Metal powder material for additive manufacturing and method for manufacturing additively manufactured object

Abstract

To provide a metal powder material for additive manufacturing, capable of generating an additive manufactured object having properties of a steel material before aging treatment of maraging steel, such as high strength and toughness and excellent moldability at the time of manufacturing by a powder bed fusion method, having appropriate hardness, having high dimensional accuracy with residual stress being released, and having properties of maraging steel by an aging treatment carried out as necessary, and to provide a method for manufacturing an additive manufactured object.SOLUTION: According to the present invention, a metal powder material for additive manufacturing is provided which comprises C (carbon): more than 0.03 mass% and 0.4 mass% or less, Si (silicon): 0.1 mass% or more and 0.5 mass% or less, Mn (manganese): 0.2 mass% or less, Co (cobalt): 0.1 mass% or less, Ni (nickel): 17 mass% or more and 19 mass% or less, Mo (molybdenum): 1.5 mass% or more and 2.5 mass% or less, Ti (titanium): 0.5 mass% or more and 2.0 mass% or less, and Al (aluminum): 0.8 mass% or more and 1.1 mass% or less, with the balance consisting of Fe (iron) and inevitable impurities.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a metal powder material for additive manufacturing and a method for manufacturing an additively manufactured product. [Background technology]

[0002] Powder fusion bonding has been known as one type of method for manufacturing metal additive manufacturing objects. Powder bed fusion bonding is also known as a type of powder fusion bonding. For example, powder bed fusion bonding is a technique in which a material layer made of metal powder material is formed on a vertically movable building table in a sealed chamber filled with an inert gas, and a laser beam is irradiated at predetermined locations on the material layer to sinter or melt the metal powder material at the irradiated locations, thereby stacking multiple solidified layers to form a desired three-dimensional object.

[0003] One metal powder material suitable for the powder fusion bonding method is one that can be used to produce maraging steel. Maraging steel has high tensile strength and high toughness, and is relatively excellent in machinability before aging treatment. In fact, powder fusion bonding uses metal powder materials that can be used to produce maraging steel or steel equivalent to maraging steel.

[0004] Patent Document 1 discloses a metal powder material that can be used to produce steel products equivalent to maraging steel. This metal powder material is highly safe because it does not contain cobalt, and is characterized by its ability to produce steel products with the properties of maraging steel. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6692339 Summary of the Invention [Problem to be solved by the invention]

[0006] In powder fusion bonding, the solidified layer formed by irradiating a material layer with laser light is very hot immediately after solidification. However, the temperature rapidly drops due to the inert gas atmosphere, heat dissipation from the already formed solidified layer, and heat dissipation from the build plate. During this process, the volume of metal shrinks due to its positive thermal expansion coefficient. However, the amount of shrinkage is limited by adhesion to the adjacent molten layer and build plate, so tensile stress remains. If this process is repeated, the tensile stress causes deformation of the molded object, sometimes resulting in cracks. Therefore, even when using this metal powder material, it has been difficult to obtain molded objects with the desired dimensional accuracy.

[0007] The present invention has been made in consideration of these circumstances, and provides a metal powder material for additive manufacturing that can produce additively manufactured objects that, when manufactured using the powder fusion bonding method, have the properties of maraging steel before aging treatment, such as high strength, toughness, and excellent formability, and that also have moderate hardness, are released from residual stress, have high dimensional accuracy, and can further acquire the properties of maraging steel through aging treatment, if necessary, as well as a method for manufacturing the additively manufactured objects. [Means for solving the problem]

[0008] According to the present invention, the following inventions are provided. [1] A metal powder material for additive manufacturing containing C (carbon): more than 0.03% by mass and not more than 0.4% by mass, Si (silicon): 0.1% by mass or more and 0.5% by mass or less, Mn (manganese): 0.2% by mass or less, Co (cobalt): 0.1% by mass or less, Ni (nickel): 17% by mass or more and 19% by mass or less, Mo (molybdenum): 1.5% by mass or more and 2.5% by mass or less, Ti (titanium): 0.5% by mass or more and 2.0% by mass or less, Al (aluminum): 0.8% by mass or more and 1.1% by mass or less, with the remainder being Fe (iron) and unavoidable impurities. [2] A method for manufacturing an additive manufacturing object, comprising: a material layer formation process in which the metal powder material for additive manufacturing described in [1] is supplied to a manufacturing area to form a material layer of a predetermined thickness; a solidification process in which a laser beam is irradiated onto a predetermined irradiation area of ​​the material layer to form a solidified layer; and a temperature adjustment process in which, each time one or more layers of the solidified layer are newly formed, the temperature of at least the newly formed upper surface layer of the solidified layer is adjusted to a first temperature, a second temperature, and again the first temperature in this order, wherein when the first temperature is T1, the second temperature is T2, the martensitic transformation start temperature of the solidified layer is Ms, and the martensitic transformation finish temperature of the solidified layer is Mf, the relationships of the following formulas (1) to (3) are all satisfied. T1≧Mf (1) T1>T2 (2) T2≦Ms (3) [3] A method for manufacturing an additive manufacturing object according to [2], further comprising a cutting step of cutting the solidified layer, in which the solidified layer is cut after the temperature of the upper surface layer is adjusted from a first temperature to a second temperature and before the temperature is adjusted from the second temperature to the first temperature. [4] A method for manufacturing an additive manufacturing object according to [2] or [3], wherein the dimensional expansion coefficient expressed by the dimension H1 of the additive manufacturing object after the upper surface layer is adjusted to a first temperature and the dimension H2 of the additive manufacturing object when the upper surface layer is adjusted from the first temperature to a second temperature and then adjusted back to the first temperature satisfies the following formula (4): 0.3<{(H2-H1) / H1}×100<1.0 (4) [5] A method for manufacturing an additive manufacturing object described in any one of [2] to [4], wherein the surface hardness of the additive manufacturing object after adjusting the temperature of the upper surface layer from the first temperature to the second temperature is Rockwell hardness HRC 39 to 45. [Effects of the Invention]

[0009] The metal powder material for additive manufacturing according to the present invention can suppress residual stress by adjusting the temperature during the process of manufacturing an additive manufactured product using the material, thereby enabling the production of an additive manufactured product with high dimensional accuracy. The additive manufactured product immediately after manufacturing can have the properties of the maraging steel before aging treatment, such as moderate hardness, high toughness, and excellent formability. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is an explanatory diagram of a layered manufacturing method using the layered manufacturing device 1. FIG. [Figure 2] 1 is an explanatory diagram of a layered manufacturing method using the layered manufacturing device 1. FIG. [Figure 3] 1 is an explanatory diagram of a layered manufacturing method using the layered manufacturing device 1. FIG. [Figure 4] 10 is a graph showing the change in the dimensional expansion coefficient with respect to the temperature of the upper surface layer in the temperature adjustment step of a manufacturing method for an additive manufacturing object using three metal powder materials with different carbon contents. [Figure 5] 10 is a graph showing the change in dimensions of the layered object relative to the temperature of the upper surface layer in the temperature adjustment step of the method for manufacturing a layered object using the metal powder material of Example 1. [Figure 6] 1 is a graph showing the warpage of a shaping plate after a layered object was manufactured using the metal powder materials of Example 1 and Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0011] The following describes embodiments of the present invention. The various features shown in the following embodiments can be combined with one another. Furthermore, each feature can be an independent invention. Furthermore, in the following embodiments, elements not specified in the claims are optional and can be omitted. Any number of "0"s (for example, one or two) may be added to the end of numerical values ​​disclosed in the following description. For example, one or two "0"s may be added after "1.4" to make it "1.40" or "1.400."

[0012] 1. Metal powder materials for additive manufacturing The metal powder material for additive manufacturing according to the present invention is characterized by having the properties of 18Ni-based maraging steel before aging treatment, such as moderate hardness immediately after manufacturing, high toughness, and excellent formability. The metal powder material contains carbon and a very small amount of cobalt. However, since 18Ni-based maraging steel is generally a nickel alloy steel that contains substantially no carbon and contains cobalt, the metal powder material for additive manufacturing is not strictly a maraging steel. Therefore, in this specification, an additively manufactured object manufactured using the metal powder material is considered to be a steel material equivalent to maraging steel.

[0013] In additive manufacturing, when the material used is a martensitic material, martensitic transformation accompanied by volume expansion occurs when certain temperature conditions are met. While detailed procedures will be described later, by adjusting the temperature during additive manufacturing and intentionally promoting martensitic transformation, it is possible to control the residual stress caused by volumetric shrinkage of the metal due to heat dissipation during manufacturing through martensitic transformation. This manufacturing method, which involves temperature adjustment, can produce objects with high dimensional accuracy. In other words, to achieve the benefits of this manufacturing method, it is necessary to use a metal powder material that has a high dimensional expansion rate due to martensitic transformation and a martensitic transformation onset temperature Ms that is easy to control.

[0014] Cobalt-free, highly safe metal powder materials equivalent to 18Ni-based maraging steel (hereinafter referred to as cobalt-less maraging steel-equivalent materials) are known, but additive manufacturing (AM) products produced using these materials have the properties of a low thermal expansion coefficient and a high martensitic transformation start temperature, Ms. Therefore, the metal powder material of the present invention is a material that, by adding mainly carbon to such cobalt-less maraging steel-equivalent materials, can be used to produce AM products that have the characteristics of a high thermal expansion coefficient and a martensitic transformation start temperature, Ms, that allows for easy control of martensitic transformation, while retaining the properties of 18Ni-based maraging steel before aging treatment in terms of hardness, toughness, and formability.

[0015] The metal powder material for additive manufacturing according to the present invention contains the following elements, with the remainder consisting mainly of iron (Fe) and unavoidable impurities.

[0016] The iron content is 76% by mass or more. The inevitable impurities are single or multiple elements whose inclusion is unavoidable to the extent that the properties of the metal powder material for additive manufacturing according to the present invention are maintained. Examples of inevitable impurities include nitrogen (N), phosphorus (P), sulfur (S), and oxygen (O). The nitrogen content is preferably 0.1% by mass or less. The phosphorus content is preferably 0.02% by mass or less. The sulfur (S) content is preferably 0.02% by mass or less. The oxygen content is preferably 0.04% by mass or less.

[0017] The nitrogen content is, for example, 0.00, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, or 0.10% by mass, and may be within a range between any two of the values ​​exemplified herein or any value less than or equal to the range. The phosphorus content is, for example, 0.00, 0.01, or 0.02% by mass, and may be within a range between any two of the values ​​exemplified herein or any value less than or equal to the range. The sulfur content is, for example, 0.00, 0.01, or 0.02% by mass, and may be within a range between any two of the values ​​exemplified herein or any value less than or equal to the range. The oxygen content is, for example, 0.00, 0.01, 0.02, 0.03, or 0.04% by mass, and may be within a range between any two of the values ​​exemplified herein or any value less than or equal to the range.

[0018] The inevitable impurities may include one or more elements other than the elements exemplified above (hereinafter, inevitable impurities other than the elements exemplified above will be referred to as "other inevitable impurities"). The content of other inevitable impurities is preferably 0.01% by mass or less.

[0019] When other inevitable impurities consist of multiple types of elements, their total content is preferably 0.5% by mass or less. The total content of other inevitable impurities is more preferably 0.2% by mass or less, and even more preferably 0.1% by mass or less. Specifically, the total content of other inevitable impurities is, for example, 0.00, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50% by mass, and may be within the range between any two of the values exemplified herein or less than any of them.

[0020] The types, component ranges, and reasons for limitation of the additive elements will be described below.

[0021] (1) 0.03% by mass < C ≤ 0.4% by mass Carbon (C) affects the martensite transformation start temperature Ms and the strength of the shaped object immediately after laminated shaping. By increasing the carbon content, it becomes possible to lower the martensite transformation start temperature Ms. Also, the higher the carbon content, the higher the hardness of the shaped object immediately after laminated shaping. If the carbon content is too low, the martensite transformation start temperature Ms becomes high, and the hardness of the shaped object immediately after laminated shaping decreases. Therefore, the carbon content is preferably more than 0.03% by mass. The C content is more preferably 0.1% by mass or more.

[0022] On the other hand, if the carbon content is too high, the hardness immediately after laminated shaping becomes excessively high, making cutting difficult, or the martensite transformation start temperature Ms becomes too low, resulting in insufficient martensite transformation. Therefore, the carbon content is preferably 0.4% by mass or less. The carbon content is more preferably 0.3% by mass or less.

[0023] Specifically, the carbon content is, for example, 0.035, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40% by mass, and may be within the range between any two of the values exemplified herein.

[0024] (2)0.1 mass%≦Si≦0.5 mass% Silicon (Si) affects machinability during cutting. Increasing the silicon content makes it easier to cut. In additive manufacturing, since it is possible to obtain a molded object that is close to the finished shape at the time of completion of manufacturing, the cutting allowance is small and high machinability is not necessary. However, if the silicon content is too low, processing costs increase due to wear and tear on the cut parts, etc. Therefore, the silicon content is preferably 0.1% by mass or more. A silicon content of 0.2% by mass or more is particularly preferred.

[0025] On the other hand, if the silicon content is too high, the thermal conductivity and toughness will be significantly reduced. Therefore, the silicon content is preferably 0.5 mass% or less, and more preferably 0.4 mass% or less.

[0026] The silicon content is specifically, for example, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, or 0.50 mass %, and may be within a range between any two of the values ​​exemplified here.

[0027] (3)Mn≦0.2% by mass Manganese (Mn) is added in small amounts, or not at all, as in the composition of 18Ni maraging steel. Even if manganese is not added, manganese may be unavoidably contained in the metal powder material. The manganese content is preferably 0.2 mass% or less. The manganese content may be substantially zero.

[0028] The manganese content is, for example, 0.00, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, or 0.20 mass%, and may be within a range between any two of the values ​​exemplified here or any of the values ​​less than or equal to these values.

[0029] (4)Co≦0.1% by mass Cobalt (Co) is expensive, and if its content is too high, it may impair the toughness of the molded object. Cobalt is also a specified chemical substance under the Industrial Safety and Health Act, and it has been pointed out that it may have some effect on the human body. Therefore, the amount of cobalt added is small, or it is not added at all. Even if cobalt is not added, it may be unavoidably contained in the metal powder material. The cobalt content is preferably 0.1% by mass or less. The cobalt content may be substantially zero.

[0030] The cobalt content is specifically, for example, 0.00, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, or 0.10 mass%, and may be within a range between any two of the values ​​exemplified here or equal to or less than any of them.

[0031] (5) 17% by mass≦Ni≦19% by mass Nickel (Ni) is a fundamental element that forms intermetallic compounds with titanium, molybdenum, etc., contributing to improved strength and making the material a metal equivalent to maraging steel. The formation of intermetallic compounds strengthens the additively manufactured product. From the viewpoint of strength, the nickel content is preferably 17% by mass or more. The nickel content is particularly preferably 17.5% by mass or more.

[0032] On the other hand, nickel is an austenite-forming element. Therefore, if the nickel content is too high, martensitic transformation is inhibited. Therefore, the nickel content is preferably 19% by mass or less. The nickel content is particularly preferably 18.5% by mass or less.

[0033] The nickel content is, for example, 17.0, 17.1, 17.2, 17.3, 17.4, 17.5, 17.6, 17.7, 17.8, 17.9, 18.0, 18.1, 18.2, 18.3, 18.4, 18.5, 18.6, 18.7, 18.8, 18.9, or 19.0 mass%, and may be within a range between any two of the values ​​exemplified here.

[0034] (6)1.5 mass%≦Mo≦2.5 mass% As mentioned above, molybdenum (Mo) forms an intermetallic compound with nickel, thereby increasing the strength of the shaped article. Therefore, the molybdenum content is preferably 1.5% by mass or more, and more preferably 2.0% by mass or more.

[0035] On the other hand, if the molybdenum content is too high, it will form coarse intermetallic compounds with iron, reducing toughness. Therefore, the molybdenum content is preferably 2.5 mass% or less, and more preferably 2.25 mass% or less.

[0036] The molybdenum content is specifically, for example, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, or 2.5 mass%, and may be within a range between any two of the values ​​exemplified here.

[0037] (7)0.5 mass%≦Ti≦2.0 mass% As mentioned above, titanium (Ti) forms an intermetallic compound with nickel, thereby increasing the strength of the shaped object. Therefore, the titanium content is preferably 0.5% by mass or more, and more preferably 0.7% by mass or more.

[0038] On the other hand, if the titanium content is too high, titanium segregation occurs, reducing toughness. Therefore, the titanium content is preferably 2.0 mass% or less, and more preferably 1.0 mass% or less.

[0039] The titanium content is specifically, for example, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, or 2.0 mass%, and may be within a range between any two of the values ​​exemplified here.

[0040] (8)0.8 mass%≦Al≦1.1 mass% Aluminum (Al) also forms an intermetallic compound with nickel, thereby increasing the strength of the additive manufacturing product. Therefore, the aluminum content is preferably 0.8% by mass or more, and more preferably 0.9% by mass or more.

[0041] On the other hand, if the aluminum content is too high, the toughness will decrease. Therefore, the aluminum content is preferably 1.1 mass % or less, and more preferably 1.0 mass % or less.

[0042] The aluminum content is specifically, for example, 0.80, 0.82, 0.84, 0.86, 0.88, 0.90, 0.92, 0.94, 0.96, 0.98, 1.00, 1.02, 1.04, 1.06, 1.08, or 1.10 mass%, and may be within a range between any two of the values ​​exemplified here.

[0043] 2. Manufacturing method for additive manufacturing In the following description, the up, down, left, and right directions are defined as shown in Fig. 1. Specifically, the up and down direction is the height direction (direction of arrow A) in which the modeling table 23 can move. The left and right direction is the driving direction of the recoater head 22 (direction of arrow B), which is perpendicular to the up and down direction.

[0044] 2.1 Additive Manufacturing Equipment 1 A method for manufacturing a layered object using a metal powder material will now be described. An example of an layered manufacturing apparatus 1 used in this manufacturing method is shown in FIGS. 1 to 3. Any device can be used as the layered manufacturing apparatus 1 as long as it can realize the manufacturing method described below. Here, the layered manufacturing apparatus 1 will be briefly described with reference to FIGS. 1 to 3. The illustrated layered manufacturing apparatus 1 includes a chamber 10, a material layer forming device 20, an irradiation device 30, a temperature adjustment device (not shown), and a machining device 50.

[0045] (1) Chamber 10 The chamber 10 covers a build region R, which is an area where a desired layered object is formed. The chamber 10 is filled with an inert gas that does not substantially react with the material layer 60 or the solidified layer 70. The chamber 10 has a window 10a at its top.

[0046] (2) Material layer forming device 20 The material layer forming device 20 has a base table 21 and a recoater head 22. The base table 21 has a building region R where a layered object is formed. A building table 23 is provided in the building region R. The building table 23 is driven by a building table drive mechanism 24 and can move up and down (in the direction of arrow A in FIG. 1). When the layered object forming device 1 is in use, a building plate 40 is placed on the building table 23, and a material layer 60 is formed thereon (FIG. 2).

[0047] The recoater head 22 is configured to be able to store and discharge the metal powder material. The recoater head 22 is movable left and right in FIG. 1 (in the direction of arrow B). The recoater head 22 is also provided with a blade 22a. While supplying the metal powder material from the recoater head 22, the blade 22a levels and flattens the metal powder material, thereby forming a material layer 60 on the building plate 40.

[0048] (3) Irradiation device 30 The irradiation device 30 irradiates the material layer 60 with laser light L (or an electron beam) to form a solidified layer 70. For example, as shown in FIGS. 1 to 3, the irradiation device 30 is provided above the chamber 10. The laser light L passes through the window 10a and irradiates the irradiation region of the material layer 60 formed in the building region R, melting and solidifying the metal powder material, thereby forming the solidified layer 70.

[0049] (4) Temperature control device (not shown) Each time one or more layers of solidified layers 70 are newly formed, the temperature adjustment device adjusts the temperature of at least the newly formed solidified layer 70 among the solidified layers 70 in the order of first temperature T1, second temperature T2, and first temperature T1. Hereinafter, the newly formed solidified layer 70, i.e., the solidified layer 70 that has never been subjected to temperature adjustment in the series of temperature adjustments at first temperature T1, second temperature T2, and first temperature T1 since its formation, will be referred to as the upper surface layer.

[0050] The first temperature T1 and the second temperature T2 are within a temperature range that can be adjusted by a temperature adjustment device. The first temperature T1, the second temperature T2, the martensitic transformation start temperature Ms of the solidified layer 70, and the martensitic transformation finish temperature Mf of the solidified layer 70 all satisfy the relationships of the following formulas (1) to (3). T1≧Mf (1) T1>T2 (2) T2≦Ms (3)

[0051] After sintering or melting, the upper surface layer contains an austenite phase before being subjected to temperature adjustment, and at least a portion of the upper surface layer is transformed into a martensite phase by temperature adjustment.

[0052] The temperature adjustment device can have any configuration as long as it is configured to be able to adjust the temperature of the upper surface layer to the first temperature T1 and the second temperature T2. The temperature adjustment device has at least one of a heater that can heat the upper surface layer and a cooler that can cool the upper surface layer, and preferably has both.

[0053] For example, the temperature adjustment device is provided within the modeling table 23. In such a configuration, the upper surface layer can be adjusted to a desired temperature via the modeling plate 40 and the underlying solidified layer 70, which are in contact with the modeling table 23. Alternatively, for example, a temperature adjustment device may be provided above the chamber 10. In this case, for example, a halogen lamp or the like may be provided as a heater, or a second irradiation device may be provided and its laser light may be used to locally adjust the temperature. In this case, for example, a blower that blows cooling gas may be provided as a cooler, or a cooling plate may be configured to directly contact the upper surface layer. In such a configuration, it is possible to directly adjust the temperature of the upper surface layer.

[0054] (5) Machining equipment 50 The machining device 50 is configured by attaching a tool (for example, an end mill) for performing machining such as cutting to a machining head, and performs machining by moving the machining head appropriately in the horizontal and vertical directions.

[0055] 2.2 Manufacturing method Each step in the method for manufacturing a layered object will be described with reference to Figures 1 to 3. In the following, the step of supplying metal powder material to the manufacturing region R to form a material layer 60 of a predetermined thickness will be referred to as the material layer forming step, the step of irradiating a predetermined irradiation region of the material layer 60 with laser light L to form a solidified layer 70 will be referred to as the solidifying step, and the step of adjusting the temperature of at least the upper surface layer of the solidified layer 70 in the order of first temperature T1, second temperature T2, and then first temperature T1 will be referred to as the temperature adjusting step.

[0056] First, the first material layer formation process is performed. As shown in Figure 1, the build plate 40 is placed on the build table 23, and the height of the build table 23 is adjusted to an appropriate position. In this state, the recoater head 22, which is filled with metal powder material, is moved from the left side to the right side of the build region R in Figure 1, thereby forming the first material layer 60 on the build plate 40.

[0057] Next, a first solidification step is performed. Laser light L is irradiated onto a predetermined portion of the material layer 60 to sinter or melt the laser irradiated position of the material layer 60, thereby obtaining a first solidified layer 70 as shown in FIG. 2. Here, the temperature of the first solidified layer 70 is adjusted to a first temperature T1. The entire upper surface layer may be adjusted to the first temperature T1, or the temperature of the upper surface layer corresponding to the irradiated position may be locally adjusted to the first temperature T1 in accordance with the scanning of the laser light L.

[0058] Next, as the second material layer formation process, the height of the modeling table 23 is lowered by the thickness (one layer) of the solidified layer 70, and the recoater head 22 is moved from the right side to the left side of the modeling area R in Figure 2, thereby forming a second material layer 60 on the solidified layer 70.

[0059] Next, in the second solidification step, a laser beam L is irradiated onto a predetermined portion of the material layer 60 to sinter or melt the laser irradiated position of the material layer 60, thereby obtaining a second solidified layer 70 as shown in Fig. 3. Here, the temperature of the second solidified layer 70 is adjusted to a first temperature T1.

[0060] By repeating the above steps, the third and subsequent solidified layers 70 are formed. When a predetermined number of solidified layers 70 have been formed, a temperature adjustment step is performed by a temperature adjustment device. First, the upper surface layer, which has been maintained at the first temperature T1, is cooled and its temperature is adjusted to the second temperature T2. Next, the upper surface layer whose temperature has been adjusted to the second temperature T2 is raised and its temperature is adjusted back to the first temperature T1. For the final layer, after the upper surface layer has been adjusted to the second temperature T2, modeling is completed without adjusting its temperature to the first temperature T1.

[0061] Furthermore, in the additive manufacturing apparatus 1 equipped with the machining device 50 as in this embodiment, a cutting process may be performed on the end surface of the solidified layer 70 using a cutting tool provided in the machining device 50 each time a predetermined number of solidified layers 70 are formed. The cutting process is preferably performed on the upper surface layer whose temperature has been adjusted to the second temperature T2. In other words, after the temperature adjustment device adjusts the temperature of the upper surface layer from the first temperature T1 to the second temperature T2, the solidified layer 70 is cut before adjusting the temperature from the second temperature T2 to the first temperature T1. Since cutting can be performed on the upper surface layer after martensitic transformation has occurred and the dimensions have stabilized, cutting can be performed with higher precision. Furthermore, spatter generated during sintering or melting may adhere to the surface of the solidified layer, creating protrusions. If the recoater head 22 collides with the protrusions during the material layer formation process, cutting may be performed on the upper surface layer to remove the protrusions.

[0062] The material layer forming process, solidifying process, temperature adjusting process, and cutting process are repeated to form the desired additive manufacturing object. The temperature adjusting process may be performed each time a solidified layer 70 is formed, or each time multiple layers are formed, or may be performed after each cutting process if a cutting process is performed. The frequency of the temperature adjusting process may be set to be variable depending on the shape of the additive manufacturing object. For example, the temperature adjusting process is normally performed every 5 mm of solidified layer 70, but the temperature adjusting process may be inserted immediately before forming a solidified layer 70 with a fragile shape.

[0063] The temperature adjustment process described above reduces residual stress caused by cooling shrinkage of the solidified layer 70, making it possible to suppress deformation of the additively manufactured object. Specifically, when additive manufacturing is performed using this metal powder material, the volume expands during martensitic transformation, mitigating volumetric shrinkage during cooling. Note that martensitic transformation occurs when the solidified layer 70 is rapidly cooled from the martensitic transformation start temperature Ms to the martensitic transformation finish temperature Mf. In other words, martensitic transformation occurs in the temperature range below the martensitic transformation start temperature Ms and above the martensitic transformation finish temperature Mf.

[0064] Furthermore, this dimensional expansion rate can be controlled by the relationship between the first temperature T1, the second temperature T2, the martensitic transformation start temperature Ms, and the martensitic transformation finish temperature Mf. In other words, the residual stress in the additively manufactured object can be controlled by adjusting these temperature relationships.

[0065] To obtain an additively manufactured object with better dimensional accuracy, a warpage measurement step may be further performed during the temperature adjustment to the second temperature T2. Because the volume expansion due to martensitic transformation varies depending on the volume and shape of the solidified layer 70 and the temperature difference during cooling to the second temperature T2, it is difficult to predict the optimal temperature setting to suppress shrinkage. Therefore, if the temperature is adjusted to the first temperature T1 and second temperature T2 initially set by the operator, warpage may occur in the build plate 40.

[0066] In the warpage measurement process, the amount of deformation when the peripheral edge of the build plate 40 warps upward (deformation in the forward warpage direction) is taken as a positive value, and the amount of deformation when the peripheral edge of the build plate 40 warps downward (deformation in the reverse warpage direction) is taken as a negative value. After the warpage measurement process is performed, the difference between the first temperature T1 and the second temperature T2 is changed in a direction that reduces the amount of warpage according to the measured magnitude of warpage. Specifically, if the deformation in the forward warpage direction is greater than a predetermined threshold, the dimensional expansion rate due to martensitic transformation is smaller than the volumetric shrinkage due to cooling, and continuing buildup at the first temperature T1 may not be able to fully suppress residual stress. Therefore, by increasing the first temperature T1, the amount of martensitic transformation increases, making it possible to reduce the deformation in the forward warpage direction.

[0067] On the other hand, if the deformation in the reverse warpage direction is greater than the predetermined threshold, the dimensional expansion rate due to martensitic transformation exceeds the volumetric shrinkage due to cooling, and continuing modeling at the first temperature T1 may cause distortion in the additively modeled object due to martensitic transformation. Therefore, by changing the first temperature T1 to a lower temperature, the amount of martensitic transformation can be reduced, thereby reducing the deformation in the reverse warpage direction.

[0068] Due to the specifications of the device, there is an upper limit to the temperature that can be adjusted as the first temperature T1. Furthermore, if the second temperature T2 is significantly higher than room temperature, there is a risk of further volumetric change occurring due to temperature changes as the layered object cools to room temperature after being adjusted to the second temperature T2 and modeling is completed. Therefore, the first temperature T1 is preferably, for example, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, or 160°C, or may be within a range between any two of the values ​​exemplified here. The second temperature T2 is preferably, for example, 0, 5, 10, 15, 20, 25, 30, 35, or 40°C, or may be within a range between any two of the values ​​exemplified here.

[0069] 3. Characteristics of additively manufactured objects made from metal powder materials 3.1 Martensitic transformation start temperature Ms and martensitic transformation finish temperature Mf The martensitic transformation start temperature Ms of the additively manufactured product manufactured using the metal powder material according to this embodiment is 70°C to 150°C. The martensitic transformation start temperature Ms is preferably 80°C to 130°C. Specific examples of the martensitic transformation start temperature Ms include 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, and 150°C, and may be within a range between any two of the values ​​exemplified here. The martensitic transformation finish temperature Mf is -40°C to 30°C. The martensitic transformation finish temperature Mf is preferably 10°C to 20°C. The martensitic transformation finish temperature Mf is specifically, for example, −40, −35, −30, −25, −20, −15, −10, −5, 0, 5, 10, 15, 20, 25, or 30° C., and may be within a range between any two of the values ​​exemplified here.

[0070] If the martensitic transformation start temperature Ms is too high, it will exceed the temperature range adjustable by the building table 23, and the additive manufacturing object will not be able to be heated sufficiently. In this case, the martensitic transformation will be almost complete immediately after the solidification step, and there is a risk that the residual stress reduction effect will not be obtained. On the other hand, if the martensitic transformation start temperature Ms is too low, the amount of martensitic transformation will be small, and there is a risk that the residual stress reduction effect will not be obtained.

[0071] The martensitic transformation start temperature Ms and martensitic transformation finish temperature Mf of the additively manufactured object using this metal powder material are within the above-mentioned temperature ranges, so that the martensitic transformation can be sufficiently controlled within the above-mentioned numerical ranges of the first temperature T1 and the second temperature T2. Therefore, in the manufacturing method according to this embodiment, the martensitic transformation caused by the temperature adjustment step can have the effect of reducing residual stress.

[0072] 3.2 Dimensional expansion rate The dimensional expansion rate of an additively-molded object manufactured using a metal powder material is calculated based on the dimension H1 of the additively-molded object at the start of the temperature adjustment process and the dimension H2 of the additively-molded object at the end of the temperature adjustment process (note that in this specification, dimension refers to the horizontal length, vertical length, and / or height of the additively-molded object). In other words, it is expressed by the dimension H1 of the additively-molded object after the upper surface layer has been adjusted to a first temperature T1 and the dimension H2 of the additively-molded object when the upper surface layer is adjusted from the first temperature T1 to a second temperature T2 and then adjusted back to the first temperature. The dimensional expansion rate satisfies the following formula (4): 0.3<{(H2-H1) / H1}×100<1.0 (4)

[0073] The dimensional expansion coefficient is specifically, for example, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, or 0.95%, and may be within a range between any two of the numerical values ​​exemplified here.

[0074] If the dimensional expansion rate due to martensitic transformation is too small, the effect of reducing residual stress cannot be obtained. On the other hand, if the dimensional expansion rate is too large, the expansion due to martensitic transformation may actually cause deformation or cracking. Therefore, an additive manufacturing product manufactured using a metal powder material whose dimensional expansion rate falls within the above range achieves the effect of reducing residual stress and has excellent dimensional accuracy.

[0075] 3.3 Hardness The surface hardness of the layered object after the temperature of the upper surface layer is adjusted from the first temperature T1 to the second temperature T2 is a Rockwell hardness of 39 to 45 HRC. Specific examples of the Rockwell hardness HRC include 39, 40, 41, 42, 43, 44, and 45, and may be within a range between any two of the values ​​exemplified here. A layered object with a Rockwell hardness HRC of 39 or higher is preferable because it has excellent strength and wear resistance.

[0076] On the other hand, if the Rockwell hardness HRC exceeds 45, the workpiece becomes too hard to cut, making the cutting process difficult. Therefore, the hardness is preferably within the above range. Furthermore, the Rockwell hardness HRC of the additive manufacturing product after aging treatment is preferably 40 to 54. Specific examples of the Rockwell hardness HRC after aging treatment are 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, and 54, and may be within a range between any two of the values ​​exemplified here.

[0077] 4. Working Example 4.1 Change in martensitic transformation start temperature Ms with carbon addition Table 1 shows the specifications for the content (mass%) of metal powder materials other than carbon used in this experiment. The amount of carbon added to the powders of the following specifications was varied, and 20mm x 20mm x 20mm additive manufacturing objects were produced using the manufacturing method described above, and the change in martensitic transformation start temperature Ms was evaluated. The dimensional expansion coefficient and Rockwell hardness HRC were also measured. [Table 1]

[0078] In this experiment, an additive manufacturing (AM) object was manufactured using three types of metal powder with carbon contents of 0.183, 0.187, and 0.2% by mass. The first temperature T1 was 160°C, and the second temperature T2 was 40°C. The temperature of the entire top surface layer of the AM object was adjusted. The dimensional expansion coefficient was measured and compared between the dimension H1 at the first temperature and the dimension H2 after cooling to the second temperature and then adjusting back to the first temperature after forming a solidified layer up to a height of 400 layers / 20 mm. In other words, in this experiment, H1 was 20 mm, and H2 was 20 mm + the amount of dimensional expansion. The rate of change during the temperature adjustment process (adjusting the temperature to the first temperature T1, the second temperature T2, and the first temperature T1) was calculated using the following equation (5). Dimensional expansion rate (%) = {(H2-H1) / H1} × 100 (5)

[0079] Rockwell hardness measurements were performed on the top surface of the additively molded object after adjustment to the second temperature T2, i.e., immediately after the completion of molding. Rockwell hardness was measured in accordance with JIS Z 2245:2011. Specifically, Rockwell hardness was measured by applying a standard test force of 98 N to the test surface of a 20 mm square (20 x 20 x 20 mm) test piece using an indenter with a diamond steel ball with a tip radius of 0.2 mm. Next, a total test force of 1471 N was applied to plastically deform the test piece, and the depth of the permanent indentation created when the test piece was returned to the standard test force was measured. Rockwell hardness was calculated from the depth of this permanent indentation using a conversion formula.

[0080] The results of this experiment are shown in Figure 4 and Table 2. Figure 4 shows the change in the dimensional expansion coefficient versus the temperature of the top layer during the temperature adjustment process of the manufacturing method for additive manufacturing using three metal powder materials with different carbon contents. To make it easier to understand, Figure 4 marks the plot for the initial first temperature T1 as "Start" and the plot when the temperature is adjusted back to the first temperature as "End."

[0081] From Figure 4, it can be seen that at the start (initial first temperature T1), the dimensional expansion rate was 0%, and as the temperature began to decrease, the dimensional expansion rate also decreased, but at a certain temperature, the dimensional expansion rate began to increase. The temperature at which this dimensional expansion rate begins to increase is the martensitic transformation start temperature Ms. Therefore, it was found that the martensitic transformation start temperature Ms decreases as the carbon content increases: approximately 150°C when the carbon content is 0.183 mass%, approximately 140°C when the carbon content is 0.187 mass%, and approximately 130°C when the carbon content is 0.2 mass%.

[0082] Furthermore, the dimensional expansion rate increases due to martensitic transformation from the martensitic transformation start temperature Ms until the upper surface layer reaches the second temperature, and after the temperature of the upper surface layer reaches the second temperature, thermal expansion occurs in the process of adjusting it back to the first temperature, resulting in volume expansion of the additive manufacturing. In this experiment, the dimensional expansion rate of all additive manufacturing objects exceeded 0.6%, indicating that sufficient volume expansion had occurred.

[0083] Table 2 shows the added carbon content (mass%) and the Rockwell hardness (HRC) of the additively molded objects fabricated using the metal powder materials. [Table 2]

[0084] Table 2 shows that all of the additively manufactured objects using the metal powder materials used in this experiment were strong yet hard enough to be machined.

[0085] 4.2 Characteristics of additively manufactured objects made from metal powder materials A metal powder material according to an embodiment of the present invention (Example 1) was produced. A metal powder material containing almost no carbon was also produced as Comparative Example 1. Table 1 shows the content of the metal powder material in Example 1 and the specifications of the metal powder material used in Comparative Example 1. [Table 3]

[0086] Using the above-described manufacturing method, a 20 mm × 20 mm × 20 mm molded object was manufactured using Example 1, and the dimensional expansion coefficient and Rockwell hardness were measured. In addition, using the above-described manufacturing method, an 80 mm × 80 mm × 10 mm layered molded object was manufactured using Example 1 and Comparative Example 1, and the warpage was measured.

[0087] In this experiment, the first temperature T1 was 140°C and the second temperature T2 was 25°C. The dimensional expansion coefficient was measured and compared between the dimension H1 at the first temperature and the dimension H2 after cooling to the second temperature when a solidified layer was formed to a height of 400 layers / 20 mm. The calculation method was the same as in the previous experiment, using equation (5) to calculate the rate of change when the temperature adjustment process (adjusting the temperature to the first temperature T1, the second temperature T2, and then the first temperature T1) was performed. The Rockwell hardness was measured using the same method as in the previous experiment. The Rockwell hardness was measured on the additively manufactured object after the second temperature adjustment, i.e., immediately after the completion of manufacturing.

[0088] Figure 5 shows the dimensional change of the additively molded object relative to the temperature of the top layer during the temperature adjustment process of the manufacturing method for additively molded objects using the metal powder material of Example 1. For ease of understanding, Figure 5 indicates "Start" on the plot for the initial first temperature T1 and "End" on the plot when the temperature was adjusted back to the first temperature. As the temperature began to decrease from the dimensions at the start (initial first temperature T1), the dimensions also decreased, but the dimensional expansion rate increased at 130°C. Therefore, the martensitic transformation start temperature Ms of Example 1 was 130°C. After the upper surface layer was adjusted to the second temperature, it was adjusted back to the first temperature. The dimensional expansion rate calculated using the aforementioned equation (4) for Example 1 was 0.82%. This indicates that the additively molded object manufactured using this powder material experienced sufficient volume expansion. The Rockwell hardness HRC of Example 1 immediately after molding was HRC43.

[0089] In addition, in the warpage measurement, the warpage of the shaping plates at the end of shaping in Example 1 and Comparative Example 1 was measured. The warpage of each shaping plate is shown in Figure 6. In this experiment, the warpage was measured as the deformation amount (mm) of the backside of the shaping plate, so deformation in the forward warping direction is expressed as a negative value and deformation in the reverse warping direction is expressed as a positive value. In other words, the shaping plate in Example 1 underwent deformation in the reverse warping direction, and the shaping plate in Comparative Example 1 underwent deformation in the forward warping direction.

[0090] In Comparative Example 1, deformation in the forward warp direction occurred, indicating that residual stress due to volumetric shrinkage could not be suppressed. On the other hand, deformation in the reverse warp direction occurred in the shaped plate of Example 1, indicating that volumetric expansion due to martensitic transformation suppressed residual stress.

[0091] In this experiment, the volume expansion due to martensitic transformation exceeded the volume contraction, resulting in reverse warpage. Therefore, by adjusting the first temperature T1 to a low value, it is thought that the amount of martensitic transformation can be reduced, deformation of the build plate can be avoided, and an additive manufacturing object with high dimensional accuracy can be obtained. [Explanation of symbols]

[0092] 1: Additive manufacturing equipment 10: Chamber 10a: Window 20: Material layer forming device 21: Base 22: Recoater head 22a: Blade 23: Modeling table 24: Building table drive mechanism 30: Irradiation device 40: Modeling plate 50: Machining equipment 60: Material layer 70: Solidified layer L: Laser light R: Printing area

Claims

1. C (carbon): more than 0.03 mass% but not more than 0.4 mass% Si (silicon): 0.1% by mass or more and 0.5% by mass or less Mn (manganese): 0.2% by mass or less Co (cobalt): 0.1% by mass or less Ni (nickel): 17% by mass or more and 19% by mass or less Mo (molybdenum): 1.5% by mass or more and 2.5% by mass or less Ti (titanium): 0.5% by mass or more and 2.0% by mass or less Al (aluminum): 0.8% by mass or more and 1.1% by mass or less The remainder is Fe (iron) and unavoidable impurities.

2. a material layer forming step of supplying the metal powder material for additive manufacturing according to claim 1 to a manufacturing region to form a material layer of a predetermined thickness; a solidification step of irradiating a predetermined irradiation area of ​​the material layer with laser light to form a solidified layer; a temperature adjusting step of adjusting the temperature of at least the upper surface layer, which is the newly formed solidified layer, of the solidified layers in the order of a first temperature, a second temperature, and again the first temperature each time one or more solidified layers are newly formed, When the first temperature is T1, the second temperature is T2, the martensitic transformation start temperature of the solidified layer is Ms, and the martensitic transformation finish temperature of the solidified layer is Mf, the relationships of the following formulas (1) to (3) are all satisfied: T1 ≧ Mf (1) T1>T2 (2) T2≦Ms (3) A method for manufacturing additively manufactured objects.

3. a cutting step of cutting the solidified layer, 3. The method for manufacturing an additively shaped object according to claim 2, wherein the cutting step cuts the solidified layer after adjusting the temperature of the upper surface layer from the first temperature to the second temperature and before adjusting the temperature from the second temperature to the first temperature.

4. a dimensional expansion coefficient expressed by a dimension H1 of the layered object after the upper surface layer is adjusted to a first temperature and a dimension H2 of the layered object when the upper surface layer is adjusted from the first temperature to a second temperature and then adjusted to the first temperature again satisfies the following formula (4): 0.3<{(H2-H1) / H1}×100<1.0 (4) The method for manufacturing a layered object according to claim 2 .

5. 3. The method for manufacturing an additive manufacturing object according to claim 2, wherein the surface hardness of the additive manufacturing object after adjusting the temperature of the upper surface layer from the first temperature to the second temperature is Rockwell hardness HRC 39 to 45.

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