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Abstract
Micro crack was one of the most serious defects in selective laser melting (SLM), weakening the mechanical properties of materials. In this paper, test specimens of Inconel 625 alloy were fabricated by progressive alternative scan strategy in SLM process. The microstructure of the crack, elements around the crack and the distribution of the grain were detected by scanning electron microscope (SEM) and electron backscattered diffraction (EBSD). The SEM results showed that a large numbers of tiny cracks occurred at room temperature, with the average length of approximately 100 μm. Internal cause of crack formation was due to the local segregation of Nb and Mo elements in the process of rapid solidification. It generated eutectic solidification (γ +Laves). At the same time, stress concentration formed around the brittle phase Laves, leading to cracking along the grain boundary. Residual thermal stress caused by high solidification rate was the direct reason. Residual stresses were reduced by substrate heating process and residual stresses were measured by X-ray stress analyzer at different substrate preheating temperatures (150 and 300℃). Residual stresses were reduced in the process and cracks were greatly eliminated by preheating at different temperatures. With the increase of temperature, the number of cracks gradually reduced. When preheated at 300℃, the number of cracks was the smallest. © 2015, Editorial Office of Chinese Journal of Rare Metals. All right reserved.
... After annealing, the average residual stress was 242 MPa, which is still at a considerably high level. To relieve the residual stress, Zhang et al. [68] preheated the LPBF substrate at 20 • C (room temperature), 150 • C, and 300 • C, and found that the residual stress decreased with the increase of preheating temperature. When the preheating temperature reached 300 • C, the residual stress (160.7 MPa) of the LPBF of IN625 alloy decreased by more than 50% compared with that without preheating. ...
... When the forming quality of the specimen is good, the defects mainly consist of tiny holes or few cracks, and most of them exist on the surface or subsurface layer of the specimen. The size of holes can be reduced by preheating the substrate [68] or conducting stress relief annealing [59], etc. Poulin et al. [62] produced up to 10% porosity in LPBF IN625 samples by intentionally seeding pores by changing the laser scanning speed to study the influence of porosity on long fatigue crack propagation behavior. By increasing the scanning speeds from 960 mm/s to 1920 mm/s (960, 1440, 1680, and 1920 mm/s), the intentionally-seeded porosities increased from 0.1% to 2.7% (0.1, 0.3, 0.9, and 2.7%). ...
The Inconel 625 (IN625) superalloy has a high strength, excellent fatigue, and creep resistance under high-temperature and high-pressure conditions, and is one of the critical materials used for manufacturing high-temperature bearing parts of aeroengines. However, the poor workability of IN625 alloy prevents IN625 superalloy to be used in wider applications, especially in applications requiring high geometrical complexity. Laser powder bed fusion (LPBF) is a powerful additive manufacturing process which can produce metal parts with high geometrical complexity and freedom. This paper reviews the studies that have been done on LPBF of IN625 focusing on the microstructure, mechanical properties, the development of residual stresses, and the mechanism of defect formation. Mechanical properties such as microhardness, tensile properties, and fatigue properties reported by different researchers are systematically summarized and analyzed. Finally, the remaining issues and suggestions on future research on LPBF of IN625 alloy parts are put forward.
... Therefore, SLM can generate finer grains and substructures within the grain, which improves the overall mechanical performance of the final components [16]. Research extensively covers SLM-formed titanium alloys [17,18], high-temperature nickel-based alloys [19,20], and iron-based alloys [21]. For aluminum alloys, research has focused on Al-Si alloys, with extensive studies already conducted on the processes, microstructures, properties, and post-treatments of AlSi10Mg [22][23][24] and 26]. ...
Hypereutectic Al-Si alloys, which have a silicon content ranging from 12% to 70%, are a new generation of casing materials for chip packaging. They have broad applications in aerospace, weaponry, and civilian communications. Selective Laser Melting (SLM) offers significant advantages in achieving near-net shaping of complex casings. This paper presents a study on the formation defects, microstructure, and room temperature tensile properties of AlSi60 alloy prepared by SLM. The results indicate that the primary forming defects in the SLM AlSi60 alloy are balling, lack of fusion, and porosity. These defects are mainly influenced by the volumetric energy density. Samples of good quality can be produced within the range of 150 J/mm3 to 250 J/mm3. However, the same volumetric energy density can result in differences in sample quality due to various combinations of process parameters. Therefore, it has been determined that a well-formed AlSi60 alloy can be obtained within a laser power range of 300 W–350 W, scanning speed of 400 mm/s–800 mm/s, and hatch spacing of 0.09 mm–0.13 mm, with a density close to 98%. The microstructure of the SLM AlSi60 alloy consists of primary Si phases with irregular shapes and sharp edges measuring 5–10 μm, eutectic Si particles of 0.5 μm, and α-Al phases, with eutectic Si dispersed within the α-Al. The SLM AlSi60 alloy exhibits fine and evenly distributed primary Si phases with an average hardness of 203 HV. No significant anisotropy in hardness values was observed in the X and Y directions. The tensile strength of the alloy reached an average of 219 MPa, with an average elongation of 2.99%. During the tensile process, cracks initiated by the primary Si phases rapidly expanded, exhibiting minor ductile fracture characteristics in the Al phases. Due to the high volume fraction of Si phases, the tensile test was dominated by brittle fracture. The tensile curve only exhibited the elastic stage.
... In addition, a building platform preheating is often performed to slow down the heat flow and thus reduce the thermal-induced stresses [25][26][27]. Another successful alternative strategy to reduce the heat flow is using support structures between the cold building platform and the component to be built [14,[28][29][30][31]. ...
Inconel 625 (IN625) superalloys can be easily fabricated by the laser-based powder bed fusion (PBF-LB/M) process, allowing the production of components with a high level of design freedom. However, one of the main drawbacks of the PBF-LB/M process is the control over thermally induced stresses and their mitigation. A standard approach to prevent distortion caused by residual stress is performing a stress-relieving (SR) heat treatment before cutting the parts from the building platform. Differently from the cast or wrought alloy, in additively manufactured IN625, the standard SR at 870 °C provokes the early formation of the undesirable δ phase. Therefore, this unsuitable precipitation observed in the PBF-LB/M material drives the attention to develop a tailored SR treatment to minimise the presence of undesirable phases. This work investigates SR at lower temperatures by simultaneously considering their effects on residual stress mitigation, microstructural evolution, and mechanical properties. A multiscale approach with cantilever and X-ray technologies was used to investigate how the residual stress level is affected by SR temperature. Moreover, microstructural analyses and phase identifications were performed by SEM, XRD, EBSD, and DSC analyses. Finally, mechanical investigations through microhardness and tensile tests were performed as well. The results revealed that for the additively manufactured IN625 parts, an alternative SR treatment able to mitigate the residual stresses without a massive formation of δ phase could be performed in a temperature range between 750 and 800 °C.
... The short-time interaction of the laser with the material, and the formation of liquid oscillations or capillary waves, favors the creation of a non-uniform microstructure in the molten pool [22]. The formation of crystal morphology is mainly affected by compositional supercooling (S), temperature gradient (G) and solidification rate (R) [23]. The directional microstructure orientation is due to the unidirectional heat flux from the center to the edge of the molten pool in SLM, and the direction of heat gradient opposite to the heat flux vector leads to directional solidification [24,25]. ...
... Yang, K. et al. prepared 18Ni300 maraging steel by SLM and aged the formed parts [17]. The microstructure was coarse martensite and was refined after heat treatment, forming dense acicular martensite, fuzzy and irregular grain boundary, and a large number of fine point austenite [18]. ...
The energy transfer process of selective laser melting (SLM) is highly complex. In this work, experiments were carried out to study the effects of SLM on the microstructure and mechanical properties of 18Ni300 martensitic steel. With the increase in laser power, the grain size of the cladding layer decreases and the microstructure becomes dense. The side hardness is higher than upper surface hardness, and the tensile strength and elongation both increase first and then decrease. When the laser power is 300 W and the scanning speed is 1,000 mm/s, the comprehensive mechanical properties are the best, as the tensile strength, microhardness, elongation at break, and elongation after fracture are 1,217 MPa, 37.5%, 37.6%, and 8.93%, respectively. EBSD (Electron Backscatter Diffraction) shows that columnar crystals grow along the growth direction (z direction) in XOZ and YOZ planes, and the grains show weak texture. There are many small-angle grain boundaries, and the grain sizes are <10 μm.
... SLM technology can reduce segregation and inhibit the development of non-equilibrium phase due to short time high speed local heat input. Therefore, M300 (SLM-M300) steel directly formed by SLM can maximize the strengthening effect of alloy elements [11,12]. ...
The energy transfer process of laser selective melting is very complex. To study the effect of la-ser selective melting on the microstructure and properties of 18Ni-300 martensitic steel, ABAQUS was used to simulate the temperature of laser cladding 18Ni-300 martensitic steel at different time points and different laser power. The results show that the cross-section shape of the molten pool changes from round to oval With the increase of laser power, the higher the peak value of temperature time curve, the greater the temperature gradient; and the laser clad-ding experiment of 18Ni-300 martensitic steel was carried out, and the microstructure and me-chanical properties of the samples under different laser power were analyzed. The results show that with the increase of laser power, the grain size of the cladding layer becomes smaller and the microstructure becomes more compact; the hardness of the side surface of the sample is higher than that of the upper surface, and the tensile strength and elongation show a trend of first increasing and then decreasing.
Nickel-based superalloys have been widely used in aerospace fields, especially for engine hot-end parts, because of their excellent high-temperature resistance. However, they are difficult to machine and process because of their special properties. High-energy beam additive manufacturing (HEB-AM) of nickel-based superalloys has shown great application potential in aerospace and other fields. However, HEB-AM of nickel-based superalloys faces serious cracking problems because of the unique characteristics of superalloys, and this has become the most significant bottleneck restricting their application. In this review, the current research status related to the types, formation mechanisms, and suppression methods of cracks in nickel-based superalloys produced by HEB-AM is described. The initiation and propagation mechanisms of cracks and their multiple influencing factors are also analyzed and discussed. Then, several possible research directions to solve the cracking problems in nickel-based superalloys produced by HEB-AM are outlined. This review provides an in-depth and comprehensive understanding of the cracking problem in AM nickel-based superalloys. It also provides valuable references for AM crack-free nickel-based superalloy components.
Laser Powder Bed Fusion (LPBF) is characterised by high design flexibility and no tooling requirement. This makes LPBF attractive to many modern manufacturing sectors (e.g. aerospace, defence, energy and automotive). Nickel-based superalloys are crucial materials in modern engineering and underpin the performance of many advanced mechanical systems. Their physical properties (high mechanical integrity at high temperature) make them difficult to process via traditional techniques. Consequently, manufacture of nickel-based superalloys using LPBF has attracted significant attention. However, components manufactured by LPBF are currently limited by their performance for use in critical applications. LPBF materials have microstructural defects, such as suboptimal grain size and morphology, and macroscale anomalies, such as lack of fusion. This results in LPBF materials performing below their wrought counterparts for various mechanical properties, such as creep, which has seldom been researched. Consequently, heat treatment of products post additive manufacture is now considered hugely important and the development of appropriate heat treatment is required to ensure material performance. Furthermore, the build time of LPBF can be slower than traditional manufacturing processes, especially for higher volumes of parts. Multi-laser machines, which have the potential to significantly reduce process time do exist, but there is currently little understanding of how interlaced scan strategies impact mechanical properties. Therefore, to permit a wider application, a deeper understanding of the mechanical behaviour, particularly of creep properties, of LPBF nickel-based superalloys needs to be achieved. Hence, the aim of this work is to establish process-structure-property relationships for the creep properties of LPBF nickel-based superalloy, to benchmark it against wrought equivalents and to provide insights on how to improve the creep performance. To do this, LPBF alloy 718 parts were fabricated using various scan strategies, build orientations and both single and multi-laser strategies, before being heat treated and creep tested. Results confirmed the necessity for heat treatment, which increased the creep life by a factor of 5. The build orientation, and its effect on the grain orientation as well as the stress state of the material were shown to be determining factors in the creep failure mechanisms. The meander scanning strategy resulted in a 58% increase in creep life compared to the stripe strategy, due to the detrimental effects of the numerous laser overlapping regions in the stripe strategy. There are numerous parameters, such as the fraction area of solidified layer, the fraction of powder underneath the layer and the interlayer rotation of the scan strategy that vary for each layer and means the layers themselves as well as the samples were heterogenous. Despite this, the creep life for any given sample was within 73h (i.e.17%) of its repeats, giving confidence in the results. The results also showed that multi-laser scan strategies have no adverse effects on the creep properties of LPBF alloy 718 at different build orientations, demonstrating the potential of using multi-laser strategies for faster build rates without compromising the mechanical properties. Indeed, it is shown that for samples built vertically (i.e. where the build direction is parallel to the loading direction), multi-laser samples outperformed their single-laser counterparts and had a similar creep life and secondary creep rate to wrought alloy (1% difference). The presence of numerous large globular carbides in the wrought alloy 718 were also identified as the reason for the material’s curtailed creep life, compared to its LPBF counterpart, despite having a similar creep rate. Finally, for a given strategy, a 24% increase in creep life compared to wrought alloy 718 was observed. This specimen was explored under thermomechanical and thermal exposure conditions for the purpose of illustrating textural and microstructural evolution and inform a potential heat treatment to improve creep performance. The results showed the instability of the LPBF microstructure in terms of grain size, precipitate density and crystallographic orientation during creep and thermal exposure, proof of the need for an appropriate heat treatment. The texture increased throughout creep testing for the wrought and LPBF alloy 718, reaching a maximum at the time of fracture. This contrasted with the thermal exposure only, where the instability of the LPBF alloy 718 microstructure was evident as the texture increased with time before decreasing and almost disappearing at the time of fracture. This also highlighted the different roles of the build and loading directions on the texture creation and evolution during creep. Finally, an ideal microstructure for improved creep performance was identified and recommendations on how to heat treat LPBF alloy 718 to reach this microstructure were given. Overall, this thesis shows that LPBF components can become more performant than wrought and conventional equivalents by developing an appropriate heat treatment and provides an insight into process-structure-property relationships for the creep properties of LPBF alloy 718. The results are promising, despite future work being required and this work demonstrates the applicability of using LPBF for critical high temperature applications.
Powder Bed Fusion (PBF) techniques constitute a family of Additive Manufacturing (AM) processes, which are characterised by high design flexibility and no tooling requirement. This makes PBF techniques attractive to many modern manufacturing sectors (e.g. aerospace, defence, energy and automotive) where some materials, such as Nickel-based superalloys, cannot be easily processed using conventional subtractive techniques. Nickel-based superalloys are crucial materials in modern engineering and underpin the performance of many advanced mechanical systems. Their physical properties (high mechanical integrity at high temperature) make them difficult to process via traditional techniques. Consequently, manufacture of nickel-based superalloys using PBF platforms has attracted significant attention. To permit a wider application, a deep understanding of their mechanical behaviour and relation to process needs to be achieved. The motivation for this paper is to provide a comprehensive review of the mechanical properties of PBF nickel-based superalloys and how process parameters affect these, and to aid practitioners in identifying the shortcomings and the opportunities in this field. Therefore, this paper aims to review research contributions regarding the microstructure and mechanical properties of nickel-based superalloys, manufactured using the two principle PBF techniques: Laser Powder Bed Fusion (LPBF) and Electron Beam Melting (EBM). The ‘target’ microstructures are introduced alongside the characteristics of those produced by PBF process, followed by an overview of the most used building processes, as well as build quality inspection techniques. A comprehensive evaluation of the mechanical properties, including tensile strength, hardness, shear strength, fatigue resistance, creep resistance and fracture toughness of PBF nickel-based superalloys are analysed. This work concludes with summary tables for data published on these properties serving as a quick reference to scholars. Characteristic process factors influencing functional performance are also discussed and compared throughout for the purpose of identifying research opportunities and directing the research community toward the end goal of achieving part integrity that extends beyond static components only.
The studies of Superalloy and Intermetallic Group of Institute of Metal Research in the past fifty years on the phase transformation phenomena in Fe-and Ni-base superalloys were reviewed. The phase transformations in the two kinds of superalloys include: the solidification reactions which occur during the solidification, e. g. L → γ + Laves, L → γ + γ' and L → γ + M3B2; precipitation of carbides, borides, silicides, GCP and TCP phases from the supersaturated γ solid solution; precipitation reactions which occur in the γ' phase; and the decomposition reactions of MC carbides.