Method for in-situ control of microstructure and properties during additive manufacturing of duplex stainless steel

Abstract

This invention discloses a method for in-situ control of microstructure and properties during the additive manufacturing process of duplex stainless steel. The method primarily involves controlling the nitrogen content in an argon-nitrogen mixture to obtain mixed gases with different thermal conductivities, thereby altering the cooling rate during additive manufacturing and achieving control over both overall and gradient grain size. During gradient control, the nitrogen content gradually increases with the stacking layer height, resulting in finer ferrite grains and increased hardness, thus allowing for changes in microstructure and properties with stacking layer height. During overall control, the nitrogen content remains stable; for example, at 100% nitrogen content, the grain size of the printed ferrite is at its minimum of 15.32 μm, contributing to improved mechanical properties. The resulting duplex stainless steel is suitable for applications requiring high strength, certain plasticity, and corrosion resistance. Furthermore, the method is simple to operate and has low manufacturing costs, making it significant for the preparation of duplex stainless steel in complex structural components and gradient materials.

Description

Technical Field

[0001] This invention belongs to the field of additive manufacturing and relates to a method for in-situ control of microstructure and properties during the additive manufacturing process of duplex stainless steel. Background Technology

[0002] Duplex stainless steel combines the high strength of the ferritic phase with the good ductility and toughness of the austenitic phase, making it widely used in industries vital to national economy and people's livelihood, such as petrochemicals, shipbuilding, energy, construction, and papermaking. However, due to the incoordination of deformation between the two phases, duplex stainless steel has poor hot working properties. Traditional processes such as casting, hot rolling, and welding can cause numerous problems in forming duplex stainless steel, severely limiting its application in complex structural components. Meanwhile, with the rapid development of industry, many structural components face the need for both surface corrosion resistance and high internal strength, making gradient materials urgently needed by the industry.

[0003] Additive manufacturing technology, through layer-by-layer deposition, avoids complex plastic processing and allows for different process parameters to be set for different deposited layers according to the service environment, providing a new solution for expanding the application of duplex stainless steel in complex structural components and gradient materials. Analysis shows that the mechanical properties and corrosion resistance of duplex stainless steel increase with decreasing grain size. Based on solidification theory, faster cooling rates result in smaller critical radii for grain nucleation, increased nucleation rate, and refined grains. Cooling rate and thermal conductivity are closely related. Higher thermal conductivity means faster heat transfer, which in turn means a larger temperature gradient and a faster cooling rate. Commonly used protective gases in additive manufacturing include argon and nitrogen, with nitrogen having a higher thermal conductivity than argon. Therefore, by controlling the nitrogen content in the mixed protective gas, gases with different thermal conductivities can be obtained, thereby altering the cooling rate during manufacturing and changing grain size and properties. This is of great significance for in-situ control of the microstructure and properties of duplex stainless steel. Summary of the Invention

[0004] To address the shortcomings of existing preparation methods, this invention proposes a method for in-situ control of microstructure and properties during the additive manufacturing process of duplex stainless steel.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A method for in-situ control of microstructure and properties during the additive manufacturing process of duplex stainless steel involves adjusting the ratio of argon and nitrogen in the introduced mixed gas to control the overall grain size and gradient grain size.

[0007] Furthermore, the diameter of the duplex stainless steel powder used is 13-52 μm.

[0008] Furthermore, the mixed gas is prepared by simultaneously introducing pure nitrogen and pure argon into a gas mixing device for mixing.

[0009] Furthermore, the outlet flow rate of the gas mixing device should be slightly lower than the sum of the inlet flow rates of the two gases by 1-2 L / min, in order to promote thorough mixing of nitrogen and argon.

[0010] Furthermore, the gas mixing device is equipped with a gas sieve, thereby changing laminar flow into turbulent flow to further promote the thorough mixing of nitrogen and argon.

[0011] Furthermore, the nitrogen content in the mixed gas is 0-100%.

[0012] Furthermore, during the preparation process, the same proportion of mixed gas is always introduced into the same stacked layer for printing, so as to keep the overall grain size consistent.

[0013] Furthermore, during the preparation process, different stacked layers are obtained by adjusting the nitrogen content in the mixed gas during the printing process to achieve different grain sizes, thereby obtaining a material with gradient grain sizes.

[0014] Furthermore, during the preparation process, the oxygen content in the environmental chamber is less than 0.1%.

[0015] Furthermore, during the preparation process, the pressure inside the environmental chamber is controlled at 1.0-2.3 MPa.

[0016] Furthermore, during the preparation process, a pulsed laser is used as the heating source with a wavelength of 1070nm, the laser power is controlled between 70-300W, the scanning speed is controlled between 200-1200mm / s, and the overlap rate is controlled between 11-90%.

[0017] Furthermore, the scanning strategy employs a zigzag pattern with 90° rotation between adjacent layers, and the layer height is controlled at 0.03 mm.

[0018] The significant advantages of this invention are:

[0019] (1) This invention obtains gases with different thermal conductivity by adjusting the nitrogen content in the argon-nitrogen mixed gas, thereby changing the cooling rate during the preparation process and achieving changes in grain size and properties. Specifically, in a single preparation process, the same proportion of mixed gas is used for the same layer to achieve overall control of grain size, and different proportions of mixed gas are used for different stacked layers to achieve gradient control of grain size, thereby improving the overall performance of the duplex stainless steel obtained by printing, and thus meeting the application requirements under different service environments.

[0020] (2) When gradient control is adopted, as the height of the stacked layer increases, the nitrogen content is gradually increased from 0% to 100%, the ferrite grain size gradually becomes finer, and the hardness gradually increases, so that the microstructure and properties can change with the height of the stacked layer. When overall control is adopted, such as when the nitrogen content is 100%, the grain size of the printed ferrite is the minimum value of 15.32μm, the tensile strength can reach 910MPa, and the elongation after fracture exceeds 13%, which further improves the mechanical properties.

[0021] (3) Compared with existing methods for controlling grain size, this method has advantages such as simple process flow, low production cost, high production efficiency and flexible control of grain size, and has significant technical advantages in controlling grain size. Attached Figure Description

[0022] Figure 1 This is a schematic diagram illustrating the control of overall grain size (a) and gradient grain size (b) in this invention.

[0023] Figure 2 The microstructure of the samples prepared with different nitrogen contents in the overall control of Example 1 is shown (austenite at the grain boundaries and ferrite in the grains), where (a) 0%, (b) 20%, and (c) 100%.

[0024] Figure 3 This is a statistical chart of grain size corresponding to samples prepared with different nitrogen contents during the overall control of Example 1.

[0025] Figure 4 The graph shows the changes in tensile strength of samples prepared with different nitrogen contents during the overall control of Example 1.

[0026] Figure 5 The graph shows the changes in elongation after fracture of samples prepared with different nitrogen contents during the overall control of Example 1.

[0027] Figure 6 The microstructure diagrams of different stacked layers prepared by adjusting the nitrogen content in Example 2 (austenite at the grain boundaries and ferrite within the grains) are shown, where (a) is at a layer thickness of 1.5 mm, (b) is at a layer thickness of 3.5 mm, (c) is at a layer thickness of 5.5 mm, (d) is at a layer thickness of 7.5 mm, (e) is at a layer thickness of 9.5 mm, and (f) is at a layer thickness of 11.5 mm.

[0028] Figure 7 This is a statistical diagram of the grain size of each stacked layer prepared by adjusting the nitrogen content in Example 2.

[0029] Figure 8 This is a diagram showing the Vickers hardness variation of each packing layer prepared by adjusting the nitrogen content in Example 2. Detailed Implementation

[0030] To make the content of this invention easier to understand, the technical solution of this invention will be further described below with reference to specific embodiments, but this invention is not limited thereto.

[0031] A method for in-situ control of microstructure and properties during the additive manufacturing process of duplex stainless steel, comprising the following steps:

[0032] 1) Before preparation, add an appropriate amount of duplex stainless steel metal powder with a diameter of 13-52μm to the powder tank;

[0033] 2) Before preparation, pure argon and pure nitrogen are introduced into the two inlets of the gas mixing device, respectively. The device has a gas sieve to ensure uniform gas mixing. The inlet flow rate is adjusted by a pressure reducing valve and a flow meter to control the content of argon and nitrogen in the outlet mixed gas.

[0034] 3) During preparation, such as Figure 1 The same proportion of mixed gas is always introduced into the same deposit layer for printing to keep the overall grain size consistent; different deposit layers have different grain sizes by adjusting the nitrogen content in the mixed gas during the printing process, thus obtaining a material with a gradient grain size.

[0035] Wherein, the duplex stainless steel powder mentioned in step 1) is 2205 duplex stainless steel powder, and the mass percentage of each element contained therein is C≤0.03%, Si≤1.0%, Mn≤2.0%, S≤0.02%, P≤0.03%, Cr: 22.0-23.0%, Ni: 4.5-6.5%, Mo: 3.0-3.5%, N: 0.14-0.2%, with the remainder being Fe and trace impurities.

[0036] Step 2) The gas mixing device is equipped with a gas screen to change laminar flow into turbulent flow; at the same time, the outlet flow rate should be slightly lower than the sum of the inlet flow rates of the two gases by 1-2 L / min.

[0037] Step 3) The nitrogen content in the mixed gas is 0-100%. The oxygen content in the environmental chamber is less than 0.1%, and the pressure is controlled at 2.0 MPa. A pulsed laser is used as the heating source with a wavelength of 1070 nm, a laser power of 230 W, a scanning speed of 400 mm / s, and an overlap rate of 66%. The scanning strategy adopts a zigzag pattern with 90° rotation between adjacent layers, and the layer height is controlled at 0.03 mm.

[0038] Example 1

[0039] In the single additive manufacturing process, printing was performed using nitrogen contents of 0%, 20%, and 100%, respectively. After cooling to room temperature, the cross-section of the metallographic sample was etched to obtain microstructure images, and the grain size was statistically analyzed using the intercept method. Room temperature tensile tests were conducted on a computer-controlled electronic universal testing machine at a tensile speed of 2 mm / min, using three parallel samples and taking the average value.

[0040] Figure 2-5 The effects of overall control on the microstructure and mechanical properties of the samples in Example 1 are shown respectively. Figure 2 , 3 It can be seen that by adjusting the nitrogen content, the ferrite grain size can be reduced from 25.87±1.76μm to 15.32±1.58μm. And from... Figure 4 , 5 It can be seen that with the increase of nitrogen content, the ferrite grains gradually become finer, which can improve tensile strength and elongation after fracture. When the nitrogen content is 100%, the tensile strength of the sample is 910.65±5.5 MPa, and the elongation after fracture is 13.84±0.3%, which are 57.55 MPa higher than the tensile strength and 5.66% higher than the elongation after fracture when the nitrogen content is 0%. This shows that when using overall control, as the nitrogen content increases, the overall grain size becomes finer, and the mechanical properties are improved.

[0041] Example 2

[0042] In the single additive manufacturing process, different nitrogen contents were sequentially used for printing layers at 0%, 4%, 8%, 12%, 20%, and 100% (the nitrogen content in the protective gas was changed every 2 mm of printing layer thickness from bottom to top). After cooling to room temperature, the cross-section of the metallographic sample was etched to obtain a microstructure image, and the grain size was statistically analyzed using the intercept method. Vickers hardness tests were performed using a Vickers hardness tester. Vickers hardness tests were conducted on the side of the metallographic sample from bottom to top at 2 mm of printing layer thickness. Three different locations were selected for testing at each printing layer thickness, and the average value was taken.

[0043] Figure 6-8 The effects of gradient modulation on the microstructure and mechanical properties of the samples in Example 2 are shown respectively. Figure 6 , 7 It can be seen that as the stacking layer height increases, and the nitrogen content is gradually increased from 0% to 100%, the ferrite grain size decreases from 28.87±3.78μm to 15.52±3.77μm. Figure 8 The Vickers hardness variation chart shows that the Vickers hardness increases from 305.69±4.7 Hv to 383.32±16.8 Hv from bottom to top. This indicates that gradient control can achieve changes in microstructure and properties with the stacking layer height, which is of great significance for the preparation of duplex stainless steel gradient materials.

[0044] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made within the scope of the claims of the present invention should be included in the scope of the present invention.

Claims

1. A method for in-situ tailoring microstructure and properties during additive manufacturing of duplex stainless steel, characterized in that, In the additive manufacturing of duplex stainless steel, the overall grain size and gradient grain size can be controlled by adjusting the ratio of argon and nitrogen in the mixed gas. During the fabrication process, the same proportion of mixed gas is consistently introduced into the same deposited layer for printing to ensure a consistent overall grain size. Different deposited layers have different grain sizes by adjusting the nitrogen content in the mixed gas during printing, thus obtaining a material with a gradient grain size. The nitrogen content in the mixed gas ranges from 0% to 100%. During the preparation process, the oxygen content in the environmental chamber is less than 0.1%, and the pressure is controlled between 1.0 and 2.3 MPa.

2. The method for in-situ tailoring of microstructure and properties during additive manufacturing of duplex stainless steel of claim 1 wherein, The diameter of the duplex stainless steel powder used is 13-52μm.

3. The method for in-situ control of microstructure and properties during the additive manufacturing process of duplex stainless steel as described in claim 1, characterized in that, The mixed gas is formed by simultaneously introducing pure nitrogen and pure argon into a gas mixing device.

4. The method for in-situ tailoring of microstructure and properties during additive manufacturing of duplex stainless steel of claim 3 wherein, The outlet flow rate of the gas mixing device is 1-2 L / min lower than the sum of the inlet flow rates of the two gases, so as to promote thorough mixing of nitrogen and argon.

5. The method for in-situ tailoring of microstructure and properties during additive manufacturing of duplex stainless steel of claim 3 wherein, The gas mixing device is equipped with a gas sieve, which transforms laminar flow into turbulent flow to further promote the thorough mixing of nitrogen and argon.

6. The method for in-situ tailoring of microstructure and properties during additive manufacturing of duplex stainless steel of claim 1 wherein, During the preparation process, a pulsed laser is used as the heating source with a wavelength of 1070nm, the laser power is controlled between 70-300W, the scanning speed is controlled between 200-1200mm / s, and the overlap rate is controlled between 11-90%.

7. The method for in-situ tailoring of microstructure and properties during additive manufacturing of duplex stainless steel of claim 6 wherein, The scanning strategy uses a zigzag pattern with 90° rotation between adjacent layers, and the layer height is controlled at 0.03mm.

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