Modification method of low-oxygen-content special steel powder for additive manufacturing

Abstract

The invention belongs to the field of powder metallurgy, and relates to a powder modification method for additive manufacturing, in particular to a method for reducing the oxygen content of powder by reducing the thickness of an oxide film on the surface of special steel powder prepared through gas atomization or plasma rotating electrode atomization for additive manufacturing and removing water vapor and inclusions attached to the surface of the powder at the same time. Atomized special steel powder to be modified is placed in a feeding device of jet mill equipment, high-pressure gas of nitrogen or argon is introduced into the jet mill equipment, the temperature of the high-pressure gas in a grinding cavity is controlled, the atomized special steel powder is subjected to high-speed airflow action, mutual collision and friction in the grinding cavity, and powder modification is achieved; the thickness of an oxidation film on the surface of the obtained jet mill modified special steel powder is obviously reduced; the inclusion content of the obtained jet mill modified special steel powder is remarkably reduced, and the sphericity degree of the powder is improved; water vapor on the surface of the obtained jet mill modified special steel powder is removed, and the oxygen content of the powder is reduced.

Description

Technical Field

[0001] The present invention belongs to the field of powder metallurgy and relates to a powder modification method for additive manufacturing, and in particular to a method for reducing the thickness of an oxide film on the surface of a special steel powder prepared by gas atomization or plasma rotating electrode atomization for additive manufacturing, while removing water vapor and inclusions attached to the surface of the powder, thereby reducing the oxygen content of the powder. Technical Background

[0002] Metal additive manufacturing (3D printing) technology can achieve precise, near-net-shape preparation of complex structural materials, solving the problems of low efficiency and waste of raw materials in the production of parts by traditional preparation methods, and has therefore attracted widespread attention. At present, most special steel powders used in additive manufacturing are prepared by gas atomization or plasma rotating electrode atomization processes. During the atomization process, the surface of the droplets is easily reacted with trace oxygen in the chamber, resulting in the formation of an oxide film on the surface of the powder. At the same time, oxide inclusions in the original molten steel or in the bar will adhere to the surface of the solidified powder particles or mix in the powder, resulting in poor sphericity of the powder, reduced fluidity, and increased oxygen content. In addition, the powder may also absorb moisture from the air during storage and transportation, especially in high humidity environments. The presence of water vapor on the powder surface will increase the viscosity of the powder, reduce the fluidity of the powder, and increase the oxygen content of the powder. The performance of the powder raw material greatly affects the performance of the printed parts. Therefore, the thinning of the oxide film on the surface of the atomized special steel powder, the removal of oxide inclusions and water vapor are crucial for the preparation of high-performance special steel structures by additive manufacturing.

[0003] At present, the thinning of the oxide film on the powder surface is mainly carried out by chemical methods, such as pickling, atmosphere reduction, etc., which have complicated steps, high costs, and difficult to handle waste liquid and waste gas. Inclusions in the powder are mainly removed by vibration screening, which is inefficient and has limited separation effect on powders and inclusions with similar particle sizes. The removal of water vapor in the powder relies on long-term vacuum drying, the amount of powder processed at a single time is limited, and the cycle is long. It can be seen that at this stage, it is urgent to develop a method that can reduce the thickness of the oxide film on the powder surface in large quantities, with high efficiency and simple process, and remove water vapor and inclusions on the surface of atomized special steel powder for additive manufacturing, so as to further improve the quality and performance of printed parts. Summary of the invention

[0004] In view of the above problems, the present invention uses air flow milling technology to modify the special steel powder atomized by gas atomization and plasma rotating electrode atomization, and removes water vapor and inclusions on the powder surface by thinning the oxide film on the powder surface, thereby improving the powder fluidity and reducing the oxygen content. The basic principle is that the high-temperature inert gas in the air flow mill equipment causes the powder to continuously collide and rub against each other in the grinding area under high grinding pressure, and the water vapor is quickly removed. The collision force and shear force between the particles thin the oxide film on the surface of the steel powder, crush and refine the brittle inclusions, and then the crushed fine particles are screened out through a grading funnel to achieve the modification of the special steel powder with low oxygen content. The present invention has a short process flow, low cost, high efficiency, and can be mass-produced.

[0005] The present invention solves the technical problem by adopting the following technical solutions:

[0006] A method for modifying special steel powder with low oxygen content for additive manufacturing, characterized in that it comprises the following steps:

[0007] Step 1) placing the atomized special steel powder to be modified in the feeding device of the airflow mill, and controlling the feeding frequency at 5-15 Hz; the atomized special steel powder has a thick oxide film on the surface, irregular inclusions in the powder, and a small amount of water vapor attached to the surface of the powder, so the powder has a high oxygen content and poor fluidity;

[0008] Step 2) introducing high-pressure gas into the jet mill, wherein the pressure range of the high-pressure gas is 0.40 MPa to 0.60 MPa, and the high-pressure gas is one of nitrogen and argon;

[0009] Step 3) controlling the temperature of the high pressure gas in the grinding chamber to be between 120°C and 200°C;

[0010] Step 4) The atomized special steel powder is subjected to high-speed airflow in the grinding chamber, causing collision and friction with each other for 10 to 25 minutes to achieve powder modification;

[0011] Step 5) Control the rotating speed of the classifying wheel to be 1000-12000 r / min, collect the modified steel powder from the discharge port of the jet mill, and then vacuum seal and package it.

[0012] Furthermore, the atomized special steel powder is prepared by gas atomization or plasma rotating electrode atomization, and the powder particle size ranges from 15 to 150 μm and is normally distributed.

[0013] Furthermore, the pressure of the high-pressure gas is controlled at 0.40 MPa~0.60 MPa, and the processing time is controlled at 10~25 minutes, and is slightly adjusted according to the average particle size of the processed steel powder, so as to thin the oxide film on the powder surface and break up the brittle inclusions without changing the morphology of the spherical steel powder.

[0014] Furthermore, the temperature of the high-pressure gas in the grinding chamber is controlled by a temperature regulating device at an air inlet of the chamber, and the temperature regulating device includes a heating element and a cooling element.

[0015] Furthermore, the morphology and particle size distribution of the modified steel powder collected remain basically unchanged, and the yield is above 95%.

[0016] Furthermore, the modified steel powder collected has a significantly reduced oxide film thickness, a significantly reduced number of inclusions in the powder, a reduced powder oxygen content, and improved powder fluidity. The powder can be directly used for additive manufacturing without screening and vacuum drying.

[0017] The technical effects of the present invention are as follows:

[0018] (1) The thickness of the oxide film on the surface of the special steel powder modified by jet milling obtained by the present invention is significantly reduced;

[0019] (2) The inclusion content of the air flow milled modified special steel powder obtained by the present invention is significantly reduced, and the sphericity of the powder is improved;

[0020] (2) The water vapor on the surface of the special steel powder modified by jet milling obtained by the present invention is removed, the oxygen content of the powder is reduced, and the fluidity is improved;

[0021] (4) The morphology and particle size distribution of the air flow milled modified special steel powder obtained in the present invention remain basically unchanged, and can be directly used for additive manufacturing without screening and vacuum drying. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The scanning electron microscope morphology images of the gas atomized steel powder before and after the air flow mill treatment in Example 1 of the present invention are as follows: (a) original powder; (b) after the air flow mill treatment.

[0023] Figure 2 Transmission electron microscope morphology images of the oxide film of the gas atomized steel powder before and after the air flow milling treatment in Example 2 of the present invention: (a) original powder; (b) after the air flow milling treatment.

[0024] Figure 3 The scanning electron microscope morphology images of the plasma rotating electrode atomized steel powder before and after the air flow milling treatment in Example 3 of the present invention are as follows: (a) original powder; (b) after the air flow milling treatment. DETAILED DESCRIPTION

[0025] The advantages and benefits of the present invention will be more apparent to those skilled in the art by reading the following detailed description of the preferred embodiments.

[0026] Example 1

[0027] 1. Using gas atomized special steel fine powder as raw material, the powder median diameter D50 is 28.0 μm, the oxygen content is 0.055wt.%, and the fluidity is 23.4 s / 50g. Scanning electron microscope observation shows that there are inclusions in the powder, such as Figure 1 As indicated by the arrow in (a).

[0028] 2. The powder raw materials were subjected to jet milling treatment, with high-purity nitrogen as the gas source, the gas pressure was 0.50 MPa, the gas was heated to 120 °C, the sorting wheel speed was 6000 r / min, and the treatment time was 12 min.

[0029] 3. The powder after jet milling is vacuum sealed and packaged, and the oxygen content, particle size distribution, fluidity and microscopic morphology of the treated powder are characterized.

[0030] 4. After testing, the median diameter D50 of the gas atomized special steel fine powder after air jet milling was 26.9 μm, the oxygen content was 0.036 wt.%, and the fluidity was 20.7 s / 50g. The particle size of the powder remained basically unchanged, while the oxygen content decreased and the fluidity increased.

[0031] 5. Scanning electron microscope observation shows that the number of powder inclusions after jet milling is significantly reduced and the sphericity of the powder is improved. Figure 1 (b) shown.

[0032] Example 2

[0033] 1. Using gas atomized special steel coarse powder as raw material, the powder median diameter D50 is 99.9 μm, the oxygen content is 0.038wt.%, the fluidity is 17.1 s / 50g, and there are many inclusions in the powder. Transmission electron microscopy observation shows that the thickness of the oxide film on the powder surface is about 7 nm. Figure 2 (a) shown.

[0034] 2. The powder raw materials were subjected to jet milling treatment, with high-purity nitrogen as the gas source, the gas pressure was 0.58 MPa, the gas was heated to 180 °C, the separation wheel speed was 2700 r / min, and the treatment time was 18 min.

[0035] 3. The powder after jet milling is vacuum sealed and packaged, and the oxygen content, particle size distribution and fluidity of the treated powder are characterized.

[0036] 4. After testing, the median diameter D50 of the gas atomized special steel coarse powder after air jet milling was 98.8 μm, the oxygen content was 0.023 wt.%, and the fluidity was 15.7 s / 50g. The particle size of the powder remained basically unchanged, while the oxygen content decreased and the fluidity increased.

[0037] 5. Transmission electron microscopy showed that the thickness of the oxide film on the powder surface after jet milling was significantly reduced to 3 nm. Figure 2 (b) shown.

[0038] Example 3

[0039] 1. Using plasma rotating electrode atomized special steel fine powder as raw material, the powder median diameter D50 is 35.3 μm, the oxygen content is 0.026 wt.%, and the fluidity is 17.6 s / 50g. Scanning electron microscope observation shows that there are irregular inclusions in the powder, such as Figure 3 As indicated by the arrow in (a).

[0040] 2. The powder raw materials were subjected to jet milling treatment, with high-purity argon as the gas source, the gas pressure was 0.48 MPa, the gas was heated to 150 °C, the separation wheel speed was 4800 r / min, and the treatment time was 10 min.

[0041] 3. The powder after jet milling is vacuum sealed and packaged, and the oxygen content, particle size distribution, fluidity and microscopic morphology of the treated powder are characterized.

[0042] 4. After testing, the median diameter D50 of the plasma rotating electrode atomized special steel fine powder after air flow milling was 35.2μm, the oxygen content was 0.020 wt.%, and the fluidity was 16.5 s / 50g. The particle size of the powder remained basically unchanged, while the oxygen content decreased and the fluidity increased.

[0043] 5. Scanning electron microscope observation shows that the number of powder inclusions after jet milling is significantly reduced, and the sphericity of the powder is increased to 99%. Figure 3 (b) shown.

Claims

1. A method for modifying special steel powder with low oxygen content for additive manufacturing, characterized in that: The following steps are involved: Step 1) placing the atomized special steel powder to be modified in the feeding device of the air flow mill, and controlling the feeding frequency at 5-15 Hz; a thick oxide film exists on the surface of the atomized special steel powder, irregular inclusions exist in the powder, and a small amount of water vapor is attached to the surface of the powder, so the oxygen content of the powder is high and the fluidity is poor; Step 2) high-pressure gas is introduced into the air flow mill, the pressure range of the high-pressure gas is 0.40 MPa-0.60 MPa, and the high-pressure gas is one of nitrogen and argon; Step 3) the temperature of the high-pressure gas in the grinding chamber is controlled to be between 120°C and 200°C; Step 4) the atomized special steel powder is subjected to the high-speed airflow in the grinding chamber, collides and rubs with each other, and the processing time is 10-25 minutes to achieve powder modification; Step 5) the rotating speed of the classifying wheel is controlled to be 1000-12000 r / min, the modified steel powder is collected from the discharge port of the air flow mill, and then vacuum-sealed and packaged.

2. The method for modifying special steel powder with low oxygen content for additive manufacturing according to claim 1, characterized in that: The atomized special steel powder is prepared by gas atomization or plasma rotating electrode atomization method, and the powder particle size ranges from 15 to 150 μm and is normally distributed.

3. The method for modifying special steel powder with low oxygen content for additive manufacturing according to claim 1, characterized in that: The pressure of the high-pressure gas is controlled at 0.40 MPa to 0.60 MPa, and the processing time is controlled at 10 to 25 minutes, and is slightly adjusted according to the average particle size of the processed steel powder, so as to thin the oxide film on the powder surface and break up brittle inclusions without changing the morphology of the spherical steel powder.

4. The method for modifying special steel powder with low oxygen content for additive manufacturing according to claim 1, characterized in that: The temperature of the high-pressure gas in the grinding chamber is controlled by a temperature regulating device at the chamber air inlet, and the temperature regulating device includes a heating element and a cooling element.

5. The method for modifying special steel powder with low oxygen content for additive manufacturing according to claim 1, characterized in that: The modified steel powder has the following advantages: the morphology and particle size distribution of the powder remain basically unchanged, and the yield is above 95%; the thickness of the oxide film on the powder surface is significantly reduced, the number of inclusions in the powder is significantly reduced, the oxygen content of the powder is reduced, and the powder fluidity is improved; The powder can be used directly in additive manufacturing without sieving and vacuum drying.

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