Spot welding is a high-speed and economical connection method, which is suitable for the manufacture of stamped and rolled sheet members that can be bonded, the joints do not require airtight, and the thickness is less than 3mm.
Spot welding is the welding of individual points on the contact surface of the joint.
Spot welding requires metal to have good plasticity.

1. Basic knowledge of spot welding
Spot welding is usually divided into two categories: double-sided spot welding and single-sided spot welding.
In double-sided spot welding, electrodes are fed from both sides of the workpiece to the weld. The typical double-sided spot welding method is the most commonly used way, when both sides of the workpiece have electrode indentation. The conductive plate with a large welding area is used as the lower electrode, which can eliminate or reduce the indentation of the workpiece below.

Often used for spot welding of decorative panels. At the same time welding two or more double-sided spot welding, the use of a transformer and the electrodes in parallel, at this time, the impedance of all current paths must be basically equal, and the surface state of each welding part, the thickness of the material, the electrode pressure must be the same, in order to ensure that the current through each welding spot is basically the same using multiple transformers double-sided spot welding. Spot welding usually uses lap joints and flanged joints, which can be composed of two or more workpieces of equal or different thickness.
When designing the spot welding structure, the reachability of the electrode must be considered, that is, the electrode must be able to easily reach the welding part of the workpiece. Factors such as edge spacing, overlap amount, point spacing, assembly clearance and solder joint strength should also be considered.
The minimum margin depends on the type of metal being welded, the thickness and the welding conditions. Smaller values are preferred for metals with high yield strength, thin parts, or strong conditions.
Point distance is the center distance between two adjacent points, and its minimum value is related to the thickness of the metal to be welded, electrical conductivity, surface cleanliness, and the diameter of the molten core.
The minimum value of the point distance is mainly to consider the shunt effect, and the point distance can be appropriately reduced when strong conditions and large electrode pressure are adopted. With thermal expansion monitoring or controllers capable of sequentially changing the current at each point, and with other devices that effectively compensate for shunt effects, point spacing can be unlimited.

The assembly gap must be as small as possible, because eliminating the gap by pressure will consume part of the electrode pressure, so that the actual welding pressure is reduced. The non-uniformity of the gap will make the welding pressure fluctuate, resulting in a significant difference in the strength of the solder joints, too large the gap will cause serious spatter, the allowable gap value depends on the workpiece stiffness and thickness, the greater the stiffness and thickness, the smaller the allowable gap, usually 0.1-2mm.

The shear strength of a single solder joint depends on the area of the molten core at the junction of the two plates. In order to ensure the joint strength, in addition to the diameter of the molten core, the welding permeability and indentation depth should also meet the requirements. The expression of the welding permeability is: η=h/ del-C ×100%. The penetration of the two plates is only allowed to be between 20-80%.

The maximum welding penetration of magnesium alloys is only allowed to 60%. Titanium alloys are allowed up to 90%. When welding workpieces of different thicknesses, the minimum welding penetration on each workpiece can be 20% of the thickness of the thin part of the joint, and the indentation depth should not exceed 15% of the thickness of the plate, if the thickness ratio of the two workpieces is greater than 2:1, or welding in inaccessible parts, and the use of flat head electrodes on one side of the workpiece, the indentation depth can be increased to 20-25%.

The strength of the spot welded joint under tensile load in the direction perpendicular to the panel is positive tensile strength. Because the sharp Angle formed between the two plates around the core can cause stress concentration, and the actual strength of the core is reduced, so the spot welding joint is generally not loaded in this way. Generally, the ratio of tensile strength and shear strength is used as the index to judge the ductility of the joint. The greater the ratio, the better the ductility of the joint.

The joint strength formed by multiple solder joints also depends on the point spacing and solder joint distribution. Small point spacing will affect the strength of the joint due to the shunt, and large point spacing will limit the number of solder joints that can be arranged. Therefore, it is necessary to take into account the point distance and the number of solder joints to obtain the maximum joint strength, and multiple rows of solder joints are best staggered rather than rectangular.
Welding to ensure the need for strength of the place welding spacing will be relatively close, but taking into account the possible welding flow of the situation, the solder joint arrangement should consider its welding unfastness, so it is not the closer the more the better, reasonable arrangement of solder joints to get better results, in general, three-layer welding solder joints than two-layer welding solder joints spacing slightly further.

The application principle of the solder joint is generally not more than 5mm, and the ratio of the thickness of the welding plate is not more than 1:3, which will cause the welding core to be not in the center of the solder joint, and even the possibility of welding through the sheet. The thickest and thinnest three-layer welding can not exceed 1:3 principle.