Spot welding is a resistance welding process that joins metals together by applying pressure while supplying electric current to the weld area. It was first introduced in 1885 when Elihu Thomson accidentally fused two copper wires together in an experiment.
Spot welding has become a staple welding process in manufacturing and assembly, especially in the sheet metal fabrication sector and automotive industry. Suitability for automation plays a big part in its rise in popularity as robotic spot welding excels at speed and efficiency.
Let’s delve deeper and further understand what resistance spot welding beholds.
What is Spot Welding?
Spot welding (also known as resistance spot welding) is a resistance welding process. This welding process is used primarily for welding two or more metal sheets together by applying pressure and heat from an electric current to the weld area.
It works by contacting copper alloy electrodes to the sheet surfaces, whereby pressure and electric current are applied and heat is generated by the passage of current through resistive materials such as low-carbon steels.
The amount of heat depends on the thermal conductivity and electrical resistance of the metal as well as the amount of time the current is applied. This heat can be expressed by the equation:
Q = I2Rt
In this equation “Q” is heat energy, “I” is current, “R” is electrical resistance, and “t” is the time for which the current is applied.
How Does Spot Welding Work?
The spot-welding process can be divided into three stages: setting the workpiece, current introduction, and solidification.
#1. The setting of the workpieces.
The first stage involves setting workpieces into the welding machine in the overlapping position. Workpieces can be of the same or different material types.
Next, the electrode tip is adjusted into the welding position. In addition, the upper and lower electrode need to be aligned precisely before applying the pressure.
2. Introducing the current.
After the correct positioning of workpieces, current starts to flow through the electrodes. While the current flow from one another electrode with high power, the resistance of the workpiece creates heat (H= I2RT) and melts the welding area.
Electrodes’ melting points should be higher than those of the workpiece. The low melting point leads to the failure of the joint because it melts before the melting of the workpiece,
#3. Solidification.
The time of current flowing in the welding spot depends on the thickness and material. After sending the current for a specified time, the next stage is cooling the welding position. The electrodes remain in the workpiece and work as support until it gets fully solidified.
Here, electrodes help to lose the heat from the joint also. After creating a tight joint, electrodes are removed from the workpieces and go for another welding point.
Materials Appropriate for Spot Welding
The material’s resistivity influences heat generation. The thermal conductivity and the heat capacity influence the heat transfer. Generally, high resistance and low thermal conductivity metals are the best for spot welding.
Steels and AHSS
Low-carbon steels, stainless steels and the new generations of advanced high strength steels (AHSS) are generally good materials for spot welding due to their low thermal conductivity and high resistance.
Spot welding zinc-coated steel is possible but it needs different welding parameters and more frequent electrode tip dressing. During the welding process, the zinc coating will be alloyed with the copper electrodes.
The copper electrode degrades more rapidly due to this surface contamination which can result in poor weld quality. Slightly high weld force is used to squeeze out the melted zinc layer and a higher welding current may be necessary to compensate the weld force.
Aluminum Alloys
Aluminum alloys have lower resistance and higher thermal conductivity which means it conducts electrical energy and heat very easily. The low melting point of aluminum alloys can cause the sheet materials sticking to electrodes.
Therefore electrode cleaning or tip dressing is critical in spot welding of aluminum alloys. To weld aluminum, a very high current and short weld time are required. It also means cooling the electrodes is very important.
Copper Alloys
Copper alloys have low resistivity and high thermal conductivity, so it is difficult to generate heat even with a high welding current, and heat will quickly transfer away.
They are rather difficult to weld with resistance spot welding. Instead, copper and copper alloy are excellent materials for electrodes.
In the electrical industries, welding or brazing copper alloy is very common for electrical circuit breakers and switches. Projection welding with contact designs is one way to concentrate the heat for resistance welding.
Also, surface coating or braze material such as brass, silver alloy or nickel with less conductivity can increase resistance at weld interface thus facilitate welding of copper alloys.
For welding copper alloys, tungsten electrodes are often used to generate more heat in electrodes and then transfer to the joint interface.
Where is Spot Welding Used?
Spot welding is a common joining method used in the production and manufacture of automobiles and electric vehicles, with the body panels of one vehicle typically featuring over 1,000 spot welds.
Resistance spot welding is also common in aerospace, rail, manufacturing, electronics and construction industries, with most spot welding work carried out by robots in large industrial settings.
When completed manually, spot welding is favored by engineers as it can be used to create complex shapes with much less effort when compared with TIG or MIG welding methods.
Advantages of Spot Welding
The advantages of Spot welding are:
- Strong and durable welds.
- Rapid and efficient process.
- Minimal distortion of workpieces.
- No need for consumable filler material.
- Suitable for various metals and alloys.
- Cost-effective and automated for mass production.
- Reliable and consistent results.
- Environmentally friendly (minimal fumes or waste).
Disadvantages of Spot Welding
The limitations of Spot Welding process are:
- Alignment of the workpiece is critical.
- Spot welding deforms parts of the base metals.
- Welding thick sheets of metal is impossible.
- Joints created might not be as strong as with some other welding methods as it only creates localised joints.
- Inadequate squeeze pressure will result in a low-quality weld.
- Shunting may result from multiple joints or from other areas in the metal sheets. (Shunting refers to the diversion of the current away from the weld area.)
- The electrode’s tip may crack because of thermal cycling.
- Spatter can occur as a result of poor handling of the equipment’s parameters.
FAQs.
how does a spot welder work?
Spot Welding machines use a method called resistance spot welding to join overlapping metals between two electrodes using electrical resistance. Squeezing the workpieces between the electrodes applies pressure, and the passage of welding current through the resistive metals generates heat.
What process is spot welding?
Spot welding is a resistance welding process that joins metal surfaces together using an electrical current. Typically used to join sheet metal together, it uses the conductive properties of a metal combined with pressure to create a bond between the metal surfaces.
When to use spot welding?
Spot welding is typically used when welding particular types of sheet metal, welded wire mesh or wire mesh. Thicker stock is more difficult to spot weld because the heat flows into the surrounding metal more easily. Spot welding can be easily identified on many sheet metal goods, such as metal buckets.
what are the advantages of spot welding?
The process can be easily automated through the use of welding robots. Multiple metal sheets can be joined at the same time. No filler material is consumed, unlike some other methods (MIG welding). Cost-effective and economical as it doesn’t use inert gases or filler metals.
What is the problem with spot welding?
Common defects that can occur with spot welding include: Splattering of welded material caused by loose metal that was burned by the heat of the welder. Indentations or cracks in the metal. Asymmetrical spot weld marks.