JP7558527B2 - Gas for metal additive manufacturing and method for manufacturing metal laminated structure - Google Patents

Description

The present invention relates to a gas for metal additive manufacturing and a method for manufacturing metal additive structures.

It is known to create layered structures using metal powder as a material. This manufacturing method is called metal additive manufacturing. It is also known to use a shielding gas during metal additive manufacturing (see, for example, Patent Document 1).

JP 2018-184634 A

Additive manufacturing using resin powder as the material is gradually becoming practical in terms of methods, conditions, and costs. On the other hand, in additive manufacturing using metal powder as the material, further improvements are desired in terms of the manufacturing speed and the physical properties of the structures produced. Therefore, the objective of this study is to provide a method for quickly and reliably performing metal additive manufacturing, and to provide a gas and manufacturing method to be used for this purpose.

The metal additive manufacturing gas according to the present disclosure is a shielding gas used in additive manufacturing of metal powder by the laser melting method, and contains 2% to 20% by volume of hydrogen gas and 80% to 98% by volume of an inert gas.

The method for manufacturing a metal laminate structure according to the present disclosure includes the steps of preparing metal powder and irradiating the metal powder with laser light in the presence of a shielding gas in a metal additive manufacturing device to melt the metal powder, the shielding gas being the aforementioned shielding gas.

The above shielding gas and manufacturing method allow metal additive manufacturing to be carried out quickly and reliably.

FIG. 1 is a flowchart showing an example of a method for manufacturing a metal laminate structure according to the present disclosure. FIG. 2 is a schematic diagram showing an example of a metal additive manufacturing apparatus. FIG. 3 is a schematic diagram showing a metal melting test device. FIG. 4 is a schematic diagram showing a chamber of a metal melting test device. FIG. 5 is a diagram showing an example of sputter scattering during metal melting by laser irradiation. FIG. 6 is a diagram showing the molten state of a metal due to laser irradiation. FIG. 7 is a diagram showing an example of a cross section of a metal solidified product obtained in a metal melting test. FIG. 8 is a diagram showing an example of a cross section of a metal laminate structure. FIG. 9 is a diagram showing the relationship between the laser scanning speed and the relative density of the metal laminate structure.

[Overview of the embodiment]
First, the embodiments of the present disclosure will be described below. The shielding gas of the present disclosure is a shielding gas used in additive manufacturing of metal powder by a laser melting method, and contains 2% by volume or more and 20% by volume or less of hydrogen gas and 80% by volume or more and 98% by volume or less of an inert gas.

Conventionally, the laser melting method is known as one of the metal additive manufacturing methods. The selective laser melting (SLM) method, which is one of the laser melting methods, is a method in which a metal powder placed on the stage of a modeling device is irradiated with a laser beam to melt the metal powder, which is then solidified to form a metal layer, and this process is repeated to produce a metal laminate structure. It is also known that in the SLM method, a shielding gas is introduced into the modeling chamber and a metal layer is formed in a shielding gas atmosphere. The shielding gas is used mainly for the purpose of excluding oxygen from the modeling chamber. This is because the presence of oxygen in the modeling chamber can cause unintended reactions such as oxidation of the metal powder. It is known to use an inert gas such as nitrogen or argon as the shielding gas.

On the other hand, further improvements in the physical properties such as mechanical strength and density, and in the manufacturing efficiency, of the metal laminated structure obtained by the SLM method have been desired. In response to this problem, the inventors have found that by using a shielding gas containing a specific ratio of hydrogen in addition to an inert gas, the continuity of the metal powder melted by the laser light is good, and a metal layer with a good solidification state is obtained. Furthermore, the inventors have found that, with the above-mentioned shielding gas, even when the stacking speed is increased, the density of the stacked structure is not reduced much, and a stacked structure with good physical properties is obtained. Although not bound by a specific theory, it is believed that the effect of the present invention is related to the fact that the shielding gas present in the vicinity of the metal powder is turned into plasma when the metal powder is irradiated with laser light, and that the energy consumed for the plasma generation differs depending on the composition of the shielding gas. With the shielding gas disclosed herein, a metal melt in a good molten state can be obtained, and even when stacking is performed at a high speed, the density of the stacked structure is not reduced much.

The inert gas may be argon gas. In metal additive manufacturing, unintended metal compounds may be formed depending on the type of shielding gas, the temperature during metal melting, and the composition (elements contained) of the metal powder. In contrast, when the inert gas is argon, there is little risk of unintended metal compounds being generated, and metal laminates can be stably manufactured under a wide range of conditions. The shielding gas may also be composed of hydrogen gas and an inert gas. However, this does not prevent the gas from containing unavoidable impurity components. When the shielding gas is composed of hydrogen gas and an inert gas, unintended reactions in the metal powder are suppressed, and a laminate structure can be stably obtained.

The manufacturing method disclosed herein also includes a step of preparing metal powder, and a step of irradiating the metal powder with laser light in the presence of a shielding gas in a metal additive manufacturing device to melt the metal powder, the shielding gas being the above-mentioned shielding gas. According to the manufacturing method disclosed herein, it is possible to obtain metal in a good molten state, and a metal laminated structure having high density after solidification and high density even when lamination is performed at high speed can be obtained.

The metal powder may be a nickel alloy or stainless steel. Nickel alloys and stainless steel are preferable because they are laser absorbent and can be melted by a laser, and can be melted without requiring excessive energy. Considering the cost benefits, nickel alloys are particularly preferable.

[Specific Example of the Embodiment]
An example of the shielding gas and manufacturing method of the present disclosure will be described below.
[Shielding gas]
The shielding gas in the present disclosure is a shielding gas used in additive manufacturing of metal powder by the laser melting method. The shielding gas in the present disclosure contains 2% by volume or more and 20% by volume or less of hydrogen gas and 80% by volume or more and 98% by volume or less of inert gas. When the ratio of hydrogen gas contained in the inert gas is 2% by volume or more and 20% by volume or less, the molten state of the metal by laser irradiation is good, there are few cracks when solidified, and a metal layer with good properties is obtained. The ratio of hydrogen gas is more preferably 3% by volume or more and 19% by volume or less, and even more preferably 7% by volume or more and 18% by volume or less.

In the past, there have been proposals to improve productivity by using hydrogen or helium as a shielding gas, which has a higher thermal conductivity than air (for example, Patent Document 1). In this proposal, the use of 100% hydrogen or helium as a shielding gas is considered. In contrast, this disclosure has found that when a shielding gas containing hydrogen at a certain concentration range is used in comparison with an inert gas, the melting and solidification state of the metal melted by the laser light is improved.

In the shielding gas of the present disclosure, the inert gas may be helium gas, argon gas, nitrogen gas, or the like. The inert gas may be one type of gas or a combination of two or more types. It is preferable to use argon gas as the inert gas. The inert gas is contained in the shielding gas at a volume ratio of 80% to 98% by volume. The ratio of the inert gas is more preferably 81% to 97% by volume, and even more preferably 82% to 93% by volume.

In the shielding gas of the present disclosure, the sum of the content ratios of the inert gas and the hydrogen gas is preferably 95% by volume or more, more preferably 97% by volume or more, and even more preferably 100% by volume, relative to the shielding gas. As long as the effect of the invention is achieved, the shielding gas may contain components other than the inert gas and the hydrogen gas. Also, it may contain trace amounts of unavoidable impurity components.

[Manufacturing equipment]
FIG. 2 is a schematic diagram of an example of a metal additive manufacturing apparatus used for the manufacturing method of the present disclosure. FIG. 2 is a schematic diagram showing an example of the configuration of a metal additive manufacturing apparatus for a metal laminate structure. As shown in FIG. 2, the manufacturing apparatus includes a laser irradiation source 1, a reflector 2, a manufacturing chamber 3, a stage 4, a shielding gas supply source 5, a circulation path 6, and a circulation device 7. The shielding gas supply source 5 and the manufacturing chamber 3 are connected by a pipe L1. The manufacturing chamber 3 is connected to a pipe L2. The shielding gas is supplied from the gas supply source 5 to the manufacturing chamber 3 through the pipe L1. The manufacturing chamber 3 can be sealed, and the inside of the manufacturing chamber 3 can be filled with the shielding gas. In addition, the filled shielding gas can be circulated through the circulation path 6 and the circulation device 7. Furthermore, excess shielding gas can be discharged from the pipe L2.

The stage 4 is provided inside the modeling chamber 3. The stage 4 can be raised and lowered in the vertical direction. Metal powder, which is the raw material, is supplied to the stage 4 from a material supply mechanism (not shown). The laser light emitted from the laser irradiation source 1 is refracted by the reflector 2 and irradiated onto the metal powder on the stage 4. The direction and angle of the reflector 2 are variable. The reflector 2 is controlled by a preset program or other control mechanism. By changing the direction and angle of the reflector 2, the laser light is irradiated onto a predetermined position of the metal powder placed on the stage 4.

The shielding gas supply source 5 is connected to the forming chamber 3 via a pipeline L1. The shielding gas supply source 5 can be configured to supply a shielding gas that is a mixed gas that is mixed in advance at a predetermined ratio. In another embodiment, the shielding gas supply source 5 may include a pipeline for supplying hydrogen gas, a pipeline for supplying an inert gas, and a pipeline for joining the pipelines.

[Production method]
1 shows a flow of a manufacturing method according to the present disclosure. The manufacturing method according to the present disclosure includes a step (S10) of preparing a metal powder, and a step (S20) of irradiating the metal powder with a laser beam in the presence of a shielding gas in a metal additive manufacturing apparatus to melt the metal powder. As the shielding gas, the above-mentioned shielding gas containing hydrogen gas and an inert gas is used.

The metal powder used in the manufacturing method of the present disclosure is not particularly limited as long as it has the effect of the invention, but for example, nickel alloy or stainless steel powder can be preferably used. Specific examples include Inconel 718, Inconel 626, SUS316, SUS316L, SUS304, SUS304L, and Hastelloy C276.

The particle size of the metal powder is not particularly limited as long as it has the effect of the invention, but for example, metal powder with a median particle size of 30 μm to 80 μm can be used, preferably 60 μm or less, and more preferably 30 μm to 60 μm. In addition, it is preferable that the metal powder has a spherical or nearly spherical shape produced by gas atomization.

In one embodiment, the metal powder that is the raw material for the metal laminate structure is stored in a tank (not shown) of a material supply mechanism provided inside the modeling chamber 3 of the modeling device. The stage 4 is fitted in the tank and moves up and down within the tank. In the metal powder preparation step S10, the stage 4 moves up and down within the metal powder filled in the tank, thereby supplying a predetermined amount of metal powder onto the stage 4. Note that the metal powder preparation step is not limited to this form. For example, a material supply mechanism may be provided above the stage 4. In this case, a predetermined amount of metal powder may be sprayed onto the stage 4 from the material supply mechanism, and then the metal powder sprayed onto the stage 4 may be homogenized to a predetermined thickness.

Prior to the additive manufacturing, the inside of the molding chamber 3 is filled with a shielding gas. Specifically, for example, the molding chamber 3 is evacuated to a vacuum (for example, 1.5×10 ⁻² Pa or less) using a vacuum pump, and then purged with the shielding gas. After purging, it is preferable to circulate the shielding gas at about 5 L/min to 10 L/min in the molding chamber 3 continuously or intermittently until the end of the additive manufacturing. In this embodiment, the flow rate of the shielding gas can be set to the above range, but the flow rate of the shielding gas is not limited to this range. The flow rate of the shielding gas can be appropriately selected depending on the device used, the size of the molding chamber, the function required of the shielding gas, and the like. For example, the flow of the shielding gas may be used to remove metal fumes generated when melting metal powder by laser irradiation, and oxygen derived from metal oxides. The flow rate of the shielding gas can be selected, for example, between 1 L/min and 400 L/min.

Then, the metal powder on the stage 4 is irradiated with a laser beam. The intensity of the laser beam is not particularly limited, but for example, a laser beam of 50 W to 500 W can be irradiated. At the same time, the reflector 2 is moved to move the irradiation position of the laser beam. By controlling the irradiation position of the laser beam according to the planar structure of the structure to be produced, a metal layer having a desired shape can be obtained. In another embodiment, the laser irradiation position can be fixed and the stage moved so that the laser beam hits different positions of the metal powder on the stage 4 in sequence. The moving speed (scanning speed) of the laser beam is not particularly limited as long as the effect of the present invention can be obtained, but for example, it can be 0.1 to 5000 mm/s, and 100 to 360 mm/s is more preferable.

The metal powder irradiated with the laser light melts. At this time, spatter occurs at the irradiation position and its vicinity, but it is preferable that the amount of spatter is small. If spatter occurs, fine metal particles may be mixed into or adhere to the metal powder in the tank or on the stage, which may adversely affect the quality of the laminated structure. Here, it has been confirmed that when the shielding gas disclosed in the present disclosure is used, there is little spatter scattered during irradiation with the laser light. In addition, the molten metal powder has fluidity, and a part or the entire molten portion is integrated. At this time, if the molten state is poor, the powder will only change into granular form and will not form a continuous layer, and only a laminated structure with low density can be obtained. In contrast, when the shielding gas disclosed in the present disclosure is used, there is a tendency for a stable and highly continuous molten metal to be formed.

Molten metal solidifies as the temperature drops, but cracks can occur during the solidification process. The occurrence of cracks is undesirable because it can make it impossible to produce a laminated structure or can cause the strength of the laminated structure to be compromised. Here, by using the shielding gas disclosed herein, solidification cracks are reduced and a metal layer in a good solidified state can be obtained.

The metal layers are stacked in sequence by repeatedly performing the process of irradiating the laser light to melt and solidify the metal powder (S20) and the process of preparing the metal powder (S10). Specifically, for example, after one metal layer is produced, the stage 4 moves downward a predetermined distance. Next, metal powder is again prepared on the stage 4 (S10), and the laser light is irradiated (S20). The stacking is repeated until a metal laminate structure of the desired dimensions is obtained, and a metal laminate structure is obtained (S30).

Note that the above manufacturing method is explained based on the SLM method (also defined as Laser-based Powder Bed Fusion of metals (PBF-LB/M)), which is a type of laser melting method, but the application of the shielding gas disclosed herein is not limited to the SLM method.

<1. Metal melting test>
[Device]
A metal melting test apparatus 30 shown in a schematic diagram in FIG. 3 was used. The metal melting test apparatus 30 in FIG. 3 includes a stage chamber 33 capable of holding metal powder therein, a laser oscillator 31, and a laser head 35 for irradiating the stage chamber 33 with a laser. A powder bucket 81 for accommodating metal powder is placed inside the stage chamber 33. An X-ray tube 71 for irradiating X-rays toward the stage chamber 33 is also included. An image intensifier 72 is provided at a position facing the X-ray tube 71 across the stage chamber 33. A camera 73 is provided connected to the image intensifier 72. The X-ray image converted into a visible image in the image intensifier 72 is recorded by the camera 73.
In the melting test apparatus 30, the stage provided in the stage chamber 33 can move in the XY directions (horizontal directions in the apparatus). The moving speed can be selected in the range of 0 to 1000 mm/s. The powder bucket 81 installed on the stage has internal dimensions of 120 mm length x 10 mm height x 3 mm width, and is filled with metal powder. The laser output of the laser oscillator 31 can be selected in the range of 0 to 500 W.

[Test 1]
(1) Inconel 718 (particle size 45 (median value) to 63 μm, manufactured by Sanyo Special Steel) was packed into the powder bucket 81 of the metal melting test device 30 and placed in the stage chamber 33. Hydrogen gas (JIS K 0512-1995) and argon gas (equivalent to JIS Z3253:2011 I1) were mixed as the shielding gas to prepare a gas containing 3 volume % hydrogen and 97 volume % argon. The chamber was purged with this shielding gas at a flow rate of 5 L/min for 10 minutes or more. Next, the inside of the chamber was placed in a gas flow state of 5 L/min.
(2) Next, while moving the powder bucket 81 in the horizontal direction at a moving speed of 7.5 mm/sec, the metal powder in the bucket 81 was irradiated with a laser beam having an output of 150 W. The spot diameter of the laser beam was set to 33 μm (design value). At the same time, the molten state of the metal was directly observed using an X-ray fluoroscopy observation device (see FIGS. 3 and 4 ) and a two-color thermometer (Thermera NIR, manufactured by Mitsui Photonics Inc.).
As a specific apparatus, as shown in FIG. 3 and FIG. 4, a laser head 35, an X-ray tube 71, an image intensifier 72, and a two-color thermometer were fixed. In addition, a powder bucket 81 containing metal powder 50 was moved horizontally in the stage chamber 33. With this configuration, the spot of the laser light was irradiated while moving relatively to the metal powder 50. In this way, the molten part of the metal powder melted by the laser light was continuously observed. The X-ray tube 71 was fixed horizontally to the metal powder sample, and the melting state and melting depth were observed. The two-color thermometer (not shown) was fixed vertically upward to the metal powder sample, and the temperature distribution during melting was measured.
(3) The appearance of the obtained solidified metal was visually observed. In addition, the solidified metal was fixed using a thermosetting resin, polished in the cross-sectional direction, and the cross-section was observed using an optical microscope.

[Tests 2 to 11]
A metal melting test was performed in the same manner as in Test 1, except that the composition of the shielding gas was as shown in Table 1. Tests 1 to 7 are examples of the present disclosure, and Tests 8 to 11 are comparative examples of the present disclosure.

[evaluation]
The temperature during melting and the amount of spatter were confirmed by a two-color thermometer. The melting state (continuity of the melt and melt depth) was confirmed by an X-ray fluoroscopy observation device. As described above in (3), the appearance of the obtained metal solidified product was visually observed, and the cross section of the metal solidified product was observed by an optical microscope.

[result]
A part of the observation results using the two-color thermometer is shown in [Figure 5]. The white parts in the image are part of the spatter. As shown in [Figure 5], when 3% H ₂ /Ar was used as the shielding gas (Test 1), the amount of spatter was clearly less than when only Ar was used as the shielding gas (Test 11). It was thought that the less spatter would ensure product quality when making an additive manufacturing object.

Observation of the molten metal using an X-ray transmission observation device confirmed that the continuity of the molten material was higher when 3% _H2 /Ar was used as the shielding gas (Test 1) than when only Ar was used as the shielding gas (Test 11). The observation results of the molten metal are shown in [Figure 6]. In Figure 6, the molten metal is shown in a white frame for ease of understanding.

Visual observation of the appearance of the obtained metal solidified product showed that in each of the cases where 7%, 10%, 16.6%, and 18% _H2 /Ar were used as the shielding gas (Tests 3 to 6), a metal solidified product with high continuity and hardness was obtained. In particular, when 10% _H2 /Ar was used as the shielding gas (Test 4), the metal solidified product was the best in terms of appearance (continuity of the solidified product) and brittleness. On the other hand, when a shielding gas with an _H2 concentration of more than 20% was used (Tests 8 to 10), the metal solidified product had low continuity and was in a scattered state.

Some of the results of cross-sectional observation of the metal solidified material by an optical microscope are shown in [Figure 7]. As shown in [Figure 7], in each case (Test 3 to Test 6) in which 7%, 10%, 16.6% and 18% _H2 /Ar were used as the shielding gas, good solidified material with little solidification cracking was obtained. In particular, when the _H2 concentration was 7% (Test 3), solidification cracking was the least. Note that solidification cracking refers to cracking that occurs when molten metal solidifies as the temperature decreases and becomes solid metal.

[Test 12]
A metal melting test was carried out in the same manner as in Test 1, except that SUS316 (45 (median) to 63 μm (manufactured by LPW Technology)) was used as the metal powder and 5% H ₂ /95% N ₂ (product name: Leakmate, manufactured by Iwatani Corporation) was used as the shielding gas.

[Test 13]
A metal melting test was carried out in the same manner as in Test 12, except that N ₂ (corresponding to JIS Z 3253:2011N1) was used as the shielding gas.

Test 12 is an example of the present disclosure, and Test 13 is a comparative example of the present disclosure. Test 12, as compared to Test 13, showed a smaller amount of spatter scattering as a result of observation with a two-color thermometer. In addition, as a result of observation of the molten metal with an X-ray transmission observation device, it was confirmed that the metal solid obtained in Test 12 had higher molten continuity than the metal solid obtained in Test 13.

<2. Metal lamination test>
[Device]
The metal additive manufacturing device shown in the schematic diagram in Fig. 2 was used. The laser output was 200 W. The laser spot diameter was 100 μm (design value).
[Metal powder]
Inconel 718 (45 (center value) to 63 μm (manufactured by Sanyo Special Steel))
[Shielding gas]
Argon gas (equivalent to JIS Z 3253:2011 I1)
3% _H2 /Ar (equivalent to JIS Z 3253:2011 I3, "Tigmate" manufactured by Iwatani Corporation)
2% _H2 /Ar (equivalent to JIS Z 3253:2011 I3, "Tigmate" manufactured by Iwatani Corporation)

[Additive manufacturing method]
(1) The powder bucket, which is the material supply mechanism, was filled with metal powder and placed in the device. The gap between the modeling stage and the coater was adjusted to within 0.05 mm, and the metal powder was spread between them.
(2) The inside of the modeling chamber was evacuated with a vacuum pump (internal pressure 1.5×10 ⁻² Pa or less), and then purged with a shielding gas for 10 minutes or more to create a gas flow state of 5 L/min.
(3) After the shielding gas was circulated, the laser was started. The laser was irradiated onto the metal powder while scanning, and the metal powder was melted and solidified to form a metal layer.
(4) After the formation of one layer was completed, the modeling stage was lowered by 0.05 mm, and the metal powder was spread on the modeling stage by a coater. At this time, the thickness of the metal powder was set to 0.05 mm. Then, the metal powder was irradiated with a laser to form a metal layer. By repeating this operation, 200 layers were stacked, and a metal laminated structure of 7.5 x 5 x 10 mm was obtained.

The above-mentioned additive manufacturing was performed using three types of shielding gas, namely, argon gas, 3% _H2 /Ar, and 2% _H2 /Ar, and the laser scanning speeds were set to 60, 90, 120, 150, 180, 270, 360, 450, 540, and 900 mm/sec.

The cross section of the obtained metal laminate structure was observed and the density was measured.
The cross-section of the metal laminated structure was observed by fixing the metal laminated structure with a thermosetting resin, polishing the metal laminated structure in the cross-sectional direction, and observing the cross-section with a stereomicroscope. The density was measured by the Archimedes method.

A cross section of a portion of the obtained metal laminate structure is shown in Fig. 8. As shown in Fig. 8, when 3% _H2 /Ar was used as the shielding gas, no significant difference was observed between the case where argon gas was used as the shielding gas, and a metal laminate structure was formed in either case.

[Figure 9] is a graph of the relative density (%) of a metal laminate structure versus the laser scanning speed. As shown in [Figure 9], when the laser scanning speed exceeds 360 mm/sec, the relative density tends to decrease. However, when a shielding gas containing 2% hydrogen gas (shown in gray) or 3% hydrogen gas (shown in black) was used, the decrease in relative density was smaller than when argon gas only (shown in white) was used. From this, it was thought that a shielding gas containing hydrogen gas enables the high-speed production of metal laminate structures with excellent physical properties.

It should be understood that the embodiments disclosed herein are illustrative in all respects and are not restrictive in any respect. The scope of the present invention is indicated by the claims, not by the meaning described above, and is intended to include all modifications within the meaning and scope of the claims.

1 Laser irradiation source, 2 Reflector, 3 Modeling chamber, 4 Stage, 5 Shielding gas supply source, 6 Circulation path, 7 Circulation device, 30 Melting test device, 31 Laser oscillator, 33 Stage chamber, 35 Laser head, 50 Metal powder, 71 X-ray tube, 72 Image intensifier, 73 Camera, 81 Powder bucket, L1, L2 Pipe

Claims (5)

A shielding gas used in additive manufacturing of metal powder by a laser melting method,
A shielding gas comprising: 7 % by volume or more and 18 % by volume or less of hydrogen gas; and 82 % by volume or more and 93 % by volume or less of an inert gas, the inert gas comprising argon gas . 2. The shielding gas according to claim 1, wherein the argon gas content is 82% by volume or more and 93% by volume or less . 3. The shielding gas according to claim 1, wherein the shielding gas comprises hydrogen gas and argon gas. Providing a metal powder;
and irradiating the metal powder with a laser beam in the presence of a shielding gas in a metal additive manufacturing apparatus to melt the metal powder.
The shielding gas is a shielding gas according to any one of claims 1 to 3.
A method for manufacturing a metal laminate structure. The method for manufacturing a metal laminate structure according to claim 4, wherein the metal powder is a nickel alloy or stainless steel.

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