Friction stir weldingFSW is a solid-phase connection technology invented by the British Welding Institute in 1991. Compared with traditional fusion welding, friction stir welding does not need to add welding wire and protective gas, no pollution, no smoke, no radiation, welding joint residual stress is low, so it has the advantages of high welding efficiency, small welding deformation, low energy consumption, simple equipment, welding process safety and so on. After nearly 30 years of development, friction stir welding has been widely used in aerospace, rail transit, ships, new energy vehicles and other fields.
2. Principle of friction stir welding
The high-speed rotating stirring head is inserted into the workpiece to be welded, the rotating stirring needle fricts with the material to be welded and plasticizes it, and the shoulder friction with the surface of the workpiece generates heat and is used to prevent the material in the plastic state from spilling. In the welding process, the workpiece should be rigidly fixed on the back plate, and the high-speed rotating edge of the stirring head side moves relative to the workpiece along the joint of the workpiece, forming a weld under the action of the forging pressure of the stirring head, and finally realizing the metallurgical combination of the welded parts. As shown in Figure 1.
figure1 Friction stir welding principle
3. Friction stir weldingApplicable material
Friction stir welding is widely used in all kinds of materials, and has successfully realized the welding of low melting point metals and alloys such as aluminum and magnesium, copper alloys, titanium alloys, steel materials, metal matrix composites and dissimilar metals (aluminum/copper, aluminum/magnesium, etc.).
4. Friction stir welding technology development
On the basis of traditional technology, friction stir welding has a variety of innovative developments: biaxial shoulder friction stir welding, static shoulder friction stir welding, friction stir spot welding, angular structure welding, friction stir surface welding, reversible welding, composite energy field friction stir welding, friction stir additive manufacturing, etc.
Dual Shoulder Friction Stir welding (BT-FSW)
Compared with the traditional FSW, BT-FSW has an upper and lower shaft shoulder structure, and the two shaft shoulders are connected by the mixing needle. The lower shaft shoulder replaces the rigid back support plate of the traditional FSW to self-support the workpiece and realize the welding of the hollow parts. The welding principle is shown in Figure 2. In the welding process, the temperature gradient in the direction of the joint thickness is reduced by the structure of the upper and lower shoulder, and the non-uniformity of the joint structure is reduced, and the root can be fully welded.
1. Upper shaft shoulder 2. Forward side 3. Fusion line 4. Backward side 5. Workpiece 6. Mixing needle 7. Lower axle shoulder
Static shoulder friction Stir welding (SS-FSW)
Static shoulder friction stir welding (SS-FSW) adopts a separate design of the shoulder and the stirring needle. During the welding process, the internal stirring needle is rotating, while the external shoulder does not rotate and only moves in the welding direction. In conventional FSW, the friction between the shoulder and the material to be welded is the main method of heat production. In contrast, static shoulder friction stir welding plays an important role in friction heat generation and material deformation because the internal stirring needle rotates and the external shaft shoulder does not rotate.
Compared with conventional FSW, the welding seam of static shoulder friction stir welding is well formed, the surface roughness can reach the level of the base material, and the friction stir welding of non-planar structural components such as fillet welds can be realized, as shown in Figure 3.
figure3 Static shoulder fillet weld
Friction stir spot welding
Similar to the conventional FSW principle, friction stir spot welding lacks the process of stirring needle and welding workpiece relative movement to produce weld, instead, relying on rotating downward pressure of the stirring head and workpiece contact friction to generate heat, after full agitation to form a single solder joint.
Friction stir spot welding can be divided into two categories: traditional in-line and backfill.
Traditional in-line friction stir spot welding includes three processes: insertion, rotation and pulling out, and a keyhole is left at the solder joint after welding. As shown in Figure 4.
figure4 In-line friction stir spot welding
Composite energy field friction stir welding
Composite energy field friction stir welding refers to the application of additional energy fields (induction heating, laser, arc, ultrasonic, etc.) in the welding process to reduce the welding load, extend the life of the stirring head, improve the weld structure, and achieve the purpose of high-melting material welding.
Friction stir additive manufacturing
The essence of friction stir additive manufacturing is the welding superposition of multi-layer materials, and its additive process is similar to FSW bonding. Different from the friction stir additive manufacturing bonding, friction stir additive manufacturing is the multi-layer and multiple bonding of materials, and it is a spatial bonding process, including the horizontal additive perpendicular to the bonding direction and the additive parallel to the material thickness direction. The friction stir additive manufacturing process is shown in Figure 5.
figure5 Friction stir additive manufacturing process
Fillet weld friction welding
Friction stir welding of aluminum alloy and magnesium alloy fillet welds is the latest research topic of friction stir welding in recent years, and it is the extension and expansion of traditional friction stir welding technology. The friction stir welding tool of fillet weld realizes the friction stir welding fillet weld of various thickness, breaking through many technical problems such as friction stir welding corner joint equipment, tooling and mixing tools.
5. Friction stir welding application industry and application advantages
In recent years, the application of friction stir welding technology in China has been greatly developed, and great progress has been made in aerospace, rail transportation, ships, power electronics, new energy vehicles and other aspects.
For aircraft structure and materials, China has carried out research on the basic process technology of friction stir welding, and cooperated with domestic aircraft design institutes and manufacturers to actively explore the feasibility of applying friction stir welding to in-service and future new generation fighter jets and large aircraft. As shown in the figure6 is displayed.
Figure 6 Friction stir welding thin-wall aircraft simulation structure
Based on the advantages of friction stir welding in the manufacture of aerospace aluminum alloy structural products and the development needs of new space launch vehicles, China friction Stir Welding Center has focused on the research and development of friction stir welding for the structure of space launch vehicle arrows during the "Tenth Five-Year Plan" period. Domestic aerospace manufacturing enterprises also actively adopt friction stir welding for aerospace storage tank structure welding. As shown in Figure 7.
figure7 Aluminum alloy fuel storage tank section simulation verification
Similar to aerospace, the application of friction stir welding in rail transit and ships is mainly the production of structural parts. For example, train body panels, drive boxes, ship decks, car doors, wheels and so on. As shown in Figure 8.
figure8 Friction stir welding processing rail train body siding and ship siding
In power electronics, friction stir welding technology is mainly used in hollow heat sinks, radiators and sensors. As shown in the figureAs shown in 9.
figure9 Water-cooled radiator
The raw materials of the new energy vehicle battery box on the market are usedThe aluminum alloy of 6 series is mainly used, and the welding thickness is generally about 2-8mm. The welding efficiency of FSW is 1~2m/min (2~4mm weld), 0.8~1.5m/min (5~8mm weld), and the welding equipment adopts constant pressure system and constant position welding function, which ensures the welding quality while welding efficiently, and can well meet the welding requirements of battery box. Figure 10As shown.
figure10 New energy vehicle battery box
6. Common friction stir welding equipment
At present, friction stir welding equipment is mainly divided into three categories: gantry friction stir welding equipment, friction stir welding machine and robot friction stir welding equipment.
The equipment has the technical characteristics of high stiffness, large thickness of weldable parts, strong welding stability and high welding efficiency. As shown in Figure 11.
figure11 Gantry friction stir welding equipment
Friction stir welding machine
Equipment that is customized for a specific size, structure, or function. Such as rocket fuel storage tank friction stir welding machine. As shown in Figure 12.
figure12 Rocket fuel storage tank friction stir welding machine
Robot friction stir welding equipment
Traditional gantry friction stir welding equipment can only complete the linear or planar two-dimensional welding requirements, can not meet the welding of complex structural parts, robot friction stir welding equipment has high flexibility, can achieve complex trajectory movement, make the welding of complex structural parts possible, can improve the degree of welding automation and production efficiency, in recent years has been more and more applications.Figure 13As shown.
Figure 13 Robot friction stir welding equipment for automatic assembly line