01. Brief description of Spot welding
Spot welding is a resistance welding method in which the weldment is assembled into a lap joint and pressed between two electrodes, and the parent metal is melted by resistance heat to form a weld.
Spot welding is mainly used in the following aspects:
1. Overlap of thin sheet stamping parts, such as automobile cabs, carriages, harvester scale screens, etc.
2. Thin sheet and steel structure and skin structure, such as carriage side walls and ceilings, trailer panels, combine harvester funnels, etc.
3. Screens and space frames and cross steel bars, etc.
02. Features of Spot welding
During spot welding, the weldment is formed into a lap joint and pressed between two electrodes. Its main features are as follows:
1. The heating time of the connection area during spot welding is very short, and the welding speed is fast.
2. Spot welding only consumes electricity, and does not require filling materials or flux, gas, etc.
3. The quality of spot welding is mainly guaranteed by the spot welding machine. Simple operation, high degree of mechanization and automation, and high productivity.
4. Low labor intensity and good working conditions.
5. Since welding is completed in a very short time, it requires a large current and pressure, so the process control is more complicated, the welding machine has a large capacity, and the equipment is expensive.
6. It is difficult to perform non-destructive testing on welds
03. Operation process of Spot welding
Before welding, the surface of the workpiece must be cleaned. The commonly used cleaning method is pickling cleaning, that is, first pickling in heated 10% sulfuric acid and then washing in hot water. The specific welding process is as follows:
(1) The workpiece joint is sent between the upper and lower electrodes of the spot welding machine and clamped;
(2) Power is turned on, so that the contact surface of the two workpieces is heated, partially melted, and a molten core is formed;
(3) After the power is turned off, the pressure is maintained so that the molten core cools and solidifies under the action of pressure to form a weld;
(4) Remove the pressure and remove the workpiece.
04. Influencing factors of Spot welding
The main influencing factors of welding quality include welding current and power-on time, electrode pressure and shunt, etc.
1. Welding current and power-on time
According to the size of the welding current and the length of the power-on time, spot welding can be divided into two types: hard specification and soft specification. The specification that passes a large current in a short time is called a hard specification. It has the advantages of high productivity, long electrode life, and small deformation of the weldment. It is suitable for welding metals with good thermal conductivity. The specification that passes a small current in a long time is called a soft specification. Its productivity is low and it is suitable for welding metals with a tendency to harden.
2. Electrode pressure
During spot welding, the pressure applied to the weldment by the electrode is called electrode pressure. The electrode pressure should be selected appropriately. When the pressure is high, it can eliminate the shrinkage and shrinkage cavity that may occur when the molten core solidifies, but the resistance and current density of the welding system are reduced, resulting in insufficient heating of the weldment, a decrease in the diameter of the weld core at the weld point, and a decrease in the strength of the weld point. The size of the electrode pressure can be selected according to the following factors:
(1) The material of the weldment. The higher the high temperature strength of the material, the greater the required electrode pressure. Therefore, when welding stainless steel and heat-resistant steel, a larger electrode pressure should be used than when welding low-carbon steel.
(2) Welding parameters. The harder the welding specification, the greater the electrode pressure.
3. Shunt
During spot welding, the current flowing outside the main welding circuit is called shunt. Shunt reduces the current flowing through the welding area, resulting in insufficient heating, causing a significant decrease in the strength of the weld, and affecting the welding quality. The factors that affect the degree of shunt are mainly the following:
(1) Weld thickness and weld spacing. As the weld spacing increases, the shunt resistance increases and the degree of shunt decreases. When a conventional spot spacing of 30 to 50 mm is used, the shunt current accounts for 25% to 40% of the total current, and as the weld thickness decreases, the degree of shunt also decreases.
(2) Surface condition of weld. When there are oxides or dirt on the weld surface, the contact resistance between the two welds increases, the current passing through the welding area decreases, and the degree of shunt increases. The workpiece can be pickled, sandblasted or polished.
05. Safety precautions of Spot welding
(1) The foot switch of the weld ing machine should have a firm protective cover to prevent accidental start-up.
(2) The working point should be equipped with a baffle to prevent working sparks from splashing.
(3) When welding, the welder should wear plain protective glasses.
(4) The place where the welding machine is placed should be kept dry, and the ground should be covered with anti-slip plates.
(5) After the welding work is completed, the power supply should be cut off, and the cooling water switch should be extended for 10 seconds before closing. When the temperature is low, the accumulated water in the waterway should be removed to prevent freezing.
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