The process of spot welding is to open the cooling water; clean the surface of the weldment, after the assembly is accurate, send it between the upper and lower electrodes, and apply pressure to make it in good contact; the contact surface of the two workpieces is heated and partially melted, forming a Nugget; keep the pressure after power off, so that the nugget cools and solidifies under pressure to form a solder joint; remove the pressure and take out the workpiece. Spot welding process parameters such as welding current, electrode pressure, energization time and electrode working surface size have a significant impact on welding quality.
The spot welding machine uses the high-temperature arc generated when the positive and negative poles are short-circuited instantaneously to melt the material to be welded between the electrodes to achieve the purpose of combining them. The structure of the spot welding machine is very simple. To put it bluntly, it is a high-power transformer that converts 220V AC into a low-voltage, high-current power supply, which can be either DC or AC. The welding transformer has its own characteristics, that is, it has the characteristics of a sharp drop in voltage.
After the electrode is ignited, the voltage drops, and the adjustment of the working voltage of the welding machine, in addition to the one-time 220/380 voltage conversion, the secondary coil also has a tap to change the voltage, and at the same time, there is an adjustable iron core that is adjusted by an iron core. The electric welding machine is generally a high-power transformer, which is made of the principle of inductance. The inductance will produce a huge voltage change when it is turned on and off, and the high-voltage arc generated when the positive and negative poles are short-circuited instantaneously is used to melt the solder on the electrode. for the purpose of combining them.
Spot welding is a resistance welding method in which the weldment is assembled and connected, and is pressed between two electrodes, and the base metal is melted by resistance heat to form a solder joint. Spot welding is mostly used for the connection of thin plates, such as aircraft skins, chimneys of aero-engines, car cab shells, etc. The welding transformer of the spot welding machine is a spot welding electric appliance, and its secondary has only one loop. The upper and lower electrodes and electrode arms are used to conduct welding current and transmit power. The cooling water circuit passes through the transformer, electrodes and other parts to avoid heat generation. When welding, the cooling water should be passed first, and then the power switch should be turned on. The quality of the electrodes directly affects the welding process, welding quality and productivity. Electrode materials are commonly made of red copper, cadmium bronze, chrome bronze, etc. The shapes of electrodes are various, mainly determined according to the shape of the weldment. When installing the electrodes, pay attention to keeping the upper and lower electrode surfaces parallel; the electrode planes should be kept clean and trimmed with emery cloth or a file. Welding Cycle The welding cycle of spot and projection welding consists of four basic stages (spot welding process):
(1) Pre-compression stage - the electrode drops to the current-on stage to ensure that the electrode presses the workpiece so that there is proper pressure between the workpieces.
(2) Welding time - the welding current passes through the workpiece and generates heat to form a nugget.
(3) Maintenance time - cut off the welding current, and continue to maintain the electrode pressure until the nugget solidifies to sufficient strength.
(4) Rest time - the electrode starts to lift until the electrode starts to descend again, starting the next welding cycle.
To improve the performance of welded joints, it is sometimes necessary to add one or more of the following to the base cycle:
(1) Increase the pre-pressure to eliminate the gap between the thick workpieces and make them fit closely.
(2) Use the preheating pulse to improve the plasticity of the metal, so that the workpiece is easy to fit closely and prevent spattering; during projection welding, this can make multiple bumps evenly contact with the flat plate before electrified welding, so as to ensure the uniform heating of each point.
