A high-speed secondary melting method for improving the performance of laser additive manufacturing aluminum alloy

Abstract

The application discloses a high-speed secondary melting method for improving the performance of laser additive manufacturing aluminum alloy, first, the optimal process parameters are used for preliminary melting of the powder bed to obtain a cladding layer with good density and apparent quality; second, the laser power and scanning rate are improved to perform secondary melting on the solidified surface to improve the microstructure of the solidified surface; then, powder is re-paved and the above process is repeated to obtain a final formed test piece. The method considers the heat accumulation of the primary laser melting in the heat transfer numerical simulation process, so that the solid-liquid interface solidification parameter prediction of the high-speed secondary melting process is closer to the actual situation. The method uses the high-power and high-scanning-rate laser to perform layer-by-layer secondary melting on the solidified surface for the first time to in-situ regulate the microstructure of the formed component, so that the comprehensive mechanical properties of the formed component are improved.

Description

Technical Field

[0001] This invention belongs to the field of additive manufacturing technology, specifically relating to a high-speed secondary melting method for controlling microstructure and improving the performance of laser additive manufacturing aluminum alloys. Background Technology

[0002] Laser powder bed fusion (LPBF) additive manufacturing technology, as a novel processing technology, has received increasing research and application. Its basic principle is the interaction between a laser and metal powder. Under the action of a rapidly moving laser, the powder transiently melts and rapidly solidifies in a molten pool, completing the forming process layer by layer along a pre-programmed path. Because the local solidification conditions within the molten pool deviate from the equilibrium state under traditional processes (such as casting), the microstructure of laser additive manufacturing exhibits distinct characteristics, and its morphology, size, and distribution are closely related to the process parameters.

[0003] Al-Si alloys have long been the most widely used and researched cast aluminum alloys due to their excellent formability and corrosion resistance. This characteristic has been extended to laser additive manufacturing technology, where the combination of these two technologies can produce engineering components with certain mechanical strength and complex shapes. However, because the reinforcing phase composition in its microstructure is relatively simple, the tensile strength has remained at a moderate level (below 400 MPa), and the elongation just meets the plasticity requirements of engineering components (around 6%–8%). Therefore, how to further improve its mechanical properties, especially by simultaneously increasing tensile strength and elongation at break, has naturally become a focus of attention and research. First, in existing laser additive manufacturing processes, subsequent heat treatment has become the most common control method, such as the widely used T6 (high-temperature solution treatment + low-temperature artificial aging) heat treatment regime. However, because the reinforcing phase Si in the microstructure is extremely sensitive to temperature, the reinforcing phase Si coarsens and grows after heat treatment. Although this change increases the elongation of the sample (to 10%–18%), the tensile strength drops sharply (to around 300 MPa). Secondly, another common method is to introduce additives / reinforcing agents, such as nanoscale TiB2, CNTs, LaB6, TiC, and graphene particles. However, this method places high demands on the initial powder preparation process. Both the input of additives / reinforcing agents and the preparation of composite powders increase the difficulty and cost of the powder preparation process. Furthermore, current research results indicate that the poor bonding effect between additives / reinforcing agents and the Al-Si alloy matrix does not significantly improve the overall mechanical properties. Thirdly, some researchers have proposed adding external auxiliary fields, such as ultrasonic vibration and static magnetic fields. However, due to the inhomogeneity of these external auxiliary fields, the microstructure inside the formed sample becomes uneven, leading to inconsistent mechanical properties in different parts of the final component.

[0004] Therefore, finding an efficient, environmentally friendly, easy-to-operate, and economical control method to further improve the comprehensive mechanical properties of laser additive manufacturing aluminum alloys, while ensuring the density and apparent quality of the formed samples, has become a challenging and valuable innovative task. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a high-speed secondary melting method for microstructure control and improving the performance of laser additive manufacturing aluminum alloys, so as to solve the problem that the prior art cannot effectively improve the tensile strength and elongation at break of Al-Si alloys at the same time.

[0006] To achieve the above objectives, the present invention employs the following technical solution:

[0007] A high-speed secondary melting method for improving the properties of laser additive manufacturing aluminum alloys includes the following steps:

[0008] Step 1: Determine the initial melting process parameters;

[0009] Step 2: Set the high-speed secondary melting process parameters. Combine the primary melting process parameters and the high-speed secondary melting parameters, and simulate the primary laser melting and high-speed secondary melting using finite element software to obtain the solidification parameters of the solid and liquid phase interfaces during the high-speed secondary melting process. The solidification parameters are the temperature gradient G and the solidification rate R. The heat storage of the primary laser melting affects the solidification parameters of the high-speed secondary melting.

[0010] Step 3: Using the solidification transformation diagram of the Al-Si alloy and the solidification parameters corresponding to the secondary melting, set the high-speed secondary melting process parameters that can obtain equiaxed crystals; the laser scanning rate in the high-speed secondary melting process parameters that can obtain equiaxed crystals is 3000-7000 mm / s and the laser power is 350-390 W.

[0011] Step 4: Melt the powder using the initial melting process parameters to form a process part. Then, use the high-speed secondary melting process parameters to obtain equiaxed crystals and perform high-speed secondary melting on the process part. Repeat the powder spreading, initial melting, and secondary melting to obtain the final aluminum alloy specimen.

[0012] A further improvement of the present invention is that:

[0013] Preferably, in step 1, the initial melting process parameters are: laser spot diameter of 100 μm, laser scanning rate of 1600 mm / s, laser power of 340 W, channel spacing of 100 μm, and powder layer thickness of 30 μm.

[0014] Preferably, in step 2, the temperature field of the molten pool during high-speed secondary melting is obtained through a finite element heat transfer numerical model, thereby obtaining the solidification parameters of the solid and liquid phase interfaces during the high-speed secondary melting process.

[0015] Preferably, in step 3, the solidification transformation diagram of the Al-Si alloy is obtained based on the thermophysical parameters of the Al-Si alloy and combined with the CET model.

[0016] Preferably, the CET model is:

[0017]

[0018] Where G represents the temperature gradient, R represents the solidification rate, and a,n are constants about the material. Represents the integral number of the equiaxed crystal, ΔT n N represents the undercooling of nucleation, N0 represents the nucleation number density, and ΔT represents the undercooling of the dendrite tip.

[0019] Preferably, in step 3, the solidification parameters corresponding to the secondary melting are introduced into the solidification transformation diagram of the Al-Si alloy to determine whether the solidification parameters are in the equiaxed crystal region. If not, they are removed directly.

[0020] Preferably, in step 3, the high-speed secondary melting process parameters are: laser scanning rate 3000 mm / s, laser power 350 W; scanning rate 4000 mm / s, power 360 W; scanning rate 5000 mm / s, power 370 W; scanning rate 6000 mm / s, power 380 W; scanning rate 7000 mm / s, power 390 W; and the channel spacing is 50 μm.

[0021] Preferably, in step 4, both the initial laser melting and the high-speed secondary melting adopt a 180° angle cross-scanning mode.

[0022] Preferably, in step 4, the angle between the initial laser melting path and the high-speed secondary melting path is 67°.

[0023] Preferably, in step 4, after each high-speed secondary melting, the sample outline is traced once, with a laser scanning rate of 1000 mm / s and a power of 200 W.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] This invention discloses a high-speed secondary melting method for improving the performance of laser additive manufacturing aluminum alloys. It proposes a method to in-situ control the microstructure of the formed component by using a high-power, high-scanning-rate laser to perform layer-by-layer secondary melting on a solidified surface, thereby improving its comprehensive mechanical properties. To accurately obtain the process parameters for high-speed secondary melting, this method incorporates the heat storage of the initial laser melting into the heat transfer numerical simulation process, making the predicted solidification parameters of the solid-liquid interface during high-speed secondary melting closer to reality. First, the powder bed is initially melted using optimal process parameters to obtain a cladding layer with good density and appearance quality. Second, the laser power and scanning rate are increased to perform secondary melting on the solidified surface to improve its microstructure. Subsequently, the powder is re-spread and the above process is repeated to obtain the final formed component. This method is performed in-situ in the forming equipment, and is simple, efficient, and applicable. Attached Figure Description

[0026] Figure 1 The results are from a finite element simulation of the molten pool temperature field. (a) Initial laser melting result; (b) High-speed secondary melting result.

[0027] Figure 2The solidification parameters of the solid-liquid interface are extracted from the finite element simulation results. (a) are the solidification parameters of the initial laser melting; (b) are the solidification parameters of the high-speed secondary melting.

[0028] Figure 3 This shows the solidification microstructure of Al-Si alloys and the regions in the diagram for conventional forming (without remelting) and high-speed secondary melting solidification parameters.

[0029] Figure 4 This is a schematic diagram of the scanning strategies for primary melting laser and high-speed secondary melting laser.

[0030] Figure 5 A photograph of a high-speed secondary melting and forming sample—longitudinal.

[0031] Figure 6 A side view of the high-speed secondary melting forming sample.

[0032] Figure 7 Comparison of the microstructure of the molded specimens—EBSD images.

[0033] Figure 8 Comparison of the microstructure of the molded specimens—SEM images.

[0034] Figure 9 Comparison of tensile properties of formed specimens—longitudinal direction.

[0035] Figure 10 A comparison diagram of the tensile properties of the formed specimens—transverse. Detailed Implementation

[0036] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0037] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. The terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, unless otherwise explicitly specified and limited, the terms "installed," "connected," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection or a detachable connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal connection of two elements. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0038] Al-Si alloys, due to their resistance to cracking during solidification and good formability, offer a wide process window in laser additive manufacturing, providing a solid foundation for further control of process parameters. On the other hand, while laser additive manufacturing allows for the control of many process parameters, the local solidification conditions at the solid-liquid interface (solid-liquid interface) – namely, the temperature gradient G and solidification rate R – have the decisive influence on the microstructure. Laser power and scanning rate dominate the influence of these solidification parameters. This invention utilizes existing mature process parameters to obtain Al-Si alloy samples with good density. Then, a high-speed secondary melting strategy is employed to perform a secondary scan of the solidified layer, thereby further improving the metallurgical quality and overall density of the formed sample, and refining the microstructure, including grains and intragranular substructures. This achieves the goal of simultaneously improving the tensile strength and elongation at break of the sample.

[0039] This invention provides a high-speed secondary melting method for improving the properties of laser additive manufacturing aluminum alloys, comprising the following steps:

[0040] Step 1: Due to the Al-Si alloy's resistance to cracking, good fluidity, and excellent formability, it has already been studied to some extent in LPBF technology. Based on previous explorations in Al-Si alloy forming, this invention determines the optimal forming process parameters: laser spot diameter of 100 μm, laser scanning rate of 1600 mm / s, laser power of 340 W, pass spacing of 100 μm, powder layer thickness of 30 μm, and a cross-scanning method with 180° rotation between adjacent passes. In this invention, these optimal process parameters are used for the initial laser melting in both finite element heat transfer numerical simulation and actual forming processes.

[0041] Step 2: Simulate the initial laser melting and high-speed secondary melting processes using finite element software. By analyzing the molten pool temperature field output by the finite element software, extract the internal temperature field information (mainly the temperature field distribution cloud map and the determination of the melting isothermal surface) to determine the solidification parameters within the molten pool during the initial melting process and the high-speed secondary melting process (mainly reflected in high scanning rate and high laser power), namely, the temperature gradient G and solidification rate R at the solid-liquid interface.

[0042] (1) The finite element method (FEM) software was used to simulate the initial laser-powder melting and high-speed laser secondary melting processes. First, the changes in the material's thermophysical parameters were programmed, embedding the thermophysical parameters of different states—powder, molten liquid, and re-solidified solid—into the model. During laser additive manufacturing, the transformation of the material's powder, liquid, and solid properties is determined by its temperature history. When the material's highest temperature is below its melting point, its properties are set to powder; when the highest temperature is above the melting point and the current temperature is below the melting point, its properties are set to solid; and when the current temperature is above the melting point, its properties are set to liquid.

[0043] (2) Then, the physical conditions of heat conduction, heat convection and heat radiation involved in the melting process are controlled by finite element software. The relevant control equations are as follows:

[0044] Q in =Q cond +Q conve +Q rad #(1)

[0045]

[0046]

[0047] In equation (1) Q in Q represents the input laser heat flux. cond Represents heat conduction, Q conve Represents thermal convection, Q rad In equation (2), T(x,y,z,t) represents temperature, t represents time, ρ represents density, C represents specific heat capacity, and K represents thermal conductivity; in equation (3), S represents the surface to which the heat flux is applied, n represents the normal vector of surface S, and h represents thermal radiation. c σ represents the convective heat transfer coefficient, T0 represents the ambient temperature of 300K, σ represents the Stefan-Boltzmann constant, ε represents the emissivity, and q represents the input heat flux. The temperature field transfer process during the forming process is simulated using the governing equation (1-3).

[0048] (3) Input the corresponding actual process parameters into the finite element software. The laser source uses a double ellipsoidal model to realize energy input:

[0049]

[0050] In equation (4), q represents the input heat flux, P represents the laser power, v represents the scanning rate, η represents the absorption rate of the laser by the alloy powder and the aluminum alloy solid surface, a, b, c, f define the volume distribution of the heat source, x, y, z represent the local coordinate axes, and t represents time.

[0051] Substituting the process parameters of the primary melting laser and the set high-speed secondary melting laser determined in step 1—laser power P and scanning rate v—into formula (4), the corresponding molten pool temperature field simulation results can be obtained through finite element software, such as... Figure 1As shown, (a) is the temperature field simulation result of the initial melting laser, and (b) is the temperature field simulation result of the high-speed secondary laser melting. It can be seen that since the secondary melting is applied immediately after the initial melting, the temperature field of the initial melting has a significant impact on the morphology of the secondary melting temperature field. Compared to the initial melting result, the secondary melting laser exhibits a long tail in its temperature field morphology due to the significantly increased scanning rate, and the bottom morphology of the molten pool is relatively flat. These differences all affect the subsequent extraction of solidification parameters in the molten pool.

[0052] (4) Extract the solidification parameters of the solid-liquid interface from the finite element simulation results: temperature gradient g and solidification rate R. The formula for extracting the temperature gradient G is:

[0053]

[0054] In equation (5), T represents temperature, and x, y, z represent local coordinate axes.

[0055] The formula for extracting the solidification rate R is:

[0056] R=Vcosα,#(6)

[0057] In equation (6), v represents the laser scanning rate and α represents the angle between the laser scanning direction and the normal direction of the molten pool.

[0058] The solidification parameters in the molten pools of the initial laser melting and high-speed secondary melting can be extracted using equations (5) and (6), respectively. Figure 2 As shown, the solidification parameters in the molten pool differ significantly under the two process conditions. First, due to the heat storage effect of the initial laser melting, the temperature gradient G in the secondary melting decreases by an order of magnitude. Second, due to the significant increase in the scanning rate of the laser in the secondary melting, the longitudinal section of the molten pool becomes longer and the bottom becomes flatter. In addition, the heat storage effect of the initial melting increases the angle α between the laser scanning direction and the normal direction of the molten pool, thus the actual solidification rate R does not increase much. However, since the crystal morphology is determined by both the temperature gradient G and the solidification rate R, the results obtained from equations (5) and (6) need to be analyzed theoretically to determine their impact on the microstructure.

[0059] Step 3: Based on the columnar-to-equiaxed transition (CET) theory, a solidification microstructure diagram of the Al-Si alloy is generated. The high-speed secondary melting solidification parameters obtained in step (4) of step 2 are mapped to the microstructure diagram. It is determined whether the solidification parameters of the high-speed secondary melting are in the equiaxed crystal region. If not, the high-speed secondary melting parameters are discarded. If they are, the secondary melting process parameters are obtained in reverse.

[0060] (1) Based on the thermophysical parameters of Al-Si alloy, the solidification microstructure diagram was generated using the CET theoretical model. The CET formula is:

[0061]

[0062] In equation (7), G represents the temperature gradient, R represents the solidification rate, and a and n are constants about the material (n = 2.5, a = 2.9 × 10⁻⁶). 6 (K 2.5 ·s·m -1 )), Represents the integral number of equiaxed crystals ( It is a fully isometric crystal. It is a fully columnar crystal, with the middle value representing the mixed region), ΔT n Represents the undercooling of nucleation (ΔT) n =2K.), N0 represents the nucleation number density (N0 = 5 × 10). 10 / m 3 ), where ΔT represents the undercooling at the dendrite tip. The solidification microstructure of the Al-Si alloy is obtained, as shown below. Figure 3 As shown.

[0063] (2) Substituting the solid-liquid interface solidification parameters G and R extracted in step 2(4) into the solidification microstructure diagram in step 3(1), it can be seen that the solidification conditions of the LPBF sample obtained under conventional process parameters are basically located in the columnar crystal region, with only a small portion located in the mixing region, and almost no CET transformation occurs. However, as analyzed in step 2(4), the heat storage generated by the initial laser melting has a significant impact on the high-speed secondary melting, causing the temperature gradient G of the secondary melting to shift downward in the solidification microstructure diagram; in addition, due to the high scanning rate characteristics of the secondary laser itself, even though the increase in the α angle limits the increase in the actual solidification rate R, the solidification rate R still tends to shift to the right in the solidification microstructure diagram compared to conventional forming parameters. In summary, after high-speed secondary melting, the solidification parameters inside the molten pool shift to the lower right corner of the solidification microstructure diagram (i.e., the region where equiaxed crystals are easily generated). Therefore, the laser process parameters for high-speed secondary melting can be obtained in reverse.

[0064] (3) The high-speed secondary melting laser process parameters determined from step (2) of step 3 are: laser scanning rate of 3000-7000mm / s and laser power of 350-390W.

[0065] Specifically, the laser scanning rate was 3000 mm / s with a power of 350 W; 4000 mm / s with a power of 360 W; 5000 mm / s with a power of 370 W; 6000 mm / s with a power of 380 W; and 7000 mm / s with a power of 390 W. The pass spacing was 50 μm, and adjacent passes were rotated 180° in a cross-scan pattern. The angle between the overall direction of the remelting laser and the overall direction of the primary melting laser was 67°. These parameters ensured that the solidified structure after the final high-speed secondary melting could transform into equiaxed crystals.

[0066] Step 4: Use LPBF equipment to prepare the sample. First, use the optimal process parameters to perform the initial melting of the powder bed; then, use the high-speed laser parameters determined in step 3 (3) to perform the secondary melting of the forming surface; then, lay the next layer of powder and repeat the above-mentioned initial melting and high-speed secondary melting process until the final component forming process is completed.

[0067] (1) Drying of molten experimental powder. Commercial AlSi10Mg alloy powder with a particle size of 15-53 μm was used in the experiment. Before the experiment, the powder was placed in a vacuum drying oven at 120℃ for 2 hours to evaporate the moisture. After the temperature in the drying oven dropped to room temperature, the powder was taken out for use.

[0068] (2) Preparation for laser additive manufacturing forming experiment. Turn on the forming experiment equipment and start the machine. Clean the experimental chamber with alcohol and reinforced industrial wipes. Place the dried powder in the powder hopper. Adjust the first layer powder spreading effect. Close the chamber door and turn on the scrubbing gas. When the oxygen content in the experimental chamber drops below 0.08% (800PPM), turn on the fan and preheat the substrate.

[0069] (3) Model establishment and processing parameter setting. Currently, there are no existing programs and models for high-speed secondary melting strategies in laser additive manufacturing equipment, so it is necessary to set them up yourself. First, a corresponding three-dimensional model is established in the modeling software, and then the parameters of the initial melting are assigned to the model, along with the corresponding optimal process parameters; second, a completely identical three-dimensional model is established at the corresponding location, and the process parameters of the high-speed secondary melting strategy are assigned to the second set of models.

[0070] Each powder bed is 30 μm thick. For the same scanning layer, both the primary and secondary melting lasers use a 180° angle cross-scanning mode, and the overall angle between the secondary and primary melting lasers is 67°. Due to the high scanning rate, the molten surface is re-melted in a very short time after the primary melting, so the time interval between the primary and secondary melting can be ignored.

[0071] The strategy of repeating the initial melting and high-speed secondary melting after re-spreading the powder is adopted, and the initial melting laser after re-spreading the powder is rotated as a whole by 67° with the secondary melting laser of the previous layer.

[0072] For each layer, after the high-speed secondary melting is completed, the sample outline is traced once, with a laser scanning rate of 1000 mm / s and a power of 200 W.

[0073] (4) Import the model into the molding equipment. Import the STL format file from the computer into the molding equipment using a USB flash drive; lower the molding platform by 1-3 layers, raise the powder hopper by 1-3 layers or drop a certain amount of powder, and move the scraper until the substrate is evenly powdered.

[0074] (5) Melt forming process. After all preparations are completed, start the program and begin the forming process; observe the process for a period of time to see if the forming is normal and make timely adjustments; the forming process may take a long time, so pay attention to the amount of powder remaining in the powder hopper and the oxygen content.

[0075] (6) Post-molding processing. After the molding process is complete, the experimental chamber can be opened after the substrate temperature drops to room temperature; clean the remaining powder in the chamber (wear gloves and a dust mask to operate), return the scraper to zero, raise the substrate to the highest point, use a brush to sweep the remaining powder in the experimental chamber into the collection tank, and use a vacuum cleaner to clean the powder; the remaining unused new powder in the powder hopper is recycled for reuse; after cleaning, the molten sample and the substrate are taken out together, and the experimental chamber is cleaned for the final time. The molding process is complete.

[0076] Because laser additive manufacturing involves numerous process parameters, a thorough understanding of the technological mechanisms is essential for targeted control of key process parameters to fundamentally improve microstructure and properties. Based on numerical simulations and theoretical calculations, this invention identifies the fundamental factors influencing microstructure: the temperature gradient G at the solid-liquid interface and the solidification rate R. This allows for the further determination of process parameters, particularly the effects of scanning rate and laser power on solidification parameters. Since high-speed secondary melting occurs after the initial laser melting, the temperature field of the initial melting process inevitably influences the temperature field of the secondary melting process. During the simulation of the melt-forming process, solidification parameters at the solid-liquid interface were extracted, and a solidification microstructure map was established using solidification principles. This allows for the quantitative analysis and setting of solidification parameters and forming control parameters, forming a solid foundation for the experimental parameters proposed in this invention.

[0077] In the actual forming process, since existing commercial LPBF equipment cannot directly achieve the strategy of high-speed secondary melting, it is necessary to further develop the forming modeling software. First, a three-dimensional forming model is established and assigned optimal process parameters to achieve the purpose of initial laser melting; then, a three-dimensional model with identical size and shape is established at the corresponding position and assigned high-speed and high-power process parameters to achieve the purpose of secondary melting.

[0078] The present invention will be further described below with reference to specific implementation processes. These descriptions are for illustrative purposes only and not for limiting the invention. The specific steps are as follows:

[0079] Example 1

[0080] The first step is to use finite element software to simulate the initial laser melting and secondary melting processes.

[0081] The laser source adopts a double ellipsoidal model, and the governing equations are as follows:

[0082]

[0083] The governing equations for the physical conditions of heat transfer, heat convection, and heat radiation are as follows:

[0084] Q in =Q cond +Q conve +Q rad #(2)

[0085]

[0086]

[0087] The boundary condition governing equations are as follows:

[0088]

[0089] The material properties are shown in Table 1 below:

[0090] Table 1. Material property parameters used in the simulation

[0091]

[0092]

[0093] The second step involves extracting the solid-liquid interface solidification parameters for the initial laser melting and high-speed secondary melting in the simulation software: the temperature gradient G and the solidification rate R. In Example 1, the temperature gradient G for the initial laser melting is 1.4 × 10⁻⁶. 6 ~6.3×10 6K / m, solidification rate R: 0.03~1.4m / s; temperature gradient G for high-speed secondary melting: 7.2×10 5 ~9.6×10 5 K / m, solidification rate R: 0.18~2.8m / s.

[0094] The third step is to establish the solidification microstructure of the Al-Si alloy. The solidification microstructure of the Al-Si alloy is plotted using the CET theoretical model, and the theoretical equations are as follows:

[0095]

[0096] The spectrum is plotted based on the above formula, such as... Figure 3 As shown.

[0097] The fourth step involves applying the solidification parameter temperature gradient G of the high-speed secondary melting solid-liquid interface obtained in the second step: 7.2 × 10⁻⁶. 5 ~9.6×10 5 K / m and solidification velocity R: 0.18~2.8m / s were substituted into the solidification microstructure diagram in step 3 and found to be within the CET transition range. Therefore, the initial laser melting process parameters used in the example (laser scanning rate 1600mm / s, laser power 340W, channel spacing 100μm) and the high-speed secondary melting process parameters (laser scanning rate 3000mm / s, laser power 350W, channel spacing 50μm) meet the requirements.

[0098] Step 5: Preparation for melt forming experiment. This invention uses commercial AlSi10Mg alloy powder for forming, with a particle size range of 15–53 μm. The chemical composition is shown in the table below:

[0099] Table 2. Chemical composition of AlSi10Mg alloy powder

[0100]

[0101]

[0102] Before drying the powder, clean the drying chamber with alcohol and reinforced industrial wiping paper; place the powder in the powder hopper, turn on the vacuum pump, wait until the drying chamber is in a vacuum state, close the gas valve, and then turn off the vacuum pump; turn on the drying chamber switch, set the drying temperature to 120℃, and the drying time to 2 hours; after the drying process is completed, wait for the temperature in the drying chamber to drop to room temperature, open the gas valve, and pressurize the drying chamber with gas. After the internal and external pressures are equal, open the drying chamber door and take out the powder; clean the drying chamber and wipe the side walls.

[0103] Step 6: Substrate and Powder Loading. Power on the machine, open the experimental chamber door, and clean the chamber with alcohol and reinforced industrial wipes. Raise the forming platform to its upper limit and install the substrate (cast ZL104 alloy). Measure the height of the substrate relative to the platform and adjust the height difference until the substrate height relative to the platform is approximately 2mm. Load the pre-baked powder into the powder hopper. Lower the forming platform by 1-3 layers and raise the powder hopper by 1-3 layers. Use a scraper to spread the powder evenly on the substrate, making appropriate adjustments until the powder is evenly distributed. After preparation, close the sample chamber door.

[0104] Step 7, Pre-operation treatment. Open the control panel, set "Gas Washing", and turn on the fan and preheat the substrate when the oxygen content in the equipment drops below 0.08% (800PPM). In this invention, the substrate preheating temperature is 35-200℃.

[0105] Step 8: Establishing the initial melting model and setting parameters. Open the modeling software and create a 3D model. In this invention, longitudinal and transverse rod-shaped samples with dimensions of 12mm×12mm×75mm (height) and 12mm×75mm×12mm (height) are created respectively. Import the 3D model into the parameter setting software and set the process parameters, as shown in the table below:

[0106] Table 3. Forming parameters of the initial laser melting model

[0107]

[0108] Step 9: Establishment and parameter setting of the high-speed secondary melting model. Based on the original 3D model, establish a 3D model with identical dimensions and positions, with longitudinal and transverse rod-shaped samples of the same size distribution: 12mm×12mm×75mm (height) and 12mm×75mm×12mm (height). Import this set of 3D models into the parameter setting software to set the high-speed secondary melting process parameters, as shown in the table below:

[0109] Table 4. Forming parameters of the high-speed secondary melting model

[0110]

[0111] Step 10, Laser Melting Process. Import the STL format model into the device, slice and preview it. If there are no errors, the forming process can begin. Click "Forming" to perform three melting operations on the first layer of powder on the substrate, which will make the sample adhere better to the substrate metal. After forming begins, observe for a period of time, and observe the amount of remaining powder and oxygen content in the powder hopper during the forming process.

[0112] Step 11, Post-forming Processing. Wait for the substrate temperature to drop to approximately room temperature before removing the sample (wear gloves and a dust mask during operation). First, clean away the powder. Lower the substrate and return the scraper to its original position. Then, raise the substrate and use a brush to sweep the powder from the substrate into the collection tank. Repeat this process of lowering and raising the substrate, using a brush and vacuum cleaner to clean away the powder. Finally, raise the substrate to the top and clean away all the powder. Replace the vacuum cleaner with a smaller one to remove the powder from the substrate screws. Then, remove the screws from the substrate and remove the substrate along with the sample. The forming process is now complete.

[0113] Step 12, Sample Analysis and Testing. The formed sample is removed from the substrate using a wire electrical discharge machining (EDM) machine. Inside the formed sample, a specimen for metallographic observation and tensile testing is cut using the same EDM machine. The sample surface is metallographically treated with sandpaper, polishing fluid, and etching solution. Microstructure is observed using an optical microscope (OM) and a scanning electron microscope (SEM). A quasi-static axial tensile test at room temperature is performed using a universal testing machine.

[0114] Example 2

[0115] In this embodiment, the first step is the same as in Embodiment 1.

[0116] The second step involves extracting the solid-liquid interface solidification parameters for the initial laser melting and high-speed secondary melting in the simulation software: temperature gradient G and solidification rate R. In this Example 2, the temperature gradient G for the initial laser melting is 1.4 × 10⁻⁶. 6 ~6.3×10 6 K / m, solidification rate R: 0.03~1.4m / s; temperature gradient G for high-speed secondary melting: 3.2×10 5 ~5.9×10 5 K / m, solidification rate R: 0.21~3.6m / s.

[0117] The third step is the same as in Example 1.

[0118] The fourth step involves applying the solidification parameter temperature gradient G of the high-speed secondary melting solid-liquid interface obtained in the second step: 3.2 × 10⁻⁶. 5 ~5.9×10 5 K / m and solidification velocity R: 0.21~3.6m / s were substituted into the solidification microstructure diagram in step 3 and found to be within the CET transition range. Therefore, the initial laser melting process parameters used in the example (laser scanning rate 1600mm / s, laser power 340W, channel spacing 100μm) and the high-speed secondary melting process parameters (laser scanning rate 4000mm / s, laser power 360W, channel spacing 50μm) meet the requirements.

[0119] Steps five through eight are the same as in Example 1.

[0120] Step 9: Establishing the high-speed secondary melting layer model and setting its parameters. Based on the existing 3D model, establish a 3D model with identical dimensions and positions, including longitudinal and transverse rod-shaped samples with the same dimensions of 12mm×12mm×75mm (height) and 12mm×75mm×12mm (height). Import this set of 3D models into the parameter setting software to set the high-speed secondary melting process parameters, as shown in the table below:

[0121] Table 5. Forming parameters of the high-speed secondary melting model

[0122]

[0123] Steps 10 through 12 are the same as in Example 1.

[0124] Example 3

[0125] In this embodiment, the first step is the same as in Embodiment 1.

[0126] The second step involves extracting the solid-liquid interface solidification parameters for the initial laser melting and high-speed secondary melting in the simulation software: the temperature gradient G and the solidification rate R. In this example 3, the temperature gradient G for the initial laser melting is 1.4 × 10⁻⁶. 6 ~6.3×10 6 K / m, solidification rate R: 0.03~1.4m / s; temperature gradient G for high-speed secondary melting: 6.2×10 4 ~2.9×10 5 K / m, solidification rate R: 0.34~4.7m / s.

[0127] The third step is the same as in Example 1.

[0128] The fourth step involves applying the solidification parameter temperature gradient G of the high-speed secondary melting solid-liquid interface obtained in the second step: 6.2 × 10⁻⁶. 4 ~2.9×10 5 K / m and solidification velocity R: 0.34~4.7m / s were substituted into the solidification microstructure diagram in step 3 and found to be within the CET transition range. Therefore, the initial laser melting process parameters used in the example (laser scanning rate 1600mm / s, laser power 340W, channel spacing 100μm) and the high-speed secondary melting process parameters (laser scanning rate 5000mm / s, laser power 370W, channel spacing 50μm) meet the requirements.

[0129] Steps five through eight are the same as in Example 1.

[0130] Step 9: Establishing the high-speed secondary melting layer model and setting its parameters. Based on the existing 3D model, establish a 3D model with identical dimensions and positions, including longitudinal and transverse rod-shaped samples with the same dimensions of 12mm×12mm×75mm (height) and 12mm×75mm×12mm (height). Import this set of 3D models into the parameter setting software to set the high-speed secondary melting process parameters, as shown in the table below:

[0131] Table 6. Forming parameters of the high-speed secondary melting model

[0132]

[0133] Steps 10 through 12 are the same as in Example 1.

[0134] Example 4

[0135] In this embodiment, the first step is the same as in Embodiment 1.

[0136] The second step involves extracting the solid-liquid interface solidification parameters for the initial laser melting and high-speed secondary melting in the simulation software: the temperature gradient G and the solidification rate R. In this example 4, the temperature gradient G for the initial laser melting is 1.4 × 10⁻⁶. 6 ~6.3×10 6 K / m, solidification rate R: 0.03~1.4m / s; temperature gradient G for high-speed secondary melting: 4.1×10 4 ~6.8×10 4 K / m, solidification rate R: 0.42~5.6m / s.

[0137] The third step is the same as in Example 1.

[0138] The fourth step involves applying the solidification parameter temperature gradient G of the high-speed secondary melting solid-liquid interface obtained in the second step: 4.1 × 10⁻⁶. 4 ~6.8×10 4 K / m and solidification velocity R: 0.42~5.6m / s were substituted into the solidification microstructure diagram in step 3 and found to be within the CET transition range. Therefore, the initial laser melting process parameters used in the example (laser scanning rate 1600mm / s, laser power 340W, channel spacing 100μm) and the high-speed secondary melting process parameters (laser scanning rate 6000mm / s, laser power 380W, channel spacing 50μm) meet the requirements.

[0139] Steps five through eight are the same as in Example 1.

[0140] Step 9: Establishing the high-speed secondary melting layer model and setting its parameters. Based on the existing 3D model, establish a 3D model with identical dimensions and positions, including longitudinal and transverse rod-shaped samples with the same dimensions of 12mm×12mm×75mm (height) and 12mm×75mm×12mm (height). Import this set of 3D models into the parameter setting software to set the high-speed secondary melting process parameters, as shown in the table below:

[0141] Table 7. Forming parameters of the high-speed secondary melting model

[0142]

[0143] Steps 10 through 12 are the same as in Example 1.

[0144] Example 5

[0145] In this embodiment, the first step is the same as in Embodiment 1.

[0146] The second step involves extracting the solid-liquid interface solidification parameters for the initial laser melting and high-speed secondary melting using simulation software: temperature gradient G and solidification rate R. In this example 5, the temperature gradient G for the initial laser melting is 1.4 × 10⁻⁶. 6 ~6.3×10 6 K / m, solidification rate R: 0.03~1.4m / s; temperature gradient G for high-speed secondary melting: 5.3×10 3 ~1.2×10 4 K / m, solidification rate R: 0.18~6.7m / s.

[0147] The third step is the same as in Example 1.

[0148] The fourth step involves applying the solidification parameter temperature gradient G of the high-speed secondary melting solid-liquid interface obtained in the second step: 5.3 × 10⁻⁶. 3 ~1.2×10 4 K / m and solidification velocity R: 0.18~6.7m / s were substituted into the solidification microstructure diagram in step 3 and found to be within the CET transition range. Therefore, the initial laser melting process parameters used in the example (laser scanning rate 1600mm / s, laser power 340W, channel spacing 100μm) and the high-speed secondary melting process parameters (laser scanning rate 7000mm / s, laser power 390W, channel spacing 50μm) meet the requirements.

[0149] Steps five through eight are the same as in Example 1.

[0150] Step 9: Establishing the high-speed secondary melting layer model and setting its parameters. Based on the existing 3D model, establish a 3D model with identical dimensions and positions, including longitudinal and transverse rod-shaped samples with the same dimensions of 12mm×12mm×75mm (height) and 12mm×75mm×12mm (height). Import this set of 3D models into the parameter setting software to set the high-speed secondary melting process parameters, as shown in the table below:

[0151] Table 8. Forming parameters of the high-speed secondary melting model

[0152]

[0153] Steps 10 through 12 are the same as in Example 1.

[0154] In this invention, the formed sample after high-speed secondary melting exhibits excellent internal metallurgical quality and surface quality, as shown in the photograph. Figure 5 (vertical) and Figure 6 As shown in the horizontal direction.

[0155] Microscopic observation and comparison revealed that: Figure 7 As shown in the EBSD image, the anisotropy of the sample's microstructure is significantly reduced after high-speed secondary melting; as... Figure 8 As shown in the SEM image, after high-speed secondary melting, the size of dendrites inside the sample grains is significantly reduced.

[0156] The results of the room temperature axial tensile test show that: Figure 9 (Comparison of longitudinal stretching results), such as Figure 10 (Comparison of transverse tensile results) Compared with the traditional unremelted specimen, the tensile strength and elongation at break of the specimen after high-speed secondary melting are significantly improved; and the anisotropy of longitudinal and transverse tensile properties is significantly reduced. Table 9 shows the comparative tensile results of Examples 1-5 and conventionally formed (unremelted) specimens.

[0157] Table 9. Comparison of Tensile Properties

[0158]

[0159]

[0160] It can be concluded that the high-speed secondary melting process parameters obtained from simulation and theoretical calculations, and applied to actual forming experiments, yielded components with good density and appearance quality. This significantly reduced microstructural anisotropy and refined grains and intragranular substructures. Furthermore, the comprehensive mechanical properties, especially tensile strength and elongation at break, were significantly improved. Therefore, the high-speed secondary melting method for improving the performance of laser additive manufacturing aluminum alloys proposed in this invention is an efficient, feasible, environmentally friendly, economical, and applicable in-situ control method.

[0161] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high-speed secondary melting method for improving the properties of laser additive manufacturing aluminum alloys, characterized in that, Includes the following steps: Step 1: Determine the initial melting process parameters; In step 1, the initial melting process parameters are: laser spot diameter of 100μm, laser scanning rate of 1600mm / s, laser power of 340W, channel spacing of 100μm, and powder layer thickness of 30μm. Step 2: Set the high-speed secondary melting process parameters. Combining the primary melting process parameters and the high-speed secondary melting parameters, simulate the primary laser melting and high-speed secondary melting using finite element software to obtain the solidification parameters of the solid and liquid phase interfaces during the high-speed secondary melting process. These solidification parameters are the temperature gradient. and solidification rate The heat storage of the initial laser melting affects the solidification parameters of the high-speed secondary melting. Step 3: Using the solidification transformation diagram of the Al-Si alloy and the solidification parameters corresponding to the secondary melting, set the high-speed secondary melting process parameters that can obtain equiaxed crystals; the laser scanning rate in the high-speed secondary melting process parameters that can obtain equiaxed crystals is 3000-7000 mm / s and the laser power is 350-390 W. Step 4: Melt the powder using the initial melting process parameters to form a process part. Then, use the high-speed secondary melting process parameters to obtain equiaxed crystals and perform high-speed secondary melting on the process part. Repeat the powder spreading, initial melting, and secondary melting to obtain the final aluminum alloy specimen.

2. The high-speed secondary melting method for improving the properties of laser additive manufacturing aluminum alloys according to claim 1, characterized in that, In step 2, the temperature field of the molten pool during high-speed secondary melting is obtained through a finite element heat transfer numerical model, thereby obtaining the solidification parameters of the solid and liquid phase interfaces during the high-speed secondary melting process.

3. The high-speed secondary melting method for improving the properties of laser additive manufacturing aluminum alloys according to claim 1, characterized in that, In step 3, the solidification transformation diagram of the Al-Si alloy is obtained based on the thermophysical parameters of the Al-Si alloy and the CET model.

4. The high-speed secondary melting method for improving the properties of laser additive manufacturing aluminum alloys according to claim 3, characterized in that, The CET model is as follows: in, Represents the temperature gradient. Represents the solidification rate. It is a constant about the material. Represents the integral number of equiaxed crystals. Represents the degree of undercooling during nucleation. Represents the nucleation number density, It represents the undercooling at the dendrite tip.

5. The high-speed secondary melting method for improving the properties of laser additive manufacturing aluminum alloys according to claim 1, characterized in that, In step 3, the solidification parameters corresponding to the secondary melting are introduced into the solidification transformation diagram of the Al-Si alloy to determine whether the solidification parameters are in the equiaxed crystal region. If not, they are removed directly.

6. The high-speed secondary melting method for improving the properties of laser additive manufacturing aluminum alloys according to claim 1, characterized in that, In step 3, the high-speed secondary melting process parameters are: laser scanning rate 3000 mm / s, laser power 350 W; scanning rate 4000 mm / s, power 360 W; scanning rate 5000 mm / s, power 370 W; scanning rate 6000 mm / s, power 380 W; scanning rate 7000 mm / s, power 390 W; and the channel spacing is 50 μm.

7. The high-speed secondary melting method for improving the properties of laser additive manufacturing aluminum alloys according to claim 1, characterized in that, In step 4, both the initial laser melting and the high-speed secondary melting adopt a 180° angle cross-scanning mode.

8. The high-speed secondary melting method for improving the properties of laser additive manufacturing aluminum alloys according to claim 1, characterized in that, In step 4, the angle between the initial laser melting path and the high-speed secondary melting path is 67°.

9. The high-speed secondary melting method for improving the properties of laser additive manufacturing aluminum alloys according to claim 1, characterized in that, In step 4, after each high-speed secondary melting, the sample outline is traced once, with a laser scanning rate of 1000 mm / s and a power of 200 W.

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