How much do you know about resistance welding ?

Resistance welding, a manufacturing process and technology for joining metals or other thermoplastic materials such as plastics by heating, is a welding process by pressure through electrodes after the parts have been joined together, and utilize resistance heat generated by the current passing through the contact surface of the joints and adjacent areas.

I. Characteristics of resistance welding

Advantage

1. When the nugget is formed, it is always surrounded by a plastic ring, the molten metal is isolated from the air, and the metallurgical process is simple.

2. Heating time is short, and heat concentration, so the heat-affected zone is small; deformation and stress are also small, not need correction and heat treatment process after welding.

3. No need for filler metals such as welding wire and rods, as well as welding materials such as oxygen, acetylene, hydrogen, etc., so the welding costs are low.

4. Simple operation, easy-to-realize mechanization and automation, and improved working conditions.

5. High productivity, no noise, and no harmful gasses. In mass production, it can be integrated into the assembly line together with other manufacturing processes. However, flash butt welding must be isolated due to the risk of spark spatter.

Disadvantage

1. There is a lack of reliable non-destructive testing methods, welding quality can only be checked by trial welding and parts destructive testing, and guaranteed by various monitoring technologies.

2. Spot and seam welded joints not only increase the weight of component, but also form angles around the weld nugget in the two plates, resulting in joints of tensile strength and shear strength are low.

3. High power, mechanization, and automation degree are higher, so the equipment cost is higher, maintenance is more difficult, and common application high-power single phase AC welder is not conducive to the balanced operation of the power grid.

II. 4 types of resistance welding methods

There are four main types of resistance welding processes: spot welding, seam welding, projection welding, and butt welding (resistance butt welding and flash butt welding).

Spot welding

Spot welding is a resistance welding process that involves welding parts into a lap joint and pressing them between two columnar electrodes, using resistance heat to melt the base metal and form a welded spot. Spot welding is mainly used for thin plate welding.

Spot welding process:

1.  Pre-pressurize to ensure good contact between parts.

2.  Discharge to form a nugget and a plastic ring.

3.  Power off forging so that the nugget continues to cool the crystallization under pressure to form dense tissue with no shrinkage hole or crack welding spot.

Seam welding

The process of seam welding is similar to spot welding. It only needs to replace the rotating disc-shaped roller electrode instead of the column electrode. Welding parts are assembled into a lap joint and placed  between the two roller electrodes. The roller electrodes pressurize the parts, rotate, and send electricity continuously or intermittently to form a continuous seam weld resistance welding process.

Seam welding is mainly used to weld structures that require regular welds and sealing. The plate thickness is generally less than 3mm.

Butt welding

Butt welding is a resistance welding process that welds together the entire contact surface.

1. Resistance butt welding

Resistance butt welding involves assembling the welding parts into butt joints so that their end faces are in close contact. Resistance heat is then used to heat the parts to the plastic state, after which the power is turned off and the top forging force is quickly applied to complete the welding process. Resistance butt welding is mainly used for simple cross-sections, diameters, or edge lengths less than 20mm, and when the parts' strength requirements are not too high.

2.Flash Butt Welding

Flash butt welding involves assembling the welding parts into butt joints, turning on the power, and gradually bringing the end face into local contact. Resistance heat is then used to heat these contact points, and under the action of high current, a flash is generated, causing the end metal to melt until the end of a certain depth range reaches a predetermined temperature. The power is then turned off, and the top forging force is quickly applied to complete the welding method.

Flash welding joint quality is better than resistance welding, and the weld mechanical properties are comparable to the base material. Before welding, there is no need to clean the pre-welded surface of the joint. Flash butt welding is commonly used for welding important parts. It can weld the same kind of metal or dissimilar metal, and it can weld 0.01mm wire or 20000mm rods and profiles.

Projection welding

Projection welding is a variant form of spot welding, where pre-treatment projections are made on a part. During projection welding, one or more nuggets can be formed at a time in the joint.

III. Introduction of resistance welder

Resistance welders are classified by welding process: spot welder, projection welder, seam welder and butt welder.

They are also classified as single-phase frequency welder, secondary rectifier welder, three-phase low frequency welder, capacitor discharge welder and inverter welder.