Tack welds are small welds that are spaced out along the workpieces. They are typically made from small beads of the same material that will be used for your final weld. The number of tack welds you need depends on the size of the materials you are joining together.
Despite being small and temporary in nature, creating tack welds is still considered a type of welding.
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. Using tack welds instead of fixtures is particularly useful for one-off jobs or low volume production work where the cost of fixtures may not be justifiable.
Tack welds not only maintain the required joint gap and orientation for the parts to be joined, but also reduce distortion in the metals during welding.
Because tack welds are small, they can easily be removed and redone if the workpieces are not correctly aligned after the first attempt.
Tack welds need to be strong enough to hold the workpieces together. Defective welds can fail if the workpieces are moved, turned or hoisted, causing the joins to tear apart with potentially harmful consequences for people and machinery.
Because tack welds are designed to support the final welding process, they should not degrade the quality of the finished weld by including defects like arc strikes, cracks, craters, hard spots or leftover slag and spatter.
Another challenge results from quenching and cooling the kinds of steels commonly used for fabricating pipes and vessels. If done too quickly, quenching and cooling can introduce defects into the base metal. This is because tack welding introduces heat to the workpiece, even though it is less than for the final weld. Rapid quenching can cause brittle and hard, crack-sensitive microstructures to form in the heat affected zone. These defects can still occur adjacent to the weld area even when the tack weld is removed by grinding.
While a high heat input may be used for a final welding process, tack welds can be created using methods such as shielded metal arc welding. This can cause crack sensitive and brittle regions to form, which may not be eliminated even by multiple high heat input passes. This cracking may increase with the final welding procedure.
As the weld metal solidifies, or when stress is placed upon the join, cracks can appear in the defective area. These cracks can be too small to be seen with the naked eye or may be hidden underneath the weld joint. Even the smallest of cracks can expand over time and eventually lead to a fracture.
In order to create an effective weld it is important to clamp the workpieces together. This can be achieved with fixtures or with tack welds. These clamping methods will stop the pieces from moving, ensuring the join is done in the correct alignment and location.
Tack welds are usually performed using the same materials and process as for the final weld, except often at a lower power level or heat input. For example, an electron beam can create tack welds ahead of a later, higher powered, final electron beam weld.
Despite being temporary, tack welding quality is still important and best practice is required to make sure the joins are good enough to serve their purpose.
As with all fusion welding processes, direction and sequence is important to control distortion in tack welds. The welds need to be capable of resisting transverse shrinkage to create adequate weld penetration.
Cracks can be avoided with post-weld heat treatments, high heat input processes, and preheating. However, these steps are not always taken with tack welds, as they are not generally seen as being as important as the final welding procedure.
Tack welding performs several important functions, including:
- Holding components in place ready for joining
- Ensuring the components are correctly aligned
- Improving the function of fixtures or eliminating the need for them entirely
- Maintaining the desired joint gap and preventing movement of the workpieces
- Helping to control distortion during final welding
- Ensuring the mechanical strength of the assembly whether hoisted, moved, or turned
These all help ensure the quality of a final weld, sometimes without the need for fixtures.
Tack welding is suitable for joining any materials that can be joined by any other joining procedure. However, the ease with which tack welding can be performed depends on factors including materials type and thickness.
Thinner materials may suffer burn-through or other aesthetically poor effects, while some materials, such as aluminium, can be difficult to tack weld due to their low melting temperature.
When to Tack Weld?
Tack welding can be used whenever you need to temporarily hold the parts of a weldment in place until the final welds are done. This can include for working on the weldment, handling or shipping purposes.
What is the Tack Weld Symbol?
There is no official tack weld symbol, but the spot weld symbol (a circle) can be used instead and placed above, below, or on the reference line to show tack welds.
What does a Tack Weld Hold?
Tack welds are used to temporarily hold two metals in place before final welding and can be used alongside or instead of other fixtures.
What is a Bridge Tack Weld?
A bridge tack weld is one that bridges the gap between bevels without penetrating the root. These can be ground away when no longer required.
What is an Ideal Tack Weld?
The ideal tack weld is one that serves its purpose and ensures the correct alignment of the workpieces, aids in preventing the metals from warping during welding, and maintains the required joint gap. This can all be achieved with the right heat, penetration and a good wire flow.
Can You Tack Weld Aluminium?
It is possible to tack weld aluminium, but a filler wire should always be used. As with welding aluminium in general, the low melting temperature makes tack welding aluminium tricky. However, it is possible if you maintain a tight arc, use a sharp electrode, and ensure there is no gap while maintaining a slight overlap.
Can You Tack Weld Stainless Steel?
Stainless steel can be tack welded but it is important to make sure the pieces are set up flush with no gap between them. Use a sharp electrode and make sure it is positioned less than 1/16” away from the joint and use a high heat for a quick burst.
Can You Tack Weld Cast Iron?
Cast iron is always difficult to weld due to the metallurgical challenges it poses. The high carbon content can cause flakes of graphite to form and cast iron must be allowed to cool slowly to avoid cracking. Welds can be made over the critical temperature for cast iron (around 1450°F), but the metal should be held at this temperature for as short a time as possible.
Can You Tack Weld Aluminium to Steel?
Welding aluminium to steel is tricky given the different metallurgies and physical properties of each material. These difficulties include thermal conductivity and melting temperatures for each metal. This can make tack welding difficult as only certain methods will work. You can find out more about welding aluminium to steel here.
Can You Tack Weld Galvanised Steel?
Galvanised steel can be tack welded but, as ever with this material, you need to be careful due to the zinc coating that becomes highly toxic when heated. Because of this, proper safety precautions should be taken, including wearing a mask, the use of a respirator, gloves and an apron. However, with the correct safety precautions and the zinc coating removed from the weld areas, galvanised steel can be welded just as you would normal steel.
Tack welding is used to temporarily hold two pieces of metal together ahead of a final welding procedure. These welds make sure the two workpieces are rigidly aligned as desired and that the joint gap and orientation are correct.
Tack welds can also reduce distortion in the metals while undergoing their final weld, reinforcing or even replacing fixtures. This is useful for low volume or one-off jobs where fixtures may not prove to be worth their cost.
Designed to be small and easily removed, tack welds still need to be strong enough to serve their purpose without compromising the base materials.