All people know what welding of metal parts is. However, not everyone knows that there is a wide variety of methods and methods of welding using various energy carriers. Spot joining of metals is a kind of contact type of welding. It differs from others in that it does not have a welded seam, and the parts are joined at several points. This connection has received the widest use in industrial enterprises, but home craftsmen are also trying to master resistance spot welding, the technology and the advantages of its use.
Principle of contact connection
The principle of resistance welding is to rapidly heat the material to a melting temperature and directly form a homogeneous metal structure at the junction.
The pulsed nature of the current during the welding process and the time of its action on the surface of the workpieces play a major role in heating the joint. A good spot weld quality can only be achieved if the parts are tightly and securely held during the crystallization of the molten metal structure.
Whenusing automatic spot welding in production, it is possible to achieve action speeds of up to six hundred contacts per minute.
Welding technology
In industrial production, resistance welding is mainly used to join sheet metal from non-ferrous alloys, various steel grades, as well as intersecting reinforcement and profile-type workpieces (angle, channel). At home, you often have to use this type of welding when repairing household appliances, installing a metal fence, welding mesh for construction.
The process of resistance spot welding can be conditionally divided into several stages:
- Before starting work, you need to prepare blanks. It is not necessary to clean the surface over the entire plane of the joint, as in welding, with a continuous seam. It will be enough to clean the intended docking points with a metal brush or fine sandpaper. Residues of paint or grease stains are removed with a special solution.
- Next, the prepared parts must be connected together in the desired position.
- After these operations, you need to place the blanks between the electrodes and press them tightly against each other.
- The next step implies the main process of work. The spot welding machine is turned on, which produces the necessary heating of the metal to a plastic state and produces the necessary deformation of the welding point.
Industrial enterprises most often use equipment capable of welding in automatic mode. And in domestic conditions, it is more efficient to usesemi-automatic spot welding.
Working modes
Depending on the connection conditions and the maximum power of the welding machine, the heating time of the junction point of the parts can vary from milliseconds to several tens of seconds. So, when doing do-it-yourself spot welding of carbon steels, which are easily hardened and capable of forming cracks, it is advisable to increase the heating time, for a slower subsequent cooling of the metal.
But it is better to weld stainless steel with a short surface heating time. This is done to reduce the possibility of damage to the anti-corrosion coating of the metal.
Tight contact of products in the places of welding provides the necessary pressure between the electrodes. This compression is entirely dependent on the thickness of the material and the characteristics of the metal to be welded. The pressure value at the end of welding is of great importance, as it is responsible for the formation of a fine-grained structure of the material at the welding point and brings the strength of the contacts closer to the strength of the base metal.
Improved crystallization of molten metal is achieved by a slight delay in the pressure force after the passage of a current pulse.
Benefits of a point connection
Compared to many other types of joints, do-it-yourself spot welding has a number of advantages:
- There is no need to use additional materials (flux, gas) in the welding process.
- Since the connection is made without the use of gas,then there are no harmful fumes.
- No slag and waste in the process.
- The efficiency of resistance spot welding is much higher than conventional joining of parts.
- The ability to achieve high quality connections in a short period of time.
- Many welders find this method the most hygienic of all welding technologies.
When following all the rules of work, it is very easy to achieve a reliable and accurate connection of products, even for a beginner.
Disadvantages of the method
Along with the advantages of spot welding, there are some disadvantages of using this method. These include the following:
- It is almost impossible to get a strong bond of dissimilar metals.
- Possibility of metal splashing when exposed to a powerful current pulse.
- The workpiece compression mechanism and the welding head device are complex in design, with simultaneous fastening in several places.
- Also, with a multi-point connection, it is necessary to complicate the design of the electrodes.
Electrode requirements
Electrodes are designed to close the electrical circuit through the metal surfaces being welded. Electrolytic cold-rolled copper and its tungsten-based alloys, as well as special bronze containing cob alt or cadmium, are the main materials for the manufacture of electrodes. All these metals correspond to the main characteristic of the electrode - resistance, i.e., they retain their dimensions, properties and shape when their surface is heated to a large extent (up to600℃).
To reduce electrode wear during operation, it is necessary to take care of their intensive water cooling.
The cross section of the electrode must match the size of the weld spot. Increasing the diameter of the electrode leads to a significant decrease in the level of heating at the welding point.
Resistance welding machine
Modern manufacturers produce many models of various spot welding machines. The cost of such models depends on the set of functionality of the device, so each welder must decide for himself for what purpose it is purchased.
Standard welding machine includes:
- converting current transformer;
- universal welding clamp;
- on and off relay;
- duration and current regulator;
- electrode compression device.
Simple household spot welders may not have a compression force regulator, so the welder has to adjust this parameter himself, based on his knowledge and experience.
All equipment can be classified:
- According to the principle of placing electrodes - opposite each other and in parallel.
- According to the method of work - for automatic and manual units.
- According to the method of movement - to suspended, mobile or stationary devices.
In industrial plants, work is done withhigh-precision automatic equipment. But for the home, a do-it-yourself spot welding machine is quite suitable.
Application of welding for batteries
Lithium-ion batteries are now widely used in household and computer devices, the correct operation of which depends on a high-quality connecting contact. At home, it is better to make spot welding for the battery with your own hands, because the price of factory units is not available to all craftsmen.
Of course, the technical characteristics of such devices will be inferior to factory devices, but they will be quite suitable for domestic use. The main thing is that in the manufacture of spot welding for batteries, consistently and very carefully consider all the requirements for a home-made device.
The principle of fastening the contact to the battery
High power pulse for a good connection of the battery terminals is not required. Therefore, before deciding how to do spot welding, you need to understand why you can not use soldering battery contacts. The bottom line is that the battery terminals are afraid of overheating, and working with a soldering iron implies high heating of the surface of the petal.
This problem can be solved by self-made spot welding for batteries, which can create a powerful current pulse up to 400-800 A. This will be enough to perform welding.
The integrity of the contact and the reliability of the connection will depend on the durationpulse (35-100 milliseconds). Longer exposure may burn the plate, resulting in loss of strength.
You can make a welding unit from a regular car battery, but a do-it-yourself machine for spot welding from a microwave will still be more functional.
Welding machine from a microwave oven
The key element of any resistance welding is a transformer that converts the input voltage to the required level. Therefore, a transformer for spot welding from a microwave oven is ideal in terms of characteristics.
If there is an old microwave oven, then first you need to carefully remove the parts from it. This procedure will look like this:
- Carefully dismantle the microwave housing and remove the transformer from the structure.
- Since the secondary winding is made of thin wire, it will have to be removed. This operation must be done with a chisel and a small hammer, while not damaging the primary winding.
- Next, you need to remove the magnetic core. Here you will have to make more effort, since on some models it is made by welding.
- The next step is to wind a new secondary winding. For winding, a wire with a cross section of 10 mm or more is suitable.
- Add two or three turns of wire to the core of the transformer so that the output voltage is 2 V.
To increasepower when making do-it-yourself spot welding from a microwave oven, you will need to install a second, similar transformer, connecting it to the first electrical circuit.
Requirements for homemade electrodes
Most often, soldering iron tips are used as electrodes, which are pre-sharpened under a cone. In the absence of such parts, you can use ordinary copper wire with a cross section of 2–5 mm. Maintenance and installation of such parts of the apparatus have their own nuances:
- In order to maintain the desired shape, the electrodes must be constantly undermined, as they are subjected to intense wear during operation.
- Periodically, the sting needs to be cleaned from plaque. And when it is completely worn out, it will have to be replaced.
- In order not to lose power, the wires to the electrodes should be kept short.
- It is better to put tips on current wires.
Last assembly step
In order for the functionality of the homemade device to be complete and safe, you need to install a switch in the clamping system. The toggle switch is connected to the primary winding of the transformer and is mounted on the handle of the clamp lever.
For domestic use, it is enough to use a lever about 60 cm long, which will increase the hand force dozens of times.
And of course, you need to think about fixing the spot welder from the microwave to the desktop. The most optimal is the fixation of the device with a clamp, which will allow you to move it to the right place for work.
Some defects in resistance welding
It is always better to learn anything from the mistakes of others. Therefore, it is necessary to familiarize yourself with the possible defects that may occur during welding:
- Cracks in the surface may appear due to violation of the temperature regime.
- When overlap welding products, breaks may occur along the edge of the joint if the weld point is located close to the edge.
- Due to poor-quality electrodes, partial or complete lack of fusion of the welding spot may result.
- But from the small diameter of the electrodes, dents may appear on the contact.
- And of course, the most common mistake for novice welders is to burn through the surface, which can happen when the electrodes are weakly compressed.
- Due to the long pulse duration and high current, external and internal cracks may form.
- If the electrodes are set incorrectly in relation to the plates to be welded, core displacement may occur.
Despite the fact that it is not difficult to purchase equipment for spot welding, a do-it-yourself device will be an assistant that fully meets the requirements of a home craftsman. And to make such a device yourself, you will not need a lot of materials and money.
