Electrode care.

Spot welding is a method in which overlapping parts are joined at one or more points. When an electric current is applied, local heating occurs, as a result of which the metal melts and sets. Unlike electric arc or gas welding, no filler material is required: it is not the electrodes that melt, but the parts themselves. There is no need to envelop it in an inert gas: the weld pool is sufficiently localized and protected from atmospheric oxygen. A welder works without a mask or gloves. This allows for better visualization and control of the process. Spot welding provides high productivity (up to 600 spots/min) at low costs. It is widely used in various sectors of the economy: from instrument making to aircraft manufacturing, as well as for domestic purposes. Not a single auto repair shop can do without spot welding.

Spot Welding Equipment

The work is performed on a special welding machine called a spotter (from the English Spot - point). Spotters can be stationary (for work in workshops) or portable. The installation operates from a 380 or 220 V power supply and generates current charges of several thousand amperes, which is significantly more than that of inverters and semi-automatic devices. Current is supplied to a copper or carbon electrode, which is pressed against the surfaces to be welded using pneumatics or a hand lever. A thermal effect occurs that lasts a few milliseconds. However, this is enough for reliable joining of surfaces. Since the exposure time is minimal, the heat does not spread further through the metal, and the welding point quickly cools down. Parts made of ordinary steel, galvanized iron, stainless steel, copper, and aluminum are subject to welding. The thickness of surfaces can be different: from the thinnest parts for instrument making to sheets 20 mm thick.

Resistance spot welding can be carried out with one electrode or two from different sides. The first method is used for welding thin surfaces or in cases where it is impossible to press on both sides. For the second method, special pliers are used to clamp parts. This option provides a more reliable fastening and is more often used for working with thick-walled workpieces.

According to the type of current, spot welding machines are divided into:

operating on alternating current;
operating on direct current;
low frequency devices;
capacitor type devices.

The choice of equipment depends on the characteristics of the technological process. The most common are AC devices.

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Electrodes for spot welding

Spot welding electrodes are different from arc welding electrodes. They not only supply current to the surfaces being welded, but also perform a pressing function and are also involved in heat removal.

The high intensity of the work process necessitates the use of material that is resistant to mechanical and chemical influences. Copper with the addition of chromium and zinc (0.7 and 0.4%, respectively) meets the most advanced requirements.

The quality of the weld point is largely determined by the diameter of the electrode. It must be at least 2 times the thickness of the parts being joined. The dimensions of the rods are regulated by GOST and range from 10 to 40 mm in diameter. Recommended electrode sizes are presented in the table. (Image 1)

For welding ordinary steels, it is advisable to use electrodes with a flat working surface, for welding high-carbon and alloy steels, copper, aluminum - with a spherical one.

Electrodes with spherical tips are more durable: they are able to produce more points before re-sharpening.

In addition, they are universal and suitable for welding any metal, but using flat ones for welding aluminum or magnesium will lead to the formation of dents.

Spot welding in hard-to-reach places is performed using curved electrodes. A welder who faces such working conditions always has a set of different shaped electrodes.

To reliably transmit current and ensure clamping, the electrodes must be tightly connected to the electrode holder. To do this, their landing parts are given the shape of a cone.

Some types of electrodes have a threaded connection or are mounted on a cylindrical surface.

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Spot Welding Parameters

The main parameters of the process are current strength, pulse duration, compression force.

The amount of heat generated, the heating rate, and the size of the welded core depend on the strength of the welding current.

Along with the current strength, the amount of heat and the size of the core are affected by the duration of the pulse. However, upon reaching a certain point, a state of equilibrium occurs when all the heat is removed from the welding zone and no longer affects the melting of the metal and the size of the core. Therefore, increasing the duration of current supply beyond this is impractical.

The compression force affects the plastic deformation of the surfaces being welded, the redistribution of heat over them, and the crystallization of the core. High compression force reduces the resistance of the electric current flowing from the electrode to the parts being welded and in the opposite direction. Thus, the current increases and the melting process accelerates. A connection made with a high compression force is highly durable. At high current loads, compression prevents splashes of molten metal. In order to relieve stress and increase core density, in some cases an additional short-term increase in the compression force is made after turning off the current.

There are soft and hard. In soft mode, the current strength is less (current density is 70-160 A/mm²), and the pulse duration can reach several seconds. This type of welding is used to join low-carbon steels and is more common at home, when work is carried out on low-power machines. In hard mode, the duration of a powerful pulse (160-300 A/mm²) ranges from 0.08 to 0.5 seconds. The parts are provided with the maximum possible compression. Fast heating and rapid cooling help maintain the welded core's anti-corrosion resistance. The hard mode is used when working with copper, aluminum, and high-alloy steels.

The selection of optimal parameters requires taking into account many factors and carrying out tests after calculations. If performing trial work is impossible or impractical (for example, for one-time welding at home), then you should adhere to the modes set out in the reference books. The recommended parameters of current strength, pulse duration and compression for welding ordinary steels are given in the table. (Image 2)

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Possible defects and their causes

A well-made point connection provides a reliable connection, the service life of which, as a rule, exceeds the service life of the product itself. However, violation of technology can lead to defects, which can be divided into 3 main groups:

insufficient dimensions of the welded core and deviation of its position relative to the joint of parts;
mechanical damage: cracks, dents, cavities;
violation of the mechanical and anti-corrosion properties of the metal in the area adjacent to the weld point.

Let's look at specific types of defects and the reasons for their occurrence:

Lack of penetration can be caused by insufficient current, excessive compression, or worn-out electrode.
External cracks occur when there is too much current, insufficient compression, or surface contamination.
The gaps at the edges are caused by the proximity of the core to them.
Indentations from electrodes occur when their working surface is too small, improper installation, excessive compression, too high a current and a long pulse.
The splash of molten metal and its filling of the space between the parts (internal splash) occurs due to insufficient compression, the formation of an air pocket in the core, and non-coaxially installed electrodes.
External splashing of molten metal onto the surface of parts can be caused by insufficient compression, too high current and time conditions, contamination of surfaces and misalignment of the electrodes. The last two factors have a negative impact on the uniformity of current distribution and metal melting.
Internal cracks and cavities occur due to excessive current and time conditions, insufficient or delayed forging compression, and surface contamination. Shrinkage cavities appear as the core cools. To prevent them, forging compression is used after stopping the current supply.
The reason for the irregular shape of the core or its displacement is the distortion or misalignment of the electrodes, or contamination of the surface of the parts.
Burn-through is a consequence of contaminated surfaces or insufficient compression. To avoid this defect, current must be applied only after compression has been achieved completely.

To identify defects, visual inspection, radiography, ultrasound, and capillary diagnostics are used.

During testing work, control over the quality of the weld point is carried out by the tearing method. The core should remain completely on one part, and a deep crater on the second.

Correction of defects depends on their nature. Mechanical cleaning of external splashes, forging during deformation, and heat treatment to relieve stress are used. More often than not, defective points are simply digested.

Spot welding is a type of resistance welding. With this method, heating the metal to its melting temperature is carried out by heat, which is generated 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 after it, the parts are compressed, resulting in mutual penetration and fusion of heated areas of the metal.

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 site (from several tens to hundreds of kg), a small melting zone.

Spot welding is most often used for overlapping sheet metal workpieces, and less often for welding rod materials. The range of thicknesses welded by it ranges from a few micrometers to 2-3 cm, but most often the thickness of the welded metal varies from tenths to 5-6 mm.

In addition to spot welding, there are other types of resistance welding (butt, seam, etc.), but 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 airliners, in particular, several million weld spots are produced.

Well-deserved popularity

The great demand for spot welding is due to a number of advantages that it has. These include: no need for welding materials (electrodes, filler materials, fluxes, etc.), minor residual deformations, simplicity and convenience of working with welding machines, neat connections (virtually no weld), environmental friendliness, cost-effectiveness, susceptibility to easy mechanization and automation, high productivity. Automatic spot welders are capable of performing up to several hundred welding cycles (welded spots) per minute.

Disadvantages include the lack of sealing of the seam and stress concentration at the welding point. Moreover, the latter can be significantly reduced or even eliminated using special technological methods.

Sequence of processes for resistance spot welding

The entire spot welding process can be divided into 3 stages.

Compression of parts causing plastic deformation of microroughnesses in the electrode-part-part-electrode chain.
Turning on a pulse of electric current, leading to heating of the metal, its melting in the joint zone and the formation of a liquid core. As current passes, the core increases in height and diameter to its maximum size. Bonds are formed in the liquid phase of the metal. In this case, plastic settlement of the contact zone continues to its final size. Compression of the parts ensures the formation of a sealing belt around the molten core, which prevents metal from splashing out from the welding zone.
Turning off the current, cooling and crystallization of the metal, ending with the formation of a cast core. When cooling, the volume of the metal decreases and residual stresses arise. The latter are an undesirable phenomenon that is combated in various ways. The force compressing the electrodes is released 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 metal, eliminating inhomogeneities in 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 SV), the duration of its pulse (t SV), the compression force of the electrodes (F SV), the dimensions and shape of the working surfaces of the electrodes (R - for a spherical shape, d E - for a flat shape ). For better clarity of the process, these parameters are presented in the form of a cyclogram reflecting their change over time.

There are 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 low current. The duration of the welding pulse ranges 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 for hard welding.

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

Depending on their shape, electrodes are divided into straight and shaped. The first ones are the most common; they are used for welding parts that allow free access of electrodes to the welded area. Their dimensions are standardized by GOST 14111-90, which sets 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 by diameter (d) and radius (R) values, respectively. The contact area of the electrode with the workpiece depends on the values of d and R, which affects the current density, pressure and size of the core. Electrodes with a spherical surface have greater durability (they can make more points before resharpening) and are less sensitive to distortions during installation than electrodes with a flat surface. Therefore, it is recommended to manufacture electrodes used in clamps with a spherical surface, as well as shaped 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 flat surface electrodes for this purpose results in excessive indentations and undercuts on the surface of the 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 were introduced.

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

The material of the electrodes is very important, determining their electrical resistance, thermal conductivity, heat resistance and mechanical strength at high temperatures. During operation, the electrodes heat up to high temperatures. The thermocyclic operating mode, 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. To ensure that the electrodes are able to withstand harsh operating conditions, they are made from special copper alloys that have heat resistance and high electrical and thermal conductivity. Pure copper is also capable of working as electrodes, but it has low durability and requires frequent regrinding of the working part.

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

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

I 2 = I NE + I w

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

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

Welding time. Welding time (tSW) refers to the duration of the current pulse when performing one weld point. Together with the current strength, it determines the amount of heat that is released in the connection area when an electric current passes through it.

As t SV increases, the penetration of parts increases and the dimensions of the molten metal core (d, h and h 1) increase. At the same time, heat removal from the melting zone increases, parts and electrodes heat up, and heat dissipates into the atmosphere. When a certain time is reached, a state of equilibrium can occur in which all the supplied energy is removed from the welding zone without increasing the penetration of parts and the size of the core. Therefore, increasing t SV 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 point, 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 calculations.

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

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

When welding in hard conditions, higher FSV values are used than in soft welding. This is due to the fact that with increasing rigidity, the power of current sources and the penetration of parts increases, which can lead to the formation of splashes of molten metal. A large compression force is precisely intended to prevent this.

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

When manufacturing the simplest resistance welding machines for home use, there is little reason to make accurate calculations of parameters. Approximate values for electrode diameter, welding current, welding time and compression force can be taken from tables available in many sources. You just need to understand that the data in the tables is somewhat overestimated (or underestimated, if you take into account the welding time) compared to those that are suitable for home devices, where soft modes are usually used.

Preparing parts for welding

The surface of parts in the area of contact between parts and at the point of contact with electrodes is cleaned of oxides and other contaminants. If cleaning is poor, power losses increase, the quality of connections deteriorates and 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 preparing the surface for welding is to remove, without damaging the metal, a relatively thick film of oxides with high and uneven electrical resistance.

Spot Welding Equipment

The differences between 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:

AC welding machines;
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 are AC welding machines.

AC resistance spot welding machines. The schematic diagram of AC spot welding machines 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 (TS). The thyristor module (CT) ensures the connection of the primary winding of the transformer to the supply voltage for the required time to form a welding pulse. Using the module, you can not only control the duration of the welding time, but also regulate the shape of the supplied pulse by changing the opening angle of the thyristors.

If the primary winding is made not of one, but of several windings, then by connecting them in different combinations with each other, you can 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 resistance spot welding machines have a set of control equipment - a power supply 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 accumulates relatively slowly in the capacitor when charging it, and then is very quickly consumed, generating a large current pulse. This allows welding to be carried out while consuming less power from the network compared to conventional spot welders.

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

Welding occurs in a very short time (hundredths and even thousandths of a second). This produces 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 thermophysical properties.

Rigid capacitor microwelding is used in the electronics 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 is the 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.

Defects in resistance spot welding

When performed with high quality, spot welding has high strength and can ensure the operation of the product for a long service life. When structures connected by multi-point, multi-row spot welding are destroyed, the destruction occurs, as a rule, along the base metal, and not at the welded points.

The quality of welding depends on the experience gained, which comes down mainly to maintaining the required duration of the current pulse based on visual observation (by color) of the weld point.

A correctly executed weld point is located in the center of the joint, has an optimal size of the cast core, does not contain pores and inclusions, does not have external or 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 parts or the position of the electrodes;
violation of metal continuity in the connection zone;
change in the properties (mechanical, anti-corrosion, etc.) of the metal of the weld point or areas adjacent to it.

The most dangerous defect is considered to be the absence of a cast zone (lack of penetration in the form of a “glue”), 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 when there are large dents from the electrodes, breaks and cracks in the overlap edge, and metal splashes. As a result of the cast zone coming to the surface, the anti-corrosion properties of the products (if any) are reduced.

Lack of penetration, complete or partial, insufficient dimensions of the cast core. Possible reasons: the welding current is low, the compression force is too high, the working surface of the electrodes is worn out. Insufficient 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 welding points, leading to a large shunt current.

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

External cracks. Reasons: 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 welding temperature regime.

The defect can be detected with the naked eye or with a magnifying glass. Capillary diagnostics is effective.

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

It is detected by external inspection - 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, excessively high forging force, incorrectly installed electrodes, too large dimensions of the cast area. The latter may be a consequence of exceeding the welding current or pulse duration.

Internal splash (release of molten metal into the gap between parts). Reasons: the permissible values of the current or the 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 has not been created or an air pocket has formed in the core, causing molten metal to flow out into the gap. The electrodes are installed incorrectly (misaligned or skewed).

Determined by ultrasonic or radiographic testing methods or external inspection (due to splashing, a gap may form between parts).

External splash (metal coming out onto the surface of the part). Possible reasons: switching on the current pulse when the electrodes are not compressed, the welding current or pulse duration is too high, insufficient compression force, misalignment 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 inspection.

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

Shrinkage cavities can occur during cooling and crystallization of the metal. To prevent their occurrence, it is necessary to increase the compression force and apply forging compression at the time of cooling of the core. Defects are detected using radiographic or ultrasonic testing methods.

Molded core is misaligned or irregularly shaped. Possible reasons: electrodes are installed incorrectly, the surface of the parts is not cleaned.

Defects are detected using radiographic or ultrasonic testing methods.

Burn-through. Reasons: the presence of a gap in the assembled parts, contamination of the surface of the parts or electrodes, absence or low compression force 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 inspection.

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

If welding is impossible (due to undesirability or inadmissibility of heating the part), instead of the defective welding point, you can put a rivet by drilling out the welding site. 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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High electrode durability and proper quality of welded spot joints are impossible without proper care of the electrodes. From 3 to 10% of a welder’s working time is spent on electrode maintenance. Proper care of the electrodes allows one pair of electrodes to perform 30...100 thousand welded 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 - stripping the electrodes directly on the machine and refilling the removed electrode on a lathe or special machine.

The frequency of stripping depends mainly on the material being welded. When welding steel with a well-prepared surface, in some cases you can do without cleaning, in others the required cleaning is performed after welding several hundred points. When welding aluminum alloys, it is necessary to clean the electrodes at 30...60 points, otherwise the electrode metal begins to stick to the metal being welded, which disrupts the welding process and also impairs the corrosion resistance of the welded joint. The same phenomenon is observed when welding other materials with a low melting point, such as magnesium.

Stripping should be carried out in such a way as to obtain a clean electrode surface without removing a large amount of metal. To simplify this operation and facilitate working conditions when stripping 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 cleaning electrodes with a flat contact surface or a piece of rubber for cleaning electrodes with a spherical working surface. Electrodes with a flat contact surface are stripped simultaneously or alternately, with a spherical contact surface - simultaneously, with a small compressive force. After cleaning, traces of abrasive dust are removed with a dry cloth.

The desire to mechanize the process of cleaning the contact surface of electrodes led to the creation of devices with an electric or pneumatic drive. In Fig. Figure 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, by the state of the surface of the product being welded, but attempts are known to determine the moment of cleaning using special devices.

With the help of software control, not only the unit to be welded, the welding current and welding time are set, but also a signal is given about the need to strip the electrodes.

It is proposed to determine the moment of stripping the electrodes 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 influence of a signal, the magnitude of which increases when the working surface of the electrode is contaminated.

Refilling the working part of a worn electrode in order to restore its original shape can be done in several ways. The least quality is the filling with a fine file. It is recommended to use special refills for these purposes. An example of a manual refill is shown in Fig. 3.

Rice. 3. Manual electrode refill:

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

It is also recommended to use special pneumatic refillers equipped with an end 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 surface of the working part of the electrode.

A good way to thread electrodes is to thread them on lathes and check the dimensions using a template.

For a large number of electrodes to be refilled, it is advisable to use special machines such as

To quickly change electrodes without damage, it is recommended to use electrodes with turnkey flats 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.

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

Electrodes for resistance welding are designed to supply current to the elements, compress them and remove the generated heat. This part is one of the most important in the equipment, since the ability to process the unit depends on its shape. The stability of the electrode determines the level of welding quality and the duration of continuous operation. Electrodes can be shaped or straight. The production of direct type elements is regulated in the GOST 14111–77 standard.

Shaped parts are characterized by the fact that their axis is offset relative to the cone (seating surface). They are used for welding assemblies and elements of complex shapes that are difficult to reach.

Design Features

Electrodes intended for resistance welding include a cylindrical part, a working part and a landing part. In the internal cavity of the element there is a special channel, which is designed to supply water that cools the electric holder.

The working part has a spherical or flat surface. Its diameter is selected in accordance with the thickness of the products being processed and the material used. The strength of the electrode is ensured by the middle part.

The landing part must have a conical shape so that the part is securely fixed in the electrical holder. It must be processed with a cleanliness of at least class 7.

Custom part properties are affected by distance from the very bottom of the cooling channel to the working edge: service life, stability, etc. If this distance is small, then the element will be cooled much more efficiently, but it will be able to withstand a much smaller number of regrinds.

Inserts based on molybdenum and tungsten are placed inside copper parts. Products made in this way are used for welding anodized or galvanized steel.

Production materials

The stability of electrodes is the ability of elements not to lose their shape and size, as well as to resist the transfer of material from welded elements and electrodes. This indicator is determined by the material and design of the welding electrode, as well as the operating conditions and mode. Wear of parts depends on the characteristics of the working tool (angle of the working surface, diameter, material, etc.). Melting, excessive heating, oxidation during operation of the electrode in a corrosive and/or humid environment, displacement or misalignment, compression deformation and other factors significantly increase the wear of working elements.

The tool material must be selected in accordance with the following rules:

Its level of electrical conductivity should be comparable to pure copper;
Effective thermal conductivity;
High degree of mechanical resistance;
Easy to process by cutting or high pressure;
Resistance to cyclic heating.

Compared to 100% copper, its alloys are more resistant to mechanical loads, which is why copper alloys are used for such products. Alloying a product with zinc, beryllium, chromium, magnesium, zirconium does not reduce electrical conductivity, but significantly increases strength, and silicon, iron and nickel increase its hardness.

Choice

In the process of selecting suitable electrodes for spot welding, special attention should be paid to the size and shape of the working element of the product. You should also take into account the characteristics of the material being processed, its thickness, the shape of the welding units and the welding mode.

Resistance welding tools have different working surfaces:

Flat;
Spherical.

Products with a spherical working surface are not particularly sensitive to bevels, which is why they are often used on suspended and radial installations, as well as for shaped electrodes with a deflection. Manufacturers from the Russian Federation recommend this particular type of electrode for processing light alloys, as they help prevent the appearance of undercuts and dents during spot welding. However, this problem can also be prevented if you use flat electrodes with an enlarged end. And electrodes equipped with hinges can even replace spherical-type electrodes, but they are recommended for welding metal sheets whose thickness does not exceed one and a half millimeters.

Dimensions of the working element tools are selected in accordance with the type and thickness of the materials being processed. The results of a study conducted by experts from the French company ARO showed that the required diameter can be calculated using the following formula:

del = 3 mm + 2t, where “t” is the thickness of the sheets to be welded.

It is more difficult to calculate the required tool diameter when the thickness of the sheets is unequal, welding materials of different types and welding a whole “package” of elements. It is clear that to work with parts of different thicknesses, the diameter of the product must be selected relative to the thinnest metal sheet.

When welding a set of elements, the diameter should be selected based on the thickness of the external elements. For welding materials of various types, the metal alloy with the minimum electrical resistivity has the least penetration. In this case, you should use a device made of material with increased thermal conductivity.

Spot welding, thanks to the advent of compact hand-held machines such as BlueWeldPlus, is becoming popular not only for industrial applications, but also in everyday life. The weak point of this technology is electrodes for resistance welding: their low durability in many cases scares off the consumer.

Reasons for the fragility of resistance electric welding electrodes

The resistance welding process consists of the following stages:

Preliminary preparation of the surface of the parts to be joined - it must not be easily cleaned of contaminants and oxides, but also very smooth in order to eliminate unevenness in the resulting electric field voltage.
Manual or mechanical clamping of welded products - with increasing clamping force, the intensity of diffusion and the mechanical strength of the weld increase.
Local melting of metals in the pressing zone by the heat of an electric current, resulting in the formation of a welding joint. Pressing the electrodes at this stage prevents the formation of welding spatter.
Turning off the current and gradually cooling the weld.

Thus, the material of electrodes for contact welding undergoes not only significant thermal stresses, but also mechanical loads. Therefore, a number of requirements are placed on it - high electrical conductivity, high thermal resistance (including from constant temperature fluctuations), increased compressive strength, low heat capacity coefficient.

A limited number of metals have this complex of properties. First of all, it is copper and alloys based on it, however, they do not always meet production requirements.

Due to the constant increase in the energy characteristics of products, many brands direct consumers to use only “their” branded electrodes, which is not always observed. As a result, the quality of welds produced using this technology decreases, and confidence in the resistance electric welding process itself is undermined.

These problems can be overcome in two ways: by improving the types and designs of welding electrodes for spot welding, and by developing new materials used for the manufacture of such electrodes. For private users, the price of the issue is also important.

Electrode materials

According to GOST 2601, the criterion for the quality of a finished seam is its tensile or shear strength. It depends on the intensity of thermal power in the electrical discharge zone, and therefore is associated primarily with the thermophysical characteristics of the electrode material.

The use of copper electrodes is ineffective for two reasons. Firstly, copper, being a highly plastic metal, does not have sufficient elasticity to completely restore the geometric shape of the electrodes during the period between operating cycles. Secondly, copper is very scarce, and frequent replacement of electrodes also causes high financial costs.

When selecting the optimal material for welding electrodes for resistance welding, they are guided by the specific electrical conductivity of the alloy. The less it differs (downward) from the electrical conductivity of pure copper - 0.0172 Ohm mm 2 /m, the better.

The most effective resistance to wear and deformation is shown by alloys containing cadmium (0.9...1.2%), magnesium (0.1...0.9%) and boron (0.02...0.03%).

The choice of material for spot welding electrodes also depends on the specific tasks of the process. Three groups can be distinguished:

Electrodes designed for resistance welding under harsh conditions (continuous alternation of cycles, surface temperatures up to 450…500ºС). They are made from bronzes containing chromium and zirconium (Br.Kh, Br.KhTsr 0.6-0.05. This group also includes nickel-silicon bronzes (Br.KN1-4), as well as bronzes additionally alloyed with titanium and beryllium (Br.NTB), used for spot welding of stainless and heat-resistant steels and alloys.
Electrodes used at contact temperatures on the surface up to 250…300ºС (welding of conventional carbon and low-alloy steels, copper and aluminum products). They are made from copper alloys of the MS and MK grades.
Electrodes for relatively light operating conditions (surface temperatures up to 120…200ºС). The materials used are cadmium bronze Br.Kd1, chromium bronze Br.X08, silicon-nickel bronze Br.NK, etc. Such electrodes can also be used for roller contact electric welding.

It should be noted that in descending order of electrical conductivity (relative to pure copper), these materials are arranged in the following sequence: Br.HTsr 0.6-0.05→MS→MK→Br.Kh→Br.Kh08→Br.NTB→Br .NK →Br.Kd1→Br.KN1-4. In particular, heating an electrode made of Br.KhTsr 0.6-0.05 bronze to the required temperature will occur approximately twice as fast as one made from Br.KN1-4 bronze.

Electrode designs

The least resistant part of the electrode is its spherical working part. The electrode is rejected if the increase in end dimensions exceeds 20% of the original dimensions. The design of the electrodes is determined by the configuration of the surface being welded. The following versions of the instrument are distinguished:

With a cylindrical working part and a conical landing part.
With conical landing and working parts, and a transition cylindrical section.
With a spherical working end.
With a beveled working end.

In addition, electrodes can be solid or composite.

When making it yourself (or re-sharpening), it is recommended to maintain the following size ratios at which the tool will have maximum durability:

To calculate the electrode diameter d, use the dependence P = (3...4)d 2, where P is the actually necessary compression of the electrodes during the resistance electric welding process. In turn, the recommended values of upsetting pressure, at which the highest quality joints are obtained, is 2.5...4.0 kg/mm 2 of the area of the resulting weld;
For electrodes with a conical working part, the optimal taper angle varies from 1:10 (for a tool with a working part diameter of up to 30...32 mm) to 1:5 - in the opposite case;
The choice of cone angle is also determined by the greatest compression force: with maximum forces, it is recommended to take a taper of 1:10, as it ensures increased longitudinal resistance of the electrode.

The main forms of electrodes for resistance welding are established by GOST 14111, therefore, when using certain size ratios, you should take into account the dimensions of the mounting space for the tool for a specific model of resistance welding machine.

Significant savings in material come from the use of composite structures. At the same time, materials with high electrical conductivity values are used to manufacture the body, and the removable working part is made of alloys with high hardness and wear resistance (including thermal). In particular, metal-ceramic alloys from the Swiss company AMRCO grades A1W or A1WC, containing 56% tungsten and 44% copper, have a similar combination of properties. Their electrical conductivity reaches 60% of the electrical conductivity of pure copper, which determines low heating losses when welding. Recommended materials can also be bronze alloys with additions of chromium and zirconium, as well as tungsten.

Electrodes for resistance welding of light alloys, where significant clamping force is not required, are made with a spherical working part, and for the contact jaws of electric spot welding machines it is advisable to use silicon bronze.

The mechanical characteristics of the electrodes must be within the following limits:

Brinell hardness, HB – 1400...2600;
Young's modulus, GPa – 80…140;
Limit bending moment, kgsm – not lower than 750...800.

Electrode structures should always be hollow to ensure efficient cooling.