The surface of the workpiece must be cleaned before welding. The common cleaning therapy is pickling, which means pickling in heated sulfuric acid with a concentration of 10% and then washing in hot water. The specific welding process is as follows:
(1) Feed the workpiece joint between the upper and lower electrodes of the spot welder and clamp it;
(2) Apply electricity to heat the contact surfaces of the two workpieces and locally melt them to form nuggets;
(3) Maintain the pressure after the power is turned off, so that the nugget is cooled and solidified under the pressure to form a solder joint;
(4) Remove the pressure and take out the workpiece.
The main influencing factors of welding quality are welding current and energizing time, electrode pressure and shunt.
1. Welding current and energizing time
According to the size of the welding current and the length of the current, spot welding can be divided into two types: hard and soft. A specification that passes a large current in a short period of 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. A specification that passes a small current for a long time is called a soft specification, and its productivity is low, which is suitable for welding metals that have a tendency to harden.
2. Electrode pressure
During spot welding, the pressure exerted by the electrode on the weldment is called electrode pressure. The electrode pressure should be selected properly. When the pressure is large, it can eliminate the shrinkage and shrinkage that may occur when the nugget solidifies. Solder joint strength decreases. The size of the electrode pressure can be selected based on the following factors:
(1) The material of the weldment. The higher the high temperature strength of the material. The greater the electrode pressure required. Therefore, when welding stainless steel and heat-resistant steel, the electrode pressure should be higher than that of 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. Shunting reduces the current flowing through the welding zone, resulting in insufficient heating, resulting in a significant decrease in the strength of the solder joint and affecting the welding quality. The factors that affect the degree of diversion are mainly the following:
(1) Weldment thickness and solder joint spacing. As the solder joint distance increases, the shunt resistance increases and the degree of shunt decreases. When a conventional pitch of 30 to 50 mm is used, the shunt current accounts for 25% to 40% of the total current, and the shunting degree decreases as the thickness of the weldment decreases.
(2) Weldment surface condition. When there are oxides or dirt on the surface of the weldment, the contact resistance between the two weldments increases, and the current passing through the welding zone decreases, that is, the degree of shunting increases, and the workpiece can be pickled, sandblasted or polished.
