rus
Issue #7/2021
M. V. Rashkovets, N. G. Kislov, A. A. Nikulina, O. G. Klimova-Korsmik
Effect of Heat Treatment on the Structure, Phase Composition and Impact Toughness of Inconel 718 Alloy Under Additive Manufacturing
Effect of Heat Treatment on the Structure, Phase Composition and Impact Toughness of Inconel 718 Alloy Under Additive Manufacturing
DOI: 10.22184/1993-7296.FRos.2021.15.7.568.575
Phase composition of the laser additively manufactured Inconel 718 alloy was investigated. The main strengthening phase of heat-treated materials conditions is gamma’ phase. δ phase deposited near Laves phase particles. Charpy impact tests show increasing of toughness in 1.5 times with the changing of building direction from parallel to transverse in as-deposited and heat-treated materials conditions. Crack propagation in the as-deposited samples is accompanied by the destruction of particles of the Laves phase and bypass them in the heat-treated material.
Phase composition of the laser additively manufactured Inconel 718 alloy was investigated. The main strengthening phase of heat-treated materials conditions is gamma’ phase. δ phase deposited near Laves phase particles. Charpy impact tests show increasing of toughness in 1.5 times with the changing of building direction from parallel to transverse in as-deposited and heat-treated materials conditions. Crack propagation in the as-deposited samples is accompanied by the destruction of particles of the Laves phase and bypass them in the heat-treated material.
Теги: additive manufacturing heat-resistance ni-base alloys heat treatment impact toughness phase composition аддитивные технологии жаропрочные никелевые сплавы термическая обработка ударная вязкость фазовый состав
Effect of Heat Treatment on the Structure, Phase Composition and Impact Toughness of Inconel 718 Alloy Under Additive Manufacturing
M.V.Rashkovets , N.G.Kislov, A.A. Nikulina, O.G. Klimova-Korsmik
Novosibirsk State Technical University, Novosibirsk, Russia
St. Petersburg Marine Technical University, St. Petersburg, Russia
Phase composition of the laser additively manufactured Inconel 718 alloy was investigated. The main strengthening phase of heat-treated materials conditions is gamma’ phase. δ phase deposited near Laves phase particles. Charpy impact tests show increasing of toughness in 1.5 times with the changing of building direction from parallel to transverse in as-deposited and heat-treated materials conditions. Crack propagation in the as-deposited samples is accompanied by the destruction of particles of the Laves phase and bypass them in the heat-treated material.
Key words: heat-resistance Ni-base alloys, additive manufacturing, phase composition, heat treatment, impact toughness
Received on: 11.10.2021
Accepted on: 25.11.2021
Introduction
The mechanical properties of additively formed materials depend on the complex ratio of a number of process parameters. Therefore, the evaluation of strength characteristics under conditions of external loading is an important applied task. A number of previously published works describe static tests, more recent studies are devoted to the evaluation of mechanical properties under dynamic and cyclic loading. A direct comparison of the mechanical properties of additively formed materials with each other is difficult, due to the high sensitivity of materials to varying technological parameters and conditions of deposition. Literature data indicate the dependence of strength characteristics in relation to the orientation of the layers and the direction of loading [1–3]. However, phase composition of materials is no less important factor. Based on this, it is especially important to know the mechanical properties of additively formed material and factors that affect the fracture.
Heat-resistant nickel-base alloys are widely used in aircraft and rocketry in the production of complex elements of gas turbine engines. Strengthening of materials occurs during heat treatment, which consists not only in the formation of the main intermetallic gamma’ and gamma’’ phases, but also in the dissolution of Laves phases, δ-phases and carbides that reduce the mechanical properties [4, 5]. There are industrial standard heat treatment modes, but also numerous studies show a wide range of temperature range and time holding during heat treatment, which can reach 32 hours [6]. All this confirms that a universal mode has not yet been found. Moreover, given the differences of the structure and phase composition of additively formed materials with the conventional manufacturing, the effect of standard heat treatment is ambiguous.
This paper presents the evaluation of structure, phase composition and mechanical properties of as-deposited and heat-treated samples.
Material and methods
The initial material of the research was spherical powder of nickel-base alloy Inconel 718 with the following chemical composition: Ni (63.4%), Cr (21%), Mo (10%), Fe (0.8%), Al (0.4%), Ti (0.4%, Nb (3%), Si (0.5%), Mn (0.4%), C (0.1%).
Direct laser deposition unit consisted of a LRM-200iD 7L Fanuc robotic complex, an LS-3 IPG Photonics laser source, a FLW D30 IPG Photonics laser head with a removable SO12 Fraunhofer IWS surfacing nozzle and a Sulzer Metco Twin 10C powder feed device to the working area. The radiation beam had a Gaussian distribution, a wavelength of 1070 nm, a collimating lens focus of 200 mm, and a collimator focus of 100 mm. Beam Parameter Product (BPP) 3.5mm × mrad. The working surface was located 36 mm behind the focus of the beam, and the spot diameter was 2.6 mm. The additive process was carried out in a protective Ar environment without preheating with the following parameters: laser power 1300 W, powder feed rate 0.8 g/s, scanning speed 25 mm/s, layer step 0.6 mm.
The industrial standard heat treatment for Inconel 718 castings and forgings, performed in a universal laboratory muffle electric furnace SNOL in an air atmosphere, was as follows: solution treatments (980°C / 1 h / air cooling) + double aging (720°C / 8 h / furnace cooling at 55°C/h to 620°C / 8 h / air cooling).
To analyze the microstructure of the deposited material, a Carl Zeiss EVO50 XVP scanning electron microscope was used. Charpy impact tests were carried out using an Instron CEAST 9050 Impact Pendulum. The schemes of experiments are shown in Figure 1.
Results and discussion
The microstructure of the as-deposited and heat-treated samples of heat-resistant nickel-base alloy Inconel 718 was characterized by a dendritic structure with the presence of fusion zones between successive layers (Fig. 2). The matrix was represented by Ni solid solution, the interdendritic regions was characterized by the presence of finely dispersed primary carbides, carbonitrides, and Laves phase. The straightening gamma’ and gamma’’ phases were not identified.
The microstructure of heat-treated sample is shown in Figure 3. The main influence on the redistribution of alloying elements probably was exerted by the presence in the matrix of carbonitrides (Nb, Ti) (N, C) during direct laser deposition process. Their being extremely stable upon subsequent high-temperature heating to 980°C [7] and still contain a significant amount of Nb. In contrast, Laves phase was partially dissolved and enriched the surrounding regions with Nb, which led to the formation of δ-phase exclusively in these regions. Since the main volume of Nb still remains in carbides and carbonitrides and also participates in the precipitation of δ-phase, the main straightening phase after heat treatment is gamma’ phase. This phase based on Ni3(Al, Ti) and characterized with a typical cubic geometry and a maximum size of about 1 μm (Fig. 3).
Charpy test of the as-deposited samples showed the expected dependence on the layer growth direction and dynamic force orientation. Сrack propagated along the columnar grains in case of scheme with longitudinal orientation. The impact toughness was 48.3 J/cm2. Fractography confirmed that the fracture occurred along the interdendritic region. Also, dimples present on the fracture surface indicate ductile fracture mode (Fig. 4 a, c). The results of impact toughness in case of transverse orientation scheme increased 1.5 times (71.7 J/cm2). The increase of impact toughness is due to the additional energy absorption manifested in crack overcomes the grains boundaries located perpendicular to cracks propagation (Fig. 4 b, d). Moreover, comparison of the results on microstructural study and fracture surfaces after Charpy test allows us to conclude that the initial fine particles of Laves phase, formed during direct laser deposition process in interdendrite regions, could contribute to crack initiation and propagation in relation to both loading schemes (Fig. 3 c, d).
The impact toughness of the heat-treated samples increased 1.5 times in relation to both loading schemes. The impact toughness was 116.7 J/cm2 and 75 J/cm2 for transverse and longitudinal orientation, respectively. The fractography of heat-treated samples has the similar mode as the as-deposited samples since recrystallization did not occur during heat treatment (Fig. 5 a, b). With this context, we can conclude that the main contribution to the strengthening of the heat-treated samples that did not undergo recrystallization during the heat treatment corresponds to precipitation of gamma’ phase. Also, considering the phase composition after heat treatment (Fig. 3), it can be assumed that cracks bypassed small Laves particles which fixed with matrix by δ-phase (Fig. 5 c, d).
Conclusions
The initial phase composition of nickel-base alloy Inconel 718 after direct laser deposition significantly affects the phase composition of heat treatment state. Heating up to 980 ºС for 1 hour leads to precipitate of δ-phase exclusively in the areas of the partially dissolved Laves phase. Subsequent double aging at temperatures of 720°C and 620°C for 8 hours gives the strengthening gamma’ phase of typical cubic geometry and maximum size of about 1 μm. The mechanical properties of the as-deposited and heat-treated samples largely depend on the layer growth direction and dynamic force orientation. The difference between the longitudinal and transverse orientation is 23.4 J / cm2. Provided that columnar structure saved after the industrial standard heat treatment of the additively formed nickel-base alloy Inconel 718, the increase 1.5 times of impact toughness is due to precipitation of strengthening gamma’ phase.
Acknowledgments
The research was carried out with the financial support of the Russian Foundation for Basic Research within the framework of scientific project No. 19-38-90131 “Investigation of the regularities of fatigue and dynamic fracture of heat-resistant alloys obtained by additive manufacturing” (2019–2021).
The studies were carried out on the equipment of the Center for Collective Use “Structure, Mechanical and Physical Properties of Materials” NSTU No. 13.CKP.21.0034.
ABOUT AUTHORS
Rashkovets M., junior researcher, Research Laboratory of Physical and Chemical Technologies and Functional Materials, Novosibirsk State Technical University, Novosibirsk, Russia.
ORCID 0000-0002-4045-0722
Kislov N., Eng. of Materials Research Department, Institute of Laser and Welding Technologies, St. Petersburg Marine Technical University,
http://en.ilwt-stu.ru/contacts, St. Petersburg, Russia.
ORCID 0000-0002-1103-5802
Nikulina A., Dr. of Sc. (Eng.), Researcher, Shared Use Center «Structure, Mechanical and Physical Properties of Materials», Novosibirsk State Technical University, Novosibirsk, Russia.
ORCID 0000-0001-9249-2273
Klimova-Korsmik O., Cand. of Science (Eng.), Head of Materials Research Department, Institute of Laser and Welding Technologies, St. Petersburg Marine Technical University, http://en.ilwt-stu.ru / contacts. St. Petersburg, Russia.
ORCID: 0000-0002-2619-8874
CONTRIBUTION BY THE MEMBERS
OF THE TEAM OF AUTHORS
Rashkovets M.: Conceptualization, planning an experiment, performing the experiment, characterizing the results, discussions, writing of the original draft; Kislov N.: Conceptualization, planning an experiment, performing the experiment; Nikulina A.: Supervision of microstructure test, editing; Klimova-Korsmik O.: Supervision of mechanical tests, editing.
CONFLICT OF INTEREST
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
M.V.Rashkovets , N.G.Kislov, A.A. Nikulina, O.G. Klimova-Korsmik
Novosibirsk State Technical University, Novosibirsk, Russia
St. Petersburg Marine Technical University, St. Petersburg, Russia
Phase composition of the laser additively manufactured Inconel 718 alloy was investigated. The main strengthening phase of heat-treated materials conditions is gamma’ phase. δ phase deposited near Laves phase particles. Charpy impact tests show increasing of toughness in 1.5 times with the changing of building direction from parallel to transverse in as-deposited and heat-treated materials conditions. Crack propagation in the as-deposited samples is accompanied by the destruction of particles of the Laves phase and bypass them in the heat-treated material.
Key words: heat-resistance Ni-base alloys, additive manufacturing, phase composition, heat treatment, impact toughness
Received on: 11.10.2021
Accepted on: 25.11.2021
Introduction
The mechanical properties of additively formed materials depend on the complex ratio of a number of process parameters. Therefore, the evaluation of strength characteristics under conditions of external loading is an important applied task. A number of previously published works describe static tests, more recent studies are devoted to the evaluation of mechanical properties under dynamic and cyclic loading. A direct comparison of the mechanical properties of additively formed materials with each other is difficult, due to the high sensitivity of materials to varying technological parameters and conditions of deposition. Literature data indicate the dependence of strength characteristics in relation to the orientation of the layers and the direction of loading [1–3]. However, phase composition of materials is no less important factor. Based on this, it is especially important to know the mechanical properties of additively formed material and factors that affect the fracture.
Heat-resistant nickel-base alloys are widely used in aircraft and rocketry in the production of complex elements of gas turbine engines. Strengthening of materials occurs during heat treatment, which consists not only in the formation of the main intermetallic gamma’ and gamma’’ phases, but also in the dissolution of Laves phases, δ-phases and carbides that reduce the mechanical properties [4, 5]. There are industrial standard heat treatment modes, but also numerous studies show a wide range of temperature range and time holding during heat treatment, which can reach 32 hours [6]. All this confirms that a universal mode has not yet been found. Moreover, given the differences of the structure and phase composition of additively formed materials with the conventional manufacturing, the effect of standard heat treatment is ambiguous.
This paper presents the evaluation of structure, phase composition and mechanical properties of as-deposited and heat-treated samples.
Material and methods
The initial material of the research was spherical powder of nickel-base alloy Inconel 718 with the following chemical composition: Ni (63.4%), Cr (21%), Mo (10%), Fe (0.8%), Al (0.4%), Ti (0.4%, Nb (3%), Si (0.5%), Mn (0.4%), C (0.1%).
Direct laser deposition unit consisted of a LRM-200iD 7L Fanuc robotic complex, an LS-3 IPG Photonics laser source, a FLW D30 IPG Photonics laser head with a removable SO12 Fraunhofer IWS surfacing nozzle and a Sulzer Metco Twin 10C powder feed device to the working area. The radiation beam had a Gaussian distribution, a wavelength of 1070 nm, a collimating lens focus of 200 mm, and a collimator focus of 100 mm. Beam Parameter Product (BPP) 3.5mm × mrad. The working surface was located 36 mm behind the focus of the beam, and the spot diameter was 2.6 mm. The additive process was carried out in a protective Ar environment without preheating with the following parameters: laser power 1300 W, powder feed rate 0.8 g/s, scanning speed 25 mm/s, layer step 0.6 mm.
The industrial standard heat treatment for Inconel 718 castings and forgings, performed in a universal laboratory muffle electric furnace SNOL in an air atmosphere, was as follows: solution treatments (980°C / 1 h / air cooling) + double aging (720°C / 8 h / furnace cooling at 55°C/h to 620°C / 8 h / air cooling).
To analyze the microstructure of the deposited material, a Carl Zeiss EVO50 XVP scanning electron microscope was used. Charpy impact tests were carried out using an Instron CEAST 9050 Impact Pendulum. The schemes of experiments are shown in Figure 1.
Results and discussion
The microstructure of the as-deposited and heat-treated samples of heat-resistant nickel-base alloy Inconel 718 was characterized by a dendritic structure with the presence of fusion zones between successive layers (Fig. 2). The matrix was represented by Ni solid solution, the interdendritic regions was characterized by the presence of finely dispersed primary carbides, carbonitrides, and Laves phase. The straightening gamma’ and gamma’’ phases were not identified.
The microstructure of heat-treated sample is shown in Figure 3. The main influence on the redistribution of alloying elements probably was exerted by the presence in the matrix of carbonitrides (Nb, Ti) (N, C) during direct laser deposition process. Their being extremely stable upon subsequent high-temperature heating to 980°C [7] and still contain a significant amount of Nb. In contrast, Laves phase was partially dissolved and enriched the surrounding regions with Nb, which led to the formation of δ-phase exclusively in these regions. Since the main volume of Nb still remains in carbides and carbonitrides and also participates in the precipitation of δ-phase, the main straightening phase after heat treatment is gamma’ phase. This phase based on Ni3(Al, Ti) and characterized with a typical cubic geometry and a maximum size of about 1 μm (Fig. 3).
Charpy test of the as-deposited samples showed the expected dependence on the layer growth direction and dynamic force orientation. Сrack propagated along the columnar grains in case of scheme with longitudinal orientation. The impact toughness was 48.3 J/cm2. Fractography confirmed that the fracture occurred along the interdendritic region. Also, dimples present on the fracture surface indicate ductile fracture mode (Fig. 4 a, c). The results of impact toughness in case of transverse orientation scheme increased 1.5 times (71.7 J/cm2). The increase of impact toughness is due to the additional energy absorption manifested in crack overcomes the grains boundaries located perpendicular to cracks propagation (Fig. 4 b, d). Moreover, comparison of the results on microstructural study and fracture surfaces after Charpy test allows us to conclude that the initial fine particles of Laves phase, formed during direct laser deposition process in interdendrite regions, could contribute to crack initiation and propagation in relation to both loading schemes (Fig. 3 c, d).
The impact toughness of the heat-treated samples increased 1.5 times in relation to both loading schemes. The impact toughness was 116.7 J/cm2 and 75 J/cm2 for transverse and longitudinal orientation, respectively. The fractography of heat-treated samples has the similar mode as the as-deposited samples since recrystallization did not occur during heat treatment (Fig. 5 a, b). With this context, we can conclude that the main contribution to the strengthening of the heat-treated samples that did not undergo recrystallization during the heat treatment corresponds to precipitation of gamma’ phase. Also, considering the phase composition after heat treatment (Fig. 3), it can be assumed that cracks bypassed small Laves particles which fixed with matrix by δ-phase (Fig. 5 c, d).
Conclusions
The initial phase composition of nickel-base alloy Inconel 718 after direct laser deposition significantly affects the phase composition of heat treatment state. Heating up to 980 ºС for 1 hour leads to precipitate of δ-phase exclusively in the areas of the partially dissolved Laves phase. Subsequent double aging at temperatures of 720°C and 620°C for 8 hours gives the strengthening gamma’ phase of typical cubic geometry and maximum size of about 1 μm. The mechanical properties of the as-deposited and heat-treated samples largely depend on the layer growth direction and dynamic force orientation. The difference between the longitudinal and transverse orientation is 23.4 J / cm2. Provided that columnar structure saved after the industrial standard heat treatment of the additively formed nickel-base alloy Inconel 718, the increase 1.5 times of impact toughness is due to precipitation of strengthening gamma’ phase.
Acknowledgments
The research was carried out with the financial support of the Russian Foundation for Basic Research within the framework of scientific project No. 19-38-90131 “Investigation of the regularities of fatigue and dynamic fracture of heat-resistant alloys obtained by additive manufacturing” (2019–2021).
The studies were carried out on the equipment of the Center for Collective Use “Structure, Mechanical and Physical Properties of Materials” NSTU No. 13.CKP.21.0034.
ABOUT AUTHORS
Rashkovets M., junior researcher, Research Laboratory of Physical and Chemical Technologies and Functional Materials, Novosibirsk State Technical University, Novosibirsk, Russia.
ORCID 0000-0002-4045-0722
Kislov N., Eng. of Materials Research Department, Institute of Laser and Welding Technologies, St. Petersburg Marine Technical University,
http://en.ilwt-stu.ru/contacts, St. Petersburg, Russia.
ORCID 0000-0002-1103-5802
Nikulina A., Dr. of Sc. (Eng.), Researcher, Shared Use Center «Structure, Mechanical and Physical Properties of Materials», Novosibirsk State Technical University, Novosibirsk, Russia.
ORCID 0000-0001-9249-2273
Klimova-Korsmik O., Cand. of Science (Eng.), Head of Materials Research Department, Institute of Laser and Welding Technologies, St. Petersburg Marine Technical University, http://en.ilwt-stu.ru / contacts. St. Petersburg, Russia.
ORCID: 0000-0002-2619-8874
CONTRIBUTION BY THE MEMBERS
OF THE TEAM OF AUTHORS
Rashkovets M.: Conceptualization, planning an experiment, performing the experiment, characterizing the results, discussions, writing of the original draft; Kislov N.: Conceptualization, planning an experiment, performing the experiment; Nikulina A.: Supervision of microstructure test, editing; Klimova-Korsmik O.: Supervision of mechanical tests, editing.
CONFLICT OF INTEREST
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Readers feedback
