Spot welding is a variation of arc welding, like plasma, TIG, MIG-MAG, or even semi-automatic welding. As such, it uses an electric arc to raise the temperature of the metals to be welded to their melting point. It uses the principle of resistance welding, and combines the rise in temperature with good mechanical pressure. Used for the assembly of sheets especially, it is also indicated on several other metals. Precise equipment makes it possible. Find out more fully below what spot welding is, how it works and all the materials and areas in which it can be applied.
Spot welding: what is it?
Using technical jargon, we can define spot welding as a pressure welding process which is based on the transformation by Joule effect of electrical energy into thermal energy, so as to cause the melting of the metals to be welded. . What to understand more simply? Metals have a resistance capacity when they are traversed by a fairly strong current, a resistance that is characterized by overheating. For welding, therefore, a high current is applied so as to generate a large amount of heat capable of melting the metal parts that you want to assemble. The invention of the welding transformer dates back to 1886 in the United States by E. THOMSON.
What is its principle?
As mentioned above, spot welding uses the principle of resistance welding. The metal parts that you want to assemble are first and foremost arranged one on top of the other. Then two non-fusible copper electrodes are applied to them, with a certain pressure. These are responsible for carrying a very intense current (several thousand amperes) to the metal parts concerned. It occurs at the level of the junction of the metal parts, a phenomenon of short-circuit which allows the metals to melt in two seconds at most; in the meantime the current will only have needed a few tenths of a second to pass. It is generally a brief operation which takes place following the following cycle.
The welding cycle
It takes place in four phases mainly.
The docking phase
The first consists of docking the electrodes as described above. These are placed on the area to be welded by exerting a certain pressure on the metal parts in question. Their arrangement varies according to the machines. Indeed, for some, a single electrode is placed on the area concerned. The other remains fixed. The docking phase ends when a current intensity is reached that can generate a resistance responsible for significant overheating. When too short, the docking phase can cause electrode wear and leave areas without resistance. Too long, it can be the cause of inadequate fusion and insufficient welding. When with an insufficient compressive force, it can be the source of an excess of resistance, of projections of metal, and of a wear of the electrodes. All settings should therefore be applied within the required normal range.
The welding stage
Once docked on the metal parts, the electrodes allow current to flow. This current produces significant heat in the area concerned by the Joule effect. The level of melting of metals will depend on its intensity. Welding should be as precise as possible. It must be carried out during a given time with a certain intensity. When these parameters are too important, it can occur metal projections or a malfunction of the electrodes. Insufficient, they would cause poor quality welding
The forging stage
Here, the electrodes are still held in place, but no more current is flowing. The molten metals therefore take time to cool and harden under pressure.
The rest phase
It consists of raising the electrodes. The welded area can therefore be released so that a new weld point can be started. The objective of this phase is to prevent possible overheating.
The welding of two high-strength steel parts, on the other hand, takes place in six phases. In addition to the four phases described here, preheating and annealing steps.
On which metals can we apply point-to-point welding
Point-to-point welding, because of its principle of operation by electrical conductivity, can only be applied to metals that can conduct electricity. It is used for the assembly of sheets having a thickness between 0.5 mm and 1cm. However, the two parts to be welded are not required to have the same characteristics. The coating and thickness of one may therefore be different from that of the other. It is widely used in the construction of automobiles.
Two main factors characterize metals or alloys that can be welded: electrical resistivity and thermal conductivity. Given these two elements, metals are divided into two main categories. The first takes into account metals which have a high electrical resistivity and a low thermal conductivity. They are mostly soldered with a low amperage, but applied over a longer period of time. Included in this category:
Mild steels and those containing few special elements weld easily. Hardening steels, on the other hand, require annealing after welding. As for stainless steels, they weld perfectly when a current of moderate intensity is applied.
It is also easily welded, but with the use of an extremely intense current.
The second category concerns metals which have low resistivity and high thermal conductivity. They therefore require a higher current intensity. Belong to this category:
This metal and its alloys only weld if an intense current is applied for a short time. Magnesium requires the same conditions.
Brass is easier to weld than aluminum. However, a high current must be applied.
Zinc and copper
It is not advisable to spot weld these two metals. The melting temperature of the first is too low while the rendering of the solder of the second is simply bad.