Metal additive manufactured article and method for producing the same

Abstract

To provide a metal additive manufactured article and its production method capable of reducing anisotropy in mechanical characteristics.MEANS FOR SOLVING THE PROBLEM: A metal additive manufactured article in which melt-solidified layers are laminated, wherein the average aspect ratio, defined as the ratio of the long axis length to the short axis length of crystal grains in a crystal orientation map image obtained by EBSD on a cross section perpendicular to the additive manufacturing surface, is 3.3 or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to metal additive manufactured products and methods for their manufacture. [Background technology]

[0002] In recent years, additive manufacturing has come into use as a method for producing metal components. Powder bed fusion, one of the additive manufacturing methods, involves spreading metal powder on a base plate mounted on a vertically movable modeling stage, and then melting and solidifying the powder using an energy beam emitted from a heat source to obtain a solidified layer. New metal powder is then spread on top of this solidified layer to obtain a further solidified layer. This process is repeated to obtain a three-dimensional metal additive product.

[0003] In such additive manufacturing methods, one of the parameters that affects the resulting metal additive product is the layer thickness of the metal powder. For example, Patent Document 1 discloses an additive manufacturing method in which a first region is manufactured from raw material powder layered at a first layer thickness of 20 μm or more and 55 μm or less, and a second region is manufactured from raw material powder layered at a second layer thickness of more than 55 μm and 85 μm or less. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-050921 Summary of the Invention [Problem to be solved by the invention]

[0005] If the layer thickness is small, it is not possible to improve the manufacturing efficiency and heat is easily input to the solidified layer formed under the metal powder. However, it is not known how to reduce the anisotropy of mechanical properties while increasing the layer thickness. High anisotropy in the mechanical properties of metal additive manufacturing products is undesirable because it reduces the strength in a specific direction.

[0006] The present invention has been made in view of the above circumstances, and provides an additive manufactured product with reduced anisotropy in mechanical properties, and a manufacturing method thereof. [Means for solving the problem]

[0007] The first invention is a metal additive manufactured product having a stack of molten and solidified layers, characterized in that the average aspect ratio, which is the ratio of the length of the long axis of the crystal grain to the length of the short axis of the crystal grain, in a crystal orientation map image obtained by the EBSD method for a cross section perpendicular to the additive manufactured surface is 3.3 or less.

[0008] In addition, for metal additive manufactured products in which melt-solidified layers are stacked, the average value of dislocation density calculated from the difference in crystal orientation between adjacent measurement points obtained by the EBSD method for a cross section perpendicular to or horizontal to the additive manufactured surface is 0.85 × 10 14 / m2 or less, and the average crystal grain size in both the vertical cross section and the horizontal cross section is 85 μm or less.

[0009] Furthermore, in the metal additively manufactured product, it is preferable that the difference between the 0.2% yield strength parallel to the additively manufactured surface and the 0.2% yield strength perpendicular to the additively manufactured surface is 17% or less of the larger 0.2% yield strength in either the parallel direction or the perpendicular direction, the difference between the tensile strength in the parallel direction and the tensile strength in the perpendicular direction is 15% or less of the larger tensile strength in either the parallel direction or the perpendicular direction, and the difference between the elongation in the parallel direction and the elongation in the perpendicular direction is 15% or less of the larger elongation in either the parallel direction or the perpendicular direction.

[0010] The method for producing a metal additive product according to the second aspect of the present invention is to provide a heat source energy density E of 20 J / mm 3 More than 85J / mm 3The method is characterized in that additive manufacturing is performed by selecting the layer thickness t from the range of more than 0.04 mm and not more than 0.12 mm, the scanning pitch a from the range of 0.09 mm or more and 0.11 mm or less, and the scanning speed v from the range of 600 mm / s or more and 1250 mm / s or less so that the layer thickness t is greater than 0.04 mm and not more than 0.12 mm, the scanning pitch a from the range of 0.09 mm or more and 0.11 mm or less, and the scanning speed v from the range of 600 mm / s or more and 1250 mm / s or less, and irradiating the laser beam or electron beam while adjusting the output power P from the range of 200 W or more and 350 W or less, and repeating melting and solidification. E=P / (v×a×t) (1) E: Heat source energy density (J / mm 3 ), P: laser beam or electron beam power (W), v: scanning speed (mm / s), a: scanning pitch (mm), t: layer thickness (mm) [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a metal additive manufactured product with reduced anisotropy in mechanical properties and a method for manufacturing the same. [Brief explanation of the drawings]

[0012] [Figure 1] This figure shows an example of an additive manufacturing device and additive manufacturing method using the Powder Bed Fusion (PBF) method. [Figure 2] (a) and (b) are IPF mapping images obtained by EBSD for the XY plane and the Z plane, respectively, of the additively manufactured product of Example 3. (c) and (d) are pole figures obtained by EBSD for the XY plane and the Z plane, respectively, of the additively manufactured product of Example 3. [Figure 3] (a) and (b) are GND density maps obtained by EBSD for the XY plane and Z plane of the additive product of Example 3, respectively. [Figure 4] (a) to (d) are diagrams showing the relationship between additive manufacturing conditions and defect rates for layer thicknesses of 0.06 mm, 0.08 mm, 0.10 mm, and 0.12 mm, which correspond to examples. (e) and (f) are diagrams showing the relationship between additive manufacturing conditions and defect rates for layer thicknesses of 0.04 mm and 0.15 mm, which correspond to comparative examples. [Figure 5](a) to (d) are graphs showing the relationship between additive manufacturing conditions and heat source energy density for layer thicknesses of 0.06 mm, 0.08 mm, 0.10 mm, and 0.12 mm corresponding to the examples. (e) and (f) are graphs showing the relationship between additive manufacturing conditions and heat source energy density for layer thicknesses of 0.04 mm and 0.15 mm corresponding to the comparative examples. [Figure 6] (a) and (b) are IPF mapping images obtained by EBSD for the XY plane and the Z plane, respectively, of the additively manufactured product of Example 4. (c) and (d) are pole figures obtained by EBSD for the XY plane and the Z plane, respectively, of the additively manufactured product of Example 4. [Figure 7] (a) and (b) are GND density maps obtained by EBSD for the XY plane and the Z plane of the additive product of Example 4, respectively. [Figure 8] (a) and (b) are IPF mapping images obtained by EBSD for the XY plane and the Z plane, respectively, of the additively manufactured product of Comparative Example 1. (c) and (d) are pole figures obtained by EBSD for the XY plane and the Z plane, respectively, of the additively manufactured product of Comparative Example 1. [Figure 9] (a) and (b) are GND density maps obtained by the EBSD method for the XY plane and the Z plane of the additive product of Comparative Example 1, respectively. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an embodiment for carrying out the present invention will be described in detail with reference to the accompanying drawings. First, an additive product of the present invention will be described, and then a manufacturing method of the additive product of the present invention will be described.

[0014] [Additive Manufacturing] The additively manufactured product of this embodiment is characterized by an average aspect ratio, which is the ratio of the length of the long axis of the crystal grain to the length of the short axis, of 3.3 or less in a crystal orientation map image obtained by the EBSD method for a cross section perpendicular to the additively manufactured surface.

[0015] (Melted solidified structure) Figure 2 shows an inverse pole figure (IPF) mapping image and a pole figure obtained by electron backscattering diffraction (EBSD) of the additively manufactured product according to this embodiment. Here, the cross section parallel to the additively manufactured surface is the XY plane, and the direction perpendicular to the additively manufactured surface (i.e., the additive manufacturing direction) is the Z direction. Figure 2(a) shows an IPF mapping image of the XY plane, and Figure 2(b) shows an IPF mapping image of a cross section parallel to the Z direction (hereinafter referred to as the Z plane). Figure 2(c) shows a pole figure of the XY plane, and Figure 2(d) shows a pole figure of a cross section parallel to the Z direction. In this case, the average aspect ratio of the crystal grains in the cross section perpendicular to the additively manufactured surface, i.e., the cross section parallel to the Z direction, of the additively manufactured product is 3.3 or less. In the additively manufactured product, the growth direction of the crystal grains is the Z direction, which is the additive manufacturing direction. Therefore, a small average aspect ratio of crystal grains in a cross section parallel to the Z direction means that crystal grains of a specific orientation do not grow large, and the crystal orientation of the structure is weak. Looking at Figures 2(c) and (d), there is no sign of a strong orientation in a specific crystal orientation. As will be discussed later, weak crystal orientation in additive manufacturing products results in low anisotropy of the mechanical properties and uniform mechanical properties. The preferred average aspect ratio is 3.0 or less, and more preferably 2.7 or less.

[0016] (average value of dislocation density) Figure 3 shows a dislocation (Geometrically Necessary Dislocations: GND) density map calculated from changes in crystal orientation, obtained by the EBSD method for the additive manufacturing product according to this embodiment. Figure 3(a) is a GND density mapping image in the XY plane, and Figure 3(b) is a GND density mapping image of a cross section parallel to the Z direction. GND density represents the area density of local dislocations within the field of view in the EBSD method. The GND density mapping image in Figure 3 shows a map of the magnitude of local dislocation density. Hereinafter, dislocation density refers to GND density. The average value of the dislocation density within a crystal grain is 0.85 × 10 in both the XY plane and the Z parallel. 14 / m 2It is preferable that the dislocation density is less than 0.05. As shown in Table 4 in the examples, the smaller the grain size, the smaller the average dislocation density within a single grain. In addition, a small dislocation density within a grain indicates a small plastic strain in the additive manufacturing product. This is also expected to improve the ductility of the additive manufacturing product. The average crystal grain size of such an additive product is preferably 85 μm or less in the XY plane and the Z plane. If the average crystal grain size is 85 μm or less, the average value of the dislocation density becomes sufficiently small as described above, and the anisotropy of the mechanical properties described below is reduced. A more preferable average crystal grain size is 80 μm or less.

[0017] As described above, additive products with structural characteristics such as low crystal orientation, low average aspect ratios of crystal grains, and low average dislocation densities reduce the anisotropy of mechanical properties. As a result, additive products with homogeneous mechanical properties are obtained. Specific mechanical properties are described below.

[0018] (mechanical properties) An additively manufactured product is obtained in which the difference between the 0.2% proof stress in the direction parallel to the additively manufactured surface and the 0.2% proof stress in the direction perpendicular to the additively manufactured surface is 100 MPa or less, more preferably 85 MPa or less, and even more preferably 70 MPa or less. Furthermore, the difference between the 0.2% proof stress in the direction parallel to the additively manufactured surface and the 0.2% proof stress in the direction perpendicular to the additively manufactured surface is preferably 17% or less of the larger 0.2% proof stress in either the parallel or perpendicular direction, more preferably 15% or less, and even more preferably 13.5% or less.

[0019] An additively manufactured product is obtained in which the difference between the tensile strength parallel to the additively manufactured surface and the tensile strength perpendicular to the additively manufactured surface is 100 MPa or less, more preferably 75 MPa or less, and even more preferably 60 MPa or less. Additionally, the difference between the tensile strength in the direction parallel to the additively manufactured surface and the tensile strength in the direction perpendicular to the additively manufactured surface is preferably 15% or less of the larger tensile strength in the parallel or perpendicular direction, more preferably 10% or less, and even more preferably 7.5% or less.

[0020] Additively manufactured articles are obtained in which the difference between the elongation parallel to the additively manufactured surface and the elongation perpendicular to the additively manufactured surface is 15% or less, more preferably 12% or less, and even more preferably 9% or less. Furthermore, the difference between the elongation in the direction parallel to the additively manufactured surface and the elongation in the direction perpendicular to the additively manufactured surface is preferably 25% or less of the larger of the elongation in the parallel direction and the perpendicular direction, more preferably 22.5% or less, and even more preferably 20% or less.

[0021] In this way, an additively manufactured product having the molten solidification structure and dislocation density characteristics of this embodiment can reduce the difference between the mechanical properties parallel to the additively manufactured surface and the mechanical properties perpendicular to the additively manufactured surface.

[0022] [Additive manufacturing method] In the manufacturing method of the additive manufacturing product of this embodiment, the heat source energy density J according to the following formula (1) is 20 J / mm 3 More than 85J / mm 3 The method is characterized in that additive manufacturing is performed by selecting a layer thickness t from the range of more than 0.04 mm and not more than 0.12 mm, a scanning pitch a from the range of 0.09 mm or more and less than 0.11 mm, and a scanning speed v from the range of 700 mm / s or more and 1250 mm / s or less, and irradiating a laser beam or electron beam while adjusting the output power P in the range of 200 W or more and 350 W or less, and repeating melting and solidification. J=P / (v×a×t) (1) J: Heat source energy density (J / mm 3 ), P: laser beam or electron beam power (W), v: scanning speed (mm / s), a: scanning pitch (mm), t: layer thickness (mm)

[0023] Although the additive manufacturing method is not particularly limited, it is preferable to use the powder bed fusion (PBF) method in that it is necessary to lay down the metal powder so that the layer thickness is more than 0.04 mm. An example of using the PBF method for additive manufacturing will be described below. Figure 1 shows an example of the schematic configuration of laser additive manufacturing (LPBF), a PBF method that uses laser light as a heat source for additive manufacturing.

[0024] The additive manufacturing process involves raising the powder supply stage 20 a predetermined distance, lowering the build stage 80 a predetermined distance, and then moving the recoater 30 in the X direction to supply metal powder 10 onto the build stage 80, forming a powder bed. Meanwhile, a laser beam 50 from a laser oscillator 40 is controlled by a galvanometer scanner 60 to irradiate a predetermined position on the powder bed, selectively melting and solidifying the powder to form a solidified layer. Next, the powder supply stage 20 is further raised a predetermined distance, and the build stage 80 is further lowered a predetermined distance. The recoater 30 then moves in the X direction to supply new metal powder 10 onto the build stage 80, selectively melting and solidifying this powder to form a new solidified layer. This process is repeated in the same manner to form a three-dimensional additive manufacturing object 70.

[0025] In the manufacturing method of the additive product of this embodiment, the heat source energy density J is controlled to obtain an additive product with weak crystal orientation and therefore low anisotropy in mechanical properties, as described above. The heat source energy density E is expressed by equation (1) using the layer thickness t of the metal powder layer, the scanning pitch a of the laser beam or electron beam, the scanning speed v, and the output P. E=P / (v×a×t) (1) E: Heat source energy density (J / mm 3 ), P: laser beam or electron beam power (W), v: scanning speed (mm / s), a: scanning pitch (mm), t: layer thickness (mm)

[0026] The layer thickness t is greater than 0.04 mm and less than or equal to 0.12 mm. The layer thickness is likely to affect the orientation of the crystal grains, the size of the crystal grains, and the average aspect ratio. Increasing the layer thickness reduces remelting in the solidified layer formed below the powder bed, thereby suppressing epitaxial growth. As a result, the growth of columnar crystals along the stacking direction (Z direction) is suppressed, reducing the average aspect ratio of the crystal grains in the Z plane. In addition, the number of remelting and solidification events in the solidified layer is reduced, thereby reducing the amount of strain accumulated in the additive product due to remelting and solidification. On the other hand, if the layer thickness is too large, defects will occur in the additive product. Therefore, the layer thickness t is greater than 0.04 mm and less than or equal to 0.12 mm. Preferably, it is 0.06 mm or more and 0.10 mm or less, and more preferably, it is 0.06 mm or more and 0.08 mm or less.

[0027] The scanning pitch a is less than 0.11 mm. The scanning pitch is likely to affect the orientation and size of crystal grains, as well as defects in the fused zone. If the scanning pitch is too large, the metal powder layer may not be sufficiently melted due to the balance with the diameter of the laser beam or electron beam. For example, in the examples described below, the beam diameter is 0.1 mm, and in this case, the scanning pitch a is less than 0.11 mm. On the other hand, as the scanning pitch increases, the number of times the solidified layer remelts and solidifies is reduced. This suppresses epitaxial growth, thereby reducing the average size and aspect ratio of the crystal grains in the Z plane, weakening the crystal orientation, and reducing the amount of strain accumulated in the additive product due to remelting and solidification. Therefore, the preferred scanning pitch is 0.09 mm or more and less than 0.11 mm.

[0028] The scanning speed v is set to 700 mm / s or more and 1250 mm / s or less. The scanning speed is likely to affect the orientation of the crystal grains, the size of the crystal grains, and defects in the molten zone. As the scanning speed increases, the cooling rate increases and the thermal impact on the additive product decreases, resulting in a smaller crystal grain size and weaker crystal grain orientation. On the other hand, if the scanning speed is too high, there is a risk that the metal powder layer will not be sufficiently melted. Therefore, the scanning speed v is set to 700 mm / s or more and 1250 mm / s or less. Preferably, it is set to 800 mm / s or more and 1250 mm / s or less, and more preferably, it is set to 800 mm / s or more and 1000 mm / s or less.

[0029] The power (P) of the laser beam or electron beam (hereinafter simply referred to as power) is 200 W or more and 350 W or less. Power is likely to affect the orientation and size of crystal grains and defects in the molten zone. Reducing the power reduces the heat input to the solidified layer, suppressing epitaxial growth. This reduces the average size and aspect ratio of crystal grains in the Z plane, weakening the crystal orientation and reducing the amount of strain accumulated in the additive product due to remelting and solidification. On the other hand, if the power is too low, the powder bed will be insufficiently melted, making defects more likely to occur. Therefore, the power should be 200 W or more and 350 W or less. Preferably, it should be 200 W or more and 300 W or less.

[0030] As a result of selecting the layer thickness t, scanning pitch a, scanning speed v, and laser beam or electron beam power P for the respective reasons as described above, the heat source energy density J calculated by equation (1) is 20 J / mm 3 More than 85J / mm 3It must be less than this. Generally, if the heat source energy density is too low, the metal powder layer may not be melted sufficiently, resulting in non-negligible defects in the additive product. On the other hand, if the heat source energy density is too high, the temperature of the molten pool may become too high, resulting in the formation of bubble-like defects. In addition, the thermal impact on the additive product may be large, causing crystal grains in a specific orientation to grow large. Therefore, the layer thickness t, scanning pitch a, scanning speed v, and power P in equation (1) above must each be selected to appropriate values. In particular, the layer thickness t must be selected appropriately, as it has the greatest effect on the average value of the size and aspect ratio of the crystal grains in the Z plane, and therefore on the crystal orientation. The heat source energy density is 20 J / mm 3 More than 85J / mm 3 The maximum is 30 J / mm. 3 More than 70J / mm 3 Less than or equal to 35 J / mm 3 More than 50J / mm 3 In particular, for Ni-based materials, the 3 Above, 30J / mm for Fe-based materials 3 It would be better to leave it at that.

[0031] In this way, if the molding conditions satisfy equation (1), an additive manufacturing product with weak crystal orientation and therefore low anisotropy in mechanical properties and few defects can be obtained. [Example]

[0032] First, additively manufactured objects (10mm x 40mm x 10mm high rectangular pillars, 10mm x 10mm x 40mm high rectangular pillars, and 10mm x 10mm x 10mm high rectangular pillars, with the height direction being the stacking direction) were fabricated using Ni-based alloy metal powder with the alloy composition (unit: mass%) shown in Table 1, and the relationship between the defect rate and the metal powder layer thickness t, laser beam scanning pitch a, scanning speed v, and laser output P was examined. This metal powder was produced by vacuum gas atomization and then sieved to a particle size of 10 to 53μm, with an average particle size (d50) of 35μm.

[0033] [Table 1]

[0034] (Relationship between molding conditions and defect rate) Figure 4 is a process map showing the relationship between each modeling condition and the defect rate of the additively manufactured product when additive manufacturing was performed under those conditions. Figure 5 also shows the relationship between each modeling condition and the heat source energy density at that time. The modeling conditions were varied as follows: layer thickness: 0.04 mm to 0.15 mm, scanning pitch: 0.09 mm to 0.11 mm, scanning speed: 600 mm / s to 1250 mm / s, and laser power: 200 W to 350 W. In Figures 4 and 5, a defect rate of less than 0.10% is considered low, and corresponding modeling conditions are shown in white. On the other hand, modeling conditions with a defect rate of 0.10% or higher are shown in gray.

[0035] The defect rate in this invention is the area ratio of defective parts determined by image processing of a cross-sectional photograph of an additive manufacturing product. The defect rate was measured using a microscope (Keyence VHX6000), which was binarized using a threshold value determined by the microscope's area ratio derivation function, and the area ratio of defective parts that appeared black was determined. The build density (%) was calculated by subtracting the area ratio of defective parts from the total area of the image (100%).

[0036] The process maps in Figures 4(a)-(d) show that the higher the scanning speed, the higher the laser power required to reduce the defect rate. This is because a high scanning speed shortens the time the laser beam irradiates a single point, resulting in a low heat source energy density unless the laser power is increased accordingly. Furthermore, the greater the layer thickness, the higher the laser power required to reduce the defect rate. This is because a larger layer thickness increases the volume of the point irradiated by the laser beam, resulting in a low heat source energy density unless the laser power is increased accordingly. In particular, when the layer thickness was 0.15 mm, the defect rate was often high even when the laser power was increased, and we determined that a layer thickness of up to 0.12 mm was preferable. From the above, it can be concluded that by increasing the laser power, additive manufacturing products can be obtained while maintaining a low defect rate, even when the layer thickness is increased.

[0037] (Relationship between layer thickness and structural and mechanical properties) Next, additive manufactured products of Examples 1 to 4 and Comparative Example 1 were produced by varying the layer thickness using metal powders with the alloy compositions (unit: mass%) shown in Table 1, and the relationship between the layer thickness and the structural and mechanical properties was investigated. This metal powder was also produced by the vacuum gas atomization method, and then sieved to have particle sizes of 10 to 53 μm and an average particle size (d50) of 35 μm.

[0038] Using a powder additive manufacturing system (EOS M290 manufactured by EOS Corporation), additive products (Examples 1 to 4, Comparative Examples 1 and 2) were produced by the LPBF method. The laser output was selected as follows based on the process map shown in Figure 2(a) to achieve the desired low defect rate.

[0039] In Example 1, the layer thickness was 0.06 mm, the laser output was 300 W, the laser scanning speed was 1200 mm / sec, and the scanning pitch was 0.09 mm.

[0040] In Example 2, the layer thickness was 0.08 mm, the laser output was 350 W, the laser scanning speed was 1150 mm / sec, and the scanning pitch was 0.09 mm.

[0041] In Example 3, the layer thickness was 0.1 mm, the laser output was 300 W, the laser scanning speed was 850 mm / sec, and the scanning pitch was 0.09 mm.

[0042] In Example 4, the layer thickness was 0.12 mm, the laser output was 300 W, the laser scanning speed was 850 mm / sec, and the scanning pitch was 0.09 mm.

[0043] In Comparative Example 1, the layer thickness was 0.04 mm, the laser power was 300 W, the laser scanning speed was 960 mm / s, and the scanning pitch was 0.11 mm. The heat source energy density E and the modeling speed calculated from these conditions, along with the defect rate of the additive manufacturing product under those conditions, are shown in Table 2.

[0044] [Table 2]

[0045] [Evaluation of mechanical properties] For Examples 1 to 4 and Comparative Example 1, tensile test specimens (parallel diameter: 3 mm, gauge length: 7 mm) were prepared in accordance with the standard test (ASTM E8). A tensile test was performed on these tensile test specimens at room temperature (22°C) using a tensile testing machine (INSTRON5982, manufactured by Instron Corporation) to determine the 0.2% yield strength, tensile strength, and elongation. The results are shown in Table 3. The tensile directions were the XY direction (parallel to the additive manufacturing surface) and the Z direction (additive manufacturing direction). The difference is the absolute value of the difference between the XY direction and the Z direction, and the percentage relative to the larger of the XY direction (parallel direction) and the Z direction (perpendicular direction) is shown in parentheses.

[0046] [Table 3]

[0047] As shown in Table 3, compared to Comparative Example 1, Examples 1 to 4 have smaller differences in 0.2% proof stress, tensile strength, and elongation between the XY and Z directions. In other words, it can be said that the anisotropy of mechanical properties is low. Specifically, the 0.2% proof stress difference in Comparative Example 1 is 17.5%, while the 0.2% proof stress difference in Example 3, which has the largest difference among the Examples, is 13.4%. Furthermore, the tensile strength difference in Comparative Example 1 is 15.7%, while the tensile strength difference in Example 3, which has the largest difference among the Examples, is 9.2%. Furthermore, the elongation difference in Comparative Example 1 is 30.0%, while the elongation difference in Example 1, which has the largest difference among the Examples, is 22.4%. This indicates that, despite the larger layer thickness in Examples 1 to 4 compared to Comparative Example 1, which has a layer thickness of 0.04 mm, the anisotropy of mechanical properties is significantly reduced. This is thought to be the result of selecting appropriate molding conditions.

[0048] [Structural observation] The average crystal grain size, crystal orientation, and GND density of the microstructure were evaluated by electron backscatter diffraction (EBSD) (JSM-7900F: manufactured by JEOL) for Examples 3 and 4 and Comparative Example 1. Each evaluation method will be described in detail below.

[0049] (Average grain size) In the structural image obtained by EBSD, grains are identified by determining the grain boundaries between adjacent measurement points where the misorientation between the measurement points is 15° or more. The identified grains are then approximated by an ellipse, and their major diameter is taken as the grain size. The ellipse approximation is shown in Figure 2(b). The average grain size is the average of the grain sizes within the field of view of the structural image obtained by EBSD.

[0050] (average aspect ratio) The aspect ratio was obtained by taking the ratio of the long and short diameters of the crystal grains in the Z-plane structure image as described above, as (long diameter) / (short diameter).The average aspect ratio was obtained by averaging this aspect ratio for the crystal grains within the field of view of the structure image.

[0051] (GND density average) The GND density, which is the area density of local dislocations within the field of view in the microstructure image obtained by the EBSD method, was averaged over the entire area of the field of view to obtain the average GND density. Such analysis can be performed, for example, using the weighted Burgers vector (WBV) method with AZtecCrystal software (Oxford Instruments).

[0052] The results of the IPF mapping image of Example 3 are shown in Figures 2(a) and (b), the pole figures are shown in (c) and (d), and the result of the GND density mapping image is shown in Figure 3. The results of the IPF mapping image of Example 4 are shown in Figures 6(a) and (b), the pole figures are shown in (c) and (d), and the result of the GND density mapping image is shown in Figure 7. The results of the IPF mapping image of Comparative Example 1 are shown in Figures 8(a) and (b), the pole figures are shown in (c) and (d), and the result of the GND density mapping image is shown in Figure 9. The average GND density and average crystal grain size of the Examples and Comparative Examples are summarized in Table 4.

[0053] [Table 4]

[0054] Table 4 shows that as the layer thickness increases, the average grain size and the average aspect ratio in the Z plane decrease. This is thought to be because the thicker the layer, the less susceptible to thermal effects during the melting and solidification process during additive manufacturing, suppressing grain growth. In particular, in the Z plane, growth in the Z direction, which is the direction in which grains tend to grow in additive manufacturing, is suppressed, resulting in a significant decrease in the average aspect ratio. In addition, the average GND density also decreased as the layer thickness increased. This is thought to be due to the smaller average grain size.

[0055] In the microstructure of Comparative Example 1 shown in FIG. 8, the preferred growth orientation of the crystals aligned in the XY and Z directions was <100> and <101> In the microstructures of Examples 3 and 4, where the layer thickness was large, the preferred growth orientation of the crystals aligned in the XY and Z directions was observed, as shown in Figs. <100> and <101> As mentioned above, this is because as the layer thickness increases, the growth of the crystal grain size is suppressed, resulting in a smaller average aspect ratio in the Z plane. The preferential growth of the crystal grains in a specific direction is suppressed, and the preferred growth orientation is weakened. <100> and <101> In this way, it is thought that increasing the layer thickness suppresses the growth of crystal grains in the preferred growth direction, and therefore weakens the crystal orientation, thereby reducing the anisotropy of the mechanical properties of the additive manufacturing product.

[0056] As described above, the low anisotropy of the mechanical properties of Examples 1 to 4 is thought to be due to the weak crystal orientation, the relatively small average aspect ratio in the Z direction of the crystal grains, and the small average value of the dislocation density.

Claims

1. A metal additive product having a laminated molten solidified layer, characterized in that the average aspect ratio, which is the ratio of the length of the major axis of the crystal grain to the length of the minor axis of the crystal grain, in a crystal orientation map image obtained by an EBSD method for a cross section perpendicular to the additively manufactured surface is 3.3 or less.

2. A metal additive manufacturing product in which melt-solidified layers are stacked, and the average value of the dislocation density calculated from the difference in crystal orientation between adjacent measurement points obtained by the EBSD method on a cross section perpendicular to the additive manufacturing surface or a cross section horizontal to the additive manufacturing surface is 0.85 × 10 １４ / m ２ and wherein the average grain size in the vertical cross section and the horizontal cross section is both 85 μm or less.

3. 3. The metal additively manufactured product of claim 1 or claim 2, characterized in that the difference between the 0.2% proof stress parallel to the additively manufactured surface and the 0.2% proof stress perpendicular to the additively manufactured surface is 17% or less of the larger 0.2% proof stress of the parallel direction or the perpendicular direction, the difference between the tensile strength in the parallel direction and the tensile strength in the perpendicular direction is 15% or less of the larger tensile strength of the parallel direction or the perpendicular direction, and the difference between the elongation in the parallel direction and the elongation in the perpendicular direction is 15% or less of the larger elongation of the parallel direction or the perpendicular direction.

4. The heat source energy density E according to the following formula (1) is 20 J / mm ３ 85J / mm or more ３ A method for producing a metal additive product, characterized in that additive manufacturing is performed by selecting a layer thickness t from the range of more than 0.04 mm and not more than 0.12 mm, a scanning pitch a from the range of 0.09 mm or more and less than 0.11 mm, and a scanning speed v from the range of 600 mm / s or more and 1250 mm / s or less so that the layer thickness t is greater than 0.04 mm and not more than 0.12 mm, a scanning pitch a from the range of 0.09 mm or more and less than 0.11 mm, and a scanning speed v from the range of 600 mm / s or more and 1250 mm / s or less, and irradiating a laser beam or electron beam while adjusting the output P in the range of 200 W or more and 350 W or less, and repeating melting and solidification, thereby obtaining a metal additive product having an average aspect ratio, which is the ratio of the length of the major axis of the crystal grain to the length of the minor axis of the crystal grain, of 3.3 or less in a crystal orientation map image obtained by the EBSD method for a cross section perpendicular to the additively manufactured surface. E=P / (v×a×t)...(1) E: Heat source energy density (J / mm ３ ), P: laser beam or electron beam power (W), v: scanning speed (mm / s), a: scanning pitch (mm), t: layer thickness (mm)

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