A method for multi-dimensional energy density region division in a selective laser melting TC4 process

Abstract

This application provides a method for dividing the multidimensional energy density region in the selective laser melting (TC4) process, belonging to the field of laser additive manufacturing technology. The energy density refers to the division of linear energy density, surface energy density, and volume energy density by adjusting the main parameters of selective laser melting (laser power, scanning speed, scanning spacing, and powder thickness). The division is divided into: incomplete melting region, low energy density spheroidization region, smooth forming region, high energy density region, and over-melting region. This division method can quickly and accurately find the corresponding experimental parameters and energy density range according to the actual engineering application or experimental purpose. When high-quality formed parts are required, the parameter combination in the smooth forming region can be directly selected. When studying defects such as spheroidization, cracks, and unmelted powder, the low energy density spheroidization region is selected. When studying defects such as spheroidization, spatter, porosity, and stress cracks, the high energy density region is selected. The incomplete melting region and the over-melting region have basically no practical application value.

Description

Technical Field

[0001] This invention belongs to the field of selective laser melting technology, specifically relating to a method for dividing the multidimensional energy density region in the selective laser melting TC4 process. Background Technology

[0002] TC4 is a commonly used structural material in industry, characterized by high strength, excellent high-temperature performance, good corrosion resistance, and strong biocompatibility. Since its development, the use of TC4 titanium alloy has been increasing, and its application range has been expanding. With technological advancements and the emergence of new processing and forming technologies, the processing cost of TC4 has gradually decreased, leading to an expansion of its application scope and fields. TC4 has a very low density, nearly half that of steel, while its strength is comparable to structural steel and cemented carbide, making it a high-quality alloy material. Selective laser melting (SLM) technology integrates a series of cutting-edge technologies from materials science, laser science, and computer science, and is the most typical and widely used new processing technology for TC4. Its process is simple, utilizing TC4 alloy powder for direct forming without the need for low-melting-point binders. SLM technology can not only significantly reduce defects in parts and increase material density, but also reduce component segregation and improve the uniformity of the microstructure. The comprehensive performance of the parts produced by SLM is comparable to that of traditional castings.

[0003] Selective laser melting forming of TC4 alloy parts is a complex process with multiple parameters. The process parameters do not affect the process independently, but interact with each other to have a comprehensive effect on the forming process. This results in a highly nonlinear relationship between the process parameters of selective laser melting and the quality of the formed parts, making it difficult to establish a regression prediction model.

[0004] Currently, research on selective laser melting (TC4) mainly includes defect research and performance research of formed parts. When conducting these two types of research, the parameters are simply listed and superimposed for experiments. When conducting defect research, simple parameter superposition not only consumes a long processing time, but also easily leads to experimental results that differ greatly from expectations, resulting in repeated experiments and wasting processing materials. Furthermore, when conducting performance research on formed parts, simple parameter superposition does not easily yield stable parts with good forming performance.

[0005] Therefore, to address the aforementioned technical issues, a method for dividing the multidimensional energy density region in the selective laser melting (TC4) process is needed. Summary of the Invention

[0006] The purpose of this invention is to provide a method for dividing the multidimensional energy density region in the selective laser melting (TC4) process, in order to solve the problems of the existing technology, which simply lists and superimposes the main parameters for experiments, resulting in an increase in the number of experiments and a complex experimental process; if an inappropriate range of process parameters is selected, it is easy to cause the experimental results to differ greatly from the expectations, resulting in repeated experiments and waste of processing materials; and the technical problem that simple parameter superposition is not easy to obtain stable parts with good forming performance when studying the performance of formed parts.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] Step 1: Using laser power P and scanning speed v as experimental parameters, conduct a single-channel experiment.

[0009] Furthermore, by combining the two parameters of a single channel, P (laser power) and v (scanning speed), the linear energy density E is obtained. L The linear energy density model is:

[0010]

[0011] Among them, E L ρ is the linear energy density, J / mm; P is the laser power, W; v is the scanning speed, mm / s.

[0012] Furthermore, the width of the melt channel was measured using an ultra-depth-of-field microscope, and the single-pass forming quality was observed. Based on the measurement and observation results, the line energy density region was divided into four areas: A: incompletely melted line energy density region, B: low-energy-density spheroidizing region, C: smoothly formed line energy density region, D: high-energy-density line energy density region, and E: over-melted line energy density region.

[0013] This step allows for the division of the line energy density region, defining different energy density regions for single-channel performance and defect studies, while also providing parameter values ​​for laser power and scanning speed for surface energy density studies.

[0014] Step 2: Select the laser power P and scanning speed v within the smoothly formed area defined in Step 1. When conducting a single-sided experiment, adjust the corresponding scanning spacing h to adjust the surface energy density. (Default powder thickness is 40μm) Set the scanning spacing h according to the overlap rate of 30%, 40%, 50%, 60%, and 70% of the melt channel width within the smoothly formed area in Step 1.

[0015] Furthermore, by combining the three parameters required for a single layer (P: laser power; v: scanning speed; h: scanning spacing), the surface energy density Es is obtained. The surface energy density model is as follows:

[0016]

[0017] Where: E S Surface energy density, J / mm 2 P is the laser power (W); v is the scanning speed (mm / s); h is the scanning distance (μm).

[0018] Furthermore, the surface roughness was measured using an ultra-depth-of-field microscope, and the single-layer surface forming quality was observed. Based on the measurement and observation results, the surface energy density region was divided into four categories: A: incompletely melted surface energy density region, B: low surface energy density spheroidized region, C: smoothly formed surface energy density region, D: high surface energy density region, and E: over-melted surface energy density region.

[0019] This step can define the surface energy density region, delineate different energy density regions for single-surface performance and defect studies, and provide parameter values ​​for laser power, scanning speed, and scanning spacing for volume energy density studies.

[0020] Step 3: After the surface energy density region is divided in Step 2, select a set of experimental parameters at the junction of the smooth forming region and the low energy density spheroidizing region, a set of experimental parameters at the junction of the smooth forming region and the high energy density region, a set of experimental parameters with the lowest surface roughness in the single-layer experiment, and a set of experimental parameters with the energy density of the smooth forming region located at the most central position. Based on these four sets of parameters, change the powder thickness to 20μm, 30μm, 40μm, 50μm, 60μm, and 70μm for bulk forming experiments.

[0021] Furthermore, by combining the four parameters required for bulk forming—P: laser power; v: scanning speed; h: scanning distance; t: powder thickness—we obtain the volume energy density Ev. The volume energy density model is as follows:

[0022]

[0023] Where: E V Volume energy density, J / mm 3 P is the laser power (W); v is the scanning speed (mm / s); h is the scanning distance (μm); t is the powder thickness (μm).

[0024] Furthermore, the surface roughness of the upper surface is measured using a super-depth-of-field microscope to observe the forming quality of the upper surface and interior. The density of the formed block is measured using the Archimedes displacement method. Based on the measurement and observation results, the volume energy density region is divided into four categories: A: incompletely melted volume energy density region, B: low-energy-density spheroidized volume energy density region, C: successfully formed volume energy density region, D: high-energy-density volume energy density region, and E: over-melted volume energy density region. This step can divide the volume energy density region and obtain the surface roughness, density, and internal defects of the formed block. The volume energy density is then further divided into incompletely melted volume energy density region, low-energy-density spheroidized volume energy density region, successfully formed volume energy density region, high-energy-density volume energy density region, and over-melted volume energy density region. Finally, the division of the line energy density region, surface energy density region, and volume energy density region for selective laser melting TC4 is completed.

[0025] Step 4: Using the parameters used in Step 1, set the same line energy density and forming conditions as the single-pass experiment, perform numerical simulation of the single-pass experiment, and compare the simulation results with the experimental results. The melt channel width and surface morphology at the same line energy density are consistent with the experimental results, thereby verifying the line energy density classification.

[0026] This step provides evidence for the division of the line energy density region, ensuring the accuracy of the line energy density division, and thus ensuring the accuracy of the line, surface, and volume energy density division throughout the entire experiment.

[0027] Compared with existing technologies, the present invention has the following advantages:

[0028] The present invention discloses a method for dividing the multidimensional energy density region in the selective laser melting (TC4) process. This method uses the default powder thickness to summarize the laser power and scanning speed into linear energy density, the laser power, scanning speed, and scanning spacing into surface energy density, and the laser power, scanning speed, scanning spacing, and powder thickness into volume energy density. This unifies multiple parameters into a single parameter, that is, multiple parameters are represented by energy density, thus solving the problem of simply listing multiple parameters for experiments.

[0029] The multi-dimensional energy density region division method of the selective laser melting (TC4) process in this invention can conveniently, quickly, and reliably obtain different energy density regions for single-pass formed parts, single-sided formed parts, and block formed parts formed by selective laser melting (TC4). When studying defects in lines, surfaces, and volumes, the energy density outside the successful forming zone corresponding to the energy density of the line, surface, or volume can be selected for experimental research. When studying the performance of formed parts in lines, surfaces, or volumes, the energy density within the successful forming zone corresponding to the energy density of the line, surface, or volume can be used for experimental research.

[0030] The multi-dimensional energy density region division method of the selective laser melting (TC4) process in this invention can quickly obtain the formation rules of defects and the types of defects corresponding to different regions when studying the incompletely melted area of ​​lines (surfaces, volumes), the low energy density spheroidized area of ​​lines, surfaces, and volumes, the high energy density area of ​​lines, surfaces, and volumes, and the over-melted area of ​​lines, surfaces, and volumes. When it is necessary to obtain stable parts with good quality performance, the parts in the smoothly formed area of ​​lines, surfaces, and volumes can be selected for relevant performance tests and applications. Attached Figure Description

[0031] Figure 1 The images show the surface and internal morphology of the incompletely melted area in the selected area laser melting in this embodiment of the invention.

[0032] Figure 2 The images show the surface and internal morphology of the low-energy-density spheroidized region in the selective laser melting embodiment of the present invention.

[0033] Figure 3 The images show the surface and internal morphology of the selected area laser melting successfully formed region in this embodiment of the invention.

[0034] Figure 4 The images show the surface and internal morphology of the high-energy-density region in the selective laser melting embodiment of the present invention, including the lines, surfaces, volumes, and internal features.

[0035] Figure 5 This describes the surface and internal morphology of the selected area laser melting zone line, surface, and volume in this embodiment of the invention.

[0036] Figure 6 This is a region division diagram of energy density in a selected laser melting line in an embodiment of the present invention;

[0037] Figure 7 This is a region division diagram of the energy density of the selected laser melting surface in an embodiment of the present invention;

[0038] Figure 8 This is a region division diagram of the energy density of the selected laser melting body in an embodiment of the present invention;

[0039] Figure 9 This is a three-dimensional powder bed model of TC4 selective laser melting in numerical simulation according to Embodiment 4 of the present invention;

[0040] Figure 10 The heat source model used in the numerical simulation of selected area laser melting in Embodiment 4 of the present invention is the Gaussian surface heat source model.

[0041] Figure 11 This is the three-dimensional instantaneous heat conduction model used in the numerical simulation of selected area laser melting in Embodiment 4 of the present invention;

[0042] Figure 12This is a comparison diagram of the melt channel width with the same line energy density in the selected area laser melting experiment and the numerical simulation in the embodiment of the present invention;

[0043] Figure 13 This is a comparison diagram of the melt channel width under different energy densities in the numerical simulation of selected area laser melting in Embodiment 4 of the present invention;

[0044] Figure 14 This is a diagram showing the internal morphology and temperature field distribution of the XOZ cross-section molten pool under different energy densities in the numerical simulation of selected area laser melting in Embodiment 4 of the present invention.

[0045] Figure 15 This is a flowchart of a method for dividing the multidimensional energy density region in a selective laser melting (TC4) process according to an embodiment of the present invention. Detailed Implementation

[0046] To make the above-mentioned objectives, advantages and technical solutions of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0047] The specific implementation of this application is illustrated in detail with the help of the following examples, but the scope of protection claimed by this invention is not limited to the specific implementation examples below. All relevant equivalent substitutions based on this principle are within the scope of protection of this invention.

[0048] Example 1.

[0049] Step 1: Combine the two parameters of a single channel, P: laser power; v: scanning speed, and the parameters are as follows:

[0050] Table 1 shows the linear energy density E. L The linear energy density model is:

[0051]

[0052] Where: E L ρ is the linear energy density, J / mm; P is the laser power, W; v is the scanning speed, mm / s.

[0053] Table 1 Single-channel experimental parameters

[0054]

[0055] Beforehand, dry the TC4 powder in an oven at 150°C for 8 hours. Prepare the TC4 board as the substrate. Before the experiment, fill the forming chamber of the selective laser melting equipment with argon gas as a protective gas to prevent the material from oxidizing during the printing process.

[0056] After completing the preparation work, a single-pass forming experiment was conducted using the above parameters with the machine's default powder thickness of 40 micrometers to obtain a single-pass formed part. After completion, the single-pass formed part was taken out after natural cooling, resulting in 35 sets of single-pass test pieces. The surface was cleaned with anhydrous ethanol to remove the surface powder of the single pass, and then air-dried.

[0057] The starting position, center position, and ending position of 35 single-pass forming parts with different linear energy densities were observed and the single-pass width was measured using an ultra-depth-of-field microscope. The average value of the three width measurements was obtained.

[0058] After measurement and observation, the final classification of linear energy density based on single-pass width and melt surface quality is as follows: Linear energy density less than 75 J / mm is classified as A: Incompletely melted zone (melt pass width < 90 μm). Typical morphology and defects of this zone are shown below. Figure 1 As shown in (a), the low linear energy density is insufficient to completely melt the powder, resulting in a discontinuous melt channel, intermittent state, spheroidization, and uneven melt channel edges. This region's linear energy density can be used when studying single-channel spheroidization and unmelted powder issues. When the linear energy density is 75-130 J / mm, it is designated as B: Low-energy-density spheroidization region (90 μm < melt channel width < 120 μm). Typical morphology and defects of a single channel in this region are shown below. Figure 2 As shown in (a), the edge of the melt channel is not smooth, and spheroidization is the most common defect in this region. When studying spheroidization of a single melt channel, the linear energy density of this region can be selected; when the energy density is 130-250 J / mm, C: Linear Energy Density Successful Forming Zone Line (120 μm < Melt Channel Width < 155 μm). The typical morphology of a single melt channel in this region is as follows: Figure 3 As shown in (a), single-pass forming yields the best results, with a uniform and ideal weld bead width, neat and smooth edges, clear texture, and virtually no balling. This region's linear energy density can be selected when a high-performance, defect-free single-pass formed part is required. When the linear energy density is between 250-350 J / mm, D represents the high energy density region (155 μm < weld bead width < 185 μm). Typical morphology and defects of single-pass forming in this region are shown below. Figure 4 As shown in (a), the single-pass spatter has splash-like protrusions on both sides of the melt channel edge, and the edge smoothness is reduced compared to the smoothly formed area. The melt channel surface has shallow depressions and cracks. When studying the single-pass spatter defects, the line energy density in this region can be selected; when the line energy density is greater than 350 J / mm, it is E: line energy density through the melt zone (melt channel width > 185 μm). The typical morphology and defects of the single-pass in this region are as follows. Figure 5 As shown in (a), the single-pass surface collapse is more severe, with longer and more numerous collapses. The splash-like protrusions on both sides of the melt channel are significantly increased. When studying defects such as single-pass collapse, the linear energy density within this region can be selected. The linear energy density region is divided as follows: Figure 6As shown in the figure, the corresponding ranges of line energy density for the five regions are marked, which makes it more intuitive to select the range of line energy density.

[0059] Example 2.

[0060] Step 2: Combine the three parameters required for a single layer: P (laser power), v (scanning speed), and h (scanning distance) to obtain the surface energy density Es. The surface energy density model is as follows:

[0061]

[0062] Where: E S Surface energy density, J / mm 2 P is the laser power (W); v is the scanning speed (mm / s); h is the scanning distance (μm).

[0063] Using the laser power and scanning speed of the smooth forming zone in Example 1, the corresponding scanning interval was adjusted during the single-sided experiment to adjust the surface energy density. (Default powder thickness 40μm) In the first step of the experiment, the width of the melt channel with different parameters was measured. The surface energy density region was divided according to the overlap rate of 30%, 40%, 50%, 60%, and 70% of the melt channel width. The experimental parameters are shown in Table 2.

[0064] Table 2 Experimental parameters for a single layer

[0065]

[0066] Beforehand, dry the TC4 powder in an oven at 150°C for 8 hours. Prepare the TC4 board as the substrate. Before the experiment, fill the forming chamber of the selective laser melting equipment with argon gas as a protective gas to prevent the material from oxidizing during the printing process.

[0067] After completing the preparation work, use the above parameters with the machine's default powder thickness of 40 micrometers to conduct a single-sided forming experiment to obtain a single-sided formed part. After completion, wait for the single-sided formed part to cool naturally before taking it out. Use anhydrous ethanol to clean the surface to remove the single-layer surface powder, and then air dry it.

[0068] The surface roughness at three points on a single-layer experimental surface was measured using an ultra-depth-of-field microscope, and the average value was obtained. The surface roughness, morphology, and defects were then comprehensively analyzed.

[0069] The final result of the surface energy density partitioning is that the surface energy density is less than 2.2 J / mm². 2 For region A: Incompletely melted area with incomplete surface energy density (roughness > 6 μm), the typical morphology and defects of a single layer in this region are as follows: Figure 1As shown in (b), the powder at the overlap of the single-sided melt channel cannot be completely melted, the edge of the melt channel is uneven, and a continuous and good melt channel morphology cannot be formed. Therefore, many large black spheroids will be generated on the surface of the melt channel overlap, and the melt channel will be distorted, the surface will be uneven, with large protrusions and depressions, and the surface will be uneven. When studying the problem of incomplete melting and spheroidization on one side, the surface energy density in this region can be selected; the surface energy density is 2.2-2.5 J / mm². 2 For region B: Low surface energy density spheroidized region (5μm < roughness < 6μm). Typical morphology and defects of a single layer in this region are as follows: Figure 2 As shown in (b), a larger scanning interval results in less or no overlap between scanning trajectories. The slower cooling rate leads to surface tension effects, causing spheroidization and other defects. A distinct raised boundary line can be clearly observed between each melt channel. When studying single-sided spheroidization, the surface energy density within this region can be selected; the surface energy density is between 2.5 and 4.2 J / mm². 2 C: Surface energy density smoothly formed region (roughness < 5 μm). Typical morphology and defects of a single layer in this region are as follows: Figure 3 As shown in (b), single-sided forming produces excellent results with very low surface roughness, no undulations, and virtually no spheroidization or powder adhesion. The weld lines are straight and exhibit a "fish-scale" pattern. When high-performance, defect-free single-sided formed parts are required, the surface energy density in this region can be selected. The surface energy density is 4.2-4.8 J / mm². 2 For D: High energy density region (5μm < roughness < 6μm), the typical morphology and defects of a single layer in this region are as follows: Figure 4 As shown in (b), there are many raised black spheres on both sides of the single-sided melt channel. The melt channels overlap tightly, exhibiting splashing and spheroidization phenomena. The surface has collapsed areas and numerous cracks. The roughness is increased compared to the area with the surface energy density in the smoothly formed region. When studying defects such as cracks in single-sided formed parts, the energy density in this region can be selected; the surface energy density is greater than 4.8 J / mm. 2 For E: Surface energy density through five regions of the molten zone (roughness > 6 μm), the typical morphology and defects of a single layer in this region are as follows: Figure 5 As shown in (b), the melt channel on one side is distorted, and there are many bright spheres splashed out from the surface. The flatness measurement results of the single side show a deeper depression. When studying defects such as collapse in single-sided formed parts, the surface energy density of this region can be selected. The surface energy density region is divided as follows: Figure 7 The figure shows the corresponding ranges of line energy density for the five regions of surface energy density, which makes it more intuitive to select the range of surface energy density.

[0070] Example 3.

[0071] Step 3: Combine the four parameters required for bulk forming: P: laser power; v: scanning speed; h: scanning distance; t: powder thickness, to obtain the volume energy density Ev. The volume energy density model is as follows:

[0072]

[0073] Where: E V Volume energy density, J / mm 3 P is the laser power (W); v is the scanning speed (mm / s); h is the scanning distance (μm); t is the powder thickness (μm).

[0074] After the surface energy density division in Example 2 was completed, a set of experimental parameters was selected from the boundary between the smooth forming region and the low energy density spheroidizing region, a set of experimental parameters from the boundary between the smooth forming region and the high energy density region, a set of experimental parameters from the single-layer experiment with the lowest surface roughness, and a set of experimental parameters from the one where the energy density of the smooth forming region is located in the middle. A total of four sets of parameters were selected as the basic parameters, and then the experiment of Example 3 was carried out. Based on the above four sets of experimental parameters, the powder thickness was changed, and the bulk forming experiment was carried out with powder thicknesses of 20μm, 30μm, 40μm, 50μm, 60μm, and 70μm. According to the experimental results, the volume energy density region division and the influence of volume energy density in different regions on the performance of the formed part were studied. The experimental parameters are shown in Table 3.

[0075] Table 3 Experimental parameters of the formed parts

[0076]

[0077] Beforehand, dry the TC4 powder in an oven at 150°C for 8 hours. Prepare the TC4 board as the substrate. Before the experiment, fill the forming chamber of the selective laser melting equipment with argon gas as a protective gas to prevent the material from oxidizing during the printing process.

[0078] After completing the preparation work, a block forming experiment was conducted using the above parameters to obtain a block forming part. After completion, the block forming part was allowed to cool naturally before being taken out. The block was then cut off from the substrate using a wire cutting machine. After cleaning with anhydrous ethanol in an ultrasonic cleaner for 20 minutes, the part was taken out and air-dried.

[0079] The density was measured using the Archimedes displacement method to obtain the density of each specimen, providing a basis for the classification of bulk energy density.

[0080] The surface roughness was measured at three points on the block part using a super depth-of-field microscope, and the average value was obtained.

[0081] After completing the roughness measurement, the block is inlaid and then polished with 200#, 400#, 600#, 800#, 1000#, 1200#, 1500# and 2000# sandpaper in sequence. Alumina polishing liquid is used for polishing and diamond polishing paste is used for grinding until a mirror effect is achieved. The internal forming condition is then observed.

[0082] Based on the surface roughness, density, and internal condition, the volume energy density was ultimately classified as follows: volume energy density less than 35 J / mm². 3 Time A: Incompletely melted zone (density <96%). Typical morphology, defects, and internal forming quality of the upper surface of this zone are as follows: Figure 1 As shown in (c) and (d), the surface of the formed part has poor metallic luster and many small black spheres. The block also contains numerous irregular defects. This is because the low volume energy density prevents the powder from completely melting, leaving unmelted powder residue inside the defects. These defects are the main reason for the low density of the formed block. When studying defects such as irregular pores inside the block, the volume energy density of this region can be used; the volume energy density is 35-42 J / mm². 3 Time B: Low energy density spheroidized region (96% < density < 99%). Typical morphology, defects, and internal forming quality of the upper surface of this region are as follows: Figure 2 As shown in (c) and (d), the edges of the melt channels on the bulk surface are still not smooth, and there is a lot of spheroidization on the surface, but there is no discontinuity. The surface roughness is high, and the internal defects are also reduced. When studying problems such as bulk spheroidization, the volume energy density in this energy density region can be selected; the volume energy density is 42-80 J / mm². 3 Time C: Volume energy density smooth forming region (99% < density). Typical morphology, defects, and internal forming quality of the upper surface of this region are as follows: Figure 3 As shown in (c) and (d), the upper surface of the block is smooth with virtually no spheroidization or powder adhesion, and the interior is almost free of defects. The overlap and melt depth are ideal, and the formed block has a high density. When studying the best properties of the block or applying it to actual production, the volume energy density within this region can be selected. The volume energy density is 80-100 J / mm². 3 Time D: High energy density region (98% < density < 99%). Typical morphology, defects, and internal forming quality of the upper surface in this region are as follows: Figure 4As shown in (c) and (d), numerous cracks perpendicular to the scanning direction were observed on the upper surface of the block. These cracks are extremely rare in other volume energy density regions. Judging from their propagation direction, the cracks primarily originate from thermal stress during the cooling process of the formed part. Observation reveals that the internal defects are mainly manifested as cracks. This is because, although the energy density is high, it is insufficient to remelt all cracks; therefore, the internal cracks accumulate layer by layer, developing into the main defects in this region. When studying defects such as cracks, the volume energy density within this region can be selected; a volume energy density greater than 100 J / mm² is recommended. 3 Time E: Volume energy density and overmelting (density <98%). Typical morphology, defects, and internal forming quality of the upper surface of this region are as follows: Figure 5 As shown in (c) and (d), the volume energy density is extremely high at this point. The remelting process causes most of the internal cracks to disappear. Since internal cracks cannot accumulate, the surface cracks are also somewhat improved in the high-energy-density zone. However, many cracks still remain on the surface. Observing the inside of the formed part, there are many nearly circular pores, but there are no similar pores on the surface. This is because some low-melting-point components vaporize inside the alloy due to the excessively high energy density, or because the molten pool is too deep. Excessive deep-pore effect prevents some gases introduced into the molten pool from escaping in time. Simultaneously, the high-energy laser causes severe remelting of the bottom layer, causing the bottom pores to merge with new pores, thus forming large-sized nearly circular pores. The volume energy density in this region can be used when studying defects such as pores. The area energy density region is divided as follows: Figure 8 As shown in the figure, the volume energy density ranges for the five regions are marked, which makes it more intuitive to select the volume energy density range.

[0083] Example 4.

[0084] Step 4: Numerical simulation of the selected area laser melting pool is performed, and the simulation is verified by comparing the single-pass width of Example 1 as the basis for line energy density partitioning. The reliability of the line energy density partitioning is verified by comparing the numerical simulation results with the experimental results. A three-dimensional powder bed model with random distribution of powder particles is established based on the discrete element method (DEM). The powder bed model is as follows: Figure 9 As shown, when establishing the powder bed model, the powder is no longer considered as a continuous volume, but rather modeled based on the actual powder particle size and its distribution ratio, resulting in the following... Figure 9 The powder bed model shown.

[0085] In this embodiment 4, the simulation uses a Gaussian surface heat source model in the simulation software, the principle of which is as follows: Figure 10 As shown, the Gaussian surface heat source is mathematically represented in the program as follows:

[0086]

[0087] Where A is the laser absorptivity of the metal powder; P is the laser power (W); R is the laser spot radius (μm); and r represents the distance from a point on the powder surface to the center of the laser spot (variable 50-103μm).

[0088] The selected laser melting temperature field distribution is based on a three-dimensional instantaneous heat conduction model, as follows: Figure 11 As shown, this three-dimensional instantaneous heat conduction model is used in the simulation to obtain the internal temperature field distribution of a single-channel simulation. The uniformity of the temperature field distribution is used to determine the quality of powder melting and forming. This three-dimensional instantaneous heat conduction model is as follows:

[0089]

[0090] Wherein, ρ, the density of the alloy, is 444 kg / m³. 3 c, specific heat capacity of the alloy, J / (K·kg); T, powder surface temperature, K; k, thermal conductivity of the alloy; Q, heat of formation per unit volume, J / m 3 .

[0091] In the simulation, gravity, surface tension, and steam recoil force are used as boundary conditions for the molten pool. The formula for surface tension is:

[0092]

[0093] Where, σ s0 σ is the surface tension coefficient at the initial temperature; S T denoted as surface tension temperature sensitivity; r is the radius of curvature of the powder surface, μm; T0 is the ambient temperature, K.

[0094] As the laser power increases, the metal vapor escapes towards the powder layer. At this time, a vertically downward recoil pressure is applied to the surface of the metal powder. The expression for the vapor recoil pressure is:

[0095]

[0096] Where P0 is the steam pressure, Pa; L lv The latent heat of vaporization of the material is expressed in J / kg; T lv Let K be the boiling point temperature of the material.

[0097] The parameters used in the simulation process of Example 4 are shown in Table 1, and the thermophysical property parameters of TC4 alloy are shown in Table 4.

[0098] Table 4. Thermophysical property parameters of TC4 alloy

[0099]

[0100]

[0101] Using the above parameters, start the simulation with computer software. After obtaining the simulation results, compare and study the simulation results with the experimental results.

[0102] The simulated variation patterns are consistent with the experimental results. For example, the simulated and experimental melt pool widths at the same energy density are compared. Figure 12 As shown, the weld pool width is also similar to the experimental results. Five groups of single-channel weld pools were simulated using the same parameters as those in the typical morphology diagram of the single-channel weld pool in Example 1. The effects of different energy densities on the weld pool width, depth, and morphology were studied. The effect of energy density on the weld pool morphology is shown in the figure below. Figure 13 and Figure 14 As shown.

[0103] The above-described embodiment 1 combines laser power and scanning speed to form a linear energy density; embodiment 2 combines laser power, scanning speed, and scanning spacing to form a surface energy density; and embodiment 3 combines laser power, scanning speed, scanning spacing, and powder thickness to form a surface energy density. This solves the problem of the cumbersome process of simply listing all parameters and completes the regional division of linear energy density, surface energy density, and volume energy density. Figure 1 , Figure 2 , Figure 3 , Figure 4 and Figure 5The explanation clearly shows the intuitive effect of dividing the energy density into lines, surfaces, and volumes. After the region division is completed, the corresponding regions can be easily found when studying defects and performance. Each region has its own characteristics. In the regions where the energy density of the lines, surfaces, and volumes is not fully melted, there are unmelted powder particles adhering to the surface of the formed part, and a large amount of spheroidization, resulting in poor continuity of the melt channel. In the regions where the energy density of the lines, surfaces, and volumes is low and spheroidized, there is a large amount of spheroidization in the lines, surfaces, and volumes, and the edges of the melt channel are distorted and deformed. In the regions where the energy density of the lines, surfaces, and volumes is successfully formed, the single channels are full, the edges of the melt channel are smooth and continuous, the single surface is smooth with a "fish scale" pattern, there is no spheroidization or collapse, the interior of the block has no pores, the surface is very smooth, there are no defects such as spheroidization, and the density is high. In the regions where the energy density of the lines, surfaces, and volumes is high, the single channels are full, the edges of the melt channel are smooth and continuous, the single surface is smooth with a "fish scale" pattern, there is no spheroidization or collapse, the block has no pores, the surface is very smooth, there are no defects such as spheroidization, and the density is high. Both single-sided and block-shaped sections exhibit significant collapse, with spatter-like protrusions along the edges of the melt channel. The smoothness of the edges is lower than in the smoothly formed area. Due to the higher energy density and extremely rapid cooling process of selective laser melting in this region, numerous cracks perpendicular to the scanning direction are observed on the surface of the formed part. In the over-melting area of ​​the linear, surface, and volume energy density, the surface collapse is more severe for single-channel, single-sided, and block-shaped sections, with longer and more numerous collapses. The spatter-like protrusions on both sides of the melt channel are significantly increased. Due to the extremely high energy density at this point, the remelting effect causes most of the internal cracks to remelt and disappear. Since internal cracks cannot accumulate, the surface cracks are also improved compared to the higher energy density area, but surface cracks still exist, and there are unexploded pores inside the part. The characteristics corresponding to each of the above regions are obvious. In practical applications or experimental research, the required linear, surface, and volume energy density regions and ranges can be quickly determined based on all the necessary characteristics of the formed part.

[0104] Example 4 uses computer simulation to corroborate the division of line energy density. Example 4 demonstrates the reliability of the line energy density region division, showing that the simulation characteristics are consistent with the experimental characteristics, and that the characteristics also differ in different regions.

[0105] In selective laser melting experiments and practical engineering applications of TC4 alloy, this embodiment divides the energy density into five regions: incomplete melting zone, low-energy-density spheroidization zone, smooth forming zone, high-energy-density zone, and over-melting zone. This allows for quick and accurate selection of different energy density regions based on actual needs, facilitating rapid and accurate engineering applications or experiments, thereby reducing the overall processing time. When studying the performance of high-quality formed parts, parameter combinations within the smooth forming zone are directly selected. For studying defects such as spheroidization, cracks, and unmelted powder, the low-energy-density spheroidization zone is chosen. For studying defects such as spheroidization, spatter, porosity, and thermal stress cracks, the high-energy-density zone is selected. This avoids numerous repetitive experiments; instead, experiments are directly conducted using the corresponding parameters, saving experimental materials.

[0106] The forming quality of incompletely melted and over-melted zones at line, surface, and volume energy densities is extremely poor, thus lacking practical application value. Defect research is crucial for a comprehensive understanding of the defect generation patterns and performance impact of selective laser melting (SLM) formed parts. Therefore, incompletely melted and over-melted zones at line, surface, and volume energy densities can be used for defect research. When studying larger defects such as incomplete melting of single-pass, single-layer, or blocky powders, the incompletely melted zones at line, surface, and volume energy densities can be directly selected. When studying defects such as collapse and internal porosity, the over-melted zones at line, surface, and volume energy densities can be directly selected. The appropriate line, surface, and volume energy densities can be directly selected based on the type of defect to be studied, eliminating the need for repeated experiments and simplifying the research process.

Claims

1. A method for dividing the multidimensional energy density region in the selective laser melting (TC4) process, characterized in that, The specific steps include: Step 1: Using laser power P and scanning speed v as experimental parameters, conduct a single-channel experiment; Combining the two parameters from a single-channel experiment, we obtain the linear energy density E. L The linear energy density model is: ； Among them, E L ρ is the linear energy density, J / mm²; P is the laser power, W; v is the scanning speed, mm / s. The width of the melt channel was measured using an ultra-depth-of-field microscope, and the single-pass forming quality was observed. Based on the measurement and observation results, the line energy density region was divided into the following areas: A1: incompletely melted line energy density region; B1: low line energy density spheroidizing region; C1: smoothly formed line energy density region; D1: high line energy density region; and E1: over-melted line energy density region. This step divides the line energy density region, defines different energy density regions for single-channel performance and defect studies, and provides parameter values ​​for laser power and scanning speed for area energy density studies. Step 2: Within the linear energy density smooth forming region defined in Step 1, select the laser power P and scanning speed v, and adjust the corresponding scanning spacing h during single-layer experiments to achieve the purpose of adjusting the surface energy density. The scanning interval h is set according to the overlap rate of 30%, 40%, 50%, 60% and 70% of the melt channel width in the smooth forming zone of the line energy density in step 1; By combining the three parameters—laser power, scanning speed, and scanning spacing—required for a single layer, the surface energy density Es is obtained. The surface energy density model is as follows: ； Among them: E S Surface energy density, J / mm 2 P represents laser power, in W. v h is the scanning speed, in mm / s; h is the scanning interval, in mm. Surface roughness was measured using an ultra-depth-of-field microscope, and the forming quality of a single layer surface was observed. Based on the measurement and observation results, the surface energy density region was divided into the following areas: A2: incompletely melted area; B2: low energy density spheroidized area; C2: smoothly formed area; D2: high energy density area; and E2: over-melted area. This step delineates the surface energy density region, defining different energy density regions for single-layer performance and defect studies, while providing parameter values ​​for laser power, scanning speed, and scanning spacing for volume energy density studies. Step 3: After the surface energy density region is divided in Step 2, select a set of experimental parameters at the boundary between the surface energy density smooth forming region and the surface energy density low energy density spheroidizing region, a set of experimental parameters at the boundary between the surface energy density smooth forming region and the surface energy density high energy density region, a set of experimental parameters with the lowest surface roughness in the single-layer experiment, and a set of experimental parameters with the energy density of the surface energy density smooth forming region located at the most central position. Based on these four sets of parameters, change the powder thickness and conduct bulk forming experiments with powder thicknesses of 20μm, 30μm, 40μm, 50μm, 60μm, and 70μm. By combining the four parameters required for bulk forming—laser power, scanning speed, scanning spacing, and powder thickness—the volume energy density Ev is obtained. The volume energy density model is as follows: ； Among them: E V Volume energy density, J / mm 3 P is the laser power (W); v is the scanning speed (mm / s); h is the scanning distance (mm); t is the powder thickness (mm). The surface roughness of the upper surface was measured using an ultra-depth-of-field microscope to observe the forming quality of the upper surface and interior. The density of the formed block was measured using the Archimedes displacement method. Based on the measurement and observation results, the volume energy density region was divided into four areas: A3: incompletely melted volume energy density region, B3: low-energy-density spheroidized volume energy density region, C3: smoothly formed volume energy density region, D3: high-energy-density volume energy density region, and E3: over-melted volume energy density region. This step divided the volume energy density region and obtained the surface roughness, density, and internal defects of the formed block. The volume energy density was then divided into incompletely melted volume energy density region, low-energy-density spheroidized volume energy density region, smoothly formed volume energy density region, high-energy-density volume energy density region, and over-melted volume energy density region. Finally, the division of the line energy density region, surface energy density region, and volume energy density region for selective laser melting TC4 was completed. Step 4: Using the parameters used in Step 1, set the same line energy density and forming conditions as the single-pass experiment, perform numerical simulation of the single-pass experiment, and compare the simulation results with the single-pass experiment results. The melt channel width and surface morphology at the same line energy density are consistent with the single-pass experiment results, thereby verifying the line energy density classification.

2. The method for dividing the multidimensional energy density region in the selective laser melting (TC4) process according to claim 1, characterized in that, In steps 1 and 2, the powder thickness remained constant at 40 μm for both single-pass and single-layer experiments.

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