JP7770751B2 - Fe-based alloy powder for additive manufacturing with excellent resistance to cracking during manufacturing and high thermal conductivity, and additive manufacturing body using the same - Google Patents

Description

The present invention relates to shaped bodies and raw material powders that can be used to make various tools, including molds, using additive manufacturing (also known as 3D printers, three-dimensional modeling, additive manufacturing, etc.). In particular, the present invention relates to powders for additive manufacturing that exhibit high resistance to cracking during molding, preventing cracking even in large shaped bodies, high thermal conductivity, and also have high mechanical properties.

Here, high resistance to cracking during molding refers to the resistance to cracking in the molded object itself (especially in areas where notches will form) or at the interface between the molded object and the base material due to thermal stresses that accompany the rapid melting and solidification that occur during additive manufacturing.

Unlike conventional manufacturing methods, additive manufacturing is capable of producing components with complex shapes and three-dimensional structures, and in recent years has seen remarkable technological development and an expansion of its range of applications. In this context, research and application of additive manufacturing to various tools is also progressing, with particular progress being made in its practical application to die-casting molds that contain complex three-dimensional cooling water pipes inside.

These tools are used to process a variety of parts, and their shapes and sizes vary depending on the processing method and part shape. Generally, additive manufacturing involves rapidly melting and solidifying raw material powder or wire by heating it for a short period of time with a narrowly focused heat source such as a laser or electron beam, and repeating this process to build up solidified layers, making it possible to manufacture parts with complex three-dimensional shapes. During this process, only a portion of the part is heated, melted, and solidified, which generates thermal stress due to localized solidification shrinkage and thermal expansion and contraction. If the material being molded or the base material is hard and brittle, it will not be able to withstand the thermal stress that occurs, resulting in cracks in the molded object itself or at the interface with the base material.

Such thermal stresses become even greater when large objects are additively manufactured, resulting in increased likelihood of cracks occurring during manufacturing. Tools generally use high-hardness alloys such as JIS standard SKD61, which are prone to cracks during manufacturing. Therefore, the application of additive manufacturing to tools has traditionally been limited to small tools with relatively low thermal stress.

In recent years, a method has been proposed in which alloys with a low M _point are used and preheated to near the M _point during additive manufacturing, thereby maintaining the soft, ductile austenite phase rather than the hard, brittle martensite phase during manufacturing, thereby suppressing molding cracking. In this way, lowering the M _point can be an indicator of improved resistance to molding cracking.

The surfaces of tools used in hot working, such as die-casting dies, rise in temperature as they come into contact with the high-temperature parts (workpieces) being machined, causing damage such as heat checking. Furthermore, areas that experience particularly rapid temperature increases can become seized. To avoid these problems, it is important to efficiently cool the die surface, and by using an alloy with high thermal conductivity as the die material, the cooling effect of water-cooled pipes placed inside the tool can be maximized all the way to the die surface.

Furthermore, when machining parts using hot tools, the tools must cool after machining one part before machining the next. If the tools can be cooled efficiently (in a short time) to the specified temperature, the part machining cycle can be shortened, which has the benefit of increasing part production efficiency.

Patent Documents 1 and 2 are examples of additively manufactured bodies made from Fe-based alloys with high build crack resistance and high thermal conductivity. The major difference between the main components of the inventions in Patent Documents 1 and 2 is the amount of Cr, with Patent Document 1 generally being on the higher Cr side compared to Patent Document 2. Furthermore, both Conventional Examples 1 and 2 restrict the variable A (= 15C + Mn + 0.5Cr + Ni) to a certain range.

Patent Document 1 regulates the content of various elements and sets variable A at more than 10 and less than 20. As mentioned above, Conventional Example 1 is in the high Cr range, so the balance between Cr and Ni is on the low Ni side due to the control of variable A, and the properties in the opposite range of low Cr (12% or less) and high Ni (4% or more) have not been fully investigated.

Patent Document 2 regulates the content of various elements and sets variable A at more than 11.5 and less than 20. As mentioned above, Conventional Example 2 is in the low Cr range, so the balance between Cr and Ni is on the high Ni side due to the control of variable A, and the properties in the opposite range of high Cr (6% or more) and low Ni (7% or less) have not been fully investigated.

Japanese Patent Application Laid-Open No. 2023-71145 JP 2023-71110 A

The problem that this invention aims to solve is to provide a powder for additive manufacturing that is resistant to cracking in the manufactured object and at the interface between the manufactured object and the base material, even in large-scale additive manufacturing, and has high thermal conductivity, as well as an additive manufacturing object made using this powder.

The inventors conducted detailed research focusing on the range of 6-12% Cr and 4-7% Ni, and surrounding areas. They discovered that by defining the balance between Cr and Ni within the region bounded by formulas A, B, and C, and by specifying the components other than Cr and Ni within desired ranges, a powder for additive manufacturing with excellent properties can be obtained.

That is, the first means for solving the problems of the present invention is:
In mass%,
C: 0.11-0.29%,
Si: 0.9% or less,
Mn: 0.9% or less,
P: 0.024% or less,
S: 0.024% or less,
Cr: 6.1-9.9%,
Ni: 3.1 to 6.9%,
The balance consists of Fe and unavoidable impurities,
Formula A: [Ni]-0.2[Cr]≧1.8,
Formula B: [Ni]-2.0[Cr]≦-8.0,
This is a powder for additive manufacturing that satisfies formula C: [Ni] + 0.6 [Cr] ≦ 11.0.
In the formula, the mass % values of each component are substituted for [Ni] and [Cr].

The second means is
O: 0.10% or less,
The powder for additive manufacturing according to the first aspect, wherein N is 0.15% or less.

The third means is
In addition to the ingredients described in either the first or second means,
As optional additional components, one or more of Mo, W, V, Al, and Cu are added in an amount of Mo+W/2: 1.4% or less;
V: 0.7% or less,
Al: 0.045% or less,
Cu: contained in a range of 4.5% or less,
The balance consists of Fe and unavoidable impurities,
Formula A: [Ni]-0.2[Cr]≧1.8,
Formula B: [Ni]-2.0[Cr]≦-8.0,
This is a powder for additive manufacturing that satisfies formula C: [Ni] + 0.6 [Cr] ≦ 11.0.
In the formula, the mass % values of each component are substituted for [Mo], [W], [Ni], and [Cr].

The fourth means is a shaped body manufactured by layered manufacturing using the powder described in any one of the first to third means.

This powder for additive manufacturing and additive manufacturing objects made using it are less likely to crack at the interface between the manufactured object and the base material, even in large-scale additive manufacturing, and have high thermal conductivity.

Before explaining the mode for carrying out the present invention, we will explain the reasons for specifying each component of the present invention. Note that % in the components refers to % by mass. We will also explain the reasons for specifying Formula A, Formula B, and Formula C.

C: 0.11-0.29%
C is an essential element for obtaining high quench-and-temper hardness by dissolving in the martensite phase, which is the matrix, and precipitating fine carbides. However, if C is less than 0.11%, high quench-and-temper hardness cannot be obtained. C is preferably 0.12% or more, more preferably 0.13% or more. On the other hand, if C exceeds 0.29%, the as-formed hardness becomes excessively high, deteriorating the forming crack resistance. C is preferably 0.24% or less, more preferably 0.19% or less.

Si: 0.9% or less (optional component)
Since Si has the effect of improving the flowability of the molten alloy, adding it appropriately can reduce the likelihood of nozzle clogging during powder production by atomization. When Si is added, it is preferable to add 0.1% or more. On the other hand, adding more than 0.9% Si reduces thermal conductivity. The Si content is preferably 0.7% or less, more preferably 0.4% or less.

Mn: 0.9% or less (optional component)
Mn has the effect of improving the flowability of the molten alloy, so adding it appropriately can make nozzle clogging less likely during powder production by atomization. When Mn is added, it is preferably added in an amount of 0.1% or more. On the other hand, adding more than 0.9% of Mn reduces thermal conductivity. The Mn content is preferably 0.4% or less, more preferably 0.3% or less.

P: 0.024% or less P is a typical impurity, but adding trace amounts has the effect of improving the flowability of the molten alloy, so adding it appropriately can make nozzle clogging less likely during powder production by atomization. The P content is preferably 0.001% or more. On the other hand, if the P content exceeds 0.024%, solidification cracking is more likely to occur during additive manufacturing. P is preferably 0.014% or less, more preferably 0.004% or less. The same effect can be obtained whether P is contained as an impurity or intentionally added.

S: 0.024% or less S is a typical impurity, but adding a small amount has the effect of improving the flowability of the molten alloy, so adding it appropriately can make nozzle clogging less likely to occur during powder production by atomization. The S content is preferably 0.001% or more. On the other hand, if the S content exceeds 0.024%, solidification cracking is more likely to occur during additive manufacturing. S is preferably 0.014% or less, more preferably 0.004% or less. The same effect can be obtained whether S is contained as an impurity or intentionally added.

Cr: 6.1-9.9%
Cr is an important essential component in the present invention, not only because it improves hardenability and corrosion resistance, but also because it reduces the _M point. Furthermore, tool steels are often used while cooled with industrial water, etc., and therefore corrosion resistance to prevent rusting is also required, making the addition of Cr essential in this respect. However, if the Cr content is less than 6.1%, a low _M point cannot be obtained. The Cr content is preferably 7.1% or more, more preferably 8.1% or more. On the other hand, if the Cr content exceeds 9.9%, the thermal conductivity will be significantly reduced. Therefore, the Cr content is preferably 9.4% or less, more preferably 9.3% or less.

Ni: 3.1-6.9%
Ni is an important essential component in the present invention, not only because it improves hardenability and corrosion resistance, but also because it reduces the _M point. Furthermore, tool steels are often used while cooled with industrial water, etc., and therefore corrosion resistance to prevent rusting is also required, making the addition of Ni essential in this respect as well. However, if the Ni content is less than 3.1%, a low _M point cannot be obtained. The Ni content is preferably 3.5% or more, more preferably 3.7% or more. On the other hand, if the Ni content exceeds 6.9%, the thermal conductivity decreases significantly. Therefore, the Ni content is preferably 5.4% or less, more preferably 4.9% or less.

O: preferably 0.100% or less O is an impurity that is inevitably mixed in mainly during powder production by gas atomization, but if it exceeds 0.100%, gas is generated during additive manufacturing, and pores remain inside the molded object. Therefore, the O content is preferably 0.050% or less, more preferably 0.030% or less. On the other hand, it is difficult to keep the O content in powder below 0.003%, so it is acceptable for O to be unavoidably contained.

N: preferably 0.150% or less N is an impurity that is inevitably mixed in mainly during powder production by gas atomization, but if it exceeds 0.150%, gas is generated during additive manufacturing, and pores remain inside the molded object. Therefore, N is preferably 0.120% or less, more preferably 0.090% or less. On the other hand, it is difficult to keep N content below 0.003% in powder, so it is acceptable for N to be unavoidably contained.

Next, we will explain why Mo, W, V, Al, and Cu are optional additional components. When adding these components, one or more of them can be added at will.

Mo+W/2: 1.4% or less Mo and W are components that promote secondary hardening during tempering and increase quench-temper hardness, and can be added as needed. When Mo and W are added, the total amount of Mo+W/2 is preferably 0.1% or more. On the other hand, if the total amount of Mo+W/2 exceeds 1.4%, the thermal conductivity decreases. From this viewpoint, when Mo and W are added, the preferred amount of addition is a total amount of Mo+W/2 of 0.2% or less.

V: 0.7% or less V is a component that promotes secondary hardening during tempering and increases quench-temper hardness. It can be added as needed, but excessive addition reduces thermal conductivity. V content is preferably 0.2% or less, and more preferably zero.

Al: 0.045% or less Al is a component that forms nitrides and suppresses grain coarsening during quenching, and can be added as needed. The Al content is preferably 0.001% or more, more preferably 0.002% or more. However, adding 0.045% or more of Al reduces toughness due to the formation of excess Al nitrides. It also reduces thermal conductivity. The Al content is preferably 0.025% or less, more preferably 0.009% or less. Note that Al is not only added intentionally, but can also be mixed in from refractories used in the melting process of gas atomization. In either case, the effect of containing Al is similar.

Cu: 4.5% or less Cu is a component that can lower the M _point without reducing the thermal conductivity, and may be added as needed to lower the M _point . It may also be added from the viewpoint of improving corrosion resistance. Cu content is preferably 0.1% or more, more preferably 0.2% or more. On the other hand, excessive addition of Cu reduces the thermal conductivity. Therefore, Cu content is preferably 2.9% or less, more preferably 0.4% or less.

Formula A: [Ni]-0.2[Cr]≧1.8,
Formula B: [Ni]-2.0[Cr]≦-8.0,
Formula C: [Ni]+0.6[Cr]≦11.0
In the formula, the mass % values of the components are substituted for [Ni] and [Cr].
The reasons for specifying the values of these formulas A, B, and C will be explained in order. By satisfying the values of these formulas A, B, and C, it is possible to identify a region surrounded by formulas A, B, and C where Cr and Ni are balanced, and it becomes possible to selectively capture powder. Note that the amounts of Cr and Ni added are larger than those of other elements, and they have a large effect on these formulas.

Formula A: 1.8 or higher Formula A ([Ni] - 0.2[Cr]) is an important parameter for controlling the _Ms point low through the balance between Cr and Ni. Cr is a carbide-forming component, and Ni is a non-carbide-forming component. The balance between these two components affects the carbide formation behavior, including secondary hardening, and the resulting composition of the martensite phase in the matrix, and also affects hardness, the _Ms point, and thermal conductivity.
If the value of [Ni]-0.2[Cr] in formula A is less than 1.8, the amount of Cr relative to Ni is excessively high, resulting in the formation of a large amount of Cr carbide and a deficiency of C in the matrix, which increases the M _point . Here, C is a component that lowers the _M point. The value of formula A is preferably 1.9 or more, more preferably 2.0 or more.

Formula B: -8.0 or less Formula B ([Ni] - 2.0[Cr]) is an important parameter for controlling high thermal conductivity through the balance between Cr and Ni. Cr is a carbide-forming component, and Ni is a non-carbide-forming component. The balance between these two components affects the carbide formation behavior, including secondary hardening, and the resulting composition of the martensite phase in the matrix, which in turn affects hardness, the _M point, and thermal conductivity.
If the value of [Ni]-2.0[Cr] in formula B exceeds -8.0, the amount of Cr relative to Ni is too low, the amount of Cr carbide produced is too small, and the amount of C in the matrix becomes excessive, resulting in low thermal conductivity. Here, C is a component that reduces thermal conductivity. The value of formula B is preferably -9.0 or less, more preferably -11.0 or less.

Formula C: 11.0 or less Formula C ([Ni] + 0.6[Cr]) is an important parameter for controlling the quenching and tempering hardness to a high level by the balance between Cr and Ni. If the amounts of both Cr and Ni added are excessively high, austenite becomes overly stabilized, and this soft austenite tends to remain without sufficient decomposition, resulting in insufficient hardness. If the value of [Ni] + 0.6[Cr] in formula C exceeds 11.0, the retained austenite becomes stable and insufficient hardness cannot be obtained. The value of formula C is preferably 10.5 or less, more preferably 10.0 or less.

Specific embodiments of the present invention will be described below using the examples and comparative examples of the present invention shown in Table 1. Please note that these examples are merely examples of embodiments of the present invention, and the scope of the claims is not limited to these examples alone.

[Production of raw material powder]
Powders of the components listed in Examples 1 to 24 in Table 1 and Comparative Examples 1 to 16 in Table 2 were obtained by gas atomization. First, the raw materials placed in an alumina crucible were melted by high-frequency heating in a vacuum and argon atmosphere. Next, this molten alloy was poured from a 5 mm diameter nozzle at the bottom of the crucible and immediately sprayed with high-pressure argon gas. This spraying breaks the molten alloy into fine droplets, which cool and solidify as they fall through the tower of the atomization device, becoming alloy powder. This alloy powder was shaken through a sieve with a mesh opening of 63 μm, and the powder that fell below the sieve was used as the raw material powder for the subsequent additive manufacturing process.

[Additive Manufacturing]
The additive manufacturing was performed using a laser-heated powder bed type device (product name: EOS-M290) under the standard manufacturing conditions for maraging steel (MS1 conditions) set in the device, with a preheating temperature of 180°C. The plate used as the base material for the manufacturing was annealed S45C, and a 20 mm x 20 mm x 20 mm block was manufactured on top of this.

[evaluation]
The molded blocks were cut using a wire cutter and used for evaluation. The hardness was measured using a Rockwell hardness tester.
The thermal conductivity was measured by a laser flash method (test piece size: diameter 5 mm x thickness 1 mm).
The _{M point} was evaluated by measuring the thermal expansion characteristics of a test piece having a diameter of 3 mm and a length of 10 mm using a Formaster tester, with the maximum heating temperature being 1050°C.
The results are shown in Tables 1 and 2.

In the evaluation of hardness, _Ms point, and thermal conductivity, a hardness of 45 to 55 HRC, an Ms point of 230° C. or less, and a thermal conductivity of 18.0 W/m·K or more were judged to be good.

In some examples, the wire-cut blocks were tempered at the following temperatures: 525°C for Example 3, 600°C for Example 9, 575°C for Example 14, and 625°C for Example 22. The tempering was performed by holding the blocks at these tempering temperatures for one hour, followed by air cooling, and this procedure was repeated twice.

All test pieces additively manufactured using the powders of Examples 1 to 24 satisfied the components and values of Formulas A, B, and C specified in the present invention. These manufactured test pieces had hardnesses of 45.6 to 54.5 HRC, which is the hardness required for machine tools and parts such as molds, but was not too hard and did not cause cracking during manufacturing. Appropriate quenched and tempered hardness was also obtained. Furthermore, the Ms point was kept low in the Examples, at 132 to 222°C, which made it easier to suppress cracking during manufacturing. Furthermore, the Examples had thermal conductivity of 18.1 W/m·k or higher, demonstrating good thermal conductivity.

In Comparative Example 1, the C content was too low, and hardness was not obtained.
In Comparative Example 2, the value of formula B is on the high side, and the amount of Cr relative to Ni is low, so the amount of Cr carbide produced is excessively small and C in the matrix is excessive, resulting in low and inferior thermal conductivity.
In Comparative Example 3, the value obtained by Formula A is not satisfied, the M _S point is too high, and the molding crack resistance is poor.
In Comparative Example 4, the value obtained by formula C is not satisfied, and the hardness is not obtained. In addition, the thermal conductivity is low and poor.
In Comparative Example 5, the value obtained by Formula A is not satisfied, the M _S point is too high, and the molding crack resistance is poor.
In Comparative Example 6, the value obtained by formula C is not satisfied, and the hardness is not obtained. In addition, the thermal conductivity is low and poor.
In Comparative Example 7, the value of formula B is on the high side, and the thermal conductivity is low and inferior.
In Comparative Example 8, the value obtained by Formula A is not satisfied, the M _S point is too high, and the molding crack resistance is poor.
In Comparative Example 9, the value obtained by formula C was not satisfied, and the hardness was not obtained. In addition, the thermal conductivity was low and poor.
In Comparative Example 10, the value obtained by Formula A is not satisfied, the M _S point is too high, and the molding crack resistance is poor.
In Comparative Example 11, the value obtained by formula C is not satisfied, and the hardness is not obtained. In addition, the thermal conductivity is low and poor.
Comparative Example 12 contains too much C, and the hardness of the as-formed product is excessively high, resulting in poor resistance to cracking during forming.
In Comparative Example 13, the Cr content was too low, and the M _S point was not lowered.
Comparative Example 14 contains an excessive amount of Cr and has a low thermal conductivity.
Comparative Example 15 had too little Ni, the value of formula A was not obtained, the M _S point was too high, and the molding crack resistance was poor.
In Comparative Example 16, the value of formula C is not satisfied, and the hardness is not obtained. In addition, the thermal conductivity is low and poor.

The powder for additive manufacturing of the present invention has a suppressed M _point temperature and is less susceptible to cracking during manufacturing, making it suitable for additive manufacturing. The additive manufactured body has excellent thermal conductivity and sufficient hardness, making it suitable for use in molds and industrial machinery such as hot work tool parts.

Claims (5)

In mass%,
C: 0.11-0.29%,
Si: 0.9% or less,
Mn: 0.9% or less,
P: 0.024% or less,
S: 0.024% or less,
Cr: 6.1-9.9%,
Ni: 3.1 to 6.9%,
The balance consists of Fe and unavoidable impurities,
Formula A: [Ni]-0.2[Cr]≧1.8,
Formula B: [Ni]-2.0[Cr]≦-8.0,
Powder for additive manufacturing that satisfies formula C: [Ni] + 0.6 [Cr] ≦ 11.0.
In the formula, the mass % values of each component are substituted for [Ni] and [Cr]. O: 0.10% or less,
The powder for additive manufacturing according to claim 1, wherein N is 0.15% or less. In addition to the components according to claim 1 or 2,
As optional additional components, one or more of Mo, W, V, Al, and Cu are added in an amount of Mo+W/2: 1.4% or less;
V: 0.7% or less,
Al: 0.045% or less,
Cu: contained in a range of 4.5% or less,
The balance consists of Fe and unavoidable impurities,
Formula A: [Ni]-0.2[Cr]≧1.8,
Formula B: [Ni]-2.0[Cr]≦-8.0,
Powder for additive manufacturing that satisfies formula C: [Ni] + 0.6 [Cr] ≦ 11.0.
In the formula, the mass % values of each component are substituted for [Mo], [W], [Ni], and [Cr]. In mass%,
C: 0.11-0.29%,
Si: 0.9% or less,
Mn: 0.9% or less,
P: 0.024% or less,
S: 0.024% or less,
Cr: 6.1-9.9%,
Ni: 3.1 to 6.9%,
The balance consists of Fe and unavoidable impurities,
Formula A: [Ni]-0.2[Cr]≧1.8,
Formula B: [Ni]-2.0[Cr]≦-8.0,
An additive manufacturing object formed from a powder for additive manufacturing that satisfies formula C: [Ni] + 0.6 [Cr] ≦ 11.0.
In the formula, the mass % values of each component are substituted for [Ni] and [Cr]. In mass%,
C: 0.11-0.29%,
Si: 0.9% or less,
Mn: 0.9% or less,
P: 0.024% or less,
S: 0.024% or less,
Cr: 6.1-9.9%,
Ni: 3.1 to 6.9%,
As an optional additional component, one or more of Mo, W, V, Al, and Cu may be added.
Mo+W/2: 1.4% or less,
V: 0.7% or less,
Al: 0.045% or less,
Cu: contained in a range of 4.5% or less,
The balance consists of Fe and unavoidable impurities,
Formula A: [Ni]-0.2[Cr]≧1.8,
Formula B: [Ni]-2.0[Cr]≦-8.0,
An additive manufacturing object formed from a powder for additive manufacturing that satisfies formula C: [Ni] + 0.6 [Cr] ≦ 11.0.
In the formula, the mass % values of each component are substituted for [Ni] and [Cr].