Sharpening tungsten electrodes. Machines and tools for sharpening electrodes for resistance spot welding Sinterleghe
Electrodes intended for contact welding are made from metal rods, the diameter of which ranges from 12 to 40 mm. Their working surface is either flat or spherical. To connect the workpieces together into a rather complex structure, they use electrodes that have an offset surface - the so-called shoe products. Such products are secured using a special shank having a cone of 1:10 or 1:5.
You can also find electrodes on sale that have a cylindrical surface, thanks to which they will be fixed to work in special structures with a conical thread. In addition to them, products are produced with a replaceable working part - it is installed on the cone using a standard union nut or simply pressed.
Electrodes for resistance welding of relief type in their shape will directly depend on the method of connection and the final shape of the product. In most cases, the size of the working surface of a given electrode does not play a special role. This is due to the fact that the contact area and the selected welding current directly depend on what shape the workpieces will have at the points of contact.
There are also electrodes for connecting elements with very complex topography. Suture equipment uses products that are a disk with a flat working surface. Moreover, these products may even have asymmetrical bevels. Such discs are fixed to the equipment by veneering or pressing.
Inside the electrodes themselves there are certain cavities through which coolant will circulate during the welding process. Electrodes for resistance spot welding are solid, so in this case the so-called external cooling is used.
To ensure that the electrode material is consumed to a minimum, the roller is made replaceable. The electrode itself is made from a special alloy based on a metal such as copper. The result is a product that has virtually no resistance to electric current, is an excellent heat conductor, and is resistant to even fairly high temperatures. In addition, when hot, this electrode will retain its original hardness, and interaction with the workpiece metal will be minimal.
Types of resistance welding equipment
The main feature of this technology is the connection of workpieces over the entire area. Optimal heating is achieved through reflow using a welding machine. However, in some cases they resort to heating due to the resistance of the part to the passage of electric current.
Resistance spot welding can occur either with metal melting or without this technological feature of the process. Resistance welding can be used to connect metal elements whose cross-section is in the range from 1 to 19 mm, and in most cases resistance welding is used, since the consumption of electrode material will be significantly lower, and the final connection is much more durable. This welding is used when performing fairly precise work, for example, in the process of producing rails to create a railway track.
Features of resistance spot welding
This technology is perfect for connecting metal elements together, and the connection is carried out both at one and at several points on these workpieces. It is extremely popular not only in industry (in particular, it is often used in agriculture, in the construction of aircraft, automobile transport, and so on), but also in everyday life.
The principle of operation of this method is quite simple: electric current, when passing through parts that are in direct contact with each other, very much heats up their edges. The heating is so strong that the metal begins to quickly melt, and the workpieces are immediately compressed with considerable force. As a result of this, a welded joint is formed.
Equipment designed to use this technology is designed to connect sheets, rods and other metal products together. The key advantages of this method are the following:
- Absence of a welded joint in the traditional sense;
- There is no need to use filler material, gas or flux;
- The equipment is very easy to use;
- The speed of work is quite high.
The main and only disadvantage of this method is that the seam is completely unsealed.
What are electrodes for resistance welding made of?
The material from which the electrodes will be made is selected depending on the requirements for the operating conditions of the product. It is worth noting that the electrodes must perfectly withstand compression, temperature changes, exposure to high temperatures, and stresses that will form inside the electrode itself, which is under serious load.
In order for the products to be of the highest quality, it is necessary that the electrode retains the original shape of its working surface, which will be in direct contact with the parts being connected. Melting of this consumable material accelerates its wear.
Usually copper is taken as the main element, and other elements are added to it - magnesium, cadmium, silver, boron, and so on. The result is a material that excellently resists even very severe physical stress. Electrodes with tungsten or molybdenum coating practically do not wear out during operation, which is why they have recently gained the greatest popularity. However, they cannot be used for welding products made of aluminum and other materials with a soft structure.
This table clearly shows the importance of electrode maintenance. This is important not only to maintain the quality of the welded joint, which is of paramount importance, but also to reduce unnecessary stress on the welding equipment. After studying the tabular data, you will be able to draw your own conclusions.
TIP PROFILE
WELDING SPOT
REQUIRED CURRENT, A
RESULT
CORRECT MAINTENANCE OF ELECTRODES FOR RESISTANCE SPOT AND RELIEF WELDING
Electrodes for projection welding
To ensure the precise alignment required for good contact and quality welds, projection welding electrodes should be positioned directly on the center line of pressure application. In addition to producing poor-quality welds, poor alignment of the electrodes can lead to damage to their surfaces [Fig. 1].
Another serious cause of poor welding is non-parallelism of the electrode surfaces. It entails uneven pressure on the electrodes, which leads to molten metal splashing out of the weld area during the welding cycle. If welding goes through the supporting part of the electrode, the reliefs are damaged and the insulation may burn out. In addition, non-parallelism leads to the biting of the electrode tips by their supporting parts during welding, resulting in a burn on the workpiece at the point of contact with displaced reliefs, and a possible shift relative to the mating parts of the welding equipment [Fig. 2].
SHOULD
... keep a supply of electrodes on the machine to minimize downtime due to electrode replacement,
... sharpen electrodes on a lathe,
...use special grade 3 copper for electrode tips.
DO NOT
... file down the electrodes (an uneven surface will lead to either partial welding or metal splashing out of the welding zone),
Electrodes for spot welding
In resistance spot welding, the thermal concentration depends on the size and shape of the electrode tips. Welding is carried out over the entire area under the tip of the electrode through which current passes. Small diameter spot welding electrode tips erode or wear away much more quickly than their projection welding counterparts and therefore must be regularly sharpened to maintain proper contact [Fig. 3].
SHOULD
... keep a supply of electrodes on the machine,
... periodically sharpen the electrodes on a specialized machine,
... change the diameter of the tips when working with different thicknesses of the metal being welded.
DO NOT
... file the electrodes (an uneven surface will lead to lack of penetration),
... store electrodes in places where damage to their surfaces is possible,
... use an adjustable wrench to remove the electrodes.
1. To ensure perfect alignment, the surfaces and axes of the electrodes must be parallel. This can be checked by inserting a piece of carbon and a piece of clean white paper between the electrodes and running the electrodes in test mode. The resulting print on paper will show the size and uniformity of the contact plane between the two surfaces.
2. Use a water jacket if necessary and place it as close to the welding surface as possible.
3. Keep the material being welded clean: free of oil, film, dirt and other foreign matter.
4. Follow the prescribed welding procedure.
WELDING ELECTRODES AND HOLDERS
RECOMMENDED |
PROHIBITED |
| 1. Use electrodes made of material suitable for your application. 2. Use standard electrodes wherever possible. 3. Use tips of the optimal diameter for the given thickness of the materials being welded. 4. Use transparent hoses to constantly monitor the flow of water through the electrodes. 5. Connect the water supply hose to the corresponding inlet on the holder so that water flows into the central cooling pipe first. 6. Cool the electrodes with water flowing at a rate of at least 7 liters per minute through each tip. 7. Make sure that the inner cooling tube of the holder is inserted into the water hole on the tip to a depth of 6mm. 8. Adjust the inner tube of the holder cooling system in height when changing to a different length of tip. 9. Make sure the top end of the holder cooling tube is cut at an angle that will not cause the tip to jam and cut off the water supply. 10. Apply a thin layer of special lubricant to the tip rod before inserting it into the holder to make it easier to pull it out. 11. Use ejector type holders for easy removal of tips and to avoid damage to tip rods. 12. Keep the tip and holder clean, smooth and free of foreign substances. 13. Grind spot welding electrodes frequently to maintain weld quality. 14. Grind the electrodes on a lathe to the original shape whenever possible. 15. Use a piece of leather or a rubber mallet when leveling the holder or tip. 16. Apply coolant to both sides of the disc when seam welding. 17. Use specially designed knurling discs to maintain proper shape of the seam welding disc electrode. |
1. Never use unknown electrodes or electrode materials. 2. Avoid specialty, offset or custom tips when the job can be done with a standard straight tip. 3. Do not use small tips for welding work with heavy large workpieces and vice versa. 4. Be sure to turn on the cooling water supply to full power before starting welding. 5. Never use a hose that does not fit tightly onto the water nipple on the holder. 6. Avoid leaking, clogging or damaging water equipment. 7. Avoid using holders with leaking or deformed tubing. 8. Never use electrode holders that do not have adjustable internal cooling tubes. 9. Do not allow the tube to become clogged due to the accumulation of impurities. A few drops of oil at reasonable intervals will help keep the tube working. 10. Do not allow electrodes to remain idle in holders for long periods of time. 11. Do not use adjustable wrenches or similar tools to remove electrodes. 12. Avoid using white lead or similar compounds to seal leaking adapters. 13. Never allow the tip of a spot welding electrode to become so flat that pointing becomes difficult. 14. Never use rough discs to sharpen electrodes. 15. Do not hit the holder or tip with a steel hammer when leveling the tool. 16. Avoid using seam welding discs that are too thin for the given thermal or physical load. 17. Do not allow the welding discs to extend beyond the workpieces being welded. |
The design of the electrodes must have a shape and dimensions that provide access to the working part of the electrode to the place where parts are welded, be adapted for convenient and reliable installation on the machine, and have high durability of the working surface.
The simplest to manufacture and operate are straight electrodes, made in accordance with GOST 14111-69 from various copper electrode alloys, depending on the grade of metal of the parts being welded.
Sometimes, for example, when welding dissimilar metals or parts with a large difference in thickness, in order to obtain high-quality connections, the electrodes must have a fairly low electrical thermal conductivity (30...40% of copper). If the entire electrode is made from such metal, it will heat up intensely from the welding current due to its high electrical resistance. In such cases, the base of the electrode is made of a copper alloy, and the working part is made of metal with the properties necessary for the normal formation of connections. Working part 3 can be replaceable (Fig. 1, a) and secured with a nut 2 on base 1. The use of electrodes of this design is convenient, as it allows you to install the desired working part when changing the thickness and grade of the metal of the parts being welded. The disadvantages of an electrode with a replaceable part are the possibility of using it only when welding parts with good approaches and insufficiently intensive cooling. Therefore, such electrodes should not be used in heavy welding conditions at high speeds.
Rice. 1 . Electrodes with a working part made of another metal
The working part of the electrodes is also made in the form of a soldered (Fig. 1, b) or pressed-in tip (Fig. 1, c). The tips are made of tungsten, molybdenum or their compositions with copper. When pressing a tungsten tip, it is necessary to grind its cylindrical surface in order to ensure reliable contact with the base of the electrode. When welding parts made of stainless steel with a thickness of 0.8...1.5 mm, the diameter of the tungsten insert 3 (Fig. 1, c) is 4...7 mm, the depth of the pressed part is 10...12 mm, and the protruding part is 1.5...2 mm. With a larger length of the protruding part, overheating and a decrease in the durability of the electrode are observed. The working surface of the insert can be flat or spherical.
When designing electrodes, special attention should be paid to the shape and dimensions of the seating part. The most common is a conical landing part, the length of which should be at least. Electrodes with a shortened cone should only be used when welding using low forces and currents. In addition to the conical fit, electrodes are sometimes fastened to threads using a union nut. This connection of electrodes can be recommended in. multi-point machines, when it is important to have the same initial distance between the electrodes, or in clamps. When using shaped electrode holders, electrodes with a cylindrical seat are also used (see Fig. 8, d).
When spot welding parts with complex contours and poor approaches to the joint, a wide variety of shaped electrodes are used, which have a more complex design than straight ones, are less convenient to use and, as a rule, have reduced durability. Therefore, it is advisable to use shaped electrodes when welding is generally impossible without them. The dimensions and shape of the shaped electrodes depend on the size and configuration of the parts, as well as the design of the electrode holders and consoles of the welding machine (Fig. 2).
Rice. 2. Various types of shaped electrodes
During operation, shaped electrodes usually experience a significant bending moment from off-axis application of force, which must be taken into account when selecting or designing electrodes. The bending moment and the usually small cross-section of the cantilever part create significant elastic deformations. In this regard, mutual displacement of the working surfaces of the electrodes is inevitable, especially if one electrode is straight and the other is shaped. Therefore, for shaped electrodes, the spherical shape of the working surface is preferable. In the case of shaped electrodes that experience large bending moments, deformation of the conical seating part and the electrode holder socket is possible. The maximum permissible bending moments for shaped electrodes made of Br.NBT bronze and electrode holders made of heat-treated bronze Br.Kh are, according to experimental data, for electrode cones with a diameter of 16, 20, 25 mm, respectively, 750, 1500 and 3200 kg× cm. If the conical part of the shaped electrode experiences a moment greater than permissible, then the maximum diameter of the cone should be increased.
When designing complex spatial shaped electrodes, it is recommended to first make a model of them from plasticine, wood or easily machined metal. This allows you to establish the most rational dimensions and shape of the shaped electrode and avoid alterations when manufacturing it directly from metal.
In Fig. 3 shows some examples of welding assemblies in places with limited access. Welding of the profile with the shell is performed using a lower electrode with an offset working surface (Fig. 3, a).
Rice. 3. Examples of using shaped electrodes
An example of using an upper electrode with oblique sharpening and a lower, shaped one is shown in Fig. 3, b. The angle of deviation of the electrode holder from the vertical axis should not be more than 30°, otherwise the conical hole of the electrode holder will be deformed. If it is impossible to install the upper electrode with a slope, then it can also be shaped. The shaped electrode is bent in two planes to reach a hard-to-reach welding spot (Fig. 3, c-d). If the machine does not have or has limited horizontal movement of the consoles for welding the parts shown in Fig. 3, e, two shaped electrodes with equal projections are used.
Sometimes shaped electrodes perceive very large bending moments. To avoid deformation of the conical seating part, the shaped electrode is additionally secured to the outer surface of the electrode holder using a clamp and a screw (Fig. 4, a). The strength of shaped electrodes with a long reach increases significantly if they are made of composite (reinforced) electrodes. For this, the main part of the electrode is made of steel, and the current-carrying part is made of a copper alloy (Fig. 4, b). The connection of current-carrying parts to each other can be made using soldering, and with a steel console - with screws. A design option is possible when a shaped electrode made of a copper alloy is supported (reinforced) with steel elements (bars), which should not form a closed ring around the electrode, since currents will be induced in it, increasing the heating of the electrode. It is advisable to fasten shaped electrodes that experience large moments in the form of an elongated cylindrical part for installation in a machine instead of an electrode holder (see Fig. 4, b).
Rice. 4. Electrodes that perceive a large bending moment:
a - with reinforcement for the outer surface of the electrode holder;
b - reinforced electrode: 1 - steel console; 2 - electrode; 3 - current supply
In most cases, spot welding uses internal cooling of the electrodes. However, if welding is performed with electrodes of small cross-section or with high heating, and the material being welded is not subject to corrosion, external cooling is used in the tongs. The supply of cooling water is carried out either by special tubes or through holes in the working part of the electrode itself. Great difficulties arise when cooling shaped electrodes, since it is not always possible to supply water directly to the working part due to the small cross-section of the cantilever part of the electrode. Sometimes cooling is performed using thin copper tubes soldered to the side surfaces of the cantilever part of a shaped electrode of a fairly large size. Considering that shaped electrodes are always cooled worse than straight electrodes, it is often necessary to significantly reduce the welding rate, preventing overheating of the working part of the shaped electrode and reducing durability.
When using pliers for welding in hard-to-reach places, as well as the need to frequently replace electrodes, use the electrode mounting shown in Fig. 5. This fastening provides good electrical contact, convenient regulation of electrode extension, good stability against lateral displacement, and quick and easy removal of electrodes. However, due to the lack of internal cooling in such electrodes, they are used when welding at low currents (up to 5...6 kA) and at a low speed.
Rice. 5. Methods for attaching electrodes
For ease of operation, electrodes with several working parts are used. These electrodes can be adjustable or rotary (Fig. 6) and significantly simplify and speed up the installation of electrodes (aligning working surfaces).
Rice. 6. Multi-position adjustable (a) and surface (b) electrodes:
1 - electrode holder; 2 - electrode
The electrodes are installed in electrode holders, which are fixed to the cantilever parts of the welding machine, transmitting compression force and current. In table For reference, the dimensions of straight electrode holders of the main types of spot welding machines are given. Electrode holders must be made of sufficiently strong copper alloys with relatively high electrical conductivity. Most often, electrode holders are made of Br.Kh bronze, which must be heat-treated to obtain the required hardness (HB not less than 110). In the case of welding steels, when low currents (5...10 kA) are used, it is advisable to make electrode holders from Br.NBT bronze or silicon-nickel bronze. These metals ensure long-term preservation of the dimensions of the conical mounting hole of the electrode holder.
Table. Dimensions of electrode holders for point machines in mm
|
Electrode holder dimensions |
MTPT-600 |
MTPT-400, MTK-75 |
MTP-300, MTP-400 |
MTK 6301, MTP-200/1200 |
MTPU-300, MTP-150/1200 MTP-200, MTP-150, MT 2507 |
MT 1607, MTP-75 MTP-100, MTPR-75 (50, 25) MTPK-25, MT 1206 |
|
Outer diameter |
||||||
|
Cone diameter for electrode |
||||||
|
Taper |
1: 10 |
1:10 |
1:10 |
The most common are straight electrode holders (Fig. 7). Inside the cavity of the electrode holder there is a tube for supplying water, the cross-section of which should be sufficient for intensive cooling of the electrode. With a tube wall thickness of 0.5...0.8 mm, its outer diameter should be 0.7...0.75 of the diameter of the electrode hole. In the case of frequent changes of electrodes, it is advisable to use electrode holders with ejectors (Fig. 7, b). The electrode is pushed out of the seat by hitting the striker 5 with a wooden hammer, which is connected to a stainless steel tube - ejector 1. The ejector and striker are returned to their original lower position by a spring 2. It is important that the end of the ejector hitting the end of the electrode does not have damage on its surface, otherwise the seating part of the electrode will quickly fail, jamming when it is removed from the electrode holder. It is convenient for operation to make the end of the electrode holder 1 in the form of a replaceable threaded bushing 2, in which the electrode 3 is installed (Fig. 7, c). This design makes it possible to make sleeve 2 from a more resistant metal and replace it when worn and install an electrode of a different diameter, and also to easily remove the electrode when jammed by knocking it out with a steel drift from inside the sleeve.
Rice. 7. Straight electrode holders:
a – normal;
b – with ejector;
c – with replaceable sleeve
If shaped electrodes are more often used when welding parts that have small dimensions of the elements being connected, then for larger sizes it is advisable to use special shaped electrode holders and simple electrodes. Shaped electrode holders can be composite and provide installation of electrodes at different angles to the vertical axis (Fig. 8, A). The advantage of such an electrode holder is the easy adjustment of the electrode extension. In some cases, the shaped electrode can be replaced with electrode holders shown in Fig. 8, b. Also of interest is the electrode holder, the tilt of which can be easily adjusted (Fig. 8, c). The design of an electrode holder bent at an angle of 90° is shown in Fig. 30, g, it allows you to attach electrodes with a cylindrical seat. A special screw clamp allows for quick attachment and removal of the electrodes. In Fig. Figure 9 shows various examples of spot welding using shaped electrode holders.
Rice. 8. Special electrode holders
Rice. 9. Examples of the use of various electrode holders
When spot welding large-sized components such as panels, it is advisable to use a four-electrode rotating head (Fig. 10). The use of such heads allows you to quadruple the operating time of the electrodes before the next stripping, without removing the panel to be welded from the working space of the machine. To do this, after each pair of electrodes is contaminated, the electrode holder 1 is rotated 90° and secured with a stopper 4. The rotating head also makes it possible to install electrodes with different shapes of the working surface for welding an assembly with parts changing, for example, stepwise in thickness, as well as to provide mechanization of stripping the electrodes with special devices. The rotary head can be used when spot welding parts with large differences in thickness and is installed on the side of the thin part. It is known that in this case the working surface of the electrode in contact with a thin part quickly wears out and is replaced by turning the head with a new one. It is convenient to use a roller as an electrode on the side of a thick part.
Rice. 10. Rotating electrode head:
1 – rotary electrode holder; 2 – body; 3 – electrode; 4 – stopper
When spot welding, the axes of the electrodes must be perpendicular to the surfaces of the parts being welded. To do this, welding of parts that have slopes (smoothly varying thickness), or are manufactured using overhead machines, in the presence of large-sized components, is performed using a self-aligning rotary electrode with a spherical support (Fig. 11, a). To prevent water leakage, the electrode has a seal in the form of a rubber ring.
Rice. 11. Self-aligning electrodes and heads:
a - rotary electrode with a flat working surface;
b - head for two-point welding: 1 - body; 2 - axis;
c - plate electrode for welding mesh: 1, 7 - machine consoles; 2-fork; 3 - flexible tires; 4-swinging electrode; 5 - welded mesh; 6 - bottom electrode
On conventional spot machines, welding of steel parts of relatively small thickness can be performed at two points at once using a two-electrode head (Fig. 11, b). Uniform distribution of forces on both electrodes is achieved by rotating the housing 1 relative to axis 2 under the action of the compression force of the machine.
To weld a mesh of steel wire with a diameter of 3...5 mm, plate electrodes can be used (Fig. 11, c). The upper electrode 4 swings on an axis to evenly distribute forces between the connections. The current supply for the purpose of its uniformity is carried out by flexible busbars 3; fork 2 and the swing axis are isolated from the electrode. When electrodes are up to 150 mm long, they can be non-oscillating.
Rice. 12. Sliding wedge electrodes inserts
When welding panels consisting of two skins and stiffeners, there must be an electrically conductive insert inside that absorbs the force of the machine electrodes. The design of the insert must ensure its tight fit to the inner surface of the parts being welded without a gap, in order to avoid deep dents on the outer surfaces of the parts and possible burns. For this purpose, a sliding insert shown in Fig. 12. The movement of the wedge 2 relative to the stationary wedge 4, ensuring their compression to the welded parts 3, is synchronized with the operation of the machine. When electrodes 1 and 5 are compressed and welding occurs, air from the pneumatic drive system of the machine enters the right cavity of the cylinder 8 mounted on the front wall of the machine and moves the wedge 2 through the rod 7, increasing the distance between the working surfaces of the wedges. When raising electrode 1, the air leaves the right one and begins to enter the left cavity of the cylinder 8, reducing the distance between the surfaces of the wedges, which allows the panel to be welded to be moved relative to the electrodes of the machine. The wedge insert is cooled by air that enters through tube 6. The use of such an insert allows you to weld parts with an internal distance between them of up to 10 mm.
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 sizes 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 the values of diameter (d) and radius (R), 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 various 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 values of F SV 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 in case of large dents from the electrodes, ruptures and cracks of the overlap edge, and metal splash. 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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Welding performed in a shielding gas environment (helium or argon) requires tungsten electrodes, which are classified as non-consumable. Due to its refractoriness, the tungsten electrode can withstand high temperatures and a long continuous service life. Currently, this welding material has a fairly extensive classification, where there is a fairly large number of types, divided by brand.
Marking and characteristics of tungsten electrodes
The marking of tungsten electrodes is specified by international standards. Therefore, it is easy to select them for the required purpose in any country, no matter where you are. It is the marking that reflects both the type of electrode chosen and its chemical composition.
The marking begins with the letter “W,” which stands for tungsten itself. In its pure form, metal is present in the product, but the characteristics of such an electrode are not very high, because it is too refractory. Alloying additives help it improve welding qualities.
- Pure tungsten rod is designated "WP". The tip of the rod is green. We can say that it belongs to the category of tungsten electrodes for welding aluminum and copper with alternating current. The tungsten content in the alloy is not less than 99.5%. Disadvantage: limitations in heat load. Therefore, the tungsten electrode (its end) “WP” is sharpened in the form of a ball.
- "C" is cerium oxide. A rod with a gray tip. It is this additive that allows the electrode to be used when working with any type of current (direct or alternating) and maintains a stable arc even at low current. Content – 2%. By the way, cerium is the only non-radioactive material from the series of rare earth metals.
- "T" - thorium dioxide. Rod with a red tip. Such electrodes are used for welding non-ferrous metals, low-alloy and carbon steels, and stainless steel. This is a commonly used electrode when carrying out argon welding. It has one drawback - the radioactivity of thorium, so it is recommended to carry out welding in open areas and in well-ventilated rooms. The welder must follow safety precautions. Note that thoriated tungsten electrodes for argon arc welding hold their shape well at the highest currents. Even the “WP” brand (pure tungsten) cannot cope with such loads. Content – 2%.
- "Y" - yttrium dioxide. A rod with a dark blue tip. It is usually used to weld critical structures from different metals: titanium, copper, stainless steel, carbon and low-alloy steels. Work is carried out only on direct current (straight polarity). The yttrium additive increases the stability of the cathode spot at the end of the electrode itself. This is precisely the reason that it can operate within a fairly wide range of welding current. Content – 2%.
- "Z" - zirconium oxide. Rod with a white tip. Used for argon welding of aluminum and copper with alternating current. This type of electrode provides a very stable arc. At the same time, the element is quite demanding regarding the cleanliness of the welding joint. Content – 0.8%.
- "L" - lanthanum oxide. There are two positions here: WL-15 and WL-20. The first rod has a golden tip, the second has a blue tip. Welding with a tungsten electrode with the addition of lanthanum oxide allows you to use both alternating and direct current. Let’s add here the ease of starting the arc (initial and during re-ignition), this type has the least wear on the end of the rod, a stable arc at the highest current levels, a low tendency to burn through, and the load-bearing capacity is twice as high as that of a pure tungsten rod. The lanthanum oxide content in WL-15 is 1.5% and in WL-20 is 2%.
The classification for digital marking is as follows. The first numbers after the letters indicate the content of alloying additives in the alloy. The second group of numbers, separated from the first by a hyphen, is the length of the tungsten rod. The most common size is 175 mm. But on the market you can also find 50 mm lengths, 75 and 150. For example, WL-15-75 is an electrode with lanthanum oxide, which contains 1.5% additive. Rod length – 75 mm. Its tip is golden.
Methods for sharpening tungsten electrodes
Sharpening tungsten electrodes is the most important component of a properly performed welding process. Therefore, all welders involved in welding in an argon environment carry out this operation very carefully. It is the shape of the tip that determines how correctly the energy transferred from the electrode to the two metals being welded will be distributed, and what the arc pressure will be. And the shape and size of the weld penetration zone, and accordingly its width and depth, will depend on these two parameters.
Attention! The parameters and shape of sharpening are selected based on the type of electrode used and the parameters of the two metal workpieces being welded.
- The working end of WP, WL electrodes is a sphere (ball).
- On WT they also make a convexity, but of a small radius. Rather, they simply indicate the roundness of the electrode.
- Other types are sharpened to a cone.
When an aluminum joint is welded, a sphere forms on its own on the electrode. Therefore, when welding aluminum, there is no need to sharpen the electrode.
What sharpening errors can lead to what?
- The sharpening width is very different from the norm, that is, it can be very wide or very narrow. In this case, the likelihood of weld failure increases greatly.
- If asymmetrical sharpening is carried out, then this guarantees the deflection of the welding arc to one side.
- The sharpening angle is too sharp - the service life of the electrode is reduced.
- The sharpening angle is too blunt - the depth of weld penetration decreases.
- The marks left by the abrasive tool are not located along the axis of the rod. Get an effect like arc wandering. That is, the stable and uniform combustion of the welded arc is disrupted.
By the way, there is a simple formula that determines the length of the sharpened area. It is equal to the diameter of the rod multiplied by a constant factor of 2.5. There is also a table that indicates the ratio of the diameter of the electrodes to the length of the sharpened end.
You need to sharpen the end of the tungsten rod crosswise, like a pencil. You can sharpen it with an electric sander or a grinder. To achieve uniform removal of metal throughout the sharpening area, you can secure the rod in the drill chuck. And rotate it at low speeds of the power tool.
Currently, manufacturers of special electrical equipment offer a machine for sharpening non-consumable tungsten electrodes. A convenient and accurate option for high-quality sharpening. The machine includes:
- Diamond disc.
- Filter for collecting dust.
- Setting the working shaft speed.
- Setting the sharpening angle. This parameter varies between 15-180°.
Research to find the optimal sharpening angle is carried out constantly. One research institute conducted a test where a WL tungsten electrode was tested for the quality of the weld by sharpening it at different angles. Several angular dimensions were selected at once: from 17 to 60°.
The exact parameters of the welding process were determined:
- Two metal sheets of corrosion-resistant steel 4 mm thick were welded.
- Welding current – 120 amperes.
- Speed – 10 m/h.
- Welding position is lower.
- Inert gas consumption – 6 l/min.
The results of the experiment are as follows. The perfect seam was obtained when a rod with a sharpening angle of 30° was used. At an angle of 17°, the weld shape was conical. At the same time, the welding process itself was unstable. The service life of the cutting electrode decreased. At large sharpening angles, the picture of the welding process also changed. At 60°, the width of the seam increased, but its depth decreased. And although the welding process itself has stabilized, it cannot be called high-quality.
As you can see, the sharpening angle plays an important role in the welding process. It doesn’t matter whether stainless steel, steel or copper electrodes are used. In any case, you need to sharpen the rod correctly, because the consequences can be extremely negative. Description of the rods by color and chemical characteristics helps to make the right choice, and at the same time choose the sharpening shape.