Contact welding

Contact self-welding, in most cases, is used to produce a series of similar parts or for the production of welded joints of parts that are large. Its principle is that the metal is heated by a current passing through it, and the current path is not homogeneous. Contact welding with your own hands requires the use of much more powerful power sources than in the case, for example, with electric arc welding, because even in small contact machines for this type of work, the current is measured in thousands of amperes, not to mention larger machines. Often, a step-down transformer with a single turn of the secondary winding and a large transformation ratio as a power source are used.

Spot contact welding is the welding of parts by means of a short pulse. The current strength of the latter should be large, and the voltage - small. Selecting the pulse duration by an experimental method, it is important to proceed from the thickness of the metal to be welded.

How is contact welding done by yourself? This requires a special apparatus. It can be made independently, but at the same time it can be welded to small parts and thin sheets. It consists of two main parts - electrical and mechanical. If you decide to assemble the welding machine yourself, then, by doing this, pay special attention to the reliability of the joints, which will determine how good the contact welding will be. Equipment that is useful for creating such a device is a control unit, a contact assembly and a welding transformer.

Most often, self-contact welding is performed in order to join the overlapping sheet parts, while the connection itself is performed at points close to it. The preforms prepared for operation are pressed with electrodes to then carry through them a high amperage current. The area of contact will be limited by the surface roughness. Those places through which the current passes, combining them, are called channels. They get very hot, which causes the central point of each of them to melt. A connection appears between the two surfaces. Strong heating promotes plastic deformation, eliminates unevenness and increases the strength of the material blanks.

This process can be performed both with metal melting (resistance) and without it (by reflow). The surface of the blanks must be carefully prepared. This is necessary, in particular, in order to ensure a uniform heating of the edge. In general, fusion welding is preferable to resistance welding, since it does not require preparation. It also makes it possible to connect parts from different types of metal to a large area.

The peculiarity of spot welding is that the pressure arising during operation increases the density and strength of the welded seam. The desired mode of operation is chosen taking into account the power of the device. The longer the heating time, the longer the cooling, and, consequently, the fewer cracks. Soft mode is used for welding high-carbon steels, as they tend to harden, leading to the formation of deformations. It provides for an increase in the heating time of the metal. Austenitic steels need to be welded in a rigid mode, which eliminates overheating of their surfaces.