The main programmable parameters of the spot or roller welding process are the current, the compression force of the electrodes, the duration of their action and the geometry working surface electrodes. The process parameters, as is customary, will be considered given if they are specified for a single cycle of the formation of a separate weld point both in the case of spot and roller welding. Due to the fact that obtaining a welded joint with given strength properties, in most cases, is identical to obtaining a joint and given dimensions of the melting zone, the core diameter and penetration will be used as a process quality criterion. This makes it possible to exclude from consideration the design of the welded joint, metallurgical features of the formation of the joint, etc.

It is known that in roller and spot welding, a sufficiently large combination of current and force values ​​is possible, which satisfy the problem of forming a cast core with specified dimensions. This indicates that the process parameters ambiguously depend on the properties of the welded metal and its thickness. Their value and tolerance field depend on the welding mode and the equipment used. In some cases, it is the equipment that determines the welding mode. All other things being equal, such as the stability of the properties of the metal, the quality of its preparation, the identity of the electrodes, etc., the most stable results in welding many metals are guaranteed on machines operating using the energy stored in capacitors. If the welding modes characteristic of capacitor machines are used when welding on low-frequency machines, then the results will be unstable. The tolerance for the spread of the current value and the duration of its action, automatically set based on the welding mode on a capacitor machine, cannot be maintained when welding on a low-frequency machine. Therefore, in order to weaken the tightness of the connection with the core dimensions of those process parameters that cannot be precisely controlled in this situation, the welding mode is changed to meet the minimum quality requirements. In the above example, the instability of the current amplitude and the duration of its action is compensated by the fact that they switch to soft modes, i.e. reduce the amplitude of the current somewhat and increase the duration of its action. Such a change is not an improvement, despite the increase in the tolerance for the amplitude of the current and the duration of its action, since the requirements for other process parameters, such as the geometry of the working surface of the electrodes, become more stringent. In addition, the frequency of filling electrodes increases, and their resistance decreases.

Preferred, recommended modes reflect both the properties of the metals being welded and the possibilities for controlling the process, i.e. advantages and disadvantages of existing equipment. Due to the fact that the substantiation and selection of the welding mode is an independent task, the methods of solving which are quite fully considered in the literature, we will consider the welding modes to be given. Permissible deviations of the process parameter will be taken equal to those deviations that are allowed for resistance welding equipment.

There are many techniques for setting process parameters through cycle parameters, including individual time intervals between commands to the actuators of the welding machine. However, from the point of view of ensuring the technological cycle of welding a single point, independent stages can be distinguished, abstracting from the technical features of control devices.

The cyclogram shown in fig. 1 reflects the features of setting process parameters through cycle parameters. We can assume that each stage and, accordingly, each value that characterizes it, is an independent parameter, since it has an excellent purpose. Obviously, at certain stages of the cyclethe tolerances for current and force will be different. Timenecessary so that the electrodes of the machine have time to move and compress the metal with a well-defined force. At this stage, there are no strict requirements for devices that count the time interval. Similarly, in cases where pre-compression is applied, the interval during which the electrodes press the metal with increased force, can also be maintained with low accuracy. These requirements also apply to devices that set the metal compression time at the end of the current, as well as to the interval corresponding to the open state of the electrodes.. As a rule, the indicated cycle intervals are not controlled under production conditions. Having established the compression forces of the electrodes and have a significant impact on the quality of welded joints and therefore are subject to mandatory control, although their permissible deviations from the specified value for. . are different.

Rice. one . A typical sequence diagram of a spot welding process

Duration The increase in forging force is one of the main characteristics of the electrode compression force drive and can have a strong influence on the formation of macrodefects in the cast joint zone. Due to the inertia of the electrode compression mechanism, the main desire is to increase the rate of force increase. At the best models of carsis not more than 0.02 seconds, counting from the moment the command is given to the actuator until the moment whenreached level 2/3 ofestablished. An important parameter of the cycle is the interval, which determines the moment of switching on the forging forcein relation to the welding current pulse. Due to the fact that even a relatively small instability of these cycle parameters significantly affects the quality of the connection, they must be periodically monitored.

Of particular importance are the time intervals of the cycle, and characterizing the current change program, as well as the magnitude of the current and . However, the accuracy of setting the cycle parameters and , may be less than and .

As a result research work and production experience in spot and roller welding, it has been established that in most cases it is possible to accept the following required accuracy (in%) of the reproduction by the welding machine of the main stages of the cycle (see Fig. 1):

The given values ​​of permissible deviations of the parameters are valid for those cases when welding is carried out in modes that are estimated as preferable. All random deviations of the parameters must be within the tolerance zone. It is assumed that the distribution of the density of probable deviations is close to the normal distribution. Using control and measuring equipment and statistically processing measurement data, it is possible in each specific case, depending on the responsibility of this product, to set the number of permissible maximum deviations of parameters. Approximately, on average, the number of points at which any of the parameters takes once the maximum permissible value should not be too large, for example, 1 time per 100 ... 200 points. The small allowable standard deviation of process parameters is explained by the fact that the probability of marriage depends on the totality of deviations of all process parameters as a whole. In addition, welding equipment, as a rule, is universal and is designed so that it is possible to weld parts not only from one specific metal, but from a combination of metals, for each of which the requirements for the accuracy of setting at least one parameter were the highest. Usually, in real conditions, the specified limit deviations of the parameters do not lead to marriage.

For example, in fig. 2 shows private data characterizing the stability of the process of welding parts with a thickness of 1.5 + 1.5 mm from alloy D16. Limit deviations of process parameters that cause an unacceptable decrease in the quality of welding are outside the tolerance range indicated above. We assume that the spread of the parameters of the welding machine does not exceed the tolerance limit. Situations in which an unacceptable decrease in quality is possible occur only when two or more parameters simultaneously take on the maximum permissible values. The following adverse events are equally likely:decreased by 5%, increased by 10%; increased by 5%, increased by 10%; and increased by 5%; and decreased by 5%; increased by 10%, decreased by 5%;decreased by 10%,increased by 5%;decreased by 15%,increased by 5%;decreased by 5%, the radius of the electrodes increased from 75 to 200 mm;increased by 10%, and the radius of the electrodes increased from 75 to 200 mm. Let the probability that in these situations a marriage occur is 0.5, and the limiting deviations of the process parameters occur on average 1 time per 50 points. Then for every thousand points, on average, at least two points will not meet the accepted standard.

Suppose that for 200 points there is one deviation of each parameter that goes beyond the tolerance limits, and with a probability of 0.9 it can be argued that a marriage appears in this case. Then the probability of the appearance of marriage increases sharply and is approximately 3% of the total number of points.

Possible random deviations in the preparatory operations, for example, the quality of surface etching has deteriorated, poor fitting of parts, there is a difference in thickness of the metal, its physical properties have changed, contribute to an increase in the total number of marriage cases.

In the statistical analysis of the production of parts from the AMg6 alloy, a scatter of process parameters was observed, estimated by standard deviations:; , the working surface of the electrodes, resistance of parts after etching. The number of points that do not meet the accepted standard amounted to 5% of the total number of points. Obviously, very high accuracy requirements are imposed on measuring and control equipment, since the maximum permissible deviations of the parameter in some cases are less than 5%. Measuring equipment should provide several classes higher accuracy. Unfortunately, when developing even specialized equipment, it is not always possible to fully satisfy these requirements. Therefore, when considering instruments and devices, comments were made about the intended purpose and scope of individual devices, which have slightly worse accuracy indicators and do not satisfy the solution of the issue as a whole, but can be successfully used in solving particular problems.

Spot welding is a type contact welding. With this method, the heating of the metal to its melting point is carried out by the heat that is formed during the passage of a large electric current 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, as a result of which mutual penetration and fusion of the heated sections of the metal occur.

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

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

In addition to spot welding, there are other types of contact 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 liners, in particular, several million weld points are produced.

Deserved popularity

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

Sequence of processes in resistance spot welding

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

At the next cycle, everything repeats again.

Basic parameters of resistance spot welding

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

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

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

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

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

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

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

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

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

I 2 \u003d I CB + I w

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

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

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

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

When accurately calculating the duration of the welding pulse, many factors must be taken into account - the thickness of the parts and the size of the weld 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 the calculation.

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

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

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

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

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

Preparation of parts for welding

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

Spot welding equipment

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

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

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

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

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

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

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

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

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

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

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

W = C U 2 /2

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

Resistance spot welding defects

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

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

Spot welding defects are divided into three types:

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

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

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

Complete or partial lack of fusion, insufficient dimensions of the cast core. Possible reasons: Welding current is low, clamping force is too high, the working surface of the electrodes is worn out. Insufficiency of the 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 the distance between the weld points, leading to a large shunt current.

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

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

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

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

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

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

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

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

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

determined by external examination.

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

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

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

Defects are detected by X-ray or ultrasonic testing.

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

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

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

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ELECTRIC SPOT WELDING

The purpose of the work: to study the technological process of electrocontact spot welding; determine its differences; get acquainted with the device of the machine MT-1606; perform welding of samples in order to determine the optimal mode.

General scheme for the formation of a compound

The entire process of compound formation conditionally consists of separate physical processes, which, depending on the role in the formation of the compound, are divided into main and accompanying ones.

In spot welding, parts 1 are assembled with an overlap or with flanging, tightly clamped between the electrodes 2 of the welding machine, heated by a short-term (0.01 ... ...12 V), as a result of which a connection is created at separate contact areas, which are called points. The creation of the connection occurs according to the scheme, which consists of stages I-III.

First stage begins from the moment of compression of parts by force Fsv, which causes plastic deformation of the microrelief in the contacts electrode - part and part - part.

The next switching on of the current I and heating of the metal facilitate the processes of leveling the microrelief, the destruction of the film surfaces, and the formation of an electrical contact.

Thermal expansion during spot welding occurs under compression conditions and is accompanied by the appearance of an uneven distribution of internal stresses, which, together with constantly acting external forces Fb, cause irreversible volumetric plastic deformations (direction of maximum deformation 3).

Thermal expansion of the metal in the area of ​​contact part - part is the cause of the formation of a gap between the parts.

Before the metal melts, the decrease in σd and the excess of metal due to the dilatometric effect are compensated by a slight dilution of the electrodes, as well as by the displacement of parts of the metal into the gap, which provides a relief on the internal contact - a sealing belt 4, which limits the spreading of the welding current.

At the first stage, the accompanying processes do not get much development due to the relatively small deformation and low temperature of the welding zone.

Second phase characterized by the melting of the metal and the formation of the core 5. As the current passes, the core grows to the maximum size - in height h and diameter d . In this case, metal 6 is mixed, surface films are removed, and metal bonds are formed in the liquid phase. The core occurs in the zone where the highest current density is achieved and the heat exchange with the electrodes is less affected.

When melting in a closed volume, the volume of the core metal increases sharply, electromagnetic forces arise and, as a result, hydrostatic pressure arises, which is determined by the general balance of stresses in the welding zone. The dilatometric effect and the overall decrease in σd is compensated by the further separation of the electrodes and the displacement of the deformed metal into the gap. This contributes to the creation of not only a relief that limits the spreading of current, but also the sealing of the cast core, preventing splashing of the metal and its contact with the atmosphere.

The inner boundary of the metal of the girdle has a temperature close to the melting point and a low value of σd; accordingly, the temperature of the outer boundary is lower, and σd is larger. The metal of the girdle is in a volume-stressed state, while the stresses tend to increase the gap between the parts. This nature of the deformation of the near-contact area of ​​the parts causes "settlement" of the metal and the appearance of dents 8 (size c) on the surface from the electrodes.

With the appearance of a molten core, there is a risk of spattering, due to thermal conductivity, the seam zone heats up, the output structure of the metal changes, mass transfer is observed in the electrode-part contact (accompanying processes).

Third stage begins after the welding current is turned off - an intensive crystallization of the core (hя, dя) occurs, which ends the creation of an integral connection of parts at the point of contact. The dot metal has a dendritic structure.

During crystallization, heat transfer to the heat-affected zone continues and the structure of the metal in it changes, the metal shrinks, as a result of which shrinkage cavities and cavities are created in it; tensile stresses arise in the core, which cause cracks and under the influence of which the destruction of a fragile point is possible.

To reduce the level of residual stresses and prevent shrinkage cracks and cavities, significant efforts Fkov are needed. High quality welding and the maximum productivity of the process for a given thickness, shape and material of products are determined by the correctness of the chosen welding mode.

The quality of the joints also depends on the welding technique, the shape of the electrodes, the quality of the assembly and surface preparation, the welding equipment, the control system, and other design and technological factors.

Spot Welding Mode Options

The main parameters of the spot welding mode are the welding current Iw (amplitude or effective value), the duration or time of passage of the current tw, the force of compressing parts with electrodes Fw, the force and duration of forging Fpr, tpr, the diameter of the working surface of the electrode de or the radius of the spherical surface of the electrode Re.

The output data for determining the listed parameters are the physical and mechanical properties of the metal and the thickness of the parts to be welded.

The modes can be set by the calculation-experimental method or experimentally. Depending on the properties of the materials for spot welding, the so-called soft or hard modes are recommended. Soft modes - small welding current and long welding time; hard mode - high welding current, short welding time.

There are many recommendations about modes (in the form of tables, nomograms, graphs). These modes are indicative and need to be checked before welding and are often adjusted taking into account the conditions of surface preparation, assembly, equipment composition, etc.

The adjustment is carried out on witness samples using the dependence of the cast core parameters on the mode parameters. For example, if the diameter is insufficient, increase the welding current Iw.

In order to avoid splashes, increase Fpr, de, Re. If the core has cracks, Fpr is increased, bringing its increase in time closer to the moment the current is turned off, and crystallization is also slowed down by modulating the trailing edge of the current. Efforts are applied before the passage of the alloy through the TEC; tpr increase with an increase in thickness and a decrease in the thermal conductivity of the metals being welded (in hard modes and high speeds its crystallization is reduced).

The quality and, in particular, the strength of the welded joint depend on the dimensions of the cast core (hя, dя), as well as the state of the metal, the degree of reduction in its strength in the seam and the heat-affected zone, the type of load, and the level of defects.

The mode parameters have a different effect on the core diameter and, accordingly, on the strength. With an increase in Ib or tb, when other parameters are constant, the strength increases first rapidly, then more slowly, with the formation of a core. But at excessive Ib and tb, the size of the core begins to decrease due to the intensification of internal splashes and the appearance of various defects. With an increase in Fb and de, the strength also first increases due to an increase in the diameter of the core, and then begins to decrease due to a sharp increase in the contact area, a decrease in current density.

With a decrease in the thickness of the parts, the density of the welding current increases. Materials with low resistivity require more current than materials with high resistivity. With high thermal conductivity and thermal diffusivity of the metal, welding is carried out in harsh conditions, that is, the time of passage of the welding current is reduced and its strength is increased.

If parts of different thicknesses are welded, the operating parameters of the mode are selected according to the thinnest of them. Welding of parts with different thicknesses (thickness ratio >1:3) is difficult (Fig.a) due to the lack of reliable penetration of a thinner part (s1

To avoid this, hard welding conditions are recommended, or electrodes with a smaller cross section should be used on the side of a thin part, or these electrodes are made of metal with a lower thermal conductivity than on the side of a thick part.

When welding parts from different materials, due to uneven heat release, the core diameter and penetration depth increase in parts with higher resistivity and lower thermal conductivity (part 2).

When welding parts using electrodes of various sizes and shapes of contact surfaces, the core is shifted to an electrode with a smaller contact surface (electrode 2), where the current density is higher.

The surface condition (contact resistance) of the parts significantly affects the distribution of heat during welding and, as a result, the size and strength of the points.

To ensure the stability of the contact resistance, parts are usually cleaned (etched or machined) before welding or coated with a thin film of oxides with a small and constant resistance value.

Typical technological process the production of welding units and the manufacture of spot welding consists of the following operations: the manufacture of blank parts, the preparation of their surfaces for welding, assembly, tacking, welding, correction, machining and corrosion protection.

For spot welding, different types of machines are used: alternating current, low-frequency, direct current, capacitor. Machine power - from 5 to 1000 kW.

AC machines are the most common in all areas of engineering, they are simpler and cheaper than other machines.

The structure of the machine MT-1606

AC machine MT-1606 is designed for spot welding of structural and high-alloy steels, titanium alloys with a thickness of 0.8 to 6.5 mm. It is also possible to weld some non-ferrous copper alloys (brass, bronze, etc.) up to 1.2 mm thick. The maximum power of the machine is 95 kW, the rated welding current is 16 kA, the maximum number of points per minute is 200.

Pneumatic system provides compression and retention of welded parts 1 in a compressed state during the entire welding cycle.

The air from the network through the air filter 13, the pressure regulator 12, the oil sprayer 11 and the electromagnetic pneumatic valve 10, depending on the position of the valve spool, passes through the throttle (10-6,10-4), which regulate the air supply rate in the cylinder cavity:
- into the lower cavity of the cylinder 4, lifting the lower piston until it stops in the upper piston 7;
- into the middle cavity 6 (through the upper hose and the upper piston rod), lowering the lower piston and compressing the parts.

The working air pressure is set using the regulator 12, controlled by the pressure gauge.

The upper piston is used to adjust the stroke of the lower. Adjustment of the stroke is carried out using the adjusting nut 9 on the rod of the upper piston. To set the working stroke of the upper electrode, it is necessary to supply air to the pneumatic cylinder (above the upper electrode) by opening the control valve 14. The upper piston will descend to the stop in the upper cylinder cover of the adjusting nut.

The valve for controlling the position of the upper piston 5 serves to supply and discharge air from the upper cavity of the cylinder. When air is released, the upper piston rises up to the stop in the cylinder cover and the electrodes will disperse to the maximum distance.

The upper electrode holder 2 is connected to the lower piston through the rod, on which the upper electrode 2 is fixed. The lower electrode holder and the electrode are stationary.

The oil sprayer 11 lubricates the moving parts. The oil from the oil sprayer is captured by the passing air and lubricates the valve, air cylinder and pistons.

Electrical diagram of the machine. The MT-1606 power source is a TR transformer, which consists of an armored magnetic circuit, primary and secondary windings. The secondary winding has one turn of a thick copper bus. By changing the number of sections of the primary coils connected to the electrical network with the switch of the steps of the PS, the power of the machine is stepwise regulated.

The AB circuit breaker turns off the machine if there is a short circuit in the machine's network or it overheats.

The thyristor switch KT has two thyristors, which are connected in anti-parallel, which makes it possible to pass alternating current to the primary winding of the transformer. Thyristors open when control pulses are applied to their control electrodes from the welding cycle controller.

On machines of this type, it is possible to smoothly adjust the power of the machine due to the synchronous phase shift of the control pulses relative to the AC half-cycle waves.

The RC cycle regulator provides automatic control machine. It is an electronic relay device that turns on and off the electromagnetic pneumatic valve and the thyristor contactor in a certain sequence, due to which, at the right time, the parts are compressed, the current is turned on and off, and the upper electrode is lifted.

In the MT-1606 machine, the electrode holders, electrodes and thyristor contactor are cooled by running water. The water supplied to cool the thyristors passes through a hydraulic valve. If the water supply stops, the hydraulic valve opens the control circuit of the thyristor and the welding current does not turn on.

How the machine works

The general cycle of welding of one point tc consists of compression of parts tco, welding tw, forging tpr and pause tp.

Compression of parts occurs when you press the pedal button of the gearbox. Compressed air is supplied through an electromagnetic pneumatic valve into the middle cavity of the cylinder, lowering down the lower piston connected to the upper electrode holder and the electrode.

After stabilization of the compression force (a given period of time tszh), the cycle regulator sends a signal to the control electrodes of the thyristors, the welding current is turned on, the circuit is closed through a column of metal sandwiched between the electrodes. At the end of t, the current is turned off.

After that, in order to crystallize the molten metal of the weld spot (in order to reduce welding stresses and deformations), the parts are left under pressure for some time (forging).

At the end of forging, the cycle regulator opens the power supply circuit of the electromagnetic pneumatic valve, the spool changes its position and air is supplied to the lower cavity of the cylinder. The lower piston rises, releasing the welded parts. The electrodes during the pause required to replace parts will be separated, and then the welding cycle is repeated.

To perform single spot welding, you need to: set the type of work switch to the "Single cycle" position, press and release the pedal once.

For execution a large number points can be operated in the " Automatic operation". The control pedal must be kept depressed all the time.

Preparation for work

1. Supply air to the machine, for which turn on the compressor, raise the pressure in the receiver to 5 atm and open the inlet valve of the machine.
2. Set the machine to the required welding mode:
   1. stroke of the upper electrode - is selected depending on the configuration of the units and parts to be welded, and is set using a nut screwed onto the rod of the upper piston (when setting the stroke, use the control valve, which after adjustment must be set to the right position);
   2. the compression force of the parts - is selected depending on the thickness and type of the material being welded, is adjusted by the air regulator screw and controlled by a pressure gauge. It must be such as to ensure good contact between parts and electrodes (the dependence of the compression force on the electrodes on the pressure on the pressure gauge is given in the table on the machine);
   3. power level (determines the amount of current) - is selected depending on the thickness and type of material being welded. It is set using three knife switches, which are located inside the machine - on the right (the dependence of the power level on the position of the switches is indicated in the machine table);
   4. times of compression, welding, forging, pause - are set using the switches of the cycle regulator located at the bottom of the machine. The time of each operation is regulated within 1-198 periods, that is, within 0.02-3.96 s, after 0.02 s (an alternating current period with a frequency of 50 Hz), units of periods - tens are set on the switches located on the left.
      The power level and compression force are selected depending on the thickness and type of material to be welded.
3. Turn on the mains switch and circuit breaker.
4. To test the operation of the machine without welding current, to do this, turn off the "Welding current" toggle switch, press the control pedal and, after a correctly completed welding cycle, turn on the toggle switch.

Method of work

1. Familiarize yourself with the essence of resistance spot welding.
2. Set the features of the formation of the core of the welding point.
3. Set the influence of the mode parameters on the parameters of the welded joint.
4. Familiarize yourself with the structure of the MT-1606 machine.
5. Carry out training welding according to the "Procedure for the operation of the machine".
6. Set the welding mode (as instructed by the teacher), weld the samples, check the strength of the welding joints.
7. Compile a report and analyze the results.

Table 1 - Record of the welding mode and testing of samples

Equipment and materials

1. Post for contact welding.
2. Machine for resistance spot welding MT-1606.
3. Breaking machine.
4. Welding consumables: sheet samples of carbon and low-alloy steel with a thickness of 0.5 ... 1.2 mm.

1. Scheme of resistance spot welding.
2. Features of the formation of the core of the point, mode parameters and their influence on the parameters of the welding joint.
3. Schematic diagram of the machine MT-1606. Technical data, specification of the main units.
4. Research results (Table 1).
5. Dependence graph F = f(tw).
6. Analysis of the obtained results. Conclusions (substantiation of the optimal welding mode).

test questions

1. Where is heat generated in spot welding?
2. Describe the welding cycle of one point, its characteristic dimensions?
3. What are the main parameters of the spot welding mode?
4. How do the mode parameters affect the quality of the connection?
5. How to avoid splashing metal without reducing the strength of the point?
6. How to change the parameters of the welding mode, if the thickness of the parts to be welded: -increased, -decreased?
7. What is forging needed for?
8. Tell us the purpose of the nodes of the electrical circuit, pneumatic circuit?
9. How to setup point machine to the maximum welding current (do it practically)?

Contact welding modes are a set of parameters that are set by the welder before starting work. The parameters of these welding modes depend on the metal product that is planned to be welded, the experience of the welder, and other things. The selected welding modes directly affect the quality of the resulting joint: incorrectly selected parameters can lead to a poor-quality seam, which can subsequently crack.

The main parameters for resistance welding will be:

• The strength of the electric current.
• Reinforcement of compression for welded parts.
• duration of current flow.

We will talk about different welding modes, and specifically the contact method of welding, further.

Welding modes and their influence on the weldability of metals.

Welding modes are divided into two main types:

• soft;
• tough.

Both types differ in the duration of the current exposure to the welded part. The hard mode of welding metal products involves a short exposure to current on the parts, while soft welding modes, on the contrary, involve a long exposure.

The choice of one or another type depends, first of all, on the metal that needs to be welded: its thickness, thermal conductivity indicators, etc. matter. Thus, hard welding modes are usually used for metals that have a greater thickness, but at the same time lower thermal conductivity. For example, the welding mode for mild steel will be much harder than for aluminum alloys.

The shape of the melting of the metal and the location of the melt zone largely depends on the processes of heat generation and heat removal that occur in the electrode and the workpiece itself. The duration of current exposure affects heat generation and heat removal, and, accordingly, the welded joint itself.

When welding in soft mode, the shape and location of the cast zone will depend directly on the electrode and the materials being welded. So, in the soft welding mode, the cast core is at the same distance from the surfaces of the part, this contributes to the fact that the irregularities formed during the welding process are shifted into the part with a large thickness.

Note that under mild welding conditions (in which the heating time of the metal product is much longer), the heat-affected zone will also be wider than with hard welding.

With hard welding, this core will be quite symmetrical with respect to both parts to be welded. During welding, it must be taken into account that the heat removal to the electrodes during hard welding is minimal, which is what makes it possible to obtain a large height of the cast zone in this welding mode (in other words, hard welding modes for parts having the same thickness give a greater penetration depth).

The quality of the obtained welded joints, made under different welding conditions, is evaluated by the following parameters:

• The seam should not have significant softening in the zone of metal joining.
• The formation of rather fragile structures in the joint zone, which can subsequently collapse, is unacceptable. This is especially true for the transition zone of the seam.
• The connection zone must be homogeneous and dense, the cast and transition zone must not have visible violations of their complexity.
• The connection must be strong enough.
• Welding work should not reduce the corrosion resistance of the metal product.
• Deformations of parts are allowed within the normal range.

Note that when performing contact welding, compliance with these conditions depends on the capabilities of your equipment for welding, the actual product that will be welded, and the experience of the welder.

Keep in mind that metals that have good weldability allow welders to use a variety of parameters to set the welding mode, and this, in turn, allows you to get better joints.

Methods of resistance welding and the formation of joints.

All methods and modes of resistance welding are based on heating parts with the help of heat, which is released when an electric current flows through them. The amount of heat released mainly depends on the strength of the current, the time of its flow through the metal, and also on the resistance of the metal itself in the welding zone.

If two or more parts are welded, compressed together, then an electric current is supplied to them through conventional electrodes. In this case, the voltage can be small, from 3 V, but the current strength can reach tens of thousands of amperes. The heat that is necessary for welding is released mainly in the parts, in the zone of contact of the parts with each other and their contact with the electrodes. At the same time, the electrical resistance of metals is of great importance in resistance welding modes.

Thus, we conclude that the choice of welding mode depends directly on the properties of the selected materials. Resistance welding modes depend on the thermal conductivity and thickness of the parts.

Note that under severe conditions, the amount of heat released is many times greater, so they are used only for metals with low thermal conductivity, for example, for steel.

Spot welding is a method in which parts are overlapped at one or more points. When an electric current is applied, local heating occurs, as a result of which the metal is melted and seized. Unlike electric arc or gas welding no filler material is required: it is not the electrodes that melt, but the parts themselves. Enveloping with an inert gas is also not necessary: ​​the weld pool is sufficiently localized and protected from the ingress of atmospheric oxygen. The welder works without a mask and gloves. This allows better visualization and control of the process. Spot welding provides high productivity (up to 600 dots/min) at low cost. It is widely used in various sectors of the economy: from instrument making to aircraft construction, as well as for domestic purposes. No car repair shop can do without spot welding.

Spot welding equipment

The work is carried out on a special welding machine, called a spotter (from the English. Spot - a point). Spotters are stationary (for work in workshops) and portable. The unit operates from a 380 or 220 V power supply and generates current charges of several thousand amperes, which is much more than that of inverters and semi-automatic devices. The current is applied to a copper or carbon electrode, which is pressed against the surfaces to be welded by pneumatics or a hand lever. There is a thermal effect lasting a few milliseconds. However, this is enough for reliable docking of surfaces. Since the exposure time is minimal, the heat does not spread further through the metal, and the weld point cools quickly. Details from ordinary steels, galvanized iron, stainless steel, copper, aluminum are subject to welding. The thickness of the surfaces can be different: from the thinnest parts for instrumentation to sheets with a thickness of 20 mm.

Contact 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 secure hold and is more commonly used for thick-walled workpieces.

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

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

The choice of equipment depends on the features technological process. The most common devices are alternating current.

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Spot welding electrodes

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

The high intensity of the working process necessitates the use of a material that is resistant to mechanical and chemical influences. Most of all, the requirements are met by copper with the addition of chromium and zinc (0.7 and 0.4%, respectively).

The quality of the weld point is largely determined by the diameter of the electrode. It should be at least 2 times the thickness of the parts to be joined. The dimensions of the rods are regulated by GOST and are from 10 to 40 mm in diameter. Recommended electrode sizes are shown in the table. (Picture 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.

Spherical tip electrodes are more durable: able to produce more points before resharpening.

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 with curved electrodes. A welder who is faced with such working conditions always has a set of different figured electrodes.

To ensure reliable current transfer and 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 are threaded or 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 nucleus are affected by the duration of the pulse. However, when a certain moment is reached, a state of equilibrium sets in, 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 the current supply beyond this is impractical.

The compression force affects the plastic deformation of the welded surfaces, the redistribution of heat over them, and the crystallization of the core. A high clamping force reduces the resistance of the electric current flowing from the electrode to the parts to be welded and vice versa. Thus, the current strength increases, the melting process accelerates. A connection made with a high compressive force is characterized by high strength. At high current loads, compression prevents splashes of molten metal. In order to relieve stress and increase the density of the core, in some cases, an additional short-term increase in the compression force is performed after the current is turned off.

Distinguish between soft and hard. In soft mode, the current strength is less (current density is 70-160 A / mm²), and the pulse duration can be up to several seconds. Such welding is used to connect low-carbon steels and is more common at home, when work is carried out on low-power devices. In hard mode, the duration of a powerful pulse (160-300 A / mm²) is from 0.08 to 0.5 seconds. Details provide the maximum possible compression. Rapid heating and rapid cooling allow the welded core to maintain anti-corrosion resistance. Hard mode is used when working with copper, aluminum, high-alloy steels.

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

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Possible defects and their causes

A well-made point provides a reliable connection, the service life of which, as a rule, exceeds the service life of the product itself. However, a violation of technology can lead to defects that 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, shells;
• violation of the mechanical and anti-corrosion properties of the metal in the area adjacent to the weld point.

Consider specific types of defects and their causes:

1. Lack of penetration can be caused by insufficient current strength, excessive compression, wear of the electrode.
2. External cracks occur with too much current, insufficient compression, dirty surfaces.
3. Breaks at the edges are due to the close location of the core to them.
4. Electrode dents occur when electrodes are too small, improperly installed, over-compressed, too high current, and too long.
5. 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 cavity in the core, and misaligned electrodes.
6. An external splash of molten metal onto the surface of parts can be caused by insufficient compression, too high current and time modes, contamination of surfaces, and skewed electrodes. The last two factors are Negative influence on the uniformity of current distribution and the melting of the metal.
7. Internal cracks and cavities occur due to excessive current and time regimes, insufficient or delayed forging compression, and contamination of surfaces. Shrinkage cavities appear at the moment of core cooling. To prevent them, forging compression is used after the current supply is stopped.
8. The reason for the irregular shape of the core or its displacement is the skew or misalignment of the electrodes, the contamination of the surface of the parts.
9. Burn-through is the result of contaminated surfaces or insufficient compression. To avoid this defect, the current must be applied only after the compression is fully secured.