What is Spot Welding?
Spot welding (or resistance spot welding), is one of a few different processes of resistance welding. This welding process can be used to weld together metal sheets, and is accomplished by providing pressure and heat suing an electric current to the area of the weld.
Spot welding is an electrical welding technique that joins two metal pieces by using the combined effects of pressure and heat through the passage of an electric current. Spot welding is best reserved for sheet metals, as the concentrated current eliminates the resistance from the weld area and aids in the fusion of the two pieces being welded together.
Spot welding is accomplished by applying copper alloy electrodes to the surfaces of the sheets, while applying both electric current and pressure. Heat is produced as the electric current passes through resistance materials like low-carbon steels.
How Does Spot Welding Work?
Spot welding is an ancient form of resistance welding where two or more pieces of metal are welded together without the use of filler material.
The welding process involves applying pressure and heat to the weld area using shaped alloy copper electrodes that carry an electrical current through the weld pieces.
The material melts and the pieces are joined together by fusing a molten nugget formed from the two metal pieces. At this point, the current is turned off, the electrodes apply force during the process, and the molten nugget solidifies forming the joint.
The heat is generated by electrical current through the copper alloy electrodes that transfers to the workpiece.
Copper alloy is selected for this application due to its high thermal conductivity and low electrical resistance compared to other materials to ensure that the heat is consistently directed at the workpiece rather than the alloy copper electrodes during the process.
The amount of generated heat depends on the type of resistance and thermal conductivity of the metal and the amount of time the current is applied. The heat can be defined as showing:
Q = I2Rt
Where “Q” is heat energy; “I” is current; “R” is electrical resistance; and “t” is the time the current is applied.
Materials Appropriate for Spot Welding
Steel is one of the easiest of materials to spot weld, because it has a lower thermal conductivity and a higher electrical resistance, especially the low-carbon grades of steels. Hardened steels (Carbon equivalence > 0.4wt%) should be avoided while spot welding since carver steel can have poor fracture toughness or crack in the welds due to the form of a hard, brittle microstructure.
Although increased temperature created when spot welding galvanized steel increases the thermal load on the welds, as compared to uncoated steel, but the higher temperature required to cadmium weld make most alloy insulated/galvanized steel more difficult to weld and create a poor weld.
There are three types of copper based electrodes for spot welding: alloyed or incoperated coating with some metal such as zince, copper or tungsten. The primary metal surface portion will become contaminated causing poor pressure and resistnace with the base material.
While welding low alloy zinc coated steels, the aspect and/or process must either be constantly updated with new electrodes or must be dressed either welding or using a powedered metal cutter to remove as many contaminants.
Essentially the only materials that are commonly spot welded include stainless steel (austenitc and ferritic grades), nickel alloys, titanium etc..
Aluminum is very conductive and thermally conductive material so it has and electrical resistance almost the same as copper; when aluminum melts, it can spot weld.
However, there is a significantly higher possibility of melting completely due drop in resistance caused by the required high level of alikated current, using two to three times higher level than steel with equivalent thickness.
Also, just as with any metals , the fay of copper alloys will rapidly degrade the surface of copper electrodes after a few welds so effectively completing a stable high quality weld is difficult. This is why aluminum spot welding has only be seen in aluminium specialist applications in the industry today.
Many new technological developments are being developed for sport welders to help promote stable high quality aluminum spot welding.
Copper or its alloys can also be welded via resistance spot welding, however, due to the rediously closeness of the heat generation in the electrodes and work piece, spot welding copper cannot be efficiently accomplished with conventional copper alloys spot welding electrodes.
The answer to forming welds with copper, is to use some type of electrode which contains an alloy that has a higher electrical resistance with a significant melting temperature far higher than copper (more than 1080 °C).
Examples of slecting electrode alloys which are used for spot welding copper include but are not limited to molybdenum and tungsten.
Where is Spot Welding Used?
Spot welding is used in a wide variety of industries including automotive, aerospace, rail, white goods, metal furniture, electronics, medical building, and construction.
As spot welding is easily automated with robots and manipulation equipment, this joining process is the most common used in high volume manufacturing lines and has – in particular – primarily been used in the construction of steel cars for over 100 years.
FAQs
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.
What is the problem with spot weld?
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.
Is spot welding stronger than MIG welding?
Spot welders are also relatively easy to use and produce strong welds. MIG welders are also used in auto body shops but are less used than spot welders. MIG welders are more versatile than spot welders, but they can be more difficult to use and produce weaker welds on thin sheets of metal.
How strong is a spot weld?
A spot weld is a very strong way to fabricate two pieces of metal. However, the particular strength of a weld is determined by many factors including the size of the weld, the thickness of the material, the reduction in thickness of the weld, and the presence of defects.
What metals cannot be spot-welded?
The principal reason is that there is no accepted definition or understanding of what constitutes a satisfactory weld. A weld that may be satisfactory for one application may not be for another. Most of the common metals may be readily spot welded. Principal exceptions are silver, copper, lead and zinc.
Is spot welding expensive?
Time- and cost-efficient for higher-volume projects
A simple, automated process requiring minimal setup, spot welding is the fastest and most cost-effective option for high-volume production.