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During the selective laser melting(SLM) process of IN718, different process parameters have a significant impact on its microstructure. To analyze the interrelationship between process parameters and microstructure, in this study, the relationship between primary dendrite arm spacing(PDAS) and process parameters was established through a solidification model, and the accuracy of the model was verified through experiments. Both the calculation results and the experimental results show that when the laser power increases, PDAS increases accordingly; while as the scanning speed and hatch distance increase, PDAS decreases accordingly; and from the bottom to the top of the molten pool, PDAS gradually increases. At the same time, the top of the molten pool is more sensitive to the change of process parameters, while the middle and bottom are relatively insensitive. These research results are of certain guiding significance for the microstructure regulation of IN718. IN718合金在选区激光熔化(selective laser melting, SLM)过程中, 不同的工艺参数会对其微观组织产生显著影响。为分析工艺参数和微观组织的相互关系, 通过凝固模型建立了一次枝晶间距(primary dendrite arm spacing, PDAS)与工艺参数之间的关系并通过实验验证了模型的准确性。计算结果与实验结果均表明: 当激光功率提高时, PDAS随之增加; 而扫描速度与扫描间距增加时, PDAS随之减小; 从熔池底部至顶部, PDAS呈逐渐增加的趋势, 同时熔池顶部对工艺参数的变化较为敏感, 而中部与底部相对迟钝。研究结果对IN718合金的微观组织调控具有一定的指导意义。

Nickel base super alloys due to their superior properties are used in different parts of gas turbines, in jet engines, and as well as in marine application from 250°C to 650°C. At such high temperature sulphur and vanadium as impurities in fuel oil get oxidised in SO2 and V2O5. Further, sulphur oxide reacts with NaCl to form Na2SO4. These salts cause high temperature corrosion which causes stress corrosion cracking of engine components of marine gas turbine. Therefore, LCF resistance of the material becomes an important consideration in design of turbine. Fatigue samples both coated with NaCl salt and uncoated samples were tested in low cycle fatigue. The fatigue tested samples were analyzed under Scanning Electron Microscope (SEM). Since micrographs analysis is an important tool with the testing of materials to evaluate the various properties of the materials. In present study micrographs analysis helped to a great extent for evaluation the Low cycle fatigue behaviour of Nickel base super alloy INCONAL 718 (IN718) at high temperature which is 550°C.

The interface bonding strength is challenging for CuCr and In718 dissimilar alloys fabricated by Laser-Based Direct Energy Deposition (DED-LB) using Powder Feedstock. Here, direct-bonded CuCr/In718 dissimilar materials (DMs) (direct-bonded specimen) and CuCr/In718 DMs with mortise and tenon structure interface (mortise-tenon specimen) were deposited by powder DED-LB. Owing to the alternating inter-track and inter-layer remelting, the defects were avoided, and the Cu elemental diffusion was obvious in the mortise-tenon specimen. Thereby, the better metallurgical bonding strength was achieved in the mortise-tenon specimen. The sandwich-shaped microstructure, including fine equiaxed and columnar grains, and the heterogeneous microstructure consisting of large columnar, short columnar, and fine equiaxed grains were formed in direct-bonded and mortise-tenon specimens, respectively. The formation mechanisms of these microstructures were unveiled, respectively. Besides, the shear strength of direct-bonded and mortise-tenon specimens was investigated. Owing to the mortise and tenon structure, the ultimate shear strength (USS) was increased by 47.18%. The synergistic enhancing mechanism of macroscopic interfacial morphology, microstructure, and elemental distribution on shear strength was revealed.

In the present study, Inconel 718 (IN718) superalloy fabricated by laser powder bed fusion (LPBF) has been characterized focusing on the effect of both homogenization and solution treatment time on grains structure, crystallographic texture, precipitates formation/dissolution and material hardness. For this purpose, a heat-treatment time window with a wide range of soaking times for both treatments was established aiming to develop the optimal post-treatment conditions for laser powder bed fused IN718. It was found that the as-printed IN718 is characterized by very fine columnar/cellular dendrites with Laves phase precipitating at the grain boundaries as well as inter-dendritic regions, which differs from the microstructure of wrought and cast materials and requires special heat-treatment conditions different from the standard treatments. The results reveal that the relatively short homogenization treatment at 1080 °C for 1 h was not enough to significantly change the as-printed grain structure and completely dissolve the segregates and Laves phase. However, a completely recrystallized IN718 material and more Laves phase dissolution were obtained after homogenization treatment for 4 h. A further increase in time of the homogenization treatment (7 h) resulted in grain growth and coarsening of carbides precipitates. The solution treatment time at 980 °C did not cause noticeable changes in the crystallographic texture and grain structure. Nevertheless, the amount of δ-phase precipitation was significantly affected by the solution treatment time. After applying the heat-treatment time window, the hardness increased by 51–72% of the as-printed condition depending on the treatment time due to the formation of γ′ and γ″ in the γ-matrix. The highest material hardness was obtained after 1 h homogenization, whereas the prolonged time treatments reduced the hardness. This study provides a comprehensive investigation of the post heat-treatments of the laser powder bed fused IN718 that can result in an optimized microstructure and mechanical behavior for particular applications.

Wire Arc Additive Manufacturing (WAAM) is one of the most appropriate additive manufacturing techniques for producing large-scale metal components with a high deposition rate and low cost. Recently, the manufacture of nickel-based alloy (IN718) using WAAM technology has received increased attention due to its wide application in industry. However, insufficient information is available on the mechanical properties of WAAM IN718 alloy, for example in high-temperature testing. In this paper, the mechanical properties of IN718 specimens manufactured by the WAAM technique have been investigated by tensile tests and hardness measurements. The specific comparison is also made with the wrought IN718 alloy, while the microstructure was assessed by scanning electron microscopy and X-ray diffraction analysis. Fractographic studies were carried out on the specimens to understand the fracture behavior. It was shown that the yield strength and hardness of WAAM IN718 alloy is higher than that of the wrought alloy IN718, while the ultimate tensile strength of the WAAM alloys is difficult to assess at lower temperatures. The microstructure analysis shows the presence of precipitates (laves phase) in WAAM IN718 alloy. Finally, the effect of precipitation on the mechanical properties of the WAAM IN718 alloy was discussed in detail.

The effects of the solution heat treatment temperature on the precipitates, grain boundary evolution and response of the microhardness of Inconel 718 (IN718) superalloy fabricated by selective laser melting (SLM) were investigated. It was found that: (1) The long-chained Laves phases formed in the as-deposited condition dissolved into the matrix when the solution temperature rises above 980 °C. (2) The width-to-length ratio was maintained at approximately 1.6 when the solution was heated from 980 °C to 1080 °C, and dropped down to 1.03 when heated to 1130 °C. (3) Low-angle grain boundaries kept the same number fraction of 65% from 980 to 1080 °C as the as-deposited condition, and decreased dramatically from 1090 to 1130 °C to 4%. (4) Annealing twin boundaries occurred at 1090 °C with a number fraction of 3%, and quickly increased to 65% when heated to 1130 °C. It is concluded that the static recrystallization of IN718 fabricated by selective laser melting (SLM) occurred at 1090 °C and fast proceeded to full recrystallization at 1130 °C. The forming of annealing twins accompanies the recrystallization process and is an effective way to refine the recrystallized grain size.

Increasing powder layer thickness could undoubtedly raise the productivity of the laser powder bed fusion (LPBF) process. On the other hand, it may cause some microstructural side-effects, which in turn influence the mechanical properties of the final part. In this work, the effects of increasing powder layer thickness from 30 to 50 μm were studied on the microstructure, tensile, and creep life of LPBF Inconel 718 (IN718). Microstructural investigations were done by optical microscope, field emission scanning electron microscope, and electron backscatter diffraction. Tensile tests were conducted at room temperature as well as 650 ֯C, and stress-rupture tests were performed in standard incremental mode. The results showed that by using 66% thicker powder layers, the intercellular spacing increased from 480 to 1000 nm and diameter of columnar grains enlarged from 38.6 to 69.3 μm. Also, coarser intercellular particles precipitated in continuous form. These microstructural changes led to slight variations in tensile properties at both room and high temperatures. On the other side, significant changes were observed in the creep life of the samples produced with different layer thicknesses. It was discovered that although the specimens produced with 30 μm layer thickness had 0.82% lower relative density, they exhibited 1.7 times longer creep life compared to those fabricated by 50 μm powder layer thickness. Furthermore, fractography showed the lack-of-fusions(LOFs) were the main crack initiators during the tensile tests, while intergranular cracking due to coalescence of spherical cavities caused the high-temperature failure during creep.

In the present study, the properties of selective laser melted IN718 superalloy specimens, prepared by using different processing parameters, were investigated. The scanning strategy (island strategy with and without 30° interlayer rotation and continuous bi-directional scanning with and without 90° interlayer rotation) and scanning speeds of 500, 700, and 1000 mm/s were selected as variables to prepare the superalloy samples. Then, the microstructure and mechanical properties (hardness and compressive strength) were characterized. The results showed, in a given scanning strategy, the density decreases as the scanning speed increases. Also, the island strategy with interlayer rotation during fabrication process leads to a higher level of density (near full density, about 8.20 g/cm3). Interlayer rotation resulted in a more uniform structure by re-melting of deposit layers in different directions during deposition of next layers in both scanning strategies. The higher values of hardness and compressive yield strength were obtained from the samples produced using continuous scanning strategy. Among the investigated samples, the sample produced with continuous scanning strategy with 90° interlayer rotation, 500 mm/s scanning speed has the highest value of hardness, 330 Vickers, and compressive yield strength, 686 MPa.

Purpose This study aims to better understand the influence of various post-treatments on the microstructure and mechanical properties of additively manufactured parts for critical applications. Design/methodology/approach In this study, Laser Powder Bed Fusion (LPBF) fabricated Inconel 718 (IN718) samples were subjected to various heat treatments, namely homogenization, solution heat treatment and double aging, to investigate their influence on the microstructure, mechanical properties and fracture mechanism at an elevated temperature of 650 °C. Homogenization treatment was performed at 1080 °C for durations ranging from 1–8 h. The solution treatment temperature varied from 980 °C to 1140 °C for 1 h, followed by double aging treatment. Findings At 650 °C, the as-built sample showed the minimum strength but demonstrated the maximum elongation to failure compared to the heat-treated samples. The strength of the IN718 superalloy increased by 20.26% to 34.81%, while ductility significantly reduced by 65.26% to 72.89% after various heat treatments compared to the as-built state. This change is attributed to the enhancement in grain boundary strength resulting from the pinning effect of the intergranular δ-phase. Originality/value The study observed that the variations in the fracture mechanism of LPBF fabricated IN718 depend on the duration and temperature of heat treatment. This research provides a thorough overview of the high-temperature mechanical properties of LPBF fabricated IN718 subjected to different homogenization times and solution treatment temperatures, correlating these effects to the corresponding changes in microstructure.