Pulse current auxiliary roll bulging forming method of micro-channel structure

Abstract

The invention discloses a pulse current auxiliary roll bulging forming method of a micro-channel structure, which comprises the following steps: step 1, pre-straining along the rolling direction of a stainless steel foil, and carrying out laser shock treatment on a selected area; secondly, the foil is placed in a pulse current auxiliary roll bulging forming device in the rolling direction to prepare a blank; and thirdly, the blank piece is placed in a high-temperature vibration mold to be subjected to shape correction and stress relief. The method is suitable for high-efficiency and low-cost preparation of the metal bipolar plate of the hydrogen fuel cell with high performance and long service life.

Description

Technical Field

[0001] This invention relates to advanced manufacturing technology for improving the reaction performance and lifespan of green hydrogen fuel cells, and in particular to a pulse current-assisted roll forming method for high-performance, long-life microfluidic structures. Background Technology

[0002] High-reactivity, long-life fuel cell metal bipolar plates are key components in the manufacture of high-efficiency hydrogen fuel cell stacks. Increasing the aspect ratio of the microchannel structure is an important way to improve its reaction performance, while reducing the number of micro-defects and increasing fatigue resistance are key measures to enhance the service life of the microchannel structure. Existing forming methods have limited achievable aspect ratios, resulting in numerous microcracks and defects, and short service life.

[0003] The information disclosed in this background section is intended only to enhance the understanding of the overall background of the invention and should not be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art. Summary of the Invention

[0004] The purpose of this invention is to provide a pulse current-assisted roller bulging forming method for high-performance, long-life microchannel structures. This method utilizes bidirectional vibration for shaping and stress relief, normal vibration to improve aspect ratio and shape uniformity, and lateral vibration friction to pre-set surface compressive stress on the microchannel sidewalls, thereby enhancing fatigue resistance and service life.

[0005] To achieve the above objectives, embodiments of the present invention provide a pulse current-assisted roll forming method for high-performance, long-life microchannel structures, characterized by comprising the following steps:

[0006] Step 1: A stainless steel foil with a grain size of 20–50 µm and a thickness of 50–200 µm is stretched along the rolling direction to a strain of 1–5%. The foil is then cut into foil blanks with a length of 300–700 mm and a width of 10–300 mm. Subsequently, a microchannel structure is selectively heated using laser in-situ along the rolling direction of the foil blank. The temperature control range is 900–950 ℃, the linear energy density is 3–6 J / mm, the scanning speed is 10–30 mm / s, the rectangular flat-top laser spot size is 0.05–0.3 mm, and the furnace is cooled to room temperature under argon protection.

[0007] Step 2: The foil blank is placed along the rolling direction in a pulse current-assisted roll forming device to prepare a microchannel structure blank. Each microchannel structure is divided into three stages with frequency conversion control of forming process parameters. The roll bending stage lasts for 0.5~2 s, with a current density of 100~150 A / mm², a frequency of 200~400 Hz, a pulse width of 50~80 μs, and a roll pressure of 200~600 N. The bulging stage lasts for 2~4 s, with a current density of 10~60 A / mm², a frequency of 100~800 Hz, a pulse width of 20~200 μs, and a roll pressure of 400~800 N. The roll pressing and bottoming stage lasts for 0.5~3 s, with a current density of 100~260 A / mm², a frequency of 300~400 Hz, a pulse width of 50~80 μs, and a roll pressure of 800~1200 N.

[0008] Step 3: Place the microchannel structure blank in a mold with a temperature of 150~400 ℃, a vibration frequency of 50~1000 Hz, and an amplitude of 5~8 µm for shaping and stress relief. The vibration direction is along the normal of the mold, the holding pressure is 100~800 N, the action time is 5~30 s, the protective gas is argon, and the shape of the shaping mold is the inner surface of the microchannel structure.

[0009] In one or more embodiments of the present invention, the selected area in step one is the curved region and the lateral waist bulging region of the microchannel, the curved region being 10~300 mm long and 50~100 μm wide, and the lateral waist bulging region being 10~300 mm long and 100~3000 μm wide.

[0010] In one or more embodiments of the present invention, 1 to 100 selection areas are arrayed on the foil blank in step one in units of microchannel structure, and the interval between each selection area is 0.5 to 5 mm.

[0011] In one or more embodiments of the present invention, the microstructure of the foil blank obtained after furnace cooling to room temperature in step one is characterized by a rolling direction / transverse grain size ratio of 1.1 to 1.3 for non-selective grains and equiaxed fine grains of 20 to 30 μm for selected grains.

[0012] In one or more embodiments of the present invention, the pulse current assisted roller expansion forming device in step two includes a high-frequency pulse power supply and a master and slave microstructure roller expansion wheel. The pulse current flows in from the master microstructure roller expansion wheel, passes through the foil blank, and flows out from the slave microstructure roller expansion wheel. The pulse output frequency of the high-frequency pulse power supply is 3~1000 Hz, the current is 1~5000A, and the pulse width is 1~2000 μs. The master and slave microstructure roller expansion wheels are provided with 8~128 U-shaped micro-protrusions along the circumference. The height of the micro-protrusion is 1~3 mm, the bottom width is 1~3 mm, the bottom radius is 1~5 mm, and the top is a semi-circular surface with a radius of 1~10 mm.

[0013] In one or more embodiments of the present invention, after the microfluidic structure blank is rolled in step two, the main and driven microstructure rolling rollers reverse direction, with a reversal speed of 1~3mm / s, a pulse current density of 100~260 A / mm², a frequency of 300~400 Hz, a pulse width of 50~80 μs, and a rolling pressure of 300~600 N.

[0014] In one or more embodiments of the present invention, the vibration described in step three is a bidirectional vibration along the normal and transverse directions of the mold, with a normal vibration frequency of 50~1000 Hz and an amplitude of 5~8 µm, and a transverse vibration frequency of 50~100 Hz and an amplitude of 1~3 µm.

[0015] Compared with the existing technology, the new process proposed in this invention can overcome the problems of small aspect ratio, many microcracks and defects, and low lifespan in the forming of high aspect ratio ultrathin wall microchannels. It has the following advantages: (1) First, the foil is pre-stretched along a specific direction to form a grain structure with a specific length-to-diameter ratio along the rolling direction and to pre-set a small amount of micro-defects. This is matched with the subsequent pulse current parameters to improve the electroplasticity, Joule heating and electron wind effects of the pulse current. (2) Laser in-situ heating treatment is used to heat-treat the foil blank in selected areas to form a fine isomorphic grain structure in the rounded corner area to eliminate bending springback, and a fine equiaxed grain structure with a small amount of defects is formed in the side waist stretching area to eliminate microcracks and defects. (3) Based on the microstructure of the selected area and the flow characteristics of the roller expansion material, the process parameters of the microchannel structure forming cycle are precisely controlled by frequency conversion in three stages: in the roller bending forming stage, dislocation slip and grain rotation are promoted to eliminate elastic stress and springback dispersion; in the expansion stage, electroplasticity and Joule heating are used to promote the uniform stretching ability of the side waist and reduce the formation rate of microcracks and microdefects; in the roller pressing and bottoming stage, the electron wind effect and Joule heating effect are used to form a molten pool at the tip of the microcrack, promote the atomic diffusion and migration on both sides of the microcrack defect, and promote rapid healing. (4) High-frequency shaping and stress relief are performed on the blank in the high-temperature vibration mold, and the hard-oriented grains are strongly rolled to unify the shaping of each channel and pre-store the residual internal stress on the surface, which greatly improves the fatigue strength and service life of the microchannel structure. (5) In the temperature range where atomic kinetic energy is greatly increased, bidirectional vibration is used to shape and relieve stress, normal vibration improves the aspect ratio and shape uniformity, and transverse vibration friction pre-sets surface compressive stress on the side wall of the microchannel to enhance fatigue strength and service life. Therefore, the method of the present invention is particularly suitable for the efficient, high-quality, and controllable manufacturing of metal bipolar plates for hydrogen fuel cells with an aspect ratio > 0.8 and a service life > 2000h. Attached Figure Description

[0016] Figure 1 This is a comparison diagram of a pulse current-assisted roller bulging forming method for a high-performance, long-life microchannel structure according to an embodiment of the present invention and a conventional method. Detailed Implementation

[0017] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.

[0018] Unless otherwise expressly stated, throughout the specification and claims, the term "comprising" or its variations such as "including" or "comprises" shall be understood to include the stated elements or components without excluding other elements or other components.

[0019] A pulse current-assisted roll forming method for high-performance, long-life microchannel structures according to a preferred embodiment of the present invention is as follows:

[0020] Step 1: A stainless steel foil with a grain size of 20–50 µm and a thickness of 50–200 µm is stretched along the rolling direction to a strain of 1–5%. The foil is then cut into foil blanks with a length of 300–700 mm and a width of 10–300 mm. Subsequently, a microchannel structure is selectively heated using laser in-situ along the rolling direction of the foil blank. The temperature control range is 900–950 ℃, the linear energy density is 3–6 J / mm, the scanning speed is 10–30 mm / s, the rectangular flat-top laser spot size is 0.05–0.3 mm, and the furnace is cooled to room temperature under argon protection.

[0021] Step 2: The foil blank is placed along the rolling direction in a pulse current-assisted roll forming device to prepare a microchannel structure blank. Each microchannel structure is divided into three stages with frequency conversion control of forming process parameters. The roll bending stage lasts for 0.5~2 s, with a current density of 100~150 A / mm², a frequency of 200~400 Hz, a pulse width of 50~80 μs, and a roll pressure of 200~600 N. The bulging stage lasts for 2~4 s, with a current density of 10~60 A / mm², a frequency of 100~800 Hz, a pulse width of 20~200 μs, and a roll pressure of 400~800 N. The roll pressing and bottoming stage lasts for 0.5~3 s, with a current density of 100~260 A / mm², a frequency of 300~400 Hz, a pulse width of 50~80 μs, and a roll pressure of 800~1200 N.

[0022] Step 3: Place the microchannel structure blank in a mold with a temperature of 150~400 ℃, a vibration frequency of 50~1000 Hz, and an amplitude of 5~8 µm for shaping and stress relief. The vibration direction is along the normal of the mold, the holding pressure is 100~800 N, the action time is 5~30 s, the protective gas is argon, and the shape of the shaping mold is the inner surface of the microchannel structure.

[0023] The selected area mentioned in step one is the curved region and the lateral waist bulging region of the microchannel. The curved region is 10~300mm long and 50~100 μm wide, and the lateral waist bulging region is 10~300 mm long and 100~3000 μm wide.

[0024] In step one, 1 to 100 selection areas are arrayed on the foil blank in units of microchannel structure, with each selection area spaced 0.5 to 5 mm apart.

[0025] The microstructure of the foil blank obtained after furnace cooling to room temperature as described in step one is as follows: the rolling direction / transverse grain size ratio of the non-selective grains is 1.1~1.3, and the shape of the selected grains is equiaxed fine grains of 20~30 μm.

[0026] The pulse current-assisted roller expansion forming device described in step two includes a high-frequency pulse power supply and master and slave microstructure roller expansion wheels. The pulse current flows in from the master microstructure roller expansion wheel, passes through the foil blank, and flows out from the slave microstructure roller expansion wheel. The pulse output frequency of the high-frequency pulse power supply is 3~1000 Hz, the current is 1~5000 A, and the pulse width is 1~2000 μs. The master and slave microstructure roller expansion wheels are provided with 8~128 U-shaped micro-protrusions along the circumference. The height of the micro-protrusions is 1~3 mm, the bottom width is 1~3 mm, the bottom radius is 1~5 mm, and the top is a semi-circular surface with a radius of 1~10 mm.

[0027] After the microchannel structure blank is rolled and expanded as described in step two, the main and driven microstructure rolling rollers reverse direction. The reversal speed is 1~3mm / s, the pulse current density is 100~260 A / mm², the frequency is 300~400 Hz, the pulse width is 50~80 μs, and the rolling pressure is 300~600 N.

[0028] The vibration described in step three is a bidirectional vibration along the normal and transverse directions of the mold. The normal vibration frequency is 50~1000Hz and the amplitude is 5~8 µm, while the transverse vibration frequency is 50~100 Hz and the amplitude is 1~3 µm.

[0029] Example 1

[0030] Taking the fabrication of 100 pieces of 316L stainless steel bipolar plates, each with a length of 300 mm, a width of 200 mm, a microchannel groove width of 1 mm, a depth-to-width ratio of 1.1, a spacing of 1 mm, a thickness of 100 μm, and a microchannel array of 80 pieces, as an example, the implementation process of the method of the present invention for fabricating high-performance, long-life microchannel structures is illustrated below:

[0031] Step 1: 316L stainless steel foil with a grain size of 20 µm and a thickness of 100 µm is stretched along the rolling direction with a strain of 3%. It is then cut into foil blanks with a length of 300 mm and a width of 200 mm. Subsequently, the microchannel structure is selectively heated by laser in-situ along the rolling direction of the foil blank. The temperature control range is 925℃, the linear energy density is 4 J / mm, the scanning speed is 20 mm / s, the rectangular flat-top spot is 0.1~0.15 mm, and argon protection is used. The foil blank is cooled to room temperature in the furnace. The microstructure characteristics of the obtained foil blank are: the rolling direction / transverse grain size ratio of the non-selected area grains is 1.2, and the shape of the selected area grains is equiaxed fine grains of 25 μm.

[0032] Step 2: The foil blank is placed along the rolling direction in a pulse current-assisted roll forming device to prepare a microchannel structure blank. Each microchannel structure is divided into three stages with frequency conversion control of forming process parameters. The roll bending stage lasts for 1 s, with a current density of 125 A / mm², a frequency of 300 Hz, a pulse width of 60 μs, and a roll pressure of 300 N. The bulging stage lasts for 3 s, with a current density of 30 A / mm², a frequency of 300 Hz, a pulse width of 100 μs, and a roll pressure of 500 N. The roll pressing and bottoming stage lasts for 1.5 s, with a current density of 160 A / mm², a frequency of 350 Hz, a pulse width of 60 μs, and a roll pressure of 900 N.

[0033] Step 3: Place the microchannel structure blank in a mold at a temperature of 300 ℃, a vibration frequency of 600 Hz, and an amplitude of 7 µm for shaping and stress relief. The vibration direction is along the normal of the mold, the holding pressure is 600 N, the action time is 15 s, the protective gas is argon, and the shape of the shaping mold is the inner surface of the microchannel structure.

[0034] Example 2

[0035] Step 1: 316L stainless steel foil with a grain size of 20 µm and a thickness of 100 µm is stretched along the rolling direction with a strain of 3%. It is then cut into foil blanks with a length of 300 mm and a width of 200 mm. Subsequently, the microchannel structure is selectively heated by laser in-situ along the rolling direction of the foil blank. The temperature control range is 925℃, the linear energy density is 4 J / mm, the scanning speed is 20 mm / s, the rectangular flat-top spot is 0.1~0.15 mm, and argon protection is used. The foil blank is cooled to room temperature in the furnace. The microstructure characteristics of the obtained foil blank are: the rolling direction / transverse grain size ratio of the non-selected area grains is 1.2, and the shape of the selected area grains is equiaxed fine grains of 25 μm.

[0036] Step 2: The foil blank is placed along the rolling direction in a pulsed current-assisted roll forming device to prepare a microchannel structure blank. Each microchannel structure is formed using three stages of frequency conversion control of the forming process parameters: the roll bending stage lasts 1 s, with a current density of 125 A / mm², a frequency of 300 Hz, a pulse width of 60 μs, and a roll pressure of 300 N; the roll forming stage lasts 3 s, with a current density of 30 A / mm², a frequency of 300 Hz, a pulse width of 100 μs, and a roll pressure of 500 N; and the roll forming stage lasts 1.5 s, with a current density of 160 A / mm², a frequency of 350 Hz, a pulse width of 60 μs, and a roll pressure of 900 N. After the roll forming of the microchannel structure blank is completed, the main and driven microstructure roll forming wheels reverse direction at a speed of 2 mm / s, with a pulse current density of 160 A / mm², a frequency of 350 Hz, and a pulse width of 60 μs. μs, roller pressing pressure is 400 N;

[0037] Step 3: Place the microchannel structure blank in a mold at a temperature of 300 ℃ and vibrate it with bidirectional ultrasonic vibration along the normal and transverse directions of the mold. The normal vibration frequency is 100 Hz and the amplitude is 6 µm, and the transverse vibration frequency is 60 Hz and the amplitude is 2 µm. The mold is used for shaping and stress relief. The holding pressure is 600 N and the action time is 15 s. The protective gas is argon. The shape of the shaping mold is the inner surface of the microchannel structure.

[0038] Comparative Example 1

[0039] One hundred 316L stainless steel bipolar plates were manufactured using a traditional stamping method, with the following steps:

[0040] Step 1: Clean the 316L stainless steel with a length of 300 mm, a width of 200 mm, and a thickness of 100 μm;

[0041] Step 2: Place the pure titanium foil in the stamping die along the rolling direction. The die surface is the inner contour of the microchannel structure.

[0042] Step 3: Start the press and move it downwards to complete the forming process, then hold the pressure at 1000N for 60 seconds;

[0043] Step 4: Pick up the item and inspect it.

[0044] Comparison results

[0045] Table 1 Comparison between the implementation method of this invention and the traditional method

[0046]

[0047] This invention proposes a pulse current-assisted roll forming method for high-performance, long-life microchannel structures. (1) First, the foil is pre-stretched along a specific direction to form a grain structure with a specific aspect ratio along the rolling direction and to pre-set micro-defects, which are matched with the subsequent pulse current parameters to improve the electroplasticity, Joule heating and electron wind effects of the pulse current. (2) The foil blank is subjected to heat treatment impact in a selected area using laser in-situ heating treatment to form a fine isomorphic grain structure in the rounded corner area to eliminate bending springback, and a fine equiaxed grain structure with micro-defects is formed in the side waist stretching area to eliminate micro-crack defects. (3) Based on the microstructure of the selected area and the flow characteristics of the roller expansion material, the process parameters of the microchannel structure forming cycle are precisely controlled by frequency conversion in three stages: in the roller bending forming stage, dislocation slip and grain rotation are promoted to eliminate elastic stress and springback dispersion; in the expansion stage, electroplasticity and Joule heating are used to promote the uniform stretching ability of the side waist and reduce the formation rate of microcracks and microdefects; in the roller pressing and bottoming stage, the electron wind effect and Joule heating effect are used to form a molten pool at the tip of the microcrack, promote the atomic diffusion and migration on both sides of the microcrack defect, and promote rapid healing. (4) High-frequency shaping and stress relief are performed on the blank in the high-temperature vibration mold, and the hard-oriented grains are strongly rolled to unify the shaping of each channel and pre-store the residual internal stress on the surface, which greatly improves the fatigue strength and service life of the microchannel structure. (5) In the temperature range where atomic kinetic energy is greatly increased, bidirectional vibration is used to shape and relieve stress, normal vibration improves the aspect ratio and shape uniformity, and transverse vibration friction pre-sets surface compressive stress on the side wall of the microchannel to enhance fatigue strength and service life. Therefore, the method of the present invention is particularly suitable for the efficient, high-quality, and controllable manufacturing of metal bipolar plates for hydrogen fuel cells with an aspect ratio > 0.8 and a service life > 2000h. Thus, as shown in Table 1, Examples 1 and 2 of the method of the present invention significantly improve the forming aspect ratio, surface accuracy, production efficiency, and service life compared to traditional methods.

[0048] The foregoing description of specific exemplary embodiments of the invention is for illustrative and explanatory purposes. These descriptions are not intended to limit the invention to the precise forms disclosed, and it will be apparent that many changes and variations can be made in accordance with the foregoing teachings. The exemplary embodiments were chosen and described in order to explain the specific principles of the invention and its practical application, thereby enabling those skilled in the art to implement and utilize various different exemplary embodiments of the invention, as well as various different choices and variations. The scope of the invention is intended to be defined by the claims and their equivalents.

Claims

1. A pulse current-assisted roller bulging forming method for a microchannel structure, characterized in that, Includes the following steps: Step 1: A stainless steel foil with a grain size of 20–50 µm and a thickness of 50–200 µm is stretched along the rolling direction to a strain of 1–5%. The foil is then cut into foil blanks with a length of 300–700 mm and a width of 10–300 mm. Subsequently, a microchannel structure is selectively heated using laser in-situ along the rolling direction of the foil blank. The temperature control range is 900–950 ℃, the linear energy density is 3–6 J / mm, the scanning speed is 10–30 mm / s, the rectangular flat-top laser spot size is 0.05–0.3 mm, and the furnace is cooled to room temperature under argon protection. Step 2: The foil blank is placed along the rolling direction in a pulse current-assisted roll forming device to prepare a microchannel structure blank. Each microchannel structure is divided into three stages with frequency conversion control of forming process parameters. The roll bending stage lasts for 0.5~2 s, with a current density of 100~150 A / mm², a frequency of 200~400 Hz, a pulse width of 50~80 μs, and a roll pressure of 200~600 N. The bulging stage lasts for 2~4 s, with a current density of 10~60 A / mm², a frequency of 100~800 Hz, a pulse width of 20~200 μs, and a roll pressure of 400~800 N. The roll pressing and bottoming stage lasts for 0.5~3 s, with a current density of 100~260 A / mm², a frequency of 300~400 Hz, a pulse width of 50~80 μs, and a roll pressure of 800~1200 N. Step 3: Place the microchannel structure blank in a mold with a temperature of 150~400 ℃, a vibration frequency of 50~1000 Hz, and an amplitude of 5~8 µm for shaping and stress relief. The vibration direction is along the normal of the mold, the holding pressure is 100~800 N, the action time is 5~30 s, the protective gas is argon, and the shape of the shaping mold is the inner surface of the microchannel structure.

2. The pulse current-assisted roller bulging forming method for a microchannel structure according to claim 1, characterized in that: The selected area mentioned in step one is the curved region and the lateral waist bulging region of the microchannel. The curved region is 10~300 mm long and 50~100 μm wide, and the lateral waist bulging region is 10~300 mm long and 100~3000 μm wide.

3. The pulse current-assisted roller bulging forming method for a microchannel structure according to claim 1, characterized in that: In step one, 1 to 100 selection areas are arrayed on the foil blank in units of microchannel structure, with each selection area spaced 0.5 to 5 mm apart.

4. The pulse current-assisted roller bulging forming method for a microchannel structure according to claim 1, characterized in that: The microstructure of the foil blank obtained after furnace cooling to room temperature as described in step one is as follows: the rolling direction / transverse grain size ratio of the non-selective grains is 1.1~1.3, and the shape of the selected grains is equiaxed fine grains of 20~30 μm.

5. The pulse current-assisted roller bulging method for microchannel structures according to claim 1, characterized in that: The pulse current-assisted roller expansion forming device described in step two includes a high-frequency pulse power supply and master and slave microstructure roller expansion wheels. The pulse current flows in from the master microstructure roller expansion wheel, passes through the foil blank, and flows out from the slave microstructure roller expansion wheel. The pulse output frequency of the high-frequency pulse power supply is 3~1000 Hz, the current is 1~5000 A, and the pulse width is 1~2000 μs. The master and slave microstructure roller expansion wheels are provided with 8~128 U-shaped micro-protrusions along the circumference. The height of the micro-protrusions is 1~3 mm, the bottom width is 1~3 mm, the bottom radius is 1~5 mm, and the top is a semi-circular surface with a radius of 1~10 mm.

6. The pulse current-assisted roller bulging method for microchannel structures according to claim 1, characterized in that: After the microchannel structure blank is rolled and expanded as described in step two, the main and driven microstructure rolling rollers reverse direction. The reversal speed is 1~3mm / s, the pulse current density is 100~260 A / mm², the frequency is 300~400 Hz, the pulse width is 50~80 μs, and the rolling pressure is 300~600 N.

7. The pulse current-assisted roller bulging forming method for a microchannel structure according to claim 1, characterized in that: The vibration described in step three is a bidirectional vibration along the normal and transverse directions of the mold. The normal vibration frequency is 50~1000 Hz and the amplitude is 5~8 µm, while the transverse vibration frequency is 50~100 Hz and the amplitude is 1~3 µm.

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