Spot welding is a welding technology, the essence of which is the use of high frequency current. It is a type of contact welding. A weld is a set of so-called weld points. The weld point is formed at the moment when the parts are fixed between two special electrodes.
The electrodes forcefully compress the metal from below and from above. During compression, simultaneously passing welding current. It is he who melts the metal, and the electrodes at the same time compress the parts, like a press, forming a welded point.
To perform this technology, special spot welding equipment is required. It can be either a compact contact spot welding machine or a stationary spot welding machine designed for industrial use. In general, this technology is successfully used in many areas. Including in everyday life.
How to work with spot welding?
Spot welding is the most common type of contact welding: about 50% of all welded structures are made using this method. It is widely used in machine and aircraft construction, which is explained by the simplicity of the method and the versatility of its use. However, there are some subtleties that you need to know in order to improve the productivity and quality of the connected parts. This will be discussed in our article.
- What is a spot welder?
- Variety of spot welding machines
- The main ways to perform SPOT welding
- Spot welding modes
- Possible difficulties
- How to determine the quality of the welded joint
What is a spot welder?
Many home welders think that a modern welding machine is a compact inverter that can be hung on a shoulder and transported anywhere. But in fact, the variety of welding machines is much greater than it may seem at first glance. There are not only manual arc or semi-automatic welding in the world, but also other technologies. For example, spot welding with the use of special devices.
In this short material, we will explain in detail what spot welding is, what a spot welding machine is and what advantages and disadvantages it may have. You will also learn in which areas this technology is used and whether it is possible to make a spot welding machine with your own hands.
Variety of spot welding machines
All devices can be divided into three categories: industrial, professional and household. Industrial devices are used exclusively in production, where it is necessary to weld parts from various metals in large volumes without restrictions on thickness. Such models are usually stationary and cumbersome. A striking example is the MT model 1928 contact spot welding machine.
Professional models are used for contact spot welding on the road, for example. Such devices are much more compact than industrial ones, they can be transported in cars and provide private services. For example, to weld profile pipes on a construction site.
It is also possible to use spot welding in domestic conditions. For this purpose, either low-power purchased devices or homemade ones are used. Yes, spot welding from improvised materials is quite possible. And it copes well with minor repairs.
The main ways to perform SPOT welding
No auto repair shop can do without the use of spot welding, for example, when repairing car bodies. To “grab” the details in this way is a matter of just a few minutes. This is very convenient when connecting metal sheets and body parts. The range of the total thickness of the welded elements ranges from 0.1 to 6 mm, however, there are industrial devices on which steel sheets up to 20 mm thick can be welded.
There are several common methods of spot welding, which mainly differ in bringing current to the surface of the parts. Let’s look at specific examples.
Situation No. 1: you need to connect two blanks. There are no obstacles, so in this case it is better to use the popular two-sided spot welding method: the parts are clamped between the electrodes with a certain force applied, and the current is supplied simultaneously from both sides.
This method is most suitable for welding small structural elements, open joints and nodes with flanging. Its advantage is the high quality of the resulting joint, since the double-sided clamping of the workpieces allows you to provide the force necessary to create a strong welded point.
The disadvantage is that the length of the forceps that clamp the welded sheets is limited. That is, if you need to weld large nodes, this method will not work.
Situation No. 2: the parts that need to be fixed are part of a closed node, so it is not possible to position the electrodes on both sides. In this case, we recommend using the method of one-sided spot welding.
It can be used to connect elements of almost any size (limited only by thickness), since there is no dependence on the length of the forceps – the electrodes are applied to one surface.
In the process, the welding current is distributed between the two parts, and heating occurs from part of the current flowing through the lower part. To improve the quality of the weld point, you can use a special copper gasket, which is installed where the electrodes will be located.
It helps to increase the parameter of the current flowing through the part, so the welding site turns out to be more durable.
In this case, remember that when working with parts of different thicknesses (the difference is more than 3 times) the current should be applied to a more massive (thick-walled) workpiece. Avoid placing the electrodes too close, as in this case there is a possibility of an external splash of molten metal.
Advantages of using this method:
- increased productivity,
- low power consumption (the area of the welding circuit of the machine is small),
- reduced deformation of parts due to the symmetry of the welding process of workpieces.
The disadvantage is that it is not always possible to provide the necessary clamping force, so the quality of such welding is inferior to double-sided.
Spot welding modes
SPOT devices are capable of operating in two modes: “hard” and “soft”. The first is characterized by a large value of welding current and a small period of its transmission, the second, on the contrary, a small value of current and a long operating time.
The main parameters of the work:
- the compressive force not only creates contact between the parts, but also deforms them to prevent the molten metal from splashing into the gap between the workpieces. First of all, it depends on the thickness of the parts being welded and the properties of the metal from which they are made.
- the current strength is also determined by the total thickness of the parts and their chemical composition.
- welding time — the size of the welded core at the point of contact depends on it.
- the diameter of the working part of the electrode (i.e. the part that is in direct contact with the parts).
Remember! Even the slightest deviation from the recommended value of at least one of the parameters will negatively affect the quality of welding.
The value of all the above parameters depends on the thickness and material of the parts being welded. Consider the modes on the example of low-carbon steel parts, since it is the most common material for the manufacture of rolled metal (sheets, channels, pipes, corners, etc.) and fasteners and is widely used in construction, in the construction of various metal structures.
For example, for workpieces with a total thickness of no more than 5 mm, a cycle with a single current pulse and a constant compression force is recommended. If the thickness value exceeds 5 mm, then several pulses will be needed.
The table shows the values of the main parameters of operation during spot welding of low-carbon steel:
|Part thickness, mm||Diameter of the working part of the electrode, mm||Compression force, kN||Current strength, kA||Welding time, sec||Minimum core diameter, mm|
However, the quality of the welded joint depends not only on the method used, but also on the material from which the workpiece is made.
In industries and factories, where most often you have to deal with parts made of alloys with high thermal conductivity (such as copper, aluminum, etc.), you need to take into account some nuances that allow you to get a high-quality welded point.
It should be remembered that elements made of such a material very quickly remove the enclosed heat, and for their bonding it is necessary to supply a current with a very high density reaching 120-300 A / sq.mm.
Another difficulty is the adhesion of aluminum particles to the electrodes and copper particles, from which the electrodes are usually made, to aluminum during operation.
In order to reduce the number of such cases, it is necessary to clean the surfaces of both blanks and electrodes very thoroughly. Please also note that due to the lower electrical resistance, spot welding of aluminum alloys is much easier than pure aluminum.
To get a high-quality result of working with such parts, you need a certain value of the welding current and welding time. In case of deviation from the required value, the connection will be unreliable and will quickly “break”.
For example, failure to comply with the recommended welding time or the compression force of the electrodes will lead to the appearance of a fragile weld point
Below is a table with the ratio of the parameters of the spot welding mode required to obtain a strong welded joint of aluminum alloy parts:
|Thickness of one piece, mm||Current value, kA||Duration of welding, sec||The value of the compression force of the electrodes, kN|
How to determine the quality of the welded joint
At each production, the quality of the welded joint must be checked. Usually they take several samples for this and destroy them.
If the welding was carried out qualitatively, then there will be a deep crater on one part, and the core of the weld point itself will remain on the second. If this is not observed, then this is the non-welding, and in this case it is necessary to increase the compression force or welding time.
The main objective indicator indicating the quality of the joint is the size of the core of the welded point (the area of the contact spot). For any material, it will be true that the diameter of the core should be equal to three thicknesses (S) of a thinner part, while the permissible spread is 20%-80% S.
If the diameter of the resulting core is less than the specified range, then this indicates insufficient penetration, and if larger, then a splash of molten metal.
Spot welding is one of the most interesting technologies. It allows you to obtain high-quality durable joints without the use of welding melting electrodes, wire, gas and flux. All that is needed is a device capable of generating high-frequency current. This is enough to melt the metal and form a welded joint.
At the same time, spot welding can be assembled with your own hands from an inverter-type welding machine (in fairness, you can basically assemble many different welding machines from an old welding inverter).
To assemble spot welding, you will still have to buy some small things, for example, a spot welding controller. And as a result, you will get a full-fledged welding machine that can become an assistant in everyday life.