Scanning method for relieving edge effect of laser melting deposition layer

Abstract

The invention belongs to the field of additive manufacturing, and relates to a scanning method for relieving the edge effect of a laser melting deposition layer. The interlayer rotation scanning method provided by the invention has higher complexity in a three-dimensional space, can effectively prevent overlarge superposition of thermal stress in the same direction, and reduces the deformation quantity of substrates and parts. In the interlayer rotation scanning mode, the direction of a heat flow changes along with continuous change of the direction of a scanning track, so that heat transfer is no longer in the one-way vertical direction, and the heat accumulation condition is improved. The problems of powder ball splashing, air hole increasing and the like caused by molten pool boiling due to too high heat input are avoided, and meanwhile the situation that the fine grain strengthening effect is weakened due to too long grain growth time is avoided. Due to the technical characteristic of high energy density of the technology, metal powder is prone to being excessively melted to cause collapse when laser scans the edge of a workpiece. The scanning method is optimized, the staying time of the laser on the edge is shortened, the heat accumulation condition of the area can be improved, and the edge effect of the printed workpiece is relieved.

Description

Technical Field

[0001] The invention belongs to the field of additive manufacturing and relates to a scanning method for alleviating the edge effect of a laser-melted deposition layer. Background Art

[0002] In today's era of science and technology advancement, the requirements for product parts in the aviation manufacturing industry are becoming increasingly stringent. Among the many technical branches in the field of additive manufacturing technology, for laser metal deposition technology (LMD), its forming characteristics are "near net forming" of large and complex components, using high-energy laser beams to achieve high-efficiency deposition, greatly improving the forming speed and saving time and cost. However, when the laser scanning trajectory moves to the edge of the workpiece during the manufacturing process, the edge coincides with the center of the beam and is in the energy peak area, and the metal powder maintains the molten state for a longer time. In addition, due to the lack of a solid deposition layer on one side of the edge position, it cannot constrain the fluid movement, resulting in an arc collapse and reducing the surface flatness of the workpiece. In the current practical application of laser melting deposition technology at home and abroad, this edge collapse effect is common. To improve this phenomenon, the scanning method can be optimized to avoid the laser staying at the edge of the part for a long time and avoid excessive heat accumulation in this area. Because multiple inflection points are added to the trajectory of the edge position, the deposition layer there is formed in batches, and the solid material that is cooled first adds a directional constraint to the fluid, thereby suppressing the degree of collapse. At the same time, it should be noted that due to the thermal shock effect at the starting point of the scan, there is a large temperature gradient at the moment of light emission, and the heat is quickly transferred to the surroundings in a very short time, which causes the height of the deposited layer at the starting point to be lower than the theoretical layer thickness. When planning the scanning path, the starting point of the light emission should be in a layer-by-layer cycle state to keep the collapse position regular, which is conducive to subsequent repair. During the processing, the workpiece is repeatedly subjected to the action of the heat cycle, and the formed structure will have certain differences. Specifically, when printing the top layer, the heat input brought by the laser energy gradually decays the effect on the bottom, the secondary heating temperature decreases layer by layer, and the difference in cooling rate at different positions causes different degrees of organizational transformation. In order to ensure uniform organization to control performance and reduce the influence of residual stress, the quality of the deposited layer should be guaranteed during the printing process to reduce repair and avoid unnecessary heat input. This also shows that in the application of laser melting deposition technology, there is a chain relationship between the performance of the workpiece and the surface flatness of the deposited layer. The purpose of "shape control" is to provide the basic guarantee of "controllability". Based on this point of view, by optimizing the scanning method, printing a "smooth and flat" deposited layer on the basis of meeting the performance requirements has become the main goal of process optimization. Summary of the invention

[0003] In response to the edge effect and light point collapse problems generated during the scanning process mentioned above, the present invention proposes a process method and verification means for optimizing the scanning method. By cyclically increasing the scanning angle of each layer, the time the laser is at the edge of the workpiece is reduced to ensure the uniformity of material deposition. The direction of heat transfer changes with the scanning direction, from vertical unidirectional transfer to uniform transfer, effectively preventing excessive heat accumulation. The continuously rotating scanning direction is more complex in three-dimensional space, preventing excessive superposition of stress in the same direction, thereby suppressing the risk of deformation and cracking of parts and substrates. During verification, simulation technology is used to predict changes in deformation variables, temperature distribution and other results. Due to changes in temperature gradients and cooling rates, there will be certain differences in the formed structure, and physical and chemical tests are required to ensure that the printed parts meet performance requirements.

[0004] The technical means adopted by the present invention are as follows:

[0005] A scanning method for mitigating edge effects of a laser melt deposited layer, comprising 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 deformation caused by excessive heat dissipation and excessive temperature gradient when scanning along the long side. The angle of the first layer is recorded as 0°, and the increment of each layer thereafter is X°. In order to facilitate the control of the regularity of the scanning trajectory and the influence of the change in the direction of heat flow on the direction of tissue growth, it is recommended that 360 is an integer multiple of the value of X. Then the angle of each subsequent layer is X°, 2X°, 3X°...(360-X)°, 360° (ie 0°). This rule is used to plan the printing trajectory of all subsequent layers. This method ensures that the laser beam does not stay at the edge of the workpiece for a long time, and also enables the workpiece to obtain better heat dissipation conditions. The higher complexity of the trajectory in three-dimensional space effectively prevents the continuous superposition of thermal stress in the same direction and reduces the deformation of the substrate.

[0008] 2. Light starting point constraints

[0009] The laser starting point often collapses due to the thermal shock effect. If its position is not constrained regularly, the collapsed position will be messy during the printing process, which is not conducive to the operator's accurate repair. In the application of laser metal deposition technology, when planning the partition of large workpieces, the printing area is often divided into multiple quadrilaterals. The first starting point is placed at a corner of the quadrilateral and is designated as A1. The starting points of the subsequent three layers are A2, A3, and A4 respectively, and then return to A1 to start the layer-by-layer cycle.

[0010] 3. Light collection point constraints

[0011] Due to the instantaneous nature of laser stopping, the delivery of metal powder cannot keep pace with it. This asynchrony will cause the powder that is continuously delivered after the laser stops at the end of the scanning track to be unable to melt due to insufficient energy, and some metal powder balls will adhere to the melt that has not completely cooled down at high temperature. When printing the next layer, if the process parameters used cannot completely melt the powder due to problems such as excessive powder feeding rate or insufficient laser power, the risk of unfused parts in the microstructure will increase. Therefore, when planning the path, a scanning method with 2 times the track spacing and reverse folding is adopted, that is, the scanning track does not collect light when it reaches the end, and reverses back to the starting point. This strategy minimizes the impact of the starting and collecting light points on the printed parts.

[0012] 4. Simulation verification technology

[0013] The simulation technology is used to predict the model distribution using the two process strategies, and the differences in deformation, stress distribution, temperature distribution, peak temperature and other results are compared and analyzed. It is summarized whether the optimized scanning method can reduce the generation of residual stress and its impact on heat dissipation conditions.

[0014] 5. Process steps

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

[0016] Step 1: Select a suitable workpiece model for layered slicing. The theoretical layer thickness is x. All slice layers are planned to have the same scanning path along the short edge. This plan is named PLAN-A.

[0017] Step 2: Slice the same model with the theoretical layer thickness x, set the rotation angle increment between layers, the light-starting point cycle layer by layer, and the light-receiving point reverse return for all slice layers, and create a new optimized scanning method, which is named PLAN-B.

[0018] Step 3: Ensure that PLAN-A and PLAN-B are output as printing programs with the same process parameters and equipment parameters except for the scanning method;

[0019] Step 4: After clamping and positioning, use inert gas to purge the processing room. After the water and oxygen content in the environment drops below the standard requirement, execute the output program to start printing;

[0020] Step 5: Use simulation technology to analyze the stress distribution, temperature distribution, and deformation variation in the output results of printed parts under the scanning trajectory of PLAN-A and PLAN-B, and summarize their influencing mechanisms;

[0021] Step 6: Compare the printed parts and observe the improvement of surface flatness and edge effect;

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

[0023] Beneficial effects of the present invention:

[0024] (1) The interlayer rotation scanning method has higher complexity in three-dimensional space, which can effectively prevent excessive superposition of thermal stress in the same direction and reduce the deformation of substrates and parts.

[0025] (2) In the interlayer rotating scanning mode, the direction of the heat flow changes due to the continuous change of the scanning track direction, resulting in the heat transfer no longer being in a unidirectional vertical direction, improving the heat accumulation. It avoids the problems of powder ball splashing and increased pores caused by excessive heat input, and also avoids the grain growth time being too long to weaken the fine grain strengthening effect.

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

[0027] (4) The 45° rotation is helpful to control the regularity of the light starting point position and avoid excessive collapse, thus reducing the number of repairs by on-site workers and saving time costs.

[0028] (5) Use simulation technology to predict stress distribution, temperature distribution, deformation and other results and only analyze trends. When applying simulation technology, a large number of parameters need to be set based on the thermoelastic-plastic method, such as material property parameters, heat transfer parameters, boundary conditions, etc. The result value will be affected by the parameter measurement and calculation errors, and it is difficult to match the actual benchmark, but trend change analysis is more applicable. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Ordinary snake scanning method;

[0030] Figure 2 45° rotation scanning method;

[0031] Figure 3 Simulation results of deformation variables using ordinary scanning method;

[0032] Figure 5 45° scanning deformation simulation results;

[0033] Figure 6 45° scanning temperature distribution simulation results;

[0034] Figure 7 Print the physical object before optimization;

[0035] Figure 8 Print the physical object after optimization;

[0036] Fig. 9 Schematic diagram of reverse reentry;

[0037] Fig.10 Macro-organization comparison chart;

[0038] Fig.11 Microstructure comparison chart. DETAILED DESCRIPTION

[0039] The present invention is further described in detail below in conjunction with the accompanying drawings so that those skilled in the art can implement the invention with reference to the description. In this embodiment, two different scanning modes and their printed products are named PLAN-A and PLAN-B respectively.

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

[0041] Step 2: Plan the scanning path for the PLAN-A model. The starting point is uniformly set to the lower left corner, and the scanning trajectories of all layers are planned as parallel short-side scanning.

[0042] Step 3: Plan the scanning path for the PLAN-B model. Set the starting point to cycle layer by layer, and use a 2-fold spacing to reverse the trajectory to ensure that the starting and ending points of the laser are at the same position. The inter-layer rotation increment is set to 45°, that is, the angles from the first layer are 0°, 45°, 90°......360°(0°) in sequence........

[0043] Step 4: Clamp the substrate and locate the zero point of the processing coordinate system. After the preparation is completed, purge the processing room, and high-purity argon is selected as the inert gas.

[0044] Step 5: Ensure an inert gas atmosphere in the box during printing, and start printing when the oxygen content is lower than 50ppm.

[0045] Step 6: Output all paths planned by the PLAN-A model through a program, and execute the program to print the PLAN-A model.

[0046] Step 7: Print the PLAN-B model as in steps 4 to 6. In this process, it should be noted that the process, equipment and other parameters in the executed program must be consistent with those when printing the PLANA model.

[0047] Step 8: Compare the surface flatness of the PLAN-A and PLAN-B models, as well as the number of repairs during the printing process, to determine whether the incremental angle improves the edge effect. Figure 7 , 8 As shown in the figure, the product printed by the scanning method in step 2 collapses significantly and the printing stability cannot be guaranteed. However, the surface flatness of the product printed by the scanning method in step 3 is greatly improved and the edge effect is improved.

[0048] Step nine, observe the stickiness of the powder at the light-collecting point.

[0049] Step 10: Simulate and analyze the scanning schemes of PLAN-A and PLAN-B, and use simufact.welding software to build the model, define the heat source, and plan the trajectory. After calculating and solving, check the simulation results and you can see Figures 3 to 6 In the experiment, the maximum deformation of the substrate was 0.91mm under the normal scanning mode, and the maximum deformation after strategy optimization was reduced to 0.69mm, the deformation was reduced by 24.2%, and the thermal stress concentration phenomenon was improved. The smaller heat influence range in the temperature results indicates that the heat dissipation conditions are better, and more heat is transferred to the gas environment. The simulation results show that the response effect of the scanning method with a 45° rotation angle is better than that of the normal scanning method.

[0050] Step 11: Through metallographic testing, analyze the differences in the formed structures under PLAN-A and PLAN-B. Fig.10 As shown in the figure, the size of β columnar crystals in the models printed by the two scanning methods is similar. Since columnar crystals grow in the opposite direction to the heat flow, the orientation of some grains may change with the heat flow, but in the full field of view of the growth direction of the two groups of parts, it is found that the trend of columnar crystal growth deviating from the Z direction is not obvious. This shows that for the overall part, the change in heat flow direction has little effect on the growth direction of columnar crystals. Further analysis, such as Fig.11 As shown in the figure, both are basket weft structures. The measured data show that the equivalent diameter of the α-shaped sheet in PLAN-A is 1.93 μm, and the β-phase content is 27.84%; the equivalent diameter of the α-shaped sheet in PLAN-B is 1.68 μm, and the β-phase content is 28.12%. In summary, the change of scanning method has little effect on the formed structure.

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

Claims

1. A scanning method for mitigating edge effects of a laser melt deposited layer, characterized in that: Here are the steps: Step 1: Select the workpiece model for layered slicing, with a theoretical layer thickness of x. Plan all slice layers to have the same scanning path along the short edge, and name this plan PLAN-A. Step 2: Slice the same model with the theoretical layer thickness x, set the rotation angle increment between layers, the light-starting point cycle layer by layer, and the light-receiving point reverse return for all slice layers, and create a new optimized scanning method, which is named PLAN-B. Step 3: Ensure that PLAN-A and PLAN-B are output as printing programs with the same process parameters and equipment parameters except for the scanning method; Step 4: After clamping and positioning, use inert gas to purge the processing room. After the water and oxygen content in the environment drops below the standard requirement, execute the output program to start printing; Step 5: Use simulation technology to analyze the stress distribution, temperature distribution, and deformation variation in the output results of printed parts under the scanning trajectory of PLAN-A and PLAN-B, and summarize their influencing mechanisms; Step 6: Compare the printed parts and observe the improvement of surface flatness and edge effect; Step 7: After cutting the printed part from the substrate, perform metallographic structure analysis and physical and chemical properties testing to ensure that the performance of the workpiece printed by the optimized scanning method still meets the standard requirements.

2. A scanning method for mitigating edge effects of a laser melt deposited layer as claimed in claim 1, characterized in that: The inter-layer rotation scanning process is adopted, as follows: Set the initial layer scanning trajectory to scan along the short edge, record the first layer angle as 0°, and then each layer increment as X°; then the subsequent angles of each layer are X°, 2X°, 3X°...(360-X)°, 360°, and follow this rule to plan the printing trajectory of all subsequent layers.

3. A scanning method for mitigating edge effects of a laser melt deposited layer as claimed in claim 1 or 2, characterized in that: The starting point constraint is adopted, as follows: Place the first light point at a corner of the quadrilateral and designate it as A1. The starting points of the subsequent three layers are A2, A3, and A4, and then return to A1 to start the cycle layer by layer.

4. A scanning method for mitigating edge effects of a laser melt deposited layer as claimed in claim 1 or 2, characterized in that: The light collection point constraint is adopted, as follows: The scanning method of 2 times the track spacing and reverse folding is adopted, that is, when the scanning track reaches the end, the light is not collected and it folds back to the starting point in the reverse direction.

5. The scanning method for mitigating edge effect of laser melting deposition layer according to claim 3, characterized in that: The light collection point constraint is adopted, as follows: The scanning method of 2 times the track spacing and reverse folding is adopted, that is, when the scanning track reaches the end, the light is not collected and it folds back to the starting point in the reverse direction.

6. A scanning method for mitigating edge effects of a laser melt deposited layer as claimed in claim 1, 2 or 5, characterized in that: Simulation technology is used to predict the model distribution using the two strategies, and the differences in deformation, stress distribution, temperature distribution, and peak temperature results are compared and analyzed. It is concluded whether the optimized scanning method can reduce the generation of residual stress and its impact on heat dissipation conditions.

7. A scanning method for mitigating edge effects of a laser melt deposited layer as claimed in claim 3, characterized in that: Simulation technology is used to predict the model distribution using the two strategies, and the differences in deformation, stress distribution, temperature distribution, and peak temperature results are compared and analyzed. It is concluded whether the optimized scanning method can reduce the generation of residual stress and its impact on heat dissipation conditions.

8. The scanning method for mitigating edge effect of laser melting deposition layer according to claim 4, characterized in that: Simulation technology is used to predict the model distribution using the two strategies, and the differences in deformation, stress distribution, temperature distribution, and peak temperature results are compared and analyzed. It is concluded whether the optimized scanning method can reduce the generation of residual stress and its impact on heat dissipation conditions.

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