What is Flux cored arc welding or FCAW?
FCAW or flux cored arc welding is a variant of Gas metal arc welding (GMAW) having a cored wired instead of solid wire being used in MIG-MAG Welding.
Setup of the FCA weld station is the key to making quality welds. It may be possible, using a poorly set-up FCA welder, to make an acceptable weld in the flat position. The FCA welding process is often forgiving; thus, welds can often be made even when the welder is not set correctly. However, such welds will have major defects such as excessive spatter, undercut, overlap, porosity, slag inclusions, and poor weld bead contours. Setup becomes even more important for out-of-position welds. Making vertical and overhead welds can be difficult for a student welder with a properly setup system, but it becomes impossible with a system that is out of adjustment.
Learning to set up and properly adjust the FCA welding system will allow you to produce high-quality welds at a high level of productivity.
FCAW is set up and manipulated in a manner similar to that of GMAW. The results of changes in electrode extension, voltage, amperage, and torch angle are essentially the same.
Types of Flux Cored Arc Welding wires
Flux Cored Welding wires are classified into three types:
- Gas shielded Types: The first group of wires is called gas shielded flux cored wires and these are meant to be used with an external gas shielding. Generally, Pure carbon dioxide is used as shielding because of its low cost.
2. The first major variation of the gas shielded flux cored wire was the self-shielded flux-cored wire. With these wires, as the name implies, no external gas shield is used, and instead, all the required shielding of the arc and the weld pool is provided by the gases formed by the breakdown of flux ingredients in the core and the slag cover on the weld metal. A certain amount of nitrogen pick-up from the atmosphere is unavoidable and therefore denitriders or nitrogen fixers such as aluminum are added to the core ingredients.
3. Another group of tubular wires is called metal-cored electrodes. These wires combine features of flux cored and gas metal arc welding wires. The continuously fed wire is cored but does not contain fluxing ingredients. Instead, the core contains only arc stabilizing compounds, deoxidizers, and metal powders. The shielding is therefore provided only by the externally supplied shielding gas as in GMAW. Metal cored arc welding electrodes are grouped with flux-cored arc welding wires in Canadian Standard CSA W 48-01, but in the United States, these wires are considered as a variation of the gas metal arc welding process.
Advantages and Applications of the Cored Wire Processes
The cored wire processes offer a:
1. high quality weld deposit with higher deposition rate and productivity than the SMAW process.
2. Higher productivity is a result of a high duty cycle, high deposition efficiency and high travel speeds.
3. Compared to GMAW, the cored wire processes are more tolerant of small deviations in welding current, voltage, tip to work distance, etc., and therefore are more likely to provide weld deposits free from incomplete fusion flaws.
Among the three cored wire variations covered here, the self-shielded flux-cored wires are better able to tolerate air currents than the others and therefore are a more suitable candidate for fieldwork. In automatic applications, very high travel speeds are possible with self-shielded wires, leading to high productivity. However, these wires should be properly selected since some formulations are not designed for multipass welds. Metal cored electrodes produce little if any slag or oxide, similar to the GMAW process. However, the metal-cored wires provide a higher deposition rate than does GMAW, and also a wider, more rounded bead shape when argon-rich gas shielding is used.
Classification of Flux-cored arc Welding (FCAW) Electrodes
The requirements for carbon steel flux-cored arc welding wires (both self-shielded and gas-shielded) in AWS Specification A5.20. Those for low alloy steels, stainless steels, and surfacing are included in AWS Specifications A5.22 and A5.22 respectively.
The classification scheme for low alloy flux-cored arc welding wires in AWS 5.29 Specification is similar to that for carbon steel wires, the main difference being the higher weld metal tensile strength and additional letters and numbers at the end used to indicate alloying elements present in the weld metal.
Similarly, metal-cored wires for low alloy steels have the same classification scheme as low alloy steel solid wires in AWS A5.28 except that S (denoting solid wire ) is replaced by C (indicating composite or metal-cored wire). In comparison, stainless steel flux-cored arc welding wires are classified based primarily on the weld metal composition and the shielding medium used during welding.
Most commonly used FCAW wire is the E71T-1. Here is what it means for each digit.
E – This implies electrode
7 – This number gives the weld’s minimum tensile strength according to square inches. You have to add four zeros to this number to obtain the minimum tensile strength. In this case, it turns out to be 70,000 pounds per square inch. This implies that the minimum tensile strength of the filler metal is 70,000 pounds per square inch.
1 – Implies the welding positions that you can adopt with this wire. There are only two numbers that are used for this digit – zero and one. Zero means that you can only assume the horizontal and flat position with this electrode. One implies that you can use the electrode in any position.
T – Abbreviation for tubular. This is the shape that you would expect in a flux-cored wire. The wire is a tube of filler metal filled with flux. In MIG electrodes, the wire is solid. There will be an S designation in this case.
1 – States the kind of flux within the electrode.
Shielding gases for Flux-cored arc Welding (FCAW)
When required, carbon dioxide is used as the shielding gas for classification purposes. However,
Argon-Carbon Dioxide (Ar-CO2) mixtures are increasingly becoming popular as their use with rutile wires provides less spatter, smoother beads and better wetting action and puddle control for out-of- position welding. Similarly, with basic wires, less spatter and smoother beads are obtained. Fewer fumes are generated when compared with 100% CO2 shielding gas. However, weld penetration is reduced to some extent.
Since Argon (Ar) is an inert gas, it does not react with elements in the arc. Use of Ar-CO2 mixtures as a shielding gas causes less oxidation of Manganese (Mn) and Silicon (Si) present in the wire, leading to higher Mn and Si content in the weld metal. This increases the weld metal tensile strength, and may also reduce the elongation values.
Similarly, the amount of hydrogen retained in the weld metal can be larger compared with the use of CO2 gas. The wires can be designed to avoid excessive increase in weld metal strength and impairment in elongation, and therefore the manufacturer should be consulted and/or procedure qualification performed before embarking on the use of Ar-CO2 mixture with flux cored wires in fabrication. The shielding gas selected does not affect the deposition rate to any significant extent.