What Is Projection Welding?

Projection welding is a high-efficiency welding method capable of simultaneous multi-point welding, which can be used instead of arc welding, brazing and bite welding. This method has fast processing speed and no consumption other than electricity, which is a great feature. Compared with spot welding, the difference between projection welding and spot welding is that the bumps are processed in advance on the plate, or the profile and chamfer on the welding part that can concentrate the current are used as the mutual contact parts during welding. Relying on bumps during welding increases the pressure and current density per unit area, which is beneficial for crushing the oxide film on the surface of the board, concentrating the heat, reducing the shunt, and reducing the center distance of spot welding. Projection welding improves productivity and reduces warpage of joints. On the vehicle body, a convex welding nut (a nut with a raised point) is generally welded to a thin plate, so that only the bolts need to be tightened during assembly, which improves assembly work efficiency. ^[1]

Projection welding is a resistance welding method in which one or more bumps are pre-machined on the bonding surface of a workpiece to make contact with the surface of another workpiece, heat it with electricity, and then collapse to form these contact points. Projection welding is a variant of spot welding. Projection welding is mainly used for welding low-carbon steel and low-alloy steel stamping parts. The most suitable thickness for plate projection welding is 0.5 ~ 4mm and less than 0.25mm. Spot welding should be used. With the development of the automotive industry, high productivity projection welding has been widely used in the manufacture of automotive parts. Projection welding

Projection welding is a resistance welding in which one or more raised points are pre-machined on the bonding surface of a workpiece to make it contact with the surface of another workpiece, pressurize and heat it, and then collapse, so that these contact points form 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, electric heating and cooling crystallization.

Pre-pressing stage. Under the pressure of the electrode, the bonding surface of the bump and the lower plate increases, which stabilizes the conductive path area of the soldering zone, destroys the oxide film on the bonding surface, and forms a good physical contact.

Power on heating stage. Composed of crushing process and nucleation process. After the bumps are crushed and the two plates are bonded, a larger heating area is formed. As the heating progresses, the melting of individual contact points gradually expands to form a molten core and plastic area of sufficient size.

cooling crystallization stage. After the welding current is cut off, the nugget begins to crystallize under pressure. The process is basically the same as that of spot welding nugget. ^[2]

(1) Welding current

The current required for each welding point during projection welding is smaller than that for the same welding point during spot welding. The maximum current is the current when no excessive metal is squeezed out under the proper electrode pressure. 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 currents required by the bumps.

(2) electrode pressure

The electrode pressure should make the bumps completely crush when the welding temperature reaches the welding temperature, and make the two pieces fit tightly. Excessive electrode pressure crushes the bumps prematurely. Loss the role of projection welding, while reducing the strength of the joint due to the reduction of the current density; too small pressure will cause severe splashing. The magnitude of 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 the points (according to the bump pressure not exceeding 10% when no power is applied).

Point electrode pressure: 500 ~ 800N when the thickness of the plate is 1mm; 5000 ~ 6000N when the thickness is 5mm.

(3) Welding time

The welding energization time refers to the energization time for welding a spot. The welding energization time for projection welding is longer than that for spot welding. If you want to shorten the welding current, you should increase the welding current, but excessive welding current will overheat the metal 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 bumps. Single spot welding

The power-on time is 0.5 to 2.5s. When the thickness of the workpiece is> 3mm, it can be energized multiple times, such as 3 to 5 times, each time it is energized 0.04 to 0.8s, and the interval is 0.06 to 0.2s to prevent individual points from overheating.

(4) Welding power

The electric power required for welding each welding point varies depending on the thickness. Generally, the workpiece is 1mm thick and the power is 40-50kW; the workpiece is 3mm thick and the power is 80-100kW.

(5) When the workpiece where the bump is located is welded with the same metal, the bump should be punched on a thicker workpiece, and when welding a different metal, the bump should be punched on a workpiece with higher conductivity. Try to achieve a thermal balance between the two workpieces. ^[3]

Projection welding is used in applications with a high thickness ratio (such as welding nuts to thin plates); and in applications where the appearance of the thin plate must be controlled (such as the surface of some appliances). The main application of projection welding is to connect some stamping

The main advantages of projection welding are:

1) Welding of several joints can be performed in one process of the welding machine at the same time, it is only limited by the ability to adjust the current and force.

2) Due to the larger current concentration and smaller shunting opportunities at the solder joints, narrower flanges can be welded, and the interval between the solder joints of the projection welds is closer than that of the spot welds.

3) The electrode contact surface used in projection welding is larger than its protrusion, and also larger than the electrode contact surface used for spot welding with a diameter equivalent to that of the nugget. Therefore, due to the lower current density, its electrodes require less maintenance than spot welding electrodes.

4) Projection welding can be used on some metals that are too thick to use RSW connection.

5) Flexible selection of the size and position of the protrusion, so that the thickness ratio to the welded workpiece can be 6 (or greater): 1. Spot welding of workpieces with thickness ratios greater than about 3: 1 is sometimes difficult.

6) The process can be used for leak-proof joints (such as circular projection welding).

Disadvantages of projection welding include:

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 the same electrode is used to weld multiple joints at one time, the alignment of the workpiece and the size of the protrusions (especially its height) must be controlled within strict tolerances in order to obtain uniform joint quality.

3) When convex welding with multiple joints is performed at the same time, the distribution of the protrusions is limited by the current shunt path, which does not necessarily match the required position. ^[4]