Weld flux is a welding agent that prevents the weld from interacting with the surrounding medium (like air). The reason why it is so important is that the base and filler material can interact with the atmosphere and cause the formation of oxides or other unwanted compounds.
Why flux is so important in welding?
During a welding process, the base metal and the filler undergo significant temperature changes in a very short amount of time. The heated metal may interact with the surrounding air and cause oxidation, which creates an oxide layer on the weld, reducing the weld strength.
And, it is not just oxygen that can create infective welds, the formation of sulfides and nitrides can also hurt the weld's strength.
As gases like nitrogen are present in abundance in the atmosphere, metals have a very high chance of getting affected with inclusions. The oxide layer takes away the corrosion resistance of the metal.
It also affects the strength of the weld. Hence, technicians and engineers look for ways to ensure that oxygen never reaches the weld seam during the welding process.
The main condition for a flux to work is that it should be inert to the metals being joined. In other words, no reaction should occur between the flux and the metals.
Hence, the selection of flux material depends on the metals used. In addition to preventing the formation of oxides, weld flux also:
- Creates a protective slag over the molten metal
- Removes impurities from the motel metal
- Reduces splatter
- Prevents hardening by slowing down cooling time etc.
Fluxes find applications in Shielded Metal Arc Welding (SMAW), Flux-Cored Arc Welding (FCAW), and Submerged Arc Welding (SAW).
Types of electrode flux
For welding, flux is not used as a separate application. They are almost always present with the electrode. Flux is coated on the electrode with a thickness of 1mm to 3mm.
Some electrodes use flux within them in a hollow cavity. In this case, the electrode covers the flux.
In the arc weld landscape, flux-cored electrodes are broadly classified into four different types based on their properties.
1. Rutile electrode
Rutile electrode coating is made from titanium oxide. They offer excellent arc control and slag control to the welder. Due to these properties, Rutile electrode coating is often known as the most welder-friendly flux types.
The amount of fumes produced from the electrode is also generally low for the Rutile electrode. Rutile electrode flux is the preferred choice for welding out-of-position welds.
2. Basic flux
Basic flux is made from calcium carbonate, calcium fluoride, magnesium carbonate, and a few other shielding compounds. The benefit fo using basic flux is that it results in better mechanical properties and low hydrogen diffusion levels.
Basic Flux is the most preferred for high strength steels. However, basic flux is much less forgiving when it comes to operational stability and out-of-place welds.
The arc is also less predictable when the basic flux is used.
3. Cellulose electrode coating
Cellulose electrode coating uses a mixture of cellulose and other organic compounds. When cellulose undergoes high temperatures in welding, it decomposes to produce carbon monoxide and hydrogen.
The production of these two gases gives the weld shielding from the atmosphere. They also provide much better penetration in welds.
However, the high rate of hydrogen production may not be suitable for welding metals that exhibit hydrogen inclusion properties.
4. Iron oxide coating
The iron oxide coating is a mixture of metallic oxides of iron, manganese, and silica. Once they are under heat, they produce a molten acidic slag.
Due to the high oxygen generation, iron oxide coating is not suitable for welding metals that easily undergo oxygen inclusion. One way to prevent oxidation of the weld is by adding deoxidizing agents with the welding core.
The difference between MIG welding and Flux welding
MIG welding or Metal Inert Gas welding is a welding process where the electrode is fed into the weld using an electrode gun. When compared with flux welding, MIG welding does not need a flux coated electrode as it uses shield gas to protect the weld pool from outside interference.
But the differences don't end there.
MIG welding equipment is generally costlier than an arc welding setup. Hence, the initial capital for MIG welding is more than arc welding.
When compared with MIG, The equipment for flux-cored arc welding is easy to transport. There are fewer components to move around as there isn’t any system required to supply a jet of air to the weld.
Ease of use
MIG welding is much easier for a beginner when compared with flux welding. The ease of use arises from the fact that the welder needs to only handle one component at a time. Flux welding often requires more skilled welders.
Indoor or Outdoor
MIG welding is not feasible outside as external factors like wind or rain can significantly reduce the weld strength. For this reason, they are mostly reserved for indoor use.
Flux welding has no such limitations. They remain operational even in less favorable conditions.
Thickness of metal
MIG welding is useful for welding metals of thin to medium thickness metals. Flux welding is effective for thicker metals thanks to its deep penetrating qualities.
Argon is one of the most popular MIG shielding gas used in the industry thanks to its affordable pricing and inert chemical characteristics.
Welding is not just the process of joining metals. A perfect weld is a result of many conscious decisions made by the welder after factoring in all the specifics involved.
Flux is an inevitable part of arc welding and choosing one carries a great deal of impact on the final quality of the weld. Welders go through detailed training to familiarize themselves with the different weld practices and the components used for each welding method.
Flux core welding may very well be one of the oldest arc welding methods. Still, it is one of the most effective and widely used metal joining methods.