What Is Spot Welding?- Definition, How Does It Work

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.

How Does Spot Welding Work?

A form of resistance welding, spot welding is one of the oldest welding processes whereby two or more sheets of metal are welded together without the use of any filler material.

The process involves applying pressure and heat to the weld area using shaped alloy copper electrodes which convey an electrical current through the weld pieces. The material melts, fusing the parts together at which point the current is turned off, pressure from the electrodes is maintained and the molten “nugget” solidifies to form the joint.

The welding heat is generated by the electric current, which is transferred to the workpiece through copper alloy electrodes. Copper is used for the electrodes as it has high thermal conductivity and low electrical resistance compared to most other metals, ensuring that the heat is generated preferentially in the workpieces rather than the electrodes.

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.

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.
Spot Welding

Materials Appropriate for Spot Welding

Due to its lower thermal conductivity and higher electrical resistance, steel is comparatively easy to spot weld, with low carbon steel being most suited to spot welding. However, high carbon content steels (Carbon equivalence > 0.4wt%) are prone to poor fracture toughness or cracking in the welds as they tend to form hard and brittle microstructures.

Galvanized steel (zinc-coated) requires slightly higher welding currents to weld than uncoated steel.  Also, with zinc alloys, the copper electrodes rapidly degrade the surface and lead to a loss of weld quality. When spot welding zinc coated steels, electrodes must either be frequently exchanged or the electrode tip surface should be ‘dressed’, where a cutter removes contaminated material to expose a clean copper surface and reshapes the electrode.

Other materials commonly spot-welded include stainless steels (in particular austenitic and ferritic grades), nickel alloys, and titanium.

Although aluminum has thermal conductivity and electrical resistance close to that of copper, the melting point for aluminum is lower, which means welding is possible. However, due to its low resistance, very high levels of the current need to be used when welding aluminum (in the order of two to three times higher than for steel of equivalent thickness).

In addition, aluminum degrades the surface of copper electrodes within a very small number of welds, meaning that stable high-quality welding is very hard to achieve. For this reason, only specialist applications of aluminum spot welding are currently found in the industry. Various new technology developments are emerging to help enable stable high-quality spot welding in aluminum.

Copper and its alloys can also be joined by resistance spot welding, although spot welding copper cannot be easily achieved with conventional copper alloy spot welding electrodes, as heat generation in the electrodes and workpiece are very similar.

The solution to welding copper is to use an electrode made of an alloy with high electrical resistance and a melting temperature far in excess of the melting point of copper (much greater than 1080°C). Electrode materials typically used for spot welding copper include molybdenum and tungsten.

Where is Spot Welding Used?

Spot welding has applications in a number of industries, including automotive, aerospace, rail, white goods, metal furniture, electronics, medical building, and construction.

Given the ease with which spot welding can be automated when combined with robots and manipulation systems, it is the most common joining process in high volume manufacturing lines and has in particular been the main joining process in the construction of steel cars for over 100 years.


What is Spot Welding?

Spot welding is a type of electric resistance weld that uses resistance to weld two or more metal sheets together using pressure and heat to the weld part. The spot-welding process uses two copper alloy electrodes to focus the welding current into a small area and hold the sheets together.

What is spot welding used for?

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.

Is spot welding easy?

Spot welding is among the earliest welding procedures and is very well-known and easy to do for even beginner welders. The procedure is totally defined and readily applies to a lot of thin metals like stainless steel, nickel alloys, and titanium.

How strong is a spot weld?

This weld has a shear strength of 1100 lbs. (499.5kg) less than the fully penetrated weld. For a full penetrating spot weld on a given plate thickness, the area of the interface will depend to a large extent on the shielding gas and the arc voltage.

How thick can you spot weld?

Spot welding is primarily used for joining parts that are normally up to 3 mm in thickness. The thickness of the parts to be welded should be equal or the ratio of thickness should be less than 3:1. The strength of the joint depends on the number and size of the welds. Spot-weld diameters range from 3 mm to 12.5 mm.

Do Spot Welders use AC or DC?

AC is more robust with longer life and is more familiar with many facilities. If you use press welders, power is not an issue, and working with normal quality standards AC is a good choice.

Can you spot weld with a MIG?

Using the Eastwood MIG Spot Weld Kit is the best way to achieve factory-looking spot welds at home using only your MIG welder. This kit will save you time and money allowing you to get that project done and on the road.

What are the disadvantages of spot welding?

Disadvantages of Spot Welding:

  • Only thin metals can be welded accurately and efficiently.
  • Installment cost is more.
  • This type of welding gets deformed.
  • Changes in power supply leads to weak welding.

What is a TAC welder?

Tack welds are used to hold two metal pieces in place ready for final welding, just as a tailor may use pins to hold two pieces of material together before sewing. These welds make sure the workpieces are correctly and rigidly aligned, reinforcing or even eliminating the use of fixtures.

What metals Cannot be spot welded?

Most of the common metals may be readily spot welded. Principal exceptions are silver, copper, lead, and zinc.

What is the difference between arc welding and spot welding?

Arc welders use sparks to melt the base metal and a filler rod, while spot welders use the electric resistance of the base metal to heat and fuse pieces. Arc welders penetrate deeply, while spot welders are best used on thin sheets of metal.

How hot does a spot welder get?

Welding current and time are used to bring the metal to welding temperature (2550 Degree F.).

How do you spot weld with a stick welder?

Can you spot weld aluminum to steel?

Aluminum alloys can be joined to steels relatively easily using techniques such as adhesive bonding, mechanical fasteners, or brazing, but when superior structural integrity is required, welding is preferred. However, welding aluminum alloys to steel are difficult.

What is the thickest metal you can spot weld?

These materials require high electric current levels to weld and a longer welding time, unlike welding steel. The other factor is the inappropriate thickness of the welding materials. Spot welding requires using materials with a thickness close to 3 mm.

How many volts are needed to spot weld?

The open-circuit voltage from the transformer is higher than this, typically in the 5-to-22-volt range. The resistance of the weld spot changes as it flows and liquefies. Modern welding equipment can monitor and adjust the weld in real-time to ensure a consistent weld.

Which type of current is used in spot welding?

Spot welding is widely used in the automotive industry for the efficient joining of metal sheets (35). Classified under resistance welding, spot welding acts by generating heat with the use of a high current, approximately about 1000 A–100 000 A.

Why AC current is used in spot welding?

Alternating current is found to be most suitable for resistance welding as it can provide any desired combination of current and voltage by means of a suitable transformer. In order to avoid surface distortion, the portion of the metal adjacent to the weld or joint should not be allowed to be overheated.

What is the difference between a plug weld and a spot weld?

A true spot weld is when two panels are pinched together and joined through an electrical current to fuse them together. A rosette or plug weld is when you drill a hole in the top layer, pinch the two panels together, and fill the hole; joining the two panels together.

How do I get a good spot weld?

What are two practical advantages of spot welding?

Spot welding is quick and easy. There is no need to use any fluxes or filler metal to create a join by spot welding, and there is no dangerous open flame. Spot welding can be performed without any special skill. Automated machines can spot weld in factories to speed up production.

What is slot weld?

A slot weld joins the surface of a piece of material to another piece through an elongated hole. The hole can be open at one end and can be partially or completely filled with weld material.

What is lap welding?

Lap welding joints are essentially a modified version of the butt joint. They are formed when two pieces of metal are placed in an overlapping pattern on top of each other. They are most commonly used to join two pieces with differing thicknesses together. Welds can be made on one or both sides.

Can I use an arc welder as a spot welder?

Base metals of various sizes can be joined using arc welding. Spot welding can be used only for welding sheet metals having thicknesses less than or nearly 3mm. The joining of metals using arc welding is a time-consuming process. Resistance spot welding can be done within minutes.

How do you show spot welding on a drawing?

The spot weld symbol is simply a circle that may be placed above, below, or centered on the reference line. When the symbol is centered on the reference line this indicates that there is no side significance.

How many types of spot welding are there?

The types are 1. ‘Pedestal Type’ Machine 2. Resistance Butt or Upset Welding 3. Flash-Butt Weld

What is the difference between MIG and spot welding?

MIG welding uses a separate piece of filler metal that is fed into the weld. This filler is melted by the electric arc, which then joins with the base metal to create the weld. Spot welding uses the material itself as the filler metal. The two pieces of metal are heated up and then pressed together to create the weld.

How thick of metal can a spot welder weld?

Almost all sport welders use copper allow electrodes because of the copper’s natural electrical conductivity. This process can be used on most types of metal, and the only real limitation is the thickness of the area where you need to do the welding, as it can be anywhere between 0.02 inches to 0.118 inches.