Processes occurring in resistance spot welding. Do-it-yourself resistance spot welding. What are the benefits of spot welding machines?

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.

With an increase in t SV, 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, ruptures 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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Resistance welding is the process of creating a monolithic weld by melting the edges of the parts being welded with an electric current and subsequent deformation by compressive force. The technology has become particularly widespread in heavy industry and serves for the continuous production of similar products.

This technology is common for serial joining of thin sheet metal

Today, at least one resistance welding machine is available in every plant, and all thanks to the advantages of technology:

• productivity – a weld point is created in no longer than 1 second;
• high stability of operation - once the device is configured, it can work for a long time without third-party intervention, maintaining the quality of work;
• low maintenance costs - this applies to consumables, the working element is contact electrodes;
• Possibility of working with the machine by low-skilled specialists.

Simple, at first glance, resistance welding technology consists of a number of procedures that must be performed. A high-quality connection can only be achieved if all technological features and process requirements are met.

Essence of the process

First, let's figure out how this system works?

The essence of electric contact welding is two inseparable physical processes - heating and pressure. When electric current passes through the connection area, heat is generated, which serves to melt the metal. To ensure sufficient heat generation, the current must reach several thousand or even tens of thousands of amperes. At the same time, some pressure is applied to the part from one or both sides, which creates a tight seam without visible or internal defects.

The joining process involves local heating of the workpieces while simultaneously pressing them

If the process is properly organized, the parts themselves are practically not subject to heating, since their resistance is minimal. As a monolithic connection is created, the resistance decreases, and at the same time the current strength. The electrodes of the welding machine, which are subject to heating, are cooled by the introduced technology using water.

Surface preparation

There are many technologies that allow you to treat the surface before using resistance welding. These include:

• cleaning from coarse dirt;
• degreasing;
• removal of oxide film;
• drying;
• passaging and neutralization.

The order and technologies themselves are determined by the specific process and type of workpiece.

In general, before welding begins, the surface must:

• ensure minimal resistance between the part and the electrode;
• ensure equal resistance along the entire length of the contact;
• The parts to be welded must have smooth surfaces without bulges or depressions.

Resistance welding machines

Equipment for resistance welding is:

• motionless;
• mobile;
• suspended or universal.

Welding is divided according to the type of current into direct and alternating current (transformer, capacitor). According to the welding methods, there are spot, seam butt and relief, which we will talk about below.

The equipment can be either stationary or portable

All spot welding devices consist of three parts:

• electrical systems;
• mechanical part;
• water cooling.

The electrical part is responsible for melting parts, controlling work and rest cycles, and also setting current modes. The mechanical component is a pneumatic or hydraulic system with various drives. If only a compression drive is installed, then we have a point type, seam drives also have rollers, and butt drives have a system for compressing and upsetting products. Water cooling consists of a primary and secondary circuit, distribution fittings, hoses, valves and relays.

Electrodes for resistance welding

In this case, the electrodes not only close the electrical circuit, but also serve as a heat remover from the welded joint, transmit mechanical load, and in some cases help move the workpiece (roller).

The sizes and shapes of electrodes for resistance welding vary depending on the equipment used and the material being welded

This use imposes a number of stringent requirements that the electrodes must meet. They must withstand temperatures over 600 degrees, pressure up to 5 kg/mm2. This is why they are made from chrome bronze, chrome zirconium bronze or cadmium bronze. But even such powerful alloys are not able to withstand the described loads for a long time and quickly fail, reducing the quality of work. The size, composition and other characteristics of the electrode are selected based on the selected mode, type of welding and thickness of the products.

Welding defects and quality control

As with any other technology, welding joints must be subject to strict control to identify all kinds of defects.

Almost everything is used here, and above all – external inspection. However, due to the pressing of the parts, it can be very difficult to identify in this way, so part of the manufactured products is selected and the parts are cut along the seam to identify errors. If a defect is detected, a batch of potentially defective products is sent for processing, and the device is calibrated.

Types of contact welding

The technology for creating a weld spot determines the division of the process into several types:

Spot resistance welding

In this case, welding occurs at one or simultaneously at several points. The strength of a seam consists of many parameters.

The spot method is the most common method

In this case, the quality of work is affected by:

• electrode shape and size;
• current strength;
• pressure force;
• duration of work and degree of surface cleaning.

Modern spot welding machines are capable of operating with an efficiency of 600 welded joints per minute. This technology is used to connect parts of precision electronics, to connect body parts of cars, airplanes, agricultural machinery, and has many other areas of use.

Relief welding

The operating principle is the same as spot welding, but the main difference is that the weld itself and the electrode have a similar relief shape. Relief is provided by the natural shape of the parts or the creation of special stampings. Like spot welding, the technology is used almost everywhere and serves as a complementary technology, capable of welding raised parts. It can be used to attach brackets or support parts to flat workpieces.

Seam welding

A multi-spot welding process in which multiple weld joints are placed closely or overlapping to form a single monolithic joint. If there is overlap between the points, then a sealed seam is obtained; if the points are close together, the seam is not sealed. Since a seam using the distance between points does not differ from that created by a spot seam, such devices are rarely used.

In industry, the most popular is an overlapping, sealed seam, which is used to create tanks, barrels, cylinders and other containers.

Butt welding

Here the parts are connected by pressing them against each other, and then the entire contact plane is melted. The technology has its own varieties and is divided into several types based on the type of metal, its thickness and the required quality of the connection.

Welding current flows through the joint of the workpieces, melts them and reliably connects them

The simplest method is resistance welding, suitable for low-melting workpieces with a small contact patch area. Reflow and preheat fusion welding is suitable for stronger metals and larger cross-sections. This method is used to weld parts of ships, anchors, etc.

The most popular and used ones are described above, but there are also the following types of spot welding:

• seam-butt welding is carried out by a rotating electrode with several contacts to close the circuit; by pulling the workpiece through such a device, you can get a leaky continuous seam consisting of many weld points;
• the relief-point part is welded according to the current relief, however, the seam does not consist of a continuous contact patch, but of many points;
• according to the Ignatiev method in which the welding current flows along the parts being welded, so the pressure does not affect the heating of the product and its welding.

Resistance welding designation in the drawing

According to the existing standard of symbols, spot welding has the following symbol on the drawings:

1. Full seam. The visible continuous seam on the general plan of the drawing is marked with the main line, the remaining structural elements with the main thin line. The hidden continuous weld is indicated by a dashed line.
2. Weld points. Visible welded joints in the general drawing are marked with a “+” symbol, while hidden ones are not marked at all.

From a visible, hidden solid seam or visible weld point there is a special line with a leader on which auxiliary symbols, standards, alphanumeric signs, etc. are marked. The designation contains the letter “K” - contact and the small letter “t” - spot, indicating the method of welding and its type. Seams that do not have a designation are marked with lines without flanges.

GOST 15878-79 Regulates the dimensions and designs of resistance welded joints

All basic information is presented on the leader line or below it, depending on the facing side (front or back). All necessary information about the seam is taken from the corresponding GOST, which is indicated in the footnote or duplicated in the table of seams.

Using spot welding? First, the parts to be joined are aligned in the desired position, placed between the electrodes of the welding machine and pressed against each other. After this, they are heated to a state of plasticity and jointly undergo subsequent plastic deformation. In industrial conditions, when using automatic equipment, the welding frequency can reach up to 600 points per minute. In order to make high-quality spot welding with your own hands at home, it is necessary to maintain a constant speed of movement of both electrodes and ensure the required amount of pressure and full contact of the parts being joined.

Spot welding - diagram

The parts are heated due to the passage of welding current in the form of a short-term pulse lasting 0.01...0.1 seconds, depending on the welding conditions. This pulse ensures the melting of the metal in the area of ​​action of the electrodes and the formation of a common liquid core of both parts, the diameter of which can range from 4 to 12 mm. After the current pulse ceases, the parts are held under pressure for some time so that the molten core cools and crystallizes.

Heating duration

Monolithic connection

The duration of heating or passage of welding current can vary from thousandths to tens of seconds and depends on the welding conditions and the power of the machine. When welding parts made of steels prone to hardening and possible cracking (for example, carbon steels), it is recommended to increase the heating time to slow down the subsequent cooling of the metal. Welding of parts made of austenitic steels must be performed, on the contrary, with the shortest possible heating time. This is done to prevent the danger of heating the outer surface of the connection point to the temperature of structural transformations, which could entail a violation of the high anti-corrosion properties of the outer layers of the metal.

Pressure force

The pressure between the electrodes should ensure reliable contact of the parts at the junction. It depends on the type of metal being welded and the thickness of the parts being joined. The pressure after heating is important, since its appropriate value ensures a fine-grained structure of the metal at the welding site, and the strength of the connection point becomes equal to the strength of the base metal.

Electrodes, technical characteristics and features of use

• The quality of welding also depends on the correct choice of the diameter of the copper electrode. The diameter of the connection point must exceed the thickness of the thinnest element of the welded joint by 2 - 3 times.
• By pressing the parts at the moment of passage of the welding pulse, the formation of a special sealing belt near the molten core is ensured, preventing the splashing of molten material from the welding zone. As a result, no additional protection measures are required at the connection point.
• To improve the crystallization of the molten metal, the electrodes must be opened with a slight delay after the welding pulse has passed.
• To obtain a high-quality and reliable weld, the surfaces to be joined must first be prepared, in particular, cleaned of rust.
• The gap between the connection points should provide a reduction in current shunting through adjacent points. For example, for welding two (three) parts with a thickness of 1 to 8 mm each, the distance between the connection points varies accordingly from 15 (20) to 60 (100) mm.

Quality of materials

• Electrodes used for spot welding must provide strength in the operating temperature range, high thermal and electrical conductivity, and ease of machining. These requirements are met by special bronzes containing cobalt or cadmium, cold-rolled electrolytic copper and copper alloys containing chromium, as well as a tungsten-based alloy.
• In terms of electrical and thermal conductivity, copper is significantly superior to bronze and alloys, but 5–7 times worse than them in terms of wear resistance. Therefore, the best alloy for the manufacture of electrodes is considered to be an alloy of the EV type, which is almost pure copper with 0.7% addition of chromium and 0.4% zinc.
  In order to reduce wear of the electrodes during operation, it is recommended to use intensive cooling with water.

Application area

At home, do-it-yourself spot welding is most often used when repairing household appliances, various works with cables, or repairing small kitchen utensils.
In industry, spot welding is used for welding sheet blanks made of steels of various grades, non-ferrous metals and alloys of various thicknesses, intersecting rods, profile blanks (angles, channels, tees, etc.).

Advantages and disadvantages of spot welding

Like any technological process, electric spot welding has its advantages and disadvantages. The first include, first of all, the mechanical strength of spot welds and high efficiency, as well as the possibility of automating welding work. A significant disadvantage is the inability to ensure tightness.

Using a homemade welding machine

For welding work at home, you can make a spot welding machine assembled by yourself. can have a wide variety of configurations - from small portable ones to quite large ones. At home, desktop versions are usually used, which can be used for welding ferrous and non-ferrous metals.

Base of the device

The main structural part of one of these welding machines is the base transformer. To do this, it is best to use a mass-produced device, for example, OSM-1. The primary winding of the transformer can be left unchanged, but it must contain at least 200 turns. The secondary winding must be replaced with a more powerful one, using wire PEV 2/1.9 or PV Z - 50. Transformer OSM – 1

Adjusting the current value in the device is not necessary. During the welding process, it is necessary to navigate by the duration of heating and control it visually by color. To make electrode holders, you can use a duralumin rod with a diameter of 30 mm.

Electrode design

The lower electrode must be made motionless and isolated from the cheeks and mounting bolts with adhesive tape and washers. To secure the electrodes in the holders, you can use two bolts or brass washers.

Then you can take some springs, say from a folding bed. The holders with electrodes should be moved back to their original position by a spring. The spot welding machine is connected to the network using a circuit breaker rated for a current of at least 20 A.

Device control

The device itself can be controlled by a magnetic starter, which can be turned on by pressing a pedal. The transformer housing and its secondary winding must be grounded. The parts to be connected must be clamped between the electrodes. The current flowing between them heats up the metal, after which the electricity is turned off, the compression force of the electrodes increases and, as a result, a welded joint is formed.

DIY spot welding videos

1. Video about the use of the GYSPOT 3502 spot welding machine, designed for removing dents using an inertial hammer, welding rivets, studs, nails, dowels, washers and bolts, removing pits and upsetting surfaces using a carbon electrode:

2. Video about using the double-sided spot welding machine GYSPOT 32D-C for repairing surfaces and

3. Do-it-yourself capacitor welding with automatic supply of hardware:

Resistance spot welding has a number of advantages - the ability of seams to withstand significant mechanical loads, low cost of equipment, the ability to create an automated process, etc.

This type of welding machine is relatively easy to assemble, which is also its advantage and makes it possible to make the machine yourself. The only drawback of this type of welding is the inability to create a sealed weld.

How to make a transformer for spot welding

The main component of the welding machine is the transformer. Providing an increased welding current is achieved by a large transformation ratio. The transformer must have a power of at least 1 kW. Transformers from microwave ovens with sufficient power are excellent for this purpose.

It is easy to find such a transformer, and a welding machine of this type can be used for welding 1 mm steel sheets. To manufacture a device with greater power, several transformer installations can be used.

The transformer contains the primary winding and magnetic circuit, which you will need. The secondary winding should be cut using a hacksaw or any other tool. In this case, it is very important to prevent damage to the magnetic core and the primary winding. If the transformer has shunts to limit the current, they must be removed.

Having removed all unnecessary (in this case) elements, you should create a secondary winding (new). In order to provide a large current, the use of thick copper wire is required, the diameter of which should be at least 1 cm. Three turns will be quite enough, ensure that the output is approximately 2 V.

More powerful DIY welding can be achieved if you connect two (or more) transformers. The main thing is to take into account the capabilities of your network, otherwise, including spot welding, you will have to deal with various troubles when lights flash, fuses trip, etc.

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Assembling spot welding with your own hands and making electrodes

Electrodes are a very important element in welding, so they should be manufactured taking into account all recommendations. To make these elements you will need copper rods. It is better to choose rods of large diameter (at least as thick as the wire). If your plans include making a welding machine with low power, you can use tips that contain powerful soldering irons.

Depending on how often resistance spot welding is used, the shape of the electrodes will be lost over time. After some time of use, they can be sharpened and, if necessary, replaced with new ones.

It is desirable that the wire that goes from the electrodes to the transformer be of a minimum length and with a minimum number of connections. The fact is that at the junction the power is partially lost. Copper lugs should be placed on the ends of the wire and the wire and electrodes should be connected through them.

Each tip should be soldered to a wire. Such measures are necessary because during welding, copper contacts can gradually oxidize. This explains the significant loss of power and failure of a home-made welding machine. Soldering the wire and the tip is quite a difficult task, which is explained by the large diameter. For this purpose, you can use tinned solder tips, which can be purchased at any specialized store.

The reason for the additional resistance that spot welding can cause may be unsoldered connections of the tips to each of the electrodes. However, this drawback cannot be corrected, because the electrodes must be periodically removed for sharpening or for complete replacement. But here it is worth noting that these connections are quite easy to clean from oxide, unlike stranded wires that are crimped with a ferrule.

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Resistance spot welding and how to control it

Spot welding is controlled by a switch and lever. In order to ensure proper contact between the parts being welded, it is necessary to provide sufficient compression force between the electrodes.

If it is necessary to weld thick sheets of iron, it is necessary to use more powerful resistance spot welding (with a greater compression force between the electrodes). It is advisable that the lever is not short, but it should be strong enough. The welding machine must have a massive base; make sure in advance that it can be attached to the table.

For the high clamping force that a do-it-yourself resistance spot welding should have, both the lever described above and a lever-screw clamp in the form of a screw tie located between the base and the lever itself can be used. If desired, you can use other methods, but they may require special equipment.

To install the switch, you should select the primary winding, or rather, its circuit. The fact is that the secondary winding circuit has too much current, which can cause additional resistance and welding of contacts.

If you decide to use a lever clamping mechanism, it is better to choose a lever to secure the switch. In this case, during operation, turning on the current and operating the lever can be done with one hand. This way it will be as convenient as possible to hold the parts to be welded.

Spot welding machines are not used as often in everyday life as arc welding machines, but sometimes it is impossible to do without them. Considering that the cost of such equipment starts from $450-$470, the profitability of its purchase is questionable.

The way out of this situation is resistance spot welding with your own hands. But, before we tell you how to make such a device yourself, let's look at what spot welding is and the technology of its operation.

Briefly about spot welding

This type of welding is contact (thermomechanical). Note that this category also includes seam and butt welding, but it is not possible to implement them at home, since complex equipment will be needed for this purpose.

The welding process includes the following steps:

• the parts are combined in the required position;
• they are secured between the electrodes of the device, which press the parts;
• heating is performed, as a result of which, due to plastic deformation, the parts are firmly connected to each other.

A production spot welding machine (such as the one shown in the photo) is capable of performing up to 600 operations within a minute.

Process technology

To heat the parts to the required temperature, a short-term pulse of high-power electric current is applied to them. As a rule, the pulse lasts from 0.01 to 0.1 seconds (the time is selected based on the characteristics of the metal from which the parts are made).

When pulsed, the metal melts and a common liquid core forms between the parts; until it hardens, the welded surfaces must be held under pressure. Due to this, as it cools, the molten core crystallizes. A drawing illustrating the welding process is shown below.

Designations:

• A – electrodes;
• B – parts to be welded;
• C – welding core.

Pressure on the parts is necessary so that, when pulsed, a sealing belt is formed along the perimeter of the molten metal core, preventing the melt from flowing outside the zone where welding occurs.

To provide better conditions for crystallization of the melt, the pressure on the parts is gradually removed. If it is necessary to “forge” the welding site in order to eliminate inhomogeneities inside the seam, increase the pressure (do this at the final stage).

Please note that to ensure a reliable connection, as well as the quality of the seam, it is first necessary to treat the surfaces of the parts in the places where welding will take place. This is done to remove oxide film or corrosion.

When it is necessary to ensure reliable connection of parts with a thickness of 1 to 1.5 mm, capacitor welding is used. The principle of its operation is as follows:

• the capacitor block is charged with a small electric current;
• the capacitors are discharged through the parts being connected (the pulse strength is sufficient to ensure the required welding mode).

This type of welding is used in those areas of industry where it is necessary to connect miniature and subminiature components (radio engineering, electronics, etc.).

Speaking about spot welding technology, it should be noted that it can be used to connect dissimilar metals together.

Examples of homemade designs

There are many examples on the Internet of creating machines that produce spot welding. Here are some of the most successful designs. Below is a diagram of a simple spot welder.

For implementation we will need the following radio components:

• R - variable resistance with a nominal value of 100 Ohms;
• C – capacitor designed for a voltage of at least 25 V with a capacity of 1000 μF;
• VD1 – thyristor KU202, the letter index can be K, L, M or N, you can also use PTL-50, but in this case the capacitance “C” must be reduced to 1000 μF;
• VD2-VD5 – diodes D232A, foreign analog – S4M;
• VD6-VD9 – D226B diodes, they can be replaced with a foreign analogue 1N4007;
• F – 5 A fuse.

It is necessary to make a digression to tell how to make the TR1 transformer. It is made on the basis of Sh40 iron, with a set thickness of 70 mm. For the primary winding you will need PEV2 wire Ø0.8 mm. The number of turns in the winding is 300.

To make a secondary winding, you will need a Ø4 mm stranded copper wire. It can be replaced with a tire, provided that its cross-section is at least 20 mm 2. The number of turns of the secondary winding is 10.

Video: do-it-yourself resistance welding

As for TR2, any of the low-power transformers (from 5 to 10 W) will be suitable for it. In this case, winding II, used to connect the backlight lamp “H”, should have an output voltage within 5-6 V, and winding III – 15 V.

The power of the manufactured device will be relatively low, ranging from 300 to 500 A, maximum pulse time up to 0.1 sec (provided that the ratings “R” and “C” are the same as in the diagram shown). This is quite enough for welding steel wire Ø0.3 mm or sheet metal if its thickness does not exceed 0.2 mm.

Let us present a diagram of a more powerful device, in which the welding electric current of the pulse will be in the range from 1.5 kA to 2 kA.

We list the components used in the circuit:

• resistance ratings: R1-1.0 kOhm, R2-4.7 kOhm, R3-1.1 kOhm;
• capacitances in the circuit: C1-1.0 µF, C2-0.25 µF. Moreover, C1 must be designed for a voltage of at least 630 V;
• VD1-VD4 diodes - D226B diodes, replacement with a foreign analogue 1N4007 is allowed, instead of diodes you can install a diode bridge, for example, KTs405A;
• thyristor VD6 - KU202N, it must be placed on a radiator with an area of ​​at least 8 cm2;
• VD6 – D237B;
• F - 10 A fuse;
• K1 is any magnetic starter that has three pairs of working contacts, and the winding is designed for ~220 V, for example, you can install PME071 MVUHLZ AC3.

Now we’ll tell you how to make transformer TR1. The LATR-9 autotransformer, such as shown in the photograph, is taken as the basis.

The winding in this autotransformer has 266 turns, it is made of copper wire Ø1.0 mm, we will use it as the primary. We carefully disassemble the structure so as not to damage the winding. We dismantle the shaft and the movable roller contact attached to it.

Next, we need to isolate the contact track; for this purpose, we clean it from dust, degrease it and varnish it. When it dries further, we insulate the entire winding using varnished cloth.

As a secondary winding we use copper wire with a cross-sectional area of ​​at least 80 mm 2. It is important that the insulation of this wire is heat resistant. When all conditions are met, we make a winding of three turns.

Setting up the assembled device comes down to calibrating the scale of the variable resistor that regulates the pulse time.

We recommend that before starting welding, you experimentally establish the optimal time for the pulse. If the duration is excessive, the parts will be burned, and if it is less than necessary, the strength of the connection will be unreliable.

As already written above, the device is capable of delivering a welding electric current of up to 2000 A, which allows you to weld steel wire Ø3 mm or sheet steel, the thickness of which does not exceed 1.1 mm.