There is a wide variety of welding methods. Among them there is such an exotic process as friction stir welding. Its distinctive feature is the lack of consumables, such as electrodes, welding wire, shielding gases. The newly developed method is becoming widespread.
Appearance story
The history of friction stir welding (STP) 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 launch the new technology into production were the Norwegian Marine Aluminum and the American Boeing. At their enterprises, they used welding equipment of the ESAB concern, which specializes in developments in the field of rotational friction welding (PCT).
Since 2003, the company has been continuously researching the possibilities of friction stir welding. For example, methods have been developed for welding aluminum alloys and their modifications, which are used in the fields of aircraft, sea vessels and railway containers.
In the aircraft industry, it was found possible to replace riveted joints with welded ones. Moreover, the welding speed by the STP method significantly exceeds the speed of the electric arc. A weld of 6 meters long can be formed in one minute, while the welding speed in the usual way reaches only 0.8-2 m / min with a part thickness of 0.5 cm.
The essence of the process
The connection of metals occurs due to heating in the welding zone by the friction method. The main welding tool for 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 to pass through the welded part. In the heating zone, the material significantly increases its ductility and, pressed by the shoulder, forms a single mass.
The next step is the movement of the rod along the welded zone. Moving forward, the collar mixes the heated metal mass, which after cooling forms a strong connection.
What affects the quality of STP
Friction welding with stirring is a constantly evolving process. But already now we can distinguish several parameters that affect the quality of the connection:
- The force created by the tool.
- Welding head feed rate.
- The size of the shoulder.
- The peripheral speed of rotation of the rod.
- Tilt angle.
- Rod feed force.
Manipulation of welding characteristics allows to achieve the connection of dissimilar metals. For example, aluminum and lithium. Due to its low density and high strength, lithium can act as an alloying component of parts made of aluminum alloys, which allows the application of this technology in the aerospace industry.
Friction welding with stirring can easily replace forging, stamping, casting, when they are used to produce parts of difficult to combine metals. For example, steels with an austenite and perlite structure, steel of aluminum or bronze.
In what areas is applied
Such areas of production as the automotive industry are constantly working on how to increase the strength properties of the product while reducing its weight. In this regard, the introduction of new materials that were previously not peculiar due to the complexity of processing is ongoing. Increasingly, power elements, such as subframes, and sometimes the entire body, are made of aluminum or its combination with other materials.
So, in 2012, Honda applied additive technologies and friction stir welding to produce subframes for its cars. They introduced a combination of steel and aluminum.
In the production of welded body elements from aluminum, burning of metal sheets may occur. This drawback is deprived of STP. In addition to the fact that electricity consumption is reduced by 1.5-2 times, the cost of consumables, such as welding wire, shielding gases, is reduced.
In addition to the production of vehicles, STP is used in the following areas:
- Construction industry: aluminum support trusses, bridge spans.
- Rail transport: frames, wheeled trolleys, wagons.
- Shipbuilding: bulkheads, structural elements.
- Aircraft industry: fuel tanks, parts of the fuselage.
- Pishcheprom: various containers for liquid products (milk, beer).
- Manufacture of electrical engineering: electric motor housings, parabolic antennas.
In addition to aluminum alloys, friction stir welding is used to obtain copper compounds, for example, in the manufacture of copper containers for the disposal of spent radioactive fuel.
Advantages of STP
The STP study made it possible to select welding modes when connecting various groups of alloys. Despite the fact that STP was originally developed for working with metals with a low melting point, such as aluminum (660 ° C), it subsequently began to be used to combine nickel (1455 ° C), titanium (1670 ° C), iron (1538 ° C )
Studies show that the seam obtained in this way is fully consistent in structure with the metal of the parts to be welded and has higher strength indicators, lower labor costs and low residual deformation.
Correctly selected welding mode ensures compliance of the weld material and the metal being welded according to the following indicators:
- fatigue strength:
- bending and tensile strength;
- impact toughness.
Advantages over other types of welding
STP has many advantages. Among them:
- Non toxic Unlike other varieties, there is no burning of an electric arc, due to which molten metal evaporates in the welding zone.
- Increase the speed of formation of the seam, thereby reducing the time of the production cycle.
- Halving energy costs.
- No need for further processing of the weld. A friction welding tool with stirring during operation creates an ideal weld that does not require stripping.
- No need for additional consumables (welding wire, industrial gases, fluxes).
- The ability to obtain metal compounds inaccessible to other types of welding.
- No special preparation of welding edges is necessary, except for cleaning and degreasing.
- Obtaining a homogeneous weld structure without pores, which makes it easier to control quality, which is regulated for friction welding with stirring GOST R ISO 857-1-2009.
How to check the quality of the weld
Welding quality is checked by two types of control. The first involves the destruction of the prototype obtained by combining two parts. The second allows verification without destruction. Applied methods such as optical control, audiometric research. It helps to determine the presence of pores and inhomogeneous inclusions that degrade the characteristics of the seam. The sound control results are a diagram that visually shows the places where the acoustic echo deviates from the norm.
Disadvantages of the method
With numerous advantages, the friction welding method has the associated disadvantages:
- Lack of mobility. STP involves the connection of stationary parts rigidly fixed in space. This imposes certain properties, such as immobility, on friction stir welding equipment.
- 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 automobile sidewalls on the conveyor, and for nothing else.
- The welding seam has a radial structure. In this regard, with certain types of deformation or during operation of the part in an aggressive environment, fatigue of the weld can accumulate.
Varieties of STP according to the principle of action
Welding processes that use friction as a basis can be divided into several types:
- Linear friction. The essence of the method is to obtain an integral connection not as a result of the action of the rotating tip, but because of the movement of the parts relative to each other. By acting on the surface at the point of contact, they create friction and, as a result, heat. Under pressure, the adjacent parts melt and a weld occurs.
- Radial welding. This method is used for the production of large-diameter tanks and railway tanks. It comes 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 tank production company Sespel. Friction welding with stirring occupies the bulk of the welding work.
- Pin welding. This variety replaces the rivet joint. This type is used for lap joints. The rotating pin at the point of contact heats the parts to be welded. The heat melts and the pin penetrates. Cooling, it creates a strong one-piece connection.
Varieties of STP by level of difficulty
Welding operations performed using friction can be conditionally 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.
So, for flat joints, the ESAB welding equipment company developed the LEGIO 2D installation. It is a customizable system for friction stir welding of various non-ferrous metals. Different size groups of equipment allow welding parts of small and large sizes. According to the marking, LEGIO equipment has several configurations that differ in the number of welding heads, the possibility of welding in several axial directions.
For welding with a difficult position in space, there are 3D robots. Such devices are installed on automobile conveyors where welds of a difficult configuration are required. One example of such robots is the ESAB Rosio installation.
Conclusion
STP compares favorably with traditional types of welding. Its widespread use not only promises economic benefits, but also preserves the health of people employed in production.