A method of laser powder bed fusion support-less manufacturing of closed overhanging structures

Abstract

The application discloses a kind of laser powder bed fusion closed overhanging structure support-free manufacturing method, comprising the following steps: step one: establish closed overhanging structure model under different energy density, obtain different energy density and different forming stage defect characteristic data;Step two: set closed overhanging structure energy partition model, energy partition model is constituted by the warping area of protruding powder layer, support-free joint area and matrix area, obtain the warping area of protruding powder layer and support-free joint area area when jointing;Step three: first energy density is endowed to warping area to relieve stress concentration at jointing position, second energy density is applied to joint area to improve lap quality, first energy density is lower than matrix area energy density, and second energy density is higher than or equal to matrix area energy density;Step four: for the energy partition model of step two, process variable is carried out to first energy density, and finally determine the best partition process.

Description

Technical Field

[0001] This invention belongs to the field of laser additive manufacturing and rapid prototyping technology, specifically relating to a method for manufacturing an unsupported closed-loop structure using laser powder bed melting. Background Technology

[0002] Laser powder bed fusion (LPBF) is a typical additive manufacturing technology that uses a high-energy laser beam guided by a digital model to melt a designated area of ​​a metal powder layer after it has been laid up. Due to its high degree of design freedom and good mechanical properties of the formed parts, this technology is widely used in the manufacture of complex structures in aerospace, engineering machinery, and other fields, especially for complex topology optimization structures that are difficult to generate using traditional design methods.

[0003] However, during actual printing, the extremely high temperature gradient around the molten pool formed by the heat source makes additively manufactured parts prone to thermal stress and warping deformation, especially in overhanging structures. Currently, the forming of overhanging structures in the industry usually requires support assistance, but this method has problems such as difficulty in removing supports, material waste, and surface roughness. In addition, for some internally enclosed structures, the inability to remove supports makes it impossible to manufacture them using the LPBF process. Therefore, supportless printing has become the trend of industry development.

[0004] However, unsupported overhang printing presents two major challenges to formability: significant thermal stress, warping, and fracture occur at high energy densities, while defects such as incomplete fusion and porosity appear at low energy densities. These defects are more pronounced in high overhang structures. For high overhang structure forming: using only high energy density results in significant warping before closure and stress concentration leading to fracture after closure. Conversely, using only low energy density leads to incomplete fusion in large overhang areas during closure, resulting in poor overlap. Therefore, using a single energy density inevitably leads to defects. How to leverage the advantages of different energy densities to address defects at various stages of high overhang structure forming becomes a problem that requires further research.

[0005] Laser powder bed technology can lead to significant defects when forming closed, unsupported overhanging structures. These defects include damaged scrapers, breakage, and internal stress fracture, which can directly cause forming failure and force printing to stop midway. The main solution to this problem is to add support to the structure to transform the closed, unsupported overhanging structure into a closed, supported overhanging structure. However, the presence of support increases material consumption and post-processing workload. At the same time, removing the support structure may damage the surface of the part. Therefore, research on forming closed, unsupported overhanging structures is crucial. Summary of the Invention

[0006] To address the shortcomings of the existing technology, this invention provides a method for manufacturing unsupported closed overhanging structures using laser powder bed melting, thereby solving the forming defects in the existing additive manufacturing process for high overhang structures.

[0007] The technical solution adopted in this invention is: a method for manufacturing a laser powder bed molten closed overhang structure without support, comprising the following steps:

[0008] Step 1: Establish closed unsupported overhanging structure models under different energy densities and obtain defect characteristic data for different energy densities and different forming stages;

[0009] Step 2: Set up a closed unsupported overhanging structure energy partition model. The energy partition model consists of a warped area of ​​the protruding powder layer, an unsupported closure area, and a matrix area. Based on the defect feature data from Step 1, the warped area of ​​the protruding powder layer and the unsupported closure area during closure are obtained through image monitoring and simulation.

[0010] Step 3: Apply a first energy density to the warped area to alleviate stress accumulation at the closure position, and apply a second energy density to the closure area to improve the overlap quality. The first energy density is lower than the energy density of the base area, and the second energy density is higher than or equal to the energy density of the base area.

[0011] Step 4: For the energy partitioning model in Step 2, perform process parameter changes on the first energy density to finally determine the optimal partitioning process.

[0012] Furthermore, step one specifically includes:

[0013] Step 1.1: High overhang structure design: Establish closed unsupported cantilever structure models under different energy densities, and design the overhang at the initial closure position of the closed unsupported cantilever structure to be ≥1mm;

[0014] Step 1.2: High overhang structure forming: The closed, unsupported overhang structure is formed by energy density in the matrix region using a laser powder bed melting device;

[0015] Step 1.3: Acquisition of structural defect feature data with high overhang: The defect feature data includes fracture defect feature data formed at the third energy density.

[0016] Furthermore, step two includes:

[0017] The closed, unsupported overhanging structure designed in step 1.1 is divided into regions. Using the high overhang structural defect feature data obtained in step 1.3, the closed, unsupported overhanging structure is divided into a warped region protruding from the powder layer, an unsupported closure region, and a matrix region.

[0018] Furthermore, step four specifically includes:

[0019] Step 4.1: First energy zone parameter variation: Set a series of parameters with laser power less than 200W and laser scanning speed higher than 1000mm / s and orthogonally combine them to assign to the first energy zone, and import them into the laser powder bed melting equipment for forming;

[0020] Step 4.2: Determination of the optimal process: The optimal process is determined by performing computed tomography (CT) characterization and online image monitoring on the energy zoning model of the closed unsupported overhanging structure after zoning parameter variation. The optimal process is the one in which no pores or lack of fusion are found inside the closed unsupported overhanging structure during CT scanning and no fracture occurs during the forming process in the closed unsupported overhanging structure during online image monitoring.

[0021] Furthermore, the laser power of the energy density in the substrate region is ≥200W, and the scanning speed is ≤1000mm / s.

[0022] Furthermore, the closed unsupported overhanging structure model in step 1.1 is a bridge-shaped structure with a height of 15mm and an inner circle radius of 42mm.

[0023] The beneficial effects of this invention are: the forming method of this invention can suppress defects in closed, supported, and overhanging structures during additive manufacturing, guide actual production, and reduce risks and costs in actual manufacturing.

[0024] In addition to the objectives, features, and advantages described above, the present invention has other objectives, features, and advantages. The invention will now be described in further detail with reference to the figures. Attached Figure Description

[0025] Figure 1 It is a design model diagram of a bridge structure;

[0026] Figure 2 This is a diagram showing the formation of a bridge-shaped structure;

[0027] Figure 3 This is a monitoring image of the initial closure layer during the laser powder bed melting process;

[0028] Figure 4 This is a diagram showing the stress-strain field distribution during the laser powder bed melting process;

[0029] Figure 5 This is a schematic diagram of the energy zoning model design;

[0030] Figure 6 These are the shaping effect diagrams under various partitioning parameters;

[0031] Figure 7 These are computed tomography (CT) scan views under various partition parameters;

[0032] Figure 8 This is a flowchart of the unsupported manufacturing method for the laser powder bed molten closed overhang structure of the present invention. Detailed Implementation

[0033] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0034] Combination Figure 8 A method for manufacturing an unsupported closed overhanging structure using laser powder bed melting includes the following steps:

[0035] Step 1: Establish closed unsupported overhanging structure models under different energy densities and obtain defect characteristic data for different energy densities and different forming stages;

[0036] Step 2: Set up a closed unsupported overhanging structure energy partition model. The energy partition model consists of a warped area of ​​the protruding powder layer, an unsupported closure area, and a matrix area. Based on the defect feature data from Step 1, the warped area of ​​the protruding powder layer and the unsupported closure area during closure are obtained through image monitoring and simulation.

[0037] Step 3: Apply a first energy density to the warped area to alleviate stress accumulation at the closure position, and apply a second energy density to the closure area to improve the overlap quality. The first energy density is lower than the energy density of the base area, and the second energy density is higher than the energy density of the base area.

[0038] Step 4: For the energy partitioning model in Step 2, adjust the process parameters for the first energy density to finally determine the optimal partitioning process.

[0039] Furthermore, step one specifically includes:

[0040] Step 1.1: High overhang structure design: Design a bridge-shaped structure with a height of 15mm and an inner circle radius of 42mm.

[0041] Step 1.2: High Overhang Structure Forming: The structure was formed using a Concept Laser M2 machine. The process parameters for each sample were as follows: 120W, 1000mm / s; 200W, 1000mm / s; 350W, 1000mm / s; 350W, 800mm / s. The laser employed a scanning strategy with 90° interlayer rotation, and the layer thickness was set to 30μm. The bridge-shaped structure was designed to initially close at layer 334; therefore, layer 334 is referred to as the initial closure layer.

[0042] Step 1.3: Acquisition of Defect Feature Data for High Overhang Structures: For the operating conditions of 200W, 1000mm / s, 350W, 1000mm / s, and 350W, 800mm / s, the samples fractured in the closed area. The fracture width was only between 1.1mm and 1.9mm, while the fracture height reached as high as 3.9mm. The fracture had a severe impact on the forming process, causing the squeegee to cut the fracture during printing, thus forcing the printing process to stop. Defect feature data was obtained by photographing the closure layer with an industrial camera. The defect feature data showed that for the sample with process parameters of 350W and 800mm / s, the warping of the un-powder-covered areas on both sides of the initial closure layer was approximately 8.7mm, and the unsupported portion at the closure position was approximately 4.6mm.

[0043] Furthermore, step two specifically includes:

[0044] Simulation analysis revealed two main causes of fracture in high-overhang structures: 1. The closure section, largely unsupported with a 0° overhang printing process, exhibited significant incomplete fusion. The weak closure section could not withstand this internal tensile stress, leading to fracture. 2. Near the closure area, the excessively low overhang angle resulted in large areas of warped residue not covered by powder before closure. After closure, this warped residue underwent remelting, exacerbating the warping trend and creating significant tensile stress at the closure point. Therefore, to address the fracture problem, it is necessary to suppress the remelting of warped residue and extract the specific locations of the warped residue protruding from the initial closure layer using online image monitoring data.

[0045] Based on the above mechanism, defects not covered by the powder layer require low-energy-density melting to suppress warping, while unsupported closure positions require high-energy-density melting to improve the overlap quality. Therefore, the energy zoning model of the closed unsupported overhanging structure is divided into a warping region protruding from the powder layer, an unsupported closure region, and a matrix region. Based on the defect characteristic data from step one, the warping region protruding from the powder layer and the unsupported closure region during closure are obtained through image monitoring and simulation. Regarding the determination of the low-energy zoning height: if the linear energy density of the matrix region is higher than 0.3 J / mm, the zoning energy density height is between 0.021 mm and 0.03 mm. If the energy density is lower than 0.3 J / mm, the zoning energy density height is between 0.012 mm and 0.021 mm.

[0046] Furthermore, step three includes:

[0047] Energy Distribution: The energy density of the matrix can be consistent with that of the unsupported overhanging structure model closed in Step 1. To effectively suppress the effects of remelting, the energy density of the warped region should be assigned a first energy density, which should be lower than that of the matrix region, for example, set to 25W, 1000mm / s, 50W, 1000mm / s, 75W, 1000mm / s, and 100W, 1000mm / s. To prevent incomplete fusion in the closure region, the energy density of the unsupported closure region should be assigned a second energy density, which should be greater than or equal to that of the matrix region. In this step, 350W, 800mm / s is used as the second energy density.

[0048] Specific operational method for energy allocation: Design different regions of the energy partitioning model in Magics commercial software, including the matrix region, warped region, and unsupported closure region. After constraining and assembling each region, import it into the Concept Laser M2 printer, where assign corresponding parameters to each region to implement the energy partitioning strategy.

[0049] Furthermore, step 4.2 of step four, determining the optimal process, includes:

[0050] External forming quality comparison: Observation revealed that the sample did not fracture at a zone energy density of 75W and 1000mm / s, while all other samples fractured. Furthermore, statistics showed that under this condition, the sample exhibited better lower surface quality, with a total thickness of 18.91mm. 3 The missing part exists on the lower surface of the closure joint.

[0051] Internal forming quality comparison: Computed tomography (CT) characterization of the specimens revealed the absence of porosity or incomplete fusion defects. This excellent internal quality can be attributed to the limited number of layers (only seven layers) used in the low-energy partitioning.

[0052] Example 1

[0053] A method for manufacturing an unsupported closed overhanging structure using laser powder bed fusion includes:

[0054] Step 1: Establish closed unsupported overhanging structure models under different energy densities, and obtain defect characteristic data for different energy densities and different forming stages, specifically:

[0055] Designed Figure 1 The bridge-shaped structure shown has a height of 15 mm and an inner radius of 42 mm. The total length of the sample is 64.75 mm, and the width is 5 mm. Table 1 shows the process parameters used.

[0056] Table 1. Process parameters of bridge-shaped specimens

[0057]

[0058] The laser employs a scanning strategy with interlayer rotation of 90° and a layer thickness of 30μm, without using an edge scanning strategy.

[0059] The model uses GH4169 as the deposition material, and the chemical composition of GH4169 powder is shown in Table 2.

[0060] Table 2. Chemical composition of GH4169

[0061]

[0062] The simulation used a substrate with dimensions of 2400μm × 1200μm × 600μm, and deposited layers with dimensions of 1200μm × 600μm × 60μm, each with a thickness of 30μm. The mesh size for the substrate region was 50μm, and the mesh size for the deposition region was 15μm.

[0063] Figure 2 The images show the forming results of the bridge-shaped samples. The left image is a camera-captured image, and the right image is a comparison of the 3D scanning deviation. Samples 1-4 (350W-800mm / s), 1-3 (350W-1000mm / s), and 1-2 (200W-1000mm / s) fractured in the closed region. The fracture widths were 1.9mm (sample 1-2), 1.4mm (sample 1-3), and 1.1mm (sample 1-4), respectively, and the fracture heights were 3.3mm, 3.5mm, and 3.9mm, respectively. Therefore, it can be concluded that as the energy density decreases, the larger the fracture width, the smaller the warpage height. Fracture has a severe impact on forming; due to the large warpage height and sharp fracture edges, the scraper is damaged, leading to uneven powder preparation. To prevent further deterioration of the powder spreading, samples 1-2, 1-3, and 1-4 were manually stopped at the 350th, 357th, and 350th layers, respectively.

[0064] To obtain defect characteristic data at different energy densities and different forming stages, extract such as Figure 3 The diagram shows the initial closure layer formation of the 334th layer, where... Figure 3 The left image shows the unsupported closure area, while Figure 3 In the left image, the warped areas not covered by the powder can be observed on both sides of the area to be joined, as indicated by the arrow.

[0065] Step 2: Set up a closed unsupported overhanging structure energy partition model. The energy partition model consists of a warped area of ​​the protruding powder layer, an unsupported closure area, and a matrix area. Based on the defect feature data from Step 1, the warped area of ​​the protruding powder layer and the unsupported closure area during closure are obtained through image monitoring and simulation.

[0066] Simulation and online image monitoring analysis revealed two main causes of fracture in structures with high overhang: 1. Figure 3 As shown in the right figure, during the initial closure of the sample, due to the excessive overhang of the sample in the initial closure layer, significant incomplete fusion occurred in the area to be closed after closure. 2. As Figure 4 The simulation results show that the sample has a large area of ​​warping residue near the closure area due to the excessively low overhang angle, which was not covered by powder before closure. Therefore, after closure, the remelting of the warping residue area exacerbates the warping trend, forming obvious tensile stress at the closure point. The weak closure point cannot withstand this internal tensile stress and thus fractures.

[0067] Based on the above mechanism, defects not covered by the powder layer require low-energy-density melting to suppress warping, while unsupported closure positions require high-energy-density melting to improve the overlap quality. Therefore, the energy zoning model of the closed unsupported overhanging structure is divided into a warping region protruding from the powder layer, an unsupported closure region, and a matrix region. Based on the defect characteristic data from step one, the warping region protruding from the powder layer and the unsupported closure region during closure are obtained through image monitoring and simulation. Regarding the determination of the low-energy zoning height: if the linear energy density of the matrix region is higher than 0.3 J / mm, the zoning energy density height is between 0.021 mm and 0.03 mm. If the energy density is lower than 0.3 J / mm, the zoning energy density height is between 0.012 mm and 0.021 mm.

[0068] Step 3: Apply a first energy density to the warped area to alleviate stress accumulation at the closure position, and apply a second energy density to the closure area to improve the overlap quality. The first energy density is lower than the energy density of the base area, and the second energy density is higher than or equal to the energy density of the base area.

[0069] For the matrix energy density, it can be consistent with the energy density of the closed, unsupported, overhanging structure model from step one. To effectively suppress the effects of remelting, the energy density of the warped region should be assigned a first energy density, which should be lower than the energy density of the matrix region. See [reference needed]. Figure 5 In this case, four process parameters were set: 25W, 1000mm / s; 50W, 1000mm / s; 75W, 1000mm / s; and 100W, 1000mm / s. To prevent incomplete fusion in the closure area, a second energy density should be assigned to the unsupported closure area. This second energy density should be greater than or equal to the energy density of the base area. In this step, 350W and 800mm / s were used as the second energy density.

[0070]

[0071]

[0072] The model was then partitioned into different areas and energy was assigned. An energy partitioning model was built in the Magics commercial software, and high-energy-density partitions were assembled with low-energy-density partitions. The data was then sliced ​​and imported into the ConceptLaser M2 metal printer, and the energy density was set.

[0073] Step 4: Based on the energy zoning model from Step 3, the optimal fracture suppression process is determined through online image analysis and offline surface scanning detection, specifically as follows:

[0074] For external forming quality, see Figure 6 The left image is a camera-captured image, and the right image is a comparison of 3D scanning deviations. It should be noted that sample 2-1 fractured at layer 339 and was intentionally stopped at layer 350. In contrast, samples 2-2 and 2-4 began to fracture at layers 373 and 366 respectively, but continued until the end of printing. Regarding the fractured sample 2-1, its fracture characteristics were relatively gentle, with the highest point of fracture only rising 3.2 mm, therefore the forming process did not damage the scraper.

[0075] For samples 2-3 that did not fracture (low energy zone: 75W, 1000mm / s), the figure shows a good upper surface and slight defects on the lower surface. Statistical analysis revealed that within the range of 250 to 450 layers, a total of 18.91mm defects were found on the lower surface of the samples. 3 The defects were identified. Therefore, the 2-3 samples with the highest surface forming accuracy at the overhang position and without fracture were selected as the optimal samples for production.

[0076] For internal forming quality, see Figure 7 The partitioned model was characterized by computed tomography (CT) scans, with the selected images being the central surface of the energy-partitioned specimen. No internal porosity or unfused defects were found. The excellent internal quality can be attributed to the limited number of low-energy partitioned layers (only seven layers). Furthermore, the high energy density following the partitioning process also facilitates the healing of unfused or porosity defects in the low-energy regions.

[0077] The above examples demonstrate that for bridge-shaped high-overhang overhang structures, the fracture problem can be solved by using low-energy zoning technology with process parameters of 75W and 1000mm / s applied to the warped residual region. Furthermore, this method can be extended to all high-overhang overhang structures, effectively solving the fracture problem by preventing remelting of the warped residual region and enhancing the connection stability at the closure joint.

[0078] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for manufacturing a laser powder bed fusion closed overhang structure without support, characterized in that, Includes the following steps: Step 1: Establish closed unsupported overhanging structure models under different energy densities and obtain defect characteristic data for different energy densities and different forming stages; Step one specifically includes: Step 1.1: High overhang structure design: Establish closed unsupported cantilever structure models under different energy densities, and design the overhang at the initial closure position of the closed unsupported cantilever structure to be ≥1mm; Step 1.2: High overhang structure forming: The closed, unsupported overhang structure is formed using a laser powder bed melting device; Step 1.3: Acquisition of structural defect feature data with high overhang: The defect feature data includes fracture defect feature data; Step 2: Set up a closed unsupported overhanging structure energy partition model. The energy partition model consists of a warped area of ​​the protruding powder layer, an unsupported closure area, and a matrix area. Based on the defect feature data from Step 1, the warped area of ​​the protruding powder layer and the unsupported closure area during closure are obtained through image monitoring and simulation. Step two includes: The closed unsupported overhanging structure designed in step 1.1 is divided into regions. Based on the high overhang structural defect feature data obtained in step 1.3, the closed unsupported overhanging structure is divided into the warped region protruding from the powder layer, the unsupported closure region, and the matrix region. Step 3: Apply a first energy density to the warped area to alleviate stress accumulation at the closure position, and apply a second energy density to the unsupported closure area to improve the overlap quality. The first energy density is lower than the energy density of the base area, and the second energy density is higher than the energy density of the base area. Step 4: For the energy partitioning model in Step 2, adjust the process parameters for the first energy density to finally determine the optimal partitioning process; The closed unsupported overhanging structure model in step 1.1 is a bridge-shaped structure with a height of 15 mm and an inner radius of 42 mm.

2. The method for manufacturing a laser powder bed fusion closed overhang structure without support according to claim 1, characterized in that, The laser power of the energy density in the substrate region is ≥200W, and the scanning speed is ≤1000mm / s.

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