Additive manufacturing method for improving powder absorptivity through ultrafast laser in-situ induction microstructure

Abstract

According to the additive manufacturing method for improving the powder absorptivity through the ultrafast laser in-situ induction microstructure, metal powder is pretreated in a selected area through ultrafast laser, a micro-nano structure close to the ultrafast laser wavelength scale is manufactured on the surface of the metal powder, the micro-nano structure can generate the near-field enhancement effect on common near-infrared laser, and the powder absorptivity is improved through the near-field enhancement effect on the common near-infrared laser. The near-infrared laser absorption rate of metal powder can be effectively increased, long-pulse laser or continuous laser is allowed to melt the area with low power, severe flowing and splashing of a molten pool caused by excessive heat input are avoided, and formation of internal air holes is reduced. The heat accumulation of peripheral materials is reduced, the thermal gradient is favorably controlled, and the residual stress is reduced; a large amount of powder is prevented from adhering to the surface and surface quality is improved.

Description

Technical Field

[0001] This invention belongs to the field of laser application technology, and in particular relates to an additive manufacturing method for improving powder absorption rate by in-situ induced microstructure using ultrafast laser. Background Technology

[0002] High thermal conductivity and low density metal additive manufacturing components have great application potential in aerospace, electronics and information technology fields. For example, copper is used in rocket engine combustion chamber liners, motor rotors, and antibacterial devices, while aluminum is used in radio frequency component manufacturing and heat dissipation systems. However, the high reflectivity of these metal powders severely restricts the quality of their laser powder bed melting manufacturing. It is known that the reflectivity of copper and aluminum to commonly used near-infrared lasers can reach 0.96 to 0.98, causing most of the incident laser energy to be reflected. This not only wastes energy but also means that the reflected high-energy laser may damage optical components.

[0003] To address the issue of high reflectivity in some metallic materials, current strategies primarily involve increasing the output power of continuous lasers, employing high-energy continuous lasers with an output power exceeding 400W for additive manufacturing. Furthermore, altering the laser wavelength can effectively improve the energy absorption rate of some materials (e.g., copper), and using green light allows for high-quality additive manufacturing of copper.

[0004] Ultrafast lasers, with their extremely short pulse widths and extremely high peak power, can induce modification on the surface of metallic materials to obtain micro- and nanostructures compared to long-pulse and continuous-wave lasers. When metals are irradiated with ultrafast lasers near their ablation threshold, a series of electromagnetic-thermal-fluidic effects occur. The extremely strong photoelectric field causes free electrons in the metal to oscillate in concert, creating localized changes in free electron density. This change excites a new electric field on the surface. Under the coupling of the excited electric field and the photoelectric field, a mixed wave propagating along the surface, called surface plasmons, is formed. This distribution of surface plasmons leads to the non-uniform deposition of laser energy at subwavelength periods. The energy of the ultrafast laser first induces a non-equilibrium state between electrons and phonons within the metal, causing a rapid increase in electron temperature. This energy is then transferred to the crystal lattice via electro-acoustic coupling, a process that occurs on the picosecond timescale. The non-uniform energy deposition generated by surface plasmons leads to a non-uniform distribution of lattice temperature, creating a complex temperature gradient within the molten pool. Ultimately, this induces the Marangoni effect within the molten pool. After cooling and nucleation, the shallow molten pool forms micro- and nanostructures on the metal surface.

[0005] Regarding the solutions for additive manufacturing of high-reflectivity powders mentioned above, when using high-energy continuous lasers to additively manufacture high-reflectivity materials, the reflected high-energy laser light has very strong energy, which can easily damage optical components and endanger the safety of equipment and experimental operators. While changing the laser wavelength is suitable for copper, materials such as aluminum are not sensitive to changes in laser processing wavelength, resulting in poor material applicability. Therefore, simply increasing laser power and changing the laser wavelength is insufficient to solve the additive forming problem of high-reflectivity metal materials; new manufacturing processes are urgently needed to overcome the bottleneck of low energy utilization in high-reflectivity metals. Summary of the Invention

[0006] To address the issues of low energy absorption and high heat requirements for melting of high-reflectivity metal powders in laser powder bed additive manufacturing, this invention provides an additive manufacturing method that enhances powder absorption by in-situ induction of microstructures using ultrafast lasers. This method utilizes the light-trapping structure and near-field enhancement effect generated by ultrafast lasers inducing micro- and nano-structures on the surface of metal powders to enhance the absorption of long-pulse lasers and continuous lasers.

[0007] An additive manufacturing method for enhancing powder absorption rate by in-situ microstructure induced by ultrafast laser includes the following steps:

[0008] S1: Based on the set laser powder bed additive slicing data and scanning path, an ultrafast laser with a difference between the output laser flux and the laser flux corresponding to the ablation threshold of the metal powder to be processed is less than a set value to scan the area to be formed of the metal powder, so that the surface of the metal powder in the area to be formed forms a microstructure, wherein the absorption rate of the metal powder with surface microstructure is greater than that of the metal powder without surface microstructure.

[0009] S2: Based on the set laser powder bed additive slicing data and scanning path, a long pulse laser or continuous laser is used to scan the metal powder with surface microstructure, thereby melting the metal powder and completing the printing of the current area to be formed;

[0010] S3: Repeat steps S1 to S2 until the additive manufacturing of metal powder is completed.

[0011] Furthermore, the set value is 5% of the laser flux corresponding to the ablation threshold of the metal powder to be processed.

[0012] Furthermore, given the output laser flux, the method for determining the equivalent number of pulses in the forming region of the metal powder using ultrafast laser scanning is as follows:

[0013] Before the additive manufacturing experiment began, different values ​​were determined for the repetition frequency, spot diameter, and scanning speed within a set range, resulting in triplets with different values; where the number of equivalent pulses = repetition frequency × spot diameter / scanning speed;

[0014] Ultrafast lasers were set according to each ternary group, so that the metal powder was scanned by different ultrafast lasers, resulting in metal powders with different absorption rates.

[0015] The number of equivalent pulses corresponding to the triplet with the maximum absorption rate is taken as the final number of equivalent pulses used in the additive manufacturing test.

[0016] Furthermore, the ultrafast laser is a femtosecond laser or a picosecond laser, and the long-pulse laser is a near-infrared long-pulse laser.

[0017] Furthermore, the ultrafast laser is a femtosecond laser or a picosecond laser, and the continuous laser is a near-infrared continuous laser.

[0018] Furthermore, the metal powder is pre-spread evenly in the forming cylinder of the powder bed system. The ultrafast laser passes through a polarizer, an attenuator, and a first mechanical shutter in sequence, and then enters the powder bed system through a dichroic mirror. Finally, it is focused onto the area of ​​the metal powder to be formed in the forming cylinder through a galvanometer and a field mirror in sequence. The long-pulse laser or continuous laser passes through a second mechanical shutter, enters the powder bed system through a dichroic mirror, and is finally focused onto the area of ​​the metal powder to be formed in the forming cylinder through a galvanometer and a field mirror in sequence.

[0019] Furthermore, after the metal powder surface in the forming area forms a microstructure, the first mechanical shutter of the ultrafast laser is closed by computer control, while the second mechanical shutter of the long pulse laser or continuous laser is opened by computer control, and scanning is performed according to the set laser powder bed additive slicing data.

[0020] Furthermore, the ultrafast laser is coaxial with the long-pulse laser or continuous laser, but does not scan the metal powder simultaneously.

[0021] Beneficial effects:

[0022] 1. This invention provides an additive manufacturing method for improving powder absorption rate by in-situ induced microstructures using ultrafast lasers. The method utilizes an ultrafast laser to pre-treat metal powder in a selected area, fabricating micro / nano structures on the metal powder surface at near-ultrafast laser wavelengths. These micro / nano structures exhibit a near-field enhancement effect on common near-infrared lasers, effectively increasing the absorption rate of the metal powder to near-infrared lasers. This allows long-pulse or continuous lasers to melt the area at lower power, avoiding excessive heat input that could cause violent flow and splashing of the molten pool, reducing internal porosity formation; reducing heat accumulation in surrounding materials, which is beneficial for controlling the thermal gradient and reducing residual stress; and preventing excessive powder adhesion to the surface, thus improving surface quality.

[0023] 2. This invention provides an additive manufacturing method for improving powder absorption rate by in-situ induced microstructure using ultrafast laser. It utilizes low-power long-pulse laser or continuous laser irradiation of the ultrafast laser pretreatment area to achieve low heat input melting, precisely control the melting area and heat accumulation, effectively reduce violent flow and splashing of the molten pool, reduce thermal gradient and stress in the forming area, reduce defects such as surface powder adhesion, and improve forming quality. Attached Figure Description

[0024] Figure 1 A schematic diagram of an apparatus for an ultrafast laser in-situ method of improving powder reflectivity in laser powder bed fusion additive manufacturing provided by the present invention;

[0025] Figure 2 A schematic diagram illustrating the principle of a laser powder bed fusion additive manufacturing method for in-situ enhancement of powder absorption rate using ultrafast laser, provided by the present invention.

[0026] Figure 3 A schematic diagram illustrating the beneficial effects of the laser powder bed fusion additive manufacturing method for in-situ enhancement of powder absorption rate by ultrafast laser provided by the present invention;

[0027] 1-Ultrafast laser system, 2-Polarizer, 3-Attenuator, 4-First mechanical shutter, 5-Dichroic mirror, 6-Reflector, 7-Galvanometer and field mirror, 8-Metal powder, 9-Powder bed system, 10-Computer control system. Detailed Implementation

[0028] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings.

[0029] Ultrafast lasers induce micro / nano structures on metal surfaces, which possess light-trapping and near-field enhancement capabilities, significantly altering the surface's absorption characteristics. Therefore, this invention utilizes ultrafast lasers to easily induce micro / nano structures on highly reflective metals such as copper and aluminum. Inducing micro / nano structures on the surface of highly reflective metal powders enhances the powder's absorption rate to both long-pulse and continuous-wave lasers, achieving low-heat-input, high-energy-absorption-rate shaping.

[0030] Specifically, the present invention provides an additive manufacturing method for improving powder absorption rate by in-situ induced microstructure using ultrafast lasers, comprising the following steps:

[0031] S1: Based on the set laser powder bed additive slicing data and scanning path, an ultrafast laser is used to scan the area to be formed of the metal powder using an ultrafast laser where the difference between the output laser flux and the laser flux corresponding to the ablation threshold of the metal powder to be processed is less than a set value. This results in the formation of microstructures on the surface of the metal powder in the area to be formed. The absorption rate of the metal powder with surface microstructures is greater than that of the metal powder without surface microstructures. The set value is 5% of the laser flux corresponding to the ablation threshold of the metal powder to be processed.

[0032] Furthermore, given the output laser flux, the method for determining the equivalent number of pulses in the forming region of the metal powder using ultrafast laser scanning is as follows:

[0033] Before the additive manufacturing experiment began, different values ​​were determined for the repetition frequency, spot diameter, and scanning speed within a set range, resulting in triplets with different values; where the number of equivalent pulses = repetition frequency × spot diameter / scanning speed;

[0034] Ultrafast lasers were set according to each ternary group, so that the metal powder was scanned by different ultrafast lasers, resulting in metal powders with different absorption rates.

[0035] The number of equivalent pulses corresponding to the triplet with the maximum absorption rate is taken as the final number of equivalent pulses used in the additive manufacturing test.

[0036] It should be noted that this invention uses near-ablation threshold energy on the laser powder surface for parameter studies to find a suitable equivalent pulse number range for the metal powder. Generally, the repetition frequency and spot diameter are kept constant, and the equivalent pulse number is changed by altering the scanning speed. Parameter studies have shown that the absorption rate of the pretreated powder can be increased by 30% to 70% without powder splashing.

[0037] In other words, this invention uses an ultrafast laser (picosecond laser or femtosecond laser) close to the ultrafast laser ablation threshold of the material to in-situ process metal powder to manufacture surface micro-nano structures within the equivalent pulse number range corresponding to the material. This reduces the reflectivity of the powder, enhances the absorption capacity of the metal powder to long-pulse lasers and continuous lasers, increases the absorption rate of the metal powder to near-infrared light by 30%-70%, reduces the power required for forming by 70%-30%, increases the energy utilization efficiency of the laser, and avoids damage to the objective lens by high-energy reflected lasers.

[0038] It should be noted that for different materials, a series of parameter experiments should be conducted in advance to determine the ablation threshold corresponding to the ultrafast laser, and the range of equivalent pulse numbers that can achieve effective powder pretreatment at the energy corresponding to that ablation threshold. For example, within a suitable range of equivalent pulse numbers, the reflectivity of copper powder can be reduced by 30% to 70%; however, below this range, the reflectivity of the powder remains essentially unchanged, and the laser absorption rate cannot be improved; above this range, due to excessive energy accumulation in the powder, the powder is ablated and dispersed, affecting the powder spreading state and resulting in poor forming quality. Ultrafast laser energies far below the ablation threshold will result in an inability to effectively change the reflectivity, while ultrafast laser energies far above the ablation threshold will result in the continuous destruction of the powder spreading state. Limiting the set value to 5% of the laser flux corresponding to the ablation threshold of the metal powder to be processed can effectively reduce reflectivity while maintaining the powder spreading state.

[0039] In other words, the pretreatment energy of ultrafast lasers is not limited to the energy corresponding to the ablation threshold. Slightly higher than the ablation threshold is beneficial for microstructure formation, but excessively high ultrafast laser energy can cause a single ultrafast pulse to disrupt the conditions for microstructure formation, reducing the absorption rate of the structure. It is worth mentioning that changing the galvanometer scanning speed is more stable than changing the process window of ultrafast laser energy. Therefore, it is best to directly limit the ultrafast laser energy within ±1.05% of the laser flux corresponding to the ablation threshold.

[0040] S2: Based on the set laser powder bed additive slicing data and scanning path, a low-power long-pulse laser or continuous laser is used to scan the metal powder with surface microstructure, thereby melting the metal powder and completing the printing of the current area to be formed.

[0041] It should be noted that the low-power long-pulse laser or continuous laser used in this invention refers to a reduction of approximately 50% in power compared to the long-pulse laser or continuous laser power required for melting metal powders without micro- or nano-structures in traditional methods. Furthermore, the power reduction varies depending on the metal material. For example, when additively manufacturing copper metal powder, under the same processing conditions, traditional methods require long-pulse lasers or continuous lasers with power exceeding 400W. However, using the method of this invention, under the same conditions, high-quality forming of copper can be achieved using a long-pulse laser or continuous laser between 100W and 200W. For other metal powders, such as Ti6Al4V, under the same processing conditions, traditional methods require long-pulse lasers or continuous lasers of approximately 200W to 300W to achieve additive manufacturing, while this invention can achieve additive manufacturing using a long-pulse laser or continuous laser of approximately 100W.

[0042] S3: Repeat steps S1 to S2 until the additive manufacturing of metal powder is completed.

[0043] Figure 1 This is a schematic diagram of an apparatus for an ultrafast laser in-situ method for improving powder reflectivity in laser powder bed fusion additive manufacturing, as proposed in this invention. It includes an ultrafast laser source, a half-wave plate, an attenuator, a mechanical shutter, a reflector, a dichroic mirror, a powder bed system, a galvanometer, a field mirror, and a computer control system. The long-pulse laser or continuous laser system is not shown. The illumination, imaging, and other systems are all integrated into the powder bed apparatus.

[0044] based on Figure 1 The system flow of the additive manufacturing method is given below:

[0045] Step 1: Import the printed model slice data, set the powder bed system powder spreading equipment parameters, and set the long pulse laser or continuous laser process parameters and scanning strategy for material melting;

[0046] Step 2: Use a polarizer to adjust the ultrafast laser to linear polarization, and use a continuously adjustable attenuator to adjust the incident ultrafast laser energy to ensure that the ultrafast laser flux after being focused by the field lens is near the laser flux corresponding to the ablation threshold of the material. After passing through the polarizer, the ultrafast laser passes through the first mechanical shutter and dichroic mirror and is then incident on the powder bed system. Finally, it is focused by the galvanometer and field lens onto the area of ​​the metal powder to be formed in the forming cylinder.

[0047] It should be noted that the present invention requires the scanning speed and repetition frequency of the ultrafast laser to be determined in advance, and the ultrafast laser scanning path to be set according to the model slice data in step one, so as to ensure that the area to be formed can be scanned by the ultrafast laser.

[0048] Step 3: The powder bed system powder spreading equipment works according to the set parameters. The forming cylinder descends to a certain height, the powder cylinder is raised and the metal powder is fed in, and the powder spreading cart spreads it evenly in the forming cylinder.

[0049] Step 4: The computer controls the opening of the first mechanical shutter of the ultrafast laser optical path, and controls the galvanometer to scan the metal powder in the area to be formed according to the ultrafast laser scanning parameters in Step 2, so as to obtain metal powder with surface micro-nano structure.

[0050] Step 5: The computer-controlled ultrafast laser first mechanical shutter closes, while the long-pulse laser or continuous laser second mechanical shutter opens. That is, the ultrafast laser and the long-pulse laser or continuous laser do not scan the metal powder simultaneously. Then, based on the slicing data, laser process parameters, and scanning strategy in Step 1, the current layer is scanned, melting the metal powder with surface micro-nano structures in the area to be formed, thus completing the printing of this layer.

[0051] In other words, after the long pulse laser or continuous laser passes through the second mechanical shutter, it is incident on the powder bed system via a dichroic mirror, and finally focused onto the area to be formed of the metal powder in the forming cylinder by the galvanometer and field mirror in sequence, and scanned according to the set laser powder bed additive slicing data.

[0052] Step Six: Repeat Steps Three, Four, and Five to complete high-quality, high-precision additive manufacturing of metal powder under low heat input conditions.

[0053] Figure 2 (a) shows a schematic diagram of ultrafast laser pulse irradiation of metal powder and the resulting metal powder with surface microstructure; (b) shows the electromagnetic field response of long-pulse laser or continuous laser acting on the surface structure. For ease of comparison, Figure 2 In (b), the background electric field strength was set to 1 V / m. The simulation showed that the electric field strength near the micro-nano structure on the powder surface was enhanced by 2.5 times, indicating that this near-field enhancement effect can improve the metal's ability to absorb laser light.

[0054] Under the same forming size requirements Figure 3 (a) is a schematic diagram of high heat input to powder bed melting of metal powder without pretreatment. Excessive heat input leads to a large heat-affected zone, violent fluctuations in the molten pool, severe splashing, and severe powder adhesion on the surface of the resulting product, which is prone to defects. Figure 3 (b) is a schematic diagram of low heat input powder bed melting of pretreated metal powder. Precise control of heat input reduces violent flow and splashing in the molten pool, resulting in a smooth surface, a small heat-affected zone, and high forming quality.

[0055] In summary, this invention provides an additive manufacturing method that uses ultrafast lasers to in-situ induce microstructures to enhance powder absorption. By using ultrafast lasers to pre-treat metal powder in selected areas to enhance absorption, it avoids the instability, high cost, and material selectivity associated with directly using short-wavelength lasers. It also avoids the process complexity, side reactions, and low processing efficiency caused by using etching or other methods to pre-treat all powder.

[0056] Of course, the present invention may have other various embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and modifications according to the present invention, but these corresponding changes and modifications should all fall within the protection scope of the appended claims.

Claims

1. A method of additive manufacturing of ultrafast laser induced in-situ microstructure to enhance powder absorption, characterized in that, The method comprises the following steps: S1: based on the set laser powder bed additive slicing data and scanning path, using ultrafast laser to scan the forming area of the metal powder, the difference between the output laser flux and the ablation threshold of the metal powder to be processed is less than the set value, so that the surface of the metal powder in the forming area forms a microstructure, wherein the absorption rate of the metal powder with surface microstructure is greater than that of the metal powder without surface microstructure; S2: based on the set laser powder bed additive slicing data and scanning path, using long pulse laser or continuous laser to scan the metal powder with surface microstructure to melt the metal powder, and complete the printing of the current forming area; S3: repeat steps S1-S2 until the metal powder additive manufacturing is completed.

2. The method of claim 1, wherein the method is a method of additive manufacturing of ultrafast laser induced microstructure in-situ to enhance powder absorption. The set value is 5% of the laser flux corresponding to the ablation threshold of the metal powder to be processed.

3. The method of claim 1, wherein the method is a method of additive manufacturing of ultrafast laser induced in-situ microstructure to enhance powder absorption. Under a given output laser flux, the determination method of the equivalent pulse number of the ultrafast laser scanning the forming area of the metal powder is as follows: Before the start of the additive manufacturing test, different values of the repetition frequency, spot diameter and scanning speed are determined in a set range to obtain different triplets; wherein the equivalent pulse number = repetition frequency × spot diameter / scanning speed; According to each triplet, the ultrafast laser is set so that the metal powder is scanned by different ultrafast lasers, and metal powders with different absorption rates are obtained; The equivalent pulse number corresponding to the triplet corresponding to the maximum absorption rate is used as the equivalent pulse number finally used in the additive manufacturing test.

4. The method of claim 1, wherein the method is a method of additive manufacturing of ultrafast laser-induced microstructures in-situ to enhance powder absorption, wherein the method comprises: The ultrafast laser is femtosecond laser or picosecond laser, and the long pulse laser is near-infrared long pulse laser.

5. The method of claim 1, wherein the method is a method of additive manufacturing of ultrafast laser induced microstructure in situ to enhance powder absorption. The ultrafast laser is femtosecond laser or picosecond laser, and the continuous laser is near-infrared continuous laser.

6. The method of claim 1, wherein the method is an ultrafast laser-induced in-situ microstructure for enhancing powder absorption in additive manufacturing. The metal powder is uniformly and flatly pre-prepared in the forming cylinder of the powder bed system, the ultrafast laser passes through the polarizer, the attenuator and the first mechanical shutter in sequence, then enters the powder bed system through the dichroic mirror, and finally focuses on the forming area of the metal powder in the forming cylinder through the galvanometer and the field lens in sequence; the long pulse laser or the continuous laser passes through the second mechanical shutter, then enters the powder bed system through the dichroic mirror, and finally focuses on the forming area of the metal powder in the forming cylinder through the galvanometer and the field lens in sequence.

7. The method of claim 1, wherein the method is a method of additive manufacturing of ultrafast laser-induced microstructures in-situ to enhance powder absorption. After the surface of the metal powder in the forming area forms a microstructure, the first mechanical shutter of the ultrafast laser is closed by computer control, and the second mechanical shutter of the long pulse laser or the continuous laser is opened by computer control, and scanning is performed according to the set laser powder bed additive slicing data.

8. The method of claim 1, wherein the method is an ultrafast laser-induced in-situ microstructure for enhancing powder absorption in additive manufacturing. The ultrafast laser and the long pulse laser or the continuous laser are coaxial and do not scan the metal powder at the same time.

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