Gas shielding prevents oxidation while arc welding, but delivering it to the weld pool requires cumbersome hoses and tanks. This also increases the risk of wind disturbing the gas flow and reducing shielding effectiveness. Flux-cored arc welding (FCAW) is an alternative solution that works well in various applications. This blog introduces FCAW and discusses when it makes sense to use flux-cored wires.
An Overview of FCAW
In conventional MIG welding an arc is formed between the electrode and the workpiece, which melts a small region of the part being welded. The electrode is consumed in the weld where it provides filler that increases joint volume and strength.
Molten metal is very prone to oxidation and contamination, which create weld defects. Two steps are taken to prevent this:
- Shielding gas is delivered around the electrode
- A welding flux is applied to the joint region
The shielding gas helps steer and focus the arc and excludes air from the weld pool, which prevents oxidation. Flux is a compound applied to the surfaces being welded. This reacts with and cleans the metal and can also generate a gas that excludes air from the weld pool.
In FCAW the filler wire contains flux. This cleans the surfaces and may, depending on composition, generate a shielding gas. Accordingly, FCAW can be either self-shielded, where the flux provides the shielding, or it can use regular gas shielding, (which requires dual-shielded equipment). Whichever variant is adopted, the process is still arc welding, but using flux-cored wires rather than solid filler and shielding gas.
Flux-cored wires are produced in a wide range of formulations. The best results are achieved by selecting one appropriate for the metals being welded.
Comparison With Other Welding Methods
FCAW has some negative characteristics, which is why it’s not a universal solution. However, it can be very useful in situations where rapid filler deposition is needed or the shielding gas is not able to perform effectively.
The downsides of FCAW are:
- Generates slag on top of the weld that requires removal
- More spatter, so more post-weld clean-up
- More smoke and fumes generated
- High cost of flux-cored wire (relative to solid filler wire)
- Puts more heat into the weld
- High cost of dual-shielded equipment (where both gas and flux-cored wire are used)
However, the beneficial attributes, and reasons for using it, are:
- Works where air movement renders gas shielding ineffective
- Slag enables welding out of position (because it holds the weld pool in place)
- Enables high weld/filler deposition rates
- Supports welds needing deep penetration
- Avoids the need for a supply of shielding gas
Applications of FCAW
The flux-cored wire process is used when:
- Metals being joined are especially dirty or contaminated
- Welding outdoors
- It’s not possible to have the weld pool horizontal and the torch vertical
- The weld design calls for deep penetration
Accordingly, it’s used mostly on large structural projects, like bridges, buildings, and ships. Pipelines, welds in thick steel or stainless steel, and similar heavy-duty fabrications are other applications. It is not used with material thinner than 20 Ga, (0.032”).
Selection Factors
Deciding whether to use FCAW is part of developing the Weld Procedure Specification (WPS). This is a document put together by a welding engineer that tells the welder how to go about making the weld. Factors the welding engineer takes into account include:
- Where the welding will be done (indoors, outdoors, somewhere with a lot of air movement)
- Depth of weld penetration needed
- Condition of the metal being welded (if a part or structure is located outdoors it can be difficult to get it really clean)
- Weld orientation
- Type of alloy: most steels are suitable but nickel and non-ferrous alloys as well as cast iron need careful selection of flux and filler
Challenges With FCAW
Before rushing to specify FCAW it’s important to understand the challenges this welding process poses for metal fabrication shops. We can discuss these under the headings of:
- Managing heat input
- Avoiding weld defects
- Clean-up
Here’s a closer look at each, along with some solutions and precautions.
Managing heat input
FCAW tends to put more heat into the weld. This can increase distortion in the welded fabrication, cause weld cracking, and alter strength and hardness. Techniques for controlling heat input are:
- Raise the travel speed
- Reduce the amps and/or volts used
- Reduce the wire feed speed
- Use a discontinuous welding technique: stitch or make intermittent welds, or use the backstepping technique
- Reduce the number of passes
- Preheat the pieces being welded
Selecting the best method, or combination of methods, takes experience, and often some trial and error. It’s prudent to make some test welds before finalizing the WPS.
Avoiding weld defects
Reducing thermal input goes a long way toward preventing defects, but it’s also helpful to control cooling. This can reduce distortion and the risk of cracking or altering material properties. Cooling can be accelerated by quenching or using fans and slowed by covering the welded fabrication with thermal blankets.
Another major concern is porosity resulting from poor shielding, and this can be exacerbated by increasing travel speed or reducing wire feed rate, as less flux is used per unit length. Using less flux also raises the risk of weld contamination as less cleaning takes place (Note that different types of flux clean with differing levels of aggressiveness.)
Clean-up
The slag generated by the flux needs to be removed from the weld. There will also be more spatter around the weld region. When planning the welding job, and especially if estimating a time for it, allowance should be made for the additional clean-up needed.
Trust Your Welding Work to Wiley
When planning a welding job it’s important to consider what could go wrong. That’s especially true when the welding work will be done outdoors, when there may be problems with metal cleanliness, or when the weld requires particularly deep penetration.
In those circumstances, the solution is often to use flux-cored arc welding. It’s a process with some challenges, but if it results in higher-quality welds, they are worth taking on.
At Wiley Metal Fabricating we have a great deal of welding experience, including with FCAW. Plus, our welders are American Welding Society (AWS) certified. Together, this means if your fabrication project has some welding challenges, we’re more than capable of handling them. Contact us with your questions or to have your project quoted.