1. Projection welding principle
Projection welding is a high-efficiency welding method that can carry out multi-point welding at the same time. It can be used to replace arc welding, brazing and biting.
This method has the characteristics of high processing speed and no other consumption except power.
Compared with spot welding, the difference between projection welding and spot welding is that convex points are processed on the plate in advance, or the profile and chamfer that can concentrate the current on the weldment are used as the mutual contact parts during welding.
During welding, the pressure and current density per unit area are improved by contacting with convex points, which is conducive to crushing the oxide film on the surface of the plate, concentrating the heat, reducing the diversion, reducing the center distance of spot welding, multi-point convex welding can be carried out at one time, improving the productivity and reducing the warpage deformation of the joint.
On the vehicle body, the projection welding nuts (nuts with bumps) are generally welded on the thin plate, so that only the bolts need to be tightened during assembly, which improves the assembly efficiency.
Projection welding is a resistance welding method in which one or more bumps are pre processed on the bonding surface of one workpiece to make it contact with the surface of another workpiece, and then heated by electricity, and then collapsed to make these contact points form solder joints.
Projection welding is a deformation of spot welding.
Projection welding is mainly used to weld stamping parts of low carbon steel and low alloy steel. The most suitable thickness for projection welding of plate is 0.5 ~ 4mm, and spot welding should be used when it is less than 0.25mm.
With the development of automobile industry, projection welding with high productivity has been widely used in automobile parts manufacturing.
2. Classification of projection welding
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.
3. Three stages of bump formation
Projection welding is a resistance welding method in which one or more convex starting points are pre processed on the bonding surface of one workpiece to make it contact with the surface of another workpiece, pressurized and heated by electricity, and then collapsed to make these contact points form solder joints.
The forming process of bump joint is similar to spot welding and seam welding, which can be divided into three stages: preloading, electrifying heating and cooling crystallization.
1. Preloading stage.
Under the action of 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. Power on heating stage.
It consists of crushing process and nucleation process.
After the bump is crushed and the two plates are bonded, a large heating zone is formed.
With the progress of heating, the melting of individual contact points is gradually expanded to form a melting core and plastic zone with sufficient size.
3. Cooling crystallization stage.
The nucleation process is basically the same as the nucleation welding process after the nucleation current is cut off
4. Factors affecting projection welding quality
Welding current the current required for each welding spot during projection welding is smaller than that for spot welding the same welding spot.
The current when too much metal will not be extruded under appropriate electrode pressure is taken as the maximum current.
The current that can melt the bump before the bump is completely crushed is the minimum current.
The material and thickness of workpiece are the main basis for selecting welding current.
In multi-point projection welding, the total welding current is the sum of the current required by the projection.
Electrode pressure the electrode pressure shall make the convex points collapse when they reach the welding temperature, and make the two workpieces fit closely.
If the electrode pressure is too high, the bump will collapse prematurely and lose the function of projection welding.
At the same time, the strength of the joint will be reduced due to the decrease of current density;
Too little pressure will cause serious splashing.
The size of electrode pressure affects heat absorption and heat dissipation at the same time.
The electrode pressure shall be determined according to the material and thickness of the workpiece.
The electrode pressure can usually be calculated as 1.5 times of the sum of all points (subject to the bump reduction of no more than 10% when the power is off).
Fault point electrode pressure: 500 ~ 800N when the plate thickness is 1mm; 5000 ~ 6000N at 5mm.
Welding power on time refers to the power on time of welding a point.
The welding power on time of projection welding is longer than that of spot welding.
In order to shorten the welding power on time, the welding current should be increased, but excessive welding current will overheat the metal and cause splashing.
For a given workpiece material and thickness, the welding energization time shall be determined according to the welding current and the stiffness of the bump.
Generally, the power on time of single point welding is 0.5 ~ 2.5s.
When the thickness of the workpiece is more than 3mm, it can be powered on for many times, such as 3 ~ 5 times, each power on for 0.04 ~ 0.8s, intermittent for 0.06 ~ 0.2S, so as to prevent overheating of individual points.
Welding power the electric power required for welding each welding spot varies according to 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.
When the workpiece where the bump is located is welded with the same metal, the bump shall be punched on the thicker workpiece.
When welding different metals, the bump shall be punched on the workpiece with high conductivity.
Try to achieve the heat balance between the two workpieces.
5. What are the advantages and disadvantages of projection welding
1. Several welding spots can be welded in one process of the welder at the same time, which is only limited by the control ability of adjusting current and force.
2. Due to the large current concentration and small shunting opportunities at the welding spot, narrow flanging can be welded, and the welding spot 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 also larger than the electrode contact surface used for spot welding with the same nugget diameter.
Therefore, due to the small current density, the electrode needs less maintenance than the spot welding electrode.
4. Projection welding can be used on some metals that are too thick to use RSW connection.
5. The size and position of the bulge can be flexibly selected, so that the thickness ratio between the bulge and the welded workpiece can be 6 (or greater): 1.
It is sometimes difficult to spot weld workpieces with a thickness ratio of about 3:1.
6. The process can be used for leak proof (such as ring projection welding) joints.
1. When forming one or more protrusions on a workpiece, additional processes will be required unless the part can be pressed to the required shape.
2. When the same electrode is used to weld multiple solder joints at one time, the alignment of the workpiece and the size of the bulge (especially its height) must be controlled within strict tolerances in order to obtain uniform solder joint quality.
3. When the projection welding of multiple solder joints is carried out at the same time, the distribution of the projection is limited by the shunt path of the current, which does not necessarily coincide with the required position.