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Abstract. This study presents a thorough literature review on the powder-bed laser additive manufacturing processes such as selective laser melting of Inconel ...
Review Article

Review on powder-bed laser additive manufacturing of Inconel 718 parts

Proc IMechE Part B: J Engineering Manufacture 2017, Vol. 231(11) 1890–1903 Ó IMechE 2016 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0954405415619883 journals.sagepub.com/home/pib

Xiaoqing Wang, Xibing Gong and Kevin Chou

Abstract This study presents a thorough literature review on the powder-bed laser additive manufacturing processes such as selective laser melting of Inconel 718 parts. This article first introduces the general aspects of powder-bed laser additive manufacturing and then discusses the unique characteristics and advantages of selective laser melting. The bulk of this study includes extensive discussions of microstructures and mechanical properties, together with the application ranges of Inconel 718 parts fabricated by selective laser melting.

Keywords Additive manufacturing, Inconel 718, laser, microstructures, mechanical properties, powder-bed

Date received: 26 March 2015; accepted: 27 October 2015

Introduction Additive manufacturing is currently one of the rapidly developing advanced manufacturing techniques in the world1,2 and has exhibited extensive application prospect.3,4 Unlike traditional machining processes that are subtractive in nature, AM is based on a different fabrication approach, namely, layer-by-layer adding and consolidating feedstock materials to form complexshaped parts. As a primary step of AM, the computeraided design model of the part to be built is mathematically sliced into thin layers. The part is then produced by the controlled consolidation of the deposited material layers. Each shaped layer represents a cross-section of the sliced computer aided design (CAD) model. AM technology accordingly offers a unique potential for freeform fabrication of complex shaped parts that cannot be easily produced by conventional manufacturing processes. Among the AM methods, laser-based AM could produce strong metallic models directly from metallic powder particles and has attracted more and more attention.5 Laser-based AM techniques have critical applications for aerospace materials such as titanium alloys, aluminide, and nickel-based alloys.6–12 Nickelbased alloys13–15 are important high-tech metallic materials to the industries because of their special properties, such as excellent refractory and corrosion resistance characteristics at both elevated and low temperatures. They also have outstanding wear resistance (anti-friction) magnetic properties (variable or constant magnetic

permeability), high electrical resistance, wear resistance, and corresponding anti-friction properties; practically zero expansion coefficient in the temperature range of 0 °C–100 °C which is very close to that of platinum and glass and so on. Among the refractory nickel-based alloys with high resistance at elevated temperatures, Inconel alloys are sufficiently resistant to the action of chlorine, fluorine, and solutions containing ions of these elements. They are also showing better resistance to progressive oxidation up to 1100 °C and in the oxidizing-sulphide atmosphere up to 850 °C as they are not vulnerable to vapor, ammonia or sulfur contained gas. Summing these properties, the Inconel alloys are intended for the manufacturing of steam and gas turbines, and parts for aerospace and nuclear industries. As a nickel-based alloy, Inconel 718 has attracted increasing attention due to its excellent creep properties, oxidation resistance, and hot corrosion resistance. It has experienced extensive development over the past four decades.16,17 Typically, the Inconel 718 super alloy has been developed and applied in wrought, cast, and powder metallurgy forms, and the Inconel 718 parts using these methods have demonstrated superior Department of Mechanical Engineering, The University of Alabama, Tuscaloosa, AL, USA Corresponding author: Kevin Chou, Department of Mechanical Engineering, The University of Alabama, 1008 NERC, 245 7th Ave, Tuscaloosa, AL 35487, USA. Email: [email protected]

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mechanical properties and performances.18–20 With the rapid development of modern industry, Inconel 718 parts with complex structures, high dimension precision, and further elevated mechanical properties are in higher demand.21 Therefore, the application of the novel non-traditional processing technology is necessary to the net shape production of Inconel 718 parts with complex configurations and high performance.

Powder-bed laser AM methods For the powder-bed laser AM methods, the precursor powder is melted by a high energy laser beam in a layerby-layer manner. Selective laser melting (SLM), embodying the most typical features of the powder-bed laser technologies, has been recognized as a promising AM technology due to its flexibility in feedstock and shapes.22–25 In SLM, a computer-controlled scanning laser beam is applied as the energy source to selectively melt the pre-spread powder particles layer by layer.26–28 Furthermore, geometrically complex components with high dimensional precision and good surface integrity can be obtained precisely by the SLM process without subsequent process requirements, with which the conventional methods cannot easily keep pace with.29,30 The SLM process utilizes an inert argon or nitrogen gas as the atmospheric environment. The schematic view of a SLM system is shown in Figure 1(a). The desired microstructures and mechanical behavior of SLM-processed parts are inevitably affected by process parameters.4 Significant research efforts are still required to focus on microstructures and properties of the fabricated parts under various processing conditions. The SLM process will be compared to the other laser technologies, such as laser rapid forming (LRF),32 direct laser deposition (DLD),10 and laser net shape manufacturing (LNSM).7 LRF, which combines laser cladding with rapid prototyping into a solid freeform fabrication process, has an important potential application in the repair of worn or damaged components with lower heat input and local heating. It provides remarkable benefits over the conventional welding processes through accurate control of the solidification microstructure and heat input. The DLD process essentially derives from a rapid prototyping process and it allows the die-less production of metal parts direct from powder. It also has application in the repairing of worn parts and manufacturing of components that feature complex internal geometries. LNSM is a laser cladding–based freeform fabrication technology using a high energy laser beam to create three-dimensional geometries by precisely cladding thin layers of metal powder on a base material. Primary applications for it include repair, overhaul and rapid prototyping, and so on.

Properties and applications Inconel 718 has great application in the industries due to its outstanding properties with retaining its superior mechanical properties in a broad range of temperatures

Figure 1. Schematic of the laser AM technologies: (a) typical SLM machine layout31 and (b) laser net shape manufacturing system.7

by virtue of solid-solution strengthening and precipitation strengthening. First, Inconel 718 is a nickel-chromium–based alloy with excellent creep properties, good tensile strength, fatigue strength, and rupture strength, which makes it widely employed in many applications, such as nuclear reactors and liquid fueled rockets.33 Second, it has excellent oxidation resistance and hot corrosion resistance,34–36 which has favored its application at a high temperature up to 700 °C and where the atmosphere is highly carburizing and oxidizing, such as combustion chambers and turbine blades, components for rings, aircraft, and land-based gas turbine engines. Due to tool over-wear and poor workpiece surface integrity,37 its high hardness and low thermal conductivity characteristics make it very difficult to manufacture finished products using conventional machining methods, especially for those having complex structures. The laser-based AM technologies will have great applications for Inconel 718 in the industry, especially for the aerospace components.

Powder characteristics and manufacturing parameters Powder characteristics The chemical compositions of the commercially available Inconel 718 powder are listed in Table 1. The

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Table 1. Nominal and measured chemical compositions of Inconel 718 powder (wt%). Elements

GA

GA

PREP

Ni Cr Fe Nb Mo Ti Al C Co Si Particle size, mm References

BAL 18.4 17.7 5.1 4.2 0.9 0.3 0.08

53.05 18.23 17.58 5.1 3.06 0.94 0.44 0.04 0.27 0.12 Fine: 44–74Coarse:74–125 Qi et al.7

54.82 18.08 17 5.17 3.1 0.89 0.53 0.03 0.17 0.08 44–149 Qi et al.7

15–45 Jia and Gu38

51.75 19.68 16–18 4.91 3.18 0.97 0.63 0.034

ASM Standard

GA: gas atomized; PREP: plasma rotating electrode processed.

powder characteristics have critical influence on the density, surface roughness, mechanical properties, and microstructures of the final produced parts. Particles should be spherical with no defects such as satellites and internal pores, because the presence of satellites will affect the flow ability of the powders and further influence the powder layers. The pores in the particles will be detrimental to the density after melting due to lack of fusion. The distribution of the powder particle sizes also plays an important role in powder-bed formation as it affects the flow ability of the powder, too. The percentage of the small particles must be balanced because batches with no small particles have less capacity for particle accommodation given that smaller particles fill the gap between larger ones. A high percentage of small particles will lead to a poor flow ability of the powder and correspondingly resulting in an inhomogeneous powder distribution over the build platform. The risk of chemical reactions between the powder material and oxygen should not be forgotten. Actually, some studies have been done in this area. Amato et al.39 found that the precursor, pre-alloyed gas atomized (GA) Inconel 718 powder used in SLM has an average particle size of 17 mm. The raw material used in SLM experiments conducted by Wang et al.21 was GA powder with a majority of the particle sizes below 50 mm. Qi et al.7 used different types of Inconel 718 powder, and the particle sizes of the coarse and fine GA powder were 74–125 mm and 44–74 mm, respectively, and the size of the plasma rotating electrode processed (PREP) powder was 44– 149 mm. The powder particles are mostly spherical in shape and an optical microscope view of a mounted, polished, and etched particle section of the microdendritic structure is shown in Figure 2(a). The microstructure of the particles shows a fine dendritic network which is caused by the rapid solidification during GA, as shown in Figure 2(b). Ardila et al.41 have done studies aiming to demonstrate how the powder can be constantly reused during several iterative fabrication processes.

Manufacturing parameters The manufacturing process parameters also significantly affect the characterizations of the parts. Therefore, it is important to understand the effects of the parameters during the building process: spot size, power, layer thickness, scanning speed, scanning pattern, and so on. In the study of Wang et al.,21 the SLM-processed Inconel 718 tensile samples were fabricated by a continuous wave IPG YLR-200 fiber laser with laser power of 170 W and beam spot size of 100 mm. The powder layer thickness was 20 mm and the laser beam was scanning at a speed of 416.7 mm/s in the strategy of zigzag scanning pattern with 90° rotating after each layer, which also have been mostly used in SLM process, as shown in Figure 3(a).21 The overlap rate of the scanning lines was 30% at the beam sport size of 100 mm, which means the distance between scanning lines (hatch spacing) was 70 mm. A fine dendritic structure was found on both cross section and vertical section, as shown in Figure 5(a) and (b), which also has been found by Amato et al.39 who reported that the dimensions of the fine equiaxed/columnar microstructure are 0.5–1 mm observed in samples from cylindrical components produced with the EOS M270 SLM system containing a 200 W Yb:YAG fiber laser with a diameter laser beam of 100 mm scanning at 800 or 1200 mm/s. Jia and Gu38 have studied the influence of laser energy densities on the surface, densification, and microstructures of the Inconel 718 parts by setting the laser power and the laser scan speed. The typical surface morphologies are shown in Figure 8. He found that the densification level was restricted at lower laser energy density (180 J/m) due to the occurrence of openpores and balling effect, as illustrated in Figure 8(a). The scanning tracks were discontinuous with largesized balls surrounded by open-pores, which lead to the densification at merely 73.6%. With increasing the laser energy density (275–330 J/m), the pores dispersed and diminished with some residual shrinkage cavities. Then

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Figure 2. Micrographs of the Inconel 718 powder: (a) characteristic morphology,21 (b) fine dendritic network,40 (c) fine GA powder, and (d) PREP powder.7

Figure 3. (a) Laser scanning strategy (Zigzag pattern, 90°)21 and (b) building strategy of the four series tensile test samples.42

the surface became relatively smooth with few scattered metallic globules to a sound surface with the near-full density of 98.4% at the recommended laser energy density of 330 J/m. In order to know the effect of orientation on the tensile strength and microstructures, Chlebus et al.42 have built four series of tensile specimens with differing orientation of their axes with respect to the build direction, as shown in Figure 3. The computer numerical control (CNC) finishing process was used after the buildup process to remove residues of support structures and to achieve a smooth, notch-free surface. Generally, the LRF, DLD, and LNSM processes will have a bigger beam spot size and layer thickness. The detail of the parameters in the studies can be seen in Table 2.

Microstructures The base phase in Inconel 718 alloy is g phase, also called g matrix, and the major precipitates are diskshaped g$ phase and spheroidal g# phase. There are also some needle/plate-like d phase, discrete metalcarbide (MC) particles, and round, island-like Laves phase.47 g$ phase, having the composition Ni3Nb and a body-centered tetragonal (bct) crystal structure, and g# phase, having the composition Ni3(Al, Ti, Nb) and a cubic (ordered face-centered) crystal structure, are the major strengthening phases which are coherent with the g matrix. An orthorhombic d phase, having the composition Ni3Nb, always precipitates at grain boundaries in a needle-like form. Laves phases are irregularly shaped

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Table 2. Manufacturing parameters have been used in the studies. References

Process

Laser type

Spot size, mm

Power, W

Layer thickness, mm

Hatch spacing, mm

Scanning speed, mm/s

Scanning pattern

Chlebus et al.42 Amato et al.39 Wang et al.21 Jia and Gu38,46 Zhao et al.32 Blackwell10 Qi et al.7

SLM SLM SLM SLM LRF DLD LNSM

CW Ytterbium Yb:YAG Fiber IPG YLR-200 Fiber YLR-200-SM CO2

180 100 100 50 3000

100 200 170 110/120/130 2350

50 ~50 20 50

160

85.7 800 (1200) 416.7 400/600 8

Zigzag, 90° Zigzag, 90° Zigzag, 90° Linear

250–550

100–400

CO2

70 50

2–8

SLM: selective laser melting; LRF: laser rapid forming; DLD: direct laser deposition; LNSM: laser net shape manufacturing.

Figure 4. Optical micrographs of Inconel 718 specimens: (a) SLM, cross section,21 (b, c) vertical section,39,42 (d) DLD, vertical section,10 (e) LRF, vertical section,32 and (f) casting.48

phases which form due to Nb segregation with the other alloying elements with a typical composition of (Ni, Fe, Cr)2(Mo, Nb, Ti), instead of g$ (Ni3Nb). This phase is detrimental to mechanical properties but it can be dissolved in the matrix by proper heat treatments.40

Within traditional manufacturing methods, such as cast and wrought, the components present coarse grain size and heavy dendritic segregation, which is caused by the low cooling rate during solidification, as shown in Figure 4(f).48 Solidification defects, such as

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Figure 5. SEM micrographs of as-deposited IN718 samples: (a) SLM, cross section,21 (b, c) SLM, vertical section,21,46 and (d) LNSM, cross section.7

shrinkage cavities and porosity, also form in the castings at the same time. In wrought products, the formation of macro-segregation, freckles, laves phase, and white spots results in a large scatter in the mechanical properties. Thus, homogenization treatment and hot isostatic pressing (HIPing) are required to homogenize the microstructure to close the internal pores and improve the casting quality.49 However, with the new AM technologies, such as SLM, the specimen shows regular fine microstructure due to the rapid cooling rate induced by high laser energy density, as shown in Figure 4(a)–(c). Under the optical microscope with lower magnifications, the SLM formation features are shown clearly. For example, the curve-like regular laser melted tracks on the cross-section correspond to laser scanning strategy, and the laminar material structure & columnar architecture throughout the vertical section are determined by the specimen building strategy, such as the scanning pattern, hatch spacing and thickness of layers. The cut ends of melted tracks in the form of a series of arcs on the vertical section are induced by the Gauss energy distribution of laser.21 At higher magnification, it can be seen that regular columnar microstructures composing of parallel dendritic cells grew along the building direction with an averaged dendrite arm spacing around 2 mm on the vertical section of the samples,21 as shown in Figure 5(b). Gong et al.50 also found that strong orientation of columnar structure

grew along the build direction and perpendicular to the melted layers determined by the vertical heat flux related to heat transfer into the substrate. But the cross section of the samples exhibits a typical equiaxed grain microstructures, as shown in Figure 5(a), which is caused by horizontal heat flux related to the movement of the heat source. Therefore, similar to the directionally solidified microstructure, such type of microstructure is a result of epitaxial, dendritic grain growth in the direction determined by heat flux direction and crystallographically favored orientation.51 If viewed in a three-dimensional section, the grains would be in a rod-shape, which has also been mentioned by Amato et al.39 and Gong and colleagues.43,50 Besides, the grains in the samples built with SLM are much smaller than other laser processing technologies, such as DLD,10 LRF,32 and LNSM,7 as can be seen from Figures 4 and 5. This is because the SLM has a smaller laser spot size (50–180 mm) and layer thickness (20–50 mm), which will form to a smaller melting pool (micro-size), as shown in Table 2. Further more, the SLM-process parameters also affect the thermal stories of IN718 parts and further influence the residual stresses level, grain size and phase composition of microstructure in as-built SLM-parts. According to the study of Jia and Gu,38 with increasing of laser energy density, the microstructures of Inconel 718 will change successively in order of coarsened columnar dendrites, clustered dendrites, to slender and

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Figure 6. SEM micrographs showing the intermetallic micro-segregation in (a) inside g-phase grains,42 (b) lower inset shows the d needles at stacking faults and carbides at higher magnification,40 and (c, d) micrograph and the respective chemical compositions of the various precipitates.40

uniformly distributed columnar dendrites. Generally, at high cooling rates, macro-segregation would be completely prevented and micro-segregation of the chemical composition in Inconel 718 could occur during the rapid dendritic/cellular growth of columnar grains resulting from the primary dendrite shape altered to a more cell-like form at high cooling rate.52 As shown in Figure 6, Laves and MC-type carbides are observed between aligned dendritic cells and in the region of an overlapping interface between two adjacent laser scanning tracks or layers. By EDS, Parimi et al.40 also observed white irregularly shaped Laves phases with size of ;1–2 mm in the inter-dendritic regions, as shown in Figure 6(c) and (d), and Wang et al.21 found d phase and Laves phase at grain boundaries, as shown in Figure 5(b). In general, d phase cannot be precipitated during the process of SLM due to the high cooling rate in SLM and the small content of Nb caused by the occurrence of abundant Laves phase. But, the precipitation of d phase occurs following aging for less than 100 h at a temperature range of 750 °C–1000 °C with maximum precipitation at 900 °C at the grain boundaries.53 Although d phase is generally detrimental for the mechanical properties, proper morphology of these precipitates at grain boundaries could improve the creep property of these materials.54

The desirable microstructure is a stable g phase strengthened with coherent and dispersive precipitates of the phases g# and g$ as IN718 parts are supposed to work in a strongly corrosive environment at low and high temperatures. Non-equilibrium structure was obtained and macro-segregation of alloy elements was sufficiently inhibited due to the inherent rapid solidification rate associated with laser deposition. To enable the precipitation of strengthening g# and g$ phases and relieve the residual stresses,44,45 post-heat treatment is necessary. After solution and double aging heat treatment, Wang et al.21 found that there are numerous niobium-rich precipitates (d and Laves phases) along the developed curved grain boundaries on both sections with the regular dendritic crystals demolished due to reheating, diffusion, and recrystallization, as shown in Figure 7(a). Generally, the d phase precipitated at grain boundaries with the dissolving of Laves, g# and g$ phases dissolving in the matrix in the solution treatment at 980 °C, and then g# and g$ phases dispersed and precipitated further in matrix to strengthen the alloy in the process of double aging treatment. Similar results have been obtained in annealed samples by Amato et al.,39 recrystallized grain boundaries are developed with the precipitating of d phase.g 00 phase precipitates parallel to the laser beam or build direction after hot isostatic pressed (HIP) treatment and g 0

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Figure 7. Micrographs in vertical section of SLM-processed IN718 samples with heat treatment: (a) Solutioned (980 °C/1 h) + Aged,21 (b) Aged, (c) Solutioned (1040 °C/1 h), and (d) Solutioned (1100 °C/1 h) and Aged.42

precipitates with its distribution in a dense field of fine g 00 precipitates in the ;50% recrystallized regions after annealed (1160 °C for 4 h) treatment. Chlebus et al.42 also compared the effect of different heating treatments, and the conclusion is that the postheat treatment is necessary for SLM-produced IN718 alloy and the corresponding parameters used should be determined by the microstructure in the as-deposited parts. The microstructure maintained its columnar nature and traces of layered build after direct aging, as shown in Figure 7(b). From the Figure 7(c), it can be seen that residual particles of the Laves phase and d phase occur on grain or sub-grain boundaries and along the layer interfaces, which states that the solution annealing parameters used appear insufficient for complete homogenization of the g phase. Complete dissolution of residual Laves particles requires the proper selection of temperature and time of the annealing. As can be seen in Figure 7(d), homogenization of g solid solution occurred (1100 °C/1 h + aged) obtained with boundaries decorated with MC carbide. This heat treatment variant was found optimal from the viewpoint of satisfying the degree of microstructure homogenization and aging effects. Thus, the recommend heat treatment is using higher temperature (i.e. 1100 °C) than that normally used to homogenize

the alloy and slow furnace-heating to avoid local subsolidus liquation of the material and possible propagation of metastable liquid along grain boundaries. This has also been mentioned by Qi et al.,7 who also studied the microstructure of Inconel 718 under different heat treatment conditions of as-deposited, direct aged, solution treatment and aging (STA), and full homogenization followed by STA. The homogenized STA heat treatment can completely dissolve the Laves phase but enabled substantial grain growth with isotropic appearance. Besides, Chlebus et al.42 also deduced that the driving force is not likely the uneven distribution of residual thermal stresses46 but more probably the higher grain boundary migration, which is much higher than that in the traditionally manufactured alloy, because the microstructure in the as-deposited parts is unstable due to high energy accumulated in the form of grain boundaries and dislocations. The detail of the heat treatment methods used in the studies is listed in Table 3. To avoid these internal stresses, one can reduce the temperature gradients by heating the substrate plate or increasing the temperature inside the processing chamber.55,56 Using a double-scanning strategy will also minimize the level of residual stresses in the parts.42 Moreover, the post-processing heat treatment56,57 could

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Table 3. Detail of the heat treatment methods used in the studies. Types

Heating methods

Heat treatments

References

SLM

Aging Solution + Aging

Chlebus et al.42

SLM SLM

Solution + Aging (double) HIP Annealing Homogenization + Solution + Aging Solution + Aging Solution + Aging Homogenization + Solution + Aging Solution + Aging Aging

720 °C/8 h (FC/100°C/h) + 620 °C/10 h (AC) 980/1040/1100 °C/1 h (WC) + 720 °C/8 h (FC/100 °C/h) + 620 °C/10 h (AC) 980 °C/1 h (AC) + 720 °C/8 h (FC) + 620 °C/8 h (AC) 1163 °C (0.1 GPa)/4 h 1160 °C/4 h 1080 °C/1.5 h (AC) + 980 °C/1 h (AC) + 720 °C/8 h (FC) + 620 °C/8 h (AC) 965 °C/1 h (WC) + 718 °C/8 h (FC, 56°C/h) + 620 °C/8 h (AC) 980 °C/1 h (AC) + 720 °C/8 h (FC) + 620 °C/8 h (AC) 1093 °C/1.5 h (AC) + 982 °C/1 h (AC) + 718 °C/8 h (FC) + 621 °C/10 h (AC) 982 °C/1 h (AC) + 718 °C/8 h (FC) + 621 °C/10 h (AC) 718 °C/8 h (FC) + 621 °C/10 h (AC)

LRF DLD DLD LNSM

Wang et al.21 Amato et al.39 Zhao et al.32 Zhang et al.20 Blackwell10 Qi et al.7

SLM: selective laser melting; LRF: laser rapid forming; DLD: direct laser deposition; LNSM: laser net shape manufacturing.

be adopted to relieve the residual stress in the final components.

Defects Even though the laser-based AM on Inconel 718 alloy has a lot of advantages compared to traditional machining methods, it also has disadvantages, such as pores, unmelted powder, and bonding defects. The pores in the volume of laser-based AM parts, which affect the density and porosity of the samples, stem from process-induced defects originating from initial powder contaminations, evaporation, or local voids after powder layer deposition. Wang et al. have observed some micro-pores on both cross section and vertical section, as shown in Figure 5(b). Zhang et al.58 found pores with a spherical shape present in the deposits. The spherical shaped pores are most likely entrapped gas bubbles in the molten metal bead that could not rise and escape to the top surface before solidification. The porosity is probably entrapped from the GA process during powder manufacturing.32 Most of the powder particles have a spherical shape with internal porosity infrequently observed in them, and very fine satellite particles attached to them.40 Another important factor causing the porosity is the laser’s high scan speed, as reported by Jia and Gu.38 During the process of manufacturing, the influence of high viscosity at a high scan speed (600 mm/s) impeded the Inconel 718 liquid from spreading out smoothly, which in turn caused the formation of openpores, as shown in Figure 8(a). Besides, the linear laser energy densities (h) also play an important part in the surface finishes as it has a critical effect of the dynamic viscosity and balling phenomenon. By comparing Figure 8(a)–(c), it can be seen that the smooth sound surface morphology without any pores or balls was obtained with the h increasing properly to 330 J/m, due to the weakened dynamic viscosity

and balling phenomenon. Niu and Chang59 have studied the instability of the scan tracks and mentioned that the balling phenomenon will occur in high energy density processes. Due to the presence of the steep thermal gradient between the center and edge of the melt pool at the surface, a variation of the surface tension, which is a function of temperature, will occur between them and further induce a Marangoni flow from a region of low surface tension to a region of high surface tension. This fluid flow will produce an extra force exerted on the molten track of the samples, which results in breaking up of the fluid to smaller volumes, such as a row of spheres, to minimize surface free energy. Also, the protective gas, argon, may be encapsulated inside the pores, with which the chamber was filled in order to avoid contamination of the processed metal with oxygen and nitrogen during AM processing. Eventually, these pores will act as strong stress raisers and finally lead to failure, especially under fatigue loading. Right now, these pore-like defects cannot be totally avoided. However, the porosity can be greatly reduced using PREP powder7,32 or fine size GA powder combined with a high h (laser power/scanning speed).7 The balling phenomenon can be inhibited by higher h when the laser wavelength l \ 2 pR, where R is the initial radius of an unperturbed liquid cylinder. At a constant laser wavelength l, lower scan speed accompanied with higher energy input could lead to a rapid increase of radius R of the cylinder, which will weaken the instability of the liquid cylinder. Therefore, at a high h, there is no need for the liquid to alter its shape to reduce the surface energy and discontinuous scanning tracks will disappear. Another method is HIPing of the final solid components, which is used to achieve reduction of pore size or even the closure of these features in traditional powder metallurgy. However, the complete closure of these pores will be hard to achieve with these established techniques. The second defect that has been studied is unmelted powder particles. Some particles (similar in size to the

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Figure 8. Porosity defects observed in the SLM-processed of Inconel 718 at various laser densities: (a) 180 J/m, (b) 275 J/m, and (c) 330 J/m.38

powder particles applied) may appear on the top surface or at the boundary of the deposited layers. Up to the melting conditions, these particles may have good metallurgical bonding or lack of bonding with the deposited metal. On occasion, these particles may also result from spattering.

Mechanical properties Mechanical properties of Inconel 718 have been frequently investigated. The studies indicate that mechanical properties of AM parts are comparable with cast or wrought Inconel 718 alloy. It is also indicated that processing parameters, such as power and speed, may cause the remarkable change in the mechanical properties.32,40 In addition, the properties will have a significant change after the post-heat treatment, such as aging, solution, and HIPing.

Tensile testing Tensile testing has been widely used to characterize the mechanical properties of Inconel 718 parts. Some researchers found that the tensile properties of AMbuilt specimens were comparable with the wrought Inconel 718 alloy.21,39 However, others presented that

the tensile strength and yield strength of the asdeposited samples are inferior to the typical wrought, while the elongations are relatively high.7,32,42 The reason for the difference could be attributed to the variation in the built parameters, which result in different structures, such as composition, structures, pore size, and porosity distribution. The detailed comparison of the mechanical properties of laser-based AM technologies versus wrought Inconel 718 is shown in Figure 9. It can be seen that the tensile strength of the asdeposited parts is comparable or even superior to the casting samples but inferior to wrought ones. Same is the case with the elastic modulus. It is quite comparable to that of the wrought counterparts. As SLM is a totally/fully powder melting manufacturing process, it can be seen that the yield strength and ultimate tensile strength of the Inconel 718 samples built with SLM21 are superior to that from the other three laser-based manufacturing technologies, LNSM, LRF, and DLD. In addition, the LNSM built parts7 have a slightly lower yield strength, which is caused by the dynamic heat transfer of the moving heat source and the layered material formation mechanism. During the building process, a thin nucleation zone at the layer interface is formed as the molten pool solidifies, which will grow into fine grains at the boundaries. As these layer

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Chlebus (SLM+STA)

Zhao (DLD+HSTA) Zhao (DLD+HSTA)

Wang (SLM+STA)

Blackwell (DLD+STA)

Figure 10. Strengths of Inconel 718 after heat treatment: (a) Yield strength and (b) Ultimate tensile strength.

Figure 9. Mechanical properties of Inconel 718: (a) Yield strength, (b) Ultimate tensile strength and (c) Elastic modulus (Young’s modulus).

interface regions are usually associated with sharp changes in grain size and degree of micro-segregations, they can be the sites of weakened tensile stress. The heat treatment has a significant effect on the tensile strength of the AM-built Inconel 718 parts. Based on the studies of Chlebus et al.,42 the heat treatment will cause the yield strength, tensile strength, and the hardness to increase by 84% (72%–95%), 38% (30%–46%), and 48%, respectively. According to Zhao et al.,32 the tensile strength of the heat-treated components increased approximately 1.5 times that of the asdeposited samples and is comparable with that of wrought Inconel 718, which has also been reported by Qi et al.7 The detailed tensile properties of Inconel after

Figure 11. Elongation compared with after heat treatment.

post-heat treatment are shown in Figure 10. However, the ductility of the samples was reduced significantly due to fine Laves particles that remained at the interdendritic regions,32 as shown in Figure 11. After homogenization treatment, the alloy presents slightly lower tensile strength of 1194 MPa, but even better elongation of 19.9%.7 Through the tensile test, Chlebus et al.42 also studied the effect of orientation on tensile strength, and he found that there is no significant difference for the B, C, and D series because of the large fraction of

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Table 4. Hardness of Inconel 718 parts built with laser-based technologies. Ref.

Methods

Test

Values

Amato et al.39

SLM SLM + HIPing SLM + Annealed SLM SLM + STA: solution treatment + double aging SLM SLM + STA: solution treatment + double aging SLM SLM LRF LRF + HSTA DMD DMD + STA

Vickers Vickers Vickers Vickers Vickers Vickers Vickers Vickers Vickers Rockwell Rockwell Vickers Vickers

3.9 GPa 5.7 GPa 4.6 GPa 3.58 GPa (Hv 365) 4.61 GPa (Hv 470) 3.07 GPa (Hv1 313) 4.54 GPa (Hv1 463) 3.88 GPa (Hv0.2 395.8) 5.6 GPa 2.3 GPa (HRC 17) 3.9 GPa (HRC 41) 2.94 GPa (Hv 300) 4.90 GPa (Hv 500)

Wang et al.21 Chlebus et al.42 Jia and Gu38,46 Amato et al.39 Zhao et al.32 Zhang et al.25

SLM: selective laser melting; HIPing: hot isostatic pressing; LRF: laser rapid forming; DMD: direct metal deposition; STA: solution treatment + aging; HSTA: homogenization + solution treatment + aging.

for the as-deposited layer or for the heat-treated layer. The detail of the hardness test results is listed in Table 4. Generally, it can be seen that the hardness of the asdeposited parts processed with SLM has higher hardness values than the parts built with other laser technologies, LRF and DMD. However, there is no big difference among them after post-heat treatments.

Other tests (oxidation test and wear test) Figure 12. Tensile strength of four series Inconel 718 specimens built with various orientation.

grains with small shape aspect ratio in the microstructure (Figure 12). The series A has the lowest tensile strength, which is caused by the perpendicular orientation of overlapping interfaces between layers to the loading direction and the weaker strengthening of the grain boundaries.

Hardness testing Some hardness tests, such as Vickers test and Rockwell test, are also applied to evaluate the hardness of laserbased AM components. The study from Chlebus et al.42 showed the hardness increased 48% after heat treatment. Amato et al.39 and Wang et al.21 have the same findings. According to Amato et al.,39 the microindentation (Vickers) hardness was 3.9 GPa for the asfabricated materials, 5.7 GPa for the HIP material, and 4.6 GPa for the annealed material. This has also been reported by Zhao et al.32 The average hardness for heattreated samples is 3.9 GPa (HRC 41), which has increased remarkably from 2.3 GPa (HRC17) for asdeposited samples. Wang et al.21 have also found that the SLM-processed parts showed direction independence without large fluctuations in various directions. Zhang et al.20 reported that the hardness of the layer produced with laser direct metal deposition (DMD) did not show obvious differences along the height of the layer, either

Some other studies have been conducted to investigate the properties of Inconel 718. Jia and Gu46 have investigated the oxidation behavior by an isothermal oxidation test. The result shows that the oxidation kinetics of SLMprocessed Inconel 718 parts follow the parabolic law and show a good high-temperature oxidation performance when the applied volumetric energy density increases from 70 to 130 J/mm3. This is contributed to the refined and uniformly distributed microstructures and the improved relative density of the parts. Besides, Jia and Gu46 also conducted the wear tests with the conclusion that the Inconel 718 parts have a considerably low friction coefficient of 0.36 without any apparent fluctuation and a decreased wear rate of 4.64 3 10 3 4 mm3/N m. The improvement of the wear performance was attributed to the influence of elevated microhardness in combination with the formation of adherent tribolayers.

Conclusion Inconel 718 is a heat-resistant, precipitation-hardening alloy that is widely used in the aerospace and power generation industries and has an excellent combination of mechanical properties. SLM has been investigated by researchers as a cost-effective alternative to fabricate Inconel 718 parts. Based on the literature review, the main conclusions can be drawn as follows: 1.

It is recommended to use PREP powder or fine GA powder (gas atomized) in order to achieve acceptable mechanical properties.

1902 2.

3.

4.

5.

The linear laser energy densities h (power/scan speed) seriously affect the dynamic viscosity and balling phenomenon which will further influence the surface finishes. A sound surface with near-full density of the as-fabricated Inconel 718 parts could be obtained by proper setting of, for example, 330 J/m. In general, as selective laser manufacturing (SLM) produced Inconel 718 parts present a fine columnar dendrites structure with internal micro-segregation in inter-dendritic regions. The recommended heating treatment is homogenization with a higher temperature following by solution treatment and double aging (HSTA). At room temperature, the tensile strength of the as-fabricated Inconel 718 parts is comparable or even superior to the casting samples but inferior to wrought ones. After post-heating treatment, the tensile strength of the Inconel 718 parts is comparable or even superior than the wrought ones.

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