Spot welding is a resistance welding procedure and is a method that does not require shielding gas. The workpieces to be joined are placed on top of each other precisely in the first step. Two electrodes press the two workpieces together mechanically and fix the parts to be welded. By supplying a strong voltage, a flow of current is generated between the two electrodes. The workpieces represent a resistance for the flowing current. This causes the metal to heat locally sufficiently to liquefy. The mechanical pressure of the electrodes causes the two workpieces to fuse together and after cooling they are inseparably joined.
Spot welding process
Aligning the workpieces
In order to join the workpieces by means of spot welding they must be precisely aligned with each other, as correction after welding is not easy.
Pressing on the electrodes
Suitable electrodes are chosen for welding the workpieces. These are mostly made of copper alloys with fractions of tungsten and molybdenum, which can withstand the high temperatures and pressures.
The current is flowing
If the electrodes have been positioned correctly, the current is switched on, which flows from one electrode to the other with a very high power. The material is heated so much that it liquefies and so both workpieces join.
The workpiece are tightly joined
The time during which the electrical current must flow through the workpieces varies, depending on the material and workpiece thickness. If the parts are tightly joined, the electrodes are removed in order to repeat the process at the next welding point.
This welding procedure can be repeated as often as necessary, depending on the workpiece size. Therefore, despite the small spot welds, a high degree of stability can be achieved.
Spot welding is generally used in sheet metalworking or, for example, for joining steel sheets in vehicle body production and vehicle production. As the force of the pressure produces internal joining of the molten metals, non-weldable raw materials can also be joined, subject to limitations. Spot welding is rarely used for aluminium.
Due to the concentration of high pressure on a relatively small area, within a very short time and high energy in the form of electrical current (up to 40,000 A), a very strong joint is produced by spot welding. In spot welding the welding control and welding parameters are decisive for durability. The influencing variables are controlled by the flow rate and temperature of the cooling water – which cools the welding electrodes -, the surface of the workpiece, the grade and thickness of the components.
In spot welding either direct current (tends to be used rarely) or alternating current with mains frequency is used. Although recently, medium frequency invert welding has established itself. In this spot welding method the welding time and the current are controlled via phase shift control (phase lag welding) by means of thyristor controllers. It is also known as MFDC welding. The welding transformer is supplied via a converter with a frequency of one to several kilohertz.
Attention must be paid to two items to check the quality of the spot welding. If the spot impression is the same colour on the inside as the machined sheet and the outer ring is brown or at most blue, it can be assumed that the material was not excessively overheated and no cracks were formed. Further the spot impression should not be deeper than 20 % of the sheet or plate thickness.
So-called spot welding guns or portable sport welders are used for spot welding. These are mainly used with mobile resistance welding equipment. Manual spot welding is used nowadays in car repair shops and in building trades. Spot welding can also be carried out by welding robots, for example, familiar in car production.
A differentiation is also made between 2 types of spot welding guns. With the cable pliers (spot welding pliers) the welding current is supplied by a welding transformer, which is set up separately. This is connected to the welding pliers via a strong and heavy-duty high-current cable (180 mm² up to 400 mm²).
In the much heavier transformer pliers the welding transformer is part of the sport welding pliers. However, the cable cross-section is only approx. 10 mm².