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Machines and tools for sharpening electrodes for resistance spot welding Sinterleghe. Choosing electrodes for contact welding

Spot welding is a type of contact welding. With this method, the heating of the metal to its melting point is carried out by heat, which is formed when a large electric current passes from one part to another through the place of their contact. Simultaneously with the passage of current and some time later after it, the parts are compressed, as a result of which mutual penetration and fusion of the heated sections of the metal occur.

The features of resistance spot welding are: short welding time (from 0.1 to several seconds), high welding current (more than 1000A), low voltage in the welding circuit (1-10V, usually 2-3V), significant force compressing the welding spot (from several tens to hundreds of kg), a small melting zone.

Spot welding is most often used for joining sheet blanks with an overlap, less often for welding rod materials. The range of thicknesses welded by it is from a few micrometers to 2-3 cm, however, most often the thickness of the metal being welded varies from tenths to 5-6 mm.

In addition to spot welding, there are other types of contact welding (butt, seam, etc.), however, spot welding is the most common. It is used in the automotive industry, construction, radio electronics, aircraft manufacturing and many other industries. During the construction of modern liners, in particular, several million weld points are produced.

Deserved popularity

The great demand for spot welding is due to a number of advantages that it has. Among them: no need for welding consumables (electrodes, filler materials, fluxes, etc.), slight residual deformations, simplicity and convenience of working with welding machines, accuracy of the connection (virtually no weld), environmental friendliness, efficiency, susceptibility to easy mechanization and automation, high performance. Spot welding machines are capable of performing up to several hundred welding cycles (spot welds) per minute.

The disadvantages include the lack of tightness of the seam and the concentration of stresses at the welding point. Moreover, the latter can be significantly reduced or even eliminated by special technological methods.

Sequence of processes in resistance spot welding

The whole process of spot welding can be divided into 3 stages.

Compression of parts, causing plastic deformation of microroughnesses in the chain electrode-part-part-electrode.
Switching on an electric current pulse, which leads to heating of the metal, its melting in the joint zone and the formation of a liquid core. As the current passes, the core increases in height and diameter to a maximum size. Bonds are formed in the liquid phase of the metal. At the same time, the plastic sedimentation of the contact zone continues to the final size. The compression of the parts ensures the formation of a sealing belt around the molten core, which prevents the metal from splashing out of the welding zone.
Turning off the current, cooling and crystallization of the metal, ending with the formation of a cast core. On cooling, the volume of the metal decreases and residual stresses arise. The latter are an undesirable phenomenon, which is fought in various ways. The force that compresses the electrodes is removed with some delay after the current is turned off. This provides the necessary conditions for better crystallization of the metal. In some cases, in the final stage of resistance spot welding, it is even recommended to increase the clamping force. It provides forging of the metal, eliminating the inhomogeneities of the seam and relieving stress.

At the next cycle, everything repeats again.

Basic parameters of resistance spot welding

The main parameters of resistance spot welding include: the strength of the welding current (I CB), the duration of its pulse (t CB), the compression force of the electrodes (F CB), the size and shape of the working surfaces of the electrodes (R - with a spherical, d E - with a flat shape ). For better visualization of the process, these parameters are presented in the form of a cyclogram reflecting their change over time.

Distinguish between hard and soft welding modes. The first is characterized by high current, short duration of the current pulse (0.08-0.5 seconds depending on the thickness of the metal) and high compression force of the electrodes. It is used for welding copper and aluminum alloys with high thermal conductivity, as well as high-alloy steels to maintain their corrosion resistance.

In the soft mode, the workpieces are heated more smoothly with a relatively small current. The duration of the welding pulse is from tenths to several seconds. Soft modes are shown for steels prone to hardening. Basically, it is soft modes that are used for resistance spot welding at home, since the power of the devices in this case may be lower than with hard welding.

Dimensions and shape of electrodes. With the help of electrodes, the welding machine is in direct contact with the parts to be welded. They not only supply current to the welding zone, but also transmit compressive force and remove heat. The shape, dimensions and material of the electrodes are the most important parameters of spot welding machines.

Depending on their shape, the electrodes are divided into straight and curly. The former are the most common, they are used for welding parts that allow free access of electrodes to the welded zone. Their sizes are standardized by GOST 14111-90, which establishes the following diameters of electrode rods: 10, 13, 16, 20, 25, 32 and 40 mm.

According to the shape of the working surface, there are electrodes with flat and spherical tips, characterized respectively by the values of the diameter (d) and radius (R). The contact area of the electrode with the workpiece depends on the value of d and R, which affects the current density, pressure, and the size of the core. Spherical surface electrodes have greater tool life (capable of making more points before regrinding) and are less susceptible to misalignment than flat surface electrodes. Therefore, with a spherical surface, it is recommended to manufacture electrodes used in tongs, as well as figured electrodes that work with large deflections. When welding light alloys (for example, aluminum, magnesium), only electrodes with a spherical surface are used. The use of electrodes with a flat surface for this purpose leads to excessive dents and undercuts on the surface of points and increased gaps between parts after welding. The dimensions of the working surface of the electrodes are selected depending on the thickness of the metals being welded. It should be noted that electrodes with a spherical surface can be used in almost all cases of spot welding, while electrodes with a flat surface are very often not applicable.

* - in the new GOST, instead of a diameter of 12 mm, 10 and 13 mm are introduced.

The landing parts of the electrodes (places connected to the electric holder) must ensure reliable transmission of the electrical impulse and the pressing force. Often they are made in the form of a cone, although there are other types of connections - along a cylindrical surface or thread.

Of great importance is the material of the electrodes, which determines their electrical resistance, thermal conductivity, thermal stability and mechanical strength at high temperatures. During operation, the electrodes heat up to high temperatures. The thermocyclic mode of operation, together with a mechanical variable load, causes increased wear of the working parts of the electrodes, resulting in a deterioration in the quality of the connections. In order for the electrodes to be able to withstand harsh working conditions, they are made from special copper alloys with high heat resistance and high electrical and thermal conductivity. Pure copper is also capable of working as electrodes, however, it has a low resistance and requires frequent regrinding of the working part.

Welding current. The strength of the welding current (I CB) is one of the main parameters of spot welding. It determines not only the amount of heat released in the welding zone, but also the gradient of its increase in time, i.e. heating rate. The dimensions of the welded core (d, h and h 1) directly depend on I WT and increase in proportion to the increase in I WT.

It should be noted that the current that flows through the welding zone (I CB) and the current flowing in the secondary circuit of the welding machine (I 2) differ from each other - and the more, the smaller the distance between the weld points. The reason for this is the shunt current (Ish) flowing outside the welding zone - including through previously made points. Thus, the current in the welding circuit of the machine must be greater than the welding current by the value of the shunt current:

I 2 \u003d I CB + I w

To determine the strength of the welding current, you can use different formulas that contain various empirical coefficients obtained empirically. In cases where an accurate determination of the welding current is not required (which happens most often), its value is taken from tables compiled for different welding modes and various materials.

Increasing the welding time allows welding with currents much lower than those given in the table for industrial devices.

welding time. The welding time (t CB) is understood as the duration of the current pulse when performing one weld point. Together with the strength of the current, it determines the amount of heat that is released in the connection zone when an electric current passes through it.

With an increase in t CB, the penetration of parts increases and the dimensions of the core of the molten metal increase (d, h and h 1). At the same time, heat removal from the melting zone also increases, parts and electrodes are heated, and heat is dissipated into the atmosphere. When a certain time is reached, a state of equilibrium may occur, in which all the input energy is removed from the welding zone, without increasing the penetration of parts and the size of the core. Therefore, an increase in t SW is advisable only up to a certain point.

When accurately calculating the duration of the welding pulse, many factors must be taken into account - the thickness of the parts and the size of the weld spot, the melting point of the metal being welded, its yield strength, heat accumulation coefficient, etc. There are complex formulas with empirical dependencies, which, if necessary, carry out the calculation.

In practice, most often the welding time is taken according to the tables, correcting, if necessary, the accepted values in one direction or another, depending on the results obtained.

Compression force. The compression force (F CB) affects many processes of resistance spot welding: plastic deformations occurring in the joint, heat release and redistribution, metal cooling and its crystallization in the core. With an increase in F CB, the deformation of the metal in the welding zone increases, the current density decreases, and the electrical resistance in the electrode-workpiece-electrode section decreases and stabilizes. Provided that the dimensions of the core remain unchanged, the strength of the weld points increases with increasing compression force.

When welding in hard conditions, higher values of F CB are used than in soft welding. This is due to the fact that with an increase in rigidity, the power of the current sources and the penetration of parts increase, which can lead to the formation of splashes of molten metal. A large compression force is just designed to prevent this.

As already noted, in order to forge a weld point in order to relieve stress and increase the density of the core, the resistance spot welding technology in some cases provides for a short-term increase in the compression force after the electric pulse is turned off. The cyclogram in this case looks as follows.

In the manufacture of the simplest resistance welding machines for home use, there is little reason to engage in accurate parameter calculations. Approximate values for electrode diameter, welding current, welding time and clamping force can be taken from tables available in many sources. It is only necessary to understand that the data in the tables are somewhat overestimated (or underestimated, if we keep in mind the welding time) compared to those that are suitable for home devices where soft modes are usually used.

Preparation of parts for welding

The surface of the parts in the zone of contact between the parts and in the place of contact with the electrodes is cleaned from oxides and other contaminants. With poor cleaning, power losses increase, the quality of the connections deteriorates and the wear of the electrodes increases. In resistance spot welding technology, sandblasting, emery wheels and metal brushes are used to clean the surface, as well as etching in special solutions.

High demands are placed on the surface quality of parts made of aluminum and magnesium alloys. The purpose of surface preparation for welding is to remove, without damage to the metal, a relatively thick film of oxides with high and uneven electrical resistance.

Spot welding equipment

The differences between the existing types of spot welding machines are determined mainly by the type of welding current and the shape of its pulse, which are produced by their power electrical circuits. According to these parameters, resistance spot welding equipment is divided into the following types:

machines for welding with alternating current;
low-frequency spot welding machines;
capacitor type machines;
DC welding machines.

Each of these types of machines has its own advantages and disadvantages in technological, technical and economic aspects. The most widely used machines for welding with alternating current.

AC resistance spot welding machines. A schematic diagram of machines for spot welding with alternating current is shown in the figure below.

The voltage at which welding is carried out is formed from the mains voltage (220/380V) using a welding transformer (TC). The thyristor module (CT) ensures the connection of the primary winding of the transformer to the supply voltage for the required time for the formation of a welding pulse. Using the module, you can not only control the duration of the welding time, but also control the shape of the applied pulse by changing the opening angle of the thyristors.

If the primary winding is made not from one, but from several windings, then by connecting them in various combinations with each other, it is possible to change the transformation ratio, obtaining different values of the output voltage and welding current on the secondary winding.

In addition to the power transformer and thyristor module, AC spot welding machines have a set of control equipment - a power source for the control system (step-down transformer), relays, logic controllers, control panels, etc.

Capacitor welding. The essence of capacitor welding is that at first, electrical energy is relatively slowly accumulated in the capacitor when it is being charged, and then it is consumed very quickly, generating a large current pulse. This allows welding to be carried out using less power from the network compared to conventional spot welding machines.

In addition to this main advantage, capacitor welding has others. With it, there is a constant controlled consumption of energy (the one that has accumulated in the capacitor) for one welded joint, which ensures the stability of the result.

Welding occurs in a very short time (hundredths and even thousandths of a second). This gives a concentrated heat release and minimizes the heat affected zone. The latter advantage allows it to be used for welding metals with high electrical and thermal conductivity (copper and aluminum alloys, silver, etc.), as well as materials with sharply different thermal properties.

Rigid capacitor micro welding is used in the radio-electronic industry.

The amount of energy stored in capacitors can be calculated using the formula:

W = C U 2 /2

where C is the capacitance of the capacitor, F; W - energy, W; U - charging voltage, V. By changing the resistance value in the charging circuit, the charging time, charging current and power consumed from the network are regulated.

Resistance spot welding defects

With high-quality performance, spot welding has high strength and is able to ensure the operation of the product for a long service life. In case of destruction of structures connected by multi-point multi-row spot welding, destruction occurs, as a rule, along the base metal, and not along weld points.

The quality of welding depends on the acquired experience, which is mainly reduced to maintaining the required duration of the current pulse on the basis of visual observation (by color) of the weld point.

A correctly made weld point is located in the center of the joint, has the optimal size of the cast core, does not contain pores and inclusions, does not have external and internal splashes and cracks, and does not create large stress concentrations. When a tensile force is applied, the destruction of the structure occurs not along the cast core, but along the base metal.

Spot welding defects are divided into three types:

deviations of the dimensions of the cast zone from the optimal ones, displacement of the core relative to the joint of the parts or the position of the electrodes;
violation of the continuity of the metal in the connection zone;
change in the properties (mechanical, anticorrosive, etc.) of the metal of the weld point or areas adjacent to it.

The most dangerous defect is the absence of a cast zone (lack of penetration in the form of "gluing"), in which the product can withstand the load at a low static load, but is destroyed under the action of a variable load and temperature fluctuations.

The strength of the connection is also reduced with large dents from the electrodes, gaps and cracks in the edge of the overlap, and splashing of metal. As a result of the exit of the cast zone to the surface, the anti-corrosion properties of the products (if any) are reduced.

Complete or partial lack of fusion, insufficient dimensions of the cast core. Possible reasons: low welding current, too high clamping force, wear of the working surface of the electrodes. The lack of welding current can be caused not only by its low value in the secondary circuit of the machine, but also by the electrode touching the vertical walls of the profile or by too close a distance between the weld points, leading to a large shunt current.

The defect is detected by external inspection, by lifting the edges of the parts with a punch, ultrasonic and radiation devices to control the quality of welding.

External cracks. Causes: too high welding current, insufficient compression force, lack of forging force, contaminated surface of parts and / or electrodes, leading to an increase in the contact resistance of parts and a violation of the temperature regime of welding.

The defect can be detected with the naked eye or with a magnifying glass. Effective capillary diagnostics.

Breaks at the edges of the lap. The reason for this defect is usually the same - the weld point is located too close to the edge of the part (insufficient overlap).

It is detected by external examination - through a magnifying glass or with the naked eye.

Deep dents from the electrode. Possible reasons: too small size (diameter or radius) of the working part of the electrode, excessive forging force, incorrectly installed electrodes, too large dimensions of the cast zone. The latter may be due to excess welding current or pulse duration.

Internal splash (outflow of molten metal into the gap between parts). Causes: Permissible values of current or duration of the welding pulse are exceeded - too large a zone of molten metal has formed. The compression force is low - a reliable sealing belt around the core was not created or an air cavity formed in the core, which caused the molten metal to flow into the gap. The electrodes are installed incorrectly (misaligned or skewed).

It is determined by the methods of ultrasonic or radiographic control or external examination (due to the splash, a gap may form between the parts).

External splash (outlet of metal to the surface of the part). Possible reasons: switching on of the current pulse with uncompressed electrodes, too high value of the welding current or pulse duration, insufficient compression force, distortion of the electrodes relative to the parts, contamination of the metal surface. The last two reasons lead to uneven current density and melting of the surface of the part.

determined by external examination.

Internal cracks and shells. Causes: The current or pulse duration is too high. The surface of the electrodes or parts is dirty. Small compression force. Missing, late or insufficient forging force.

Shrinkage cavities can occur during the cooling and crystallization of the metal. To prevent their occurrence, it is necessary to increase the compression force and apply forging compression at the moment of core cooling. Defects are detected by X-ray or ultrasonic testing.

Displacement of the cast core or its irregular shape. Possible reasons: electrodes are installed incorrectly, the surface of the parts is not cleaned.

Defects are detected by X-ray or ultrasonic testing.

burn. Causes: the presence of a gap in the assembled parts, contamination of the surface of the parts or electrodes, the absence or low force of compression of the electrodes during the current pulse. To avoid burn-through, current should only be applied after full compression force has been applied. determined by external examination.

Correction of defects. The method of correcting defects depends on their nature. The simplest is repeated spot or other welding. It is recommended to cut or drill the defective place.

If it is impossible to weld (due to the undesirability or inadmissibility of heating the part), instead of a defective weld spot, you can put a rivet by drilling out the welding spot. Other correction methods are also used - cleaning the surface in case of external splashes, heat treatment to relieve stress, straightening and forging when the entire product is deformed.

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UNION OF SOVIET SOCIALIST REPUBLICS 1-5 V 23 K 11/10 INVENTION 4b ".,".,.;.;,: 1 ore for resistance spot welding. The purpose of the invention is to simplify the design and improve the cleanliness of the machined surface, On both ends of the tool 1, teeth 7 are located parallel to each other, Each of the teeth 7 is made with two cutting edges 8 and a supporting surface 5 between them, The device is clamped between the electrodes 4 by force , developed by the machine drive for contact welding. When the device is rotated, the cutting edges 8 cut off the metal layer, and the support surfaces 5 smooth the work area along the entire working end of the electrode. 4 il, eknoKTmo- bogoSTATE COMMITTEE ON INVENTIONS AND DISCOVERIES OF THE USSR State Committee for Science and Technology AUTHOR'S WITNESS (56) Author's certificate USSRM 490579, class. V 23 V 29/14, 1974. Sliozberg S.KChuloshnikov P.LEtrodes for contact welding, L.; Mechanical Engineering, 1972, p. 79, fig. 44 a, (54) DEVICE FOR SHARPENING ELRODS MACHINES FOR RESISTANCE WELDING (57) The invention relates to welding and can be used in the development of 1595635 A 1 The invention relates to welding and can be used in the development of equipment for resistance spot welding. improved surface finish. FIG. 1 schematically shows a device for sharpening the spherical working surface of the electrode, axial section; in fig. 2 - the same, top view; in Fig. 3 - a device for sharpening flat-conical and flat-conical with a ledge of the working surfaces of the electrode, an example of execution; in Fig. 4 - the same, top view. The device for sharpening the electrode consists of a tool 1 installed in a holder 2 with a handle 3 (Fig. 2), or the handle 3 is fixed directly on the tool 1 itself (Fig. 4), In the tool 1, a recess is made on both ends that defines the profile of the processed surface of the electrode 4 and forms a support surface 5. At the ends of the tool 1, grooves b are made, forming parallel teeth 7 on the support surface with two cutting edges 8,U of the tool 1, intended for processing electrodes with a working with a flat-conical or flat-conical surface with a ledge shape (Fig. 3 and 4), the grooves b are placed symmetrically relative to the longitudinal axis and centering blind holes 9 are made on the ends. The electrodes are sharpened as follows .. The device is clamped between the electrodes 4 installed in the electrode holders of the welding machine, welding force, while the electrodes rest on the supporting surfaces 5 on the teeth 7 of the tool 1. The device is centered on the electrodes. At the same time, sections of the supporting surface 5, taking the force from the electrodes, crush the protrusions on the surfaces and elastically deform the electrode material. Turning the device with the handle 3 around the electrodes of the edge 8 cuts off the metal layer, 5 The surface of the electrodes being processed along the entire length of the cutting edge fits snugly to the sections 5 of the supporting surface; since the cutting edge is part of the supporting surface, the sections of the supporting surface 5 sliding along the electrodes 10 under load the end face of the tooth 7, thereby achieving a high surface finish. When the cutting edge 15 is located exactly along the axis of the tool 1, the entire surface of the electrode end face is machined and smoothed. for sharpening electrodes allows you to process the working surfaces of the electrodes without readjusting the machine in terms of force. This achieves high purity and precision processing. The simplicity of the design of the device ensures low manufacturing costs when using commercial equipment 30 that, in order to simplify the design and improve the cleanliness of the machined surface, the teeth are parallel to each other, and each 40 of the teeth is made with two cutting edges and a supporting surface between them to smooth the working surface of the electrode. Production and publishing plant "Patent", Uzhgorod, Gagarina st., 101 Order 2876 Circulation 645

Application

4440071, 03.05.1988

ENTERPRISE PYa G-4086

KRASNOV FELIX IVANOVYCH

IPC / Tags

Link Code

Device for sharpening electrodes of resistance spot welding machines

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We decided to separate the story about electrode holders and electrodes for spot welding in a separate article due to the large amount of material on this topic.

Electrode holders for spot welding machines

Electrode holders are used to install electrodes, regulate the distance between them, supply welding current to the electrodes and remove heat generated during welding. The shape and design of the electrode holders is determined by the shape of the welded assembly. As a rule, the electrode holder is a copper or brass tube with a conical hole for installing the electrode. This hole can be made along the axis of the electrode holder, perpendicular to the axis or at an angle. Often the same machine can be equipped with several options for electrode holders for each type of electrode, depending on the shape of the parts to be welded. In some low-power machines, electrode holders may not be included at all, since their functions are performed by welding trunks.
In standard machines, straight electrode holders are most often used (Fig. 1), as the simplest ones. Electrodes of various shapes can be installed in them. In the case of welding large parts with limited access to the welding site, it is advisable to use shaped electrode holders with simple straight-shaped electrodes. They are fastened in electrode holders due to a conical fit, pins or screws. Removal of the electrode from the holder is carried out by light tapping with a wooden hammer or a special extractor.

Spot welding electrodes

Spot welding electrodes are used to compress parts, supply welding current to parts and remove heat generated during welding. This is one of the most critical elements of the welding circuit of a spot welding machine, because the shape of the electrode determines the possibility of welding a particular node, and its durability determines the quality of welding and the duration of the machine's trouble-free operation. There are straight (Fig. 4) and curly electrodes (Fig. 5). Some examples of the use of straight electrodes are shown in Table 1. Many straight electrodes are manufactured in accordance with GOST 14111-77 or OST 16.0.801.407-87.

For shaped electrodes, the axis passing through the center of the working surface is significantly shifted relative to the axis of the seating surface (cone). They are used for welding parts of complex shape and assemblies in hard-to-reach places.

Design of electrodes for spot welding

The electrode for spot welding (Fig. 6) structurally consists of a working part (1), a middle (cylindrical) part (2) and a landing part (3). Inside the body of the electrode, there is an internal channel, into which the tube for supplying the cooling water of the electrode holder is inserted.
The working part (1) of the electrode has a flat or spherical surface; the diameter of the working surface d el or the radius of the sphere R el is chosen depending on the material and thickness of the parts to be welded. The cone angle of the working part is usually 30°.
The middle part (2) ensures the strength of the electrode and the possibility of using extractors or other tools for dismantling the electrodes. Manufacturers use different methods to calculate electrode sizes. In the USSR, according to OST 16.0.801.407-87, standard series were established:

D el = 12, 16, 20, 35, 32, 40 mm

L = 35, 45, 55, 70, 90, 110mm

Depending on the maximum compression force of the machine:

D el \u003d (0.4 - 0.6) √ F el (mm).

Where: F el - the maximum compression force of the machine (daN).

The landing part (3) must have a taper for a tight fit in the electrode holder and to prevent leakage of cooling water. For electrodes with a diameter of 12-25 mm, the taper is 1:10, for electrodes with a diameter of 32-40 mm, the taper is 1:5. The length of the conical part is not less than 1.25D el. The landing part is processed with a purity of at least the 7th class (R z 1.25).

The diameter of the internal cooling channel is determined by the flow rate of cooling water and sufficient compressive strength of the electrode and is:

d 0 \u003d (0.4 - 0.6) D el (mm).

The distance from the working surface of the electrode to the bottom of the inner channel largely affects the operational characteristics of the electrode: resistance, service life. The smaller this distance, the better the cooling of the electrode, but the less regrinding the electrode can withstand. According to experimental data:

h = (0.75 - 0.80) D el (mm).

Refractory inserts made of tungsten W or molybdenum Mo (Fig. 4g) are pressed into copper electrodes or soldered with silver-containing solders; such electrodes are used when welding galvanized or anodized steels. Electrodes with a replaceable working part (Fig. 4i) and with a ball joint (Fig. 4k) are used when welding parts from different materials or parts of different thicknesses. The replaceable working part is made of tungsten, molybdenum or their alloys with copper and is attached to the electrode with a union nut. Steel or brass electrodes with a pressed copper sheath (Fig. 4h) or copper electrodes with a steel spring-loaded sleeve are also used.

Materials for spot welding electrodes

The resistance of electrodes is their ability to maintain the dimensions and shape of the working surface (end), to resist the mutual transfer of the metal of the electrodes and the parts to be welded (contamination of the working surface of the electrode). It depends on the design and material of the electrode, the diameter of its cylindrical part, the angle of the cone, the properties and thickness of the material being welded, the welding mode, and the electrode cooling conditions. The wear of the electrodes depends on the design of the electrodes (material, diameter of the cylindrical part, the angle of the cone of the working surface) and the parameters of the welding mode. Overheating, melting, oxidation during operation in a humid or corrosive environment, deformation of the electrodes with high compression forces, distortion or displacement of the electrodes increase their wear.

The electrode material is selected taking into account the following requirements:

electrical conductivity comparable to that of pure copper;
good thermal conductivity;
mechanical strength;
machinability by pressure and cutting;
resistance to softening under cyclic heating.

Compared with pure copper, alloys based on it have 3-5 times greater resistance to mechanical stress, therefore, copper alloys are used for spot welding electrodes with their seemingly mutually exclusive requirements. Alloying with cadmium Cd, chromium Cr, beryllium Be, aluminum Al, zinc Zn, zirconium Zr, magnesium Mg does not reduce electrical conductivity, but increases strength in the heated state, while iron Fe, nickel Ni and silicon Si increase hardness and mechanical strength. Examples of the use of some copper alloys for spot welding electrodes are shown in table 2.

Selection of electrodes for spot welding

When choosing electrodes, the main parameters are the shape and dimensions of the working surface of the electrode. In this case, it is necessary to take into account the grade of the material to be welded, the combination of thicknesses of the sheets to be welded, the shape of the welded assembly, the requirements for the surface after welding, and the design parameters of the welding mode.

There are the following types of shape of the working surface of the electrode:

with flat ones (characterized by the diameter of the working surface d el);
with spherical (characterized by radius R el) surfaces.

Electrodes with a spherical surface are less sensitive to distortions, therefore they are recommended for use on machines of the radial type and hanging machines (pliers) and for shaped electrodes operating with a large deflection. Russian manufacturers recommend using only electrodes with a spherical surface for welding light alloys, which makes it possible to avoid dents and undercuts along the edges of the weld point (see Fig. 7). But you can avoid dents and undercuts by using flat electrodes with an enlarged end. The same hinged electrodes avoid distortion and can therefore replace spherical electrodes (Fig. 8). However, these electrodes are mainly recommended for welding sheets with a thickness of ≤1.2 mm.

According to GOST 15878-79, the dimensions of the working surface of the electrode are selected depending on the thickness and grade of the materials being welded (see Table 3). After examining the cross section of the weld spot, it becomes clear that there is a direct relationship between the diameter of the electrode and the diameter of the core of the weld spot. The electrode diameter determines the area of the contact surface, which corresponds to the fictitious diameter of the resistance conductor r between the sheets to be welded. The contact resistance R will be inversely proportional to this diameter and inversely proportional to the preliminary compression of the electrodes to smooth out surface microroughness. Research by ARO (France) showed that the calculation of the diameter of the working surface of the electrode can be carried out according to the empirical formula:

d el = 2t + 3 mm.

Where t is the nominal thickness of the sheets to be welded.

It is most difficult to calculate the electrode diameter when the thickness of the sheets being welded is unequal, when a package of three or more parts is welded, and when dissimilar materials are welded. Obviously, when welding parts of different thicknesses, the electrode diameter must be selected relative to the thinner sheet. Using the formula for calculating the electrode diameter, which is proportional to the thickness of the sheet being welded, we form a fictitious conductor with a tapering diameter, which, in turn, moves the heating spot to the contact point of these two sheets (Fig. 10).

When welding a package of parts at the same time, the choice of the diameter of the working surface of the electrode is made according to the thickness of the outer parts. When welding dissimilar materials with different thermophysical characteristics, less penetration is observed in a metal with a lower electrical resistivity. In this case, an electrode with a large diameter of the working surface d el or made of a material with higher thermal conductivity (for example, BrX chromium bronze) is used on the side of the metal part with lower resistance.

Valery Raisky
Magazine "Equipment: market, offer, prices", No. 05, May 2005

Literature:

Knorozov B.V., Usova L.F., Tretyakov A.V. Technology of metals and materials science. - M., Metallurgy, 1987.
Handbook of the machine builder. T. 5, book. 1. Ed. Satel E.A. - M., Mashgiz, 1963.

Most of the metal products that surround us are made using resistance welding. There are various types of welding, but contact welding allows you to create fairly strong and aesthetically beautiful seams. Since the metal is not welded by the traditional method, this process requires resistance welding electrodes.

Resistance welding is possible only for welding two metal parts superimposed on one another, they cannot be butt-joined by this method. At that moment, when both parts are clamped by the conductive elements of the welding machine, an electric current is briefly applied, which melts the parts directly at the point of compression. This is mainly due to the current resistance.

Electrode designs

Electrodes are also used to work with electric arc welding, but they are fundamentally different from conductive elements for resistance welding, and are not suitable for this type of work. Since at the time of welding the parts are squeezed by the contact parts of the welding machine, the resistance welding electrodes are able to conduct electric current, withstand the compression load and remove heat.

The diameter of the electrodes determines how firmly and efficiently the parts will be welded. Their diameter should be 2 times thicker than the welded joint. According to state standards, they come in diameters from 10 to 40 mm.

The metal being welded determines the shape of the electrode used. These elements, having a flat working surface, are used for welding ordinary steels. The spherical shape is ideal for joining copper, aluminium, high carbon and alloy steels.

The spherical shape is the most resistant to combustion. Due to their shape, they are able to make more welds before sharpening. In addition, the use of this form allows you to cook any metal. At the same time, if aluminum or magnesium is welded with a flat surface, dents will form.

The electrode seat is often cone-shaped or threaded. This design avoids current loss and effectively performs compression of parts. The landing cone can be short, but they are used at low forces and low currents. If a threaded fastener is used, then often through a union nut. Threaded fastening is especially important in special multi-point machines, since the same gap between the claws is required.

To perform welding in the depth of the part, electrodes of a curved configuration are used. There is a variety of curved shapes, so if you are constantly working in such conditions, it is necessary to have a selection of different shapes. However, they are inconvenient to use, and they have a lower resistance than straight ones, so they are resorted to last.

Since the pressure on the shaped electrode is not along its axis, it is subject to bending during heating, and this must be remembered when choosing its shape. In addition, at such moments, it is possible to shift the working surface of a curved electrode in relation to a flat one. Therefore, in such situations, a spherical working surface is usually used. Non-axial load also affects the seat of the electrode holder. Therefore, with excessive load, it is necessary to use electrodes with an increased cone diameter.

When welding in the depth of a part, a straight electrode can be used if it is tilted vertically. However, the angle of inclination should be no more than 30 °, since with a greater degree of inclination, deformation of the electrode holder occurs. In such situations, two curved conductive elements are used.

The use of a clamp at the point of attachment of the figured electrode allows to reduce the load on the cone and extend the service life of the seat of the welding machine. When developing a curly electrode, you must first make a drawing, then make a trial model from plasticine or wood, and only after that proceed with its manufacture.

In industrial welding, cooling of the contact part is used. Often such cooling occurs through the internal channel, but if the electrode is small in diameter or there is increased heating, then the coolant is supplied from the outside. However, external cooling is allowed provided that the parts to be welded are not susceptible to corrosion.

The hardest thing is to cool the figured electrode because of its design. For its cooling, thin copper tubes are used, which are located on the side parts. However, even under these conditions, it does not cool well enough, so it cannot cook at the same pace as a straight electrode. Otherwise, it overheats and the service life is reduced.

Welding in the depth of a small part is done with shaped electrodes, and with large parts it is preferable to use shaped holders. The advantage of this method is the ability to adjust the length of the electrode.

During contact welding, the axis of the two electrodes must be 90 about in relation to the surface of the part. Therefore, when large-sized parts with a slope are welded, rotary, self-aligning holders are used, and welding is performed with a spherical working surface.

Steel mesh with a diameter of up to 5 mm is welded with a plate electrode. Uniform distribution of the load is achieved by free rotation around its axis of the upper conductive contact.

Although the spherical shape of the working surface is the most stable of the other shapes, nevertheless, due to thermal and power loads, it loses its original shape. If the working surface of the contact increases by 20% of the original size, then it is considered unsuitable and must be sharpened. Grinding of resistance welding electrodes is carried out in accordance with GOST 14111.

Materials of electrodes for contact welding

One of the decisive factors in the quality of the weld is the tensile strength. This is determined by the temperature of the weld spot and depends on the thermophysical properties of the conductor material.

Copper in its pure form is inefficient, because it is a very ductile metal and does not have the necessary elasticity to restore its geometric shape between weld cycles. In addition, the cost of the material is relatively high, and with such properties, the electrodes would require regular replacement, which would increase the cost of the process.

The use of hardened copper was also unsuccessful, since a decrease in the recrystallization temperature leads to the fact that with each subsequent weld point, the wear of the working surface will increase. In turn, copper alloys with a number of other metals proved to be effective. For example, cadmium, beryllium, magnesium and zinc added hardness to the alloy during heating. At the same time, iron, nickel, chromium and silicon allow you to withstand frequent thermal loads and keep the pace of work.

The electrical conductivity of copper is 0.0172 Ohm * mm 2 / m. The lower this indicator, the more suitable it is as an electrode material for resistance welding.

If you need to weld elements from different metals or parts of different thicknesses, then the electrical and thermal conductivity of the electrode should be up to 40% of this property of pure copper. However, if the entire conductor is made of such an alloy, then it will heat up quickly enough, since it has a high resistance.

Using the technology of composite structures, you can achieve tangible cost savings. In such designs, the materials used in the base are selected with a high electrical conductivity, and the outer or replaceable part is made of heat and wear resistant alloys. For example, cermet alloys, consisting of 44% copper and 56% tungsten. The electrical conductivity of such an alloy is 60% of the electrical conductivity of copper, which allows heating the weld point with minimal effort.

Depending on the working conditions and tasks, alloys are divided into:

Difficult conditions. The electrodes operating at temperatures up to 500 ° C are made of bronze, chromium and zirconium alloys. For welding stainless steel, alloys of bronze alloyed with titanium and beryllium are used.
Average load. Welding of standard carbon, copper and aluminum parts is carried out with electrodes from alloys in which the grade of copper for electrodes is capable of operating at temperatures up to 300 ° C.
Lightly loaded. Alloys, which include cadmium, chromium and silicon nickel bronze, are capable of operating at temperatures up to 200 ° C

Spot welding electrodes

The process of spot welding explains itself from its own name. Accordingly, a mini-welding seam is one point, the size of which is determined by the diameter of the working surface of the electrode.

Electrodes for resistance spot welding are rods made of alloys based on copper. The diameter of the working surface is determined by GOST 14111-90, and is manufactured in the range from 10-40 mm. Spot welding electrodes are carefully selected because they have different properties. They are made with both spherical and flat working surfaces.

Do-it-yourself spot welding electrodes can theoretically be made, but you need to be sure that the alloy meets the stated requirements. In addition, you need to withstand all sizes, which is not so easy at home. Therefore, when purchasing factory conductive elements, you can count on high-quality welding work.

Spot welding has a lot of advantages, including an aesthetic welding spot, ease of operation of the welding machine and high productivity. There is also one drawback, namely the lack of a sealed weld.

Seam welding electrodes

One of the varieties of resistance welding is seam welding. However, seam welding electrodes are also an alloy of metals, only in the form of a roller.

Rollers for seam welding are of the following types:

without bevel;
with a bevel on one side;
beveled on both sides.

The configuration of the part to be welded determines which roller shape should be used. In hard-to-reach places, it is unacceptable to use a roller with a bevel on both sides. In this case, a roller without bevels or with a bevel on one side is suitable. In turn, a roller with a bevel on both sides presses the parts more efficiently and cools faster.

The use of roller welding helps to achieve tight welds, which allows them to be used in the manufacture of containers and reservoirs.

So, resistance welding allows you to produce high-tech seams, but in order to achieve a high-quality result, you must carefully follow the values \u200b\u200bindicated in the tables. Which welding to choose, spot or seam, depends on your needs.

High electrode durability and good spot weld quality are not possible without proper care of the electrodes. From 3 to 10% of the welder's working time is spent on electrode maintenance. Proper care of the electrodes allows one pair of electrodes to make 30 ... 100 thousand weld points, while the consumption of the electrode alloy is only 5 ... 20 g per thousand welded points.

Caring for the electrodes of point machines consists of two operations - cleaning the electrodes directly on the machine and filling the removed electrode on a lathe or a special machine.

The frequency of stripping depends mainly on the material to be welded. When welding steel with a well-prepared surface, in some cases it is possible to do without stripping, in others the required stripping is carried out after welding several hundred points. When welding aluminum alloys, electrode cleaning is required after 30 ... The same phenomenon is observed when welding other materials with a lower melting point, such as, for example, magnesium.

Stripping should be carried out in such a way that, without removing a large amount of metal, a clean electrode surface is obtained. To simplify this operation and facilitate working conditions when stripping the electrodes, special devices are used.

The simplest device is shown in Fig. 1. It is a spatula with double-sided recesses into which sandpaper is inserted. The spatula is inserted between the compressed electrodes, and when rotated around the axis of the electrodes, it cleans their contact surfaces.

Rice. 1. Device for manual stripping of electrodes:

1 - skin; 2 - spherical recess.

Instead of such a spatula, you can use a steel plate for stripping electrodes with a flat contact surface or a piece of rubber for stripping electrodes with a spherical working surface. Electrodes with a flat contact surface are cleaned simultaneously or alternately, with a spherical one - simultaneously, with a small compressive force. After stripping, traces of abrasive dust are removed with a dry rag.

The desire to mechanize the process of cleaning the contact surface of the electrodes led to the creation of devices with an electric or pneumatic drive. On fig. 2 shows a pneumatic machine for stripping electrodes.

Rice. 2. Angle pneumatic electrode stripping machine

The need for cleaning the contact surface is determined visually, according to the state of the surface of the product to be welded, but attempts are known to determine the moment of cleaning with the help of special devices.

With the help of software control, not only the installation of the welded unit, welding current and welding time is carried out, but also a signal is given about the need to clean the electrodes.

It is proposed to determine the moment of electrode stripping by comparing the brightness of the light beam reflected from the contact surface of the electrode with the brightness of the beam reflected from the surface of the standard. This method also makes it possible to stop the welding process under the action of a signal, the magnitude of which increases when the working surface of the electrode is contaminated.

Filling the working part of a worn electrode in order to restore its original shape can be done in several ways. The least quality is refilling with a small file. It is recommended to use special filling stations for these purposes. An example of a manual primer is shown in fig. 3.

Rice. 3. Manual electrode filler:

1 - body; 2 - screws. 3 - incisors; 4 - handle.

It is also recommended to use special pneumatic fillers equipped with a face mill, the profile of the cutting part of which corresponds to the profile of the working part of the electrode. A special cutter is inserted into the chuck of a conventional hand drill and allows you to simultaneously process the conical and flat surfaces of the working part of the electrode.

A good way to thread electrodes is threading on lathes with a gauge check.

With a large number of refillable electrodes, it is advisable to use special machines of the type.

To quickly change the electrodes without damage, it is recommended to use turnkey flattened electrodes or use special pullers.

The simplest puller (Fig. 4) is a screw clamp of a special design.

Rice. 4. Puller of the simplest design:

1 - body; 2 - dies; 3 - clamping screw.

Recovery of worn electrodes for spot welding has not previously been practiced. Recently, a technology has been developed for the restoration of electrodes of spot welding machines by arc surfacing. The hardness, electrical conductivity and resistance of the regenerated electrodes correspond to the properties of electrodes made from rods. Application of the method of recovery of the electrode by surfacing for only one multi-point machine allows saving up to 500 kg of bronze per year.