Tack welding is used to bond base metals together using short beads. This aligns the work pieces together in the correct position ready for the final welding.
Because the purpose of tack welding is to hold metal pieces in place so that the final weld can be completed, it is important that the tack sizes do not exceed the size of the final weld.
Ideal for precise work and for small projects, tack welds are typically placed at small intervals along the work pieces. This spacing, typically 25-30mm (approximately one inch), should be enough to hold the metal pieces in place, but small enough to remain inconspicuous in the final weld.
If the work pieces are improperly aligned, the tack welds can be easily removed so that the metals can be realigned and re-tacked. This is the same should a defect be detected before the final weld takes place, preventing any defects from arising once the final weld is completed.
Tack welding is an important pre-welding procedure that can save time, effort and material wastage in the assembly of parts.
You can use MIG (metal inert gas) welding to lay down tack welds. With the MIG process, two seconds should be long enough to form molten puddles that are strong enough to hold the two work pieces together. Once the tack welds are completed, you can lay the final welding bead.
You can find out more about tack welding in our dedicated FAQ, but next we will look at spot welding…
Spot welding (also called resistance spot welding) is a resistance welding process that is mainly used to join two or more metal sheets through the application of heat and pressure to the weld area. The process uses copper alloy electrodes that are applied with pressure to the surfaces of the metal sheets to be joined. Electric current passes through the electrodes creating heat in the resistive materials, melting the metal underneath the electrodes and forming a bond.
Although most spot welding is done using a spot welding machine that applies pressure and electric current to the metals, it is possible to spot weld with a MIG welder. This requires preparation to drill or chisel out a hole in the upper layer of material before clamping it to the lower layer and then creating a weld to completely fill the hole.
Spot welding is one of the oldest known welding techniques and you can find out more about spot welding in our dedicated FAQ.
Although some people get the two confused, tack welding and spot welding are actually quite different processes.
Tack welds are a preliminary step that is used to position and secure pieces so a final weld can be done. Spot welds, on the other hand, are used as a final permanent join.
In summary, the difference between spot welding and tack welding is as follows:
- Tack welding can be used as a preliminary process in any welding project
- Tack welding takes place before the actual final welds are completed, including any spot welding
- Tack welding is a temporary process that ensures that any parts to be welded are held securely and properly aligned, while spot welding finally joins the parts together
The uses of the two techniques further demonstrate their differences.
Tack welding is widely used for a range of applications whenever there is a requirement to temporarily hold parts of a weldment in place ready for the final weld to be completed. Tack welds are also used for handling and shipping purposes.
Spot welding, meanwhile, is widely used with low carbon steels as they show a greater resistance to electricity. In addition, low carbon steels have a lower thermal conductivity when compared to the copper electrode used to create the join. High carbon steels and aluminium alloys may form weak welds that won’t last long. Spot welding is also not ideal for small sections of base material. However, with no need for filler materials or fluxes, spot welding is widely used as a fast and easy joining process for wire mesh, sheet metal, or welded wire mesh. Best used on thinner foils and sheets, this process should be avoided with thicknesses of 6mm or more. Common applications for spot welding include joining sheet metals for automotive manufacturing, as well as for electronics, construction, aerospace, and metal furnishings.
Spot welding and tack welding are both common processes that can be used to attach metals. However, beyond this, they are very different processes, used for different applications and purposes.
The difference between a spot weld and a tack weld are that a tack weld is a temporary initial step to position work pieces and ensure the correct alignment for the final weld. Tack welds can be pulled apart and reapplied should the alignment not be correct. By contrast, spot welding create a permanent, final joins in the materials and is mostly used for sheet metals. As a result, spot welds cannot be easily removed unlike with the more temporary tack welds.
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