Selective laser melting forming method for large-size aviation high-strength aluminum alloy thin-wall special-shaped grating

Abstract

The invention discloses a selective laser melting forming method for a large-size aviation high-strength aluminum alloy thin-wall special-shaped grating. The selective laser melting forming method comprises the steps of part process model design, part placement and support design, selective laser melting forming, heat treatment, support removal and part grinding. In the step of designing the part process model, thickening and chamfering the thin-wall area of the tip of the lower skin of the grating; in the step of part placement and support design, a grid opening overhanging included angle of a grid horizontal section, an included angle in the moving direction of a scraper and a grid lower surface inclination angle are set; a sheet-shaped support is arranged between the grid opening with the grid opening suspension included angle of 60-120 degrees and the tip end of the grid lower skin; in the step of selective laser melting forming, technological parameters of selective laser melting forming are set. The forming quality and the forming precision of the large-size aviation high-strength aluminum alloy thin-wall special-shaped grating are improved, the internal defect density can be controlled to be 0.1% or below, and the forming precision can be controlled to be within + / -0.5 mm.

Description

Technical Field

[0001] This invention belongs to the field of additive manufacturing technology, specifically relating to a method for laser selective melting forming of large-size aerospace high-strength aluminum alloy thin-walled irregular-shaped grids. Background Technology

[0002] With the increasing demands for comprehensive performance of aircraft, the structure of aircraft bodies needs to shift towards lightweight, robustness, and structural-functional integration, resulting in increasingly complex aircraft parts structures. Traditional processes are unable to meet the requirements for efficient and high-precision integrated manufacturing.

[0003] Selective laser melting (SLM) technology is suitable for the integrated precision forming of complex metal components and is a hot research direction in the current aerospace manufacturing field. Grille parts are crucial components of the aircraft's intake and exhaust systems, and their forming quality and accuracy significantly impact the aircraft's flight performance. Currently, irregularly shaped thin-walled grilles are mainly prepared using SLM, with materials including AlSi10Mg aluminum alloy and TC4 titanium alloy. However, with the improvement of aircraft performance, the mechanical properties of AlSi10Mg aluminum alloy are insufficient to meet manufacturing requirements, and TC4 titanium alloy, due to its higher density than aluminum alloy, is difficult to meet the demands of lightweight manufacturing. Therefore, improving the quality of high-strength aluminum alloy thin-walled irregularly shaped grilles formed by SLM has become an urgent problem to be solved. High-strength aluminum alloys have poor toughness and plasticity, and the thermal stress during SLM is high, making them prone to deformation and cracking when preparing large-size thin-walled irregularly shaped grilles. Summary of the Invention

[0004] The purpose of this invention is to provide a laser selective melting forming method for large-size aerospace high-strength aluminum alloy thin-walled irregular-shaped grilles, in order to solve the problem of cracking that easily occurs when forming high-strength aluminum alloy thin-walled irregular-shaped grilles using the laser selective melting forming method.

[0005] This invention is achieved through the following technical solution: A method for laser selective melting forming of large-size high-strength aluminum alloy thin-walled irregular-shaped grids for aerospace applications, including the steps of part process model design, part placement and support design, laser selective melting forming, heat treatment, support removal and part grinding; In the process model design of the part, the thin-walled area at the tip of the lower skin of the grille is thickened and chamfered. In the steps of component placement and support design, the angle between the grid opening overhang and the scraper movement direction of the grid horizontal section and the tilt angle of the grid lower surface are set to prevent the scraper from scratching the grid; and a sheet support is set between the grid opening with a grid opening overhang angle of 60°~120° and the tip of the grid lower surface. In the laser selective melting forming step, the laser selective melting forming process parameters are set, including setting the grid solid printing process parameters, reducing internal defects of the part by setting the scanning spacing; setting the surface printing process parameters, setting the recognition range of the grid lower skin area, and setting the lower skin contour scanning energy input to improve the lower skin strength.

[0006] In some embodiments of the present invention, thickening and chamfering the thin-walled region at the tip of the lower skin of the grille includes: When the overhang angle of the grille opening is 0~40°, the thin-walled area at the tip of the lower skin is not thickened, and the chamfer is set to R10~15mm; When the included angle of the grille opening is 40°~60°, the thin-walled area at the tip of the lower skin is thickened by 0.1~0.2mm, and the chamfer is set to R15~20mm; When the sag angle of the grille opening is 60°~120°, the thin-walled area at the tip of the lower skin is thickened by 0.2~0.3mm, and the chamfer is set to R20~30mm.

[0007] In some embodiments of the present invention, the thickened region is located outside the grille.

[0008] In some embodiments of the present invention, the sheet support is T-shaped, and its ends are respectively connected to the grid opening at the grid opening overhang angle and the two tips of the grid lower skin.

[0009] In some embodiments of the present invention, the thickness of the sheet support is set to 0.1~0.2 mm.

[0010] In some embodiments of the present invention, the angle between the horizontal plane of the sheet support and the scraper is 0~30°, and the vertical plane deviates from the scraper's movement direction by 20°~40°.

[0011] In some embodiments of the present invention, setting the grating solid printing process parameters includes: The scanning interval is set to 0.5~0.1mm; And / or, laser power 350~380W, scanning speed 1000~1200mm / s, scanning spacing 0.05~0.1mm, layer thickness 30~50μm, scanning strip width 7~10mm, strip overlap width 0.1~0.2mm.

[0012] In some embodiments of the present invention, setting surface printing process parameters includes: The thickness of the upper skin is 0.05~0.1mm, and the printing process parameters are set as follows: laser power 150~200W, scanning speed 1000~1200mm / s, and scanning spacing 0.1mm; And / or, the thickness of the lower skin is 0.1~0.3mm, and the printing process parameters are set as follows: laser power 200~250W, scanning speed 1000~1200mm / s, scanning spacing 0.1mm; And / or, set the contour printing process parameters as follows: outer ring laser power 150~200W, scanning speed 1200~1500mm / s, additional outer ring laser power 150~200W, scanning speed 1200~1500mm / s.

[0013] In some embodiments of the present invention, the sheet support printing process parameters are set, including: When setting up sheet support printing, only one scan line is used; Laser power 200~250W, scanning speed 1200~1500mm / s; grid sheet support does not scan the contour, process parameters are laser power 250~300W, scanning speed 1200~1500mm / s, scanning spacing 0.12~0.15mm.

[0014] In some embodiments of the present invention, two annealing heat treatments are employed. The first annealing heat treatment is performed at a temperature of 180~250℃ for 4~12 hours. The second annealing heat treatment is performed by raising the temperature to 280~320℃ and holding it at that temperature for 1~4 hours, followed by air cooling.

[0015] Compared with the prior art, the present invention has the following advantages and beneficial effects: This invention addresses the cracking problem that easily occurs in the laser selective melting forming of high-strength aluminum alloy thin-walled irregular-shaped gratings. It optimizes the forming process from multiple aspects, including grating filling process parameters, surface process parameters, support design, and heat treatment, thereby improving the forming quality and accuracy of large-size aerospace high-strength aluminum alloy thin-walled irregular-shaped gratings. The internal defect density can be controlled below 0.1%, and the forming accuracy can be controlled within ±0.5mm, providing reliable technical support for the engineering application of large-size aerospace high-strength aluminum alloy thin-walled irregular-shaped gratings. Attached Figure Description

[0016] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a schematic diagram of the grille structure according to an embodiment of the present invention.

[0018] Figure 2 This is a schematic diagram of the cross-section of the grille in an embodiment of the present invention.

[0019] Figure 3 This is a schematic diagram showing the cross-sectional details of the grille in an embodiment of the present invention.

[0020] Figure 4 This is a schematic diagram of the included angle of the grille opening in an embodiment of the present invention.

[0021] Figure 5 This is a schematic diagram of the thickened area of ​​the grille in an embodiment of the present invention.

[0022] Figure 6 This is a schematic diagram of the sheet-like support structure in an embodiment of the present invention.

[0023] in: 1. Grille; 11. Grille opening; 12. Grille opening overhang angle; 13. Upper skin; 14. Lower skin; 15. Outline; 16. Grille body; 17. Sheet support; 18. Tip of lower skin of grille; 19. Angle between grille opening overhang angle and scraper movement direction of horizontal cross section of grille. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments.

[0025] This invention addresses the problem of cracking that easily occurs in the laser selective melting forming of high-strength aluminum alloy thin-walled irregular-shaped grilles. It optimizes the forming process from multiple aspects, including grille filling process parameters, surface process parameters, support design, and heat treatment, thereby improving the forming quality and accuracy of large-size aerospace high-strength aluminum alloy thin-walled irregular-shaped grilles.

[0026] The high-strength aluminum alloys of this invention include, but are not limited to, AlMgScZr and AlMgErZr. The grid tilt angle of the thin-walled irregular-shaped grid is ≥45°.

[0027] The laser selective melting forming process of grid 1 includes part process model design, part placement and support design, laser selective melting forming, heat treatment, support removal and part grinding.

[0028] Reference Figure 1 The diagram shows a grid structure with several opening areas, referred to as grid openings 11; the downward angle along the forming direction is the grid opening overhang angle 12.

[0029] In some embodiments of the present invention, during the part process model design step, the thin-walled area at the tip of the lower skin of the grille with a large angle (e.g., greater than 40°) is thickened and chamfered to prevent cracking in the tip area of ​​the lower skin of the grille.

[0030] Reference Figure 2 and 3 The upper skin 13 is the surface facing upwards along the forming direction; the lower skin 14 is the surface facing downwards along the forming direction. The area between the lower skin and the upper skin is the grid entity 16 filling area, and their outer edges together form the grid outline 15.

[0031] Specifically, when the sag angle of the grille opening is 0~40°, the thin-walled area at the tip of the lower skin is not thickened, and the chamfer is set to R10~15mm; When the included angle of the grille opening is 40°~60°, the thin-walled area at the tip of the lower skin is thickened by 0.1~0.2mm, and the chamfer is set to R15~20mm; When the sag angle of the grille opening is 60°~120°, the thin-walled area at the tip of the lower skin is thickened by 0.2~0.3mm, and the chamfer is set to R20~30mm.

[0032] The thickened area is the outer side of the grille, such as Figure 4 and Figure 5 As shown, this is to prevent the thickening from affecting the airflow in and out of the grille.

[0033] Because the grid openings of thin-walled grid-type parts have a rhomboid shape in cross-section, the edges of the parts are prone to bulging under the influence of thermal stress during laser selective melting and forming, which can easily cause friction between the printed parts and the squeegee.

[0034] To address this technical problem, in some embodiments, the placement method in the steps of component placement and support design is set as follows: like Figure 6 As shown, the angle between the grid opening and the scraper movement direction is controlled between 19° and 0°, and the tilt angle of the lower surface of the grid is controlled between 0° and 40° to prevent the scraper from scratching the grid.

[0035] When the included angle of the grid opening in the horizontal cross section of the grid is small compared to the direction of the scraper's movement, the contact area between the scraper and the grid body is small, which can reduce the risk of scratches.

[0036] The following scheme is adopted for the grid support: A sheet-like support 17 is provided between the grille opening 11 with a hanging angle of 60~120° and the tip 18 of the lower surface of the grille. The sheet-like support 17 is T-shaped. Figure 6 As shown, its ends are connected to the grid opening at the grid opening overhang angle and the two tips of the grid lower skin, respectively.

[0037] The sheet support 17 has a thickness of 0.1~0.2mm. The sheet support is set to provide a heat dissipation channel for the tip of the lower surface of the grille, so as to reduce the concentration of thermal stress and prevent cracking in the tip area of ​​the lower surface of the grille.

[0038] To prevent the T-shaped support from rubbing against the scraper, the angle between its horizontal plane and the scraper is 0~30°, and the vertical plane is offset from the scraper's direction of movement by 20°~40°. For example... Figure 6 As shown, 17-1 is the horizontal plane of the T-shaped support, and 17-2 is the vertical plane of the T-shaped support.

[0039] Due to the thin-walled support inside the grille opening, the vertical surface of its T-shaped structure needs to be offset by 20°~40° in the direction of the scraper's movement to reduce the risk of being scratched by the scraper.

[0040] In some embodiments, setting laser selective melting forming process parameters includes: Setting the process parameters for grid solid printing: Reduce the scanning spacing to reduce internal defects in the parts. Set the scanning spacing to 0.5~0.1mm.

[0041] Surface printing process parameter settings: Increase the recognition range of the lower skin area of ​​the grid and increase the energy input of the lower skin contour scanning to improve the strength of the lower skin and prevent cracking at the tip of the lower skin.

[0042] Setting the sheet support printing process parameters: Set to high scanning speed and low energy input to improve support forming efficiency and reduce the difficulty of subsequent support removal process.

[0043] In some embodiments, the grid solid printing process parameters are set as follows: laser power 350~380W, scanning speed 1000~1200mm / s, scanning spacing 0.05~0.1mm, layer thickness 30~50μm, scanning strip width 7~10mm, and strip overlap width 0.1~0.2mm.

[0044] In some embodiments, the surface printing process parameters are set as follows: The thickness of the upper skin is 0.05~0.1mm, and the printing process parameters are set as follows: laser power 150~200W, scanning speed 1000~1200mm / s, and scanning spacing 0.1mm; The thickness of the lower skin is 0.1~0.3mm, and the printing process parameters are set as follows: laser power 200~250W, scanning speed 1000~1200mm / s, and scanning spacing 0.1mm; The contour printing process parameters are set as follows: outer ring laser power 150~200W, scanning speed 1200~1500mm / s, additional outer ring laser power 150~200W, scanning speed 1200~1500mm / s; the outer ring is the scanning line of the part contour, and the additional outer ring is the contour scanning line that deviates outward from the outer ring by 0.1~0.2mm.

[0045] In some embodiments, the setting of sheet support printing process parameters includes: When printing sheet supports, only one scan line is used to reduce the difficulty of removing the sheet supports; one scan line ensures that the support thickness is controlled within 0.08~0.15mm, making it convenient for fitters to quickly remove the supports.

[0046] The printing process parameters for sheet supports are set as follows: laser power 200~250W, scanning speed 1200~1500mm / s; the grid sheet support does not scan the contour to improve the support forming efficiency, and the other process parameters are laser power 250~300W, scanning speed 1200~1500mm / s, and scanning spacing 0.12~0.15mm.

[0047] In some embodiments, a double annealing heat treatment is employed to achieve stress relief and enhanced mechanical properties of the grille; The parameters for the first annealing heat treatment were set as follows: temperature 180~250℃, time 4~12h, in order to reduce the thermal stress of the parts and at the same time make the grid elongation ≥15% to prevent the parts from deforming and cracking during the heat treatment process. The parameters for the second annealing heat treatment were set as follows: the temperature was raised to 280~320℃, the holding time was 1~4h, and then air-cooled to improve the strength of the grid, so that the room temperature tensile strength of the grid is ≥500MPa and the elongation is ≥10%.

[0048] During the first annealing heat treatment, a grid shaping fixture can be used to reduce the amount of grid surface deformation.

[0049] The following section uses the forming of a large-size irregularly shaped AlMgScZr high-strength aluminum alloy grille as an example to illustrate the method of the present invention in detail.

[0050] A laser selective melting method for forming large-size irregularly shaped AlMgScZr high-strength aluminum alloy grids includes the following steps: 1. First, identify and thicken the thin-walled area at the tip of the lower surface of the grille parts at large angles, and then chamfer it, as shown in the attached document. Figure 1 As shown; When the sag angle of the grille opening is 0~40°, the tip is not thickened, and the chamfer is R10~15mm; when the sag angle of the grille opening is 40~60°, the tip is thickened by 0.1~0.2mm, and the chamfer is R15~20mm; when the sag angle of the grille opening is 60~120°, the tip is thickened by 0.2~0.3mm, and the chamfer is R20~30mm.

[0051] The thickened area is on the outside of the grille to prevent the thickening from affecting the airflow into and out of the grille.

[0052] 2. Optimize the placement direction of the grid parts in the slicing software. The placement method is to control the angle between the sharp corner of the horizontal section of the grid and the direction of scraper movement between 0 and 30°, and the tilt angle of the lower surface of the grid between 0 and 40° to prevent the scraper from scratching the grid and the grid sheet support.

[0053] 3. Add sheet supports to the part in the slicing software. The sheet support design involves adding sheet supports at the lower end of the grille opening with an included angle of 60~120° and at the tip of the lower surface of the grille. The sheet supports have a T-shaped cross-section and a thickness of 0.1~0.2mm. Their purpose is to provide a heat dissipation channel for the tip of the lower surface of the grille, reduce thermal stress concentration, and prevent cracking in the tip area of ​​the lower surface of the grille. (See attached image.) Figure 6 As shown, dot matrix supports are added to the non-grid area to reduce the deformation of the grid parts' outer surface.

[0054] 4. Set the printing process parameters for laser selective melting and forming. The printing parameters for printing the grid entity are set as follows: laser power 380W, scanning speed 1200mm / s, scanning spacing 0.1mm, layer thickness 30μm, scanning strip width 7mm, and strip overlap 0.1mm. The thickness of the upper skin is 0.05mm, and the printing process parameters are set as follows: laser power 150W, scanning speed 1200mm / s, and scanning spacing 0.1mm; The thickness of the lower skin is 0.3mm, and the printing process parameters are set as follows: laser power 250W, scanning speed 1200mm / s, and scanning spacing 0.1mm; The printing process parameters for the outline are set as follows: laser power of 150W and scanning speed of 1200mm / s for the outer ring, and laser power of 200W and scanning speed of 1500mm / s for the additional outer ring. The printing process parameters for the sheet support were set as follows: laser power 200W, scanning speed 1500mm / s; The grid-like support does not scan the contour to improve the support forming efficiency. The process parameters are set as follows: laser power 250W, scanning speed 1500mm / s, and scanning spacing 0.15mm.

[0055] 5. After printing, clean the powder off the surface of the parts, and then put the grid parts together with the substrate into an air heat treatment furnace for stress relief annealing at 180°C for 8 hours. During heat treatment, clamp the parts in a conformal fixture to prevent deformation.

[0056] 6. After stress-relief annealing, raise the heat treatment temperature to 300℃, hold for 2 hours, and then air-cool to remove the grid parts.

[0057] 7. Remove the supports and grind the grating parts to obtain the final grating parts.

[0058] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., used to indicate the orientation or positional relationship are based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship that the product of this invention is usually placed in during use. They are only for the convenience of describing this invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0059] Furthermore, the use of terms such as "horizontal" and "vertical" in the description of this invention does not imply that the components are required to be absolutely horizontal or suspended, but rather that they can be slightly tilted. For example, "horizontal" simply means that its direction is more horizontal than "vertical," and does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0060] In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention in light of the specific circumstances.

[0061] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications or equivalent changes made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for laser selective melting forming of large-size, high-strength, thin-walled, irregularly shaped aerospace aluminum alloy grids, characterized in that... The process includes steps such as part process model design, part placement and support design, laser selective melting forming, heat treatment, support removal and part grinding. In the process model design of the part, the thin-walled area at the tip of the lower skin of the grille is thickened and chamfered. In the steps of component placement and support design, the angle between the grid opening overhang and the scraper movement direction of the grid horizontal section and the tilt angle of the grid lower surface are set to prevent the scraper from scratching the grid; and a sheet support is set between the grid opening with a grid opening overhang angle of 60°~120° and the tip of the grid lower surface. In the laser selective melting forming step, the laser selective melting forming process parameters are set, including setting the grid solid printing process parameters, reducing internal defects of the part by setting the scanning spacing; setting the surface printing process parameters, setting the recognition range of the grid lower skin area, and setting the lower skin contour scanning energy input to improve the lower skin strength.

2. The method for laser selective melting and forming of large-size high-strength aerospace aluminum alloy thin-walled irregular-shaped grids according to claim 1, characterized in that, The process of thickening and chamfering the thin-walled area at the tip of the lower skin of the grille includes: When the overhang angle of the grille opening is 0~40°, the thin-walled area at the tip of the lower skin is not thickened, and the chamfer is set to R10~15mm; When the included angle of the grille opening is 40°~60°, the thin-walled area at the tip of the lower skin is thickened by 0.1~0.2mm, and the chamfer is set to R15~20mm; When the sag angle of the grille opening is 60°~120°, the thin-walled area at the tip of the lower skin is thickened by 0.2~0.3mm, and the chamfer is set to R20~30mm.

3. The method for laser selective melting and forming of large-size aerospace high-strength aluminum alloy thin-walled irregular-shaped grids according to claim 2, characterized in that, The thickened area is located on the outside of the grille.

4. The method for laser selective melting and forming of large-size aerospace high-strength aluminum alloy thin-walled irregular-shaped grids according to claim 1, characterized in that, The sheet support is T-shaped, with its ends connected to the grid opening at the overhang angle and the two sharp points of the lower surface of the grid, respectively.

5. The method for laser selective melting and forming of large-size aerospace high-strength aluminum alloy thin-walled irregular-shaped grids according to claim 4, characterized in that, The thickness of the sheet support is set to 0.1~0.2mm.

6. The method for laser selective melting and forming of large-size aerospace high-strength aluminum alloy thin-walled irregular-shaped grids according to claim 4, characterized in that, The angle between the horizontal plane of the sheet support and the scraper is 0~30°, and the vertical plane deviates from the scraper's movement direction by 20°~40°.

7. The method for laser selective melting and forming of large-size aerospace high-strength aluminum alloy thin-walled irregular-shaped grids according to claim 1, characterized in that, Set the grating solid printing process parameters, including: The scanning interval is set to 0.5~0.1mm; And / or, laser power 350~380W, scanning speed 1000~1200mm / s, scanning spacing 0.05~0.1mm, layer thickness 30~50μm, scanning strip width 7~10mm, strip overlap width 0.1~0.2mm.

8. The method for laser selective melting and forming of large-size aerospace high-strength aluminum alloy thin-walled irregular-shaped grids according to claim 1, characterized in that, Set the surface printing process parameters, including: The thickness of the upper skin is 0.05~0.1mm, and the printing process parameters are set as follows: laser power 150~200W, scanning speed 1000~1200mm / s, and scanning spacing 0.1mm; And / or, the thickness of the lower skin is 0.1~0.3mm, and the printing process parameters are set as follows: laser power 200~250W, scanning speed 1000~1200mm / s, scanning spacing 0.1mm; And / or, set the contour printing process parameters as follows: outer ring laser power 150~200W, scanning speed 1200~1500mm / s, additional outer ring laser power 150~200W, scanning speed 1200~1500mm / s.

9. The method for laser selective melting and forming of large-size aerospace high-strength aluminum alloy thin-walled irregular-shaped grids according to claim 1, characterized in that, Set the sheet support printing process parameters, including: When setting up sheet support printing, only one scan line is used; Laser power 200~250W, scanning speed 1200~1500mm / s; grid sheet support does not scan the contour, process parameters are laser power 250~300W, scanning speed 1200~1500mm / s, scanning spacing 0.12~0.15mm.

10. The method for laser selective melting and forming of large-size aerospace high-strength aluminum alloy thin-walled irregular-shaped grids according to claim 1, characterized in that, Two annealing heat treatments were used. The first annealing heat treatment was carried out at a temperature of 180~250℃ for 4~12 hours. The second annealing heat treatment was carried out at a temperature of 280~320℃ for 1~4 hours, followed by air cooling.

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