Stirring head for magnesium alloy friction stir welding and machining method of stirring head

Abstract

The invention discloses a stirring head for magnesium alloy friction stir welding and a machining method of the stirring head. The machining method comprises the following steps that S1, a blank meeting the requirement of a stirring needle is obtained; heating and annealing the blank; s2, the conical cutter handle, the shaft shoulder, the stirring needle and the thread groove are machined through a numerical control machine tool, and the conical cutter handle, the shaft shoulder and the stirring needle are sequentially welded into a whole through a rotary friction welding method; s3, the stirring head is subjected to heat treatment; s4, a nitriding / nitrocarburizing process is adopted, a WC-Co coating is deposited through laser cladding or chromium aluminum nitride is deposited through physical vapor deposition, and surface strengthening treatment is conducted on the shaft shoulder needle and the stirring needle; and S5, carrying out nondestructive testing on the stirring head to obtain a finished product. According to the machining method, the diameter of the shaft shoulder of the stirring head is increased, and the welding speed of magnesium alloy is increased by 100%; the large-shaft-shoulder stirring needle can promote better mutual fusion of base materials, and the welding quality is improved; the wear resistance, the high-temperature strength and the fatigue resistance are high.

Description

Technical Field

[0001] This invention belongs to the field of friction stir welding technology, specifically relating to a stirring head for friction stir welding of magnesium alloys and its processing method. Background Technology

[0002] Magnesium alloys are alloys composed of magnesium as the base metal and other elements. The density of magnesium alloys is only 1.74–1.8 g / cm³. 3 Magnesium-aluminum alloys contain 64% of the mass of aluminum alloys and 25% of the mass of steel, but their specific strength (strength to density ratio) is significantly higher than that of aluminum alloys and engineering plastics. They also have a higher specific modulus of elasticity, better heat dissipation, better vibration damping, and greater impact load capacity than aluminum alloys. Furthermore, they exhibit good resistance to organic matter and alkali corrosion, making them particularly suitable for lightweight applications such as aerospace and new energy vehicles. Currently, magnesium-aluminum alloys are the most widely used, followed by magnesium-manganese alloys and magnesium-zinc-zirconium alloys.

[0003] Friction stir welding is a solid-state joining process that uses the heat generated by the rotational friction of the stirring head and the heat of plastic deformation as the welding heat source, resulting in a solid-state bond between the plates. The original grains in the fusion region of the joint are transformed into fine equiaxed recrystallized grains. Because the welding temperature does not reach the melting point of magnesium alloys during friction stir welding, traditional fusion welding defects such as porosity, cracks, inclusions, and joint weakening can be fundamentally avoided.

[0004] Friction stir welding requires precise selection of welding parameters (rotation speed, welding speed, tilt angle, and penetration depth), as well as the structure of the stirring head (shape of the stirring pin and shoulder). The stirring head is a key component of the friction stir welding equipment. During welding, the stirring head generates heat through friction with the surrounding material, breaks up the oxide layer on the workpiece surface, and transfers ductile material. Its structural design determines various process parameters and weld quality throughout the entire welding process. The stirring head mainly consists of two parts: the shoulder and the stirring pin. Factors such as the shoulder diameter, the shape and size of the stirring pin directly affect the heat input and plastic flow of the material during friction stir welding, thus influencing the joint microstructure, the size and distribution of the metal oxide layer, and mechanical properties.

[0005] Currently, the same stirring pin is used in friction stir welding of magnesium alloys as in friction stir welding of aluminum alloys. However, magnesium alloys have the characteristics of slow thermal conductivity and fast heat dissipation, requiring a greater heat input than aluminum alloys during friction stir welding, resulting in a slower welding speed when welding magnesium alloys. Summary of the Invention

[0006] The purpose of this invention is to provide a stirring head for magnesium alloy friction stir welding and its processing method, which improves the efficiency and quality of friction welding. Under the condition of the same stirring pin length, a large-diameter shoulder stirring pin is used to solve the problems of slow welding speed and low efficiency in existing magnesium alloy friction stir welding technology.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A stirring head for friction stir welding of magnesium alloys is provided, wherein a conical tool holder, a shoulder, and a stirring pin are sequentially welded together to form a single unit using a rotary friction welding method; a threaded groove is machined on the shoulder using a CNC machine tool; wherein the diameter of the shoulder is 4 to 5 times the diameter of the stirring pin.

[0008] In conventional aluminum alloy friction stir welding, the shoulder diameter of the stirring pin is 2 to 3 times the pin diameter. For example, when the stirring pin is 2.5 mm long and 3.5 mm in diameter, the matching shoulder diameter is 10 mm. However, for the magnesium alloy stirring head of this invention, the shoulder uses a larger proportional diameter, 4 to 5 times the stirring pin diameter, which can significantly increase the heat generation during friction stir welding. For example, when the stirring pin is 2.5 mm long and 3.5 mm in diameter, the preferred shoulder diameter for the magnesium alloy-specific stirring pin is 16 mm. This increases the heat generation by 40 times at the same rotational speed, thus achieving higher welding speeds at lower spindle speeds. Furthermore, the increased stirring area results in a more uniform composition in the stirring zone, improving weld strength. The present invention discloses a processing method for a stirring head for friction stir welding of magnesium alloys, comprising the following steps: S1, obtaining a blank that meets the requirements of the stirring pin; heating and annealing the blank; S2, processing a conical tool holder, a shoulder, a stirring pin, and a threaded groove using a CNC machine tool, and sequentially welding the conical tool holder, the shoulder, and the stirring pin into a single unit using a rotary friction welding method; S3, heat-treating the stirring head; S4, performing surface strengthening treatment on the shoulder pin and the stirring pin using a nitriding / nitrocarburizing process, laser cladding deposition of WC-Co coating, or physical vapor deposition of chromium aluminum nitride; S5, non-destructively inspecting the stirring head for defects to obtain the finished product.

[0009] The conical tool holder, shoulder, and stirring pin can be made of different materials, such as all of them being made of high-temperature alloys (e.g., Inconel 718) or tool steel (e.g., H13). Preferably, the conical tool holder is made of high-temperature alloys (e.g., Inconel 718) or tool steel (e.g., H13), and the shoulder and stirring pin are made of tungsten steel.

[0010] Preferably, in step S1, the annealing is performed by heating the billet to 850-900°C and holding it at that temperature for 3-4 hours, cooling it in the furnace to below 500°C, and then removing it from the furnace and cooling it to room temperature in the air.

[0011] Preferably, in step S3, the heat treatment involves different quenching and tempering processes depending on the material of different parts. Preferably, the conical tool holder is kept in the tempered state after the pre-friction welding heat treatment, and the shoulder and stirring pin are induction hardened using electromagnetic induction; then the stirring head undergoes three overall tempering treatments. This heat treatment method can maximize the hardness and wear resistance of the shoulder and stirring pin, while maintaining high strength and toughness of the conical tool holder, improving fatigue resistance, and thus extending the effective lifespan of the tool.

[0012] More preferably, the induction hardening process conditions are: heating rate 50-100℃ / s; first heating to 500-650℃, holding for 8-12 min; then heating to 800-850℃, holding for 8-12 min; finally heating to 1260-1280℃, holding for 10-20 min, and then cooling in quenching oil to room temperature. Preferably, the three-stage overall tempering process conditions are: heating rate 30-50℃ / min; first tempering temperature 320-340℃, holding for 50-70 min followed by air cooling; then heating to 540-560℃ for the second and third temperings, each holding for 50-70 min followed by air cooling, with an interval of less than 4 hours between each tempering.

[0013] In step S4, the surface strengthening treatment method is one or more of the following three methods: (1) nitriding or nitrocarburizing at a treatment temperature of 500-550℃ to increase the surface hardness to HV 1000-1200 and enhance wear resistance; (2) laser cladding to deposit a WC-Co coating on the surface of the shoulder pin and the stirring pin; (3) physical vapor deposition (PVD) of a chromium aluminum nitride (CrAlN) layer on the surface of the shoulder pin and the stirring pin to extend the life of the stirring pin. In step S5, ultrasonic non-destructive testing is used to detect whether there are cracks or pores inside the stirring head to ensure that the needle body is free of defects; magnetic particle testing is used to check for surface micro-cracks.

[0014] Compared with the prior art, the beneficial effects of the present invention are: This invention increases the diameter of the stirring head shoulder, resulting in higher heat generation during magnesium alloy welding. This allows for a 100% increase in welding speed while maintaining welding quality. Simultaneously, leveraging the high fluidity of magnesium alloys at high temperatures, the larger shoulder of the stirring pin promotes better fusion of the base materials, improving weld quality. The device also exhibits high wear resistance, high-temperature strength, and fatigue resistance. The processing method of this invention involves friction welding followed by quenching and tempering, and finally, the addition of a wear-resistant coating. This results in low overall internal stress in the stirring head, refined grain structure, and high coating stability, thereby increasing wear resistance and service life. Attached Figure Description

[0015] Figure 1This is a schematic diagram of the overall structure of the stirring head used for friction stir welding of magnesium alloys; in the figure: (1) cylindrical tool holder; (2) shoulder; (3) stirring pin.

[0016] Figure 2 This is a schematic diagram of the friction welding position of the conical tool holder (1) and the shoulder (2) of the stirring head used for friction stir welding of magnesium alloys. Detailed Implementation

[0017] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] like Figure 1 As shown. A stirring head for friction stir welding of magnesium alloys is formed by sequentially welding a conical tool holder 1, a shoulder 2, and a stirring pin 3 into a single unit using a rotary friction welding method; a threaded groove 4 is machined on the shoulder 2 using a CNC machine tool; wherein: the stirring pin 3 has a length of 2.5 mm and a diameter of 3.5 mm, and the shoulder 2 has a diameter of 16 mm.

[0019] Example 1 A method for processing a stirring head for friction stir welding of magnesium alloys includes the following steps: S1. Obtain a blank that meets the requirements of the stirring needle: the conical tool holder 1 is made of tool steel H13, and the shoulder 2 and the stirring needle 3 are made of tungsten steel; heat and anneal the blank: heat the blank to 900°C and hold it for 3 hours, cool it to 490°C in the furnace, and then take it out of the furnace and cool it to room temperature in the air; S2, the conical tool holder 1, shoulder 2, stirring pin 3 and threaded groove 4 are machined using a CNC machine tool. The stirring pin 3 is 2.5mm long and 3.5mm in diameter. The shoulder 2 is 16mm in diameter. The threaded groove 4 is used to enhance the mixing effect of the material. The conical tool holder 1, shoulder 2 and stirring pin 3 are welded together in sequence to form a whole using a rotary friction welding method. S3. Heat treatment of the stirring head: The conical handle 1 is kept in the tempered state after the friction welding heat treatment. The shoulder 2 and stirring needle 3 are induction hardened using electromagnetic induction. Then, the stirring head is subjected to three overall tempering treatments. The induction hardening process conditions are: heating rate 80℃ / s, first heating to 600℃ and holding for 10 min; then heating to 850℃ and holding for 10 min; finally heating to 1270℃ and holding for 15 min, and then cooling to room temperature in quenching oil. The process conditions for the three overall tempering treatments are: tempering heating rate 40℃ / min, first tempering temperature 330℃, holding for 1 h and then air cooling; then heating to 550℃ for the second tempering, holding for 1 h and then air cooling; then heating to 560℃ for the third tempering, holding for 1 h and then air cooling; the interval between each tempering is 3 h. S4, firstly, a nitriding process (processing temperature 550℃) is used to nitrid the surface of the stirring head to increase the surface hardness to HV1100; then, physical vapor deposition (PVD) is used to deposit chromium aluminum nitride (CrAlN) on the surface of the stirring pin. S5. Ultrasonic non-destructive testing is used to detect whether there are cracks or pores inside the stirring head to ensure that the needle body is free of defects; magnetic particle testing is used to check for micro-cracks on the surface; thus, the finished product is obtained.

[0020] The welding performance of the stirring head was verified according to the ISO25239-4 standard. The stirring head is more suitable for magnesium alloy friction stir welding and has better welding performance than ordinary aluminum alloy stirring heads. During the verification process, it also has a longer service life than ordinary aluminum alloy stirring heads. The working life of ordinary stirring heads is 1,500 meters, while that of this type of stirring head can reach 4,000 meters, indicating that the wear resistance of the stirring head of the present invention has been greatly improved.

Claims

1. A stirring head for friction stir welding of magnesium alloys, comprising sequentially welding a conical tool holder, a shoulder, and a stirring pin into a single unit using a rotary friction welding method; wherein a threaded groove is machined on the shoulder using a CNC machine tool; wherein: The diameter of the shoulder is 4 to 5 times the diameter of the stirring needle.

2. The stirring head for friction stir welding of magnesium alloys according to claim 1, characterized in that, The stirring needle is 2.5 mm long and 3.5 mm in diameter, and the shoulder diameter is 16 mm.

3. The method for processing a stirring head for friction stir welding of magnesium alloys according to claim 1 or 2, characterized in that, Includes the following steps: S1. Obtain a blank that meets the requirements of the stirring pin; heat and anneal the blank; S2. Use a CNC machine tool to process a conical tool holder, shoulder, stirring pin, and threaded groove, and weld the conical tool holder, shoulder, and stirring pin into a single unit using rotary friction welding; S3. Perform heat treatment on the stirring head; S4. Use nitriding / nitrocarburizing process, laser cladding deposition of WC-Co coating, or physical vapor deposition of chromium aluminum nitride to perform surface strengthening treatment on the shoulder pin and stirring pin; S5. Non-destructive testing of the stirring head to ensure there are no defects yields the finished product.

4. The processing method of the stirring head for magnesium alloy friction stir welding according to claim 3, characterized in that, The conical tool holder is made of high-temperature alloy or tool steel, and the shoulder and stirring needle are made of tungsten steel.

5. The method for processing a stirring head for friction stir welding of magnesium alloys according to claim 3, characterized in that, In step S1, the annealing is performed by heating the billet to 850-900°C and holding it at that temperature for 3-4 hours, cooling it in the furnace to below 500°C, and then removing it from the furnace and cooling it to room temperature in the air.

6. The method for processing a stirring head for friction stir welding of magnesium alloys according to claim 3, characterized in that, In step S3, the heat treatment is as follows: the conical tool holder is kept in the tempered state of the pre-friction welding heat treatment, and the shoulder and stirring needle are induction hardened by electromagnetic induction; then the stirring head is subjected to three overall tempering treatments.

7. The method for processing a stirring head for friction stir welding of magnesium alloys according to claim 6, characterized in that, The induction hardening process conditions are as follows: heating rate 50-100℃ / s; first heat to 500-650℃ and hold for 8-12 min; then heat to 800-850℃ and hold for 8-12 min; finally heat to 1260-1280℃ and hold for 10-20 min, then cool in quenching oil to room temperature.

8. The method for processing a stirring head for friction stir welding of magnesium alloys according to claim 6, characterized in that, The process conditions for the three overall tempering treatments are as follows: heating rate 30-50℃ / min; first tempering temperature 320-340℃, holding for 50-70min followed by air cooling; then heating to 540-560℃ for the second and third temperings, holding for 50-70min each time followed by air cooling, with an interval of less than 4h between each tempering.

9. The method for processing a stirring head for friction stir welding of magnesium alloys according to claim 1, characterized in that, In step S4, the surface strengthening treatment method is one or more of the following three methods: (1) Nitriding or nitrocarburizing is used at a treatment temperature of 500-550°C to increase the surface hardness to HV 1000-1200 and enhance wear resistance; (2) Laser cladding is performed to deposit a WC-Co coating on the surface of the shoulder pin and the stirring pin; (3) Physical vapor deposition (PVD) of chromium aluminum nitride (CrAlN) is performed on the surface of the shoulder pin and the stirring pin to extend the life of the stirring pin.

10. The method for processing a stirring head for friction stir welding of magnesium alloys according to claim 1, characterized in that, In step S5, ultrasonic non-destructive testing is used to detect whether there are cracks or pores inside the stirring head; magnetic particle testing is used to check for surface micro-cracks.

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