There is a wide variety of welding methods. Among them is such an exotic process as friction stir welding. Its distinguishing feature is the absence of consumables such as electrodes, welding wire, shielding gases. A newly developed method is gaining widespread acceptance.
History of Appearance
The history of friction stir welding (FSW) began in 1991. It was an innovative development of the British Welding Institute (TWI). A few years later, the technology was used in the construction of aircraft and ships.
The first companies to put the new technology into production were the Norwegian Marine Aluminum and the American Boeing. They used welding equipment from the ESAB concern, which specializes in developments in the field of rotational friction welding (PCT), in their enterprises.
Since 2003, the company has been continuously researching the possibilities of friction stir welding. For example, there weremethods have been developed for welding aluminum alloys and their modifications, which are used in the construction of aircraft, ships and railway containers.
In the aircraft industry, it was found possible to replace riveted joints with welded ones. Moreover, the speed of welding by the FSW method significantly exceeds the speed of the electric arc. A 6m long weld can be formed in one minute, while conventional welding speed is only 0.8-2m/min for a part thickness of 0.5cm.
The essence of the process
Metal joining occurs due to heating in the welding zone by the friction method. The main welding tool of friction stir welding is a metal rod, consisting of two halves: a shoulder and a shoulder.
With its protruding part, the rotating rod is immersed in the material, causing strong heating. Its supply is limited by the shoulder, not allowing the workpiece to be welded to pass through. In the heating zone, the material significantly increases its plasticity and, pressed down by the shoulder, forms a single mass.
The next step is the movement of the rod along the welded zone. Moving forward, the shoulder mixes the heated metal mass, which after cooling forms a strong connection.
What affects the quality of STP
Friction stir welding is a constantly evolving process. But already now there are several parameters that affect the quality of the connection:
- Force generated by the tool.
- Feed ratewelding head.
- The value of the shoulder.
- The circumferential speed of rotation of the rod.
- Tilt angle.
- Feeding force of the rod.
Manipulation of the characteristics of welding allows you to achieve the connection of dissimilar metals. For example, aluminum and lithium. Lithium, due to its low density and high strength, can act as an alloying component of aluminum alloy parts, which allows this technology to be used in the aerospace industry.
Friction stir welding can easily replace forging, stamping, casting when they are used to produce parts from difficult-to-match metals. For example, steels with austenite and pearlite structure, steels made of aluminum or bronze.
In what areas is used
Industries such as the automotive industry are constantly working on how to increase the strength properties of the product while reducing its weight. In this regard, there is a continuous introduction of new materials that were previously uncharacteristic due to the complexity of processing. Increasingly, structural elements such as subframes and sometimes entire bodies are made from aluminum or a combination of aluminum.
Thus, in 2012, Honda applied additive manufacturing and friction stir welding to produce subframes for its vehicles. They introduced a combination of steel and aluminum.
Burn-through of metal sheets may occur during the production of body weldments from aluminum. This shortcoming is deprived of STP. Besides thatelectricity consumption is reduced by 1.5-2 times, the cost of consumables such as welding wire, shielding gases is reduced.
Besides car production, STP is used in the following areas:
- Construction industry: aluminum support trusses, bridge spans.
- Rail transport: frames, wheeled bogies, wagons.
- Shipbuilding: bulkheads, structural elements.
- Aircraft: fuel tanks, fuselage parts.
- Food industry: various containers for liquid products (milk, beer).
- Electrical production: motor housings, parabolic antennas.
In addition to aluminum alloys, friction stir welding is used to obtain copper compounds, for example, in the production of copper containers for the disposal of spent radioactive fuel.
STP benefits
Studying the FSW made it possible to select welding modes when joining different groups of alloys. Despite the fact that initially FSW was developed to work with metals with a low melting point, such as aluminum (660 ° C), later it began to be used to join nickel (1455 ° C), titanium (1670 ° C), iron (1538 ° C).
Studies show that the weld obtained in this way is fully consistent in its structure with the metal of the parts to be welded and has higher strength indicators, lower labor costs and low residual deformation.
Correctthe selected welding mode guarantees the conformity of the weld material and the metal being welded according to the following indicators:
- fatigue strength:
- flexural and tensile strength;
- toughness.
Advantages over other types of welding
STP has many advantages. Among them:
- Non-toxic. Unlike other varieties, there is no electric arc burning, due to which the molten metal evaporates in the welding zone.
- Increased seam formation speed, resulting in faster cycle times.
- Reducing energy costs by half.
- No need for further processing of the weld. The Friction Stir Tool produces a perfect weld without the need for stripping.
- No need for additional consumables (welding wire, industrial gases, fluxes).
- The ability to obtain metal joints that are not available for other types of welding.
- No special preparation of welding edges is necessary, except for cleaning and degreasing.
- Obtaining a homogeneous weld structure without pores, resulting in easier quality control, which is regulated for friction stir welding GOST R ISO 857-1-2009.
How the quality of a weld is checked
The quality of welding is checked by two types of control. The first involves the destruction of the prototype resulting fromconnection of two parts. The second allows verification without destruction. Methods such as optical control, audiometric examination are used. It helps to determine the presence of pores and inhomogeneous inclusions that degrade the characteristics of the seam. The results of the sound control are a diagram that clearly shows the places where the acoustic echo deviates from the norm.
Disadvantages of the method
With numerous advantages, the friction welding method has accompanying disadvantages:
- Lack of mobility. STP involves the connection of fixed parts, rigidly fixed in space. This imposes certain properties on friction stir welding equipment, such as immobility.
- Low versatility. Bulky equipment is configured to perform the same type of operations. In this regard, devices for welding are designed for specific tasks. For example, for welding car sidewalls on a conveyor, and for nothing else.
- The welding seam has a radial structure. In this regard, with certain types of deformation or when the part is operated in an aggressive environment, weld fatigue may accumulate.
Varieties of STP according to the principle of action
Welding processes based on friction can be divided into several types:
- Linear friction. The essence of the method is to obtain a permanent connection not as a result of the action of a rotating tip, but due to the movement of parts relative to each other. Acting on the surface at the point of contact, they createfriction and consequently high temperatures. Under pressure, the adjoining parts are melted, and a welded joint is formed.
- Radial welding. This method is used for the production of large diameter containers, railway tanks. It boils down to the fact that the joints of the parts are heated by a rotating ring dressed on the outside. By friction, it causes a temperature close to the melting point. An example of an enterprise using this technology is the Cheboksary company producing tanks Sespel. Friction stir welding takes up the bulk of welding work.
- Stud welding. This variety replaces the rivet connection. This type is used for overlap connections. The rotating pin at the point of contact heats the parts to be welded. From the high temperature, melting occurs, and the pin penetrates inside. Cooling down, it creates a strong permanent connection.
Varieties of STP by difficulty level
Welding operations performed using friction can be divided into planar and volumetric. The main difference between these varieties is that in the first case, the weld is formed in two-dimensional space, and in the second - in three-dimensional space.
Thus, for planar joints, the welding equipment manufacturer ESAB has developed the 2D LEGIO machine. It is a customizable system for friction stir welding of various non-ferrous metals. Different size groupsequipment allows you to weld parts of small and large sizes. According to the marking, LEGIO equipment has several layouts, which differ in the number of welding heads, the ability to weld in several axial directions.
There are 3D robots for welding jobs with complex positions in space. Such devices are installed on automobile conveyors, where welds of a complex configuration are required. One example of such robots is ESAB's Rosio.
Conclusion
STP compares favorably with traditional types of welding. Its widespread use not only promises economic benefits, but also the preservation of the he alth of people employed in production.
