Porous structure

Abstract

To provide porous structures with controlled structural and mechanical properties.SOLUTION: A porous structure comprises a plurality of beam-like members connected to each other at nodes. Each beam-like member is arranged in the porous structure so that the overall structure avoids repetition of identical unit structures. The length of each beam-like member and the number of beam-like members branching from a node are distributed within predetermined upper and lower limits. At least some of the numerous nodes in the porous structure may be arranged so that each beam-like member extends isotropically in three dimensions from the same node.SELECTED DRAWING: Figure 3B

Description

[Technical Field]

[0001] The present invention relates to a porous structure. [Background technology]

[0002] In recent years, porous structures with numerous pores have been attracting attention in fields requiring advanced functionality, such as biomedical engineering and aerospace engineering. Development of design methods for porous structures that can be manufactured using additive manufacturing technology and have desired properties is underway. For example, Patent Document 1 discloses a method for designing porous structures for implants by fusing cells consisting of multiple struts and nodes with other cells. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-200630 Summary of the Invention [Problem to be solved by the invention]

[0004] The design method of Patent Document 1 does not control the properties of each strut, such as stiffness, orientation, and branching, and therefore cannot control the structural properties of the porous structure, resulting in a porous structure with different mechanical properties depending on the load direction. If such a porous structure is applied to, for example, an implant, it may be damaged if a sudden load is applied from an unexpected direction. This problem is not limited to when a porous structure is applied to an implant, but also exists when a porous structure is applied to other objects.

[0005] The present invention has been made based on the above background, and has an object to provide a porous structure having controlled structural and mechanical properties. [Means for solving the problem]

[0006] In order to achieve the above object, the porous structure according to the present invention comprises: A porous structure comprising a plurality of beam-like members connected to each other at nodes, The beam-like members are arranged so that the same unit structure is not repeated in the porous structure, The length of the beam member and the number of beam members branching from the node are distributed within a range of a predetermined upper limit and a predetermined lower limit, respectively.

[0007] At least some of the numerous nodes included in the porous structure may be nodes arranged so that the beam-like members extend three-dimensionally isotropically from the same node. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a porous structure having controlled structural and mechanical properties. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic diagram showing the configuration of a manufacturing system for a porous structure according to an embodiment of the present invention. [Figure 2] 1 is a diagram showing an example of a beam generated by a design apparatus according to an embodiment of the present invention; [Figure 3A] 1 is a perspective view showing a network structure configured by beams generated by a design device according to an embodiment of the present invention; [Figure 3B] FIG. 3B is a diagram showing how the beam in FIG. 3A is converted into a volume. [Figure 4] 1 is a block diagram showing a hardware configuration of a design apparatus according to an embodiment of the present invention; [Figure 5A] FIG. 4 is a diagram illustrating an example of a data table of a parameter storage unit according to the embodiment of the present invention. [Figure 5B] FIG. 4 is a diagram showing an example of a data table of a probability distribution storage unit according to the embodiment of the present invention; [Figure 6A] 10 is a graph showing an example of a probability density function of a beam length. [Figure 6B]10 is a graph showing an example of a probability mass function of the branching number. [Figure 7] FIG. 10 is a diagram showing a specific example of a beam arrangement having three-dimensional isotropy. [Figure 8] 10A to 10C are diagrams showing a procedure in which the design device according to the embodiment of the present invention joins two nodes into one. [Figure 9] 1 is a flowchart showing a flow of a design process according to an embodiment of the present invention. [Figure 10] 10 is a flowchart showing a flow of a beam generation process according to an embodiment of the present invention. [Figure 11] 10A and 10B are diagrams showing a procedure in which a design device according to a modified example of the present invention joins two nodes into one. [Figure 12] FIG. 10 is a diagram showing an example of a beam orientation angle generated by a design device according to a modified example of the present invention. [Figure 13] FIG. 10 is a front view showing how a porous structure according to a modified example of the present invention is deformed by a compressive load. [Figure 14A] FIG. 2 is a photograph of the appearance of a test piece of the porous structure of the present invention in an example. [Figure 14B] FIG. 1 is a photograph of the appearance of a test piece of a diamond lattice structure in an example. [Figure 15] 1 is a graph showing the stress-strain relationship of each test piece in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, a design device, a design method, a program, a porous structure, and a manufacturing method thereof according to embodiments of the present invention will be described in detail with reference to the drawings. In each drawing, the same or equivalent parts are designated by the same reference numerals.

[0011] FIG. 1 is a schematic diagram showing the configuration of a manufacturing system 1 for a porous structure according to an embodiment. The manufacturing system 1 is a system that generates a model of a porous structure and manufactures the porous structure based on the generated model. The manufacturing system 1 includes a design device 100 and a manufacturing device 200. The design device 100 and the manufacturing device 200 are connected to each other so that they can communicate with each other via a wired or wireless communication line.

[0012] The design device 100 generates a porous structure model composed of a large number of beam-like members arranged in a three-dimensional network in an arbitrary design space. The porous structure model is obtained by converting the large number of beams that make up the network structure into a volume, and may be either a solid model or a surface model. The network structure is composed of a large number of beams arranged in a three-dimensional network, and the beams are straight lines with no volume and nodes at both ends.

[0013] FIG. 2 is a diagram illustrating an example of beams generated by the design apparatus 100 according to the embodiment. FIG. 2 illustrates a three-dimensional space viewed from directly above, with each beam distributed in the three-dimensional space. Each beam is represented by a solid or dotted line, and each node, including the origin, is represented by a dot. The generation of a new beam begins from a single origin set in the design space. The next new beam is generated at the node located at the tip of each of the four existing cantilever beams initially generated, as shown by the dotted lines in FIG. 2. Each new beam is generated so that all beams belonging to the same node extend isotropically in three dimensions. By repeatedly generating new beams branching from the node located at the tip of the existing cantilever beam, a network structure consisting of numerous beams gradually grows.

[0014] To randomly orient each beam in the network structure, the parameters of the beam length, number of branches, and rotation angle are randomly set based on a predetermined rule, for example, a probability distribution. Specifically, the beam length is randomly set for each beam, and the number of branches and rotation angle are randomly set for each node. The beam length is the length between the nodes at both ends of the beam. The number of branches is the number of beams branching from one node, including both new beams and existing beams. In the example of Figure 2, the number of branches at the starting point is 4, and the number of branches at the other nodes is 3. The rotation angle is the angle when the new beam is rotated around an axis extending in the longitudinal direction of the existing beam.

[0015] FIG. 3A is a perspective view showing a network structure composed of beams generated by the design apparatus 100 according to the embodiment, and FIG. 3B is a diagram showing the process of volumetrically transforming the beams shown in FIG. 3A. In FIG. 3A, as an example, a network structure is generated within a 25 mm × 25 mm × 25 mm design space. For example, by starting the generation of new beams from the starting point (0,0,0), and then repeatedly generating new beams from the nodes at the tip of the existing cantilever beams, multiple beams are generated to expand the entire design space, as shown in FIG. 3A. Next, by volumetrically transforming each beam to have a volume, as shown in FIG. 3B, a porous structure model is generated. To volumetrically transform the beams, for example, each beam can be converted into a beam-like member with the same cross section at least in its middle.

[0016] Returning to FIG. 1, the manufacturing apparatus 200 is an apparatus that manufactures a porous structure based on CAD (Computer Aided Design) data of the model generated by the design apparatus 100. The manufacturing apparatus 200 is, for example, an additive manufacturing machine. The additive manufacturing machine uses, for example, a laser powder bed fusion method that repeats a series of steps of forming a thin layer of metal powder on a base plate and selectively melting the metal powder layer with laser light.

[0017] 4 is a block diagram showing a hardware configuration of a design apparatus 100 according to an embodiment. The design apparatus 100 is, for example, a general-purpose computer. The design apparatus 100 includes an operation unit 110, a display unit 120, a communication unit 130, a storage unit 140, and a control unit 150. The units of the design apparatus 100 are connected to each other via an internal bus (not shown). Below, an example will be described in which the design apparatus 100 randomly changes the beam length for each beam generation, and the number of branches and rotation angle for each node, and converts each beam into a cylindrical body with a constant diameter for volume generation.

[0018] The operation unit 110 receives instructions from the user and supplies an operation signal corresponding to the received operation to the control unit 150. The operation unit 110 includes input devices such as a mouse and a keyboard.

[0019] The display unit 120 includes a display drive circuit and displays various images to the user based on data supplied from the control unit 150. The display unit 120 includes a display device such as a liquid crystal display. The display unit 120 displays, for example, the porous structure model generated by the design device 100.

[0020] The communication unit 130 is a communication interface for communicating with external devices by the design apparatus 100. The communication unit 130 communicates with external devices via a communication network such as the Internet or an input / output terminal such as a USB (Universal Serial Bus).

[0021] The storage unit 140 includes, for example, a random access memory (RAM), a read only memory (ROM), a flash memory, and a hard disk. The storage unit 140 stores programs executed by the control unit 150 and various data. The storage unit 140 also temporarily stores various information and functions as a work memory for the control unit 150 to execute processing. The storage unit 140 also includes a parameter storage unit 141, a probability distribution storage unit 142, and a model storage unit 143.

[0022] 5A is a diagram showing an example of a data table of the parameter storage unit 141 according to the embodiment. The parameter storage unit 141 stores the beam diameter of the network structure representing the porous structure, the search range for node junctions, the search range for node density, and the node density threshold. The beam diameter is the diameter of the cylinder when the beam is converted into a volume and into a cylinder, and is limited to a range that allows additive manufacturing. The search range for node junctions, the search range for node density, and the node density threshold are all parameters related to the conditions for terminating the generation of a new beam at the node on the tip side of the new beam, and will be described in detail below.

[0023] 5B is a diagram showing an example of a data table of the probability distribution storage unit 142 according to the embodiment. The probability distribution storage unit 142 stores a probability density function of the beam length and a probability mass function of the branching number. It is preferable that an upper limit and a lower limit are set for the beam length and the branching number, respectively. As an example, the probability density function of the beam length is expressed by a gamma probability distribution with an upper limit and a lower limit set as shown in FIG. 6A, and the probability mass distribution of the branching number is expressed by a Poisson probability distribution that is discretely distributed within a branching number range of 3 to 6 as shown in FIG. 6B. The probability density function of the beam length is set in consideration of the beam diameter stored in the parameter storage unit 141.

[0024] The volume density of a porous structure is one of the design indices that significantly affects the mechanical properties of the porous structure. To increase the volume density, it is advisable to do at least one of the following: increase the beam diameter, shorten the average beam length, or increase the average number of branches. Alternatively, the nodal density threshold may be increased. Increasing the volume density can increase the rigidity of the porous structure, for example.

[0025] 4 , the model storage unit 143 stores CAD data indicating the boundary surfaces of the design space set by the user and CAD data indicating the porous structure model generated by the design device 100. The boundary surfaces of the design space may be generated by the user using a three-dimensional CAD, for example, and the CAD data of the boundary surfaces generated by the three-dimensional CAD may be imported into the model storage unit 143.

[0026] The control unit 150 includes a processor and controls each unit of the design apparatus 100. The processor is, for example, a CPU (Central Processing Unit). The control unit 150 executes the design process of FIG. 9 and the beam generation process of FIG. 10 by executing a program stored in the storage unit 140. Functionally, the control unit 150 includes an acquisition unit 151, a beam generation unit 152, a node joint unit 153, a beam deletion unit 154, a model generation unit 155, and an output unit 156.

[0027] The acquisition unit 151 acquires CAD data indicating the boundary surface of the design space set by the user, the beam diameter, the search range of node junctions, the search range and node density threshold, the probability density function of the beam length, and the probability mass function of the branch number, and stores them in one of the parameter storage unit 141, the probability distribution storage unit 142, and the model storage unit 143. Acquisition of data by the acquisition unit 151 also includes reading out data stored in the storage unit 140.

[0028] The beam generating unit 152 repeatedly generates a plurality of new beams branching from a node on the tip side of an existing beam so that a large number of beams that constitute a network structure of a porous structure are generated within the design space acquired by the acquiring unit 151. Specifically, as shown in Fig. 2, beam generation starts at a starting point set within the design space, and then the generation of new beams branching from a node on the tip side of the generated cantilever-shaped existing beam is repeated.

[0029] Each beam branching from the same node is generated to extend isotropically in three dimensions according to the number of branches assigned. For example, if the number of branches is 3, the branching angle, which is the angle between adjacent beams among the multiple beams belonging to the same node, should be set to 120°, as shown in the upper part of Figure 7. If the number of branches is 4, the beams are arranged in a four-legged block, as shown in the lower part of Figure 7, so the branching angle should be set to 109.5°.

[0030] The beam generation unit 152 randomly changes the beam length at each node on the tip side of an existing beam, and the number of branches and rotation angle at each node, based on preset rules. The beam length and number of branches are changed based on the probability density function and probability mass function stored in the probability distribution storage unit 142 of FIG. 5B, and the rotation angle is changed with equal probability. To randomly change the beam length, number of branches, and rotation angle, random numbers may be generated according to rules set for each. The random numbers include pseudo-random numbers. By randomly changing the beam length, number of branches, and rotation angle at each beam generation, the beam length and orientation of each beam are randomly distributed, resulting in structural isotropy in the network structure when viewed macroscopically.

[0031] The beam generation unit 152 sequentially assigns a node number i to each node each time a beam is generated. Different node numbers i are sequentially assigned to the nodes at the tip ends of multiple new beams branching from the same node. For example, if the number of branches is four, a different node number i is assigned to each of the three newly generated nodes. The node number i is used to determine the order in which nodes are joined in the node joining process described below.

[0032] The beam generation unit 152 determines for each node whether a new beam can be generated or whether to terminate the generation of the new beam. The beam generation unit 152 terminates the generation of the new beam at the node when the termination conditions for new beam generation are met. The generation of the new beam at each node is terminated when one of the following four termination conditions (1) to (4) is met. For ease of understanding, the node at the tip of the new beam will be referred to as the "new node" below.

[0033] Termination condition (1) When multiple new beams branching from a node on the tip side of an existing beam are generated, the generation of the beam at that node is terminated.

[0034] Termination condition (2) If a new node exists outside the design space, beam generation at that new node is terminated. By setting termination condition (2), after repeated beam generation is completed, a network structure is generated that slightly extends beyond the boundary surface of the design space. Since the beam having a node for which beam generation was terminated under termination condition (2) extends outside the design space, the portion extending outside the design space is deleted in the process described below.

[0035] Termination condition (3) If the new node does not satisfy the termination condition (2) and there is a node within the search range from the new node that satisfies the preset weldable condition, the generation of the beam at that new node is terminated. New nodes that satisfy the termination condition (3) are joined to weldable nodes in the process described below. The search range is, for example, a spherical region centered on the new node and is expressed by the radius of the spherical region. The nodes that satisfy the weldable condition during beam generation are only those that satisfy the termination condition (1) or (2) at the time of determining whether the new node satisfies the termination condition (3), and do not include the nodes at both ends of the existing beam into which the new beam to which the new node belongs is branched.

[0036] Termination condition (4) If the new node does not satisfy termination conditions (2) and (3) and the node density within the search range from the new node is greater than the node density threshold, the beam generation at the new node is terminated. The search range is, for example, a spherical region centered on the new node and is expressed by the radius of the spherical region. The node density is the number of nodes per unit volume within the search range that can generate a new beam at the time when it is determined whether the new node satisfies termination condition (4), or nodes that satisfy termination condition (1) or (2). Termination condition (4) is set because further beam generation at a new node that satisfies this condition would result in beams becoming congested in the surrounding area. Termination condition (4) may be omitted if the search range for node junctions and the probability distribution of the beam length and number of branches are appropriately set. The above is the condition for ending beam generation.

[0037] The node joining unit 153 joins a node for which generation of a new beam has been completed as determined by the beam generation unit 152 to a node that can be joined that is closest to the node. The node joining unit 153 joins a node for which generation of a new beam has been completed does not necessarily match the node that can be joined that was determined to be closest to the new node at the time when it was determined that a node that satisfies the joinable conditions exists. This is because, for example, after it is determined that a node that satisfies the joinable conditions exists, a new node that satisfies the joinable conditions may appear closer to the new node.

[0038] To explain the joining process for individual nodes using the specific example of Figure 8, first, for a node for which the beam generation unit 152 has finished generating a new beam because it determines that a node satisfying the joinable conditions exists within the search range, the closest node that can be joined is searched for. As shown in the area surrounded by the dashed line in Figure 8, a node for which the beam generation unit 152 has finished generating a new beam because it determines that a node satisfying the joinable conditions exists within the search range is joined to the closest node that can be joined. Specifically, the node for which the new beam generation has finished is moved so that it overlaps with the closest node that can be joined, and the inclination and length of the beam to which the node belongs are also changed in accordance with the movement of the node.

[0039] The joining of nodes is performed according to the following procedure after the beam generation unit 152 has completed the generation of repeated beams. For nodes for which the beam generation unit 152 has completed the generation of a new beam as it is determined that a node satisfying the joinable conditions exists within the search range, the node with node number i is searched for a joinable node in ascending order of the set node number i, and the node with node number i is joined to the closest joinable node among the joinable nodes. In this case, the joinable node is a node that satisfies the termination condition (1) or (2) at the time the beam generation unit 152 has completed the generation of repeated beams, and does not include nodes belonging to the beam to which the node with node number i belongs or to a beam directly connected to it. Furthermore, if there are multiple joinable nodes at the same distance, the node with the smallest node number i is selected and joined. By performing the bonding process using the above procedure, it is possible to minimize the number of node connections, thereby maintaining the structural isotropy of the network structure.

[0040] The beam deletion unit 154 deletes some of the beams existing outside the design space and all of the cantilever beams existing inside the design space and not in contact with the boundary surface of the design space, in the network structure obtained by the node connection unit 153. Specifically, the beam deletion unit 154 deletes the part outside the design space of a cantilever beam whose part satisfies the termination condition (2) of beam generation in the beam generation unit 152 and is outside the design space acquired by the acquisition unit 151. In addition, the beam deletion unit 154 deletes cantilever beams existing inside the network structure obtained by the node connection unit 153. Such cantilever beams are deleted because they do not contribute to the transmission of load as a structure.

[0041] The model generation unit 155 generates a porous structure model by converting each beam of the network structure from which some of the beams constituting the network structure have been deleted by the beam deletion unit 154 into a volume. Specifically, all of the beams constituting the network structure are converted into cylinders having the beam diameter acquired by the acquisition unit 151, and CAD data representing the porous structure model is generated by setting a sphere with the beam diameter as a diameter at each node. By assigning a sphere to each node when converting into a volume, the surface properties at each node can be improved.

[0042] The output unit 156 outputs the porous structure model generated by the model generation unit 155. The output unit 156, for example, causes the display unit 120 to display CAD data of the porous structure model generated by the model generation unit 155. Furthermore, the output unit 156, for example, controls the communication unit 130 to transmit the CAD data of the porous structure model generated by the model generation unit 155 to the manufacturing apparatus 200. The above is the hardware configuration of the design apparatus 100.

[0043] (Design Processing) Next, the design process executed by the control unit 150 of the design device 100 according to the embodiment will be described with reference to Fig. 9. The design process is a process for generating CAD data of a porous structure model in a design space specified by a user. The design process starts when the user starts the application of the design device 100.

[0044] The design device 100 requests instructions from the user regarding CAD data and various parameters that indicate the boundary surface of the design space. The various parameters include the beam diameter, the search range for node junctions, the search range for node density, the node density threshold, the beam length, and the number of branches. The user sets the CAD data and various parameters that indicate the boundary surface of the design space in response to the request. At this time, the beam length is set using a probability density function, and the number of branches is set using a probability mass function. The acquisition unit 151 acquires the CAD data and various parameters that indicate the boundary surface of the design space set by the user, and stores them in either the parameter storage unit 141 in FIG. 5A, the probability distribution storage unit 142, or the model storage unit 143 in FIG. 5B, respectively (step S1).

[0045] Next, the beam generating unit 152 executes a beam generation process in which the beam generation is repeated until the beam is expanded to cover the entire design space acquired in the process of step S1 (step S2). Hereinafter, the flow of the beam generation process executed by the beam generating unit 152 will be described with reference to FIG.

[0046] (Beam generation processing) First, the beam generation unit 152 randomly sets the beam length, number of branches, and rotation angle, and generates an initial beam at the origin based on the set beam length, number of branches, and rotation angle (step S21). The origin is preset within the design space by the user. The beam length and number of branches are set based on the probability density function of the beam length and the probability mass function of the number of branches stored in the probability distribution storage unit 142 of FIG. 5B, and the rotation angle is set randomly with equal probability. Each beam belonging to the same node is generated so as to extend three-dimensionally isotropically according to the assigned number of branches. In the example of FIG. 2, four beams are generated from the origin, but the number of branches at the origin may be other than four. After beam generation, the origin is set to "beam generation completed" and a new node is set to "beam generation possible." Thereafter, when generating a beam at each node, a node number i is assigned to the new node in order. The node number i at the origin is 1. Different node numbers i are assigned to the nodes at the tip ends of multiple new beams generated from the same node in order.

[0047] Next, the beam generation unit 152 determines whether or not a node capable of generating a new beam exists (step S22). If it is determined that a node capable of generating a new beam exists (step S22; Yes), the beam generation unit 152 randomly sets the beam length, number of branches, and rotation angle, and performs beam generation at the node capable of generating a beam with the smallest node number based on the set beam length, number of branches, and rotation angle (step S23). At this time, the beam length is set randomly for each beam, and the number of branches and rotation angle are set randomly for each node. After beam generation, the node is set to "beam generation completed." On the other hand, if it is determined that a node of a beam capable of generating a new beam does not exist (step S22; No), the process returns.

[0048] After the process of step S23 is completed, the beam generation unit 152 determines whether the new node generated in the process of step S23 satisfies a condition for terminating the generation of a new beam at the node (step S24). Specifically, the condition for terminating the generation of a new beam at each node may be any of the following: when multiple new beams branching from a node at the tip of an existing beam are generated; when the new node is outside the design space; when a node satisfying the joinable condition exists within the search range of the new node; and when the node density in the search range of the new node is greater than the node density threshold. The node joining search range, node density search range, and node density threshold may be read from the parameter storage unit 141 in FIG. 5A.

[0049] If it is determined that the new node satisfies the new beam generation termination condition (step S24; Yes), the beam generation unit 152 sets the new node to "beam generation terminated" (step S25) and returns the process to step S22. On the other hand, if it is determined that the new node does not satisfy the new beam generation termination condition (step S24; No), the beam generation unit 152 sets the new node to "beam generation possible" (step S26) and returns the process to step S22. The above is the flow of the beam generation process.

[0050] 9, the node joining unit 153 joins a node for which generation of a new beam has been completed as a result of determining in step S2 that a node satisfying the joinable conditions exists within the search range to the closest joinable node (step S3). Specifically, for nodes for which it has been determined in step S2 that a node satisfying the joinable conditions exists within the search range, the unit searches for a joinable node among the nodes with node number i in ascending order of node number i, and joins the closest joinable node among the joinable nodes to the node with node number i. If there are multiple joinable nodes at the same distance, the unit simply selects the node with the smallest node number i.

[0051] Next, the beam deletion unit 154 deletes some of the beams that exist outside the design space and all of the cantilever-shaped beams that exist inside the design space and do not contact the boundary surface of the design space from the network structure obtained by the processing of step S3 (step S4).

[0052] Next, the model generation unit 155 generates CAD data of the porous structure model by converting each beam of the network structure obtained in the processing of step S4 into a volume, and stores the data in the model storage unit 143 (step S5). In the volume conversion, each beam is converted into a cylinder whose diameter is the beam diameter stored in the parameter storage unit 141 of Fig. 5A, and a sphere whose diameter is the beam diameter is set at each node.

[0053] Next, the output unit 156 outputs the porous structure model generated in the processing of step S5 to the outside (step S6), and ends the processing. For example, when the user instructs display of the model, the output unit 156 may cause the porous structure model to be displayed on the display unit 120. Furthermore, when the user instructs manufacturing of an actual porous structure in the manufacturing apparatus 200, the output unit 156 may transmit CAD data of the porous structure to the manufacturing apparatus 200 via the communication unit 130. The above is the flow of the design process.

[0054] When the manufacturing apparatus 200 acquires the CAD data of the porous structure from the design apparatus 100, it performs slicing to determine a tool path based on the CAD data. The tool path is the path along which the laser beam of the manufacturing apparatus 200 moves. The manufacturing apparatus 200 then performs additive manufacturing based on the slicing results to obtain an actual porous structure based on the porous structure model. The porous structure obtained by the above process is not limited to one that strictly reflects the shape of the porous structure model, but is also expected to be influenced by the dimensional accuracy achievable in additive manufacturing. For example, the portion where multiple cylindrical bodies are connected in the porous structure may be rounded.

[0055] The porous structure obtained by the manufacturing apparatus 200 according to the embodiment is a porous structure comprising a plurality of rod-shaped members connected to each other at nodes, and the rod-shaped members are arranged so that the same unit structure does not occur repeatedly in the porous structure. The rod-shaped members are an example of beam-shaped members connecting nodes. The length of the rod-shaped members and the number of rod-shaped members branching from the nodes are set so as to be distributed within respective upper and lower limits, and the rod-shaped members are arranged without being limited to a specific extension direction. Furthermore, in the porous structure, it is preferable that the arrangement of the rod-shaped members is configured so as not to coincide with the arrangement of Voronoi edges constructed by Voronoi tessellation of the design space. The porous structure may have the same shape at least in the middle portion of each rod-shaped member.

[0056] The porous structure has a large number of nodes, some of which are arranged so that each rod-shaped member extends isotropically in three dimensions from the same node. "Each rod-shaped member extends isotropically in three dimensions" means that, for N rod-shaped members extending from the same node, when N points are uniformly or equally spaced on a unit sphere centered on the origin, the N rod-shaped members are arranged along the direction of a unit vector connecting the origin and each point. "Uniformly spaced N points" refers to, for example, an arrangement in which the minimum value of the spherical distance between the points is maximized among all possible arrangements of N points. For example, when N = 3 or 4, the arrangement of each rod-shaped member when N points are uniformly spaced corresponds to the beam arrangement shown in the specific example of FIG. 7. "Equal" with respect to the same node also means, for example, that the absolute value of the difference between the angles formed by each rod-shaped member and the angles formed when the points are uniformly spaced on a unit sphere is within 10°, preferably within 5°.

[0057] Because the porous structure has the above-described configuration, it has the following advantages over regular lattice structures (e.g., diamond-shaped lattice structures) composed of repeated unit structures. First, the porous structure according to the embodiment does not have the structural anisotropy that regular lattice structures have, and as a result, is mechanically isotropic, and can therefore withstand sudden loads in unexpected directions. Furthermore, the porous structure according to the embodiment does not have imperfect unit structures on the surface of the design space, as regular lattice structures do, and therefore localized fractures on the surface of the design space can be prevented. In addition, the porous structure according to the embodiment suppresses stress reduction and fluctuations after the initial maximum compressive stress under compressive load, which regular lattice structures exhibit. This allows for a high design stress and suppresses fracture progression. Therefore, the porous structure is suitable for application to technical fields where fatal fracture progression is not permitted, such as biomedical engineering and aerospace engineering.

[0058] As described above, the design device 100 according to the embodiment includes a beam generation unit 152 that repeatedly generates multiple new beams branching from a node at the tip of an existing beam while changing at least one of the beam length, the number of branches, and the rotation angle around the axis of the existing beam based on preset rules, and terminates beam generation at the node at the tip of the new beam if a node that satisfies preset weldable conditions is found within a search range from the node at the tip of the new beam, and a node joining unit 153 that selects multiple nodes that are not directly connected to each other by beams from the many nodes generated by the beam generation unit 152 based on preset rules and joins them together. This makes it easy to design a porous structure model that has mechanical isotropy in any design space.

[0059] The present invention is not limited to the above-described embodiment, and the following modifications are possible.

[0060] (Variation) In the above embodiment, the beam diameter, the search range for nodal junctions, the search range and nodal density threshold for nodal density, the probability density function for the beam length, and the probability mass function for the number of branches are set by the user, but the present invention is not limited to this. For example, the design device 100 may generate the above parameters and probability distributions based on conditions specified by the user, or the design device 100 may acquire those generated by an external computer.

[0061] In the above embodiment, the beam length, number of branches, and rotation angle are changed for each beam generation, but the present invention is not limited to this. For example, any one of the beam length, number of branches, and rotation angle may be changed for each beam generation, or two of the beam length, number of branches, and rotation angle may be changed for each beam generation. Furthermore, it is not limited to randomly setting the same parameters for all beam generation, and the type of parameters randomly set for each beam generation may be changed each time. In addition, the beam length, number of branches, and rotation angle may be changed for each node during beam generation.

[0062] In the above embodiment, the beam length is expressed using a gamma probability distribution, and the number of branches is expressed using a Poisson probability distribution. However, the present invention is not limited to this. The beam length and the number of branches may be expressed using a uniform probability distribution, for example. Furthermore, the beam length and the number of branches do not necessarily have to be generated randomly based on the probability distribution, but may be set based on a predetermined rule within a numerical range set for each beam generation.

[0063] In the above embodiment, a new beam is generated so that the existing beam and the new beam belonging to the same node are three-dimensionally isotropic, but the present invention is not limited to this.A new beam may be generated so that the angles formed by adjacent beams among multiple beams belonging to the same node are different from each other.

[0064] In the above embodiment, only one origin is set in the design space, but the present invention is not limited to this. Two or more origins may be set in the design space. In this case, the network structure may be integrated by connecting the beams belonging to each origin. For example, if two origins are set in the design space, and a node at the tip of a beam belonging to one origin and a node at the tip of a beam belonging to the other origin are within a distance threshold, the two may be joined into a single node to integrate the network structure.

[0065] In the above embodiment, the condition for terminating the generation of a new beam is set in advance and the generation of the beam is controlled based on this, but the present invention is not limited to this. For example, the parameter storage unit 141 may store a target number of generations for beam generation, and new beams may be repeatedly generated simultaneously from all nodes on the tip side of an existing cantilever beam, with the condition for terminating the generation of a new beam being when the number of repetitions reaches the target number of generations. The number of generations is the number of times a beam is generated at the same timing, and the target number of generations is the target number of generations.

[0066] In the above embodiment, after generating a network structure consisting of many beams, two nodes defined by the node joining condition are joined into one, but the present invention is not limited to this. For example, when a new beam is generated, two nodes defined by the new beam generation termination condition (3) may be joined into one.

[0067] In the above embodiment, the node that has completed generation of a new beam is moved so as to overlap with the node that satisfies the weldable condition and is closest to the node, as shown in Fig. 8, but the present invention is not limited to this. For example, as shown by the "symbol ★" in Fig. 11, a new node may be set at the point where the extension line of the beam to which the node that has completed generation of a new beam belongs intersects with the extension line of one of the beams to which the node that satisfies the weldable condition and is closest to the node belongs.

[0068] In the above embodiment, when the beam generating unit 152 determines that a node satisfying the weldable condition exists within the search range from the node on the tip side of the new beam and the generation of the new beam is completed, the node is joined to the closest weldable node, but the present invention is not limited to this. For example, when the beam generating unit 152 determines that a node satisfying the weldable condition exists within the search range and the generation of the new beam is completed, the node may be joined to the second-closest weldable node, rather than the closest weldable node.

[0069] In the above embodiment, the cantilever beams inside the structure that do not contact the boundary surface of the design space are deleted, but the present invention is not limited to this. For example, if it is desired to increase the volume density of the porous structure, the process of deleting the cantilever beams (step S4) may be omitted.

[0070] In the above embodiment, cantilever beams are present on the surface of the structure that contacts the boundary of the design space. However, the present invention is not limited to this. For example, a network structure may also be formed on the surface by connecting the tips of adjacent cantilever beams on the surface of the structure via an additional beam. Alternatively, multiple surfaces may be attached to cover the tip surfaces of the cantilever beams along the boundary of the design space. The surface attached to the tip surface of the cantilever beam may be flat or curved. This ensures strength even at the surface of the porous structure and suppresses macroscopic deformation. The above method is also useful in improving the manufacturability of porous structures, since the length and orientation angle of the cantilever beams that can be manufactured using a manufacturing device can be relaxed by supporting them at both ends.

[0071] In the above embodiment, the network structure is generated so as to extend slightly beyond the boundary surface of the design space, but the present invention is not limited to this. For example, a spherical space containing the design space may be set, a network structure may be generated within this spherical space, and then the portion of the network structure outside the boundary surface of the design space may be deleted.

[0072] In the above embodiment, a network structure consisting of many beams is volumized by converting each beam into a cylinder and setting spheres of the same diameter at the nodes, but the present invention is not limited to this. For example, the cross-sectional shape of the beam-like member obtained by volumizing the beam may be formed into an ellipse or a polygon such as a triangle, square, or rectangle. Furthermore, the beam-like member is not limited to a rod-like member, and may be, for example, a plate-like member.

[0073] In the above embodiment, a porous structure having structural isotropy was created, but the present invention is not limited to this. Structural anisotropy may be intentionally imparted by adjusting the beam and branching in the porous structure model according to the orientation angle. The beam orientation angle is a parameter that represents the beam's attitude relative to a reference coordinate system set in the design space. For example, if the reference coordinate system is an orthogonal coordinate system consisting of X, Y, and Z axes, it can be expressed as the angle relative to the Z axis as shown in Figure 12.

[0074] To impart structural anisotropy to the model, for example, after the process of generating a structurally isotropic network structure (step S4) is completed, additional processing may be performed to impart structural anisotropy to the network structure. To impart structural anisotropy, for example, the orientation direction of all beams relative to the reference coordinate system may be changed, specifically, the entire network structure may be stretched or contracted in one direction. Alternatively, beams within a preset orientation angle range may be deleted, or new beams with orientation angles within a preset range may be added.

[0075] As another method, a process for imparting structural anisotropy to the model may be performed during beam generation in the beam generation process (step S2). For example, the beam length may be extended or shortened depending on the orientation angle during beam generation, or at least one of the number of branches and the rotation angle may be changed. Specifically, a weighting coefficient according to the orientation angle may be preset, and this weighting coefficient may be multiplied by either the probability density function of the beam length and the probability mass function of the number of branches read from the probability distribution storage unit 142, or the probability density function of the rotation angle stored in the storage unit 140. The weighting coefficient may be, for example, a value obtained by adding 0.5 to the cosine of the angle with the Z axis.

[0076] As an example of expanding or contracting the beam length according to the orientation angle, the weighting coefficient can be set to 0.5 plus the cosine of the angle with the Z axis, and the beam length determined by the probability density function can be multiplied by the weighting coefficient. In this case, the beam oriented to the Z axis will be 1.5 times the length determined by the probability density function, and the beam oriented to the XY plane will be 0.5 times the length. This allows the porous structure model to be endowed with structural anisotropy, with a beam length distribution from the Z axis to the X and Y axes.

[0077] Another method for imparting structural anisotropy to a model during beam generation involves shortening the lengths of beams whose orientation angle with the Z axis is greater than a threshold and whose length is greater than a threshold by a certain percentage or to a certain length, and extending the beams connected to these beams. This method can also be applied to, for example, eliminating beams with orientation angles and lengths that are difficult to manufacture using manufacturing equipment when stacking in the Z axis direction by additive manufacturing, and is therefore useful for improving the manufacturability of porous structures.

[0078] As another method, a process for giving the model structural anisotropy may be performed in the process for determining the end of new beam generation in the beam generation process (step S2) and in the node joining process (step S3). For example, when searching for nodes within the search range that satisfy the weldable conditions in the process of step S2, it is possible to determine whether the orientation angle of the beam when joined to each weldable node is within a preset orientation angle range, and to end the generation of the new beam if there is a weldable node that falls within the orientation angle range. Next, in the process of step S3, it is possible to join the new node to the node that is closest to the new node among the weldable nodes that are within the preset orientation angle range.

[0079] The porous structure obtained by the above method has beams arranged in various directions, unlike lattice structures in which the beam orientation angles are limited to a few types, and therefore can achieve structural anisotropy in which the mechanical properties change gradually depending on the direction of the load. Furthermore, the above method can also be applied to excluding beams that are difficult to manufacture using manufacturing equipment from the model, which is also useful in terms of improving the manufacturability of porous structures.

[0080] In addition, in relation to the above-mentioned modification, a model of a porous structure may be intentionally provided with a gradient structure. A gradient structure is a type of structure with structural anisotropy, in which at least one of the parameters affecting the structural characteristics, such as beam length, node junction search range, number of branches, node density search range, node density threshold, and beam diameter, is distributed within the model. By providing a gradient structure in a porous structure, the mechanical properties can be varied depending on the region, allowing for intended control of deformation and fracture behavior. For example, the gradient structure may include a weak region in the porous structure that is prone to deformation or fracture and a strong region that is resistant to deformation or fracture. As an example, as shown in FIG. 13, a weak region 11 of a porous structure 10 can be sandwiched between a pair of strong regions 12, so that the weak region 11 deforms first when a compressive load is applied in the direction in which these regions overlap.

[0081] To give the model a gradient structure, for example, in the beam generation process (step S2), it is advisable to give any of the beam length, the search range of the nodal junction, and the number of branches a distribution according to the coordinates in the design space. To give the beam length a distribution according to the coordinates in the design space, specifically, a weighting factor dependent on the coordinates in the design space is set and multiplied by the probability density function of the beam length read from the probability distribution storage unit 142. As an example of the weighting factor, the origin of the reference coordinate system can be set at the center of the porous structure, the weighting factor at the origin can be set to 0.5, and the weighting factor at the maximum and minimum Z-axis coordinates can be set to 1.5, and then linearly changed along the Z-axis coordinate. In this case, a beam length distribution according to the Z-axis coordinate can be given to the porous structure model.

[0082] As another method, each beam may be converted into a truncated cone rod-shaped member during the volumetric modeling process (step S5), and the diameter of each rod-shaped member may be changed in the longitudinal direction according to the coordinates in the design space during this conversion process. Specifically, the origin of the reference coordinate system is set at the center of the porous structure, and the diameter is set in advance to change linearly from the origin toward the maximum and minimum Z-axis coordinate positions. The diameters in the Z-axis coordinates of the nodes at both ends of each beam are then converted into truncated cone rod-shaped members as the diameters of the upper and lower surfaces of the truncated cone. In this case, the porous structure model can be given a distribution of beam diameter and volume density according to the Z-axis coordinate.

[0083] As another method, in the processing of step S24 that constitutes the beam generation process (step S2), the node density threshold, which is one of the conditions for terminating the generation of a new beam, may be changed according to the coordinates in the design space, thereby changing the node density distribution according to the coordinates in the design space.

[0084] The gradient porous structure obtained by the above-mentioned method has controllable deformation and fracture behavior, and compared to an isotropic porous structure of the same or equivalent mass, it has the ability to change the elastic modulus, offset stress, and plateau stress while maintaining high energy absorption properties, making it suitable, for example, as an impact absorbing material.

[0085] In the above embodiment, various data are stored in the storage unit 140 of the design apparatus 100, but the present invention is not limited to this. For example, all or part of the various data may be stored in an external control device or computer via a communication network.

[0086] In the above embodiment, the design apparatus 100 operates based on the program stored in the storage unit 140, but the present invention is not limited to this. For example, the functional configuration realized by the program may be realized by hardware.

[0087] In the above embodiment, the design apparatus 100 is a general-purpose computer, but the present invention is not limited to this. For example, the design apparatus 100 may be realized by a dedicated system or a computer provided on the cloud.

[0088] In the above embodiment, the processing performed by the design device 100 is realized by an apparatus having the above-described physical configuration executing a program stored in the memory unit 140, but the present invention may also be realized as a program or as a storage medium on which the program is recorded.

[0089] In addition, a program for executing the above-mentioned processing operations may be stored and distributed on a non-transitory computer-readable recording medium such as a flexible disk, a CD-ROM (Compact Disk Read-Only Memory), a DVD (Digital Versatile Disk), or an MO (Magneto-Optical Disk), and the program may be installed on a computer to configure an apparatus that executes the above-mentioned processing operations.

[0090] In the above embodiment, a metal porous structure is manufactured using laser powder bed fusion. However, the present invention is not limited to this and may be applied to the manufacture of porous structures made of ceramic or resin materials. For example, a resin porous structure may be manufactured using material extrusion (MEX). The manufactured porous structure can be used for a wide variety of purposes, such as medical devices (e.g., implants), structural materials for transportation equipment, building materials, and shock absorbing materials.

[0091] The above-described embodiments are merely examples, and the present invention is not limited to these. Various embodiments are possible within the scope of the invention as set forth in the claims. The components described in the embodiments and modifications can be freely combined. Furthermore, inventions equivalent to the inventions set forth in the claims are also included in the present invention.

[0092] The present invention will be specifically described below with reference to examples, although the present invention is not limited to these examples.

[0093] (Example) In the examples, a porous structure was fabricated using the method according to the above embodiment, and its mechanical properties were evaluated. First, a porous structure was designed in a design space of 25 mm x 25 mm x 25 mm and manufactured using additive manufacturing. Specifically, a network-like skeleton was created by arbitrarily assigning the length and number of branches of each beam constituting the skeleton of the porous structure according to a probability distribution. Next, a volume was assigned to the network structure by assigning cylindrical volumes of a constant diameter to all beams in the network structure and spherical volumes of the same diameter to the nodes, thereby generating a porous structure model.

[0094] The probability density function of the beam length of each beam that makes up the skeleton of the porous structure was given by a gamma probability distribution, with upper and lower limits set. In this case, the cross-sectional dimensions of the beam to be manufactured were given within the range of beam lengths that could be horizontally formed using the manufacturing equipment. The probability mass function of the number of branches was given by a Poisson probability distribution, with the number of branches limited to 3 or 4. The probability density functions of the beam diameter and beam length, and the probability mass function of the number of branches were each set so that the volume density of the porous structure was approximately 40%. It was confirmed that the designed porous structure had no bias in the beam orientation direction when viewed macroscopically, and no structural anisotropy.

[0095] Next, a porous structure was fabricated using laser powder bed fusion (LPBF) in an additive manufacturing system (LUMEX Avance-25, manufactured by Matsuura Machinery Works, Ltd.). Maraging steel powder (Matsuura Maraging II, manufactured by Matsuura Machinery Works, Ltd.) was used as the material. After additive manufacturing, six surfaces of the sample were machined to produce a 20 mm × 20 mm × 20 mm test piece, as shown in Figure 14A. For comparison, a standard diamond lattice structure test piece with the same dimensions and volumetric density was fabricated, as shown in Figure 14B.

[0096] A compression fracture test was conducted on both specimens in accordance with ISO 13314. A mechanical testing machine (Shimadzu AG-250kND) was used for the compression fracture test. The results are shown in Figure 15. In the macroscopic stress-strain relationship of the diamond lattice structure, after the initial maximum compressive stress was reached, the stress significantly decreased, and fracture progressed with layered fracture, repeating large wavy fluctuations. On the other hand, in the porous structure of the present invention, the decrease and fluctuation of stress after the initial maximum compressive stress were suppressed, and it was confirmed that the absorbed energy was greatly improved. Therefore, it can be seen that the porous structure of the present invention can suppress the progression of fracture under compressive load, allowing the design stress to be set high and the absorbed energy to be improved.

[0097] This application is based on Japanese Patent Application No. 2022-58004 filed on March 31, 2022, including its specification, claims, drawings and abstract. The disclosure of the above-mentioned Japanese patent application is incorporated herein by reference in its entirety. [Industrial Applicability]

[0098] The porous structure of the present invention is useful because its structural and mechanical properties are controlled. [Explanation of symbols]

[0099] 1. Manufacturing System 10 Porous structure 11 Weak Areas 12 Strong Area 100 Design equipment 110 Operation section 120 Display section 130 Communications Department 140 Storage section 141 Parameter storage unit 142 Probability distribution memory unit 143 Model Memory Unit 150 control section 151 Acquisition Department 152 Beam generation unit 153 Node Joint 154 Beam deletion section 155 Model Generation Unit 156 Output section 200 Manufacturing equipment

Claims

1. A porous structure comprising a plurality of beam-like members connected to each other at nodes, The beam-like members are arranged so that the same unit structure is not repeated in the porous structure, The length of the beam-like member and the number of the beam-like members branching from the node are distributed within a range of a predetermined upper limit and a predetermined lower limit, respectively. Porous structure.

2. At least a portion of the numerous nodes provided in the porous structure are nodes arranged so that each beam-shaped member extends three-dimensionally isotropically from the same node. The porous structure according to claim 1 .

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