A thermoforming method for complex-shaped metal tubular fittings

Abstract

A method for thermoforming complex-shaped metal tubular fittings includes the following steps: Step 1: Selecting appropriate metal tubing or sheet metal to form the required tubular blank based on the shape and size characteristics of the metal tubular fitting; Step 2: Placing thermally expanded graphite into the tubular blank and compacting it to obtain a tubular blank filled with thermally expanded graphite; Step 3: Placing the tubular blank into the lower mold and then closing the upper mold; Step 4: Heating the entire or partial area of ​​the tubular blank filled with thermally expanded graphite to the initial expansion temperature to obtain a heated tubular blank; Step 5: Axially compressing the tubular blank, causing the material in the non-deformation zone to flow into the deformation zone; Step 6: Continuing to load to the forming temperature, the tubular blank continues to maintain axial compression and radial diameter increase until it completely fits the cavity of the rigid mold, at which point axial compression is stopped; Step 7: Stopping heating and cooling to room temperature; Step 8: Removing the thermally expanded graphite to obtain a complex-shaped metal tubular fitting. This forming method improves the forming quality and efficiency of such parts.

Description

Technical Field

[0001] This invention belongs to the field of processing technology for complex-shaped metal pipes, and specifically relates to a thermoforming method for complex-shaped metal pipes. This method uses thermally expandable graphite as a force transmission medium and utilizes its thermal expansion properties to thermoform complex-shaped metal pipes. Background Technology

[0002] Complex-shaped metal pipe fittings, such as reducing pipe fittings, branching pipe fittings, and pipe fittings with local protrusions, are common structural components in the aerospace, automotive, and other fields. Traditional manufacturing mainly uses bulging methods to increase the diameter of the pipe by radial loading, such as rigid die bulging, polyurethane bulging, and fluid bulging. If the pipe shape is complex, quality problems such as thinning and cracking are prone to occur. In such cases, axial loading is required in conjunction with radial loads to increase the diameter of the pipe blank while relying on the non-deformation zone to supplement the deformation zone. Methods such as high-pressure forming in fluid and polyurethane forming are also needed.

[0003] In addition, if the shape of the pipe is too complex, not only is it necessary to supplement the deformation zone with material from the non-deformation zone, but it is also necessary to utilize the high-temperature softening effect of the material under certain temperature conditions to achieve the forming of such parts, such as high-pressure forming in a hot medium. However, both internal high-pressure forming and high-pressure forming in a hot medium require the matching of axial and radial loads, making the operation and control process relatively complex.

[0004] Thermally expandable graphite is a material formed by artificially synthesizing natural graphite flakes by inserting other molecules, atoms, or ions into the interlayers. Under high-temperature conditions, the intercalations within the thermally expandable graphite rapidly absorb heat and vaporize, generating a thrust on the graphite flake structure, thus causing significant volume expansion. It can be used for the hot forming of lightweight alloy sheet metal parts, and also as a filler in the hot bending process of complex pipe fittings. Summary of the Invention

[0005] To simplify the process and facilitate operation, this invention proposes a method for thermoforming complex-shaped metal tubes. Under certain temperature conditions, only axial loading is applied to the tube blank, while the radial load is provided by the high-temperature expansion of thermally expanded graphite. Specifically, thermally expanded graphite is placed into the tube blank, and the graphite is further compacted within the blank. Then, both ends of the tube blank containing the expanded graphite are sealed to prevent the graphite from overflowing during heating. After heating to a certain temperature, the thermally expanded graphite begins to expand due to the temperature effect, acting radially on the tube blank to increase its diameter. Simultaneously, axial compression of the tube blank causes material from the non-deformation zone to flow into the deformation zone, suppressing wall thickness reduction caused by the increased diameter of the tube blank under the expansion force of the thermally expanded graphite, thus forming a complex-shaped metal tube.

[0006] Compared to other forming methods, thermally expandable graphite exhibits superior heat resistance and a wide expansion temperature range, making it suitable for hot forming of difficult-to-deform metal tubes such as aluminum alloys, magnesium alloys, and titanium alloys. Furthermore, the large expansion capacity of thermally expandable graphite at high temperatures meets the requirements for deforming variable-diameter tubes, branched tubes, and tubes with localized protrusions. Therefore, by heating to the initial metal forming temperature and applying axial force, complex-shaped metal tubes can be formed.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A method for thermoforming complex-shaped metal tubular fittings includes the following steps:

[0009] Step 1: Based on the shape and size characteristics of the metal fittings, select the corresponding metal pipes or plates to make the required pipe blanks;

[0010] Step 2: Place the thermally expanded graphite into the tube blank, and then compact the thermally expanded graphite in the tube blank to obtain a tube blank filled with thermally expanded graphite.

[0011] Step 3: Place the tube blank into the lower mold, and then close the upper mold;

[0012] Step 4: Heat the entire or partial area of ​​the tube blank filled with thermally expanding graphite to the initial expansion temperature using a heating device to obtain the heated tube blank;

[0013] Step 5: Control the punch to axially compress the tube blank, causing the material in the non-deformation zone of the tube blank to flow into the deformation zone;

[0014] Step 6: Continue loading to the forming temperature, the tube blank continues to be axially compressed and the radial diameter increases until it is completely fitted with the cavity of the rigid mold, and the punch stops moving;

[0015] Step 7: Stop heating and cool to room temperature;

[0016] Step 8: Remove the punch, open the mold, and take out the thermally expanded graphite to obtain a metal tube with a complex shape.

[0017] In step 1, the blank is prepared by either pipe cutting or sheet rolling.

[0018] In step 2, the amount of thermally expanded graphite used is determined based on the size of the formed part, and the internal volume of the hollow part is calculated to determine the required volume of thermally expanded graphite.

[0019] The thermally expanded graphite includes graphite obtained from natural graphite flakes by acid washing with acidic substances or graphite obtained by oxidation intercalation with strong oxidizing substances.

[0020] In step 4, the initial temperature for hot forming of the pipe fitting is the lowest temperature at which the thermally expanded graphite deforms the pipe fitting under a closed, high-temperature state, but it does not exceed the initial temperature for hot forming of the pipe fitting. The specific measurement method is as follows: place the thermally expanded graphite into the pipe blank and seal both ends; heat the pipe blank and the thermally expanded graphite; and measure the temperature at which the change in the pipe blank diameter reaches 2% as the initial temperature for hot forming of the pipe fitting.

[0021] The initial expansion temperature of the thermally expandable graphite ranges from 200℃ to 900℃. In the hot forming of complex metal tubes, the initial expansion temperature of the selected thermally expandable graphite should be lower than the hot forming temperature of the tube blank, and it should be ensured that the expansion force generated by the thermally expandable graphite is greater than the force required to increase the diameter of the tube blank.

[0022] In step 4, the forming temperature is set by a temperature control device and measured by a temperature sensor. The temperature sensor transmits the measured temperature to the temperature control system. When the measured temperature has not reached the set forming temperature, the temperature control system controls the heating device to heat; when the measured temperature reaches the set forming temperature, the temperature control system controls the heating device to maintain the temperature. The temperature control device includes a temperature sensor and a temperature control system. The temperature sensor is used to measure the forming temperature, and the temperature sensor and the temperature control system are connected. The temperature control system and the heating device are connected.

[0023] In step 5, the tube blank is axially compressed, and the compression amount is generally 5-20%.

[0024] The punch and drive mechanism are connected.

[0025] The heating device is a heating element or a heating furnace. The heating element is installed inside the rigid mold and is equipped with an asbestos insulation layer, which is wrapped around the outer surface of the rigid mold.

[0026] When heating is performed using the aforementioned heating furnace, the rigid mold is placed inside the furnace chamber.

[0027] The temperature sensor is a thermocouple, which is used to determine the forming temperature. It is installed in a rigid mold or inside a heating furnace.

[0028] The technical effects of this invention are as follows:

[0029] This invention discloses a thermoforming method for complex-shaped metal tubing using thermally expanded graphite as the forming medium. During the forming process, the tubing blank and thermally expanded graphite are heated simultaneously. Under these warm conditions, the plasticity of the metal tubing increases, its deformation resistance decreases, and it is easier to deform. Thermally expanded graphite possesses excellent heat resistance and thermal conductivity; it expands when the heating temperature reaches its initial expansion temperature, providing the radial load required to increase the diameter of the tubing blank. This thermoforming method using thermally expanded graphite as the forming medium is applicable not only to complex-shaped metal tubing such as reducing pipes, branched pipes, and pipes with local protrusions, but also to metal profiles requiring thermoforming. Compared to high-pressure forming methods using a hot medium, this method simplifies the operation and process of forming complex-shaped metal tubing, improving the quality and efficiency of forming such parts. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the thermoforming apparatus (initial state) for complex-shaped metal tubes using thermally expanded graphite as a forming medium in Embodiment 1 of the present invention.

[0031] Figure 2 This is a schematic diagram of the thermoforming apparatus (forming state) for complex-shaped metal tubes using thermally expanded graphite as the forming medium in Embodiment 1 of the present invention.

[0032] Figure 3 This is a schematic diagram of the structure of a thermoforming apparatus (initial state) for complex-shaped metal tubes using thermally expanded graphite as a forming medium in Embodiment 2 of the present invention.

[0033] Figure 4 This is a schematic diagram of the thermoforming apparatus (forming state) for a complex-shaped metal tube using thermally expanded graphite as a forming medium in Embodiment 2 of the present invention.

[0034] Figure 5 This is a schematic diagram of the thermoforming apparatus (initial state) for complex-shaped metal pipe fittings using heat-resistant mineral oil as the forming medium in Comparative Example 1 of the present invention.

[0035] Figure 6 This is a schematic diagram of the thermoforming apparatus (forming state) for a complex-shaped metal tube using heat-resistant mineral oil as the forming medium in Comparative Example 1 of the present invention.

[0036] Figure 7 This is a schematic diagram of the structure of the complex-shaped metal tube forming device (initial state) using polyurethane rubber as the forming medium in Comparative Example 2 of the present invention.

[0037] Figure 8 This is a schematic diagram of the structure of a complex-shaped metal tube forming device (forming state) using polyurethane rubber as the forming medium in Comparative Example 2 of the present invention.

[0038] 1- Rigid mold, 2- Tube blank, 3- Punch, 4- Thermally expanded graphite, 5- Heating element, 6- Thermocouple, 7- Temperature control system, 8- Heating furnace, 9- Polyurethane rubber. Detailed Implementation

[0039] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0040] In the following embodiments, the initial temperature for thermoforming the tube is the lowest temperature at which the thermally expanded graphite 4 deforms the tube under a closed, high-temperature state. The measurement method is as follows: the expanded graphite is placed into the tube blank 2 and both ends are sealed. The tube blank 2 and the expanded graphite are heated, and the temperature at which the diameter change of the tube blank 2 reaches 2% is measured as the initial temperature for thermoforming the tube.

[0041] Example 1

[0042] A thermoforming apparatus for complex-shaped metal tubes, such as Figure 1 and Figure 2 As shown, the forming device includes: a rigid mold 1, a punch 3, a heating element 5, a thermocouple 6, and a temperature control system 7; the rigid mold 1 is equipped with the heating element 5 and the thermocouple 6, the output end of the thermocouple 6 is connected to the input end of the temperature control system 7, and the output end of the temperature control system 7 is connected to the input end of the heating element 5. The rigid mold 1 includes an upper mold and a lower mold, the upper mold is fastened to the lower mold to form a cavity, a tube blank 2 is provided in the cavity, the tube blank 2 is filled with thermally expanded graphite 4, and punches 3 are provided at both ends of the tube blank 2 in the cavity of the rigid mold 1, and axial pressure is applied by the punches 3 at both ends to compress both ends of the tube blank 2.

[0043] In this embodiment, the tube blank 2 is made of aluminum alloy 5A06, with an outer diameter of 50mm, a wall thickness of 2mm, and a length of 180mm. The tube blank 2 is made into a tube with an outer diameter of 80mm and a length of 156mm, and the forming temperature is 250℃.

[0044] A method for thermoforming complex-shaped metal tubular parts using thermally expanded graphite as the forming medium, employing the aforementioned forming apparatus, includes the following steps:

[0045] Step 1: The metal pipe fittings are laser-cut into the required metal pipe blank 2;

[0046] Step 2: Place the thermally expanded graphite 4 into the metal tube blank 2, and then compact the thermally expanded graphite 4.

[0047] The forming temperature of aluminum alloy 5A06 tube blank 2 is determined to be 250℃. Therefore, thermally expandable graphite 4 with an initial expansion temperature of 180℃ is selected. The initial expansion temperature of thermally expandable graphite 4 differs from the forming temperature of tube blank 2 by 70℃.

[0048] Step 3: Place the metal tube blank 2 into the rigid mold 1, and then close the rigid mold 1;

[0049] Step 4: In this embodiment, the heating device uses heating element 5, which is placed inside the rigid mold 1 and kept warm. Considering the properties of thermally expanding graphite 4 and the forming temperature of aluminum alloy tube blank 2, the initial forming temperature is set through temperature control system 7. Thermocouple 6 measures the temperature inside the heating device during the heating process and heats the temperature to 200°C to obtain the heated tube blank 2.

[0050] Step 5: Slowly compress the punches 3 at both ends axially, so that the material in the non-deformation zone of the tube blank 2 flows into the deformation zone.

[0051] Step 6: Continue heating to the forming temperature of 250℃. The tube blank 2 continues to be axially compressed and its radial diameter increases until it is completely fitted with the cavity of the rigid mold 1. The punch 3 then stops moving.

[0052] Step 7: Stop heating and cool to room temperature;

[0053] Step 8: Remove punch 3, open rigid mold 1, remove thermally expanded graphite 4 from the metal tube, and obtain a metal tube with a complex shape.

[0054] Example 2

[0055] A thermoforming apparatus for complex-shaped metal tubes, such as Figure 3 and Figure 4 As shown, the forming device includes: a rigid mold 1, punches 3, thermocouples 6, a temperature control system 7, and a heating furnace 8. The rigid mold 1 is equipped with thermocouples 6. The output end of thermocouples 6 is connected to the input end of the temperature control system 7 located on the operating table. The output end of the temperature control system 7 is connected to the input end of the heating furnace 8. The initial forming temperature is set by the temperature control system 7. The thermocouples 6 measure the temperature inside the device during the heating process. The heating furnace 8 heats the forming mold and the tube blank 2 to the forming temperature required for the experiment. The rigid mold 1 is located inside the heating furnace 8. The rigid mold 1 includes an upper mold and a lower mold. The upper mold is fastened to the lower mold to form a cavity. The tube blank 2 is placed in the cavity. The tube blank 2 is filled with thermally expanded graphite 4. The two punches 3 pass through the heating furnace 8 and abut against both ends of the tube blank 2. The punches 3 at both ends apply axial pressure and compress both ends of the tube blank 2.

[0056] In this embodiment, the tube blank 2 is made of aluminum alloy 5A06, with an outer diameter of 50mm, a wall thickness of 2mm, and a length of 180mm. The tube blank 2 is made into a part with an outer diameter of 80mm and a length of 156mm, and the forming temperature is 250℃.

[0057] A method for thermoforming complex-shaped metal tubular parts using thermally expanded graphite as the forming medium, employing the aforementioned forming apparatus, includes the following steps:

[0058] Step 1: The metal pipe fittings are laser-cut into the required metal pipe blank 2;

[0059] Step 2: Place the thermally expanded graphite 4 into the metal tube blank 2, and then compact the thermally expanded graphite 4.

[0060] The forming temperature of aluminum alloy 5A06 tube blank 2 is determined to be 250℃. Therefore, thermally expandable graphite 4 with an initial expansion temperature of 180℃ is selected. The initial expansion temperature of thermally expandable graphite 4 differs from the forming temperature of tube blank 2 by 70℃.

[0061] Step 3: Place the metal tube blank 2 into the rigid mold 1, and then close the rigid mold 1;

[0062] Step 4: In this embodiment, the heating device selected is heating furnace 8. Considering the properties of thermally expanding graphite 4 and the forming temperature of aluminum alloy tube blank 2, the initial forming temperature is set by temperature control system 7, and thermocouple 6 measures the temperature inside the device during the heating process. The temperature is heated to 180°C to obtain the heated tube blank 2.

[0063] Step 5: Slowly compress the punches 3 at both ends axially, so that the material in the non-deformation zone of the tube blank 2 flows into the deformation zone.

[0064] Step 6: Continue heating to the forming temperature of 250℃. The tube blank 2 continues to be axially compressed and its radial diameter increases until it is completely fitted into the rigid mold 1. The punch 3 then stops moving.

[0065] Step 7: Stop heating and cool to room temperature;

[0066] Step 8: Remove punch 3, open rigid mold 1, remove thermally expanded graphite 4 from the metal tube, and obtain a metal tube with a complex shape.

[0067] Comparative Example 1

[0068] A thermoforming apparatus for complex-shaped metal tubular fittings filled with heat-resistant mineral oil, such as Figure 5 and Figure 6As shown, the forming device includes: a rigid mold 1, a punch 3, a heating element 5, a thermocouple 6, and a temperature control system 7; the rigid mold 1 is equipped with the heating element 5 and the thermocouple 6, the output end of the thermocouple 6 is connected to the input end of the temperature control system 7, and the output end of the temperature control system 7 is connected to the input end of the heating element 5. The rigid mold 1 includes an upper mold and a lower mold, the upper mold is fastened to the lower mold to form a cavity, a tube blank 2 is provided in the cavity, and punches 3 are provided at both ends of the tube blank 2 located in the cavity of the rigid mold 1. The punches 3 are axially provided with fluid channels for introducing heated heat-resistant mineral oil into the tube blank 2, and the axial pressure is applied by the punches 3 at both ends to compress both ends of the tube blank 2.

[0069] In this embodiment, the tube blank 2 is made of aluminum alloy 5A06, with an outer diameter of 50mm, a wall thickness of 2mm, and a length of 180mm. The tube blank 2 is made into a part with an outer diameter of 80mm and a length of 156mm. The forming temperature is 250℃, and the heat medium is heat-resistant mineral oil.

[0070] A method for thermoforming complex-shaped metal tubular parts using heat-resistant mineral oil as the forming medium, employing the aforementioned forming apparatus, includes the following steps:

[0071] Step 1: The metal pipe fittings are laser-cut into the required metal pipe blank 2;

[0072] Step 2: Place the metal tube blank 2 into the rigid mold 1 and close the rigid mold 1. Then, push the axial punches 3 at both ends horizontally to form a seal.

[0073] Step 3: Apply ultra-high pressure of 20-200 MPa inside the pipe fitting;

[0074] Step 4: In this embodiment, the heating device uses heating element 5, which is placed inside the rigid mold 1 and insulated. Considering the properties of the heat-resistant mineral oil and the forming temperature of the aluminum alloy tube blank 2, the initial forming temperature is set through the temperature control system 7, and the thermocouple 6 measures the temperature inside the device during the heating process, raising the temperature to 250°C.

[0075] Step 5: Inject the heated heat-resistant mineral oil into the tube blank 2 through the fluid channel, and at the same time, axially compress the punches 3 at both ends, so that the material in the non-deformation zone of the tube blank 2 flows into the deformation zone and into the rigid mold 1 that is completely in contact with it. Then stop the movement of the punches 3 and the injection of mineral oil.

[0076] Step 6: Stop heating and cool to room temperature;

[0077] Step 7: Remove punch 3, open rigid mold 1, pour out the heat-resistant mineral oil in the pipe, and obtain a complex-shaped metal pipe.

[0078] Observing the formed complex-shaped metal tubes, unlike in Example 1, the heat-resistant mineral oil used as the forming medium presents challenges in heat transfer, sealing, and part removal. The heat-resistant mineral oil requires high pressure to be applied inside the tube blank 2 to generate radial force. However, under the same conditions, the thermally expanding graphite 4 absorbs heat and expands, providing the radial force needed to increase the diameter of the tube blank 2, preventing wrinkling and cross-sectional distortion, improving forming quality, and simplifying the operation.

[0079] Comparative Example 2

[0080] A forming apparatus for complex-shaped metal tubes using polyurethane rubber as the forming medium, such as Figure 7 and Figure 8 As shown, the forming device includes: a rigid mold 1, a punch 3, and polyurethane rubber 9; the rigid mold 1 includes an upper mold and a lower mold, the upper mold is fastened to the lower mold to form a cavity, a tube blank 2 is provided in the cavity, the tube blank 2 is filled with polyurethane rubber 9, and punches 3 are provided at both ends of the tube blank 2 located in the cavity of the rigid mold 1, and pressure is applied by the punches 3 at both ends to axially compress both ends of the tube blank 2 simultaneously.

[0081] In this embodiment, the tube blank 2 is made of aluminum alloy 5A06, with an outer diameter of 50mm, a wall thickness of 2mm, and a length of 180mm. The tube blank 2 is made into a part with an outer diameter of 80mm and a length of 156mm.

[0082] A method for forming complex-shaped metal tubular components using polyurethane rubber as the forming medium, employing the aforementioned forming apparatus, includes the following steps:

[0083] Step 1: The metal pipe fittings are laser-cut into the required metal pipe blank 2;

[0084] Step 2: Place the metal tube blank 2 filled with polyurethane rubber 9 into the rigid mold 1, close the rigid mold 1, and then push the punches 3 at both ends horizontally to form a seal;

[0085] Step 3: Apply axial pressure through the punches 3 at both ends to make the material in the non-deformation zone of the tube blank 2 flow into the deformation zone until it is completely in contact with the rigid mold 1, and then stop the movement of the punches 3.

[0086] Step 4: Remove the punch 3, open the rigid mold 1, and take out the polyurethane rubber 9 from the pipe to obtain a complex-shaped metal pipe.

[0087] The complex-shaped metal tubular parts were observed and compared with those in Example 1. The forming medium inside the tube blank 2 was changed from thermally expanded graphite 4 to polyurethane rubber 9. The heat resistance temperature of polyurethane rubber 9 is generally below 200℃, and it will age when heated during operation, affecting the quality of the formed parts. Therefore, the forming process is cold forming at room temperature. However, the mold fitting accuracy of cold-formed parts is lower than that of hot-formed parts. Therefore, in Example 1, hot forming using thermally expanded graphite 4 is more beneficial for improving the forming accuracy of the parts than forming using polyurethane rubber 9.

[0088] Comparative Example 3

[0089] The molding apparatus of Comparative Example 3 is the same as that of Example 1, but the molding method differs from that of Example 1 in that:

[0090] In step 2, the forming temperature of aluminum alloy 5A06 tube blank 2 is determined to be 250℃, and thermal expansion graphite 4 with an initial expansion temperature of 250℃ is selected.

[0091] During the static heating process, at a temperature of 250℃, the diameter of the tube blank 2 containing thermally expanded graphite 4 changes by 2%. However, due to the small expansion force generated by thermally expanded graphite 4 under the forming temperature conditions, it is unable to provide the required radial force for increasing the diameter of the tube blank 2 for forming complex tubes. As a result, axial compression and wrinkling occur after the tube blank 2 is formed.

[0092] Comparative Example 4

[0093] The molding apparatus and molding method of Comparative Example 4 are the same as those of Example 1, except that:

[0094] The tube blank 2 uses titanium alloy TC4, the forming temperature is 550-600℃, and thermal expansion graphite 4 with an initial expansion temperature of 350℃ is selected.

[0095] During static heating at 350℃, the diameter change of the tube blank 2 filled with thermally expanded graphite titanium TC4 reached 2%. When this thermally expanded graphite 4 is filled into the tube blank 2 and compressed, the initial expansion temperature of the thermally expanded graphite 4 differs from the forming temperature of the tube blank 2 by 200℃. The pressure generated by the thermally expanded graphite 4 within the tube blank 2 exceeds the pressure required for plastic forming, resulting in a rapid increase in the diameter of the tube blank 2 and ultimately causing the tube to crack.

Claims

1. A method for thermoforming complex-shaped metal tubular fittings, characterized in that, Includes the following steps: Step 1: Based on the shape and size characteristics of the metal fittings, select the corresponding metal pipes or plates to make the required pipe blanks; Step 2: Place the thermally expanded graphite into the tube blank, and then compact the thermally expanded graphite in the tube blank to obtain a tube blank filled with thermally expanded graphite. Step 3: Place the tube blank into the lower mold, and then close the upper mold; Step 4: Heat the entire or partial area of ​​the tube blank filled with thermally expanded graphite to the tube blank thermoforming start temperature using a heating device to obtain the heated tube blank; Step 5: Control the punch to axially compress the tube blank, causing the material in the non-deformation zone of the tube blank to flow into the deformation zone; Step 6: Continue loading to the forming temperature, the tube blank continues to be axially compressed and the radial diameter increases until it is completely fitted with the cavity of the rigid mold, and the punch stops moving; Step 7: Stop heating and cool to room temperature; Step 8: Remove the punch, open the mold, and take out the thermally expanded graphite to obtain a metal tube with a complex shape; In step 4, the starting temperature of tube blank hot forming is the lowest temperature at which the thermally expanded graphite deforms the tube blank under closed high temperature conditions, but it is lower than the tube blank hot forming temperature. The specific measurement method is as follows: put the thermally expanded graphite into the tube blank and seal both ends, heat the tube blank and the thermally expanded graphite, and measure the temperature at which the change in the diameter of the tube blank reaches 2% as the starting temperature of tube blank hot forming. In step 5, the tube blank is axially compressed by 5-20%. The initial expansion temperature of thermally expanded graphite differs from the forming temperature of the tube blank by 70°C.

2. The method for thermoforming complex-shaped metal tubular fittings according to claim 1, characterized in that: In step 1, the blank is prepared by either pipe cutting or sheet rolling.

3. The method for thermoforming complex-shaped metal tubular fittings according to claim 1, characterized in that: In step 2, the amount of thermally expanded graphite used is determined based on the size of the formed part, and the internal volume of the hollow part is calculated to determine the required volume of thermally expanded graphite.

4. The method for thermoforming complex-shaped metal tubular fittings according to claim 1, characterized in that: The thermally expanded graphite includes graphite obtained from natural graphite flakes by acid washing with acidic substances or graphite obtained by oxidative intercalation with strong oxidizing substances.

5. The method for thermoforming complex-shaped metal tubular fittings according to claim 1, characterized in that: In step 4, the forming temperature is set by a temperature control device and measured by a temperature sensor. The temperature sensor transmits the measured temperature to the temperature control system. When the measured temperature does not reach the set forming temperature, the temperature control system controls the heating device to heat. When the measured temperature reaches the set forming temperature, the temperature control system controls the heating device to maintain the temperature. The temperature control device includes a temperature sensor and a temperature control system. The temperature sensor is used to measure the forming temperature. The temperature sensor and the temperature control system are connected. The temperature control system and the heating device are connected.

6. The method for thermoforming complex-shaped metal tubular fittings according to claim 1, characterized in that: The heating device described in step 4 is a heating element or a heating furnace. The heating element is installed inside the rigid mold, and the outer surface of the rigid mold is covered with an insulation layer. When the heating furnace is used for heating, the rigid mold is placed in the furnace chamber.

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