KR102507347B1 - Method of heat treatment for improving strength and ductility of additive manufactured superalloy - Google Patents

Abstract

The present invention includes the steps of solution heat treating a niobium (Nb)-containing nickel-based heat-resistant superalloy manufactured by additive manufacturing (AM) at 1010 to 1050 ° C. for 5 minutes or more, (b) the niobium (Nb)-containing nickel group After cooling the superheat-resistant alloy to 700°C at 20°C/min or more, aging the superalloy by holding it at 700°C for 4 to 12 hours, and (c) air-cooling the niobium (Nb)-containing nickel-based superheat-resistant alloy. It relates to a heat treatment method for improving the strength and ductility of a nickel-base superalloy manufactured by additive manufacturing, and a nickel-base superalloy heat-treated thereby.

Description

Heat treatment method for improving strength and ductility of additively manufactured superalloys

The present invention relates to a heat treatment method for improving strength and ductility of a nickel-based heat-resistant superalloy manufactured through additive manufacturing (AM) and a nickel-based heat-resistant superalloy heat-treated thereby.

In the case of Alloy 625 (Ni-21.5Cr-2.5Fe-9Mo-3.5Nb-0.2Ti-0.2Al-0.06C), a nickel-base superalloy used as a material for high-temperature parts in the aerospace/defense field, the fourth industrial revolution In line with this, additive manufacturing using 3D printing is being actively applied.

At this time, in the case of the laminated nickel-base superalloy, residual stress is excessively accumulated due to thermal deformation and a microstructure oriented in a specific crystallographic direction appears, showing a tendency to be weak in mechanical properties. In addition, since high-density dislocations and segregation also remain, it is required to secure the soundness of mechanical properties through microstructure stabilization through subsequent heat treatment.

However, in the case of superalloys manufactured through additive manufacturing, heat treatment conditions for achieving proper microstructure and physical properties have not been established, so the standard used for superalloys manufactured by conventional manufacturing methods (rolling or forging after casting) Heat treatment conditions are practically applied as they are.

Therefore, there is a demand for development of an optimal heat treatment method capable of improving both strength and ductility while being economical for the laminated heat-resistant superalloy.

US Patent Publication No. 2016-0138400 (published date: 2016. 05. 19.) Korean Patent Publication No. 10-2015-0116632 (published date: 2015. 10. 16.)

The technical problem to be solved by the present invention is to contribute to the stabilization of the additive manufacturing quality of the superalloy, a customized post-heat treatment method capable of imparting a stable microstructure and excellent strength and ductility to an additively manufactured nickel-base superalloy, and Accordingly, a heat-treated nickel-base superalloy is provided.

In order to achieve the above technical problem, the present invention provides (a) solution heat treatment of a niobium (Nb)-containing nickel-based superalloy manufactured by additive manufacturing (AM) at 1010 to 1050 ° C. for 5 minutes or more; (b) aging the niobium (Nb)-containing nickel-based superalloy by cooling it to 700°C at a rate of 20°C/min or more, and then holding it at 700°C for 4 to 12 hours, and (c) the niobium (Nb) ) A heat treatment method for improving the strength and ductility of a nickel-based superalloy manufactured by additive manufacturing, including air-cooling the containing nickel-base superalloy, is proposed.

In addition, the niobium (Nb)-containing nickel-based heat-resistant superalloy manufactured by the additive manufacturing method is prepared using a powder bed fusion (PBF) heat treatment of the nickel-based superalloy manufactured by the additive manufacturing method, characterized in that suggest a way

In addition, the niobium (Nb)-containing nickel-based heat-resistant superalloy produced by the additive manufacturing method is produced by a high-energy direct irradiation method (Direct Energy Deposition, DED), characterized in that the heat treatment of the nickel-based heat-resistant superalloy produced by the additive manufacturing method suggest a way

In addition, as a preferred example of a heat treatment method for a nickel-based superalloy according to the present invention, (a) Alloy 625 (Ni-21.5Cr-2.5Fe-9Mo-3.5Nb) manufactured by a laser melting method (Selective Laser Meling, SLM) -0.2Ti-0.2Al-0.06C) solution heat treatment of the alloy at 1038 ° C for 1 hour, (b) cooling the solution heat treated Alloy 625 alloy to 700 ° C at 25 ° C / min, then 700 ° C Aging by holding for 6 hours, and (c) air-cooling the niobium (Nb)-containing nickel-base superalloy for improving strength and ductility of a nickel-base superalloy manufactured by additive manufacturing Heat treatment method is proposed.

In addition, the present invention proposes a nickel-base heat-resistant superalloy heat-treated according to the above method in another aspect of the invention.

The post-heat treatment method for the nickel-base superalloy manufactured according to the present invention can simultaneously improve the strength and ductility of the laminated Alloy 625 alloy, as well as increase the cooling rate above a certain value immediately after the solution heat treatment. After cooling to a specific temperature, by performing aging treatment at that temperature, the economic feasibility of the heat treatment process can also be secured.

1 is a process flow diagram sequentially showing each step of a heat treatment method according to the present invention.
2 is a schematic diagram showing a path of a heat treatment method according to the present invention.
Figure 3a is a scanning electron microscope (SEM) image of the Alloy 625 alloy subjected to solution heat treatment in the present example.
Figure 3b is an electron backscatter diffraction (EBSD) image of the alloy 625 alloy subjected to solution heat treatment in the present example.

In describing the present invention, if it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the gist of the present invention, the detailed description will be omitted.

Embodiments according to the concept of the present invention may be applied with various changes and may have various forms, so specific embodiments are illustrated in the drawings and described in detail in the present specification or application. However, this is not intended to limit the embodiments according to the concept of the present invention to a specific disclosed form, and should be understood to include all changes, equivalents, or substitutes included in the spirit and technical scope of the present invention.

Terms used in this specification are only used to describe specific embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly dictates otherwise. In this specification, terms such as "comprise" or "having" are intended to designate that the described feature, number, step, operation, component, part, or combination thereof exists, but one or more other features or numbers However, it should be understood that it does not preclude the presence or addition of steps, operations, components, parts, or combinations thereof.

Hereinafter, the present invention will be described in detail.

1 is a process flow chart sequentially showing each step of a heat treatment method for improving the strength and ductility of a superheat-resistant superalloy manufactured according to the present invention, and FIG. 2 shows the strength and It is a schematic diagram of a heat treatment profile showing a heat treatment method for improving ductility.

As shown in FIGS. 1 and 2, the heat treatment method for improving the strength and ductility of the laminated superheat-resistant alloy according to the present invention is first made of alloy 625, etc. It is maintained at 1010 to 1050 ° C. for 5 minutes or more for solution treatment of the alloy (S100).

At this time, the niobium (Nb)-containing nickel-based heat-resistant superalloy manufactured by the additive manufacturing method is a high energy direct irradiation method (Direct Energy Deposition, DED) defined in ASTM (American Society for Testing and Materials) F42 and ISO TC261 (Additive Manufacturing) ) or powder bed fusion (PBF).

The DED method is a method (DMD, MMAAM, AFS, LENS, EBF, etc.) that forms a locally dissolved pool by irradiating a metal surface with a laser and at the same time supplies metal powder to produce a shape, and the PBF method is a powder It is a method (SLM, EBM, etc.) that produces shapes by selectively melting them using a high heat energy source (laser or electron beam) in a chamber.

Nickel-based superheat-resistant alloys such as Alloy 625 alloy manufactured through additive manufacturing methods such as SLM have excessive residual stress and segregation due to repeated rapid heating and quenching during the additive manufacturing process, high dislocation density, and columnar crystal grains oriented in a specific direction. Due to the structures, it exhibits a considerably lower elongation compared to the wrought product.

Therefore, in this step (a), solution heat treatment is performed at 1010 to 1050 ° C. for 5 minutes or more to cause recrystallization in the alloy, thereby eliminating segregation generated in the alloy during additive manufacturing and inducing a uniform microstructure.

The reason why the solution heat treatment temperature is limited to 1010 to 1050 ° C. in this step (a) is that when the solution heat treatment temperature exceeds 1050 ° C., there is no additional benefit due to the increase in the heat treatment temperature, and rather, the grain coarsening is severe, so the heat treatment Recrystallization does not occur when the solution heat treatment temperature is less than 1010°C, and the textured columnar grain structure is maintained as it is after additive manufacturing, greatly degrading the mechanical properties. This is because there is a possibility that anisotropy may exist.

On the other hand, in this step (a), the solution treatment time requires sufficient dissolution of carbides in the alloy produced by the additive manufacturing method, removal of segregation zones, and adequate grain growth to reduce the area of fragile grain boundaries. , preferably 5 minutes or longer.

Next, in the step (b) of the heat treatment method according to the present invention, in order to age the nickel-base superalloy treated in the previous step, the temperature is cooled from the solution heat treatment temperature to 700 °C at a cooling rate of 20 °C/min or more. After that, aging treatment is performed by maintaining at 700 ° C. for 4 to 12 hours (S200).

At this time, after the completion of the solution heat treatment in the step (a), it is possible to promote the economic efficiency of the process by cooling to 700 ° C. immediately from the solution heat treatment temperature without cooling to room temperature and then performing the aging treatment.

In this step (b), by performing aging treatment at 700 ° C for 4 to 12 hours, the strength of the alloy can be improved by precipitating the strengthening phase γ″ (Ni ₃ Nb) and fine carbides in the nickel-based superheat-resistant alloy. By cooling at a cooling rate of 20°C/min or more to the processing temperature of 700°C, coarsening of crystal grains and γ″ reinforced phase that may occur during cooling can be prevented.

Finally, in the step (c) of the heat treatment method according to the present invention, the aged niobium (Nb)-containing nickel-base heat-resistant superalloy is air-cooled (about 100° C./min) to terminate the heat treatment (S300).

The reason why air cooling is required after aging treatment is that the residual stress generated during additive manufacturing is not completely resolved even after heat treatment. When water cooling is performed, deformation of the sample occurs. This is because coarsening of carbides occurs.

The above-described post-heat treatment method for the nickel-based superheat-resistant alloy produced by the laminate according to the present invention can simultaneously improve the strength and ductility of the laminated Alloy 625 alloy, as well as cool to a certain value or more immediately after the solution heat treatment. After cooling to a specific temperature at a rate, by performing aging treatment at that temperature, the economic feasibility of the heat treatment process can also be secured.

Hereinafter, the present invention will be described in more detail by way of examples.

Embodiments according to the present specification may be modified in many different forms, and the scope of the present specification is not construed as being limited to the embodiments detailed below. The embodiments herein are provided to more completely explain the present specification to those skilled in the art.

<Example>

For Alloy 625 specimens prepared by the laser melting method (Selective Laser Meling, SLM), only solution heat treatment was performed (Comparative Example) or solution heat treatment and aging treatment were performed according to the heat treatment conditions shown in Table 1 below (Examples) .

< Table 1 >

For reference, prior to establishing the optimal solution heat treatment conditions in this embodiment, the candidate conditions were set to a temperature range of 870 to 1207 ° C. As a result of heat treatment by temperature in the above temperature range, it was confirmed that recrystallization occurred at a temperature of 1000 ° C. or higher, and it was confirmed that the same tendency appeared at all temperatures above that. Therefore, considering the efficiency, it is expected that residual stress relief, homogenization and recrystallization effects can be secured at the same time during heat treatment at 1038 ° C / 1 hr, and 1038 ° C / 1 hr was selected as the final condition for solution heat treatment.

3a is a scanning electron microscope (SEM) image of a solution heat treated Alloy 625 specimen. Referring to this, it can be seen that recrystallization occurred uniformly throughout the alloy through the solution heat treatment.

In addition, Figure 3b is an electron backscatter diffraction (EBSD) image of the alloy 625 specimen subjected to solution heat treatment, and it can be confirmed that recrystallization occurs evenly and well in the alloy specimen through Figure 3b, and the average size of the recrystallized crystal grains (average grain size, AGS) was confirmed to be 27 μm.

As shown in Table 1 above, in the case of the specimen (comparative example) subjected to only solution heat treatment at 1038 ° C / 1 hr, it was confirmed that the tensile strength was 849 MPa, which was significantly less than the target reference condition of 890 MPa, which was strengthened by high-temperature heat treatment It is the result of phase γ″ and the dissolution of fine carbides.

On the other hand, as shown in Table 1, the specimen (Example) subjected to aging heat treatment at 700 ° C. for 6 hours after solution heat treatment in order to induce precipitation of γ″ cast image and fine carbides showed a significantly improved tensile strength of 914 MPa, as shown in Table 1. , it was confirmed that the elongation rate was also remarkably improved.

Claims (5)

(a) Alloy 625 (Ni-21.5Cr-2.5Fe-9Mo-3.5Nb-0.2Ti-0.2A1-0.06C) alloy prepared by Selective Laser Meling (SLM) was melted at 1038℃ for 1 hour. embodied processing;
(b) cooling the solution heat-treated Alloy 625 alloy from the solution heat treatment temperature to 700 ° C. at a rate of 25 ° C./min, and then aging it by maintaining it at 700 ° C. for 6 hours; and
(c) air-cooling the aged niobium (Nb)-containing nickel-based superalloy;
A heat treatment method for improving strength and ductility of nickel-based heat-resistant superalloys manufactured by additive manufacturing. delete delete delete A nickel-base superalloy heat-treated according to the method according to claim 1.

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