Technology, applications, and attributes
March 14, 2011
Even though metal-cored wires have been gaining popularity in recent years, there seems to be some mystery and misconceptions surrounding this filler metal. To better understand these wires it helps to consider the technology behind them and the applications for which they are best suited. Additionally, it is important to know the general characteristics of metal-cored wires, along with the ways they can provide potential improvements in the welding operation.
Every welding power source, component, and filler metal has its unique benefits, as well as certain limitations. Metal-cored wires are no different.
While certainly not a new technology, metal-cored wires have been gaining popularity in recent years, particularly in the manufacturing, fabricating, and repair industries. Still, among the general public, there seems to be some mystery and misconceptions surrounding this filler metal.
To gain a better understanding of metal-cored wires, it helps to consider the technology behind these filler metals and the applications they are best-suited for. It is also important to learn about the general characteristics of metal-cored wires, along with the potential ways they can improve welding operations.
Metal-cored wires have a different structure and composition than solid wires, which result in distinctive operating characteristics. The wires consist of a hollow metal sheath filled with metallic powders and/or alloys, including iron, that are designed to provide various characteristics such as arc stabilization and high tensile strength.
With their tubular structure, metal-cored wires carry the welding current through the outside metal sheath to the workpiece instead of through the entire cross section, as solid wires do. At equivalent amperage settings, metal-cored wires also carry higher current densities. They produce a broad, cone-shaped arc and a wide penetration profile, as well as a high burnoff rate that can help increase the deposition rate and provide a faster travel speed.
Metal-cored wires operate using the spray transfer mode of droplet transfer with shielding gas mixtures high in argon (a minimum of 75 percent argon is recommended). Using a constant-voltage (CV) power source, the wires are capable of flat, horizontal, vertical-down, and overhead welding. They also can be used for vertical-up welding with a pulsing-capable power source or with a CV power source adjusted to the short-circuit mode. Generally speaking, the wires operate at a lower deposition rate in the vertical-up position than flux-cored wires and at about the same deposition rate as solid wires in the same position.
Typically, metal-cored wires are available in diameters from 0.035 to 0.093 in.
Metal-cored wires are appropriate for welding on mild, low-alloy, and stainless steel. They work particularly well in the automotive industry for welding components such as chassis and steel wheels, primarily because of their ability to provide a wide bead profile and high travel speed.
In the manufacturing and fabrication industries, metal-cored wires are suitable for welding agricultural and heavy equipment, as well as railcars, mostly because of their ability to weld through the rust and mill scale (the fine oxide layer found on hot-rolled steels) typically encountered in these applications (see Figure 1). The wires can weld on these 0.25-in. and thicker materials, as well as the thinner materials found in the food and chemical industries. The wires, however, are not recommended for welding sheet metal.
Other applications in which metal-cored wires can be used include welding piping or other components with poor fit-up, applications prone to burn-through, and those requiring aesthetic bead appearances. They also are suitable for single- and multipass welding using a robotic or automatic welding process.
Metal-cored wires can be an alternative to submerged-arc or gas-shielded, flux-cored welding processes in some applications. They also might be suitable in applications that typically employ solid wire, such as those requiring single-pass welding for welds that are 3 in. or longer welds in the flat and horizontal positions using the spray transfer mode.
When switching from solid wires to metal-cored wires, welders often can increase the wire diameter by one size, allowing them to standardize on a single wire diameter within their facility and permitting welding on many joint sizes and material thicknesses.
Because of the way that metal-cored wires carry the welding current and burn off, they create very little spatter and slag and offer good gap bridging. Certain metal-cored wires have a chemical composition that helps minimize silicon deposits at the toes of the weld, while others are formulated to provide a specific chemistry or to increase tensile or impact strengths. Most can minimize porosity and provide good side-wall fusion to reduce instances of undercut in the final weldment and rework on defective parts.
Like other filler metals, metal-cored wires adhere to the same strict industry standards. All carry an American Welding Society (AWS) classification, and many are compliant with standards set forth by agencies such as American Bureau of Shipping (ABS), the American Petroleum Institute (API), and Det Norske Veritas (DNV).
On average, metal-cored wires cost more per pound than other filler metals, particularly solid wire. Companies typically select them because of their potential to reduce costs, especially in pre- and postweld areas.
Preweld operations are often used for grinding, sandblasting, and degreasing materials in preparation for welding. Metal-cored wires often can eliminate the need for these activities because they can weld through mill scale and rust. They also create little spatter, which can eliminate the need for antispatter and separate cleaning operations before painting. In certain cases, the elimination of such pre- or postweld operations can allow companies to reallocate labor elsewhere in the welding operation.