A method of unsupported laser selective melting forming of overhanging structures

Abstract

This invention relates to a supportless laser selective melting forming method for overhanging structures, belonging to the field of metal additive manufacturing technology. The method includes: determining a pre-forming layer; defining a final forming layer and defining a pre-forming layer downwards within the projection area of ​​the final forming layer; performing point exposures on the overhanging projection area from the pre-forming layer adjacent to the final forming layer to the pre-forming layer adjacent to the substrate, with a non-monotonic sequence coverage; forming layer by layer from the pre-forming layer adjacent to the final forming layer to the pre-forming layer adjacent to the substrate, using point exposures to input energy within the overlapping area of ​​the overhanging structure projection during the forming process; actively avoiding exposure points of the previous layer during the point exposure of the middle pre-forming layer; and using densification parameters in the final forming layer to complete the final forming of the overhanging structure. This invention provides an effective solution for the supportless manufacturing of key components such as aero-engine impellers.

Description

Technical Field

[0001] This invention belongs to the field of metal additive manufacturing technology, and particularly relates to a method for unsupported laser selective melting forming of a suspended structure. Background Technology

[0002] Key components in aero-engines, such as closed impellers, typically contain numerous complex overhang structures and internal flow channels. When manufacturing such parts using traditional selective laser melting (SLM) technology, the main technical challenges are as follows: For structures with extremely small overhang angles, the molten pool is prone to flow and seepage due to gravity, leading to high surface roughness, loss of precision, and even forming failure. To prevent molten material from falling and ensure forming accuracy, traditional processes require the design and forming of internal support structures below the overhang area. However, these internal support structures are extremely difficult to remove subsequently, especially for difficult-to-machine materials such as titanium alloys, and in the narrow flow channel areas inside closed impellers. Removing the supports is not only difficult but also easily damages the part itself, increasing manufacturing costs and time, and may affect the fatigue performance of the part due to surface damage.

[0003] Currently, while some research attempts to achieve supportless forming by optimizing process parameters—for example, for titanium alloy closed impellers, reducing the laser power on the lower surface (e.g., 60W) to decrease warping deformation, thereby achieving supportless forming of overhanging surfaces at specific angles (e.g., below 30°)—existing supportless processes, while alleviating the aforementioned problems to some extent, still generally suffer from limitations such as narrow process windows, poor forming consistency, and insufficient adaptability to large-sized overhangs at low angles. Especially for the more complex and diverse overhanging structures in aero-engine impellers, particularly large-sized, low-angle overhanging surfaces, problems such as poor molten pool stability, easy powder adhesion, high lower surface roughness, and potential melt seepage, warping deformation, and even forming failure are still prevalent. Existing technologies often struggle to achieve high-quality supportless overhanging surfaces while ensuring the density and dimensional accuracy of the formed parts.

[0004] Therefore, there is an urgent need to develop a targeted, stable, and effective unsupported laser selective melting forming method for improving the forming quality of the overhanging structure of aero-engine impeller parts. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a supportless laser selective melting forming method for overhanging structures. This method is suitable for the direct forming and manufacturing of components in aero-engines with complex internal flow channels, closed impellers, or other low-angle overhanging structures where support removal is difficult. It is particularly suitable for overhanging structures with a projected area Qn ≤ 16 mm². 2 For parts with a projected length Ln ≤ 4 mm, the suspension angle is 0° to 15°.

[0006] A method for unsupported laser selective melting forming of a suspended structure, specifically including the following steps:

[0007] Determine the pre-forming layer: Define the final forming layer where the overhang structure is located as Slice-n, and define the pre-forming layer downward within the projection area of ​​the final forming layer; The coverage of point exposure of the overhang projection area from the pre-forming layer adjacent to the final forming layer to the pre-forming layer adjacent to the substrate is a non-monotonic sequence.

[0008] From the pre-forming layer immediately adjacent to the final forming layer to the pre-forming layer immediately adjacent to the substrate, forming is performed layer by layer. During the forming process, energy is input in the overlapping area of ​​the projection of the suspended structure using point exposure. During the point exposure process of the middle pre-forming layer, the exposure points of the previous layer are actively avoided.

[0009] The final forming layer employs densification parameters to complete the final forming of the overhanging structure.

[0010] The number of pre-forming layers is at least five adjacent forming layers.

[0011] When determining the pre-forming layer, the overhanging structure in the 3D model of the part to be processed is identified, the layers are automatically sliced ​​and the projection contour of each layer is calculated, and the projection length L and projection area Q are calculated layer by layer.

[0012] The final forming of the final forming layer is accomplished using a fully densified scanning strategy.

[0013] The number of pre-forming layers is five, which are defined sequentially downwards from the projection area of ​​the final forming layer as: Slice-n-1, Slice-n-2, Slice-n-3, Slice-n-4, and Slice-n-5.

[0014] During the sequential layer-by-layer forming process from Slice-n-5, Slice-n-4, Slice-n-3, Slice-n-2 to Slice-n-1, the coverage areas of the overlapping regions of the projected overhanging structure are 40%, 80%, 40%, 80%, and 100%, respectively.

[0015] In the sequential layer-by-layer forming process from Slice-n-5, Slice-n-4, Slice-n-3, Slice-n-2 to Slice-n-1, the laser diameter is 70-110μm, the laser power is 120-180W, and the spot exposure spacing is 350-450μm.

[0016] During the sequential layer-by-layer forming process from Slice-n-5, Slice-n-4, Slice-n-3, Slice-n-2 to Slice-n-1, the exposure times are 50-100μs, 50-100μs, 50-100μs, 125-175μs, and 50-100μs, respectively.

[0017] During the Slice-n-5 layer forming process, there are no overlapping areas between the exposure areas at each point;

[0018] The center of the point exposure of the Slice-n-4 layer coincides with that of the Slice-n-5 layer, and a 0-15μm overlap area is formed between the point exposure areas;

[0019] The center of the Slice-n-3 layer exposure should avoid the already exposed points of the Slice-n-4 layer and form an overlap area of ​​10-50μm with the Slice-n-4 layer exposure area.

[0020] The center of the spot exposure in Slice-n-2 layer coincides with that in Slice-n-3 layer, and an overlap area of ​​0-15μm is formed between the spot exposure areas.

[0021] The final pre-forming layer performs point exposure on 100% of the overhang projection area; the exposure parameters used have an energy density of 50% to 75% of the Slice-n layer densification parameters.

[0022] By employing the above technical solution, the present invention has at least the following beneficial effects:

[0023] (1) This application provides a non-monotonic area sequence design:

[0024] An innovative exposure area ratio sequence of 40% → 80% → 40% → 80% → 100% is adopted. This non-monotonic design avoids continuous high heat input, effectively controls thermal stress accumulation, and prevents warping; at the same time, the secondary local exposure (40%) of the Slice-n-3 layer as the middle layer further enhances the structural stability of the central part of the pre-support and disperses stress.

[0025] (2) This application provides precise control over the overlap amount and the arrangement of points:

[0026] In this application, a 0-15μm micro-overlap is formed between the Slice-n-4 layer and the Slice-n-2 layer's spot exposure area, and a 10-50μm overlap is formed between the Slice-n-3 layer and the underlying layer. This design avoids an excessively large heat-affected zone due to excessive overlap while ensuring interlayer metallurgical bonding and energy transfer. Furthermore, in this application, the spot exposure center of the Slice-n-3 layer avoids existing spots in the Slice-n-4 layer, breaking the continuous fragile interface and enhancing the structure's resistance to sagging.

[0027] (3) This application provides energy density gradient settings and material compatibility:

[0028] This application controls the energy density of the Slice-n-1 layer, which is closest to the forming layer, to 50-75% of the densification parameter. This ensures that the layer has sufficient strength to support the molten pool above, while avoiding over-sintering that would make it difficult to be fully melted. In a specific embodiment, each layer is configured with a matching range of laser power, exposure time, and dot pitch parameters based on the material properties of the titanium alloy, improving the feasibility and repeatability of the process.

[0029] (4) This application provides a clear definition of the applicable structural constraints:

[0030] This application explicitly states that this method is preferably applicable to titanium alloy and high-temperature alloy suspension structures with a projected area Qn ≤ 16mm², a projected length Ln ≤ 4mm, and a suspension angle of 0°-15°. This limitation allows for more targeted design of process parameters, ensuring optimal results can be achieved within the aforementioned key dimensional range.

[0031] In summary, by employing a non-monotonic exposure area sequence (40%-80%-40%-80%-100%) and precisely controlled overlap, a pre-support structure with gradient strength can be formed in the lower layer. This effectively supports the upper molten metal, preventing seepage and deformation, while ensuring complete fusion during final forming, resulting in a high-quality overhanging surface (after sandblasting and polishing, an Ra value below 12.5 μm can be achieved). This method is particularly suitable for forming titanium alloy overhanging structures with a projected area not exceeding 16 mm², a projected length not exceeding 4 mm, and an overhang angle of 0°-15°, providing an effective solution for the supportless manufacturing of key components such as aero-engine impellers. Attached Figure Description

[0032] Figure 1 This is a schematic diagram illustrating the forming principle of the unsupported laser selective melting forming method for the suspended structure provided by the present invention; (where α is the suspension angle of the part, L is the projected length, T is the layer thickness, S1 is the Slice-n-1 layer, S2 is the Slice-n-2 layer, S3 is the Slice-n-3 layer, S4 is the Slice-n-4 layer, and S5 is the Slice-n-5 layer).

[0033] Figure 2 This is a schematic diagram of the pre-forming layer after printing the Slice-n-5 layer in Embodiment 1 of the present invention;

[0034] Figure 3 This is an illustration of the pre-forming layer after printing the Slice-n-4 layer in Embodiment 1 of the present invention;

[0035] Figure 4 This is a schematic diagram of the pre-forming layer after printing the Slice-n-3 layer in Embodiment 1 of the present invention;

[0036] Figure 5 This is a schematic diagram of the pre-forming layer after printing the Slice-n-2 layer in Embodiment 1 of the present invention;

[0037] Figure 6 This is a schematic diagram of the pre-forming layer after printing the Slice-n-1 layer in Embodiment 1 of the present invention;

[0038] Figure 7 This is a side view of the pre-forming layer after printing in Embodiment 1 of the present invention;

[0039] In the picture:

[0040] 1. Components; 2. Substrate. Detailed Implementation

[0041] To better explain and facilitate understanding of the present invention, the technical solution and effects of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0042] Example 1

[0043] The part 1 to be processed in this embodiment is a suspension structure of an aero-engine impeller with a suspension angle α of 8°.

[0044] Combination Figures 1-7 As shown in this embodiment, a method for unsupported laser selective melting forming of a suspended structure is provided. The core of this method lies in using multi-layer progressive discontinuous point exposure pre-support. Energy and geometry-controllable non-dense point exposure is performed in five preceding layers (Slice-n-1, Slice-n-2, Slice-n-3, Slice-n-4, and Slice-n-5) below the final forming layer (Slice-n). This constructs a transition support region with gradient intensity and a good fusion interface, thereby achieving high-quality unsupported forming. In this embodiment, part 1 has a projected area of ​​Qn = 10 mm² for the suspended structure of the final forming layer, and a projected length Ln = 3.2 mm for the final forming layer. The specific steps include:

[0045] Step 1: Determining the pre-forming layer.

[0046] Step 1.1: Model Processing:

[0047] The Materialise Magics software identifies the overhanging structure in the 3D model of the part to be processed, automatically slices it, calculates the projection contour of each layer, and calculates the projection length L and projection area Q layer by layer.

[0048] Step 1.2: Setting parameters for the pre-forming layer: In the equipment operating software, precisely set the point exposure parameters for the pre-forming layer of the target overhanging area.

[0049] The final shaping layer containing the overhanging structure is defined as Slice-n, and five preliminary shaping layers are defined downwards within its projected area Qn: Slice-n-1, Slice-n-2, Slice-n-3, Slice-n-4, and Slice-n-5. Slice-n-5 melts localized powder particles into discontinuous monomers to prevent warping. Slice-n-4 enlarges the monomers formed by Slice-n-5, initially connecting them and minimizing overlap to prevent warping. Slice-n-3 fills the remaining gaps in Slice-n-4, forming a stable support layer. Slice-n-2 further stabilizes the support layer based on Slice-n-3. Slice-n-1 provides the initial shaping of the entire overhanging surface. Slice-n achieves densification based on Slice-n-1. Exposure points on each layer should be evenly distributed.

[0050] During the forming process of Slice-n-5 to Slice-n-1 layers, energy input is performed within the suspended projection area using a point exposure method. The exposure area ratio, laser parameters, and point arrangement of each layer are adjusted layer by layer according to preset rules. Specifically:

[0051] In the Slice-n-5 layer, 40% of the area of ​​the overhanging projection region is exposed at a point, and there is no overlap between the exposed point regions.

[0052] In the Slice-n-4 layer, 80% of the overhang projection area is exposed by spot exposure. The center of the spot exposure coincides with the center of the spot exposure in the Slice-n-5 layer, and an overlap area of ​​0-15μm is formed between the spot exposure areas.

[0053] In the Slice-n-3 layer, 40% of the overhang projection area is exposed. The center of the exposure point avoids the already exposed points in the Slice-n-4 layer and forms an overlap area of ​​10-50μm with the exposure area of ​​the Slice-n-4 layer.

[0054] 80% of the overhang projection area is exposed in the Slice-n-2 layer. The center of the exposure coincides with the center of the exposure in the Slice-n-3 layer, and an overlap area of ​​0-15μm is formed between the exposure area and the exposure area in the Slice-n-3 layer.

[0055] Point exposures were performed on 100% of the overhang projection area of ​​the Slice-n-1 layer, with the exposure parameters and energy density being 50-75% of the densification parameters of the Slice-n layer.

[0056] The forming is completed using a fully densified scanning strategy in the Slice-n layer; a suitable overhang structure is required, with an overhang angle of 0°-15°, a projected area Qn ≤ 16 mm², and a projected length Ln ≤ 4 mm.

[0057] Step 2: Turn on the equipment, spread powder layer by layer on the substrate 2 and execute the printing program.

[0058] This embodiment uses a Renishaw AM250 laser selective melting system, equipped with a laser system and control system supporting spot exposure. The optical system can achieve spot exposure with a diameter of 70-110 μm. The control system executes the layer-by-layer spot exposure strategy. The built-in module can automatically identify the overhanging structure, calculate the projected length L and projected area Q, and generate the spot exposure path and parameters for Slice-n-5 to Slice-n-1 layers. Additive manufacturing is performed using titanium alloy TC4 powder with a particle size range of 15-53 μm. The substrate 2 preheating temperature is 80℃, and the forming chamber atmosphere is high-purity argon with an oxygen content of less than 0.1%.

[0059] Step 3: Based on the settings in Step 1, print each pre-forming layer in the order of Slice-n-5, Slice-n-4, Slice-n-3, Slice-n-2, and Slice-n-1.

[0060] When printing to the Slice-n-5 layer, the laser system uniformly distributes spot exposures over 40% of the area within the overhanging projection region Qn, generating a dot matrix with a 40% coverage rate and a dot spacing of 400 μm. Each exposed area is independent, non-overlapping, and does not overlap. The laser spot diameter is 80 μm, the power is 150 W, and the exposure time is 75 μs.

[0061] When printing the Slice-n-4 layer, the laser system performs spot exposure on 80% of the area within the same projection region Qn, with a spot spacing of 400 μm. It is ensured that the center of each spot exposure on the Slice-n-4 layer precisely coincides with the center of the spot exposure on the Slice-n-5 layer. An overlap of approximately 10 μm is formed between the laser spot and the spot exposure area of ​​the Slice-n-5 layer by adjusting the laser spot overlap. The laser spot diameter is 80 μm, the power is 150 W, and the printing time is 150 μs.

[0062] When printing the Slice-n-3 layer, within the same projection area Qn, the laser system first identifies the coordinates of all exposure points on the Slice-n-4 layer. Then, it performs spot exposure on 40% of the area to generate a new dot matrix with a dot spacing of 400 μm. The center position of the spot exposure is required to actively avoid the already exposed points on the Slice-n-4 layer, and an overlap of approximately 30 μm is formed between the laser spot and the spot exposure area of ​​the Slice-n-4 layer by adjusting the laser spot overlap. The laser spot diameter is 80 μm, the power is 150 W, and the exposure time is 75 μs.

[0063] When printing the Slice-n-2 layer, 80% of the area within the same projection region Qn is dot-exposed with a dot pitch of 400 μm. The center of the dot exposure coincides with the center of the dot exposure of the Slice-n-3 layer, forming a 5 μm overlap area between them. The spot diameter is 80 μm, the power is 150 W, and the printing time is 150 μs.

[0064] When printing the Slice-n-1 layer, 100% coverage of the projection area is achieved by spot exposure of 100% of the projection area, with a spot spacing of 100 μm, and the energy density is controlled at 65% of the Slice-n layer densification parameters. Specifically, the spot diameter is 80 μm, the power is 165 W, and the exposure time is 100 μs.

[0065] Step 4: Print the final formed layer.

[0066] The Slice-n layer was printed using a standard continuous scanning mode with a scanning plate diameter of 80 μm. Densification parameters optimized for TC4 titanium alloy were employed to completely scan and melt the entire overhanging area. The printing parameters were: power 250 W, scanning speed 900 mm / s, and scanning spacing 110 μm.

[0067] Step 5: Cut part 1 off the substrate 2 and perform sandblasting and polishing on the overhanging structure surface.

[0068] Step 6: Post-processing and effect verification.

[0069] After printing, the formed impeller part 1 is removed from the substrate 2 by wire cutting. After sandblasting and ultrasonic cleaning, loose powder on the surface is removed. Sandblasting is performed using 120-mesh white corundum at a pressure of 0.4 MPa; ultrasonic cleaning is performed using anhydrous ethanol as the medium for 30 minutes.

[0070] Macroscopic observation: Visual and stereomicroscopic observation showed that the overhanging surface was generally flat and uniformly transitioned, with no visible powder adhesion, melt sagging, or warping deformation.

[0071] Quantitative measurement: The unsupported, overhanging surface was measured using a surface roughness tester. Five measurement points were randomly selected, and the average value was calculated.

[0072] In this embodiment, a MarSurf GD 25 surface roughness tester was used to measure the overhanging surface. The measurement length was 4 mm, and the evaluation length was 3.2 mm. Five measurement points were evenly selected in the overhanging area, and the Ra values ​​were 11.3 μm, 12.1 μm, 11.5 μm, 12.2 μm, and 11.6 μm, with an average value of 11.7 μm. Visually, the surface was smooth and there was no visible powder adhesion.

[0073] Example 2

[0074] The part 1 to be processed in this embodiment is a suspension structure of an aero-engine impeller with a suspension angle α of 12°.

[0075] This embodiment provides a method for unsupported laser selective melting forming of a suspended structure. The core of this method lies in multi-layer progressive discontinuous point exposure pre-support. Energy and geometry-controllable non-dense point exposure is performed in five preceding layers (Slice-n-1, Slice-n-2, Slice-n-3, Slice-n-4, and Slice-n-5) below the final forming layer (Slice-n). This constructs a transition support region with gradient intensity and a good fusion interface, thereby achieving high-quality unsupported forming. In this embodiment, part 1 has a projected area of ​​Qn = 14.5 mm² for the suspended structure of the final forming layer, and a projected length Ln = 3.8 mm. The specific steps include:

[0076] Step 1: Determining the pre-forming layer.

[0077] Step 1.1: Model Processing:

[0078] The Materialise Magics software identifies the overhanging structure in the 3D model of the part to be processed, automatically slices it, calculates the projection contour of each layer, and calculates the projection length L and projection area Q layer by layer.

[0079] Step 1.2: Parameter settings: In the equipment operation software, precisely set the point exposure parameters for the pre-forming layer of the target overhanging area.

[0080] The forming layer containing the overhanging structure is defined as Slice-n, and five preceding forming layers are defined downward within its projection area Qn, namely Slice-n-1, Slice-n-2, Slice-n-3, Slice-n-4, and Slice-n-5.

[0081] During the forming process of Slice-n-5 to Slice-n-1 layers, energy input is performed within the suspended projection area using a point exposure method. The exposure area ratio, laser parameters, and point arrangement of each layer are adjusted layer by layer according to preset rules. Specifically:

[0082] In the Slice-n-5 layer, 40% of the area of ​​the overhanging projection region is exposed at a point, and there is no overlap between the exposed point regions.

[0083] In the Slice-n-4 layer, 80% of the overhang projection area is exposed by spot exposure. The center of the spot exposure coincides with the center of the spot exposure in the Slice-n-5 layer, and an overlap area of ​​0-15μm is formed between the spot exposure areas.

[0084] In the Slice-n-3 layer, 40% of the overhang projection area is exposed. The center of the exposure point avoids the already exposed points in the Slice-n-4 layer and forms an overlap area of ​​10-50μm with the exposure area of ​​the Slice-n-4 layer.

[0085] 80% of the overhang projection area is exposed in the Slice-n-2 layer. The center of the exposure coincides with the center of the exposure in the Slice-n-3 layer, and an overlap area of ​​0-15μm is formed between the exposure area and the exposure area in the Slice-n-3 layer.

[0086] Point exposures were performed on 100% of the overhang projection area of ​​the Slice-n-1 layer, with the exposure parameters and energy density being 50-75% of the densification parameters of the Slice-n layer.

[0087] The forming is completed using a fully densified scanning strategy in the Slice-n layer; a suitable overhang structure is required, with an overhang angle of 0°-15°, a projected area Qn ≤ 16 mm², and a projected length Ln ≤ 4 mm.

[0088] Step 2: Turn on the equipment, spread powder layer by layer on the substrate 2 and execute the printing program.

[0089] The equipment used in this embodiment is a Renishaw AM250 laser selective melting system, equipped with a laser system and control software that supports spot exposure. Additive manufacturing is performed using titanium alloy TC4 powder with a particle size range of 15-53 μm. The substrate 2 is preheated to 80°C, and the forming chamber atmosphere is high-purity argon with an oxygen content of less than 0.1%.

[0090] Step 3: Based on the settings in Step 1, print each pre-forming layer in the order of Slice-n-5, Slice-n-4, Slice-n-3, Slice-n-2, and Slice-n-1.

[0091] When printing to the Slice-n-5 layer, the laser system uniformly distributes spot exposures over 40% of the area within the overhanging projection region Qn, generating a dot matrix with a 40% coverage rate and a dot spacing of 450 μm. Each exposed area is independent, non-overlapping, and does not overlap. The laser spot diameter is 80 μm, the power is 120 W, and the exposure time is 50 μs.

[0092] When printing the Slice-n-4 layer, the laser system performs spot exposure on 80% of the area within the same projection region Qn, with a spot spacing of 450 μm. It is ensured that the center position of each spot exposure on the Slice-n-4 layer precisely coincides with the center position of the spot exposure on the Slice-n-5 layer. An overlap of approximately 5 μm is formed between the laser spot and the spot exposure area of ​​the Slice-n-5 layer by adjusting the laser spot overlap. The laser spot diameter is 80 μm, the power is 120 W, and the printing time is 125 μs.

[0093] When printing the Slice-n-3 layer, within the same projection area Qn, the laser system first identifies the coordinates of all exposure points in the Slice-n-4 layer. Then, it performs spot exposure on 40% of the area to generate a new dot matrix with a dot spacing of 450 μm. The center of the spot exposure actively avoids the already exposed points in the Slice-n-4 layer, and the overlap with the spot exposure area of ​​the Slice-n-4 layer is adjusted to form an overlap of approximately 15 μm. The spot diameter is 80 μm, the power is 120 W, and the exposure time is 50 μs.

[0094] When printing the Slice-n-2 layer, 80% of the area within the same projection region Qn is dot-exposed with a dot pitch of 450 μm. The center of the dot exposure coincides with the center of the dot exposure of the Slice-n-3 layer, forming a 5 μm overlap area with the dot exposure area of ​​the Slice-n-3 layer. The spot diameter is 80 μm, the power is 120 W, and the printing time is 125 μs.

[0095] When printing the Slice-n-1 layer, 100% coverage of the projection area is achieved by dot exposure of 100% of the projection area, with a dot pitch of 150 μm, and the energy density is controlled at 65% of the Slice-n layer densification parameters. Specifically, the spot diameter is 80 μm, the power is 150 W, and the exposure time is 75 μs.

[0096] Step 4: Print the final formed layer.

[0097] The Slice-n layer was printed using a standard continuous scanning mode with a scanning plate diameter of 80 μm. Densification parameters optimized for TC4 titanium alloy were employed to completely scan and melt the entire overhanging area. The printing parameters were: power 250 W, scanning speed 1000 mm / s, and scanning spacing 110 μm.

[0098] Step 5: Cut part 1 off the substrate 2 and perform sandblasting and polishing on the overhanging structure surface.

[0099] Step 6: Post-processing and effect verification.

[0100] After printing, the formed impeller part 1 is removed from the substrate 2 by wire cutting. After sandblasting and ultrasonic cleaning, loose powder on the surface is removed. Sandblasting is performed using 120-mesh white corundum at a pressure of 0.4 MPa; ultrasonic cleaning is performed using anhydrous ethanol as the medium for 30 minutes.

[0101] Macroscopic observation: Visual and stereomicroscopic observation showed that the overhanging surface was generally flat and uniformly transitioned, with no visible powder adhesion, melt sagging, or warping deformation.

[0102] Quantitative measurement: The unsupported, overhanging surface was measured using a surface roughness tester. Five measurement points were randomly selected, and the average value was calculated.

[0103] In this embodiment, a MarSurf GD 25 surface roughness tester was used to measure the overhanging surface. The measurement length was 4 mm, and the evaluation length was 3.2 mm. Five measurement points were evenly selected in the overhanging area, and the Ra values ​​were 12.8 μm, 12.1 μm, 13.2 μm, 12.4 μm, and 12.0 μm, with an average value of 12.5 μm. Visual observation revealed slight ripples on the surface, but no powder adhesion.

[0104] Comparative Example 1

[0105] The experimental object in this embodiment is a suspended structure with the same structure as in Embodiment 1. The difference between this embodiment and Embodiment 1 lies in the parameter settings of the pre-forming layer and the process of layer-by-layer powder application and printing based on the parameter settings. The remaining methods, steps, and related parameter settings are the same as in Embodiment 1.

[0106] In this embodiment, the same parameters are used for all layers from Slice-n-5 to Slice-n-1, with a power of 100 W, a time of 80 μs, a dot pitch of 400 μm, and the exposure area ratio of each layer being 50% of the overhanging projection area Qn. The exposure center positions of all points coincide, and there is no active misalignment requirement. The parameters of the Slice-n layer are exactly the same as those in Example 1.

[0107] The results analysis of this embodiment are as follows:

[0108] Slight dust splashing was observed during the molding process.

[0109] After forming, five measurement points were evenly selected in the overhanging area, and the Ra values ​​were 47.8 μm, 48.5 μm, 49.1 μm, 47.5 μm, and 48.1 μm, with an average value of 48.2 μm. Obvious powder adhesion and melt sagging were visually observed.

[0110] Comparative Example 2

[0111] The experimental object in this embodiment is a suspended structure with the same structure as in Embodiment 2. The difference between this embodiment and Embodiment 2 lies in the parameter settings of the pre-forming layer and the process of layer-by-layer powder application and printing based on the parameter settings. The remaining methods, steps, and related parameter settings are the same as in Embodiment 2.

[0112] In this embodiment, a monotonically increasing area ratio strategy is adopted from Slice-n-5 to Slice-n-1. Specifically,

[0113] Slice-n-5 layers: 20% area exposure;

[0114] Slice-n-4 layers: 40% area exposure;

[0115] Slice-n-3 layers: 60% area exposure;

[0116] Slice-n-2 layer: 80% area exposure;

[0117] Slice-n-1 layer: 100% area exposure;

[0118] All exposure centers coincide, with no active offset.

[0119] The remaining parameters for each layer are the same as in Example 2.

[0120] The results analysis of this embodiment are as follows:

[0121] Slight dust splashing was observed during the molding process.

[0122] After forming, five measurement points were evenly selected in the overhanging area, with Ra values ​​of 42.1 μm, 40.8 μm, 42.5 μm, 41.9 μm, and 41.7 μm, respectively, and an average value of 41.8 μm. Local sintering spheroidization and slight warping were observed.

[0123] The comparison and conclusions between the two embodiments and the comparative example are shown in Table 1 below:

[0124] Table 1. Comparison and conclusions between the two embodiments and the comparative example:

[0125] .

[0126] Experimental results show that the specific non-monotonic area sequence of 40%-80%-40%-80%-100% used in this invention, combined with precise point misalignment design, can effectively improve the surface quality of the overhang, with a Ra value significantly lower than that of the comparative examples. Comparative examples 1 and 2, lacking this refined energy control and structural design, exhibit significantly poorer surface quality. This demonstrates the technical advantages of this invention in the unsupported forming of overhang structures for aero-engine impellers.

Claims

1. A method for unsupported laser selective melting forming of a suspended structure, characterized in that, Specifically, the following steps are included: Determine the pre-forming layer: Define the final forming layer where the overhanging structure is located as Slice-n, and define the pre-forming layer downward within the projection area of ​​the final forming layer; The coverage of point exposure of the overhanging projection area from the pre-forming layer immediately adjacent to the final forming layer to the pre-forming layer immediately adjacent to the substrate is a non-monotonic sequence. From the pre-forming layer adjacent to the final forming layer to the pre-forming layer adjacent to the substrate, forming is carried out layer by layer. During the forming process, energy is input in the overlapping area of ​​the projection of the suspended structure using spot exposure. Actively avoid the exposure points of the layer above during the point exposure process of the middle pre-forming layer; The final forming layer employs densification parameters to complete the final forming of the overhanging structure. The number of pre-forming layers is five, which are defined sequentially downwards from the projection area of ​​the final forming layer as: Slice-n-1, Slice-n-2, Slice-n-3, Slice-n-4, and Slice-n-5; During the sequential layer-by-layer forming process from Slice-n-5, Slice-n-4, Slice-n-3, Slice-n-2 to Slice-n-1, the coverage areas of the overlapping regions of the projected overhanging structure are 40%, 80%, 40%, 80%, and 100%, respectively. In the sequential layer-by-layer forming process from Slice-n-5, Slice-n-4, Slice-n-3, Slice-n-2 to Slice-n-1, the laser diameter is 70-110μm, the laser power is 120-180W, and the spot exposure spacing is 350-450μm. During the sequential layer-by-layer forming process from Slice-n-5, Slice-n-4, Slice-n-3, Slice-n-2 to Slice-n-1, the exposure times are 50-100μs, 50-100μs, 50-100μs, 125-175μs, and 50-100μs, respectively. During the Slice-n-5 layer forming process, there are no overlapping areas between the exposure areas at each point; The center of the point exposure of the Slice-n-4 layer coincides with that of the Slice-n-5 layer, and a 0-15μm overlap area is formed between the point exposure areas; The center of the Slice-n-3 layer exposure should avoid the already exposed points of the Slice-n-4 layer and form an overlap area of ​​10-50μm with the Slice-n-4 layer exposure area. The center of the spot exposure in Slice-n-2 layer coincides with that in Slice-n-3 layer, and an overlap area of ​​0-15μm is formed between the spot exposure areas.

2. The method for unsupported laser selective melting forming of a suspended structure according to claim 1, characterized in that: The number of pre-forming layers is at least five adjacent forming layers.

3. The method for unsupported laser selective melting forming of a suspended structure according to claim 1, characterized in that: When determining the pre-forming layer, the overhanging structure in the 3D model of the part to be processed is identified, the layers are automatically sliced ​​and the projection contour of each layer is calculated, and the projection length L and projection area Q are calculated layer by layer.

4. The unsupported laser selective melting forming method for a suspended structure according to claim 1, characterized in that: The final forming of the final forming layer is accomplished using a fully densified scanning strategy.

5. The method for unsupported laser selective melting forming of a suspended structure according to claim 1, characterized in that: The final pre-forming layer performs point exposure on 100% of the overhang projection area; the exposure parameters used have an energy density of 50% to 75% of the Slice-n layer densification parameters.

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