A method for strengthening aluminum alloy friction stir welds using magnetic nanoparticles

Abstract

The present invention belongs to a stir friction welding method, and specifically relates to a method for reinforcing aluminum alloy stir friction welds with magnetic nanoparticles. The aluminum alloy plates to be welded are butted and fixed, and magnetic nanoparticles are plasma-coated on the surfaces of the welds on both sides of the butt joint; immediately following the stir friction welding, the stirring head and the shaft shoulder rotate together at high speed and move forward at a certain speed, and under the action of the vortex grooves on the lower surface of the shaft shoulder and the asymmetric spiral grooves on the surface of the stirring head, the welded base material and the cladding layer are evenly mixed to form a weld. The magnetic stirrer simultaneously magnetically stirs the nanoparticles in the weld area, accelerates the movement of the nanoparticles, and improves their distribution uniformity. In the method of the present application, magnetic nanoparticles are used as the reinforcing material for the stir friction weld, and are simultaneously stirred evenly with a magnetic stirrer to obtain higher mechanical properties. Compared with nanoparticle-reinforced welds that do not use magnetic stirring, the nanoparticles in the magnetically stirred nanoparticle-reinforced welds are more evenly distributed and have better mechanical properties.

Description

Technical Field

[0001] The invention relates to a friction stir welding method, in particular to a method for reinforcing an aluminum alloy friction stir weld with magnetic nanoparticles. Background Art

[0002] Friction stir welding (FSW) is a solid-state joining technique developed by the Welding Institute in the United Kingdom in 1991. It uses the heat generated by friction between a high-speed rotating stirrer and the materials being joined to soften and mix the materials, forming a dense, solid-state weld. Compared to traditional fusion welding, FSW is smoke-free, arc-free, requires no welding wire or shielding gas, and produces joints free of pores and cracks. It has been widely used in aerospace, aviation, and automotive manufacturing.

[0003] Among high-strength aluminum alloys, 7000 series aluminum alloys possess the highest strength and play a crucial role in the aviation industry, serving as one of its primary structural materials. However, during the fusion welding process, their constituent elements, such as aluminum, zinc, magnesium, and copper, flow unevenly and can even be burned. This can lead to thermal cracking in the welded product and welded joints that fail to meet performance requirements. Consequently, 7000 series aluminum alloys are considered non-fusion-weldable. Consequently, friction stir welding (FSW) is the primary joining method for 7000 series aluminum alloys.

[0004] The 7000 series is an age-hardened alloy that has been aged before welding. The material contains a large number of GP zones and ƞ The (MgZn2) precipitate phase is the primary strengthening phase in the material. During friction stir welding, the high-speed rotation of the stirrer generates a large amount of heat due to friction with the material, raising the temperature in the core area of ​​the joint to over 400°C, or even higher, approaching the material's solution temperature. At this point, the precipitate in the core dissolves, while the precipitate in the heat-affected zone coarsens due to the heat, resulting in a decrease in joint performance.

[0005] During the post-weld cooling process, due to the high quenching sensitivity of 7000 series aluminum alloys, when the cooling rate is low, the supersaturated solid solution in the core area is prone to precipitate coarse equilibrium phases. ƞ (Mg2Zn), greatly reducing the number of solute atoms. Although the precipitation form of the precipitate phase in the matrix can be controlled by appropriate heat treatment to improve the degree of precipitation strengthening. However, in most cases, the workpiece is limited by space after friction stir welding, or other parts except the weld are not suitable for heat treatment. In order to solve this type of problem, CN211921652 U proposes a large-scale friction stir welding weld heat treatment device to achieve local heat treatment of the weld. However, the local heating of the component by the induction coil will cause the grains of the weld adjacent to the base material to grow due to heat, thereby reducing the overall mechanical properties of the weld.

[0006] Studies have shown that adding suitable nanoparticles to stir friction welding welds can effectively improve weld quality and mechanical properties. CN108817642 A processes grooves on the butt joint surfaces of the plates to be welded, adds reinforcing phase particles into the grooves, and then stir friction welding is performed to improve the mechanical properties of non-heat-treated strengthened aluminum alloy stir friction welded joints. CN106862750 A achieves surface nano-crystallization by supersonic particle bombardment of the surface of the substrate to be welded, and then stir friction welding is performed to improve the performance of weak links such as the thermo-mechanically affected zone. CN102560472 B uses a synchronous powder feeding stirring head to stir friction process the metal workpiece to synthesize nano-composite materials in situ. CN102350585 B feeds the configured nanopowder into the connection area through the nozzle on the stirring head during the stir friction welding process. After stirring, the elements in the nanopowder are evenly dispersed in the welding area, thereby obtaining a welding area with good mechanical properties. CN109807562 B uses wire arc welding followed by friction stir welding to effectively eliminate dendrite growth and refine grain size during the additive forming process of Al-Mg-Si alloys. CN112719653 A uses a plasma arc welding torch to ablate the processed sheet metal in a nitrogen and oxygen environment before performing friction stir welding to enhance the performance of ultrafine-grained materials. These methods share the commonality of introducing nanoparticles into the weld seam through the rotation of the stirring head, but this can lead to uneven distribution of the nanoparticles, making it difficult to achieve the desired effect. Summary of the Invention

[0007] The purpose of the present invention is to solve the problem of uneven distribution of nanoparticles in nanoparticle-reinforced stir friction welds. A method for strengthening aluminum alloy stir friction welds with magnetic nanoparticles is provided. The method adopts plasma cladding of magnetic nanoparticles, combined with stir friction welding to form a nanoparticle-reinforced weld, and simultaneously applies magnetic stirring to make the nanoparticles evenly distributed, thereby further improving the mechanical properties of the weld.

[0008] The technical solution to achieve the objectives of the present invention is a method for reinforcing aluminum alloy friction stir welds with magnetic nanoparticles. Reduced iron powder and aluminum powder are subjected to a high-energy ball milling process to synthesize magnetic nano-Fe3Al powder for later use. The resulting magnetic nano-Fe3Al powder is then clad onto the surfaces of the aluminum alloy plates to be welded on both sides of the butt joint using plasma cladding technology, completing the friction stir welding process immediately. During the friction stir welding process, a magnetic stirrer mounted on the stir head further stirs the magnetic nano-Fe3Al particles entering the weld, ensuring uniform distribution and improving the mechanical properties of the weld.

[0009] Furthermore, the method specifically comprises the following steps:

[0010] Step 1: Reduced iron powder and aluminum powder are mixed in a certain proportion, and a high-energy ball milling process is used to complete mechanical alloying of the mixed powder to obtain magnetic nano-Fe3Al powder;

[0011] Step 2: Place the weldment 4 and weldment 6 in a butt joint and clamp them, move the stirring head 3 and plasma spray gun 9 to a suitable distance above the butt joint, ensure that their center lines are aligned with the butt joint, and the plasma spray gun 9 is in front of the stirring head 3;

[0012] Step 3: Start the tool handle 1, rotate the shaft shoulder 2 and the stirring head 3 and move downward until the lower surface of the shaft shoulder 2 presses against the upper surfaces of the weldment 4 and the weldment 6, and the stirring head 3 enters the interior of the material to be welded and keeps rotating;

[0013] Step 4: Start the plasma spray gun 9 to plasma-clad the magnetic nano-Fe3Al powder obtained in step 1 onto the upper surfaces of both sides of the joint between the weldment 4 and the weldment 6 to form a cladding layer 8;

[0014] Step 5: Move the tool handle 1 and the plasma spray gun 9 forward along the butt joint at the same speed;

[0015] Step 6: The magnetic stirrer 7 installed above the shaft shoulder 2 performs magnetic stirring on the magnetic nano-Fe3Al particles entering the weld zone 5 in step 5, and then cools naturally.

[0016] Furthermore, the lower surface of the shoulder 2 described in step 3 is designed with vortex-pattern grooves 21 and circumferentially offset grooves 22 , which can gather the nano-cladding layer 8 toward the center of the shoulder 2 .

[0017] Furthermore, the surface of the stirring head 3 described in step 3 is designed with asymmetric spiral grooves, one side of which is a positive spiral groove 31, which allows the material to flow from top to bottom; the other side is a reverse spiral groove 32, which allows the material to flow from bottom to top.

[0018] As a preferred solution, the ball milling time of the reduced iron powder and aluminum powder in step 1 should be more than 25 hours, and the particle size of the formed magnetic nanoparticles is about 7 nanometers.

[0019] As a preferred solution, the ratio of reduced iron powder and aluminum powder in step 1 is 72 / 78 according to the Fe / Al atomic ratio.

[0020] As a preferred solution, the rotation speed of the stirring head 3 in step 3 is 500~1000rpm.

[0021] As a preferred solution, the forward moving speed of the stirring head 3 in step 3 is 100~400mm / min.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] This invention improves the performance of friction stir welding (FSW) of aluminum alloys by using magnetic nano-Fe3Al powder as a friction stir weld reinforcement, particularly for 2000 and 7000 series aluminum alloys, which are difficult to process using conventional fusion welding. The magnetic nano-Fe3Al particles, inherently high in strength and hardness, are introduced into the weld by high-speed stirring and then stirred by a magnetic stirrer, becoming evenly distributed throughout the weld. This significantly strengthens the weld, offsetting the degradation of weld performance caused by the precipitation of equilibrium phases during natural aging at high temperatures. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a schematic diagram of the principle of using magnetic nanoparticles to enhance friction stir welding in the present invention.

[0025] Figure 2 for Figure 1 Schematic diagram of a cross section passing through the center line of the stirring head and perpendicular to the weld direction.

[0026] Figure 3 for Figure 2 Schematic diagram of the vortex-pattern grooves on the lower surface of the center shoulder.

[0027] Figure 4 for Figure 2 Schematic diagram of the asymmetric spiral grooves on the surface of the stirring head.

[0028] Description of Reference Numerals

[0029] 1-Handle, 2-Shaft Shoulder, 3-Stirring Needle, 4, 6-Weldment, 5-Weld Seam, 7-Magnetic Stirrer, 8-Plasma Cladding, 9-Plasma Spray Gun. DETAILED DESCRIPTION

[0030] The present invention is described in detail below in conjunction with specific embodiments. The following examples will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It is particularly noted that, under the premise of the present invention, some variations and improvements are made, which all fall within the scope of protection of the present invention.

[0031] Example 1:

[0032] The materials of weldment 4 and weldment 6 are both 7075-T6 aluminum alloy. The nano-reinforced material used for plasma cladding is nano-scale Fe3Al magnetic powder particles prepared by mechanical alloying process of reduced iron powder and aluminum powder. The processing method is as follows: (1) Reduced iron powder and aluminum powder are mixed according to the Fe / Al atomic ratio of 72 / 78, and the mixed powder is mechanically alloyed by high-energy ball milling process. The ball milling time exceeds 25 hours to obtain particles with a particle size of 7nm, and magnetic nano Fe3Al powder is prepared for use; (2) Weldment 4 and weldment 6 are placed horizontally on the same horizontal plane and clamped; (3) The stirring head 3 is moved to the welded part of weldment 4 and weldment 6 so that the center line is aligned with their joint; (4) The plasma spray gun 9 is vertically aligned with the joint of weldment 4 and weldment 6, and aligned with the stirring head 3 maintains a small distance; (5) controls the rotation of the tool handle 1 so that the stirring head 3 is inserted downward into the welded parts 4 and 6 at a speed of 800 r / min until the lower end surface of the shaft shoulder 2 is in close contact with the upper surface of the welded parts 4 and 6, then stops inserting and keeps rotating; (6) turns on the plasma spray gun 9 and starts to clad the upper surfaces on both sides of the joint between the welded parts 4 and 6 to form a magnetic nano Fe3Al particle cladding layer 8; (6) simultaneously moves the tool handle 1 and the plasma spray gun 9 in the horizontal direction along the joint between the welded parts 4 and 6 until the welding is completed, and then cools naturally.

[0033] Example 1:

[0034] The materials of weldment 4 and weldment 6 are both 2024-T6 aluminum alloy, and the nano-reinforced material used for plasma cladding is a soft magnetic nano-iron-silicon alloy FeSi particles. The processing method is as follows: (1) Reduced iron powder and silicon powder are mixed according to the Fe / Si atomic ratio of 60 / 40, and the mixed powder is mechanically alloyed by high-energy ball milling process. The ball milling time is 120 hours to obtain particles with a particle size of 7nm, and magnetic nano-FeSi powder is prepared for use; (2) Weldment 4 and weldment 6 are placed horizontally on the same horizontal plane and clamped; (3) The stirring head 3 is moved to the welded part of weldment 4 and weldment 6 so that the center line is aligned with their joint; (4) The plasma spray gun 9 is vertically aligned with the joint of weldment 4 and weldment 6, and is aligned with the stirring head 3 Maintain a small distance; (5) Control the rotation of the tool handle 1 so that the stirring head 3 is inserted downward into the welded parts 4 and 6 at a speed of 800 r / min until the lower end surface of the shaft shoulder 2 is in close contact with the upper surface of the weld 4 and the weld 6, then stop inserting and keep rotating; (6) Turn on the plasma spray gun 9 and start cladding the upper surfaces on both sides of the joint between the weld 4 and the weld 6 to form a soft magnetic nano-iron silicon alloy FeSi cladding layer 8; (6) Move the tool handle 1 and the plasma spray gun 9 simultaneously in the horizontal direction along the joint between the weld 4 and the weld 6 until the welding is completed, and then cool naturally.

Claims

1. A method for reinforcing aluminum alloy friction stir welds with magnetic nanoparticles, characterized in that: Specifically, the method comprises the following steps: Step 1: Mix reduced iron powder and aluminum powder in proportion, and use high-energy ball milling process to complete mechanical alloying of the mixed powder to obtain magnetic nano Fe3Al powder; Step 2: Place and clamp the weldment 1 (4) and the weldment 2 (6) together, and move the stirring head (3) and the plasma spray gun (9) to a suitable distance above the butt joint, respectively, to ensure that their center lines are aligned with the butt joint, and the plasma spray gun (9) is in front of the stirring head (3); Step 3: Start the tool handle (1) to rotate the shaft shoulder (2) and the stirring head (3) and move downward until the lower surface of the shaft shoulder (2) presses against the upper surface of the weldment 1 (4) and the weldment 2 (6), and the stirring head (3) Entering the interior of the material to be welded and keeping rotating; Step 4: Starting the plasma spray gun (9), and cladding the magnetic nanopowder obtained in Step 1 onto the upper surfaces of both sides of the butt joint of the weldment 1 (4) and the weldment 2 (6) through plasma, to form a cladding layer (8); Step 5: Simultaneously moving the tool handle (1) and the plasma spray gun (9) forward along the butt joint at the same speed, and stirring and mixing the base material to be welded and the cladding layer (8) under the action of the shaft shoulder (2) and the stirring head (3) to form a weld; Step 6: The magnetic stirrer (7) installed above the shaft shoulder (2) magnetically stirs the magnetic nanoparticles entering the weld area in Step 5 in the semi-solid weld, and then cools naturally; The diameter of the shaft shoulder (2) in step 3 is slightly larger than the width of the plasma cladding layer (8) in step 5; the magnetic stirrer (7) in step 6 is a low-frequency magnetic field generating device with a frequency of 4-10 Hz, which generates a rotating magnetic field; the magnetic stirrer (7) in step 6 moves forward with the tool handle (1) during operation, but does not rotate; the lower surface of the shaft shoulder (2) in step 3 is designed with vortex-patterned grooves (21) and circumferentially offset grooves (22); the surface of the stirring head (3) in step 3 is designed with asymmetric spiral grooves, including positive spiral grooves (31) and negative spiral grooves (32).

2. The method for reinforcing aluminum alloy friction stir welds with magnetic nanoparticles according to claim 1, characterized in that: The aluminum alloy includes 2000 series and 7000 series super-hard aluminum alloys.

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