Tack welds may seem insignificant and transient, but they are far more important than what most people think. Read on to find out more about the purpose that they serve and the problems that can arise with improper tack welding.
What is a tack weld? Tack welds are small and temporary welds to hold parts together for final welding. With tack welds, fixtures may not be needed for holding parts rigidly together. Tack welds ensure good welding quality by maintaining the right alignment and gap between the components that are being welded.
This article will explain the basics of tack welding and how to ensure high-quality, tack weld joints.
Why do you need tack welds?
To start welding parts together, you have to clamp them to suitable fixtures. This is a necessary step because even the slightest movement can change the gap that must be welded. The alignment can also be thrown off. This can adversely affect weld quality.
To prevent this undesirable movement, you can apply tack welds to temporarily hold the pieces together at the right alignment and location so that the final welding can be carried out without any problems.
Tack welds are useful because they eliminate the need for fixtures. This is particularly useful for one-time jobs and low-volume production where the cost of fixtures is not justified.
As you might expect, tack welds are typically small. The general rule is that small tack welds are applied at a certain distance to each other so that the edges can be held rigidly.
Another advantage of tack welding is that even if you find that the pieces are not correctly aligned, you can easily remove the tack welds and redo them correctly without too much trouble.
In most cases, tack welding is performed using the same process as that for the final weld. For instance, aluminum alloy parts that are to be joined using friction stir welding can also be tack welded with the same process that employs an appropriately sized tool.
Likewise, electron beam tack welds can be created using a reduced power setting, and then final electron beam welding can be performed at higher power.
Since tack welds are temporary, one may be misled into thinking that their quality is not too important or that best practices for welding are not required. This is not true.
Tack welding requires the same level of care as the final welding procedure. If not done right, tack welds may fail to serve their purpose.
Here are the different functions that tack welds must perform:
- Tack welds keep the pieces adequately aligned and hold them rigidly in the correct location
- May eliminate the need for fixtures
- Can reinforce the function of fixtures
- Reduces distortion that can arise from welding
- Maintains the joint gap at the right value
- Can maintain the assembly at the right orientation even if it is overturned, manipulated, moved or hoisted
Risks of defective tack welding
If the tack welds are defective, they may not be strong enough to complete their functions. If such tack welds are present in an assembly, then these temporary joints can tear if the assembly is moved or turned in any way. Parts of the assembly may fall or rollover. This can potentially harm people and property.
Tack welding should be performed in a way such that it does not degrade the final weld joint or interfere with the final welding process. It should not create weld defects like craters, arc strikes, hard spots, cracks, and leftover slag.
Several kinds of steels used to fabricate vessels and pipes can suffer defects as a result of rapid quenching and cooling. This factor must be taken into account while creating tack welds because these joints introduce heat into the base metal. Although the heat input for tack welds is less than that for final welding, it is nevertheless significant.
The right level of heat is necessary in such cases so that the possibility of developing brittle and hard microstructures is minimized.
With rapid quenching, crack-sensitive, hard, and brittle microstructures can develop in the heat-affected zone. This is potentially hazardous because even if the tack weld is removed by grinding, the region adjacent to tack welds will still have these defects.
This defective region can suffer cracks while the weld metal is solidifying or when there is stress on the assembly. Since these cracks may be located underneath the weld joint, they cannot be seen even by careful inspection. Some of these cracks may even be too small to be seen. They are hazardous because they can grow into much larger cracks over time, which may eventually fracture.
How to ensure high tack weld quality
To control the quality of tack welds, welding codes state that tack welding must be performed only fully certified welders who are aware of appropriate welding techniques. This requirement applies to all kinds of welding processes. This is required by the ASME Boiler and Pressure Vessel Code, among other standards.
Procedures for controlling distortion
For all fusion welding tasks, the direction and sequence of tack welds are crucial for controlling distortion. Not only should tack welds be strong enough to maintain the right joint gap, but they must also be capable of resisting transverse shrinkage to promote adequate welding penetration.
Several steps can be undertaken to eliminate the creation of cracks as a result of tack welds. Preventative measures include post-weld heat treatment, high heat input processes, and preheat.
Unfortunately, tack welds do not usually receive the same level of care and attention as the final welding procedure.
For the final welding process, a high heat input method may be chosen. However, tack welds may be created with shielded metal arc welding, for instance. As a result of this, crack sensitive and brittle regions may develop in the roots of these tack welds. These cracks may be left behind even if the tack welds are removed. These cracks may not be eliminated even with multiple passes with high heat input procedures. These cracks may go deeper into the base metal as a result of the final welding procedure.
American Welding Society standard for tack welding
The AWS A3.0 Standard Terms and Definitions explains tack welds as joints that hold the parts in correct alignment until the final welding is done. This definition does not imply that tack welds have a temporary nature. Nor does it imply that they must be removed after they have served their purpose.
Regardless of their time duration, they must be made properly to prevent crack formation, as described above. These seemingly unimportant welds may become the weakest point in a structure and lead to failure in applications that involve cyclic loading.
The AWS standard does not define the size or length of the tack weld. Although small intermittent weld beads may be applied along the length of the joint to be welded, it may not always be sufficient.
After creating tack welds in the weld joint, you can make passes over them during the final welding. You can also make tack welds outside the weld joint. Welds created inside the weld joint can be remelted during final welding so that they are part of the finished weld joint. Even if part of the tack welds remains within the joints, these tack welds may remain a part of the finished weld. Tack welds made outside of the weld joint may be either removed or allowed to remain in place.
The tack weld should be strong enough to resist forces to which it may be subjected. The tack welds should be appropriately sized so that they can manage the weight of the components that they are holding. Tack welds are often made when the assembly is being preheated as preparation for the final welding process.
You should create tack welds of sufficient size so that they can resist the stress and strain associated with thermal expansion, which can be quite large. Like other welds, the strength of these tack welds varies in proportion to the length and throat size. A larger throat can be used for a stronger tack weld. The longer length also increases the strength of tack welds. A combination of both may be employed according to the situation.
Contrary to popular misconception, a continuous tack weld can be applied along the length of the weld joint. This is normally done where intermittent tack joints will not suffice.
For high strength steels and heavier steel sections, preheating is necessary. In most cases, you must preheat these types of components before applying tack welds if the tack weld is to be remelted, then the geometry of the tack weld such that it facilitates remelting. Remelting is easier when the tack weld is small.
How do you tack weld with a stick welder? Leave the device set to exactly what you will normally use to weld. Do a little puddle, only enough to hold items in place. If you find those aren’t large enough to hold, do bigger ones, but you’ll need the tacks large enough to hold, yet small enough that you could still change the material to pull or push it straight back to where you need if necessary. The idea is to get the tacks little enough that you can just burn off them once you go to perform the weld.
How strong is a tack weld? The intent of a tack weld would be to maintain sections of a join in appropriate alignment briefly until the last welds are created. Even though the dimensions of tack welds aren’t specified, they’re generally between 1/2″ into 3/4″ in length, but not over 1″ in length.
Can you tack weld aluminum? You can tack weld aluminum, but you should always use a filler metal. Tack welding aluminum may be a tricky procedure because aluminum includes a rather low melting temperature and will not react well to many welding procedures.