Optimizing Metal-Cored Performance
Metal-cored wire is among the more versatile and flexible welding wires available for today’s manufacturing requirements. It can be used to weld a wide variety of material thicknesses with equal success and offers the opportunity to substantially increase deposition rates and travel speeds and produce highly aesthetic bead appearances and very little to no spatter compared to solid and flux cored wires. Plus, a single wire size can be used on a wide range of material thicknesses simply by adjusting welding parameters, technique and transfer process.
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Metal-cored wire costs roughly 33 percent more than solid wire, but it also carries the potential to increase travel speeds and deposition rates by 25 percent or more. Given that labor comprises roughly 85 percent of a welding operation’s costs, the time savings can greatly outweigh the additional expenditures. However, in order for metal-cored wire to be worth the investment, you will need to chose the proper wire and adjust your welding technique to achieve optimum performance.
What to Consider
The good news is that very rarely is metal-cored wire a bad choice for applications with material thicknesses ranging from 3/16 thru 4 inches, being welded in the flat and horizontal positions. If you are experiencing a problem with your metal-cored welding operation or it has not increased welding productivity compared to solid wire, it is almost certainly a result of using incorrect welding parameters, wire model or technique.
For example, a ropy bead with poor penetration and a convex profile can typically be solved by adjusting wire feed speed, voltage, travel speed and possibly even the gas mixture. Similarly, burn through or other problems caused by excessive heat and force can also be solved by adjusting the same variables.
Finding that metal-cored “sweet spot,” where the welds look and penetrate perfectly and the travel speeds are high, however, can be a bit tricky. Yet, if there is a cardinal rule in optimizing the performance of your metal-cored welding, it is experimentation. What works for one company or welder might not work for the next. Getting the best results from your metal-cored wire will require fine tuning a number of variables to achieve the ideal combination of travel speed, deposition and weld quality. Your filler metal manufacturing representative or welding distributor can assist you with fine tuning these variables.
Wire Selection One of the most critical factors in metal-cored welding is selecting the proper wire. Because there are many varieties of wire under the same AWS classification, each with very different characteristics, choosing the right one for your operation may require a little investigation.
Available in diameters ranging from .035-in. — 3/32-in., metal-cored wire is available for carbon steel, low alloy steel and stainless steel applications. Metal-cored wire is a hollow metal tube filled with a blend of metal, mineral and chemical powders. The majority of the core is iron, and additional elements are added to produce specific results based on the materials and applications for which the wires are designed.
Welding operations that manufacture painted equipment, such as bulldozers, cranes, fork lifts or tractors, for example, may want to select a metal-cored wire that produces very little spatter, that minimizes the formation of silicon islands or brings the islands to the center of the weld bead, where they are easily removed. With labor accounting for an average of 85 percent of a welding operation’s expenses, this type of metal-cored wire can contribute significantly to a company’s bottom line by reducing the amount of time spent on cleaning weldments and other activities that do not contribute to the throughput cycle.
Likewise, operations that weld material with mill scale and other surface contaminants should look to a wire with sufficient deoxidizers, specifically manganese and silicon, to bring those contaminants to the surface of the weld so that they don’t remain trapped in the bead. Choosing a wire with a strong “driving” arc will also provide better penetration through mill scale and other contaminants.
Shielding Gas
Shielding gas plays a major role in the success of metal-cored welding. The wrong shielding gas mix can lead to insufficient penetration, burn through, excessive spatter and silicon deposits or a number of other imperfections.
Thicker materials, which often contain mill scale, rust and other surface contaminants, may require an arc with more force and drive to break through those contaminants. In such cases, argon / CO2 mixtures will provide a broader penetration profile and the arc drive necessary to weld thicker materials. Higher levels of CO2 will create more spatter, so it is generally recommended to use as low of a percentage of CO2 as necessary for the given application. (Note: the arc stabilizing and spatter reducing benefits of Argon do not become significant until it is at least 75% of the overall gas mixture ).
Like solid and gas-shielded flux-cored wire, metal-cored wire can be used with either 100 percent CO2 or an Argon/CO2 mixture with the richer argon mixtures being the most common. Metal-cored wire can also be used with a small amount of oxygen as part of an Argon/oxygen mixture. The reactivity of the oxygen with the arc intensifies its heat and accelerates the spray transfer process without producing any additional drive. The result is that an Argon/oxygen mixture produces a softer arc with high deposition rates, which allows metal-cored wire to weld very thin materials at high travel speeds and without burn through that might otherwise be caused by an Argon/CO2 mixture.
For mild steel applications, a 95 Argon/5 oxygen mixture is commonly recommended. Stainless steel applications usually use less oxygen—a 98 Argon/2 oxygen mixed gas is often used.
Joint Configuration and Material Thickness
Joint configuration will dictate a number of factors when using the metal-cored process, including wire selection, gas mixture and welding parameters. Deep V-Grooves, often required for thick sections, for example, typically require a hard-driving arc. Such an arc can be achieved through proper wire selection, 75/25 Argon/ CO2 shielding gas and proper voltage settings.
Maximizing your metal-cored welding performance also depends heavily on the application in which it is being used. Although it is often prized for its ability to achieve high deposition rates and high travel speeds with a spray transfer process on thick material, metal-cored wire can perform very well on thin material as well. When welding thin material, typically 11 gauge and thinner, a spray transfer process may introduce too much heat and penetrate too deeply into the base material, causing burn through or warping. In these instances, a short circuit or globular transfer or pulse process often provide successful results (usually at the expense of travel speed, however).
Applications that require bridging gaps and welding in a vertical or overhead position can benefit from the lower heat input and calmer weld pool created by a pulse process or standard short circuit transfer with a metal-cored wire. Other variables, such as gas mixture can also be adjusted to achieve ideal results when metal-cored welding on thin material.
Tip Recess
Because of the heat it creates, metal-cored welding usually requires the contact tip to be recessed from 1/8-in. to 1/4-in. inside the nozzle. Keeping the contact tip recessed keeps it cooler, which in turn increases consumable life and prevents the electrical conductivity from degrading.
A recessed tip also has the effect of producing a longer wire stick out, which increases the voltage passing through the wire and better facilitates the spray transfer process.
In some instances, robotic welding applications will use an extended contact tip as a way of accessing confined joints.
Wire Feed Speed Maintaining proper wire feed speeds, particularly in relation to travel speeds, is another important factor in optimizing metal-cored performance. Proper wire feed speeds depend on the material type, its thickness, electrical parameters and several other factors. Like solid and flux-cored wire, running a travel speed that is too fast for the wire feed speed can result in a ropy, convex bead with poor penetration and tie-in to the base material.
Conversely, running a wire feed speed that is too fast can cause an unstable arc with excessive spatter and a turbulent weld pool. The resultant weld bead will not spread out properly and may result in poor penetration and undercut, a condition where the weld metal is not fully bonded with the base metal. Also, consider using a semi-spray transfer to prevent excessive voltage and undercut during high-speed welding.
A Final Thought Of course, metal-cored wire will not be the perfect wire for every application. If not used to its full potential, many of the characteristic benefits of metal-cored wire — travel speed, deposition and bead appearance — can be lost. Welded to its full potential, however, metal-cored wire carries the potential to dramatically improve weld cycle times, bead appearance and overall productivity, and to considerably reduce a welding operation’s costs. Getting to that point will require consideration of a number of factors as well as a little bit of old fashioned trial and error to find what works best for your application.