FeCo alloy metal additive fabrication structure and manufacturing method

Abstract

To provide an additively manufactured body using an FeCo-based alloy formed by additive manufacturing, which exhibits excellent soft magnetic properties by controlling the crystal grains to a suitable state. [Solution] A metal additively fabricated body made of an FeCo alloy in which, by mass%, Co: 45.0~55.0%, V ≤ 0~2.5% (including 0%), the remainder being Fe and unavoidable impurities, and the content of small-angle grain boundaries is 10% or less.

Description

Technical Field

[0001] The present invention relates to a laminated structure formed by laminating using an FeCo-based alloy suitable for a rapid melting and rapid solidification process of a three-dimensional laminated manufacturing method, and a method for manufacturing the laminated structure.

Background Art

[0002] Permendur having a representative composition of Fe49Co2V (the numerical values are in mass %) is an FeCo-based alloy known as a soft magnetic material with low coercive force and high saturation magnetic flux density. And Permendur is used in small and high-output motors, solenoid valves requiring high magnetic force, pole pieces of electronic devices, etc. However, since Permendur is a difficult-to-machine material that is embrittled by the precipitation of the B2 regular phase, it is not easy to process a steel material containing the B2 regular phase into a component.

[0003] It would be convenient if parts could be manufactured in a near-net shape by metal additive manufacturing. However, simply performing additive manufacturing with the component composition of Permendur does not exhibit the original soft magnetic properties of Permendur. For example, in a summary collection of research publications applying metal additive manufacturing to Fe50Co (see Non-Patent Document 1), although X-ray diffraction data of the additive manufactured object is shown, although peaks other than the α phase are observed, the B2 regular phase is not recognized in those peaks, and the soft magnetic properties such as those of the melted material of Permendur are not exhibited, and it could not be said to be sufficient as a formed body.

Prior Art Documents

Non-Patent Documents

[0004]

Non-Patent Document 1

Summary of the Invention

[0005] The present invention aims to provide a metal additively fabricated structure made of an FeCo-based alloy that exhibits excellent soft magnetic properties.

[0006] To achieve excellent soft magnetic properties, it is necessary to coarse the crystal grains, but in metal additive manufacturing, the crystal grains become finer. Simply applying Permendur to metal additive manufacturing will not result in soft magnetic properties like those of molten materials. Therefore, it is considered necessary to consider coarsening the crystal grains in order to obtain excellent soft magnetic properties, but the fabrication of additively manufactured bodies with intentionally coarse crystal grains using Permendur has not been attempted until now.

[0007] Therefore, the problem that the present invention aims to solve is to provide an additively manufactured body made of an FeCo-based alloy formed by additive manufacturing, which exhibits excellent soft magnetic properties by controlling the crystal grains to a suitable state, and a method for manufacturing the same. [Means for solving the problem]

[0008] The inventors discovered that by controlling the heat treatment conditions of the additively fabricated body, they could obtain a metal additively fabricated body with a small-angle grain boundary content of 10% or less. Furthermore, they found that by reducing the proportion of small-angle grain boundaries and coarsening the crystal grain boundaries, they could obtain an additively fabricated FeCo-based alloy with excellent soft magnetic properties.

[0009] Therefore, the first means for solving the problems of the present invention is a metal additively fabricated body made of an FeCo alloy consisting of, by mass%, Co: 45.0~55.0%, V ≤ 0~2.5% (including 0%), with the remainder being Fe and unavoidable impurities, and having a content of small-angle grain boundaries of 10% or less.

[0010] The second method is a metal additively fabricated body according to the first method, wherein the average crystal grain size determined by EBSD is 10 μm or more.

[0011] The third method is a metal additively fabricated body according to the first or second method, having a hardness of 99 HRB or less.

[0012] The fourth method is a method for manufacturing a metal additive body made of an FeCo alloy, which includes a heat treatment step in which alloy powder made of an FeCo alloy is layered and rapidly melted and solidified to form a metal additive body, the metal additive body is held at 1000 to 1170°C for 1 to 8 hours, and then held at 800 to 980°C for 3 to 10 hours before being cooled to room temperature.

[0013] Another method is a method for manufacturing a metal additive body made of an FeCo alloy having a small-angle grain boundary content of 10% or less, comprising a heat treatment step in which the metal additive body is held at 1000 to 1170°C for 1 to 8 hours, then held at 800 to 980°C for 3 to 10 hours, and then cooled to room temperature. [Effects of the Invention]

[0014] According to the means of the present invention, it is possible to obtain a metal additively manufactured FeCo alloy body that, while being manufactured by additive manufacturing, has a low coercivity of 200 A / m or less, a high permeability of 5000 emu or more, and excellent soft magnetic properties, similar to that of a molten material.

[0015] Furthermore, by annealing the additively manufactured body in the two-step process described in the present invention, a metal additively manufactured body made of an FeCo-based alloy having a small-angle grain boundary content of 10% or less can be obtained. Therefore, by coarsening the grain boundaries, a metal additively manufactured body with excellent soft magnetic properties can be obtained. [Brief explanation of the drawing]

[0016] [Figure 1]The IQ map created based on the measurement results by EBSD is shown in grayscale. (a) is Example 1, and (b) is Comparative Example 1. The original figure is in color, where the part showing the grain boundaries in blue (the thick gray grain boundaries in Figure 1) is the large-angle grain boundaries, and the part shown in red (the slightly lighter gray grain boundaries in Figure 1) is the small-angle grain boundaries. [Figure 2] It is a figure showing only the small-angle grain boundary part among the map figures of FIGS. 1(a) and (b). It is the part of the small-angle grain boundary shown in red in the original figure and appears as the slightly lighter gray grain boundary in the grayscale Figure 1. [Figure 3] It is a figure showing only the large-angle grain boundary part among the map figures of FIGS. 1(a) and (b). It is the part of the large-angle grain boundary shown in blue in the original figure and appears as the thick gray grain boundary in the grayscale Figure 1. [Figure 4] It is a graph showing the relationship between hardness and coercive force. [Figure 5] It is a graph showing the relationship between the reciprocal of crystal grain size and coercive force. [Figure 6] It is a graph showing the relationship between the small-angle grain boundary content rate and the maximum magnetic permeability. [Figure 7] It is an explanatory diagram showing an example of the procedure of two-step annealing with the vertical axis being temperature and the horizontal axis being time. [Figure 8] It is an explanatory diagram of the two-step annealing and other procedures.

Embodiments for Carrying out the Invention

[0019] The FeCo-based alloy is an alloy system mainly composed of Fe and Co and the balance consisting of inevitable impurities. In the FeCo-based alloy referred to in the present invention, it also includes the case where a small amount of V is contained as an optional additive component.

[0020] Co: 45.0 to 55.0% Co is a basic component for obtaining a magnetic material. Therefore, in the present invention, Co is set to 45.0 to 55.0%. When Co is less than 45%, the magnetic permeability becomes low and the coercive force also becomes high. From this perspective, the lower limit of Co is preferably 47.0% or more, more preferably 48.0% or more, and still more preferably 49.0% or more. On the other hand, when Co exceeds 55%, the saturation magnetic flux density becomes small and the coercive force also becomes high. From this perspective, the upper limit of Co is preferably 53.0% or less, more preferably 52.0% or less, and still more preferably 51.0% or less.

[0021] V: 0 to 2.5% V is a component that can be optionally added to the present invention as a component for improving workability, but V may be 0%. When the B2 regular phase is excessive, the toughness is inferior and the workability deteriorates, so adding V improves the workability. However, when V becomes excessive, in addition to the saturation magnetic flux density decreasing, the coercive force also increases. Therefore, the addition of V is set to 0 to 2.5% or less (V includes 0%).

[0022] In addition, since the present invention is a laminated formed body by a laminated manufacturing method, the shape of near net shape can be formed in advance, so the amount of V added can be reduced by prioritizing soft magnetic properties over workability. From this perspective, V is 2.0% or less, more preferably 1.0% or less, and still more preferably 0.5% or less. Also, V may be 0%.

[0023] Balance: Fe and inevitable impurities Fe generally has a high saturation magnetic moment, and by adding Co, it is possible to achieve excellent saturation magnetic flux density as an FeCo alloy.

[0024] [Powder] This section describes the additive manufacturing powder used for additively manufactured bodies made of the FeCo alloy of the present invention. FeCo alloy powder used for additive manufacturing can be obtained by various methods, including water atomization, single-roll quenching, twin-roll quenching, gas atomization, disc atomization, and centrifugal atomization. However, gas atomized powder is preferred from the viewpoint of spheroidization and other factors. Furthermore, from the viewpoint of the fluidity and packing efficiency of the powder used for additive manufacturing, it is desirable that the average particle size of the FeCo alloy powder be 10 μm or more and 100 μm or less on a volume average basis. Therefore, the following description will use gas atomized powder as an example.

[0025] [molding] One method for fabricating objects is a rapid melting and quenching solidification process, which involves melting and solidifying metal powder. Specific examples of this process include three-dimensional additive manufacturing, thermal spraying, laser coating, and cladding. In particular, the FeCo alloy powder of the present invention is suitable for powder bed fusion bonding three-dimensional additive manufacturing, enabling the formation of large-sized objects at high density.

[0026] As a three-dimensional additive manufacturing method, for example, a 3D printer can be used to obtain near-net-shape objects with complex three-dimensional shapes. In the powder bed fusion method of additive manufacturing, a laser beam or electron beam is irradiated onto the FeCo-based alloy powder of the present invention that is spread out. The particles are rapidly heated and rapidly melted by the irradiation. The molten particles then rapidly solidify. Through this melting and solidification, the particles bond together. The irradiation is selectively applied to a portion of the spread out FeCo-based alloy powder. The parts of the spread out powder that are not irradiated do not melt. A bonding layer is formed only in the irradiated parts.

[0027] A thin layer of FeCo alloy powder is then spread on top of the bonding layer. A laser beam or electron beam is irradiated onto a portion of this FeCo alloy powder. The irradiation causes the particles to melt rapidly. The melted particles then solidify rapidly. This melting and solidification process causes the particles in the powder to bond together, forming a new bonding layer. This new bonding layer also bonds with the existing bonding layer.

[0028] Through repeated irradiation-induced bonding, an aggregate of bonded layers gradually grows. This growth results in a three-dimensional object. This additive manufacturing method allows for the easy creation of objects with complex shapes.

[0029] [Heat treatment] By using FeCo-based alloy powder in additive manufacturing, instead of using the formed unheat-treated object as is, an annealing heat treatment process is performed on the unheat-treated object to obtain an additive manufacturing object with the desired properties of the present invention.

[0030] When FeCo alloys are used in conventional processes such as forging, they are annealed at temperatures below 900°C, which is in the α single-phase region.

[0031] In this invention, the soft magnetic properties of a metal additively manufactured body can be greatly improved by performing a two-stage annealing process.

[0032] The first annealing stage involves setting the heat treatment temperature (T1) to 1000-1170°C and holding it for a holding time (t1) of 1-8 hours. By setting the heat treatment temperature (T1) to 1000°C or higher, which is in the γ region, fine crystal grains are coarsened, thereby obtaining a molded body with coarse crystal grains. From the viewpoint of coarsening, a heat treatment temperature of 1050°C or higher is preferable, 1100°C or higher is more preferable, and 1150°C or higher is even preferable.

[0033] Subsequently, the material is cooled from T1 at a cooling rate of 100 to 1000°C / hr. Alternatively, as illustrated in Figure 7, the material may be cooled from T1 to the second annealing heat treatment temperature (T2) and the second annealing stage may be carried out immediately, or, as illustrated in Figure 8, the material may be cooled from T1 to room temperature and then heated again to the T2 temperature for the second annealing stage.

[0034] The heat treatment temperature (T2) for the second stage of annealing should be in the range of 800 to 980°C, and the material should be held for a holding time (t2) of 3 to 8 hours. By setting T2 to a range of 800-980°C, the proportion of small-angle grain boundaries can be reduced, resulting in coarser crystal grains. In the second annealing stage, heat treatment in the α region (below 980°C) further reduces the proportion of small-angle grain boundaries, making the crystal grains even coarser. The closer the heat treatment temperature is to the upper limit of the α region, the greater the effect of reducing the proportion of small-angle grain boundaries. Therefore, it is desirable to determine the heat treatment temperature for the second stage in consideration of the upper limit of the α region. For example, if the α region is 980°C or lower, the heat treatment temperature for the second annealing stage is preferably 840 to 980°C, more preferably 880 to 980°C, and even more preferably 920 to 980°C.

[0035] By keeping the cooling rate after annealing below 2000°C / hr, not only is distortion due to rapid cooling suppressed, but the precipitation of the B2 ordered phase can be promoted, thereby improving the soft magnetic properties. From these viewpoints, a cooling rate of 600°C / hr or less is preferred, and 100°C / hr or less is more preferred.

[0036] In the first heat treatment stage, the majority of the material consists of large-angle grain boundaries, but some small-angle grain boundaries remain.

[0037] In the second stage, annealing in the alpha region aligns the crystal orientations of slightly misaligned grains, causing them to coarseen as they merge into one large grain. In other words, the proportion of small-angle grain boundaries decreases, resulting in coarser grain size.

[0038] Whether the annealing heat treatment is performed in two consecutive stages as shown in Figure 7, or divided into two steps as shown in Figure 8, the effect of improving magnetic properties can be obtained in the same way.

[0039] In this application, boundaries with a crystal orientation difference of 5° or more are identified as crystal grain boundaries, and the crystal grain size is calculated. Furthermore, it is determined whether the grain boundary is a small-angle grain boundary or a large-angle grain boundary based on the difference in the surrounding area of ​​adjacent crystals.

[0040] A small-angle grain boundary is a grain boundary where the difference in orientation between adjacent crystals is small. Specifically, a small-angle grain boundary is defined as one where the difference in crystal orientation between adjacent crystal grains is less than 5 to 15°. A large-angle grain boundary is a grain boundary where the difference in crystal orientation between adjacent grains is 15° or more.

[0041] [Evaluation of grain boundaries using EBSD] EBSD (Electron Backscatter Diffraction) is an electron beam diffraction method that uses a detector mounted on a scanning electron microscope (SEM). By irradiating the surface of a sample tilted at approximately 60-70° with an electron beam, diffracted electron beams are obtained from each crystal plane in the region below approximately 50 nm from the sample surface, according to Bragg's law (λ=2dsinθ). By analyzing the resulting diffraction pattern, information on the orientation of crystalline samples can be obtained. The diffraction pattern can be fitted to the crystal information of the target material by detecting bands using the Hough transform method. Unlike the average crystal information obtained by XRD, EBSD allows for the evaluation of the crystal orientation of individual crystal grains. Therefore, grain boundaries can be determined based on the crystal orientation from the EBSD measurement results.

[0042] [Crystal grain size] Average grain size: 10μm or more The grain size is determined based on the grain boundaries determined by EBSD measurements. Larger grain sizes result in lower coercivity, higher maximum permeability, and improved soft magnetic properties. In additive manufacturing methods using rapid quenching and solidification, the grain size tends to be finer. Therefore, by controlling the average grain size to 10 μm or more through heat treatment of the additively manufactured body, excellent soft magnetic properties can be obtained. The grain size is preferably 10.5 μm or more, more preferably 11.0 μm or more, and even more preferably 48.0 μm or more. The average grain size can be calculated by measuring the area of ​​the crystals within each particle using image analysis software and averaging their equivalent circle diameters.

[0043] [Percentage of small angle grain boundaries] The proportion (content) of small-angle grain boundaries can be measured and calculated by EBSD. The smaller the proportion of small-angle grain boundaries, the coarser the crystal grains can be made, resulting in superior soft magnetic properties. By setting the proportion (content) of small-angle grain boundaries to 10.0% or less, the coercivity is reduced, the maximum permeability is increased, and the soft magnetic properties can be improved. The proportion of small-angle grain boundaries is preferably 9.0% or less, more preferably 8.0% or less, and even more preferably 5.5% or less.

[0044] [Annealed hardness] Hardness: 99 HRB or less By setting the annealed hardness of the additively fabricated body to 99 HRB or less, it is possible to obtain a fabricated body with low strain and excellent soft magnetic properties of coarse crystal grains. Therefore, the annealed hardness is set to 99.0 HRB or less. Preferably, the annealed hardness is 98.0 HRB or less, more preferably 97.0 HRB or less, and even more preferably 97.0 HRB or less.

[0045] [Coercivity] The coercivity can be measured using a BH tracer. Since FeCo alloys are used as soft magnetic materials, a lower coercivity is preferable. Therefore, the preferred coercivity of the additively manufactured body of the present invention is 200 A / m or less. A more preferred coercivity of the additively manufactured body of the present invention is 180 A / m or less, and an even more preferred coercivity is 145 A / m or less.

[0046] [Maximum permeability] The maximum magnetic permeability can be measured using a BH tracer. Since FeCo alloys are used as soft magnetic materials, a lower maximum magnetic permeability is preferable. Therefore, the preferred maximum magnetic permeability of the additively manufactured body of the present invention is 5000 emu or higher. A more preferred coercivity of the additively manufactured body of the present invention is 6000 emu or higher, and an even more preferred coercivity is 8000 emu or higher.

[0047] [Examples] Tables 1 and 2 show the chemical composition of the powders used in Examples 1 to 12 and Comparative Examples 1 to 4 of the present invention. The composition of the additively fabricated bodies using these powders is also substantially the same.

[0048] [Table 1]

[0049] [Table 2]

[0050] (Made from powder) The powders of Examples 1-12 and Comparative Examples 1-4 were prepared by the vacuum melting inert gas atomization method. Specifically, in a vacuum, raw materials having the predetermined compositions listed in Tables 1 and 2 were heated by high-frequency induction heating in an alumina crucible to melt them. After melting, the molten metal was dropped from a nozzle with a diameter of 5 mm below the crucible, and high-pressure argon gas was sprayed onto the molten metal to obtain powder, which was then classified using a -63 μm sieve.

[0051] (About additive manufacturing) Using these powders as raw materials, additive manufacturing was performed using a three-dimensional additive manufacturing system (EOS-M280) to obtain an additively manufactured body consisting of a 10 × 10 × 10 mm rectangular parallelepiped. The manufacturing conditions were based on the system's standard parameter MS1 (layer thickness 40 μm), but with the output changed to 300 W and the scanning speed to 1100 mm / s. This additively manufactured body was used as a test specimen for coercivity measurement.

[0052] (Regarding heat treatment conditions) Table 1 shows the heat treatment conditions for the example additive-built bodies, and Table 2 shows the heat treatment conditions for the comparative example additive-built bodies. Annealing was performed under furnace cooling conditions in a vacuum atmosphere, and the cooling rate was set to 100°C / hr in all cases.

[0053] (Evaluation of crystal grain size) The grain size was evaluated by EBSD. A JEOL JSM-7001F was used, and measurements were performed under the following conditions. Acceleration voltage: 20kV Observation magnification: 100x EBSD measurement area: 800 μm × 800 μm Measurement step: 1 μm / step EBSD measurement phase: α-Fe

[0054] The grain size is calculated from the IQ map measured by EBSD. Boundaries where the crystal orientation differs by 5° or more were identified as grain boundaries, and the grain size was calculated. Grain boundaries with crystal orientations between 5° and less than 15° were classified as small-angle grain boundaries, and those with crystal orientations of 15° or more were classified as large-angle grain boundaries, and these were color-coded on the IQ map.

[0055] (Regarding the proportion of grain boundaries with small inclination angles) Based on the crystal orientation determined by EBSD measurements, small-angle and large-angle grain boundaries within an 800 μm × 800 μm measurement area were identified, and then the length of each grain boundary was calculated. The proportion (content) of small-angle grain boundaries is the ratio of the length of small-angle grain boundaries to the total length of all grain boundaries. (Proportion of small-angle grain boundaries) = (Total length of small-angle grain boundaries) / ((Total length of small-angle grain boundaries) + (Total length of large-angle grain boundaries))

[0056] (Regarding the measurement of annealed hardness) The Rockwell hardness (HRB) of the heat-treated additively manufactured bodies was measured using a Rockwell hardness tester in accordance with JIS Z2245.

[0057] (Measurement of coercivity and permeability) Coercivity and permeability were measured using a DC magnetization characteristic tester. Measurement device: Metron Giken Co., Ltd. DC magnetization characteristic tester MTR-1335 Specimen dimensions: Outer diameter 17.5 mm x Inner diameter 12.5 mm x Thickness 4 mm Applied magnetic field range: -50 Oe to 50 Oe

[0058] The results of each measurement are shown in Tables 1 and 2. Examples 1 to 12 of the present invention are additively manufactured metal bodies made of FeCo alloy produced by a two-stage annealing process, with the first stage annealing at 1100 to 1170°C and the second stage at 800 to 980°C. These additively manufactured bodies have an average grain size of 10 μm or more, a small-angle grain boundary content of 10% or less, a coercivity of 200 A / m or less, a maximum permeability of 5000 emu or more, and a hardness of 99 HRB or less, exhibiting excellent soft magnetic properties.

[0059] Comparative Example 1 was annealed at a higher temperature than usual, 1100°C, but the average grain size of the additively manufactured body was less than 10 μm, indicating insufficient coarsening. It also had a high content of small-angle grain boundaries, a hardness exceeding 99 HRB, a low maximum permeability, and excessively high coercivity, resulting in insufficient soft magnetic properties. Comparative Example 2 is a material annealed in Permendur at a typical temperature of 850°C. While 850°C is generally recommended because higher temperatures generate face-centered cubic gamma phases, degrading magnetism, materials manufactured using additive manufacturing tend to have fine grain sizes. As a result, the material after heat treatment had small grain sizes, a high content of small-angle grain boundaries, excessive hardness, and excessively high maximum permeability and coercivity, thus failing to exhibit soft magnetic properties. Comparative Example 3 had a low annealing temperature of 900°C in the first stage, and although it was repeated twice at the same temperature, the average grain size was small and coarsening was insufficient, the content of small-angle grain boundaries was high, the hardness was above 99 HRB, the maximum permeability was low, and the coercivity was too high, resulting in insufficient soft magnetic properties. In Comparative Example 4, the first stage of annealing was performed at a higher temperature than usual, but the annealing temperature in the second stage was too low. As a result, the proportion of small-angle grain boundaries was high, and coarse crystal grains could not be obtained. Consequently, the maximum permeability was low, the coercivity was too high, and soft magnetic properties could not be sufficiently obtained. [Industrial applicability]

[0060] Metal additively manufactured using the FeCo alloy according to the present invention exhibits excellent soft magnetic properties, making it applicable to components such as small, high-power motors, solenoid valves requiring high magnetic force, and pole pieces for electronic equipment.

Claims

1. A metal additively fabricated body made of an FeCo alloy consisting of, by mass%, 45.0-55.0% Co, V ≤ 0-2.5% (including 0%), with the remainder being Fe and unavoidable impurities, and having a small-angle grain boundary content of 10% or less.

2. The metal additive body according to claim 1, wherein the average crystal grain size determined by EBSD is 10 μm or more.

3. A metal additively fabricated body according to claim 1 or claim 2, wherein the hardness is 99 HRB or less.

4. By layering alloy powders made of FeCo-based alloys and rapidly melting and solidifying them, a metal additive manufacturing structure is formed. After holding the metal additively fabricated body at 1000-1170°C for 1-8 hours, Furthermore, it includes a heat treatment step in which the material is held at 800-980°C for 3-10 hours before being cooled to room temperature. A method for manufacturing a metal additive body made of FeCo-based alloy.

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