Development of FCAW

To discuss the development of Flux Cored Arc Welding it can be best to review some history of both the Gas Metal and Shielded Metal Arc Welding processes. The 1920s saw the first use of a mechanically fed continuous wire electrode, the beginnings of the Gas Metal Arc Welding process (GMAW). This same time period saw the development of coated stick (SMAW) welding electrodes. Coated stick electrodes found rapid acceptance across a wide range of industries due to their versatility, portability, and quality weld metal.

For a time, interest in the use of Gas Metal Arc Welding was overshadowed by the widespread use of coated stick electrodes. However, by the late 1940s, GMAW had become widely applied. There was a growing incentive to find a way to combine the benefits of both processes: GMAW with its continuous wire electrode and operator appeal and SMAW with the cleaning and metallurgical benefits of its flux coating.

At first, there were efforts to coat continuous electrodes, but any attempt to roll the coated wire electrode into a coil cracked and ruined the flux. Then, at an American Welding Society exposition in May of 1954, the FCAW process debuted to the public (Lincoln, 1994). A continuous wire electrode consisting of a tubular metal sheath filled with powdered flux had succeeded in marrying the productive benefits of GMAW with the metallurgical benefits of SMAW.

The earliest FCAW electrodes required a supplemental shielding gas in the form of carbon dioxide, which remains one of the most popular shielding gasses for the process to this day. Then in 1956, the Lincoln Electric company debuted its Innershield® process, the first FCAW electrodes to be completely self-shielded, with no external gasses required. This has led to FCAW being split into two sub-processes, each with its own special considerations, FCAW-G or gas shielded FCAW, and FCAW-S or self shielded FCAW. FCAW-G, often referred to by the Lincoln Electric trade name Dualshield®, is still frequently used in indoor shop welding environments. While FCAW-S has gained widespread use in outdoor field-welding environments.

Basics of the Process

Flux Cored Arc Welding is a high production process which is widely employed to weld a variety of ferrous materials, particularly in industries where the materials being joined are heavy-weight and thick. As with other arc welding processes, the heat of an electrical arc is used to melt and fuse two pieces of base metal with the addition of weld filler metal provided by a consumable welding electrode.

The most distinguishing characteristic of the FCAW process is the electrode from which it gets its name. The electrode is hollow (referred to as a tubular electrode), with its exterior consisting of a metal sheath and its interior filled with powdered flux. The flux elements in FCAW serve basically the same functions as in SMAW: deoxidizing and denitrifying, shielding, alloying, arc stabilization, and slag formation. Unlike SMAW electrodes, FCAW electrodes come in the form of long continuous coils, ranging in size from just a few pounds to large five-hundred-pound coils packaged in drums. The electrodes are available in a range of diameters from as small as 0.035” up to ⅛”. The larger diameter wires can deposit a large amount of weld metal in a short amount of time, while smaller diameter wire is more suitable for thinner materials and vertical or overhead welding.

The benefits of continuous wire electrodes, particularly in high-production environments, cannot be understated. The development of coated stick electrodes and SMAW led to great strides in weld quality. However, the slow welding speed, downtime to change electrodes, and ‘stub loss’ gave rise to a motivation to combine the advantages of SMAW with those of a continuous mechanically fed electrode. FCAW is capable of producing welds with comparable or superior quality to those of the SMAW process in much shorter times and with less welder training.

These advantages are further buttressed by the fact that FCAW is considered a semi-automatic process. That is, the electrode is fed into the weld pool mechanically by a wire feeding unit, less operator skill is required as compared to SMAW, where the electrode is fed into the weld pool manually. The mechanization of feeding the electrode also results in much faster travel speeds along the joint and much higher weld metal deposition rates as compared to SMAW. All other aspects of welding, electrode angle, electrode manipulation, and travel speed are still under the direct control of the welder. Additionally, in manual processes, like SMAW and GTAW, the welder is in direct control of the arc length; in FCAW and GMAW, the arc length is largely determined by settings at the power source and wire-feeder. However, the welder must still maintain a constant electrical stickout. electrical stickout is the length of wire electrode extending out of the tip of the welding gun. The further the gun is held from the work during welding, the longer the stickout. Electrical stickout greatly impacts shielding, welding current, and arc stability. Though FCAW is most commonly employed in this semi-automatic fashion, the process also lends itself well to mechanization or full automation.

That the electrode is continuous and mechanically fed is not the only characteristic that gives it advantages over other processes. Gas Metal Arc Welding also employs a continuous, mechanically-fed wire electrode, but unlike FCAW, the electrode is solid, uncoated, and totally reliant on a shielding gas to protect the weld puddle from the atmosphere. This reliance on shielding gas makes GMAW unsuitable for outdoor use. The shielding provided by the flux in FCAW electrodes has seen the process gain wide outdoor use across a number of industries, particularly FCAW-S. Beyond providing shielding, the flux also contains deoxidizers and denitrifiers, which serve to soak up contaminants that would otherwise lead to weld defects. This reduces the amount of cleaning required on the base metal prior to welding. Alloying elements are also present in the flux, improving weld quality and allowing the electrode to be tailored to weld a variety of materials. The flux also produces a slag coating, which solidifies atop the weld metal; this layer of slag slows the cooling rate of the solidifying weld metal and protects it from atmospheric contamination as it cools. Other elements in the flux aid in arc starting and serve to stabilize the welding arc, improving weld quality and increasing operator appeal.

Though Flux Cored Arc Welding boasts many great advantages, the process is not without limitations. FCAW is a somewhat dirty process producing large amounts of welding fumes, especially for self-shielded electrodes. Not only can these fumes cause visibility issues, obscuring the welder’s view of the puddle, but they can become a health concern if proper care is not taken to ventilate the welding environment properly.

The layer of slag produced by the process also introduces a few special considerations. Firstly, the slag must be cleaned between each successive pass of a multiple pass weld; even single pass welds must have their slag removed to permit any visual inspection of the weld; this cleaning requirement is also somewhat limiting to the process’s ability to be fully automated. Secondly, as with any welding process that produces slag, the possibility of slag inclusions is introduced. A slag inclusion is a cavity within the weld caused by the metal solidifying around a pocket of slag. Slag inclusions are discussed in greater depth in a later chapter.

Finally, though FCAW is employed to weld a wide variety of ferrous materials, including cast iron and stainless steel, it is mostly limited to these metals. Commonly welded non-ferrous metals like aluminum, titanium, copper, and copper alloys (brass, bronze, etc.) must be joined by another process. Nickel-based alloys, such as Hastelloy and Inconel™, are the exception to FCAW’s general limitation to ferrous metals.

Uses of FCAW in Industry Today

As mentioned throughout this chapter, Flux Cored Arc Welding has found use in various industries. Though FCAW is limited to ferrous metals, these metals, steel in particular, make up the majority of materials joined by arc welding in our present day. Fast travel speeds, high weld metal deposition rates, and suitability for outdoor welding environments have made Flux Core extremely popular in structural fabrication and construction. FCAW is also used in the maritime, railroad, automotive, mining, and heavy equipment manufacturing industries. These applications would typically be considered ‘heavy industry’ and share a need for quality welds produced quickly and at an economical cost.

Another characteristic that ties these industries together is that they all employ a variety of steels. Later in this book, we will learn that the alloy content, the carbon content in particular, is an important determinant of how difficult a steel is to weld. FCAW can be successfully employed to weld a wide variety of carbon and low alloy steels. Hardened and abrasion-resistant steels may also be readily welded with Flux Core, provided the proper electrode and welding techniques are used. There are also FCAW electrodes for welding stainless steel and nickel-alloys.

Attributions

1. Figure 9.1: Roy Bearden performs flux cored arc welding on an M1 tank by Mark Cleghorn in the Public Domain; United States government work
2. Figure 9.2: FCAW Weld Pool Diagram by Nicholas Malara, for WA Open ProfTech, © SBCTC, CC BY 4.0