All people know what welding of metal parts is. However, not everyone knows that there is a wide variety of welding methods and methods using various energy sources. The point connection of metals is a type of contact welding. It differs from others in that it does not have a weld, and the parts are docked at several points. This compound has been most widely used in industrial enterprises, but home craftsmen are also trying to master contact spot welding, the technology and the advantages of its use.
Principle of contact connection
The principle of contact welding is to quickly heat the material to the melting point and directly form a metal structure of a uniform nature at the junction.
The pulsed nature of the current during the welding process and the time it affects the surface of the workpieces play a major role in heating the junction. Good quality of the weld point can only be achieved by tightly and reliably holding parts for the period of crystallization of the molten metal structure.
When using automatic spot welding in production, it is possible to achieve a speed of action of up to six hundred contacts per minute.
Welding technology
In industrial production, contact welding is mainly used for joining sheet metal from non-ferrous alloys, various steel grades, as well as intersecting fittings and profile-type workpieces (corner, channel). At home, it is often necessary to apply this type of welding when repairing household appliances, installing a metal fence, welding mesh for construction.
The process of contact spot welding can be divided into several stages:
- Before you start, you need to prepare blanks. It is not necessary to clean the surface along the entire joint plane, as in welding, with a continuous seam. It will be enough to clean the alleged docking points with a metal brush or fine sandpaper. Remains of paint or grease stains are removed with a special solution.
- Further, the prepared parts must be interconnected in the desired position.
- After these operations, the workpieces must be installed between the electrodes and pressed tightly against each other.
- The next stage involves 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.
In industrial enterprises, equipment most often used is capable of welding automatically. And in domestic conditions, the use of semi-automatic spot welding is more efficient.
Modes of work
Depending on the conditions of connection and the maximum power of the welding machine, the duration of the heating of the junction of parts can vary from milliseconds to several tens of seconds. So, when doing spot welding of carbon steels with their own hands, which are easily hardened and capable of cracking, it is advisable to increase the heating time for a slower subsequent cooling of the metal.
But it is better to weld a stainless steel with a short duration of heating the surface. This is done to reduce the possibility of violation of the anti-corrosion coating of the metal.
The tight contact of the products at the weld points provides the necessary pressure between the electrodes. This compression is completely dependent on the thickness of the material and the characteristics of the metal being welded. The pressure at the end of welding is of great importance, since it is responsible for the formation of a fine-grained structure of the material at the welding point and brings contact strength closer to the strength of the base metal.
Improving the crystallization of molten metal is achieved due to a small delay in the pressure force after the passage of a current pulse.
Point Connection Benefits
Compared to many other types of joints, do-it-yourself spot welding has a number of advantages:
- During the welding process, it is not necessary to use additional materials (flux, gas).
- Since the connection is carried out without the use of gas, there are no harmful fumes.
- Lack of slags and waste in the course of work.
- The efficiency of contact spot welding is much higher than the usual joining of parts.
- The ability to achieve a high quality connection in a short period of time.
- Many welders consider this method the most hygienic in comparison with other welding technologies.
If you follow 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 to using this method. These include the following:
- It is almost impossible to obtain a strong connection of dissimilar metals.
- The ability to spray metal when exposed to a powerful current pulse.
- The compression mechanism of the workpieces and the device of the head for welding have a complex structure with simultaneous fastening in several places.
- Also, with multipoint connection, it is necessary to complicate the design of the electrodes.
Electrode Requirements
The electrodes are designed to close the electrical circuit through the welded metal surface. The main material for the manufacture of electrodes are electrolytic cold-rolled copper and its alloys on a tungsten basis, as well as special bronze containing cobalt or cadmium. All these metals correspond to the basic characteristic of the electrode — resistance, that is, they retain the dimensions, properties, and shape when the surface is highly heated (up to 600 ℃).
To reduce the wear of the electrodes during operation, it is necessary to take care of their intensive water cooling.
The cross section of the electrode should correspond to the size of the weld point. An increase in the diameter of the electrode leads to a significant decrease in the level of heating at the weld point.
Resistance welding machine
Modern manufacturers produce many models of various spot welding machines. The cost of such models depends on the set of functional capabilities of the device, so each welder must decide for himself what purpose it is purchased for.
The standard welding machine includes:
- converting current transformer;
- universal welding clamp;
- on and off relay;
- regulator of duration and current strength;
- device for compressing electrodes.
Simple household appliances for spot welding 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:
- By the principle of electrode placement - opposite each other and in parallel.
- By the method of work - on automatic and manual units.
- By the method of movement - to suspended, mobile or stationary devices.
In industrial enterprises, work is performed using high-precision automatic equipment. But for the home, a do-it-yourself spot welding machine is quite suitable.
Welding application for batteries
In household and computer devices, lithium-ion batteries are currently widely used, the proper functioning 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 will prove to be quite suitable for domestic use. The main thing is that in the manufacture of spot welding for batteries, carefully and carefully consider all the requirements for a home-made device.
The principle of fastening the contact to the battery
High pulse power for a good connection of the battery terminals is not required. Therefore, before solving the question of how to do spot welding, you need to understand why soldering of battery contacts cannot be used. The bottom line is that the terminals of the batteries 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 hand-made spot welding for batteries, which can create a powerful current pulse of up to 400-800 A. This will be quite enough to perform welding.
Contact integrity and connection reliability will depend on the pulse duration (35-100 milliseconds). In the case of a longer exposure, the plate may burn out, resulting in a loss of strength.
It is possible to make a unit for welding from a conventional battery for a car, but a machine made by yourself for spot welding from a microwave will be more functional.
Microwave Oven Welding Machine
A key element of any resistance welding is a transformer, which converts the input voltage to the required level. Therefore, a transformer for spot welding from a microwave is ideally suited for its characteristics.
If you have an old microwave, you must first carefully remove the parts from it. This procedure will look like this:
- Carefully disassemble the microwave housing and remove the transformer from the structure.
- Since the secondary winding is made of a small cross-section wire, it will have to be removed. This operation must be carried out with the help of a chisel and a small hammer, while the primary winding cannot be damaged.
- Next, you need to remove the magnetic circuit. 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 is suitable.
- We add two or three turns of wire to the core of the transformer so that the output has a voltage of 2 V.
To increase the power in the manufacture of do-it -yourself spot welding from a microwave, you will need to install a second, similar transformer, connecting it to the electric circuit with the first.
Homemade Electrode Requirements
Most often, soldering iron tips are used as electrodes, which are pre-ground under a cone. In the absence of such parts, ordinary copper wire with a cross section of 2–5 mm can be used. Maintenance and installation of such parts of the device have their own nuances:
- To maintain the required shape, the electrodes must be constantly grinded, as they are intensively subjected to wear during operation.
- Periodically, the sting must be cleaned from plaque. And with complete wear it will have to be replaced.
- In order not to lose power, wires suitable for the electrodes should be short.
- On current wires it is better to put on tips.
Last build stage
In order for the functionality of the home-made device to be complete and safe, you need to install a switch in the clamp system. The toggle switch is connected to the primary winding of the transformer and is mounted on the handle of the clamp lever.
For household use, the use of a lever with a length of about 60 cm is sufficient, which will increase the effort of the hands tens of times.
And of course, you need to think about fixing the spot welding from the microwave to the desktop. The most optimal is fixing the device using a clamp, which will allow you to move it to the right place for work.
Some defects of resistance welding
It is always better to learn any business from the mistakes of others. Therefore, it is necessary to familiarize yourself with possible defects that may occur during welding:
- Cracks on the surface may appear due to violation of the temperature regime.
- When welding lap welds, breaks may occur along the joint edge if the weld point is close to the edge.
- Due to poor-quality electrodes, a partial or complete lack of penetration of the welding site can result.
- But from a small diameter of the electrodes, dents can appear on the contact.
- And of course, for novice welders, the most common mistake is a burn through of the surface, which can happen with weak compression of the electrodes.
- Due to the long pulse duration and strong current, external and internal cracks can form.
- If the electrodes are installed incorrectly with respect to the plates being welded, a core displacement may occur.
Despite the fact that it is not difficult to purchase equipment for spot welding, the machine made by hand will still be an assistant that fully meets the requirements of a home craftsman. And to make such a device yourself, you do not need a lot of materials and money.