High-performance repairing method for double-beam laser-assisted cold spraying aluminum alloy defective part

Abstract

The invention discloses a double-beam laser assisted cold spraying aluminum alloy defective part high-performance repairing method which comprises the following steps: firstly, preprocessing an aluminum alloy defective part to obtain a preprocessed aluminum alloy defective part; secondly, cold spraying repairing treatment is conducted on the preprocessed aluminum alloy defect part, meanwhile, low-power-density laser is emitted to the preprocessed aluminum alloy defect part to assist in cold spraying repairing, and therefore initial repairing aluminum alloy is obtained; then, high-power-density laser is used for conducting micro-scale laser shock peening treatment on the aluminum alloy, and the pre-repaired aluminum alloy is obtained; and then, the laser-assisted cold spraying treatment and the laser shock peening treatment flow are repeated until repairing is completed, and at the moment, repairing of the aluminum alloy defective part can be completed. On the basis of a cold spraying technology, two laser beams with different powers are carried, compared with a traditional technology, the repairing effect can be improved, heat influence on components is avoided, and therefore the method is very suitable for in-situ repairing of the aviation aluminum alloy.

Description

Technical Field

[0001] The present invention belongs to the technical field of additive repair of aviation aluminum alloy structural parts, and specifically relates to a high-performance repair method for double-beam laser-assisted cold-sprayed aluminum alloy defective parts. Background Art

[0002] As an important nonferrous metal material, aluminum alloy is widely used in the aerospace field due to its advantages such as high specific strength, easy processing, good corrosion resistance, good electrical and thermal conductivity, etc. However, since aviation equipment often serves in a high load, high vibration, and high corrosion environment, aluminum alloy parts can easily fail and be scrapped due to wear or corrosion, which will not only cause huge economic losses, but also may cause equipment failure, resulting in major safety accidents. In addition, due to the complex shape of aviation parts and the high difficulty of casting, the scrap rate of parts production is usually high. If they cannot be effectively repaired, it will also cause huge waste. Therefore, the repair of aluminum alloy is of great significance in the aerospace field.

[0003] At present, the repair of aluminum alloy structural parts is mostly based on technologies such as laser cladding and thermal spraying; among them, the aforementioned technologies have disadvantages such as high heat input and large thermal impact on the substrate area. Therefore, they are not suitable for in-situ repair of components made of heat-sensitive materials such as aviation aluminum alloys; therefore, based on the aforementioned deficiencies, there is an urgent need to provide a high-performance repair method suitable for aviation aluminum alloys that can avoid high heat input and impact on the substrate, and can restore the fatigue performance of the defective parts to the original standard to a large extent. Summary of the invention

[0004] The purpose of the present invention is to provide a high-performance repair method for dual-beam laser-assisted cold sprayed aluminum alloy defective parts, so as to solve the problem that the prior art cannot be applied to in-situ repair of components made of heat-sensitive materials such as aviation aluminum alloys due to the disadvantages of high heat input and large thermal impact on the substrate area.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] In a first aspect, a high-performance repair method for a double-beam laser-assisted cold-sprayed aluminum alloy defective part is provided, comprising:

[0007] Performing pre-processing on the area to be repaired of the aluminum alloy defective part to obtain a pre-processed aluminum alloy defective part;

[0008] Performing a cold spray repair process on the pre-processed aluminum alloy defective part, and during the cold spray repair process, emitting a first laser to the pre-processed aluminum alloy defective part, so as to obtain an initial repaired aluminum alloy after the cold spray repair process is completed;

[0009] emitting a second laser to the initial repair aluminum alloy, so as to perform microscale laser shock strengthening treatment on the initial repair aluminum alloy using the second laser to obtain a pre-repaired aluminum alloy, wherein the first laser is a low power density laser, and the second laser is a high power density laser;

[0010] The pre-repaired aluminum alloy is repeatedly subjected to cold spray repair treatment and micro-scale laser shock strengthening treatment until a preset condition is reached, so as to obtain a repaired aluminum alloy defective part when the preset condition is reached.

[0011] Based on the above disclosed content, the present invention first pre-processes the area to be repaired of the aluminum alloy defective part to obtain a pre-processed aluminum alloy defective part; then, the pre-processed aluminum alloy defective part is cold sprayed for repair, and a low-power density laser is simultaneously emitted to the pre-processed aluminum alloy defective part to assist in the cold spray repair, thereby obtaining an initial repaired aluminum alloy after the cold spray repair; then, a high-power density laser is used to perform micro-scale laser shock strengthening treatment on it to obtain a pre-repaired aluminum alloy; then, the aforementioned laser-assisted cold spray treatment and micro-scale laser shock strengthening treatment process are repeated until the repair meets the preset conditions, at which time, the repair of the aluminum alloy defective part can be completed to obtain the repaired aluminum alloy.

[0012] Through the above design, the present invention is equipped with two laser beams of different powers to repair defects on the basis of cold spraying technology. First, the thermal effect of low-power density laser is used to in-situ heat the repair interface, soften the substrate / coating surface, improve the cold spray powder deposition efficiency, reduce the coating porosity, and thus improve the coating quality; then, the shot peening effect of high-power density laser is used to strengthen the repair coating layer by layer, and the cold spraying treatment of low-power density laser and the laser shock strengthening treatment of high-power density laser are repeatedly performed. In this way, in this process, Uniform laser is used to assist cold spraying to repair the stress field of the coating, and at the same time, a residual compressive stress field with a large value and uniform distribution inside the material is generated, thereby overcoming the problem of uneven stress of the cold spraying repair coating. The residual compressive stress layer can inhibit the further initiation of cracks in the specimen to be repaired, and can improve the fatigue performance of the specimen itself. In addition, the present invention adopts cold spraying + laser-assisted technology to repair defects. Compared with traditional technologies, it does not have high heat input and will not produce thermal effects on the substrate area. Therefore, it is very suitable for in-situ repair of heat-sensitive material components such as aviation aluminum alloys.

[0013] In a possible design, the area to be repaired of the aluminum alloy defective part is pre-processed to obtain a pre-processed aluminum alloy defective part, including:

[0014] Grinding and polishing the area to be repaired of the aluminum alloy defective part to obtain a polished aluminum alloy defective part;

[0015] Cleaning the polished aluminum alloy defective part to obtain a cleaned aluminum alloy defective part;

[0016] The cleaned aluminum alloy defective part is dried to obtain a pre-processed aluminum alloy defective part after drying.

[0017] In a possible design, compressed hot air is used to dry the cleaned aluminum alloy defective part, so as to obtain the pre-processed aluminum alloy defective part after drying.

[0018] In a possible design, sandpaper is used to grind and polish the area to be repaired of the aluminum alloy defective part to obtain a polished aluminum alloy defective part.

[0019] In a possible design, the mesh number of the sandpaper is between 600 mesh and 2000 mesh.

[0020] In one possible design, the sandpaper has a grit of 1200.

[0021] In one possible design, the pre-processed aluminum alloy defective part is subjected to a cold spray repair process, and during the cold spray repair process, a first laser is emitted to the pre-processed aluminum alloy defective part to obtain an initial repaired aluminum alloy after the cold spray repair process is completed, including:

[0022] According to the component performance of the pre-processed aluminum alloy defective part, a cold spray repair parameter and a first laser parameter are obtained;

[0023] According to the cold spray repair parameters, controlling the cold spray equipment to perform cold spray repair processing on the pre-processed aluminum alloy defective parts;

[0024] During the cold spray repair process, based on the first laser parameters, the first laser emitting device is controlled to generate the first laser, and the first laser is emitted to the pre-processed aluminum alloy defective part at a preset temperature to obtain the initial repaired aluminum alloy after the cold spray repair process is completed.

[0025] In one possible design, the cold spray repair parameters include the distance between the cold spray nozzle and the pre-processed aluminum alloy defective part, the powder incident angle, the cold spray gas pressure, the cold spray gas temperature and the repair path, and the first laser parameters include laser power and spot diameter.

[0026] In one possible design, emitting a second laser to the initial repair aluminum alloy includes:

[0027] According to the performance of the component of the initially repaired aluminum alloy, obtaining second laser parameters, wherein the second laser parameters include laser energy, spot diameter, spot overlap rate, impact times and processing path;

[0028] According to the second laser parameters, the second laser emitting device is controlled to generate a second laser, and the second laser is emitted to the initial repair aluminum alloy at a preset temperature, so as to perform microscale laser shock strengthening treatment on the initial repair aluminum alloy using the second laser.

[0029] In a possible design, after obtaining the repaired aluminum alloy defective part, the method further includes:

[0030] The repaired aluminum alloy defective part is subjected to grinding and polishing treatment, cleaning treatment and drying treatment in sequence, so as to obtain the actually repaired aluminum alloy defective part after drying treatment.

[0031] Beneficial effects:

[0032] (1) Based on the cold spraying technology, the present invention uses two laser beams with different powers to repair defects. First, the thermal effect of the low-power density laser is used to in-situ heat the repair interface, soften the substrate / coating surface, improve the cold spray powder deposition efficiency, reduce the coating porosity, and thus improve the coating quality; then, the shot peening effect of the high-power density laser is used to strengthen the repair coating layer by layer, and the low-power density laser cold spraying treatment and high-power density laser laser shock strengthening treatment processes are repeated continuously. In this way, in this process, Uniform laser is used to assist cold spraying to repair the stress field of the coating, and at the same time, a residual compressive stress field with a large value and uniform distribution inside the material is generated, thereby overcoming the problem of uneven stress of the cold spraying repair coating, and the residual compressive stress layer can inhibit the further initiation of cracks in the specimen to be repaired, and can improve the fatigue performance of the specimen itself; in addition, the present invention adopts cold spraying + laser-assisted technology to repair defects. Compared with traditional technologies, it does not have high heat input and will not produce thermal effects on the substrate area. Therefore, it is very suitable for in-situ repair of heat-sensitive material components such as aviation aluminum alloys. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 A schematic flow chart of the steps of a high-performance repair method for a double-beam laser-assisted cold-sprayed aluminum alloy defective part provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0034] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the present invention will be briefly introduced below in combination with the drawings and the description of the embodiments or the prior art. Obviously, the following description of the structure of the drawings is only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work. It should be noted that the description of these embodiments is used to help understand the present invention, but does not constitute a limitation of the present invention.

[0035] It should be understood that although the terms first, second, etc. may be used herein to describe various units, these units should not be limited by these terms. These terms are only used to distinguish one unit from another unit. For example, a first unit can be referred to as a second unit, and similarly, a second unit can be referred to as a first unit without departing from the scope of the exemplary embodiments of the present invention.

[0036] It should be understood that the term "and / or" that may appear in this article is merely a description of the association relationship between associated objects, indicating that three relationships may exist. For example, A and / or B can represent three situations: A exists alone, B exists alone, and A and B exist at the same time. The term " / and" that may appear in this article describes another type of association object relationship, indicating that two relationships may exist. For example, A / and B can represent two situations: A exists alone, and A and B exist alone. In addition, the character " / " that may appear in this article generally indicates that the previous and next associated objects are in an "or" relationship.

[0037] Example:

[0038] See also Figure 1 As shown, the present embodiment provides a high-performance repair method for a double-beam laser-assisted cold-sprayed aluminum alloy defective part. Based on the cold-spraying technology, two laser beams of different powers are used. First, the thermal effect of the low-power-density laser is used to in-situ heat the repair interface, soften the substrate / coating surface, improve the cold-sprayed powder deposition efficiency, reduce the coating porosity, and thus improve the coating quality; then, the shot peening effect of the high-power-density laser is used to strengthen the repair coating layer by layer, and during the repair process, the aforementioned process is repeated continuously, so that a uniform laser can be generated to assist the cold-sprayed repair coating stress field, and at the same time, a residual compressive stress field with a large value and uniform distribution inside the material can also be generated; based on this, the problem of uneven stress of the cold-sprayed repair coating can be overcome, and the residual compressive stress layer can also inhibit the further initiation of cracks in the specimen to be repaired, thereby improving the fatigue performance of the specimen itself; in addition, compared with traditional technologies, this method does not have high heat input and will not have a thermal impact on the substrate area, so it is very suitable for in-situ repair of heat-sensitive material components such as aviation aluminum alloys.

[0039] The repair process of the aforementioned repair method may be, but is not limited to, steps S1 to S4 as shown below.

[0040] S1. Pre-processing the area to be repaired of the aluminum alloy defective part to obtain a pre-processed aluminum alloy defective part; in specific applications, for example but not limited to the following steps S11 to S13 can be used to complete the pre-processing of the area to be repaired.

[0041] S11. Grinding and polishing the area to be repaired of the aluminum alloy defective part to obtain a polished aluminum alloy defective part; in this embodiment, grinding and polishing is performed on the area to be repaired of the aforementioned aluminum alloy defective part, mainly to remove irregular scratches, protrusions and other defects on the surface of the defective component, so that the roughness of the component surface reaches a machinable level.

[0042] In specific applications, for example, but not limited to, sandpaper is used to grind and polish the area to be repaired of the aforementioned aluminum alloy defective part, so as to obtain a polished aluminum alloy defective part; optionally, the mesh number of the selected sandpaper is between 600 mesh and 2000 mesh; among them, the present embodiment preferably uses 1200 mesh sandpaper to grind and polish the area to be repaired of the aluminum alloy defective part; of course, the mesh number of the sandpaper can be specifically set according to actual use, and is not limited to the aforementioned example.

[0043] After the aluminum alloy defective part is polished and polished, it can be cleaned, and the process can be but not limited to the following step S12.

[0044] S12. Cleaning the polished aluminum alloy defective part to obtain a cleaned aluminum alloy defective part; in this embodiment, cleaning the polished aluminum alloy defective part is mainly used to remove wear chips and other impurities on the surface and defective position, thereby ensuring the cleanliness of the workpiece surface to facilitate subsequent repair.

[0045] After the workpiece is cleaned, a drying process may be performed, and the process may be, but is not limited to, as shown in the following step S13.

[0046] S13. Drying the cleaned aluminum alloy defective parts to obtain pre-processed aluminum alloy defective parts after drying. In specific implementation, for example but not limited to using compressed hot air to dry the cleaned aluminum alloy defective parts, so as to obtain the pre-processed aluminum alloy defective parts after drying.

[0047] In this way, through the aforementioned steps S11 to S13, the pre-processing of the aluminum alloy defective parts can be completed, and then the repair process can be carried out; wherein, this embodiment combines two laser beams of different powers on the basis of cold spraying technology to achieve the repair of aluminum alloy, and the process can be but not limited to as shown in the following steps S2 and S3.

[0048] S2. Perform cold spray repair treatment on the pre-processed aluminum alloy defective part, and during the cold spray repair treatment, emit a first laser to the pre-processed aluminum alloy defective part to obtain an initial repaired aluminum alloy after the cold spray repair treatment is completed; in this embodiment, first, according to the size and shape of the pre-processed aluminum alloy defective part, a matching fixture is selected to fix it on the laser-assisted cold spray platform; then, according to the component performance of the pre-processed aluminum alloy defective part, cold spray repair parameters and first laser parameters are obtained; then, according to the cold spray repair parameters, the cold spray equipment on the laser-assisted cold spray platform is controlled to perform cold spray repair treatment on the pre-processed aluminum alloy defective part; at the same time, during the cold spray repair treatment, based on the first laser parameters, the first laser emitting device is controlled to generate a first laser, and the first laser is emitted to the pre-processed aluminum alloy defective part at a preset temperature to obtain an initial repaired aluminum alloy after the cold spray repair treatment is completed.

[0049] In a specific implementation, for example, the cold spray repair parameters may include but are not limited to: the distance between the cold spray nozzle and the pre-processed aluminum alloy defective part, the powder incident angle, the cold spray gas pressure, the cold spray gas temperature and the repair path, and the first laser parameter may include but is not limited to the laser power and the spot diameter; of course, a component performance parameter table may be pre-set (the table stores the first laser parameters and cold spray repair parameters corresponding to different component performances), and when in use, it can be matched in the table according to the construction performance of the aforementioned pre-processed aluminum alloy.

[0050] At the same time, when using the first laser-assisted cold spray repair with low power density, it is carried out in room temperature atmospheric environment, that is, the aforementioned preset temperature is normal temperature (that is, 25 degrees Celsius); at the same time, lasers with power density less than or equal to 100w / cm2 are usually judged as low power density lasers.

[0051] Thus, based on the aforementioned step S2, the thermal effect of the low power density laser can be used to in-situ heat the repair interface, thereby softening the substrate / coating surface, improving the cold spray powder deposition efficiency, reducing the coating porosity, and achieving the purpose of improving the coating quality.

[0052] After the pre-processed aluminum alloy defective parts are repaired layer by layer by laser-assisted cold spraying under normal temperature atmospheric environment, high-power density laser impact strengthening treatment can be performed, and the process can be but not limited to as shown in the following step S3.

[0053] S3. Emit a second laser to the initial repair aluminum alloy, so as to utilize the second laser to perform micro-scale laser shock strengthening treatment on the initial repair aluminum alloy to obtain a pre-repaired aluminum alloy, wherein the first laser is a low-power density laser, and the second laser is a high-power density laser. In specific implementation, the second laser parameters are first obtained according to the component performance of the initial repair aluminum alloy. Then, according to the second laser parameters, the second laser emitting device is controlled to generate the second laser, and the second laser is emitted to the initial repair aluminum alloy at a preset temperature, so as to utilize the second laser to perform micro-scale laser shock strengthening treatment on the initial repair aluminum alloy to obtain a pre-repaired aluminum alloy.

[0054] In this embodiment, the second laser parameters may include but are not limited to: laser energy, spot diameter, spot overlap rate, number of impacts and processing path; of course, the acquisition method is the same as the first laser parameters, which will not be repeated here; at the same time, the emission environment of the second laser is also in a normal temperature atmospheric environment, and the example may include but is not limited to judging a laser with a power density greater than or equal to 6.9GW / cm2 as a high power density laser.

[0055] Based on this, the shot peening effect of high-power density laser can be used to strengthen the repair coating layer by layer, thereby improving the repair effect; in this way, after completing a laser-assisted cold spray repair and high-power density laser impact strengthening treatment, it can be repeated according to the repair standard until the repair standard is met; wherein, the cyclic repair process can be but is not limited to as shown in the following step S4.

[0056] S4. Repeat the cold spray repair treatment and micro-scale laser shock strengthening treatment on the pre-repaired aluminum alloy until the preset conditions are met, so as to obtain the repaired aluminum alloy defective parts when the preset conditions are met; in this embodiment, the preset conditions refer to the repair meeting the repair standards, such as fatigue performance reaching preset performance, etc.; wherein, the specific repair standards can be specifically set according to actual use, and are not limited to the aforementioned examples; in this way, after cyclically executing the aforementioned steps S2 and S3, the repair of the aluminum alloy defective parts can be completed to obtain the repaired aluminum alloy.

[0057] In addition, in the present embodiment, after obtaining the repaired aluminum alloy defective part, it is necessary to perform grinding and polishing treatment, cleaning treatment and drying treatment on the repaired aluminum alloy defective part in sequence, so as to obtain the actually repaired aluminum alloy defective part after drying treatment; wherein, 600-2000 mesh sandpaper is still used for grinding and polishing until the original morphology and roughness of the component are restored; then, it is cleaned to remove impurities such as surface debris; finally, it is placed under flowing compressed hot air for drying, so that after drying, the repair process is terminated.

[0058] Thus, a high-performance repair method for aluminum alloy defective parts assisted by dual-beam lasers is described in detail in the aforementioned steps S1 to S4. The present invention is based on the cold spraying technology and is equipped with two lasers of different powers. First, the thermal effect of the low-power density laser is used to in-situ heat the repair interface, soften the substrate / coating surface, improve the cold spray powder deposition efficiency, reduce the coating porosity, and thus improve the coating quality; then, the shot peening effect of the high-power density laser is used to strengthen the repair coating layer by layer, and during the repair process, the aforementioned process is repeated continuously, so that a uniform laser can be generated to assist the cold spray repair coating stress field, and at the same time, a residual compressive stress field with a large value and uniform distribution inside the material can also be generated; based on this, the problem of uneven stress of the cold spray repair coating can be overcome, and the residual compressive stress layer can also inhibit the further initiation of cracks in the specimen to be repaired, thereby improving the fatigue performance of the specimen itself; in addition, compared with traditional technologies, the present invention does not have high heat input and will not have a thermal impact on the substrate area, so it is very suitable for in-situ repair of heat-sensitive material components such as aviation aluminum alloys.

[0059] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the protection scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A high-performance repair method for double-beam laser-assisted cold spraying aluminum alloy defective parts, characterized in that: include: Performing pre-processing on the area to be repaired of the aluminum alloy defective part to obtain a pre-processed aluminum alloy defective part; Performing a cold spray repair process on the pre-processed aluminum alloy defective part, and during the cold spray repair process, emitting a first laser to the pre-processed aluminum alloy defective part, so as to obtain an initial repaired aluminum alloy after the cold spray repair process is completed; emitting a second laser to the initial repair aluminum alloy, so as to perform microscale laser shock strengthening treatment on the initial repair aluminum alloy using the second laser to obtain a pre-repaired aluminum alloy, wherein the first laser is a low power density laser, and the second laser is a high power density laser; The pre-repaired aluminum alloy is repeatedly subjected to cold spray repair treatment and micro-scale laser shock strengthening treatment until a preset condition is reached, so as to obtain a repaired aluminum alloy defective part when the preset condition is reached.

2. The method according to claim 1, characterized in that The area to be repaired of the aluminum alloy defective part is pre-processed to obtain a pre-processed aluminum alloy defective part, comprising: Grinding and polishing the area to be repaired of the aluminum alloy defective part to obtain a polished aluminum alloy defective part; Cleaning the polished aluminum alloy defective part to obtain a cleaned aluminum alloy defective part; The cleaned aluminum alloy defective part is dried to obtain a pre-processed aluminum alloy defective part after the drying process.

3. The method according to claim 2, characterized in that The cleaned aluminum alloy defective part is dried using compressed hot air to obtain the pre-processed aluminum alloy defective part after drying.

4. The method according to claim 2, characterized in that: The area to be repaired of the aluminum alloy defective part is polished by using sandpaper to obtain a polished aluminum alloy defective part.

5. The method according to claim 4, characterized in that The mesh number of the sandpaper is between 600 mesh and 2000 mesh.

6. The method according to claim 5, characterized in that The mesh number of the sandpaper is 1200 meshes.

7. The method according to claim 1, characterized in that The pre-processed aluminum alloy defective part is subjected to cold spray repair treatment, and during the cold spray repair treatment, a first laser is emitted to the pre-processed aluminum alloy defective part to obtain an initial repaired aluminum alloy after the cold spray repair treatment is completed, comprising: According to the component performance of the pre-processed aluminum alloy defective part, a cold spray repair parameter and a first laser parameter are obtained; According to the cold spray repair parameters, controlling the cold spray equipment to perform cold spray repair processing on the pre-processed aluminum alloy defective parts; During the cold spray repair process, based on the first laser parameters, the first laser emitting device is controlled to generate the first laser, and the first laser is emitted to the pre-processed aluminum alloy defective part at a preset temperature to obtain the initial repaired aluminum alloy after the cold spray repair process is completed.

8. The method according to claim 7, characterized in that The cold spray repair parameters include the distance between the cold spray nozzle and the pre-processed aluminum alloy defective part, the powder incident angle, the cold spray gas pressure, the cold spray gas temperature and the repair path, and the first laser parameters include the laser power and the spot diameter.

9. The method according to claim 1, characterized in that: The step of emitting a second laser to the initially repaired aluminum alloy comprises: According to the performance of the component of the initially repaired aluminum alloy, obtaining second laser parameters, wherein the second laser parameters include laser energy, spot diameter, spot overlap rate, impact times and processing path; According to the second laser parameters, the second laser emitting device is controlled to generate a second laser, and the second laser is emitted to the initial repair aluminum alloy at a preset temperature, so as to perform microscale laser shock strengthening treatment on the initial repair aluminum alloy using the second laser.

10. The method according to claim 1, characterized in that After obtaining the repaired aluminum alloy defective part, the method further comprises: The repaired aluminum alloy defective part is subjected to grinding and polishing treatment, cleaning treatment and drying treatment in sequence, so as to obtain the actually repaired aluminum alloy defective part after drying treatment.

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