A scanning method for mitigating edge effects in laser melting deposition layers

Abstract

The present application belongs to the field of additive manufacturing, and relates to a scanning method for slowing down the edge effect of laser melting and depositing layers. The interlayer rotation scanning method has higher complexity in three-dimensional space, can effectively prevent the thermal stress from being excessively large in the same direction, and reduce the deformation amount of the substrate and the part. Due to the continuous change of the scanning track direction in the interlayer rotation scanning mode, the heat flow direction also changes, so that the heat transfer is no longer in the unidirectional vertical direction, and the heat accumulation condition is improved. The problems such as powder ball splashing, pore increase and the like caused by the boiling of the molten pool due to the excessively high heat input are avoided, and the grain growth time is also avoided from being excessively long to weaken the fine-grain strengthening effect. Due to the process characteristics of high energy density of the technology itself, the metal powder is easy to be excessively melted when the laser scans the edge of the workpiece, leading to collapse. The optimization of the scanning method reduces the time length of the laser staying at the edge, which can improve the heat accumulation condition of the region and slow down the edge effect of the printed workpiece.

Description

Technical Field

[0001] This invention belongs to the field of additive manufacturing and relates to a scanning method for mitigating the edge effect of laser melting deposition layers. Background Technology

[0002] In today's technologically advanced era, the standards for product parts in the aerospace manufacturing industry are becoming increasingly stringent. Among the many branches of additive manufacturing technology, laser metal deposition (LMD) is characterized by its near-net-shape forming of large and complex components. It utilizes high-energy laser beams to achieve high-efficiency deposition, significantly increasing forming speed and saving time and costs. However, during the manufacturing process, when the laser scanning trajectory reaches the edge of the workpiece, the edge line coincides with the beam center and is in the energy peak region, prolonging the time the metal powder remains in a molten state. Furthermore, because there is no solid deposition layer to restrain the fluid movement on one side of the edge, an arc-shaped collapse occurs, reducing the surface flatness of the workpiece. This edge collapse effect is prevalent in the practical applications of laser melting deposition technology both domestically and internationally. To improve this phenomenon, the scanning method can be optimized to avoid the laser remaining at the edge of the part for extended periods, preventing excessive heat accumulation in that area. By adding multiple inflection points to the trajectory at the edge line, the deposition layer is formed in batches, and the preferentially cooled solid material provides directional constraint on the fluid, thereby suppressing the degree of collapse. It should be noted that a thermal shock effect occurs at the scanning starting point, resulting in a significant temperature gradient at the moment of light emission, with heat rapidly spreading to the surrounding area. This leads to a deposition layer height at the starting point being lower than the theoretical layer thickness. When planning the scanning path, the starting point should be in a layer-by-layer cyclic state to maintain a regular pattern of collapse locations, facilitating subsequent repairs. During processing, the workpiece is repeatedly subjected to thermal cycling, resulting in variations in the formed microstructure. Specifically, the heat input from the laser energy during the top layer printing gradually diminishes on the bottom, the secondary heating temperature decreases layer by layer, and differences in cooling rates at different locations cause varying degrees of microstructure transformation. To ensure uniform microstructure for performance control and reduce the impact of residual stress, the quality of the deposition layer should be maintained during printing to minimize repairs and avoid unnecessary heat input. This also illustrates a chain relationship between workpiece performance and the surface smoothness of the deposition layer in laser melting deposition technology applications; "shape control" aims to provide a fundamental guarantee for "performance control." Based on this viewpoint, optimizing the scanning method to print a "smooth and flat" deposition layer while meeting performance requirements becomes the main goal of process optimization. Summary of the Invention

[0003] To address the edge effects and laser point collapse issues arising during the scanning process mentioned above, this invention proposes an optimized scanning method and verification techniques. By cyclically increasing the scanning angle of each layer, the duration of the laser's contact with the workpiece edge is reduced, ensuring uniform material deposition. The heat transfer direction changes with the scanning direction, shifting from vertical unidirectional transfer to uniform transfer, effectively preventing excessive heat accumulation. The continuously rotating scanning direction increases complexity in three-dimensional space, preventing excessive stress superposition in the same direction, thereby suppressing the risk of deformation and cracking of the parts and substrate. During verification, simulation technology is used to predict changes in deformation, temperature distribution, and other results. Due to variations in temperature gradient and cooling rate, the formed microstructure will differ, necessitating physicochemical testing to ensure the printed parts meet performance requirements.

[0004] The technical means employed in this invention are as follows:

[0005] A scanning method for mitigating edge effects of laser-melted deposited layers comprises the following steps:

[0006] 1. Interlayer Rotation Scanning Process

[0007] The initial layer scanning trajectory is set to scan along the short side to avoid excessive heat dissipation and large temperature gradients that could lead to deformation when scanning along the long side. The angle of the first layer is recorded as 0°, and each subsequent layer increments by X°. To facilitate control over the regularity of the scanning trajectory and the impact of changes in heat flow direction on the tissue growth direction, it is recommended that 360 be an integer multiple of X. Therefore, the angles of subsequent layers are X°, 2X°, 3X°...(360-X)°, 360° (i.e., 0°), following this pattern to plan the printing trajectory for all subsequent layers. This method ensures that the laser beam does not linger at the workpiece edge for an extended period and also provides better heat dissipation for the workpiece. The increased complexity of the trajectory in three-dimensional space effectively prevents thermal stress from continuously accumulating in the same direction, reducing substrate deformation.

[0008] 2. Light source constraint

[0009] Due to thermal shock, laser initiation points often collapse. If their positions are not systematically constrained, the collapses can become chaotic during printing, hindering accurate repairs by the operator. In laser metal deposition technology, when planning the partitioning of large workpieces, the printing area is often divided into multiple quadrilaterals. The first initiation point is placed at one corner of a quadrilateral and designated A1. The initiation points for the subsequent three layers are A2, A3, and A4, respectively, before returning to A1 to begin the layer-by-layer cycle.

[0010] 3. Light collection point constraint

[0011] Due to the instantaneous nature of the laser's cessation, the delivery of metal powder cannot be synchronized. This synchronicity means that powder continuously delivered after the laser stops at the end of the scanning trajectory will not melt due to insufficient energy, and some metal powder spheres will adhere to the melt that has not yet fully cooled at high temperatures. When printing the next layer, if the process parameters used are insufficient to completely melt the powder due to problems such as excessive powder feed rate or insufficient laser power, the risk of incomplete fusion in the part's microstructure increases. Therefore, in path planning, a scanning method with twice the trajectory spacing and reverse reversal is adopted. That is, the light is not stopped at the end of the scanning trajectory, and it is reversed back to the starting point. This strategy minimizes the impact of the start and stop points on the printed parts.

[0012] 4. Simulation verification technology

[0013] Simulation technology was used to predict the model distribution using two process strategies. The differences in deformation, stress distribution, temperature distribution, peak temperature and other results were compared and analyzed. The results showed that the optimized scanning method could reduce the generation of residual stress and its impact on heat dissipation conditions.

[0014] 5. Process Steps

[0015] The optimization and verification sequence of the aforementioned process scheme is as follows:

[0016] Step 1: Select a suitable part model for layer slicing. The theoretical layer thickness is x. Plan the same scanning path along the short side for all sliced ​​layers. Name this scheme PLAN-A.

[0017] Step 2: Slice the same model into layers with the theoretical layer thickness x. Set the rotation angle, interlayer increment, cyclical start point, and reverse folding of the receiver point for all slices to create an optimized new scanning method. Name this scheme PLAN-B.

[0018] Step 3: Ensure that PLAN-A and PLAN-B schemes, except for the scanning method, use the same process parameters and equipment parameters to output the printing program;

[0019] Step 4: After clamping and positioning, the processing chamber is purged with inert gas until the water and oxygen content in the environment drops below the standard requirements. Then, the output program is executed to start printing.

[0020] Step 5: Analyze the stress distribution, temperature distribution, and deformation changes in the printed parts output results under the scanning trajectories of PLAN-A and PLAN-B schemes using simulation technology, and summarize their influencing mechanisms;

[0021] Step Six: Compare the printed parts and observe the improvement in surface smoothness and edge effect;

[0022] Step 7: After cutting the printed part from the substrate, perform metallographic analysis and physicochemical property testing to ensure that the performance of the workpiece printed by the optimized scanning method still meets the standard requirements.

[0023] The beneficial effects of this invention are:

[0024] (1) The scanning method of interlayer rotation has higher complexity in three-dimensional space, which can effectively prevent thermal stress from being superimposed too much in the same direction and reduce the deformation of the substrate and parts.

[0025] (2) In the interlayer rotation scanning method, the direction of heat flow changes as the scanning trajectory changes, so the heat transfer is no longer in a unidirectional vertical direction, which improves the heat accumulation. This avoids problems such as powder ball splashing and increased porosity caused by excessive heat input leading to molten pool boiling, and also avoids the weakening effect of fine grain strengthening due to excessive grain growth time.

[0026] (3) Due to the high energy density of the process, the metal powder is prone to over-melting and collapse when the laser scans the edge of the workpiece. Optimizing the scanning method to reduce the time the laser stays at the edge can improve the heat accumulation in this area and reduce the edge effect of the printed workpiece.

[0027] (4) Rotating at a 45° angle helps to control the regularity of the light-emitting starting point position and avoid excessive collapse, reducing the number of repairs required by on-site workers and saving time and costs.

[0028] (5) Using simulation technology to predict stress distribution, temperature distribution, deformation, etc., and only analyzing trends, requires setting a large number of parameters, such as material property parameters, heat transfer parameters, boundary conditions, etc., when applying simulation technology based on the thermo-elastic-plastic method. The results will be affected by the error of parameter measurement and calculation, making it difficult to fit with reality, but it is more suitable for trend change analysis. Attached Figure Description

[0029] Figure 1 Standard serpentine scanning method;

[0030] Figure 2 45° rotation scanning method;

[0031] Figure 3 Simulation results of deformation in ordinary scanning mode;

[0032] Figure 5 Simulation results of deformation in 45° scanning mode;

[0033] Figure 6 Simulation results of temperature distribution using a 45° scanning method;

[0034] Figure 7 Printed physical sample before optimization;

[0035] Figure 8 Optimized printout;

[0036] Figure 9 Reverse turnaround diagram;

[0037] Figure 10 Macro-organizational comparison chart;

[0038] Figure 11 Comparison of microstructures. Detailed Implementation

[0039] The present invention will now be described in further detail with reference to the accompanying drawings, so that those skilled in the art can implement it based on the description. In this embodiment, the two different scanning methods and their printed parts are designated as PLAN-A and PLAN-B, respectively.

[0040] Step 1: Select a substrate size of 200mm×100mm×6mm and a model size of 50mm×50mm×75mm. The material is TA15 titanium alloy. Before printing, put the metal powder into a drying oven, heat it to 120℃ and keep it warm for 2 hours.

[0041] Step 2: Perform scanning path planning on the PLAN-A model, uniformly setting the starting point to the lower left corner, and planning the scanning trajectory of all layers as parallel short-side scanning.

[0042] Step 3: Plan the scanning path for the PLAN-B model. Set the starting point to a layer-by-layer loop, and use a double-interval reverse trajectory to ensure the start and end points of the laser are at the same position. Set the inter-layer rotation increment to 45°, meaning the angles starting from the first layer are 0°, 45°, 90°...360° (0°)...

[0043] Step four: Clamp the substrate and locate the zero point of the machining coordinate system. After preparation, purge the machining chamber with high-purity argon as the inert gas.

[0044] Step 5: Ensure an inert gas atmosphere inside the chamber during printing. Start printing when the oxygen content is below 50 ppm.

[0045] Step six involves outputting all paths planned in the PLAN-A model and then executing the program to print the PLAN-A model.

[0046] Step seven, the same as steps four through six, is to print the PLAN-B model. It is important to note that the process, equipment, and other parameters in the executed program must be consistent with those used when printing the PLAN A model.

[0047] Step 8: Compare the surface smoothness of PLAN-A and PLAN-B models, as well as the number of repairs during printing, to determine whether the incremental angle improves the edge effect. For example... Figure 7 , 8 As shown, the part printed using the scanning method in step two exhibits significant collapse, failing to guarantee printing stability. In contrast, the part printed using the scanning method in step three shows a substantial improvement in surface flatness and reduces edge effects.

[0048] Step 9: Observe the adhesion of the powder at the light-receiving point.

[0049] Step 10: Perform simulation analysis on the PLAN-A and PLAN-B scanning schemes. Use simufact.welding software to build the model, define the heat source, and plan the trajectory. After calculation and solution, view the simulation results; you can see... Figures 3-6 In the simulation, the maximum substrate deformation was 0.91 mm under the ordinary scanning method. After strategy optimization, the maximum deformation was reduced to 0.69 mm, a 24.2% reduction, and the thermal stress concentration phenomenon was improved. The smaller heat influence range in the temperature results indicates better heat dissipation conditions, with more heat being transferred to the gas environment. The simulation results show that the 45° angle rotation scanning method has a better response performance than the ordinary scanning method.

[0050] Step 11: Metallographic testing was conducted to analyze the differences in the microstructure formed under PLAN-A and PLAN-B schemes. As can be seen... Figure 10 As shown, the β-columnar crystal sizes of the printed models obtained by the two scanning methods are roughly the same. Since the columnar crystals grow in the opposite direction to the heat flow, the orientation of some grains may change with the heat flow. However, in the full field of view of the growth direction of both sets of parts, the trend of columnar crystal growth deviating from the Z-direction is not obvious. This indicates that for the overall part, the change in the heat flow direction has little impact on the growth direction of the columnar crystals. Further analysis, such as... Figure 11 As shown, both are basketweave structures. Measurements revealed that in PLAN-A, the equivalent diameter of the lamellar α phase was 1.93 μm, and the β phase content was 27.84%; in PLAN-B, the equivalent diameter of the α phase was 1.68 μm, and the β phase content was 28.12%. In conclusion, the change in scanning method has little effect on the formed structure.

[0051] Step 12: Conduct physicochemical performance tests and compare the performance differences under PLAN-A and PLAN-B schemes to ensure that they still meet the standard requirements.

Claims

1. A scanning method for mitigating edge effects of laser-melted deposited layers, characterized in that, The steps are as follows: Step 1: Select the part model and slice it into layers. The theoretical layer thickness is x. Plan the same scanning path along the short side for all sliced ​​layers. Name this scheme PLAN-A. Step 2: Slice the same model into layers with the theoretical layer thickness x. Set the rotation angle, interlayer increment, cyclical start point, and reverse folding of the receiver point for all slices to create an optimized new scanning method. Name this scheme PLAN-B. Step 3: Ensure that PLAN-A and PLAN-B schemes, except for the scanning method, use the same process parameters and equipment parameters to output the printing program; Step 4: After clamping and positioning, the processing chamber is purged with inert gas until the water and oxygen content in the environment drops below the standard requirements. Then, the output program is executed to start printing. Step 5: Analyze the stress distribution, temperature distribution, and deformation changes in the printed parts output results under the scanning trajectories of PLAN-A and PLAN-B schemes using simulation technology, and summarize their influencing mechanisms; Step Six: Compare the printed parts and observe the improvement in surface smoothness and edge effect; Step 7: After cutting the printed part from the substrate, perform metallographic analysis and physicochemical property testing to ensure that the performance of the workpiece printed by the optimized scanning method still meets the standard requirements; Interlayer rotational scanning technology is employed, as detailed below: Set the initial layer scanning trajectory to scan along the short side, record the first layer angle as 0°, and then each subsequent layer increment as X°; then the subsequent layer angles are X°, 2X°, 3X°... (360-X)°, 360°. Use this pattern to plan the printing trajectory of all subsequent layers. The starting point constraint is adopted, as follows: Place the first light source at one corner of the quadrilateral and designate it as A1. The starting points for the subsequent three layers are A2, A3, and A4, respectively, and then return to A1 to start the cycle layer by layer. The light-receiving point constraint is adopted, as follows: A scanning method with twice the track spacing and reverse reversal is adopted, that is, the light is not collected when the scanning track reaches the end, and it is reversed back to the starting point.

2. The scanning method for mitigating edge effects of laser-melted deposited layers as described in claim 1, characterized in that, Simulation technology was used to predict the model distribution using two strategies. The differences in deformation, stress distribution, temperature distribution, and peak temperature results were compared and analyzed. The results showed that the optimized scanning method could reduce the generation of residual stress and its impact on heat dissipation conditions.

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