Ultrasonic welding of metals is a process in which an inseparable compound in the solid phase is obtained. The formation of juvenile areas (in which connections are formed) and the contact between them occur under the influence of a special tool. It provides the combined action of relative alternating tangential displacements of small amplitude and compressive normal force on the workpiece. Let us consider in more detail what constitutes ultrasonic welding technology.
Connection mechanism
Small amplitude displacements occur between parts with an ultrasonic frequency. Due to them, microroughnesses on the surface of parts undergo plastic deformation. Along with this, pollution is evacuated from the compound zone. Ultrasonic mechanical vibrations are transmitted to the weld area from the tool from the outside of the workpiece. The whole process is organized in such a way as to prevent slipping of the fixture and support on the surfaces of the parts. During the passage of vibrations through the workpieces, energy is dissipated. This is ensured by external friction between the surfaces at the initial stage of welding and internal friction in the material located between the support and the tool after the formation of the setting area. In this connection, the temperature rises, which facilitates deformation.
Material behavior specifics
The tangential movements between the parts and the stresses that are caused by them and act together with compression from the welding force provide localization of intense plastic deformation in small volumes in the near-surface layers. The whole process is accompanied by grinding and mechanical evacuation of oxide films and other contaminants. Ultrasonic welding provides a reduction in yield strength, which facilitates plastic deformation.
Process features
Ultrasonic welding contributes to the formation of the necessary conditions for the connection. This is ensured by mechanical vibrations of the converter. Thanks to the energy of vibration, complex shear, compression, and stress are created. Plastic deformation occurs when the elastic limits of materials are exceeded. Obtaining a strong connection is ensured by increasing the area of direct contact after the evacuation of surface oxides, organic and adsorbed films.
The use of ultrasound
Ultrasound is widely used in the scientific field. With it, scientists investigate a number of physical properties of substances and phenomena. In industry, ultrasound is used for degreasing and cleaning products, working with difficult to process materials. In addition, fluctuations favorably affect crystallizing melts. Ultrasound provides for their degassing and grinding of grain, increasing the mechanical properties of cast materials. Fluctuations contribute to the removal of residual stresses. They are also widely used to increase the speed of slow-moving chemical reactions. Ultrasonic welding can be used for various purposes. Fluctuations can become a source of energy for the formation of suture and point joints. Under the influence of ultrasound on the weld pool during crystallization, the mechanical properties of the joint are improved by grinding the weld structure and intensive removal of gases. Due to the fact that vibrations actively remove impurities, artificial and natural films, it is possible to combine parts with an oxidized, varnished, etc. surface. Ultrasound helps reduce or eliminate the intrinsic stresses that appear during welding. Due to vibrations, it is possible to stabilize the components of the structure of the compound. This, in turn, helps to prevent the likelihood of spontaneous deformation of structures subsequently. Ultrasonic welding has recently been increasingly used. This is due to the undoubted advantages of this method of connection in comparison with cold and contact methods. Especially often used are ultrasonic vibrations in microelectronics.
A promising area is ultrasonic welding of
polymeric materials. Some of them cannot be combined by any other method. At industrial enterprises, ultrasonic welding of thin-walled aluminum profiles, foil, wire is currently carried out. This method is especially effective for joining products from heterogeneous raw materials. Ultrasonic welding of aluminum is used in the manufacture of household appliances. This method is effective in splicing sheet materials (nickel, copper, alloys). Ultrasonic welding of plastics has found application in the manufacture of optical instruments and fine mechanics. Currently, machines for connecting various elements of microcircuits have been created and introduced into production. Devices are equipped with automatic devices, due to which productivity is significantly increased.
Ultrasonic power
Ultrasonic welding of plastic provides a one-piece connection due to the combined action of high-frequency mechanical vibrations and a relatively small compressive force. This method has much in common with the cold method. The power of ultrasound, which can be transmitted through the medium, will depend on the physical properties of the latter. If the tensile strengths in the compression zones are exceeded, the solid material will collapse. In similar situations, cavitation occurs in liquids, accompanied by the appearance of small bubbles and their subsequent collapse. Together with the latter process, local pressures arise. This phenomenon is used in the cleaning and processing of products.
Device Nodes
Ultrasonic welding of plastic is carried out using special machines. The following nodes are present in them:
- Source of power.
- Oscillating mechanical system.
- Control equipment.
- Pressure actuator.
The oscillation system is used to convert electric energy into mechanical energy for its subsequent transfer to the connection section, its concentration and obtaining the required value of the radiator speed. This node contains:
- Electromechanical converter with windings. It is enclosed in a metal case and is cooled by water.
- Transformer of elastic vibrations.
- Welding tip.
- Support with pressure mechanism.
The system is mounted using a diaphragm. Ultrasound radiation occurs only at the time of welding. The process occurs under the influence of oscillations, pressure applied at right angles to the surface, and thermal effect.
Method Features
Ultrasonic welding is most effective for plastic materials. Products made of copper, nickel, gold, silver, etc., can be connected both with each other and with other low-plastic products. With increasing hardness, ultrasonic weldability deteriorates. Refractory products made of tungsten, niobium, zirconium, tantalum, molybdenum are effectively combined using ultrasound. Ultrasonic welding of polymers is considered a relatively new method. Such products can also be connected both with each other and with other solid parts. As for metal, it can be combined with glass, semiconductors, and ceramics. You can also connect preforms through the interlayer. For example, steel products are welded to each other through aluminum plastic. Due to the short duration of stay at elevated temperature, a high-quality compound of dissimilar products is obtained. The properties of raw materials undergo minor changes. The absence of impurities is one of the advantages that ultrasonic welding has. Harmful factors for humans are also absent. When combined, favorable hygienic conditions are created. Product links are characterized by chemical homogeneity.
Connection Features
Welding of metal is carried out, as a rule, by an overlap method. At the same time, various design elements are added. Welding can be carried out by points (one or several), a continuous seam or in a closed circle. In some cases, when preforming the end of a workpiece from wire, T-joints are made with a plane. It is possible to carry out ultrasonic welding of several materials at the same time (package).
Thickness of parts
It is limited by the upper limit. With an increase in the thickness of the metal billet, it is necessary to apply vibrations with a larger amplitude. This will compensate for the loss of energy. The increase in amplitude, in turn, is possible up to a certain limit. Limitations are associated with the likelihood of fatigue cracks, large dents from the tool. In such cases, you should evaluate how appropriate ultrasonic welding will be. In practice, the method is used with a thickness of products from 3 ... 4 microns to 05 ... 1 mm. Welding can also be used for parts with a diameter of 0.01 ... 05 mm. The thickness of the second product may be significantly greater than the first.
Possible problems
When applying the method of ultrasonic welding, it is necessary to take into account the probability of fatigue failure of existing compounds in products. During the process, the workpieces can be turned relative to each other. As mentioned above, dents remain on the surface of the material from the tool. The device itself has a limited service life due to erosion of its working plane. At individual points, the product material is welded to the tool. This leads to wear on the device. Equipment repair is accompanied by a number of difficulties. They are connected with the fact that the instrument itself acts as an element of a non-separable unified unit design, the configuration and dimensions of which are designed exactly for the operating frequency.
Product Preparation and Mode Settings
Before performing ultrasonic welding, any complex measures with the surface of parts are not necessary. If desired, the stability of the quality of the compound can be improved. To do this, it is only advisable to degrease the product with a solvent. For joining ductile metals, the optimal cycle is considered to be a delayed pulse relative to the start of ultrasound. With a relatively high hardness of the product, it is advisable to wait for a little heat before turning on the ultrasound.
Welding patterns
There are several of them. Technological schemes of ultrasonic welding differ in the nature of the oscillation of the tool. They can be torsional, bending, longitudinal. Also, the schemes are distinguished depending on the spatial position of the device relative to the surface of the welded part, as well as on the method of transferring compressive forces to the products and the design features of the support element. For contour, suture and point joints, options with bending and longitudinal vibrations are used. Ultrasonic exposure can be combined with local pulsed heating of parts from a separate heat source. In this case, a number of advantages can be achieved. First of all, it is possible to reduce the amplitude of the oscillations, as well as the strength and time of their transmission. The energy properties of the heat pulse and the period of its imposition on ultrasound act as additional process parameters.
Thermal effect
Ultrasonic welding is accompanied by an increase in temperature at the joint. The occurrence of heat is caused by the appearance of friction on the surfaces of the contacting products, as well as by plastic deformations. They, in fact, accompany the formation of a welded joint. The temperature at the contact area will depend on the strength parameters. The main one is the degree of hardness of the material. In addition, its thermophysical properties are very important: thermal conductivity and heat capacity. The selected welding mode also affects the temperature level. As practice shows, the emerging thermal effect does not act as a determining condition. This is due to the fact that the maximum strength of the joints in the products is achieved earlier than the temperature rises to the maximum level. It is possible to reduce the duration of transmission of ultrasonic vibrations by preheating the parts. This will also contribute to enhancing the bond strength.
Conclusion
Ultrasonic welding is currently in some industrial sectors an indispensable method of joining parts. This method is especially common in microelectronics. Ultrasound allows you to connect a variety of plastic and solid materials. Today, scientific work is being actively conducted to improve welding tools and technologies.