JP7462185B2 - How to create 3D fired objects - Google Patents

Description

Article 30, Paragraph 2 of the Patent Act applied. Published on the Laser Society of Japan 40th Annual Conference Lecture Abstracts website on January 20, 2020.

Application of Article 30, Paragraph 2 of the Patent Act Presented at the 40th Annual Meeting of the Laser Society of Japan held at the Sendai International Center on January 21, 2020

The present invention relates to a method for producing three-dimensional fired products using soil materials.

Powder bed fusion (PBF) is a method for producing a 3D object by irradiating a powder of resin, metal, etc. with a laser, which is a heat source, to sinter it. Powder bed fusion is a method for producing a 3D object by repeatedly performing a process in which a powdered material is thinly laid out to form a powder layer, and then irradiating the areas of the powder layer to be sintered with a laser.

However, the laser used as the heat source in powder bed fusion bonding has a small irradiation diameter and provides rapid localized heating, which can easily cause temperature variations on the surface of the powder layer. For this reason, for example, in Patent Document 1, the laser irradiation path on the powder layer is set so that the temperature reached by laser irradiation is constant, and the irradiation position is controlled.

Specifically, the first irradiation pass is set with a scan pitch that is at least twice the laser irradiation diameter, and the second to Nth irradiation passes are set to complement the first irradiation pass by shifting this scan pitch by a distance that is 1/N. The laser is then irradiated in order starting from the first irradiation pass, but if the powder layer retains a history of temperature rise due to the preceding laser irradiation during the laser irradiation, a rest period during which no laser light is irradiated is provided to keep the reached temperature constant.

JP 2019-151050 A

The method of Patent Document 1 is suitable when granulated material with a uniform and small particle size (for example, about 100 μm) is used as the material for the powder layer. However, when attempting to apply this method to a sand layer made of sand, for example, various problems arise.

Specifically, the grain size of sand is large, up to about 2 mm, and is not uniform, so the sand layer formed by spreading sand is not of a uniform thickness, with some parts being thicker than others. Also, compared to powder layers made of metal or resin, which are commonly used materials for powder bed fusion bonding, sand layers have a low thermal conductivity and a high absorption coefficient. The absorption coefficient refers to the proportion of radiation absorbed per unit length as it travels through a material when it is irradiated with radiation.

For this reason, even if the temperature reached at the surface of the sand layer is constant, not only is it difficult for the temperature of the entire sand layer to be uniform, but it is also easy for the bottom surface to not be heated and melted. To address this, it is possible to increase the output of the laser, but if this is used to rapidly heat the sand, small grains and minerals with low boiling points will evaporate, making it difficult to produce high-quality objects.

The present invention has been made in consideration of these problems, and its main objective is to provide a method for producing three-dimensional fired products that can produce high-quality three-dimensional fired products using soil materials.

In order to achieve this object, the present invention provides a method for producing a three-dimensional sintered product, which comprises: forming a particle layer by spreading out an earthen material; irradiating a surface of the particle layer with a laser beam along an irradiation path; and repeating a melting and solidifying process for melting and solidifying the particle layer while stacking the particle layers; the irradiation path includes an irradiation line indicating the direction in which the laser beam travels; the subsequent irradiation line is shifted in the width direction relative to the preceding irradiation line while being overlapped substantially parallel to the preceding irradiation line; and the region of the particle layer that will become the three-dimensional sintered product is set to be irradiated with the laser beam a plurality of times; and the laser beam is irradiated along the preceding irradiation line and then irradiated along the subsequent irradiation line. a heat transfer period from the surface of the particle layer in the depth direction is provided between the irradiation of the particle layer and the irradiation of the soil material, the soil material being sand in which quartz accounts for the largest proportion, and the amount of shift of the irradiation line in the width direction is calculated by calculating the energy density that can be applied by one irradiation of the particle layer based on the following formula, dividing the calculated energy density by the material specific heat and material density of quartz to obtain the temperature rise per irradiation, determining the number of repeated irradiations that will exceed the melting point but not reach a temperature at which volatilization occurs based on the obtained temperature rise and the melting point and boiling point of the particle layer, and then dividing the scanning pitch of the laser light by the determined number of repeated irradiations .
Ed = P / (w x h x v)
Ed: Energy density (J/mm ³ )
P: Laser output (W)
w: Scanning pitch (irradiation diameter: mm)
h: Layer pitch (mm)
v: Scanning speed (mm/sec)

The method for producing a three-dimensional fired product of the present invention is also characterized in that the number of times the area to be the three-dimensional fired product is irradiated with the laser light is determined based on the temperature rise of the particle layer resulting from irradiating the laser light once along the irradiation line.

According to the method for producing a three-dimensional fired product of the present invention, a period of heat transfer from the surface of the particle layer in the depth direction is provided between irradiating the laser light along the preceding irradiation line and irradiating the laser light along the following irradiation line. As a result, the heat supplied by irradiating the laser light along the preceding irradiation line is transferred in the depth direction of the particle layer, and then the operation of irradiating the laser light along the following irradiation line is repeated.

Therefore, compared to powders such as resins and metals that are generally used as materials in powder bed fusion (PBF), a particle layer made of soil material with low thermal conductivity and particle size distribution can be heated uniformly all the way to the bottom without concentrating heat on the surface, thereby raising the temperature.

This makes it possible to melt and solidify the particle layer all the way to the bottom surface, producing a high-quality three-dimensional fired product. In addition, since there is no need to preheat the particle layer, and no mold is required for molding, a simple configuration that only requires irradiating laser light makes it possible to easily produce a three-dimensional fired product with the desired shape.

Furthermore, the number of times to irradiate the area of the particle layer that will become the three-dimensional sintered product is determined based on the temperature rise of the particle layer caused by irradiating the laser light once along the irradiation line. This makes it possible to produce a three-dimensional sintered product in a short period of time while efficiently supplying heat with the energy density required to melt the particle layer to the particle layer without waste.

According to the present invention, by alternating periods of irradiating the particle layer with laser light and periods of heat transfer in the particle layer, it is possible to uniformly heat the particle layer from the top to the bottom, increasing the temperature and melting and solidifying the particle layer, which was previously considered difficult to handle as a material for powder bed fusion (PBF), and to produce a high-quality three-dimensional fired product.

1 is a diagram showing an outline of a powder sintering lamination method according to an embodiment of the present invention; FIG. 2 is a diagram showing an apparatus used to produce a three-dimensional fired product in an embodiment of the present invention. 1A to 1C are diagrams showing a method (part 1) for producing a three-dimensional fired product in an embodiment of the present invention. FIG. 2 is a diagram showing the particle size distribution of sand used as a raw material in an embodiment of the present invention. 11A to 11C are diagrams showing a method (part 2) for producing a three-dimensional fired product in an embodiment of the present invention. 11A to 11C are diagrams showing a method (part 3) for producing a three-dimensional fired product in an embodiment of the present invention. 11A to 11C are diagrams showing a method (part 4) for producing a three-dimensional fired product in an embodiment of the present invention. FIG. 2 is a diagram showing a three-dimensional fired product according to an embodiment of the present invention. FIG. 13 is a diagram showing the test results of a uniaxial compressive strength test of a three-dimensional fired product according to an embodiment of the present invention.

The method for producing three-dimensional fired products of the present invention involves melting and solidifying soil material using the powder bed fusion method to produce high-quality three-dimensional fired products. Below, we will explain the method for producing three-dimensional fired products with reference to Figures 1 to 9, but first we will provide an overview of the powder bed fusion method.

Powder Bed Fusion (PBF)
Powder bed fusion is a method for irradiating a powdered material with a laser beam L as a heat source to sinter it and create a three-dimensional object. Specifically, as shown in Fig. 1(a), a powdered material is spread evenly in a predetermined layer thickness on a stage S that can move up and down in a chamber C to form a first powder layer 31. Next, while the first powder layer 31 is heated, a laser beam L is irradiated along an irradiation path 20 as shown in Fig. 1(b) to melt and solidify a desired area of the first powder layer 31.

After this, as shown in FIG. 1(c), a powdered material is spread evenly on the first powder layer 31 to create the second powder layer 32. Using the same procedure as above, the second powder layer 32 is melted and solidified, and the first and second powder layers 31 and 32 are bonded together. This process is repeated until the height of the object to be produced is reached.

As shown in FIG. 2, the above-mentioned irradiation with the laser light L is performed using a laser generator 41 and a scanner 42. The laser generator 41 irradiates the powder layer 30 with the laser light L, which serves as a heat source, and employs a so-called fiber laser. The scanner 42 irradiates the laser light L along an irradiation path 20 set on the powder layer 30, and employs a so-called galvano scanner.

<How to create 3D fired objects>
Based on the above-mentioned powder bed fusion method, the procedure for melting and solidifying a particle layer 10 in which a soil material having a particle size distribution is spread evenly instead of the powder layer 30 to produce a three-dimensional fired product 50 as shown in FIG. 8 will be described in detail below with reference to FIGS. 3 to 8.

It is not necessary to preheat the particle layer 10, and therefore the chamber C for keeping the particle layer 10 warm is also not required. In addition, when producing the three-dimensional fired product 50 to be produced, the irradiation area E to be irradiated with the laser light L on the first particle layer 11 is, for example, a rectangle with the X direction and the Y direction perpendicular to each other, as shown in FIG. 3(a).

Furthermore, the irradiation path 20 when irradiating the laser light L to the intended irradiation area E may be a path that travels back and forth in the X direction while proceeding in the Y direction, as shown in FIG. 3(b), or a path that travels in the Y direction while sequentially repeating movements in one direction in the X direction, as shown in FIG. 3(c). In this embodiment, an example is given of the case where the laser light L travels in one direction in the X direction, as shown in FIG. 3(c).

The material to be melted and solidified is a soil material with a grain size distribution, and can be any of sand, gravel, sand mixed with gravel, and gravel mixed with sand. It is preferable that the soil material with a grain size distribution has a grain size of about 2 mm or less. In this embodiment, the grain size distribution shown in Figure 4 is the area surrounded by the dashed line or a distribution along this line, and sand with a maximum grain size of about 1.75 mm and the largest proportion of quartz is used as an example.

<Melting and solidifying step of first particle layer 11>
First, as shown in Fig. 5(a), sand, which is a material to be melted and solidified, is poured onto the stage S to form the first particle layer 11. At this time, the layer pitch h of the first particle layer 11 is set to the maximum grain size of the sand. Therefore, in this embodiment, the layer pitch h = 1.75 mm. Next, the above-mentioned irradiation path 20 is set within the planned irradiation area E, and a laser beam with an irradiation diameter of 1 mm is irradiated along this path onto the surface of the first particle layer 11.

When setting the irradiation path 20, first, the irradiation method of the laser light L is selected taking into consideration the following points. That is, it is necessary to take into consideration that the sand constituting the first particle layer 11 has a large and non-uniform particle size and a low thermal conductivity compared to powders of resins, metals, and the like that are widely used as materials for powder bed fusion bonding.

To melt sand with a large grain size and low thermal conductivity, it is necessary to supply heat with a high energy density, so if a one-pass method (irradiation only once per location) is used, it becomes necessary to irradiate the first particle layer 11 with high-power laser light L. However, when such a method is used, there is a risk that the upper part of the sand with large grain size that forms the first particle layer 11 will volatilize, and the entire sand with small grain size will volatilize.

Therefore, a method is adopted in which the irradiation target area E set in the first particle layer 11 is irradiated with laser light L having a laser output P of about 200 W by a multi-pass method to melt and solidify. The multi-pass method is a method in which the same position in the first particle layer 11 is irradiated with laser light L multiple times by overlapping irradiation lines P that have a width equivalent to the irradiation diameter of the laser light L and extend in the X direction while shifting them in the width direction (Y direction) by an amount of deviation A, as shown in Figure 5 (b).

When the multi-pass method is employed, a heat transfer period of the first particle layer 11 is provided each time the same region of the first particle layer 11 is irradiated with laser light L. The heat transfer period is preferably set to the time required for the heat supplied by irradiating the surface of the first particle layer 11 with laser light L to reach the bottom surface, and is appropriately set to a period during which the temperature does not drop to the temperature before irradiation.

As a result, the first particle layer 11 alternates between periods during which heat is supplied to the surface and periods during which the supplied heat is conducted to the bottom surface of the first particle layer 11, uniformly raising the temperature of the entire first particle layer 11.

The irradiation path 20 of the laser light L can be set by determining the amount of deviation A in the irradiation width direction (Y direction) between the leading and trailing irradiation lines P among the multiple irradiation lines P extending in the X direction.

<Calculation method of deviation amount A>
When the surface of the first particle layer 11 is irradiated with laser light L by the multi-pass method, a method for determining the amount of deviation A in the width direction (Y direction) between the preceding and succeeding irradiation lines P extending in the X direction will be described below.

First, the energy density Ed that can be imparted to the first particle layer 11 by one irradiation is calculated based on the following formula (1). Assuming that the scanning speed v is 1000 m/sec, the laser output P is 200 W, and the scanning pitch w is 1 mm and the layer pitch h is 1.75 mm, the energy density Ed per irradiation is 0.114 J/ ^mm3 .

Ed = P / (w × h × v) (1)
Ed: Energy density (J/mm ³ )
P: Laser output (W)
w: Scanning pitch (irradiation diameter: mm)
h: Layer pitch (mm)
v: Scanning speed (mm/sec)

Next, the temperature rise ΔT of the first particle layer 11 per irradiation is calculated based on the following formula (2). By substituting the energy density Ed = 0.114 J/mm ³ , the material specific heat γ of quartz = 0.71, and the material density d = 2.65 × 10 ^-3 g/mm ³ , the temperature rise ΔT = 61 °C.

ΔT=Ed/(γ×d) (2)
Ed: Energy density (J/mm ³ )
γ: Material specific heat (J/gK)
d: Material density (g/mm3)

As mentioned above, the sand that forms the first particle layer 11 contains a large amount of quartz, which has a melting point of 1713°C and a boiling point of 2230°C. Therefore, when the same area of the first particle layer 11 is repeatedly irradiated with a heat transfer period in between, the number of irradiations required to raise the temperature to the range of 1713 to 2230°C is calculated based on the temperature increase ΔT.

The same location needs to be repeatedly irradiated 29 times (1713°C/61°C) to exceed the melting point, and about 37 times (2230°C/61°C) to raise the temperature close to the boiling point. If the temperature of the first particle layer 11 exceeds the boiling point at this time, volatilization may occur, which may affect the quality of the three-dimensional fired product 50 after production. Therefore, when determining the number of irradiations to raise the temperature of the first particle layer 11, adjustments are made so that the first particle layer 11 does not reach a temperature at which volatilization occurs.

Therefore, in this embodiment, for safety reasons, the multi-pass method is adopted and the same location is repeatedly irradiated 35 times. The width of the irradiation line P corresponds to the irradiation diameter W, which is 1 mm as described above.

Therefore, assuming a scanning speed v = 1000 m/sec and a laser output P = 200 W, the irradiation path 20 of the laser light L with a scanning pitch w = 1 mm moves the laser light L in the X direction of the intended irradiation area E, and then moves it in the Y direction by a deviation amount A = 0.028 mm (1 mm/35 times). After this, the laser light L is moved again in the X direction of the intended irradiation area E, and this operation is repeated to cover the entire intended irradiation area E.

Laser light L is irradiated to the irradiation area E in the first particle layer 11 along the irradiation path 20 determined by the above calculation method. Then, as shown in FIG. 5(b), the temperature in the area in the Y direction after the position where 35 irradiations have been repeated in the irradiation area E rises to about 2135°C (61°C x 35 times) from the surface to the bottom.

In this way, the heat of the energy density Ed required to melt the first particle layer 11 is supplied in multiple batches to the surface of the first particle layer 11 with a heat transfer period in between. This allows the first particle layer 11, which is made of sand with a large, non-uniform particle size and low thermal conductivity, to be heated uniformly all the way to the bottom surface without concentrating heat on the surface, raising the temperature and allowing efficient melting and solidification.

The range in which the three-dimensional fired product 50 is produced is set within the range in the planned irradiation area E that is irradiated 35 times with the laser light L. The energy density Ed imparted to the first particle layer 11 by one irradiation can be appropriately adjusted by changing the laser output P and the scanning speed v.

<Melting and solidifying step of second particle layer 12>
After the melting and solidifying process is completed, the first particle layer 11 has a state in which the sand particles have solidified into a solidified body 111 as shown in Fig. 6(a) in the irradiation planned area E. Therefore, the second particle layer 12 is formed by spreading sand evenly in the same manner as the first particle layer 11 so as to fill the gaps between the solidified bodies 111 as shown in Fig. 6(b).

After this, as shown in FIG. 7(a), the second particle layer 12 is irradiated with laser light L in the same procedure as the melting and solidifying process of the first particle layer 11, and as shown in FIG. 7(b), the sand particles that constituted the second particle layer 12 solidify to form a solidified body 121. In this way, although some gaps are formed between the solidified body 111 of the first particle layer 11 and the solidified body 121 of the second particle layer 12, both are firmly melted and solidified.

The above process is repeated until the desired height is reached, as shown in Figure 8, to produce a three-dimensional fired product 50 made by melting and solidifying the sand.

The above method makes it possible to produce a high-quality three-dimensional fired product 50 that maintains the desired density without causing volatilization of the particle layer 10. In addition, since there is no need to preheat the particle layer 10 and no mold is required for molding, it is possible to easily produce a three-dimensional fired product 50 having the desired shape with a simple configuration that only requires irradiation with laser light L.

Furthermore, the number of times to irradiate the area of the particle layer 10 that will become the three-dimensional sintered product 50 is determined based on the temperature rise Δt of the particle layer 10 caused by irradiating the laser light L once along the irradiation line P, so it is possible to produce the three-dimensional sintered product 50 in a short period of time while efficiently supplying heat with the energy density Ed required to melt the particle layer 10 without waste.

<Compressive strength test>
FIG. 9 shows the results of a uniaxial compressive strength test carried out on a three-dimensional sintered body 50 produced using sand as a raw material based on the above-described production method.

In the experiment, crushed Osawa stone, which contains a large amount of quartz, was used as the material to be melted and solidified. The particle size distribution of the crushed Osawa stone was adjusted in advance, and it was similar to the sand that constitutes the particle layer 10 shown as an example of the method for producing a three-dimensional fired product. As shown in Figure 4, the particle size ranges from a few μm to about 2 mm, and it is distributed along the vicinity of the area surrounded by the dashed line. The method for producing the specimen to measure the uniaxial compressive strength is as follows.

Crushed Osawa stone is spread out in a circular shape to form a particle layer, and laser light L is irradiated in a spiral pattern from the outer edge towards the centre. This process is repeated for multiple layers to produce a test specimen of a specified height. As shown in Figure 9 (b), seven types of test specimens were created by appropriately changing the layer thickness (layer pitch h), sweep speed (scanning speed v), laser output P, etc., and the test specimen density and uniaxial compressive strength were calculated for each. The sweep speed of the laser light L was adjusted so that it increased as it moved from the outer edge to the centre of the circularly spread particle layer, so that the temperature rise of the particle layer was uniform at the outer edge and centre.

Looking at the compressive strength of each specimen in Figure 9(b), for example, it can be seen that specimen number 4 exhibited a compressive strength of 14.9 N/ ^mm2 , which is similar to that of JIS ordinary brick type 2 (15 N/ ^mm2 ). Also, looking at Figure 9(a), a correlation was observed between specimen density and compressive strength, and it can be seen that compressive strength increases by increasing the density of the specimen.

As a result, when producing a three-dimensional fired product 50 as shown in Figure 8, it is expected that the compressive strength can be further improved by appropriately adjusting the method of scattering sand when forming the particle layer 10 and the irradiation conditions of the laser light L (e.g., laser output P, sweep speed (scanning speed) v, etc.) to reduce the gap between the solidified bodies 111, 121 and increase the density.

The method for producing three-dimensional fired products using the soil material of the present invention is not limited to the above embodiment, and various modifications are possible without departing from the spirit of the present invention.

For example, in this embodiment, sand containing a large amount of quartz is used as the material, but this is not limited to this. Any soil material such as sand, gravel, sand mixed with gravel, or gravel mixed with sand may be used, and the minerals contained are not limited in any way. The deviation amount A when setting the irradiation path 20 of the laser light L may also be set appropriately taking into consideration the melting point and boiling point of the sand used.

Furthermore, in this embodiment, when setting the irradiation path 20, the case where the irradiation line P extends linearly in the X direction is given as an example, but this is not limited to this, and it may be set to any shape such as an arc or a spiral.

10 Particle layer 11 First particle layer 111 Coagulated body 12 Second particle layer 121 Coagulated body 20 Irradiation path 30 Powder layer 31 First powder layer 32 Second powder layer 41 Laser generator 42 Scanner 50 Three-dimensional sintered product S Stage L Laser light P Irradiation line C Chamber E Planned irradiation area (area to become three-dimensional sintered product)

Claims (2)

A method for producing a three-dimensional fired product, comprising the steps of: spreading out an earthen material to form a particle layer; irradiating a surface of the particle layer with a laser beam along an irradiation path; and repeating a melting and solidifying process for melting and solidifying the particle layer while stacking the particle layers;
The irradiation path includes an irradiation line indicating the traveling direction of the laser light, and the subsequent irradiation line is overlapped with the preceding irradiation line while being shifted in the width direction, so that the region in the particle layer that becomes the three-dimensional fired product is irradiated multiple times with the laser light.
A heat transfer period from the surface of the particle layer to a depth direction is provided between the time when the laser light is irradiated along the preceding irradiation line and the time when the laser light is irradiated along the following irradiation line ,
The soil material is sand with a highest proportion of quartz;
The amount of shift of the irradiation line in the width direction is
The energy density that can be applied to the particle layer by one irradiation is calculated based on the following formula, and the calculated energy density is divided by the material specific heat and material density of quartz to obtain the temperature rise per irradiation.
Based on the obtained temperature rise and the melting point and boiling point of the particle layer, a number of repeated irradiation times is determined that exceeds the melting point but does not reach a temperature at which volatilization occurs, and
A method for producing a three-dimensional fired product, characterized in that the scanning pitch of the laser light is divided by the determined number of repeated irradiations to calculate the scanning pitch .
Ed = P / (w x h x v)
Ed: Energy density (J/mm ³ )
P: Laser output (W)
w: Scanning pitch (irradiation diameter: mm)
h: Layer pitch (mm)
v: Scanning speed (mm/sec) A method for producing a three-dimensional fired product according to claim 1,
The number of times that the area to be the three-dimensional fired product is irradiated with the laser light is
A method for producing a three-dimensional fired product, characterized in that the temperature is determined based on the increase in temperature of the particle layer caused by irradiating the laser light once along the irradiation line.

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