First, the spot welding method:
Spot welding is usually divided into two categories: double-sided spot welding and single-sided spot welding. In double-sided spot welding, the electrodes are fed from the sides of the workpiece to the weld. A typical double-sided spot welding method is shown in Figure 11-5. In the figure, a is the most common way, when there are electrode indentations on both sides of the workpiece. In the figure, b indicates that the conductive plate of the large welded area is used as the lower electrode, which can eliminate or reduce the indentation of the workpiece below. Commonly used for spot welding of decorative panels. In the figure, c is double-sided spot welding for welding two or more spot welds at the same time. The electrodes are connected in parallel using a transformer. At this time, the impedances of all current paths must be substantially equal, and the surface state and material of each welded portion. The thickness and electrode pressure must be the same to ensure that the current through each solder joint is basically the same. In the figure, d is double-sided multi-point spot welding using multiple transformers, so that the shortage of c can be avoided.
For single-sided spot welding, the electrode is fed from the same side of the workpiece to the weld. The typical single-sided spot welding method is shown in Figure 11-6. In the figure, a is single-sided single-point spot welding, and the electrode without solder joint is used. Large diameter and large contact surfaces to reduce current density. In the figure, b is a single-sided double spot welding without splitting, in which case the welding current flows through the welding zone. In the figure, C has a split single-sided double-point spot welding, and the current flowing through the upper workpiece does not pass through the weld zone to form a wind flow. In order to provide a low resistance path for the welding current, a copper pad is placed under the workpiece. In the figure, d is used when the distance l between the two solder joints is large, for example, when welding the skeleton member and the composite plate, in order to avoid the warpage of the composite plate caused by improper heating and to reduce the resistance between the two electrodes, a special The copper bridge A is pressed against the workpiece at the same time as the electrode.
In mass production, single-sided multi-point spot welding is widely used. At this time, it can be powered by a transformer, each pair of electrodes is pressed against the workpiece (Fig. 11-7a), and each pair of electrodes can be powered by a separate transformer, and all the electrodes are pressed against the workpiece at the same time (Fig. 11- 7b). The latter type has more advantages and is more widely used. The advantages are: each transformer can be placed closest to the connected electrode, thus.
The power and size can be significantly reduced; the process parameters of each solder joint can be adjusted separately; all solder joints can be welded at the same time, and the productivity is high; all the electrodes are pressed against the workpiece at the same time, which can reduce deformation; multiple transformers can be energized at the same time to ensure three-phase Load balancing.
Second, spot welding process parameter selection
Usually, according to the material and thickness of the workpiece, referring to the welding condition table of the material, the shape and size of the end face of the electrode are first determined. Secondly, the electrode pressure and welding time are preliminarily selected, then the welding current is adjusted, and the sample is welded at different currents. After checking the diameter of the nugget meets the requirements, the electrode pressure, welding time and current are adjusted within an appropriate range to carry out the sample. Welding and inspection until the quality of the solder joints fully meets the requirements specified in the technical conditions. The most commonly used method for inspecting specimens is the tearing method. The mark of the quality solder joint is: a round hole in one piece of the torn sample and a round boss on the other piece. Thick plates or quenched materials sometimes cannot tear round holes and bosses, but the diameter of the nugget can be judged by the sheared fracture. If necessary, low-power measurement, pull test and X-ray test are also required to determine the penetration rate, shear strength and presence or absence of shrinkage cavities, cracks, and the like.
When selecting the process parameters from the sample, the difference between the sample and the workpiece in the shunt, the influence of the ferromagnetic material, and the assembly gap should be fully considered and adjusted appropriately.
Third, unequal thickness and spot welding of different materials
When unequal thickness or different material spot welding, the nugget will be asymmetrical to its interface, but to the thick plate or the side of the conductive and poor thermal conductivity, the result of the offset will make the thin piece or conductive, thermal conductivity The good workpiece penetration rate is reduced and the solder joint strength is reduced. The nugget offset is caused by the difference in heat generation and heat dissipation conditions of the two workpieces. When the thickness is not equal, the thick part has a large resistance and the interface is far away from the electrode, so the heat is generated more and the heat is less, so that the nugget is biased toward the thick part; when the materials are different, the material with poor conductivity and thermal conductivity is easy to heat and heat is difficult to be used. The nugget is also biased towards this material
The principle of adjusting the nugget offset is to increase the heat generation of a thin plate or a conductive or thermally conductive workpiece to reduce heat dissipation. Common methods are:
(1) The strong influence is used to increase the influence of the contact resistance heat generation between the workpieces, and the influence of the heat dissipation of the electrodes is lowered. Capacitor energy storage welders use large currents and short energization times to weld workpieces with large thickness ratios.
(2) Electrodes with different contact surface diameters are used with small diameters on the side of thin parts or workpieces with good electrical conductivity and thermal conductivity to increase the current density on this side and reduce the influence of electrode heat dissipation.
(3) Use a different electrode material sheet or a conductive or thermally conductive workpiece on the side of the workpiece to use a copper alloy with poor thermal conductivity to reduce heat loss on this side.
(4) Using a process gasket to place a gasket made of a metal with poor thermal conductivity (thickness of 0.2-0.3 mm) on the side of a thin piece or a conductive or thermally conductive workpiece to reduce heat dissipation on this side. .
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