A method for the pretreatment of metal powders for additive manufacturing

Abstract

The application discloses a pretreatment method of metal powder for additive manufacturing, and comprises the following steps: loading the metal powder into a powder cylinder of an additive manufacturing device, heating a forming base plate to a preset temperature by using a high-energy heat source; layer-by-layer pretreating the metal powder on a forming cylinder platform under a high-vacuum and inert atmosphere environment until all the metal powder in the powder cylinder is treated; taking out the metal powder after the metal powder is cooled to a certain temperature in the furnace, and performing screening; and determining the resistivity value of the pretreated metal powder and the preset resistivity value. By the method, the conductivity of the powder can be improved without affecting the purity of the powder, the "blowing powder" phenomenon can be inhibited, and the forming process can be stably performed.

Description

Technical Field

[0001] This invention relates to the field of additive manufacturing technology, and more specifically to a pretreatment method for metal powders used in additive manufacturing. Background Technology

[0002] Powder bed electron beam additive manufacturing is an additive manufacturing method that uses an electron beam as a high-energy heat source. Electrons are accelerated by a high-voltage electric field and deflected by deflection coils, causing the high-energy electron beam to bombard metal powder, thus melting and shaping the powder layer by layer. This technology offers advantages such as high energy utilization, a high-vacuum and clean forming environment, and low forming stress, and is widely used in aerospace, automotive, and biomedical fields.

[0003] However, powder bed electron beam additive manufacturing technology faces a problem during the forming process: "powder blowing," which occurs when powder particles deviate from their original positions under the influence of the electron beam, affecting the printing process or even halting it. The surface of metal powder is an electrically insulating oxide layer. Powder particles accumulate through point contacts. After the electron beam irradiates the metal powder surface, charges accumulate on the oxide layer, awaiting redistribution. If the accumulated charges cannot be dissipated in time, excessive Coulomb repulsion between the powder particles leads to "powder blowing." Clearly, the conductivity of the metal powder plays a crucial role.

[0004] Currently, to solve the "powder blowing" problem, metal powders are typically prepared using methods such as gas atomization and plasma rotating electrode methods. By controlling the oxygen content of the powder and grading the powder particle size, combined with appropriate preheating and melting processes, stable forming can be achieved. For example, patent CN108580881A proposes to uniformly mix metal powder with a conductive material (a mixture of graphene and carbon fiber) through ball milling. Under heating, graphene has a certain binding property, which can form linear and surface conductivity, allowing the negative charge on the surface of the metal powder to be transferred rapidly, improving conductivity and thus solving the powder blowing problem. However, this process introduces impurity elements and causes changes in powder morphology and oxygen content due to ball milling, which can affect the performance of the formed parts. Furthermore, this technology cannot completely solve the powder blowing problem. Summary of the Invention

[0005] This invention provides a pretreatment method for metal powder used in additive manufacturing. This method can improve the conductivity of the powder and suppress the "powder blowing" phenomenon without affecting the purity of the powder, thus ensuring the stable progress of the forming process.

[0006] The technical solution provided by this invention is as follows:

[0007] A pretreatment method for metal powder used in additive manufacturing includes the following steps:

[0008] Step 1: Load the metal powder into the powder cylinder of the additive manufacturing device, and use a high-energy heat source to heat the formed substrate to the preset temperature;

[0009] Step 2: Use a powder spreading device to spread the metal powder evenly on the forming substrate. In a high vacuum and inert atmosphere, pre-treat the metal powder on the forming cylinder platform layer by layer until all the metal powder in the powder cylinder has been treated.

[0010] Step 3: After the metal powder cools to a certain temperature in the furnace, the metal powder is taken out and sieved to obtain the first powder, which is a pre-treated metal powder.

[0011] Step 4: Test the resistivity value of the first powder to obtain a first resistivity value. Compare the first resistivity value with a preset second resistivity value. If the first resistivity value is less than or equal to the second resistivity value, the first powder can be directly used for the stable forming of additive manufacturing parts. If the first resistivity value is greater than the second resistivity value, the first powder is pre-treated and resistivity is determined again until the resistivity value of the metal powder after the second pre-treatment is less than or equal to the second resistivity value, or the number of times the first powder is pre-treated and resistivity is determined again reaches the target value.

[0012] Furthermore, in step one: the metal powder is an intermetallic compound powder, which is a TiAl alloy powder or a NiTi alloy powder.

[0013] Furthermore, in step one, the metal powder is prepared by gas atomization or plasma rotating electrode method;

[0014] The high-energy heat source is an electron beam or a combination of an electron beam and a laser.

[0015] The vacuum degree of the forming chamber of the additive manufacturing apparatus is 1.0 to 2.5 × 10⁻⁶. -1 Pa, the forming chamber is filled with helium or argon, and the vacuum degree of the forming chamber can be adjusted by adjusting the flow rate of the inert atmosphere.

[0016] Further, in step one, the high-energy heat source is an electron beam, and the electron beam is used to heat the forming substrate. The preheating current is 10-40mA, the electron beam scanning speed is 10-15m / s, the preheating time is 50-70min, and the preset temperature of the forming substrate is 950-1200℃.

[0017] In step two, the pretreatment of metal powder by electron beam is carried out by electron beam scanning of the powder bed. The electron beam scanning speed is 10-15 m / s, the scanning interval is 0.7-1.5 mm, the electron beam spot defocusing amount is 0.2-0.4 V, the preheating current is 40-48 mA, the preheating time is 30-80 s, the number of preheating stages is 2-10, and the initial preheating current is 10-30 mA.

[0018] Furthermore, in step two, during the layer-by-layer pretreatment of the metal powder, the forming substrate is lowered by a certain height after each layer of metal powder is treated. The lowering height of each layer of the forming substrate is 50-120 μm, and the amount of powder taken from each layer is 50-120 μm.

[0019] Furthermore, in step three, after the metal powder is cooled to a certain temperature in the furnace, the metal powder is taken out and sieved, specifically as follows:

[0020] After the metal powder cools down to ≤60℃ in the furnace, the metal powder is cleaned out and sieved using a sieve with the corresponding mesh size according to the required particle size.

[0021] Furthermore, in step four, the resistivity value of the first powder is tested using a four-probe method.

[0022] Furthermore, in step four, the second resistivity value is 400–800 mΩ·cm.

[0023] Furthermore, in step four, the number of times the first powder is pretreated and resistivity determined again reaches the target value specifically includes:

[0024] The number of times n is to pre-treat the first powder again is determined based on the first resistivity value and the second resistivity value of the first powder, where n is an integer of 2 ≤ n ≤ 5.

[0025] Compared with the prior art, the beneficial effects of the present invention are:

[0026] 1. This invention uses a high-energy heat source to pretreat metal powder in a high vacuum and inert atmosphere, which improves the conductivity of the metal powder and effectively suppresses the occurrence of "powder blowing" phenomenon, thereby achieving stable powder bed formation.

[0027] 2. This invention will not introduce other impurities into the metal powder, and the pretreated metal powder still meets the requirements for powder printing. Attached Figure Description

[0028] Figure 1 This is a schematic diagram illustrating the use of electron beam pretreatment of powder in an embodiment of the present invention;

[0029] Figure 2This is a statistical chart showing the number of powder blowing times during the pretreatment of Ti-48Al-2Cr-2Nb alloy powder in this embodiment of the invention.

[0030] Figure 3 This is a graph showing the resistivity of Ti-48Al-2Cr-2Nb alloy powder before and after pretreatment with pressure in an embodiment of the present invention.

[0031] Figure 4 The images shown are XRD patterns of Ti-48Al-2Cr-2Nb alloy powder before and after pretreatment in this embodiment of the invention.

[0032] Figure 5 The images shown are surface morphology diagrams of Ti-48Al-2Cr-2Nb alloy powder before and after pretreatment in an embodiment of the present invention, wherein (a) is the surface morphology diagram of Ti-48Al-2Cr-2Nb alloy powder before pretreatment and (b) is the surface morphology diagram of Ti-48Al-2Cr-2Nb alloy powder after pretreatment. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of this application, not all embodiments. The components of the embodiments of this application described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0034] Therefore, the detailed description of the embodiments of this application provided below with reference to the accompanying drawings is intended merely to illustrate selected embodiments of this application and is not intended to limit the scope of protection claimed by this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

[0035] This invention provides a pretreatment method for metal powder used in additive manufacturing, comprising the following steps:

[0036] Step 1: Load the metal powder into the powder cylinder of the additive manufacturing device, and use a high-energy heat source to heat the formed substrate to the preset temperature;

[0037] Step 2: Use a powder spreading device to spread the metal powder evenly on the forming substrate. In a high vacuum and inert atmosphere, pre-treat the metal powder on the forming cylinder platform layer by layer until all the metal powder in the powder cylinder has been treated.

[0038] Step 3: After the metal powder cools to a certain temperature in the furnace, the metal powder is taken out and sieved to obtain the first powder, which is a pre-treated metal powder.

[0039] Step 4: Test the resistivity value of the first powder to obtain the first resistivity value. Compare the first resistivity value with the preset second resistivity value. If the first resistivity value is less than or equal to the second resistivity value, the first powder can be directly used for the stable forming of additive manufacturing parts. If the first resistivity value is greater than the second resistivity value, the first powder is pre-treated and resistivity is determined again until the resistivity value of the metal powder after the second pre-treatment is less than or equal to the second resistivity value, or the number of times the first powder is pre-treated and resistivity is determined again reaches the target value.

[0040] The pretreatment of the metal powder is completed through steps one through four.

[0041] Optionally, in step one: the metal powder is an intermetallic compound powder, such as TiAl alloy powder or NiTi alloy powder.

[0042] Optionally, in step one, the metal powder is prepared by gas atomization or plasma rotating electrode method.

[0043] The high-energy heat source is an electron beam, a combination of an electron beam and a laser, or other high-energy heat sources.

[0044] The vacuum degree of the forming chamber of the additive manufacturing apparatus is 1.0–2.5 × 10⁻⁶. -1 Pa, the forming chamber is filled with inert gases such as helium or argon, and the vacuum level of the forming chamber can be adjusted by regulating the flow rate of the inert atmosphere.

[0045] It should be emphasized that in this application, the vacuum degree of the forming chamber can be adjusted by adjusting the flow rate of the inert atmosphere. The inert atmosphere can protect the powder from oxidation on the one hand, and neutralize the electrons generated when the electron beam is lowered, thereby reducing the risk of powder blowing.

[0046] Optionally, the high-energy heat source used in step one is an electron beam. The electron beam is used to heat the forming substrate. The preheating current is 10-40mA, the electron beam scanning speed is 10-15m / s, the preheating time is 50-70min, and the preset temperature of the forming substrate is 950-1200℃.

[0047] In step two, the pretreatment of the metal powder using an electron beam is performed by scanning the powder bed with an electron beam. The electron beam scanning speed is 10–15 m / s, the scanning interval is 0.7–1.5 mm, the electron beam spot defocusing amount is 0.2–0.4 V, the preheating current is 40–48 mA, the preheating time is 30–80 s, the number of preheating stages is 2–10, and the initial preheating current is 10–30 mA. These parameter ranges have been determined through numerous optimizations to ensure that powder blowing does not occur during the pretreatment process and that the pretreatment process can proceed stably.

[0048] The absolute value of the defocusing amount indicates the degree of focusing of the electron beam spot (unit: volts, V). The smaller the absolute value of the defocusing amount, the more focused the electron beam spot (the degree of focusing is highest at 0V). When the defocusing amount is 0.2-0.4V, the electron beam spot is in a defocused state, which is beneficial for the removal of charge from the powder bed. The number of preheating stages indicates the number of steps in the change of electron beam current. For example, when the number of preheating stages is 4, the changes in preheating current are: 10mA (initial preheating current), 19.5mA, 29mA, 38.5mA, 48mA (preheating current), and the preheating time at each step current is 1-3s. After the preheating current reaches 48mA, the preheating current is kept constant. The stepped preheating method can reduce the impact of the electron beam on the powder bed, and cause micro-sintering of the powder bed under a small preheating current, thereby improving the conductivity of the powder bed to resist the impact of a large preheating current on the powder bed and reducing the risk of "powder blowing".

[0049] Optionally, in step two, during the layer-by-layer pretreatment of the metal powder, the forming substrate is lowered by a certain height after each layer of metal powder is treated. The lowering height of each layer of the forming substrate is 50–120 μm, and the amount of powder taken per layer (the lifting height of the powder cylinder per layer) is 50–120 μm. As the number of powder pretreatments increases, the conductivity and anti-collapse ability of the powder improve, and the lowering height or the amount of powder taken per layer of the forming substrate can be increased. For example, the lowering height of each layer of the forming substrate during pretreatment can be fixed at 50 μm, 80 μm, etc., and the amount of powder taken can be gradually increased, but it is not limited to this.

[0050] Optionally, in step three, after the metal powder cools down to a certain temperature with the furnace, the metal powder is taken out and sieved. Specifically, after the metal powder cools down to a temperature ≤60℃ with the furnace, the metal powder is cleaned out and sieved using a sieve with the corresponding mesh size according to the metal powder particle size requirements.

[0051] Optionally, in step four, the resistivity value of the first powder is tested using a four-probe method.

[0052] Optionally, in step four, the second resistivity value is 400–800 mΩ·cm.

[0053] Optionally, in step four, the number of times the first powder undergoes pretreatment and resistivity determination reaches the target value. Specifically, this involves determining the number of times, n, the first powder will be pretreated again based on its first and second resistivity values, where n is an integer between 2 and 5. Here, n represents the number of times the powder pretreatment and resistivity comparison are repeated. The value of n is an empirical value, ensuring that after n treatments, the first resistivity value of the metal powder is less than the second resistivity value. In actual production, repeating this process 2-3 times is sufficient to complete the pretreatment of the metal powder.

[0054] Example 1

[0055] This embodiment provides a pretreatment method for metal powder used in additive manufacturing, including the following steps:

[0056] Step 101: Load Ti-48Al-2Cr-2Nb alloy powder into the powder cylinder of the additive manufacturing device. After evacuating the forming chamber, purge it with helium gas to a vacuum level of 2.3 × 10⁻⁶. -1 Pa, using an electron beam to heat the shaped substrate to 1100°C.

[0057] Step 102: Use a powder spreading device to spread the alloy powder evenly on the forming substrate, and use an electron beam high-energy heat source to pre-treat the alloy powder on the forming cylinder platform layer by layer until all the alloy powder in the powder cylinder has been treated. Figure 1 This is a schematic diagram of pre-treating alloy powder layer by layer on a forming cylinder platform using an electron beam high-energy heat source.

[0058] Step 103: Cool the alloy powder to be treated in the furnace to below 60°C, remove the alloy powder, and sieve it using a 100-mesh sieve to obtain pretreated -100-mesh alloy powder.

[0059] Step 104: Test the pretreated alloy powder using the four-probe method to obtain the first resistivity value. Compare the first resistivity value with the preset resistivity value. If the first resistivity value is greater than the preset resistivity value (800 mΩ·cm), continue the powder pretreatment and repeat steps one to three. After pretreatment n times (preferably, 2≤n≤5, where n is an integer), if the resistivity value is less than the preset resistivity value (800 mΩ·cm), it can be used for the stable forming of additive manufacturing parts.

[0060] In step 101, the Ti-48Al-2Cr-2Nb alloy powder is prepared using a plasma rotating electrode method.

[0061] In step 101, the resistivity of the initial Ti-48Al-2Cr-2Nb alloy powder is 6700 mΩ·cm, which is much higher than the resistivity threshold (800 mΩ·cm).

[0062] In step 101, the preheating current of the forming substrate varies in the range of 15 to 40 mA, showing a gradual increasing trend. The electron beam scanning speed is 12 m / s, and the preheating time is 60 min.

[0063] The parameters that need to be set in step 102 are: electron beam scanning speed 12m / s, scanning spacing 0.8mm; electron beam spot defocusing amount 0.35V, preheating current 48mA, preheating time 45s, preheating stage number 8, and initial preheating current 10mA.

[0064] In step 102, the powder pretreatment process differs from the preheating process in solid part printing. Due to the poor conductivity of the original powder, improper pretreatment process parameters can cause "powder blowing," preventing the pretreatment process from proceeding. Therefore, during pretreatment, it is necessary to increase the helium filling rate (the vacuum pressure in the forming chamber is 2.3 × 10⁻⁶). -1 On the one hand, increasing the number of preheating stages (8 stages) and the amount of decoking (0.35V) reduces the risk of "powder blowing" and enables the powder pretreatment to proceed stably.

[0065] In step 102, the descent height of each layer of the forming substrate is 50 μm, and the powder amount taken per layer (the lifting height of each layer of the powder cylinder) is 80 μm.

[0066] In step 103, the treated alloy powder is furnace cooled under vacuum and a protective atmosphere to prevent oxidation.

[0067] In step 104, when n=2, the resistivity of the processed alloy powder is 224mΩ·cm, which is lower than the preset resistivity threshold and can be directly used for the stable forming of additive manufacturing parts.

[0068] In step 104, for alloy powder that can be stably formed into solid parts after pretreatment, the risk of "powder blowing" is reduced due to the improved conductivity of the alloy powder. Therefore, when forming solid parts, on the one hand, the amount of helium gas introduced can be appropriately reduced to reduce gas consumption; on the other hand, the number of preheating stages (4 stages) and the defocusing amount (0.25V) can be reduced, and the initial preheating current (20mA) can be increased to reduce the preheating time, thereby improving the printing efficiency.

[0069] Figure 2 The powder blowing phenomenon of Ti-48Al-2Cr-2Nb alloy powder before and after pretreatment was statistically analyzed. It was found that after two pretreatments, the printing process proceeded stably, and powder blowing no longer occurred. Comparing the resistivity changes of the powder before and after pretreatment, the resistivity of the powder decreased significantly after two pretreatments. Figure 3 ).

[0070] XRD analysis was performed on the phase composition of TiAl alloy powders before and after pretreatment, such as... Figure 4 As shown, the results indicate that the peak intensity of the γ phase in the powder is significantly increased after pretreatment, corresponding to an increase in the γ phase content and a decrease in the α phase content. The resistivity of the γ phase (TiAl phase) is lower than that of the α phase (Ti3Al phase), meaning that the higher the γ phase content in the powder, the better the powder conductivity. The surface morphology of the powder before and after pretreatment is shown in the figure. Figure 5 As shown, sintering necks formed between a small number of powder particles after pretreatment, which facilitates the dispersion of charge on the powder bed. It is noteworthy that the sintering necks between these small amounts of powder particles do not significantly affect the mechanical properties of the formed parts.

[0071] In summary, the reasons why electron beam pretreatment of intermetallic compound powders can suppress the "powder blowing" phenomenon may be as follows: Firstly, the increased content of the γ phase in the powder after pretreatment due to its good conductivity improves the powder's conductivity to some extent. Secondly, the formation of sintering necks between powder particles enhances the conductivity of the powder bed.

[0072] The above description is merely the preferred embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A pretreatment method for metal powder used in additive manufacturing, characterized in that, Includes the following steps: Step 1: Load the metal powder into the powder cylinder of the additive manufacturing device, and use a high-energy heat source to heat the formed substrate to the preset temperature; Step 2: Use a powder spreading device to spread the metal powder evenly on the forming substrate. In a high vacuum and inert atmosphere, pre-treat the metal powder on the forming cylinder platform layer by layer until all the metal powder in the powder cylinder has been treated. Step 3: After the metal powder cools to a certain temperature in the furnace, the metal powder is taken out and sieved to obtain the first powder, which is a pre-treated metal powder. Step 4: Test the resistivity of the first powder to obtain a first resistivity value. Compare the first resistivity value with a preset second resistivity value. If the first resistivity value is less than or equal to the second resistivity value, the first powder can be directly used for the stable forming of additive manufacturing parts. If the first resistivity value is greater than the second resistivity value, the first powder is pre-treated and resistivity is determined again until the resistivity value of the metal powder after the second pre-treatment is less than or equal to the second resistivity value, or the number of times the first powder is pre-treated and resistivity is determined again reaches the target value. The second resistivity value is 400–800 mΩ·cm; The number of times the first powder is pretreated and resistivity is determined again to reach the target value is specifically as follows: The number of times n is to pre-treat the first powder again is determined based on the first resistivity value and the second resistivity value of the first powder, where n is an integer of 2 ≤ n ≤ 5; In step one, the high-energy heat source is an electron beam. The electron beam is used to heat the forming substrate. The preheating current is 10-40mA, the electron beam scanning speed is 10-15m / s, the preheating time is 50-70min, and the preset temperature of the forming substrate is 950-1200℃. In step two, the pretreatment of metal powder by electron beam is carried out by electron beam scanning of the powder bed. The electron beam scanning speed is 10-15 m / s, the scanning interval is 0.7-1.5 mm, the electron beam spot defocusing amount is 0.2-0.4 V, the preheating current is 40-48 mA, the preheating time is 30-80 s, the number of preheating stages is 2-10, and the initial preheating current is 10-30 mA.

2. The pretreatment method for metal powder used in additive manufacturing according to claim 1, characterized in that, In step one: the metal powder is an intermetallic compound powder, which is TiAl alloy powder or NiTi alloy powder.

3. The pretreatment method for metal powder used in additive manufacturing according to claim 1, characterized in that, In step one, the metal powder is prepared by gas atomization or plasma rotating electrode method; The high-energy heat source is an electron beam or a combination of an electron beam and a laser. The vacuum degree of the forming chamber of the additive manufacturing apparatus is 1.0 to 2.5 × 10⁻⁶. -1 Pa, the forming chamber is filled with helium or argon, and the vacuum degree of the forming chamber is adjusted by regulating the flow rate of the inert atmosphere.

4. The pretreatment method for additive manufacturing metal powder according to any one of claims 1-3, characterized in that, In step two, during the layer-by-layer pretreatment of the metal powder, the forming substrate is lowered by a certain height after each layer of metal powder is treated. The lowering height of each layer of the forming substrate is 50-120 μm, and the amount of powder taken from each layer is 50-120 μm.

5. The pretreatment method for metal powder used in additive manufacturing according to claim 4, characterized in that, In step three, after the metal powder cools to a certain temperature in the furnace, it is taken out and sieved, specifically as follows: After the metal powder cools down to ≤60℃ in the furnace, the metal powder is cleaned out and sieved using a sieve with the corresponding mesh size according to the required particle size.

6. The pretreatment method for metal powder used in additive manufacturing according to claim 4, characterized in that, In step four, the resistivity value of the first powder is tested using a four-probe method.

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