Projection Welding Principle
Projection welding is a high-efficiency welding method capable of simultaneous multi-point welding, which can be used to replace arc welding, brazing and clinching. The processing speed of this method is fast, and there is no other consumption except electricity, which is a great feature. Compared with spot welding, the difference between projection welding and spot welding is that bumps are processed on the plate in advance, or the profile and chamfer on the weldment that can concentrate the current are used as the mutual contact parts during welding.
Contact by bumps during welding increases the pressure and current density per unit area, which is conducive to crushing the oxide film on the surface of the board, concentrating heat, reducing shunting, and reducing the center distance of spot welding, and multiple points can be performed at one time. Projection welding increases productivity and reduces joint warpage. On the body, the projection welding nut (nut with bumps) is generally welded on the thin plate, so that only the bolts need to be tightened during assembly, which improves the assembly work efficiency.
Projection welding is a resistance welding method that pre-processes one or more bumps on the bonding surface of a workpiece, makes it in contact with the surface of another workpiece and is heated by electricity, and then collapses to form a solder joint at these contact points.
Projection welding is a variant of spot welding. Projection welding is mainly used for welding stamping parts of low carbon steel and low alloy steel. The most suitable thickness of plate projection welding is 0.5~4mm and less than 0.25mm, and spot welding should be used. With the development of the automobile industry, high-productivity projection welding has been widely used in the manufacture of auto parts.
Projection welding classification
Projection welding is divided into single-point projection welding, multi-point projection welding, ring welding, T-shaped welding, roll projection welding, and wire cross welding.
The three stages of bump formation
Projection welding is to pre-process one or more raised points on the bonding surface of a workpiece, make it in contact with the surface of another workpiece, pressurize it and heat it with electricity, and then collapse it, so that these contact points form resistance welding of solder joints. method.
The formation process of bump joints is similar to spot welding and seam welding, and can be divided into three stages: pre-pressing, electrified heating and cooling, and crystallization.
1. Preloading stage. Under the action of the electrode pressure, the bonding surface between the bump and the lower plate increases, which stabilizes the conductive path area of the welding area, destroys the oxide film on the bonding surface and forms a good physical contact.
2. Electric heating stage. It consists of a crushing process and a nucleation process. After the bumps are crushed and the two plates are laminated, a larger heating zone is formed. As the heating progresses, the melting of individual contact points gradually expands to form a melting core and a plastic zone of sufficient size.
3. Cooling crystallization stage. After cutting off the welding current, the nugget begins to crystallize under the action of pressure, and the process is basically the same as the crystallization process of the spot welding nugget.
Factors Affecting Projection Welding Quality
Welding current
The current required for each solder joint during projection welding is smaller than that for spot welding the same solder joint, and the current that does not extrude too much metal under the appropriate electrode pressure is taken as the maximum current.
The current that can melt the bump before the bump is completely crushed is taken as the minimum current. The material and thickness of the workpiece are the main basis for selecting the welding current. In multi-point projection welding, the total welding current is the sum of the current required for the bump.
Electrode pressure
The electrode pressure should make the bumps all collapse when they reach the welding temperature, and make the two workpieces closely fit. Excessive electrode pressure will prematurely crush the bumps. The effect of projection welding is lost, and the joint strength is reduced due to the reduction of the current density; if the pressure is too small, it will cause serious splashing.
The magnitude of the electrode pressure affects both heat absorption and heat dissipation. The size of the electrode pressure should be determined according to the material and thickness of the workpiece.
The electrode pressure can usually be calculated as 1.5 times the sum of each point (the bump is pressed down by no more than 10% when the power is not turned on).
Sui point electrode pressure: when the thickness of the plate is 1mm, it is 500-800N; when the thickness is 5mm, it is 5000-6000N.
Welding power-on time
Welding power-on time refers to the power-on time of welding a point, and the welding power-on time of projection welding is longer than that of spot welding. If you want to shorten the welding power-on time, the welding current should be increased, but excessive welding current will cause the metal to overheat and cause splashing. For a given workpiece material and thickness, the welding energization time should be determined based on the welding current and the stiffness of the bump.
Usually, the power-on time of single-point welding is 0.5 to 2.5s. When the thickness of the workpiece is more than 3mm, it can be energized for many times, such as 3 to 5 times, each energization is 0.04 to 0.8s, and the interval is 0.06 to 0.2s to prevent overheating of individual points.
Welding power
The electric power required for welding each solder joint varies depending on the thickness. Generally, the thickness of the workpiece is 1mm, and the power is 40-50kW; the thickness of the workpiece is 3mm, and the power is 80-100kW.
When welding the same kind of metal to the workpiece where the bump is located, the bump should be punched on the thicker workpiece, and when welding dissimilar metals, the bump should be punched on the workpiece with higher conductivity. Try to achieve thermal balance between the two workpieces.
What are the advantages and disadvantages of choosing projection welding?
advantage
1. Several welding points can be welded in one process of the welding machine at the same time, which is only limited by the control ability to adjust the current and force.
2. Due to the large current concentration and small shunt opportunity at the solder joints, narrower flanges can be welded, and the solder joint spacing of projection welding is closer than that of spot welding.
3. The electrode contact surface used in projection welding is larger than its protrusion, and it is also larger than the spot welding electrode contact surface used for the diameter of the welding nugget. Therefore, its electrodes also require less maintenance than spot welding electrodes due to the lower current density.
4. Projection welding can be used on metals that are too thick to use RSW connections.
5. Flexible selection of bump size and position, so that the thickness ratio to the welded workpiece can be 6 (or greater): 1. Workpieces with a thickness ratio greater than about 3:1 are sometimes difficult to spot weld.
6. Its process can be used for leak-proof joints (such as annular projection welding).
Shortcoming
1. When forming one or more protrusions on a workpiece, additional steps will be required unless the part can be pressed into the desired shape.
2. When using the same electrode to weld multiple solder joints at a time, the alignment of the workpiece and the size of the bump (especially its height) must be controlled within strict tolerances to obtain uniform solder joint quality.
3. When the projection welding of multiple solder joints is performed at the same time, the distribution of the projections is limited by the shunt path of the current, which does not necessarily match the required position.
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