JP7552196B2 - Alloy composition, method for producing alloy composition, and mold - Google Patents

Description

The present invention relates to an alloy composition suitable for use as a repair material for dies used, for example, when casting aluminum alloys.

For example, JIS (Japanese Industrial Standards) SKD61 is used for dies used in low pressure casting, gravity casting, and die casting of aluminum alloys. When casting is performed repeatedly using the same die, damage occurs to the die. Known causes of damage include melting and galling. When damage occurs, the damaged area is repaired by welding to build up the material. A preferable repair material is an alloy that has a high melting point and excellent melting resistance with creep resistance at high temperatures. An example of this alloy is disclosed in Patent Document 1.

Special Publication No. 1-502680

The alloy disclosed in Patent Document 1 has a high melting point and excellent high-temperature deformation resistance, so it is expected to have resistance to melting damage compared to aluminum alloys. However, the alloy disclosed in Patent Document 1 is insufficient to prevent galling, which is one of the causes of damage to dies. In other words, high hardness is required to prevent galling, but the alloy disclosed in Patent Document 1 has a hardness of about HV 300 to 370 and HRC of about 30 to 38, which is low.

In view of the above, the present invention aims to provide an alloy composition that is resistant to melting damage when casting an aluminum alloy, has high hardness, and can suppress the occurrence of galling, a method for manufacturing the alloy composition, and a mold that uses this alloy composition.

The present invention provides an additively manufactured body that has a Mo-Cr dendritic structure and a Ni-Al interdendritic structure that fills the periphery of the Mo-Cr dendritic structure, and in which fine particles having a B2 ordered phase with a maximum particle size of 300 nm or less are precipitated within the Mo-Cr dendritic structure, and that has an alloy composition that has excellent resistance to corrosion and galling.

In the alloy composition of the present invention, when emphasis is placed on resistance to corrosion, the following chemical composition I is adopted, and when emphasis is placed on the effects of improving galling resistance due to high hardness and lowering the melting point of the alloy composition, the following chemical composition II is adopted.
<Chemical Composition I>
When Mo+Cr+Ni+Al=100 at. %, Ni+Al=15 to 50 at. %, Mo+Cr=50 to 85 at. %
<Chemical Composition II>
When Mo+Cr+Ni+Al=100 at. %, Ni+Al=40 to 70 at. %, Mo+Cr=30 to 60 at. %

When chemical composition I is used in the alloy composition of the present invention, the ratio of the area of the Mo-Cr dendritic structure to the entire structure is 50-85%. When chemical composition II is used in the alloy composition of the present invention, the ratio of the area of the Mo-Cr dendritic structure to the entire structure is 50-70%.

In the alloy composition of the present invention, when chemical composition I is adopted, preferably, Ni is 7.5 to 25 at. %, Al is 7.5 to 25 at. %, Cr is 10 to 25 at. %, and the balance is Mo and inevitable impurities.

When chemical composition II is used in the alloy composition of the present invention, preferably, Ni is 20-35 at. %, Al is 20-35 at. %, Cr is 10-50 at. %, and the balance is Mo and unavoidable impurities.

The ranges of chemical compositions I and II overlap in part. This overlapping range, i.e., Ni+Al = 40-50 at. %, Mo+Cr = 50-60 at. %, is most preferable from the standpoint of improving corrosion resistance and hardness, and lowering the melting point of the alloy composition.

In the alloy composition of the present invention, preferably, there are regions in the Mo-Cr dendritic structure where the Cr/Mo ratio is different. An example of a region in the Mo-Cr dendritic structure where the Cr/Mo ratio is different is a form in which the Cr/Mo ratio is higher at the edge of the dendritic structure than at the center.

In the alloy composition of the present invention, the maximum arm width or arm spacing of the Mo-Cr dendritic structure is preferably 10 μm or less.

In the alloy composition of the present invention, the Rockwell hardness is preferably 45 HRC or more.

As a method for producing the alloy composition described above, the present invention proposes a method for melting and solidifying a raw material powder containing one or both of a simple metal powder of Mo, Cr, Ni or Al, and an alloy powder consisting of two or more metals selected from Mo, Cr, Ni and Al.

The present invention also proposes a method for manufacturing the alloy composition described above, which involves melting and solidifying raw material powders containing one or both of a single metal powder of Mo, Cr, Ni or Al, and an alloy powder consisting of two or more metals selected from Mo, Cr, Ni and Al, and additive manufacturing. Additive manufacturing is effective in refining the Mo-Cr dendritic structure.

The manufacturing method of the present invention can also be applied to a method for repairing cladding dies in which raw material powders containing one or both of a single metal powder of Mo, Cr, Ni or Al, and an alloy powder consisting of two or more metals selected from Mo, Cr, Ni and Al are melted and solidified.

In the manufacturing method of the present invention, the base material is preferably preheated to 400°C to 800°C during additive manufacturing.

The present invention also relates to a mold that uses the alloy composition as a repair material. The alloy composition of the present invention can also be applied to alloy powders and alloy ingots for additive manufacturing.

According to the alloy composition of the present invention, the Mo-Cr dendritic structure mainly improves the corrosion resistance. Also, the Ni-Al interdendritic structure mainly increases the hardness. This makes it possible to obtain an alloy composition that has both corrosion resistance and high hardness. With this alloy composition, for example, the occurrence of galling in a die during casting of an aluminum alloy can be suppressed.

1 is a photograph showing the microstructure of an alloy composition according to an embodiment of the present invention, taken with a scanning electron microscope (SEM). FIG. 1 is a diagram for explaining the arm width W1 of a primary dendrite, the arm width W2 of a secondary dendrite, and the arm spacing W3 of a secondary dendrite in a dendritic structure according to the present invention, and is a diagram for determining the arm width W1 of a primary dendrite, the arm width W2 of a secondary dendrite, and the arm spacing W3 of a secondary dendrite based on a model of a dendritic crystal. 1 is a microstructure photograph for identifying the arm width W1 of a primary dendrite and the arm width W2 of a secondary dendrite based on an alloy composition according to an embodiment of the present invention. 1 is a microstructure photograph for identifying the arm width W1 of a primary dendrite and the arm width W2 of a secondary dendrite based on an alloy composition according to an embodiment of the present invention. 1 is a diagram showing a manufacturing procedure of an alloy composition according to an embodiment of the present invention. 4 is a photograph showing the microstructure of samples in which the solidification rate of the alloy composition according to the present embodiment is changed. 1 is a graph comparing the hardness of an alloy composition according to an embodiment of the present invention with that of a conventional alloy. 1 is a photograph (SEM observation image) showing the microstructure of the alloy composition according to this example (sample No. 5: ingot). 1 is a photograph (SEM observation image) showing the microstructure of the alloy composition according to this example (sample No. 5: ingot). 1 is a photograph (SEM observation image) showing the microstructure of the alloy composition according to this example (sample No. 5: ingot). 1 is a photograph (SEM observation image) showing the microstructure of the alloy composition according to this example (sample No. 5: ingot). 1 is a photograph (SEM observation image) showing the microstructure of the alloy composition according to this example (sample No. 6: ingot). 1 is a photograph (SEM observation image) showing the microstructure of the alloy composition according to this example (sample No. 6: ingot). 1 is a photograph (SEM observation image) showing the microstructure of the alloy composition according to this example (sample No. 6: ingot). 1 is a photograph (SEM observation image) showing the microstructure of the alloy composition according to this example (sample No. 6: ingot). FIG. 1 is a diagram showing the results of crystal structure analysis by X-ray diffraction (XRD) of the alloy compositions according to the present embodiment (samples Nos. 5 to 7: ingots). 1 is a photograph (SEM observation image) showing the microstructure of an alloy composition according to this example (sample No. 8: additive manufacturing body). 1 is a photograph (SEM observation image) showing the microstructure of an alloy composition according to this example (sample No. 8: additive manufacturing body). 1 is a photograph (SEM observation image) showing the microstructure of an alloy composition according to this example (sample No. 8: additive manufacturing body). 1 is a photograph (SEM observation image) showing the microstructure of an alloy composition according to this example (sample No. 9: additive manufacturing body). 1 is a photograph (SEM observation image) showing the microstructure of an alloy composition according to this example (sample No. 9: additive manufacturing body). 1 is a photograph (SEM observation image) showing the microstructure of an alloy composition according to this example (sample No. 9: additive manufacturing body). FIG. 1 is a diagram showing the results of crystal structure analysis by X-ray diffraction (XRD) of the alloy compositions according to the present examples (sample No. 7: ingot, sample Nos. 8 and 9: layered manufactured bodies). FIG. 1 is a diagram showing the results of evaluating the element distribution in the dendritic structure of the alloy composition according to the present embodiment (sample No. 7: ingot). FIG. 1 is a diagram showing the results of evaluating the element distribution in the dendritic structure of the alloy composition according to the present embodiment (sample No. 7: ingot). 1 is a TEM image of Sample No. 8 (layer-by-layer manufactured body) observed with a TEM (Transmission Electron Microscope). This is a result of electron beam diffraction in the dendritic structure in the region A shown in FIG. 13 . 1 is a STEM image of the above-mentioned region A of Sample No. 8 (layer-by-layer manufactured body) observed by a STEM (Scanning Transmission Electron Microscope). 14 is an STEM image of the interdendritic structure in region B shown in FIG. 13, in which region B of sample No. 8 (layer-by-layer manufactured product) was observed with an STEM. 14 is an STEM image of the interdendritic structure in region B shown in FIG. 13, in which region B of sample No. 8 (layer-by-layer manufactured product) was observed with an STEM.

Hereinafter, an embodiment of the present invention will be described with reference to the attached drawings. Note that below, a numerical range expressed using "~" means that the numerical values before and after "~" are included as the lower and upper limits. In addition, the upper and lower limits can be combined in any way.

When the alloy composition according to this embodiment is used in a casting mold, it has resistance to melting and a hardness that can suppress galling. These properties are obtained solely due to its structure.

[Organization]
As shown in FIG. 1, the alloy composition 1 according to the present embodiment has two phases, a dendritic structure 3 and an interdendritic structure 5. The dendritic structure is also called a dendrite structure. In FIG. 1, the dendritic structure 3 is shown in white with high brightness, and the interdendritic structure 5 is shown in gray with low brightness. The dendritic structure 3 forms a Mo-Cr phase mainly composed of Mo and Cr, and constitutes the main phase of the alloy composition 1. The interdendritic structure 5 forms a Ni-Al phase mainly composed of Ni and Al, and constitutes a subphase responsible for hardening the entire alloy. The interdendritic structure is also called an interdendritic structure, intercolumnar structure, etc.

[Dendrite structure]
The dendritic structure 3 imparts corrosion resistance to the alloy composition 1 according to this embodiment. In the dendritic structure 3, Mo has a high melting point of 2623°C, and if corrosion resistance alone is taken into consideration, the main phase would consist of Mo alone. However, when used as a casting mold, oxidation resistance is also required in addition to corrosion resistance. Therefore, in order to improve oxidation resistance, Cr is contained in addition to Mo. The melting point of Cr is 1907°C, which is lower than that of Mo, but it contributes to oxidation resistance by forming a very thin and dense protective oxide (Cr ₂ O ₃ ) on the surface.

The dendritic structure 3 is mainly composed of a body-centered cubic (bcc) lattice structure, but partially contains a B2 ordered phase. The B2 ordered phase has a crystal structure in which the first adjacent atoms are ordered with the bcc as the basic lattice, and the B2 ordered phase in the present invention is an ordered phase mainly composed of Ni and Al.

In the dendritic structure 3, the greater the amount of Mo, the higher the corrosion resistance, and the greater the amount of Cr, the higher the oxidation resistance. The ratio of Mo to Cr can be determined according to the required properties. In other words, if corrosion resistance is particularly required, the ratio of Mo should be increased, and if oxidation resistance is particularly required, the ratio of Cr should be increased. However, oxidation resistance can be obtained in the dendritic structure 3 even with the addition of a small amount of Cr.

The dendritic structure 3 mainly contains a Mo-Cr solid solution with a high content of the top two elements, Mo and Cr, and does not preclude the inclusion of Ni and Al and unavoidable impurities contained in the interdendritic structure 5. Hereinafter, in this specification, the dendritic structure may be referred to as a Mo-Cr dendritic structure. As shown in the examples described later, the Mo-Cr dendritic structure should not be interpreted as containing only Mo and Cr, and the inclusion of elements such as Al and Ni is permitted.

In the Mo-Cr dendritic structure 3, Cr is concentrated at the edge of the dendritic structure, and the Cr concentration at the edge is high. This is shown in Figure 1 where the color of the edge of the primary dendritic arm and the secondary dendritic arm is dark. The dendritic edge is the connection area between the interdendritic structure and the dendritic structure, and it is desirable for the composition to be gradient in order to provide continuity of the structure. The gradient in composition and continuity between the interdendritic structure and the dendritic structure means that the properties of Mo, Cr, Ni, and Al, such as corrosion resistance, oxidation resistance, and high hardness, are continuously gradient, that is, it means that there is little difference in properties depending on the location. The concentration distribution of each element in the dendritic structure can be evaluated by EPMA (Electron Probe Micro Analyzer). First, a mirror-polished sample is prepared, and an area analysis is performed on an area (for example, 50 μm x 50 μm) that is sufficiently filled with dendritic structure. The obtained analysis results are converted into a concentration display. Linear analysis is performed at any position across the dendritic structure to evaluate the element distribution. The Cr/Mo ratio is calculated at the dendrite edge and center.

In the alloy composition of the present invention, when the Cr/Mo ratios of the dendrite edge and center are compared, it was confirmed that the Cr/Mo mass ratio (weight ratio) of the dendrite edge is higher than that of the center. In this way, the element distribution inside the dendritic structure can be evaluated. In addition, the Cr content ratio increases in the edge, and the Ni and Al contents also increase. Solid solution strengthening or precipitation strengthening of NiAl in the dendrite edge is effective as an alloy resistant to Al corrosion.

The B2 ordered phase present in the dendritic structure mainly composed of bcc can be confirmed by high magnification observation with a TEM (Transmission Electron Microscope). For example, a test piece with a thickness of about 100 nm can be cut out by a microsampling method using a Focused Ion Beam (FIB) for observation. In the TEM observation, a limited-area diffraction of φ200 nm is performed targeting the dendritic structure, and the diffraction pattern showing the bcc structure and the spots of the slightly existing B2 ordered phase can be identified from the analysis results of the obtained electron beam diffraction image. In addition, the distribution state of the B2 ordered phase can be confirmed by capturing a dark-field image in accordance with the spots of the B2 ordered phase. In the alloy composition of the present invention, it has been confirmed that a granular B2 ordered phase with a maximum diameter (maximum grain size) of approximately 170 nm or less is present (see FIG. 15). This B2 ordered phase may precipitate during cooling to room temperature after solidification, or may precipitate by heat treatment after solidification. When it precipitates during cooling after solidification, the slower the cooling rate, the greater the amount of precipitation, or the larger the size of the B2 ordered phase. For example, the amount of precipitation increases when the additive manufacturing process is performed while preheating at 400°C to 800°C. These minute precipitates are stable from room temperature to relatively high temperatures and act to hinder the movement of dislocations, thereby increasing the deformation resistance of the dendritic structure. In other words, it is believed that the presence of these precipitates contributes to the increased hardness and high softening resistance of the alloy composition of the present invention. Therefore, the maximum grain size of the B2 ordered phase is preferably 300 nm or less, and more preferably 200 nm or less.

The size (grain size) of the B2 ordered phase (precipitated particles) can be determined by the maximum length of the precipitated particles observed in the observation area. For example, the observation method can be performed by measuring the maximum length of the precipitated particles observed in any observation area using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). In addition, the equivalent circle diameter, that is, the area of the precipitated particles observed in any observation area, can be measured, and the diameter of a perfect circle having the same area as the precipitated particle can be calculated and used as the diameter of the precipitated particle. Furthermore, the average grain size of the precipitated particles can be determined by dividing the diameter of each precipitated particle in the observation area obtained by the above method by the number of precipitated particles observed.

[Interdendritic structure]
Next, the interdendritic structure according to this embodiment imparts mechanical strength, particularly hardness, to the alloy composition according to this embodiment. Hardness is required to deal with galling that occurs during casting. The interdendritic structure 5 is made of a solid solution or intermetallic compound mainly composed of Ni and Al. The interdendritic structure 5 is preferably made of a NiAl intermetallic compound containing Ni and Al in a ratio of approximately 1:1. This intermetallic compound has a body-centered cubic (bcc) lattice structure and a B2 type crystal structure similar thereto. These intermetallic compounds are excellent in high-temperature strength and oxidation resistance, and are suitable for dies for casting aluminum alloys. The melting point of the NiAl intermetallic compound is 1638°C, and the hardness is approximately 41 HRC. In addition to NiAl, the interdendritic structure 5 containing Ni _and Al can also be constituted by an intermetallic compound of a well-symmetric L12 type ordered structure with a face-centered cubic basic lattice, Ni 3 Al. The hardness of Ni ₃ Al is approximately 44 HRC.

There are several types of Ni-Al intermetallic compounds, and for example, the hardness of Ni ₂ Al ₃ is about 62 HRC. These intermetallic compounds are generated in an additive manufacturing process, and do not undergo significant changes even when exposed to a temperature range of about 500°C to 700°C. Therefore, when the alloy composition of the present application is applied to a mold to which molten Al at about 700°C is repeatedly attached, the hardness of the alloy composition does not change, that is, the softening resistance is high and the hardness is maintained. In short, the alloy composition of the present application has a stable structure even at 700°C, so that it is unlikely to change under repeated loads at this temperature, and it is also an advantageous feature over conventional materials that the hardness hardly decreases even when used for a long period of time.

For example, SKD61 is a typical conventional material used for aluminum die-casting dies. In practice, SKD61 is often used with a hardness exceeding 40HRC, but in most cases it is tempered to 45HRC by heat treatment. The tempering temperature is about 600°C to 650°C, so if the SKD61 base material is left as is and repeatedly exposed to molten aluminum at 700°C, which exceeds the tempering temperature, it will soften. For this reason, nitriding treatment is sometimes performed on the surface of the SKD61 base material (for example, nitriding treatment to improve the surface layer of 50 to 200μm to about 800 to 1000Hv), but when used for aluminum die-casting dies, there is a problem that the nitriding layer gradually disappears due to repeated heat effects, and eventually the SKD61 base material will soften.

The interdendritic structure 5 mainly contains Ni-Al intermetallic compounds with high contents of the top two elements Ni and Al, and does not preclude the inclusion of Mo, Cr, and unavoidable impurities contained in the dendritic structure 3. Hereinafter, in this specification, the interdendritic structure may be referred to as Ni-Al interdendritic structure. As shown in the examples described later, the Ni-Al interdendritic structure should not be interpreted as containing only Ni and Al, and the inclusion of trace amounts of elements such as Mo and Cr is permitted.

As shown in FIG. 1, the interdendritic structure 5 exists so as to fill the gaps between the Mo-Cr dendritic structures 3. Since the interdendritic structure 5 contains Al, it is preferable that the amount of Al in the alloy composition is small from the viewpoint of resistance to melting damage. However, in order to deal with galling, it is preferable that the material constituting the mold has high hardness. Therefore, in this embodiment, hardening by forming the Ni-Al interdendritic structure 5 is utilized. The hardness of the alloy in this embodiment is desirably higher than the hardness of SKD61 used as the base material of the mold before heat treatment, in order to improve galling resistance. That is, the alloy in this embodiment is 45 HRC or more, further 50 or more, preferably 55 HRC or more. If the hardness is too high, it will cause a decrease in toughness, so the upper limit is 70 HRC, preferably 65 HRC or less.

[Ratio of main phase to subphase]
In the alloy composition according to the present embodiment, when the corrosion resistance is important, the ratio of the area occupied by the dendritic structure 3 to the entire alloy is preferably 50 to 85%. If the area ratio of the dendritic structure 3 is less than 50%, sufficient corrosion resistance may not be obtained. The corrosion resistance is largely influenced by the ratio of the area occupied by the dendritic structure 3, but is also influenced by the degree of fineness of the dendritic structure 3. In other words, the finer the dendritic structure 3, the better the corrosion resistance. On the other hand, if the area ratio of the dendritic structure 3 exceeds 85%, the interdendritic structure 5 as a hardening phase becomes less, and sufficient hardness may not be obtained. The hardness is largely influenced by the ratio of the area occupied by the interdendritic structure 5, but is also influenced by the degree of fineness of the dendritic structure 3. In other words, even with the same composition, the finer the dendritic structure 3, the better the hardness.

As described above, it is preferable to adjust the ratio of the area occupied by the dendritic structure 3 and the interdendritic structure 5, taking into consideration the resistance to melting and hardness, by changing the composition ratio of the raw materials, changing the cooling rate during shaping, etc.

The ratio of the area of the dendritic structure 3 to the total area of the alloy can be obtained by mirror-polishing the cross section of the alloy, observing it with a SEM, and converting the SEM image into two shades of gray.

In the alloy composition according to this embodiment, when hardness and a low melting point are more important than resistance to corrosion, it is preferable that the area ratio of the dendritic structure 3 to the entire alloy is 50 to 70%. In other words, when the area ratio of the dendritic structure 3 to the entire alloy is 50 to 70%, the area ratio of the interdendritic structure 5 as a hardening phase to the entire alloy is 30 to 50%, so that sufficient hardness is easily obtained. In addition, since Mo, the main component of the dendritic structure 3, has a high melting point, the melting point of the alloy composition can be lowered by setting the area ratio of the dendritic structure 3 to the entire alloy to 50 to 70%.

[Arm width and arm spacing of dendritic tissue]
As described above, the finer the dendritic structure, the better the corrosion resistance. Also, the finer the dendritic structure, the higher the hardness. In this embodiment, the degree of fineness of the dendritic structure is specified by the arm width or arm spacing of the dendritic structure.

Figure 2(a) shows a modeled dendrite. A dendrite is formed first as a primary dendrite 10, which can be called a trunk, and then a secondary dendrite 20, which can be called a branch, is formed on the primary dendrite 10 so as to be perpendicular to its axis. In this embodiment, the maximum arm widths W1, W2 and arm spacing W3 of the dendritic structure composed of the primary dendrite and the secondary dendrite are preferably both 50 μm or less. More preferably, the maximum arm widths W1, W2 and arm spacing W3 are 30 μm or less, even more preferably, the maximum arm widths W1, W2 and arm spacing W3 are 20 μm or less, and even more preferably, the maximum arm widths W1, W2 and arm spacing W3 are 10 μm or less. The maximum arm width W1 can be determined as the maximum value of the width perpendicular to the extension direction of the primary dendrite arm in the field of view of the SEM image taken at a magnification of 300 times. Similarly, the maximum arm width W2 and arm spacing W3 can be determined by taking the maximum value of the width in the direction perpendicular to the extension direction of the secondary arborescent arms as the maximum arm width W2 and the maximum value of the distance between the center lines of adjacent secondary arborescent arms in the field of view of an SEM image taken at 300x magnification as the maximum arm width W2 and the arm spacing W3, respectively.

Figure 2(b) shows the arm widths W1 and W2 of sample No. 1 in the embodiment described below. The maximum arm width W1 of sample No. 1 is approximately 20 μm, and the maximum arm width W2 is approximately 45 μm. Hereinafter, the maximum arm widths W1 and W2 may be simply referred to as arm widths W1 and W2.

Figure 2(c) shows a microstructure photograph of sample No. 4 of the embodiment described later, and it can be seen that both arm width W1 and arm width W2 are 10 μm or less. Thus, the structure of sample No. 4 is finer than that of sample No. 1. From the two photographs of Figures 2(b) and (c), it can be seen that if the arm widths W1 and W2 of the dendritic structure are small, that is, if the dendritic structure is fine, the interdendritic structure filling the gaps between the dendritic structures will also be fine, and the arm spacing W3 will be small. This means that even if the ratio of the dendritic structure and the interdendritic structure in the entire alloy is the same, if the arm widths W1 and W2 and the arm spacing W3 are small, each dendritic structure and interdendritic structure will be small.

As mentioned above, compared to MoCr and NiAl, NiAl has inferior corrosion resistance. However, if the Ni-Al interdendritic structure is refined, the area contributing to corrosion is reduced and only a portion of the surface layer is corroded, which results in a reduced amount of elution and improved corrosion resistance for the entire alloy composition. Refining the structure in this way also leads to improved hardness as mentioned above, which contributes to improved corrosion resistance and hardness, which are necessary to suppress damage to the die.

Here, it is known that the corrosion occurs through the following two mechanisms: (1) and (2).

(1) Physical corrosion (cavitation erosion)
When the molten metal is suddenly accelerated in a part inside the mold where the shape changes suddenly, cavitation occurs, resulting in melting damage. Cavitation is a physical phenomenon in which bubbles appear and disappear in a short period of time due to pressure differences in the flow of liquid. When the bubbles disappear, stress is generated by the pressure shock that acts on the surface, mechanically damaging it, so improving the strength of the surface is an effective countermeasure against damage. In other words, improving hardness is effective in improving melting damage resistance.

(2) Chemical corrosion (corrosion due to chemical compound formation)
When molten aluminum alloy comes into contact with the mold surface, aluminum diffusion occurs, and the aluminum alloy is deposited on the mold surface. The deposited surface portion of the mold falls off together with the deposited aluminum alloy. To prevent this type of chemical corrosion, it is effective to use a material that is low in reactivity with aluminum. In other words, it is effective to use a material that does not have a eutectic point with aluminum and has a high melting point.

Next, we will explain the chemical composition I and chemical composition II of the alloy composition having the structure described above.

Chemical composition I has a higher total amount of Mo and Cr than chemical composition II (the upper limit of Mo + Cr is 85 at.%), and is a composition that emphasizes corrosion resistance.

Chemical composition II has a higher total amount of Ni and Al than chemical composition I (the upper limit of Ni + Al is 70 at.%). Chemical composition II is a composition that emphasizes the effects of improving galling resistance due to high hardness and lowering the melting point of the alloy composition.

When the amount of Ni + Al is increased, the hardness of the alloy composition according to this embodiment increases, but the corrosion resistance may decrease. When the amount of Mo + Cr is increased, the corrosion resistance of the alloy composition according to this embodiment increases, but the hardness decreases and the melting point increases. In consideration of these trends, chemical compositions I and II can be appropriately adopted depending on the manufacturing method and application of the alloy composition.

[Chemical composition of alloy composition I]
Ni+Al=15-50at. %Mo+Cr=50-85at. %
When Mo+Cr+Ni+Al=100 at. %,
Ni: 7.5 to 25 at. % Al: 7.5 to 25 at. %
Cr: 10-25 at. % Remainder: Mo and unavoidable impurities The amount of Ni+Al affects the amount of interdendritic structure. That is, when the amount of Ni+Al is 15 at. % or more, hardness for suppressing mechanical damage is imparted. When the amount of Ni+Al exceeds 50 at. %, the amount of dendritic structure consisting of Mo and Cr, which are mainly responsible for corrosion resistance, decreases. Therefore, Ni+Al is preferably 15-50 at. %. The content of Ni+Al is more preferably 20-45 at. %, and further preferably 30-45 at. %.

Each of Ni and Al can be contained in the range of 7.5 to 25 at. %. If the content of each is less than 7.5 at. %, the amount of interdendritic structure is small and hardness may be insufficient. If the content exceeds 25 at. %, the amount of dendritic structure consisting of Mo and Cr is small and corrosion resistance may be insufficient. The amount of each of Ni and Al is more preferably 10 to 25 at. %, even more preferably 15 to 25 at. %, and even more preferably 20 to 25 at. %.

Next, the amount of Mo + Cr affects the corrosion resistance. In other words, if the amount of Mo + Cr is less than 50 at. %, the amount of dendritic structure, which is mainly responsible for corrosion resistance, will be reduced. If the amount of Mo + Cr exceeds 85 at. %, the melting point of the raw material will be high, and there is a concern that the following problems will occur. In addition, the amount of interdendritic structure, which is responsible for precipitation hardening, will be reduced. Therefore, it is preferable that Mo + Cr is 50 to 85 at. %. The content of Mo + Cr is more preferably 50 to 80 at. %, and even more preferably 50 to 70 at. %.

<Problems when the amount of Mo+Cr exceeds 85 at. % and the melting point of the raw material becomes high>
As described later, the alloy powder according to the present embodiment can be manufactured by, for example, an atomization method. During the atomization process, which is a powder manufacturing process, the high-melting-point raw material does not melt well in the crucible, resulting in unmelted material, making it difficult to control the composition of the molten metal.

Even if alloy powder can be produced by atomization, if laser light is irradiated onto the obtained alloy powder to perform additive manufacturing, melting, defoaming, and deposition are unlikely to proceed sufficiently, and there is a high possibility that a defective molded body will be formed. In other words, even if a high-melting-point raw material with a Mo+Cr content of more than 85 at. % can be atomized, the resulting powder will have poor moldability. Even if the output of the high-frequency melting furnace is increased to melt the high-melting-point raw material, the crucible (usually made of alumina, zirconia, etc.) cannot withstand the temperature and will be damaged, preventing the melt from being discharged and making it impossible to obtain an alloy. Even if damage to the crucible can be avoided, the temperature of the path from the crucible to the discharge nozzle must also be kept high, which consumes a lot of power. If the temperature of the discharge nozzle is low, problems such as clogging at the time of discharging the melt will occur.

Cr is contained in the range of 10 to 25 at. %, and Mo is defined as the remainder after subtracting Ni, Al, and Cr from the entire alloy. If the Cr content is less than 10 at. %, oxidation resistance may be insufficient, and if it exceeds 25 at. %, the amount of Mo responsible for corrosion resistance will be reduced. In other words, if corrosion resistance is important, the Mo content should be increased, and if oxidation resistance is important, the Cr content should be increased. As the Cr content increases, the melting point will gradually decrease.

The more preferred amount of Cr is 12 to 20 at. %, and even more preferred is 12 to 18 at. %.

C, B, and Si lower the melting point of the alloy, while also forming hard particles such as carbides, borides, and borosilicates with Mo, Cr, Ni, and Al, which contribute to improving wear resistance. Si is also a chemical component added as a deoxidizer, and has the effect of increasing the cleanliness of the molten metal. This effect is achieved when the C, B, and Si content is 0.01 at. % or more, but if it is added in excess of 8.0 at. %, the amount of hard particles increases and cracks are more likely to occur during additive manufacturing. Therefore, when C, B, and Si are actively added to chemical composition I, the content should be 0.01 to 8.0 at. %.

[Chemical composition of alloy composition II]
When emphasis is placed on improving the galling resistance and lowering the melting point of the raw materials, the alloy composition preferably has the following chemical composition.

Ni+Al=40-70at. %Mo+Cr=30-60at. %
When Mo+Cr+Ni+Al=100 at. %,
Ni: 20 to 35 at. % Al: 20-35 at. %
Cr: 10 to 50 at. % Balance: Mo and unavoidable impurities

When hardness is important, the amount of Ni+Al, which affects the amount of interdendritic structure, is set to 40 at. % or more. However, if the amount of Ni+Al exceeds 70 at. %, the amount of Mo-Cr dendritic structure, which is mainly responsible for corrosion resistance, decreases. Therefore, in order to achieve both the desired hardness and corrosion resistance, it is preferable that Ni+Al is 40 to 70 at. %. In chemical composition II, the content of Ni+Al is more preferably 40 to 65 at. %, and even more preferably 40 to 55 at. %.

Ni and Al can each be contained in the range of 20 to 35 at. %. By containing Ni and Al in this range, the hardness can be made 50 HRC or more. In chemical composition II, the amount of Ni and Al is more preferably 20 to 30 at. %, and even more preferably 20 to 25 at. %.

The amount of Mo+Cr, which affects the corrosion resistance, is 30 to 60 at. % in chemical composition II. If the amount of Mo+Cr is less than 30 at. %, the amount of dendritic structure, which is mainly responsible for corrosion resistance, decreases, making it difficult to obtain the desired corrosion resistance. On the other hand, if the amount of Mo+Cr exceeds 60 at. %, the amount of interdendritic structure decreases, so if hardness is important, it is preferable to set the upper limit of the amount of Mo+Cr to 60 at. %. In chemical composition II, the content of Mo+Cr is more preferably 35 to 60 at. %, and even more preferably 40 to 60 at. %.

Cr is contained in the range of 10 to 50 at. %, and Mo is defined as the remainder after subtracting Ni, Al, and Cr from the entire alloy. If the Cr content is less than 10 at. %, oxidation resistance may be insufficient, and if it exceeds 50 at. %, the amount of Mo responsible for corrosion resistance will be reduced. In other words, if corrosion resistance is important, the Mo content should be increased, and if oxidation resistance is important, the Cr content should be increased.

As mentioned above, the melting point gradually decreases as the Cr content increases, but when the Cr content reaches about 50 at. %, the melting point begins to rise again. Therefore, if lowering the melting point of the alloy composition is important, a more preferable Cr amount is 15 to 45 at. %, and an even more preferable Cr amount is 20 to 40 at. %. The advantages of lowering the melting point of the alloy composition include the following:

Benefits of lowering the melting point of the alloy composition
The alloy powder according to the present embodiment can be manufactured by, for example, an atomization method. When the melting point of the raw material is low, the raw material melts well in a short time in the crucible during atomization, so that a molten metal of uniform composition is easily obtained, and the melt can be discharged without clogging the discharge nozzle. If the raw material can be melted at a low temperature, the life of the crucible is extended, and the production cost is reduced. When layered manufacturing is performed using atomized powder, the raw material is melted without remaining unmelted by the output of a general laser light, and degassing and deposition proceed, so that a shaped body without defects is easily formed. In other words, powder produced by atomizing a raw material with a low melting point has good moldability.

C, B, and Si lower the melting point of the alloy, while forming hard particles such as carbides, borides, and borosilicates with Mo, Cr, Ni, and Al, which contribute to improving wear resistance. Si is also a chemical component added as a deoxidizer, and has the effect of increasing the cleanliness of the molten metal. This effect is achieved when the C, B, and Si content is 0.01 at. % or more, but if it is added in excess of 8.0 at. %, the amount of hard particles increases and cracks are more likely to occur during additive manufacturing. Therefore, when C, B, and Si are actively added to chemical composition II, the content is set to 0.01 to 8.0 at. %.

As described above, chemical compositions I and II can be appropriately adopted depending on the manufacturing method and application of the alloy composition.

The ranges of chemical compositions I and II overlap in part. This range, i.e., Ni+Al = 40-50 at. %, Mo+Cr = 50-60 at. %, is most preferable from the standpoint of improving corrosion resistance and hardness, and lowering the melting point of the alloy composition.

[Form of alloy composition and manufacturing method]
The alloy composition according to the present embodiment is used, for example, as an alloy ingot or an additive manufacturing product as described below, or as an alloy powder for obtaining the same. As an example, the alloy powder is also used as a raw material for powder deposition, which is a type of additive manufacturing method.

As a manufacturing method of the alloy composition of this embodiment, a manufacturing method can be used in which a raw material powder containing one or both of a simple metal powder consisting of at least one of Mo, Cr, Ni, and Al, and an alloy powder consisting of at least two metals selected from Mo, Cr, Ni, and Al is melted and solidified. The simple metal powder may be one type, two types, or more, and the alloy powder may be one type or two types. As a specific example, a mixture of simple metal powders Mo powder and Cr powder and alloy powder NiAl powder can be used as the raw material alloy. Alternatively, a mixture of alloy powders Ni ₃ Al and MoCr alloy can be used as the raw material alloy. Since Mo and Cr are completely solid-soluble, the composition of the MoCr alloy can be freely adjusted. Since the Cr-rich CrMo alloy has a low melting point, it is easy to trigger the formation of a molten pool.

When the ratio of Cr in the MoCr alloy is 80-90 at. % (i.e., when the ratio of Mo is 10-20 at. %), the melting point of the MoCr alloy is the lowest, at about 1700°C. The fact that it is easy to trigger the formation of a molten pool means the following. When a laser beam is irradiated onto a MoCr alloy with a low melting point, melting begins from this composition powder. The liquefied MoCr alloy has an active metal surface and has high fluidity, so it wets the surrounding NiAl powder while increasing the contact area, transferring heat, and alloying at this contact point also causes a low melting point, making it easy to promote melting. In this way, mixing composition powders with a low melting point improves the ease and stability of forming a molten pool. Therefore, by mixing a Cr-rich CrMo alloy with NiAl powder, it is possible to stably form a molten pool, and improves the formability.

It is also essential that the manufacturing method of this embodiment includes melting and solidification steps, an example of which is described below.

[Method of manufacturing alloy powder]
An example of a method for producing this alloy powder will be described with reference to Fig. 3. The gist of this method is that raw material powder is granulated using a spray dryer, and then sintered to obtain alloy powder.

This manufacturing method includes a raw material preparation step S101, a raw material mixing step S103, a granulation step S105, a firing step S107, and an alloying step S109. In the raw material preparation step S101, for example, Mo powder, Cr powder, Ni powder, and Al powder are prepared as raw materials according to the composition of the alloy composition to be obtained. The raw materials are not limited to metal powder as a single element, and may be, for example, MoCr alloy powder, NiAl alloy powder, or Mo powder, Cr powder, and NiAl alloy powder. The particle size of these raw material powders may be appropriately selected according to the particle size of the alloy powder to be obtained.

Next, in the raw material mixing step S103, the raw material powder prepared in the raw material preparation step S101 is wet-mixed with wax such as paraffin. For mixing, a known device such as an attritor can be used, and in addition to the raw material powder and wax, a dispersion medium such as ethanol can be added to the attritor and wet-mixed to obtain a mixed powder slurry.

Next, in the granulation process S105, the slurry obtained in the raw material mixing process is sprayed and dried using a spray dryer to granulate the mixture powder.

Next, in the firing step S107, the powder mixture granulated in the granulation step S105 is put into a drying furnace and degreased at a degreasing temperature of 400°C to 600°C, and then fired at a firing temperature of 600°C or higher. The degreasing temperature is a temperature at which the wax used can be removed, and the firing temperature is a temperature at which the powder particles of the mixture are solidified. In the granulated powder that has been fired, the raw material powders are bonded together, but are not alloyed.

Next, in the alloying step S109, the granulated powder that has been through the sintering step S107 is exposed to a temperature higher than the sintering temperature in the sintering step S107 to alloy it. For example, thermal plasma-droplet-refining (PDR), which involves passing the powder through a high-temperature region such as a plasma, can be used for this alloying. The granulated powder is instantly melted and solidified by the alloying process using PDR. Alternatively, after the sintering step S107, the temperature can be further raised to alloy the contact points between the powders for the purpose of alloying. This powder has the strength required for use in additive manufacturing.

Here, thermal plasma is an ionized gas in which the molecules in the gas are dissociated into atoms by adding energy to the gas, and the atoms are further ionized into ions and electrons. Compared to conventional manufacturing methods in which metal pieces are heated in an electric furnace, it is possible to heat the metal pieces to extremely high temperatures, specifically to temperatures of 5000°C or higher in the high-temperature areas. In this thermal plasma that creates an extremely high-temperature atmosphere, even powders with a high melting point, such as Mo, can be instantly melted.

In addition, because thermal plasma can generate an extremely high temperature atmosphere locally, a steep temperature gradient can be formed between the thermal plasma area and the surrounding atmosphere. This steep temperature gradient causes metal pieces to instantly melt in the high temperature part of the thermal plasma, and their own surface tension causes them to become spherical. The spherical metal pieces are quickly cooled below their melting point by the surrounding atmosphere, and solidify to form metal spheres.

The above steps produce the alloy powder according to this embodiment. In this way, the granulated powder that has been solidified by removing the wax through the firing step S107 is instantly heated in the alloying step S109 to melt and solidify. As a result, the dendritic structure of the resulting alloy powder is fine. Furthermore, the surface tension of the resulting alloy composition gives each particle a nearly spherical shape, and the particle surfaces are smooth.

[Other powder manufacturing methods]
The alloy powder according to the present embodiment can be manufactured by the atomization method. In the atomization method, molten metal is scattered as droplets by the kinetic energy of a high-pressure atomizing medium and solidified to manufacture powder. Depending on the atomizing medium applied, the atomization method is classified into water atomization method, gas atomization method, jet atomization method, and the like. Any of the atomization methods can be adopted to manufacture the alloy powder according to the present embodiment. The alloy powder obtained by the atomization method is also melted and solidified, and the solidification speed is fast, so that the dendritic structure can be made fine.

In the water atomization method, molten metal is allowed to flow down from the bottom of a tundish, and high-pressure water is sprayed onto the molten metal flow as an atomizing medium, which is then atomized using the kinetic energy of the water. The water atomization method has a faster cooling rate during solidification than other atomization methods. However, the resulting powder has an irregular shape.

In gas atomization, a high-pressure gas, such as an inert gas such as nitrogen or argon, or air, is used as the atomizing medium. Powder produced by gas atomization tends to become spherical. This is thought to be mainly because the cooling rate of gas is slower than that of water, and the molten particles that have been turned into droplets become spherical due to surface tension before solidifying.

The jet atomization method uses a combustion flame such as kerosene as the atomizing medium, and injects a high-temperature flame jet at a speed exceeding the speed of sound into the molten metal, accelerating the molten metal for a relatively long period of time and pulverizing it. This powder tends to become spherical, and a finer particle size distribution can be obtained.

The EiGA method (electrode induction melting gas atomization method) is a method in which an ingot is prepared, and this is used as the electrode material, melted in an induction coil, and directly atomized. MoCr and NiAl have different melting points, so depending on the composition they may separate in the atomization furnace. In this case, however, by preparing the ingot in a furnace with a strong stirring force and applying the EIGA method, it is possible to obtain powder with a homogeneous composition.

The manufacturing method of the alloy powder described above is one example of the present invention, and the present invention can manufacture alloy powder by other manufacturing methods.

[Method of manufacturing alloy ingot]
The alloy ingot can be obtained most typically by a melting and casting method in which molten metal is obtained in a melting furnace, then poured into a predetermined mold and solidified. Examples of melting furnaces include furnaces that convert electric energy into thermal energy to melt the material, electric resistance furnaces that use Joule heat, low-frequency induction furnaces that use electromagnetic induction currents, high-frequency induction furnaces that use eddy currents, and arc melting furnaces. The alloy ingot obtained by the melting and casting method has a shape that conforms to the shape of the mold, and can have various shapes such as a flat plate or a rectangular parallelepiped.

The alloy ingot according to this embodiment includes a component manufactured by additive manufacturing. The additive manufacturing method referred to here involves melting and solidifying raw powder to form a solidified layer. This operation is repeated to stack the solidified layers to form a component of a desired shape, and the concept includes powder cladding. For example, damaged parts of an aluminum alloy casting mold can be repaired by powder cladding with the alloy composition according to this embodiment. This repaired part constitutes the alloy ingot according to this embodiment.

For powder build-up, for example, plasma powder build-up welding and laser powder build-up welding can be used. Plasma powder build-up welding uses plasma as the heat source. Since plasma powder build-up welding is performed in an inert argon atmosphere, it is possible to form a build-up layer with a smooth surface and few voids inside.

Laser powder build-up welding uses a laser as a heat source. Laser powder build-up welding has the advantage that the heat input area can be narrowed, making it possible to build up thin objects. In addition, with laser powder build-up welding, the area that is heated is small, so the temperature gradient with the base material is large, and after the laser light passes through, the temperature drops sharply and the material is rapidly cooled. Therefore, since it melts and solidifies, it is advantageous for refining the dendritic structure.

Laser metal deposition (LMD) can be used for laser powder buildup welding. In LMD, a buildup layer is formed on the surface of a substrate by supplying raw material powder while irradiating the substrate with laser light.

As the raw powder for powder cladding, alloy powder produced by the above-mentioned method or a method other than the above-mentioned method can be used.

In addition, if the powder overlay welding machine can supply multiple types of powder, Mo powder, Cr powder, Ni powder, and Al powder can be used as the raw materials, and MoCr alloy powder and NiAl alloy powder can also be used as the raw materials. In this case, the raw material powder is alloyed by melting and solidifying during the overlay process. With this method, it is possible to create a gradient composition from the base material to the surface. A gradient composition is a composition in which the ratio of the base material components to MoCr and NiAl is changed, resulting in different hardness and composition ratios from the base material to the surface. Eliminating the difference in hardness between the base material and the modeling material (i.e. the overlay alloy material) leads to a reduction in cracks between the modeling material and the base material.

In addition, if corrosion resistance and galling resistance are required in adjacent areas of the surface, different ratios of MoCr and NiAl can be added to each area. In this way, overlay alloys with different compositions can be formed in areas of the mold that require corrosion resistance and areas that require galling resistance, respectively, resulting in a high-performance mold with excellent wear resistance.

[Application]
The composition of the present invention is preferably used as a repair material for dies for casting aluminum alloys. For example, it is used for low-pressure casting dies for molding aluminum wheels and dies for aluminum cylinder heads of engines. It is also widely used for other members that are required to withstand high temperatures. For example, it can be used for bearings that may generate frictional heat due to high-speed rotation.

The manufacturing method of the alloy composition of the present invention can also be applied to a method for repairing cladding dies.

[Example 1]
Next, an example of the present invention will be described based on an embodiment.

The following four types of raw material powders were prepared and mixed to obtain the alloy composition shown below.

Mo powder: particle size 3-8μm
Cr powder: particle size 1-5mm
Ni powder: particle size 8-15mm
Al powder: particle size 40μm or less Alloy composition:
Mo; 45at. %Cr; 15at. %Ni; 20at. % Al; 20at. %

The above raw material powders were melted in a high-frequency induction melting furnace to obtain an alloy ingot. This melted and cast alloy ingot was designated Sample No. 1. The liquidus temperature of Sample No. 1 was determined to be 1630°C. The liquidus temperature was calculated using Thermo-calc (thermodynamics database: SSOL6).

The alloy ingot obtained by subjecting sample No. 1 to a heat treatment at 1200°C for 2 hours was designated sample No. 2. The alloy ingot as melted and cast was irradiated with laser light to partially remelt and solidify. The remelting conditions were as follows. The alloy ingot with the slower scanning speed was designated sample No. 3, and the alloy ingot with the faster scanning speed was designated sample No. 4. The size of samples No. 1 to 4 produced in Example 1 was all 20 mm in diameter x 50 mm in length.

Remelting conditions: Equipment: IPG YLR-2000-S 2kW fiber laser Output: 1200W
Side gas: Ar
Laser light incidence angle: 10°
Laser beam scanning width: 13 mm
Laser light scanning speed: 100 mm/min (sample No. 3)
500 mm/min (Sample No. 4) For Samples No. 1, 3, and 4, microstructure observation was performed using a scanning electron microscope. The results are shown in Figure 4. For Samples No. 3 and 4, the observation was performed on the parts that had been remelted and solidified. The same was true for hardness.

The Vickers hardness of samples No. 1 to 4 and the conventional alloy was measured at room temperature with a Vickers hardness tester under a load of 1000 gf and a holding time of 15 seconds. The measurement was performed 10 times, and the average value of 8 points excluding the maximum and minimum values was recorded. The measured Vickers hardness (HV) was converted to Rockwell hardness (HRC). For the conversion, ASTM (American Society for Testing and Materials) E140 Table 2 was referenced. The results are shown in Figure 5. The conventional alloy was DENSIMET 185 by Plansee of Austria. DENSIMET is a registered trademark of Plansee.

The positions for observing the structure and measuring the arm width are on the cross section of the sample, that is, on a two-dimensional surface. When the sample is an alloy ingot, a sample with a cross section for observation and measurement is cut from the center of the ingot. In the case of remelting and solidification (samples No. 3 and No. 4), a cross section parallel to the scanning direction of the laser is formed, and the cross section is polished and then observed. In addition, backscattered electron images were taken, and the arm widths W1 and W2 of the primary and secondary dendrites were obtained from the dendritic structure and interdendritic structure. The arm width of the primary dendrite was measured as the maximum width in the growth direction of the dendritic structure. The arm width of the secondary dendrite was measured at 10 arbitrary points and the average value was calculated. In addition, as shown in Figure 4, it can be seen that all of samples No. 1, 3, and 4 have a composite structure of dendritic structure and interdendritic structure. This structure is observed due to the difference in contrast caused by the difference in composition. Here, the backscattered electron images were converted into two gradations, and the area ratio of each color was derived using image analysis software.

First, the results of the structural observation showed that the structure became finer in the order of sample No. 1, 3, and 4. This was due to the influence of the cooling rate during solidification. In other words, sample No. 1, which was obtained by melting in a high-frequency induction melting furnace and then casting, had a slower cooling rate during solidification compared to the local remelting and solidification caused by the irradiation of laser light. Also, for sample No. 3 and sample No. 4, the faster the scanning speed of the laser light, the faster the cooling rate during solidification, so sample No. 4 had a finer structure than sample No. 3. The measurement results for the arm widths W1 and W2 of sample Nos. 1, 3, and 4 were as follows.

Sample No. 1: W1 = 20 μm, W2 = 30 μm, area ratio of dendritic tissue = 68%
Sample No. 3: W1 = 6 μm, W2 = 10 μm, area ratio of dendritic tissue = 62%
Sample No. 4: W1 = 6 μm, W2 = 4 μm, area ratio of dendritic structure = 70%
Next, as shown in FIG. 5, all of the samples No. 1 to No. 4 according to this embodiment have a hardness of 50 HRC or more, and all except for sample No. 2 have a hardness of 55 HRC or more. Therefore, when the samples No. 1 to No. 4 according to this embodiment are used as repair materials for aluminum alloy casting dies, they show high resistance to galling. This is because the area ratio of the dendritic structure is 85%. %, and the hard Ni-Al interdendritic structure is finely present. In particular, samples No. 3 and No. 4, in which the structure is refined by remelting and solidification, have a hardness of 60 HRC or more. It has excellent resistance to galling.

In addition, from Sample No. 1 and Sample No. 2, the hardness of the alloy composition of this embodiment can be adjusted by subjecting it to high-temperature heat treatment.

The composition of the dendritic structure and interdendritic structure of sample No. 1 was analyzed by energy dispersive X-ray spectroscopy (EDX), and the results are as follows:
Dendritic structure:
Mo; 72.2 at. %Cr; 24.6at. %
Ni; 1.8at. % Al; 1.4at. %
Interdendritic tissue Ni; 49.9at. % Al; 45.2 at. %
Mo; 0.8at. %Cr; 4.1at. %
From the above compositional analysis results, it is recognized that the interdendritic structure is mainly composed of NiAl intermetallic compounds having a composition ratio of Ni and Al of 1:1.

[Example 2]
Alloy ingots were obtained in the same manner as for sample No. 1, except that the raw material powders prepared in Example 1 were mixed to obtain the following three types of alloy compositions. These melted and cast alloy ingots were designated as samples No. 5, 6, and 7. The size of each of samples No. 5 to 7 prepared in Example 2 was 20 mm in diameter and 50 mm in length.
Alloy composition of sample No. 5:
Mo; 15at. %Cr; 35at. % Ni; 25at. % Al; 25at. %
Alloy composition of sample No. 6:
Mo; 10 at. %Cr; 50at. %Ni; 20at. % Al; 20at. %
Alloy composition of sample No. 7:
Mo; 19at. %Cr; 19at. % Ni; 28at. % Al; 34at. %

[Comparative Example]
As a comparative example, the raw material powders prepared in Example 1 were mixed to obtain the following alloy composition. An attempt was made to obtain an alloy ingot using this mixed powder in the same procedure as in Sample No. 1, but melting in a high-frequency induction melting furnace was not possible, and an alloy ingot could not be obtained.

Mo; 65at. %Cr; 25at. %Ni; 5at. % Al; 5at. %

[Considerations on Samples No. 1, 5 to 7, and Comparative Example]
The liquidus temperatures of Samples Nos. 5, 6, and 7 and Comparative Example 1 were determined by Thermo-calc (thermodynamic database: SSOL6). In addition, the Rockwell hardness (HRC) of Samples Nos. 5 to 7 was determined in the same manner as above. The results are summarized in Table 1.

As shown in Table 1, samples No. 5 to 7 have a lower liquidus temperature than sample No. 1, and the alloy composition has a lower melting point.

In particular, sample No. 5 (liquidus temperature: 1505°C) has a hardness of 55 HRC or more, while having a liquidus temperature 125°C lower than that of sample No. 1 (liquidus temperature: 1630°C). Therefore, increasing the amount of Cr relative to Mo, setting the amount of Ni + Al to 50 at. %, and the amount of Mo + Cr to 50 at. %, is effective in achieving both the desired hardness and a low melting point of the alloy composition.

Next, comparing sample No. 1 and sample No. 6, both have a Ni+Al content of 40 at. % and a Mo+Cr content of 60 at. %, but sample No. 6 (Mo: 10 at. %, Cr: 50 at. %) has a lower liquidus temperature than sample No. 1 (Mo: 45 at. %, Cr: 15 at. %). Therefore, if the improvement of oxidation resistance and the lower melting point of the alloy composition due to the inclusion of Cr are important, the Cr content should be set to be greater than the Mo content. With sample No. 6, it is possible to obtain a hardness of 50 HRC or more while keeping the liquidus temperature at 1550°C or less.

Comparing sample No. 5 (Ni + Al = 50 at.%) with sample No. 7 (Ni + Al = 62 at.%), sample No. 5 has a lower liquidus temperature and a higher hardness than sample No. 7. Therefore, it was found that the upper limit of the total amount of Ni and Al is 70 at.%, further 60 at.%, and more preferably 50 at.%.

Comparative Example 1, in which the total amount of Mo and Cr is 90 at. % and the total amount of Ni and Al is 10 at. %, has a high liquidus temperature of 2130°C, and cannot be melted in a high-frequency induction melting furnace, and an alloy ingot cannot be obtained. From this result, the upper limit of the total amount of Mo and Cr should be 85 at. %, and the lower limit of the total amount of Ni and Al should be 15 at. %. The upper limit of the total amount of Mo and Cr is preferably 80 at. %, more preferably 70 at. %, and even more preferably 60 at. %. The lower limit of the total amount of Ni and Al is preferably 20 at. %, more preferably 30 at. %, and even more preferably 40 at. %.

The arm widths W1 and W2 and the area ratio of dendritic structures were measured for samples No. 5, 6, and 7 using the same procedure as in Example 1. The measurement results were as follows.

Sample No. 5: W1 = 11 μm, W2 = 9 μm, area ratio of dendritic tissue = 58%
Sample No. 6: W1 = 17 μm, W2 = 11 μm, area ratio of dendritic tissue = 66%
Sample No. 7: W1 = 8 μm, W2 = 7 μm, area ratio of dendritic tissue = 57%
The measurement results for Sample No. 1 and the measurement results for Samples No. 5, 6, and 7 are shown in Table 2.

Figure 6 shows an SEM image of sample No. 5, and Figure 7 shows an SEM image of sample No. 6. As shown in Figures 6 and 7, samples No. 5 and 6 also have a composite structure of dendritic and interdendritic structures.

Also, by comparing Figures 6 and 7, it can be seen that Sample No. 5 (Figure 6) has a finer structure than Sample No. 6 (Figure 7). This corresponds to the result shown in Table 2 that both arm widths W1 and W2 are smaller in Sample No. 5. Also, as shown in Table 1, Sample No. 5 has a greater amount of Ni+Al than Sample No. 6, and is therefore harder. This is because Sample No. 5 has a higher proportion of Ni-Al interdendritic structures in the alloy composition than Sample No. 6, and the Ni-Al interdendritic structures that contribute to improved hardness are appropriately dispersed within the fine structure.

The crystal structure of samples No. 5 to No. 7 was analyzed by X-ray diffraction (XRD). The results are shown in Figure 8. As shown in Figure 8, it was confirmed that samples No. 5 to No. 7 contain a mixture of bcc phase and NiAl (B2) phase.

Among samples No. 5 to 7, sample No. 7, which has the highest total amount of Ni and Al at 62 at. %, has the highest peak intensity of the NiAl (B2) phase. However, as shown in Table 1, sample No. 5 (Ni + Al = 50 at. %) has a higher hardness than sample No. 7. From this result, it was found that hardness is correlated not only with the proportion of Ni-Al interdendritic structures in the alloy composition, but also with the refinement of the structure. It is presumed that the finer the structure of the alloy composition and the more appropriately the Ni-Al interdendritic structures are dispersed, the more advantageous it is for obtaining high hardness.

[Example 3]
In Example 3, two layered objects were produced by layered manufacturing using laser metal deposition (LMD) (samples No. 8 and 9). The size of samples No. 8 and 9 was 4 mm x 20 mm x 40 mm. In the following, the layered object may be simply referred to as the object.

The following four types of raw alloy powders for additive manufacturing were prepared. The raw powders were prepared for melting and melted in a normal high-frequency vacuum melting furnace to produce master alloys, which were then produced by gas atomization in an argon atmosphere. Powders with particle sizes of 53 to 150 μm were classified from the atomized powder and used for additive manufacturing. The d10, d50, and d90 of the classified powders were d10: 58 μm, d50: 92 μm, and d90: 148 μm, respectively.

Alloy composition of samples No. 8 and 9:
Mo; 19at. %Cr; 19at. % Ni; 28at. % Al; 34at. %
Additive manufacturing conditions: Equipment: Lasertec 65 manufactured by DMG Mori Seiki Co., Ltd. Overlay welding by laser metal deposition (LMD) Base material: Alloy 718
Laser light scanning speed: 1000 mm/min
Raw material powder supply amount: 14g/min
Laser light output: 1.6 kW (sample No. 8)
2.0 kW (Sample No. 9)
[Example 4]
In Example 4, a shaped body was produced by preheat additive manufacturing using LMD (sample No. 10).
The size and alloy composition of sample No. 10 are the same as those of samples No. 8 and No. 9.

<Additive manufacturing conditions>
Equipment: Lasertec 65 manufactured by DMG Mori Seiki Co., Ltd. Overlay welding by laser metal deposition (LMD) Base material: Alloy 718
Laser light scanning speed: 1000 mm/min
Raw material powder supply amount: 14g/min
Laser light output: 1.6 kW
Preheat temperature: 500°C
[Considerations on Samples No. 8, 9, and 10 (shaped bodies) and Sample No. 7 (ingot)]
The liquidus temperature and Rockwell hardness (HRC) of Samples No. 8 and No. 9 were determined in the same manner as in Examples 1 and 2 (the polished cross section was measured at two points and the average was calculated). The results are shown in Table 3 together with the results of Sample No. 7 (ingot) prepared in Example 2.

As shown in Table 3, samples No. 7 to 9 have the same alloy composition, and therefore all have a liquidus temperature of 1540°C. The alloy compositions of samples No. 7 to 9 fall within the range of chemical composition II, and with chemical composition II, it is possible to obtain a hardness of 45 HRC or more, and even close to or equal to 50 HRC, while keeping the liquidus temperature at 1600°C or less, and even 1550°C or less. In other words, it has been confirmed that chemical composition II is an effective composition when producing an alloy composition by additive manufacturing.

The ingot (sample No. 7) and the shaped body (sample No. 8) were subjected to heat treatment (heat treatment conditions: held in vacuum at 700°C for 10 hours) to determine the hardness after the heat treatment. The results are shown in Table 3. The "hardness after heat treatment" was determined by converting the Vickers hardness (HV) measured by the same method as above into Rockwell hardness (HRC). Comparing the "hardness" and "hardness after heat treatment" in Table 3, it was confirmed that there was almost no change in hardness before and after the heat treatment, and that the samples before and after the heat treatment had the same hardness. In other words, with this alloy composition, the structure created during the solidification process was stable at high temperatures, so it is considered that the softening resistance was high. In other words, an alloy composition within the range of chemical composition II is also suitable for components that require heat resistance, such as die-casting dies that are prone to Al melting.

Figure 9 shows an SEM image of sample No. 8 (laser light output: 1.6 kW), and Figure 10 shows an SEM image of sample No. 9 (laser light output: 2.0 kW). Table 4 shows the arm widths W1 and W2 and the area ratio of dendritic structures measured for sample No. 8 and sample No. 9, along with the results for samples No. 5 to 7. The method for measuring arm widths W1 and W2 is as described in Example 1. The area ratio of dendritic structures for sample Nos. 8 and 9 was observed after polishing the cross section parallel to the modeling direction (side surface of the layered model).

As shown in Figures 9 and 10, Samples No. 8 and 9, which were produced by the additive manufacturing method, also have a composite structure of dendritic and interdendritic structures. Comparing Figures 9 and 10, it can be seen that Sample No. 8 (Figure 9) has a finer structure than Sample No. 9 (Figure 10). This is because Sample No. 8, which has a lower laser light output (laser light output: 1.6 kW), has a faster cooling rate than Sample No. 9 (laser light output: 2.0 W), and therefore forms a finer dendritic structure (dendrite).

Furthermore, by comparing Fig. 6 (sample No. 5, ingot) and Fig. 7 (sample No. 6, ingot) shown in Example 2 with Fig. 9 (sample No. 8, additively manufactured body) and Fig. 10 (sample No. 8, additively manufactured body) of this example, it was confirmed that the structure of the additively manufactured body was significantly finer than that of the ingot. This is also supported by a comparison of the arm widths W1 and W2 of samples No. 8 and 9 (manufactured bodies) shown in Table 4 with the arm widths W1 and W2 of samples No. 1, 5 to 7 (ingots). In samples No. 1, 5 to 7 (ingots) obtained by casting and solidification, the arm width W1 ranges from 8 to 20 μm and the arm width W2 ranges from 7 to 30 μm, and the dendritic structure is thicker.

On the other hand, in samples No. 8 and 9 obtained by additive manufacturing, the arm widths W1 and W2 can both be suppressed to 10 μm or less, and even 6 μm or less. In sample No. 8, the arm widths W1 and W2 can both be suppressed to 3 μm or less. This reduces the mean free path of the Ni-Al interdendritic structure, resulting in small and dispersed areas with low corrosion resistance, which prevents localized corrosion and is believed to result in improved corrosion resistance of the entire alloy composition. In other words, the Mo-Cr dendritic structure, which has high corrosion resistance, exists in a mesh shape, and Ni-Al interdendritic structure, which has lower corrosion resistance than the Mo-Cr dendritic structure, exists to fill the gaps, so it is presumed that the corrosion resistance of samples No. 8 and 9 obtained by additive manufacturing is improved.

The area ratio of the dendritic structure of the molded bodies, Samples No. 8 and 9 (Sample No. 8: 59%, Sample No. 9: 56%), is lower than the area ratio of the dendritic structure of Sample No. 1 (ingot) (68%). However, since the additively molded bodies have very fine dendritic structures as described above, it is believed that Samples No. 8 and 9 (molded bodies) can achieve the same corrosion resistance as Sample No. 1 (ingot).

As mentioned above, the liquidus temperature of sample No. 1 (ingot) is 1630°C, while the liquidus temperature of samples No. 8 and 9 (modeled bodies) is 1540°C. In other words, the liquidus temperature of samples No. 8 and 9 is 90°C lower than that of sample No. 1. The lower melting point of the alloy composition is advantageous when applying the alloy composition to additive manufacturing methods such as laser metal deposition (LMD). If the temperature of the alloy composition to be modeled is high, the temperature of the molten pool during modeling becomes high, that is, the temperature difference with room temperature is large, and the amount of shrinkage that occurs during cooling is large, making the model prone to cracking. With a lower melting point, the temperature difference is small, so the material is less susceptible to the effects of thermal shrinkage that occurs after modeling, and the formation of beads welded to each other proceeds smoothly, making stable modeling possible. In addition, the low melting point of the alloy composition allows the raw materials to melt without remaining unmelted when subjected to typical laser light output, and degassing and deposition are promoted, making it easier to form defect-free shaped bodies.

A compositional analysis of the dendritic and interdendritic structures of samples No. 8 and 9 was performed using Energy Dispersive X-ray Spectroscopy (EDX). The results are as follows. From the EDX results, it was found that the dendritic structure also contains Ni and Al. It was also confirmed that there is almost no Mo in the interdendritic structure, but that there is a trace amount of Cr.

Comparing the EDX results of sample No. 1 (ingot) produced in Example 1 with the EDX results of samples No. 8 and 9 (shaped bodies) produced in this example, the shaped bodies have a greater amount of Al and Ni present in the dendritic structure. For this reason, it is believed that the B2 ordered phase, mainly composed of Ni and Al, is also present in the dendritic structure of the shaped bodies. The B2 ordered phase will be discussed later.

<Sample No. 8>
Dendritic structure:
Mo; 46.7 at. % Cr; 33.1at. %
Ni; 4.4at. % Al; 15.8 at. %
Interdendritic tissue Ni; 47.0 at. % Al; 49.7at. %
Mo; 0.5at. %Cr; 2.8at. %
<Sample No. 9>
Dendritic structure:
Mo; 53.4 at. %Cr; 29.3at. %
Ni; 2.8at. % Al; 14.5at. %
Interdendritic tissue Ni; 47.7at. % Al; 49.7at. %
Mo;0at. %Cr; 2.6at. %
Fig. 11 is a diagram showing the results of crystal structure analysis by XRD of Samples No. 8 and 9 prepared in this example. Note that Fig. 11 also shows the results of XRD of Sample No. 7, which has the same composition as Samples No. 8 and 9.

As shown in Figure 11, it was confirmed that, like sample No. 7, samples No. 8 and 9 also contain a mixture of the bcc phase and NiAl (B2) phase. Since samples No. 7 to 9 have the same composition, no significant differences were observed in the peak positions of the bcc phase and NiAl (B2) phase between the samples. However, when comparing sample No. 8 and sample No. 9, sample No. 8 (laser light output: 1.6 kW), which has a lower laser light output, has a higher maximum peak intensity of the bcc phase and NiAl (B2) phase.

Figure 12 shows the results of the element distribution evaluation of the dendritic structure for sample No. 7 (ingot). The results of the surface analysis by EPMA are shown in the left figure of Figure 12. For the surface analysis by EPMA, a JXA8500F manufactured by JEOL was used. The analysis conditions were acceleration voltage: 15 kV, irradiation current: 0.02 A, beam diameter: φ0.1 μm, step size: 0.2 μm, irradiation time: 30 msec, number of spots: 250 x 250 points, and surface analysis was performed in a range of 50 μm x 50 μm. The obtained analysis results were converted to concentration display (mass%), and line analysis evaluation was performed at any position so as to cross the dendritic structure, thereby evaluating the element distribution of the dendritic structure. Linear analysis allows the evaluation of the chemical composition at each analysis point. Here, Cr and Mo were focused on, and their concentration ratio Cr/Mo ratio was evaluated.

As a result, as shown in the right diagram of Figure 12, it was confirmed that in the dendritic structure of sample No. 7, there are regions where the Cr/Mo ratio differs between the edge and center of the dendrite. When this analysis evaluation was performed on sample No. 7, the Cr/Mo ratio in the center was approximately 0.4, while there were regions at the edge where the Cr/Mo ratio was 0.6 or more.

The right diagram in Figure 12 also shows that Al and Ni are contained within the Mo-Cr dendritic structure.

Sample No. 8 (shaped body), which has the same chemical composition as Sample No. 7, was also evaluated for element distribution in the dendritic structure in the same manner as Sample No. 7 (ingot). As a result, it was confirmed that the dendritic structure of Sample No. 8 also had regions where the Cr/Mo ratio differed between the dendrite edge and the dendrite center. Sample No. 8 also had regions where the Cr/Mo ratio in the center was approximately 0.4, but the Cr/Mo ratio was 0.6 or more at the edge.

To evaluate the microstructure within the dendritic structure, high-magnification observation was performed using a TEM (Transmission Electron Microscope) (magnification: 3000 times). Figure 13 shows a TEM image of sample No. 8 (laminated manufactured body, before heat treatment). In Figure 13, the dark gray area A is the dendritic structure, and the light gray area B is the interdendritic structure. Figure 14 shows the results of electron beam diffraction in the dendritic structure within area A in Figure 13. From the analysis of the electron beam diffraction image, the main phase of the dendritic structure was identified as a bcc structure, but slight spots of the B2 ordered phase were confirmed within the dendritic structure.

The conditions for electron diffraction are as follows:
Equipment model: Hitachi High-Tech Corporation Model HF-2100
Acceleration voltage: 200 kV
Selected area diffraction method Selected area: φ200 nm
15 is a scanning transmission electron microscope (STEM) image of the above-mentioned region A of sample No. 8 (laminated product). The left side is a bright-field image (BF-STEM image), and the right side is a dark-field image (DF-STEM image). The dark-field image was captured by adjusting the image so that diffraction of the B2 ordered phase shown in FIG. 14 occurs.

From the dark field image in FIG. 15, it can be seen that the B2 ordered phase (precipitated particles) is distributed as a granular structure with the maximum length of the large particles being about 170 nm. The area of the dark field image in FIG. 15 (area of the observation area) was about 5.76 μm ² (2.4 μm × 2.4 μm). The B2 ordered phase is formed when the molten part during additive manufacturing is quenched, the MoCr alloy structure with a high melting point solidifies while forming a dendritic structure, and the NiAl alloy with a relatively low melting point is extruded between the dendrites and solidifies, and Ni and Al taken into the MoCr dendritic structure precipitate during the cooling process or by heat treatment. It is believed that the hardness of the dendritic structure is improved due to the presence of such fine precipitates. Since this B2 ordered phase is stable even in a high-temperature environment of about 700 ° C used in an Al die casting mold, as shown in Table 3, sample No. No. 8 (layer-by-layer manufactured body) showed almost no change in hardness before and after the heat treatment at 700° C., and is therefore considered to have had high resistance to softening.

The TEM and STEM sample preparation procedures and observation conditions are as follows:

Figure 16 is a scanning transmission electron microscope (STEM) image of the above-mentioned region B of sample No. 8 (laminated product). The interdendritic structure of region B is a NiAl alloy, containing almost no Mo or Cr. Therefore, the B2 ordered phase seen in the dendritic structure was not observed.

<TEM and STEM sample preparation procedure>
One surface of sample No. 8 (layer-by-layer manufactured body) was polished to prepare a test piece having a thickness of about 100 nm by a microsampling method using a Focused Ion Beam (FIB). For the microsampling, a Hitachi High-Technologies Corporation FB-2100 model was used.

<TEM and STEM observation conditions>
Sample thickness: 100 nm
Equipment model: Hitachi High-Tech Corporation Model HF-2100
Acceleration voltage: 200 kV The preferred embodiments and examples of the present invention have been described above, but in addition to the above, it is possible to select and/or change the configurations given in the above embodiments to other configurations as appropriate without departing from the spirit of the present invention.

1 alloy composition 3 dendritic structure 5 interdendritic structure 10 primary dendrite 20 secondary dendrite W1 primary dendrite arm width W2 secondary dendrite arm width W3 secondary dendrite arm spacing

Claims (11)

A Mo-Cr dendritic structure;
and a Ni-Al interdendritic structure surrounding the Mo-Cr dendritic structure.
When Mo+Cr+Ni+Al=100 at. %,
Ni is 20 to 35 at. %,
Al is 20 to 35 at. %,
Cr is 10 to 50 at. %,
The balance is Mo and inevitable impurities.
Ni+Al=40-70 at. %, Mo+Cr=30-60 at. %,
An alloy composition, characterized in that it is an additively manufactured body in which fine particles having a B2 ordered phase with a maximum particle size of 300 nm or less are precipitated within the Mo-Cr-based dendritic structure. 2. The alloy composition according to claim 1 , wherein the Mo-Cr dendritic structure accounts for 50 to 70% of the area of the entire structure. The alloy composition according to claim 1 or 2 , wherein Ni+Al=40-50 at. %, Mo+Cr=50-60 at. %. The alloy composition according to any one of claims 1 to 3 , further comprising C, B and Si in a total amount of 0.01 to 8.0 at. %. The alloy composition according to any one of claims 1 to 4 , wherein within the Mo-Cr dendritic structure, there are regions with different Cr/Mo ratios. The alloy composition of claim 5 , wherein the Cr/Mo ratio in the Mo-Cr based dendritic structure is higher at the dendritic edge than at the center of the dendritic structure. The alloy composition according to any one of claims 1 to 6 , wherein the Mo-Cr dendritic structure has a maximum arm width or arm spacing of 10 µm or less. The alloy composition according to any one of claims 1 to 7 , having a Rockwell hardness of 45 HRC or more. The method for producing the alloy composition according to any one of claims 1 to 8, wherein a raw material powder containing one or both of a simple metal powder of Mo, Cr, Ni or Al, and an alloy powder consisting of two or more metals selected from Mo, Cr, Ni, and Al is melted and solidified to perform layered manufacturing. The method according to claim 9 , wherein the additive manufacturing is performed while preheating the base material to 400°C to 800°C. A mold using the alloy composition according to any one of claims 1 to 8 as a repair material.

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