secondary phases (γ'/γ" and Laves).

Effects of Static Magnetic Field on the Microstructure of Selective Laser Melted Inconel 625 Superalloy: Numerical and Experiment Investigations

A number of researchers have reported that a static magnetic field (SMF) will affect the process of selective laser melting (SLM), which is achieved mainly through affecting molten pool evolution and microstructure growth. However, its underlying mechanism has not been fully understood. In this work, we conducted a comprehensive investigation of the influence of SMF on the SLM Inconel 625 superalloy through experiments and multi-scale numerical simulation. The multi-scale numerical models of the SLM process include the molten pool and the dendrite in the mushy zone. For the molten pool simulation, the simulation results are in good agreement with the experimental results regarding the pool size. Under the influence of the Lorentz force, the dimension of the molten pool, the flow field, and the temperature field do not have an obvious change. For the dendrite simulation, the dendrite size obtained in the experiment is employed for setting up the dendrite geometry in the dendrite numerical simulation, and our findings show that the applied magnetic field mainly influences the dendrite growth owing to thermoelectric magnetic force (TEMF) on the solid–liquid interface rather than the Lorentz force inside the molten pool. Since the TEMF on the solid–liquid interface is affected by the interaction between the SMF and thermal gradient at different locations, we changed the SLM parameters and SMF to investigate the effect on the TEMF. The simulation shows that the thermoelectric current is highest at the solid–liquid interface, resulting in a maximum TEMF at the solid–liquid interface and, as a result, affecting the dendrite morphology and promoting the columnar to equiaxed transition (CET), which is also shown in the experiment results under 0.1 T. Furthermore, it is known that the thermoelectric magnetic convection (TEMC) around the dendrite can homogenize the laves phase distribution. This agrees well with the experimental results, which show reduced Nb precipitation from 8.65% to 4.34% under the SMF of 0.1 T. The present work can provide potential guidance for microstructure control in the SLM process using an external SMF.

Precipitation behavior of δ phase and its effect on stress rupture properties of selective laser-melted Inconel 718 superalloy

The selective laser-melted (SLM) Inconel 718(IN718) specimens were heat-treated with different solution regimes after the hot isostatic pressing (HIP). The results show that δ phase forms at grain boundary and presents needle-like pattern, and the size and volume fraction of δ phase increase with the increase of holding time and the decrease of solution temperature. The equilibrium volume fraction of needle-like δ phase in SLM-built IN718 is higher than that of short rod-like δ phase in forged-IN718 under the same heat treatment. The SLM-built IN718 specimen without δ phase at grain boundary has the best stress rupture properties, while the specimen with the largest volume fraction of δ phase at grain boundary exhibits the worst stress rupture properties. This is attributed to that the needle-like δ phase at grain boundary promotes the nucleation of void-formed cracks and accelerates the deformation damage. The analysis of the responses to the coordinated deformation capacity of grain orientation and grain size indicates that there exist the heterogeneous deformation and stress/strain concentration at grain boundary. The existence of δ phase influences the coordinated deformation capacity and promotes the stress/strain concentration around grain boundary region. Especially, the needle-like δ phase at grain boundary leads to the more detrimental effect on the rupture life and ductility of SLM-built IN718 alloy on compared with the forged-IN718 alloy.

Integrated Computational Materials Engineering to Predict Dimensions for Steady-State and Transient Melt-Pool Formation in the Selective Laser Melting of Inconel 625

Integrated Computational Materials Engineering to Predict Dimensions for Steady-State and Transient Melt-Pool Formation in the Selective Laser Melting of Inconel 625

Influence of laser parameters on segregation of Nb during selective laser melting of Inconel 718

A transient three-dimensional powder-scale model was established for understanding the flow field and mass transfer within the molten pool during the selective laser melting (SLM) of Inconel 718 alloy by considering some important physical phenomena, such as, a transition from powder to solid, nonlinearities produced by temperature-dependent materials’ properties, and fluid flow in the calculation. The influence of laser power or scanning speed on the flow field and cooling rate was discussed in detail. The simulation results reveal that the motion of molten pool and higher cooling rate promote the mass transfer and benefit the solute distribution by increasing laser power. However, with increasing the scanning speed, the melt flow speed and cooling rate are elevated, resulting in an agglomeration of the solute elements, which is ascribed to the shorter dwelling time of liquid. Therefore, the segregation of Nb can be effectively suppressed by increasing laser power or decreasing scanning speed, which can decrease the dwelling time of liquid.

Effect of powder reuse on powder characteristics and properties of Inconel 718 parts produced by selective laser melting

Abstract Recycling metallic powders used in the selective laser melting (SLM) process is beneficial to reduce the manufacturing cost and promote resource sustainability. However, current research about the effect of powder reuse on the final SLM part is still limited. In this work, a systematic study was made to explore the recycled powder characteristics, microstructures and mechanical properties of the SLM Inconel 718 (In718) part. Results show that the powder reuse cycles can increase the particle size but not change its chemical composition. With the increase of reuse times, the apparent density increases and the flowability becomes better. The In718 parts fabricated by the recycled powder exhibit similar microstructure characteristic with dark austenite matrix and some white Laves phases. X-ray computed tomography (CT) was used to characterize the pores inside the parts. More tiny pores (deq≤50) and higher pore sphericity (ϕs≥0.6) are formed as the reuse times increase. More importantly, powder reuse between 1st-14th times has no negative effect on the mechanical properties of the SLM In718 parts. Room-temperature tensile test on both virgin and recycled powders shows a similar room-temperature tensile properties, such as 1025 MPa ultimate tensile strength, 750 MPa yield strength and 27%–30% elongation, achieving the application requirements of In718 alloy in the ASTM standard. This work provides a useful insight into the powder reuse of In718 alloy for SLM process.

An investigation into melting modes in selective laser melting of Inconel 625 powder: single track geometry and porosity

The laser process parameters used in selective laser melting (SLM) affect the mode of melting and the process characteristics such as the porosity. Some parameters may have a higher effect on the single-track characteristics than the others. This study aims at determining both the individual and combined effects of laser parameters on the single-track geometry and porosity for Inconel 625 alloy powder. The single tracks with linear energy densities (LED) ranging from 0.16 to 0.98 J/mm are fabricated to explore the effect of LED on melting modes and the keyhole porosity. A micro-CT scanner is used to measure the pores formed inside the tracks, and the results showed that the porosity increased with an increase in LED in the keyhole mode. Besides, metallography is performed to reveal the transverse melt pool boundary and to differentiate the melting modes. The transverse melt pool boundary shows that the depth of the melt pool increases significantly compared to the width with an increase in LED, which signifies the change in the melting from the conduction to keyhole mode. Further, another set of experiments is performed by using different combinations of laser power and scan speed from the same LEDs of 0.48 J/mm and 0.75 J/mm. The melt pool boundary shows that within the same LED, the depth of the melt pool increases significantly with an increase in the laser power. However, for the same LED, the single-track porosity increased up to a certain level; then, the porosity decreased with the further increase in the power and speed. The physics behind such phenomenon is explained by the particle-level SLM process simulations developed using FLOW-3D. The simulation results show that the change in the melting mode from the conduction to keyhole for the same LED is due to the increased vaporization with an increase in the laser power, which enhanced the vapor pressure.

Process parameter optimization for selective laser melting of Inconel 718 superalloy and the effects of subsequent heat treatment on the microstructural evolution and mechanical properties

• Laser power, scanning speed and overlap rate were coupled into energy density. • Heat treatments led to the correspondingly microstructural evolution. • The microhardness, tensile strength and wear resistance of SLMed samples were investigated. The Inconel 718 superalloy (IN718) was fabricated by selective laser melting (SLM) successfully in this work. The optimization of process parameters and effects of three heat treatment processes on the microstructures and mechanical properties of samples were also systematically investigated. The relationship equation between relative density (RD) of SLMed samples and energy density (ED) coupled by laser parameters was determined. After the solution aging (SA) heat treatment, a large amount of needle-like δ phases precipitated and the precipitation of ultrafine spherical γ'/γ” strengthening phases as well as complete recrystallization appeared after homogenization + solution aging (HSA) heat treatment. The overall performances of SLMed IN718 samples were improved significantly using the HSA treatment, with the increase of tensile strength from 946 MPa to 1570 MPa.

Comparative Assessment of Physics-Based Computational Models on the NIST Benchmark Study of Molten Pool Dimensions and Microstructure for Selective Laser Melting of Inconel 625

Comparative Assessment of Physics-Based Computational Models on the NIST Benchmark Study of Molten Pool Dimensions and Microstructure for Selective Laser Melting of Inconel 625