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Article

Effect of Mo Content on Microstructure and Mechanical Properties of Laser Melting Deposited Inconel 690 Alloy

by
Chen Liu
1,2,*,
Wenbo Yao
1,2,
Shuo Shang
1,2,
Kuaikuai Guo
1,2,
Hang Sun
1,2 and
Changsheng Liu
1,2,*
1
School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China
2
Key Laboratory for Anisotropy and Texture of Materials, Ministry of Education, Northeastern University, Shenyang 110819, China
*
Authors to whom correspondence should be addressed.
Coatings 2023, 13(2), 340; https://doi.org/10.3390/coatings13020340
Submission received: 31 December 2022 / Revised: 24 January 2023 / Accepted: 31 January 2023 / Published: 2 February 2023
(This article belongs to the Special Issue Laser Surface Engineering)
Browse Figures
Versions Notes

Abstract

:
Inconel 690 alloy is widely used in nuclear power, petrochemical, aerospace, and other fields due to its excellent high-temperature mechanical properties and corrosion resistance. The Inconel 690 alloy with different Mo content was fabricated by laser melting deposition (LMD). The effects of Mo content on the microstructure and mechanical properties were investigated. The microstructure of as-deposited Inconel 690 is composed of columnar dendrites grown epitaxially, and M23C6 carbides are precipitated in the grain boundaries. With the increase of Mo content, the amount of precipitated carbide increases gradually. At the same time, the grain boundary becomes convoluted. The tensile test at room temperature shows that the high Mo content in the as-deposited Inconel 690 increases the ultimate strength but decreases the ductility. Compared with low Mo content, the alloy with high Mo deposition has better mechanical properties. The present study provides a new method to achieve the preparation of Inconel 690 alloy with excellent integrated mechanical properties.

1. Introduction

Nickel-based alloys have been widely used in petrochemical, energy engineering, ocean engineering, aerospace, and other fields, due to their excellent corrosion resistance, high-temperature strength, and creep resistance [1]. Inconel 690 alloy is widely used for steam generator tubes, tube sheets, baffles, and hardware in nuclear power plants. Compared with Inconel 600, the Cr content is increased to 29 wt.%, which further improves the resistance to stress corrosion and intergranular corrosion [2]. Inconel 690 has a face-centered cubic (FCC) crystal structure and a strong tendency to harden at rapid strain rates, which makes the machining and forming of Inconel 690 alloy difficult [3]. At present, Inconel 690 alloy components are mainly manufactured using the traditional subtractive manufacturing method, with the disadvantages of long production cycles and high costs [4,5].
Laser melting deposition (LMD) is a new cost-effective alternative for manufacturing full-density, near-net-shape, and defect-free components [6,7]. LMD has the potential to solve the problems of conventional manufacturing methods such as space constraints, material waste, and high machining costs associated with components with complex shapes. It has been used in many industries, including the aerospace, petrochemical, nuclear, and automotive fields [8,9,10]. Further, various metallic materials such as titanium alloys, nickel-based alloys, steel, and cobalt-based alloys were successfully fabricated using the LMD process [11,12,13,14].
Several studies have investigated laser additive manufacturing of nickel-based alloy systems. So far, most of these related studies have focused on conventional nickel-based high-temperature alloys, such as Inconel 718 and Inconel 625 [15,16,17,18,19]. However, the design of these alloys does not consider the particular characteristics of laser additive manufacturing (LAM), which are prone to porous metallurgical defects, under-fusion, and cracks [20,21,22].
It has been reported that the mechanical properties of LMD-deposited materials are improved compared to pure alloys. Due to the rapid solidification, the material’s microstructure is much finer than conventional methods such as inert tungsten gas. Segura et al. [23] uses electron beam powder bed fusion (EPBF) to fabricate 690 alloys on 316 L stainless steel, which has shown improved mechanical properties and reduced corrosion susceptibility. Wilson et al. [24] embeds TiC reinforced particles into Inconel 690 by laser melting deposition to build functional gradient metal matrix composites, in which microhardness and wear resistance increase with TiC content. Zheng et al. [25,26] showed that the appropriate increase of carbon content is conducive to improving the mechanical properties of NiCrFe-7A additively manufactured metals. Mo et al. [27,28,29] improved the overall mechanical properties of the weld metal by adjusting the content of Ti, Nb, and B elements in NiCrFe-7A welding material. Zhang et al. [29] showed that the added Nb and Mo could be increased Inconel 690 weld metal mechanical properties and reduce the ductility-dip cracking tendency, where Mo mainly plays the solid solution strengthening. Although the above work has investigated the effect of different elements on the properties of Inconel 690 metal, there is limited research on the effect of molybdenum (Mo) content on the properties of Inconel 690 alloys, primarily additive manufacturing metals.
In this work, we intend to manufacture high-strength Inconel 690 alloy with the addition of Mo by additive manufacturing. The effect of Mo addition on the microstructure and mechanical properties of Inconel 690 fabricated by LMD was investigated. The strengthening mechanism of Mo in the deposited Inconel 690-Mo and the morphology and distribution of molybdenum were discussed. High-performance LMD-fabricated Inconel 690 at room temperature was achieved by Mo addition.

2. Materials and Methods

Inconel 690 powder is manufactured by self-developed gas-atomized equipment, and its chemical composition is shown in Table 1. Figure 1 shows the scanning electron microscopy (SEM) morphology of Inconel 690 alloy powder with a particle size distribution between 50–100 um and the chemical composition of pure Mo powder (>99.9% purity) with a particle size distribution (20–50 um). Pure Mo powder and Inconel 690 powder were mixed in a tumbler for 1 h at design content (1.5 wt.%, 2.5 wt.%, 3.5 wt.%). The powder was dried in a vacuum drying oven at 120 °C for 2 h to remove the moisture adsorbed on the surface of the particles.
The experiments were performed on a self-made LMD system. The system mainly consists of a laser generation system, a powder feeding system, a three-dimensional six-axis robotics system, and an atmosphere-controlling system. The schematic diagram of the experimental process of LMD is shown in Figure 2a. The building direction and laser scanning direction of the LMD are shown in Figure 2b. The LMD system with coaxial powder injection was used to fabricate Inconel 690 samples. During LMD, an argon gas stream was used as a delivery and protective atmosphere. Based on a series of preliminary experiments [30,31], the optimized process parameters used in this work are listed in Table 2. Inconel 690 plates with dimensions of 150 × 100 × 10 mm were used as substrates.
Inconel 690 metallographic specimens for microstructure analysis were cut into 10 × 10 × 4.5 mm squares by electro-discharge machining. The metallographic specimens were ground on 240–3000# silicon carbide sandpaper and polished with 2.5 μm diamond polishing paste. Electrochemically etched with 10% oxalic acid at 5 V DC for 10–15 s. Metallographic analysis was performed using an optical microscope (OLYMPUS GX71).
The microstructure of the longitudinal sections of the deposited samples was investigated by scanning electron microscopy (JSM-7001F) and energy dispersive spectroscopy (EDS). The phase of Inconel 690 alloy samples was analyzed by SmartLab 9 type X-ray diffraction (XRD) with a scanning speed of 5°/min. The hardness of the samples was measured using a Vickers hardness (WILSON-401MVD) with a force of 1.96 N and a dwell time of 15 s. Three tensile plate specimens were cut perpendicular to the fabricating direction, as shown in Figure 2b. The geometric size of the tensile specimen is shown in Figure 2c. Tensile testing was performed on an electronic universal testing machine (AG-Xplus) with a 100 kN capacity. The engineering stress versus strain curves was recorded. Then, the fracture morphology after the tensile test was observed by SEM.

3. Results and Discussion

3.1. Microstructural Observation

Figure 3 shows the melt pool morphology in the YZ cross-section of the deposited sample. Due to the Gaussian distribution of the laser energy, the typical fish scale shape of the melt pool can be observed in Figure 3a. Each melt pool had a dimensional width of about 1.6–2.0 mm and a depth of about 0.4–0.5 mm. Figure 3c shows the melt pool at high magnification. A typical columnar dendritic microstructure was observed in the melt pool. The dendritic structures grew perpendicular to the direction of the melt pool boundary, which was also the direction of the highest temperature gradient. As shown in Figure 3b, the microstructure of Inconel 690 in the deposited state consists mainly of columnar crystals and intercrystalline dendritic substructures that grew epitaxially along the deposition direction through multiple deposited layers. It had a columnar microstructure achieved by epitaxial growth due to the high solidification rate (103 K/s) and high-temperature gradient (104 K/mm) during the deposition process [32]. During solidification, the average growth of the melt pool was approximately antiparallel to the heat flow direction and perpendicular to the solid-liquid interface. This microstructure was also found in other grades of laser-deposited nickel-based alloys [33]. From Figure 3d, it can be noticed that there was a transparent layer boundary in the deposited state samples. This phenomenon is common in additively manufactured nickel-based high-temperature alloys and titanium alloys, and the mechanism of layer boundary formation has been discussed in the literature [34,35]. In addition, the microstructure pictures show that there are almost no defects, such as pores and cracks.
Figure 4 shows the optical microstructure of the XY cross-section of deposited Inconel 690 with different molybdenum contents. As shown in Figure 4d, the deposited Inconel 690 microstructure is a typical austenite organization with a large amount of M23C6-type carbides at the grain boundaries. There is no significant change in the size of the dendritic arms with different Mo contents. With the increase of Mo content, the precipitates on the grain boundaries change from intermittent to continuous precipitation, while it can be observed that the grain boundaries become more meandered.
The microstructure of the intergranular and intercrystalline precipitates of Inconel 690 in the deposited state was observed by SEM, as shown in Figure 5a. The precipitates are mainly distributed at the grain boundaries and interdigitate. As shown in Figure 5b, it is clear from the EDS results that the M23C6 carbide is precipitated continuously on the grain boundaries.
The X-ray diffraction curves of the samples with different Mo contents are shown in Figure 6. XRD results show that the deposited state microstructures of all four compositional alloys consist of γ, and the characteristic peaks of (111), (200), (220), and (311) can be observed. No diffraction peaks of the precipitated phases were detected for the deposited state Inconel 690, most likely due to the precipitate content being below the detection limit of the device. As shown in Figure 6b, by zooming in on the (100) crystal plane diffraction peaks, it can be seen that the diffraction peaks of the γ matrix are left shifted in the corresponding angle with increasing Mo content. From Bragg’s law (2dsinθ = nλ), it can be obtained that the crystal plane spacing of the γ matrix increases, and the corresponding diffraction angle decreases slightly. The strengthening mechanism of Inconel 690 is mainly solid solution strengthening. With increasing Mo content, more Mo elements solid solution to the cell parameters become larger, and the crystalline surface, spacing becomes larger. The solution element atoms lead to lattice distortion when Ni atoms are replaced.

3.2. Mechanical Properties

The average hardness values of Inconel 690 in the deposited state for the four Mo contents are shown in Figure 7. The hardness values of the samples increase with the increase of Mo elements. The hardness of the samples increased to 232.54 HV with the addition of 3.5 wt.% Mo. This is mainly attributed to the solid solution strengthening effect, where the lattice distortion caused by the solid solution of Mo elements increases the resistance to dislocation movement and makes the slip deformation of the metal more difficult, thus increasing the hardness of the alloy.
Figure 8a shows the engineering stress-strain curves of Inconel 690 in the deposited state with different Mo contents, and the corresponding room temperature tensile properties results are shown in Figure 8b. The strength of the deposited Inconel 690 continues to increase with increasing Mo content, while the elongation decreases continuously after reaching a maximum of 1.5 wt.% Mo. At 2.5 wt.% Mo content, the yield strength of deposited Inconel 690 reached a maximum value of 418.3 ± 44 MPa. At 3.5 wt.% Mo, the ultimate tensile strength of deposited Inconel 690 reached a maximum value of 640.2 ± 13 MPa. The tensile test results indicate that Mo addition increases the ultimate tensile strength and yield strength but decreases the elongation of the LMD deposited state. Solid solution strengthening is the main contributor to the strength increase due to the increase in Mo content. The continuous precipitation of M23C6 carbides at grain boundaries after exceeding 2.5 wt.% Mo leads to a decrease in plasticity. The yield strength, tensile strength, and elongation of Inconel 690 in the deposited state were 331.0 MPa, 574.4 MPa, and 35.3%, respectively, which met the requirements of Inconel 690 welded joints (tensile strength > 552 MPa, elongation > 30%). Comparing the mechanical properties of Inconel 690 fabricated by WAAM based on overlay welding [36], Inconel 690 fabricated by LMD showed higher yield strength, tensile strength, and elongation. The application of additive manufacturing technologies in the nuclear material Inconel 690 alloy has been proven feasible and effective, especially the LMD technology based on powder filled. According to the literature [37], heat treatment of nickel-based alloys in the deposited state can significantly improve their elongation, and we will study this issue in our following work.
Figure 9 shows the SEM fractograms of the tensile tests. The dimple fracture is the main feature observed in all fractures. With increased Mo addition, the toughness nests become large and shallow. At Mo content of 2.5 wt.% and 3.5 wt.%, detrending steps appear. This indicates that increasing Mo content decreases as-deposited Inconel 690 of plasticity.

4. Conclusions

In this work, Inconel 690 deposited samples with different Mo contents were rapidly fabricated at a low cost based on the particular advantage of the adjustable chemical composition of LMD raw material powder. The effect of Mo content on the microstructure and mechanical properties was revealed. The following conclusions were drawn:
  • Columnar dendrites were the main feature of the microstructure of Inconel 690 in the deposited state. With the increase of Mo content, the size and number of precipitates at grain boundaries become larger and larger, and the grain boundaries become curved.
  • Deposited Inconel 690 mainly consists of γ phase and M23C6 phase precipitated at the grain boundary. With the increased Mo content, the XRD diffraction peak was shifted to the left, more Mo elements were solidified into the γ-phase lattice structure.
  • With the increase of Mo content, the hardness, yield strength, and tensile strength of as-deposited Inconel 690 increased and the elongation decreased. The Mo element in the as-deposited Inconel 690 mainly strengthened the solid solution. The continuous precipitation of M23C6 at the grain boundaries is the major contributor to the decrease in plasticity.
This work systematically establishes the relationship between the Mo content on the microstructural characteristics and mechanical properties of the as-deposited Inconel 690. These results are expected to provide process reference and theoretical support for Inconel 690 additive manufacturing, such as integrated heat transfer structure, complex flow channel instrumentation valve body, and other high-value-added and difficult-to-process parts.

Author Contributions

Conceptualization, C.L. (Chen Liu) and W.Y.; methodology, C.L. (Changsheng Liu); software, W.Y. and H.S.; validation, C.L. (Chen Liu), K.G. and S.S.; formal analysis, C.L. (Chen Liu); investigation, W.Y.; resources, W.Y.; data curation, K.G. and H.S.; writing—original draft preparation, C.L. (Chen Liu); writing—review and editing, C.L. (Chen Liu); visualization, H.S. and W.Y.; supervision, C.L. (Changsheng Liu) and S.S.; project administration, C.L. (Changsheng Liu); funding acquisition, C.L. (Changsheng Liu). All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Joint Funds of National Natural Science Foundation–Liaoning [grant numbers U1508213].

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data are available from the authors upon reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. SEM morphology of Inconel 690 powder for LMD, with the upper inset showing microdendritic microstructure.
Figure 1. SEM morphology of Inconel 690 powder for LMD, with the upper inset showing microdendritic microstructure.
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Figure 2. (a) Schematic diagrams of the LMD process; (b) sample location and build direction; (c) geometric size of the tensile specimen in mm.
Figure 2. (a) Schematic diagrams of the LMD process; (b) sample location and build direction; (c) geometric size of the tensile specimen in mm.
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Figure 3. Macrostructure of different sections of the deposited Inconel 690 samples: (a) YZ; (b) XZ; (c) YZ enlarged; (d) XZ enlarged.
Figure 3. Macrostructure of different sections of the deposited Inconel 690 samples: (a) YZ; (b) XZ; (c) YZ enlarged; (d) XZ enlarged.
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Figure 4. Microstructure of samples with different Mo contents in the deposited state: (a) Inconel 690; (b) 1.5 wt.% Mo; (c) 2.5 wt.% Mo; (d) 3.5 wt.% Mo.
Figure 4. Microstructure of samples with different Mo contents in the deposited state: (a) Inconel 690; (b) 1.5 wt.% Mo; (c) 2.5 wt.% Mo; (d) 3.5 wt.% Mo.
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Figure 5. SEM micrographs and compositional analysis of M23C6: (a) M23C6 precipitated on grains boundary; (b) EDS results of M23C6.
Figure 5. SEM micrographs and compositional analysis of M23C6: (a) M23C6 precipitated on grains boundary; (b) EDS results of M23C6.
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Figure 6. (a) X-ray diffraction curves of samples with different Mo contents; (b) Magnified image of (100) peaks shifted to the left.
Figure 6. (a) X-ray diffraction curves of samples with different Mo contents; (b) Magnified image of (100) peaks shifted to the left.
Coatings 13 00340 g006
Coatings 13 00340 g007 550
Figure 7. The hardness of as-deposited Inconel 690 with different Mo content.
Figure 7. The hardness of as-deposited Inconel 690 with different Mo content.
Coatings 13 00340 g007
Coatings 13 00340 g008 550
Figure 8. Comparison of tensile properties of different samples: (a) Engineering stress-strain curves; (b) Comparison of yield strength, ultimate tensile strength, and elongation.
Figure 8. Comparison of tensile properties of different samples: (a) Engineering stress-strain curves; (b) Comparison of yield strength, ultimate tensile strength, and elongation.
Coatings 13 00340 g008
Coatings 13 00340 g009 550
Figure 9. Room temperature tensile SEM fractogram of the deposited state of Inconel 690: (a) Inconel 690; (b) 1.5 wt.% Mo; (c) 2.5 wt.% Mo; (d) 3.5 wt.% Mo.
Figure 9. Room temperature tensile SEM fractogram of the deposited state of Inconel 690: (a) Inconel 690; (b) 1.5 wt.% Mo; (c) 2.5 wt.% Mo; (d) 3.5 wt.% Mo.
Coatings 13 00340 g009
Table
Table 1. Chemical compositions of Inconel 690 powders (wt.%).
Table 1. Chemical compositions of Inconel 690 powders (wt.%).
ElementNiCrFeAlTiMnMo
Content (wt.%)Bal.29.6610.100.150.170.220.52
Table
Table 2. The LMD processing parameters were used in the experiment.
Table 2. The LMD processing parameters were used in the experiment.
Laser Power [W]Scanning Speed [mm/s]Powder Feeding Rate [g/min]Layer Thickness [mm]Overlapping Ratio [%]Scanning Strategy
24008.83.50.5550Single direction
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MDPI and ACS Style

Liu, C.; Yao, W.; Shang, S.; Guo, K.; Sun, H.; Liu, C. Effect of Mo Content on Microstructure and Mechanical Properties of Laser Melting Deposited Inconel 690 Alloy. Coatings 2023, 13, 340. https://doi.org/10.3390/coatings13020340

AMA Style

Liu C, Yao W, Shang S, Guo K, Sun H, Liu C. Effect of Mo Content on Microstructure and Mechanical Properties of Laser Melting Deposited Inconel 690 Alloy. Coatings. 2023; 13(2):340. https://doi.org/10.3390/coatings13020340

Chicago/Turabian Style

Liu, Chen, Wenbo Yao, Shuo Shang, Kuaikuai Guo, Hang Sun, and Changsheng Liu. 2023. "Effect of Mo Content on Microstructure and Mechanical Properties of Laser Melting Deposited Inconel 690 Alloy" Coatings 13, no. 2: 340. https://doi.org/10.3390/coatings13020340

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