1. Projection welding principle
Projection welding is an efficient welding method that allows multipoint welding simultaneously. Can be used as an alternative to arc welding, brazing and biting.
This method has the advantage of high processing speed with low consumption, requiring only energy.
Compared to spot welding, projection welding differs in that it involves pre-processing convex points on the plate or using profiles and chamfers that concentrate current on the welding material as contact points during welding.
During the welding process, the pressure and current density per unit area are increased through contact with the convex points, which helps to eliminate the oxide film on the plate surface, concentrate heat, reduce drift, and decrease the center distance in spot welding. This allows multipoint convex welding to be performed in one go, improving productivity and reducing joint warping deformation.
In automotive body manufacturing, projection welding nuts (lugged nuts) are typically welded to thin plates, allowing for easy assembly, requiring only tightening of the bolts, which improves assembly efficiency.
Projection welding is a resistance welding technique where one or more raised projections are preprocessed onto the surface of a part to make contact with the surface of another part. An electrical current is then applied, causing the projections to heat up and eventually collapse, forming a welded joint.
Projection welding is a variation of spot welding.
It is mainly used for welding low carbon steel and low alloy steel stamped parts. The ideal thickness range for projection welding a plate is between 0.5 and 4 mm, while spot welding is recommended for thicknesses less than 0.25 mm.
With the growth of the automotive industry, projection welding with its high productivity has become widely used in the production of automotive components.
2. Classification of projection welding
Projection welding can be classified into several types, including single-point projection welding, multi-point projection welding, ring welding, T-shaped welding, roll projection welding and wire cross welding.
3. Three stages of collision formation
Projection welding is a resistance welding method that involves preprocessing one or more convex starting points on the surface of a part to make contact with the surface of another part. The workpieces are then pressurized and heated by electricity, causing the convex points to collapse and form a welded joint.
Collision joint formation is similar to spot welding and seam welding and can be divided into three stages: preload, electrical heating, and cooling crystallization.
1. Preloading stage.
Under the influence of electrode pressure, the bonding surface between the projection and the bottom plate expands, stabilizing the conductive area of the welding zone and breaking the oxide film on the bonding surface, resulting in a strong physical connection.
2. Turn on the heating stage.
The welding process consists of two stages: the crushing stage and the nucleation stage.
After the projection is crushed and the two plates are glued together, a large heating area is formed.
As heating progresses, the fusion of the individual contact points expands, resulting in the formation of a fusion core and a plastic zone of sufficient size.
3. Cooling crystallization stage.
The nucleation process is similar to the nucleation welding process after the nucleation current is turned off.
4. Factors affecting the quality of projection welding
Welding current: The amount of current required for each welding point in projection welding is less than the amount required for spot welding of the same point. The maximum current is considered to be the current that does not cause excessive metal extrusion under appropriate electrode pressure. The minimum current is the current that can melt the projection before it is completely crushed. The choice of welding current is mainly based on the material and thickness of the workpiece. In multipoint projection welding, the total welding current is the sum of the current required by each projection.
Electrode pressure: Electrode pressure should cause the projections to collapse when they reach welding temperature and ensure a tight fit between the two work pieces. If the electrode pressure is too high, the projections will collapse prematurely, reducing the effectiveness of projection welding and weakening the joint strength due to decreased current density. On the other hand, too little pressure can result in excessive splashing. The size of the electrode pressure affects both heat absorption and heat dissipation. The electrode pressure should be determined based on the material and thickness of the workpiece. Normally it can be calculated as 1.5 times the sum of all points, as long as the projection reduction is not more than 10% when the power is turned off.
Electrode pressure failure point: The electrode pressure failure point is between 500 and 800 N for a plate thickness of 1 mm and between 5000 and 6000 N for a plate thickness of 5 mm.
Welding On Time: This refers to the time that welding power is applied to a point. The welding activation time for projection welding is longer than that for spot welding. To reduce welding time, the welding current must be increased, but excessive current may cause overheating and spatter. For a given material and part thickness, the welding energization time should be determined based on the welding current and projection stiffness. Typically, the connection time for single point welding is between 0.5 to 2.5 seconds. For workpieces thicker than 3mm, various switching times, such as 3 to 5 times, each lasting 0.04 to 0.8 seconds, with intermittent periods of 0.06 to 0.2 seconds, can be used to prevent overheating of individual points.
Welding power: The electrical energy required to weld each point varies according to the thickness of the part. For a 1 mm thick part, the required power is between 40 to 50 KW, and for a 3 mm thick part, the required power is between 80 to 100 kW. When welding parts with the same metal, the projection must be punched into the thickest part. When welding dissimilar metals, the projection must be punched into the part with the highest conductivity to obtain thermal balance between the two parts.
5. What are the advantages and disadvantages of projection welding
Advantages of projection welding:
- Multiple welding points can be welded in one process, limited only by the controllability to adjust current and force.
- Projection welding allows the welding of narrow flanges as it has a large current concentration and limited opportunities for deviation at the welding point, with closer spacing between welding points compared to spot welding.
- The electrode contact surface in projection welding is larger than its projection and also larger than the electrode contact surface used in spot welding with the same nugget diameter, which results in less electrode maintenance due to current density reduced.
- Projection welding can be applied to metals too thick for resistance spot welding (RSW) connections.
- The size and position of the projection can be flexibly selected, allowing a thickness ratio between the projection and the workpiece of 6 (or greater) to 1. This is sometimes difficult to achieve with spot welding for workpieces. work with a thickness ratio of about 3. to 1.
- The process can be used to leak-proof joints, such as in ring projection welding.
Disadvantages of projection welding:
- Additional processes will be required unless the part can be pressed into the desired shape by forming one or more protrusions on a workpiece.
- Workpiece alignment and control of protuberance size (especially its height) must be maintained within strict tolerances to ensure uniform weld joint quality when using the same electrode to weld multiple weld joints at once.
- When performing projection welding of multiple weld joints simultaneously, the projection distribution is restricted by the current bypass path, which may not be aligned with the desired position.
