(As seen in The Fabricator, May 2005)
Part I: Consumables for MIG/GMAW
Editor's Note: This article, which covers consumables for gas metal arc welding, is the first installation of a two-part article. Part II will discuss gas tungsten arc welding consumables.
Many factors influence gas metal arc welding (GMAW) consumable selection. Chief among the factors is productivity. Because 85 percent of the cost of making a weld is for labor, you need to choose consumables based on how well they improve welder productivity and minimize downtime. To get the longest life out of GMAW consumables, you need to know what to look for when making a decision. Three important consumables are contact tips, nozzles, and liners.
|Using tapered, nonthreaded contact tips, nozzles and diffusers can help welders fit into spaces that have limited accessibility.
GMAW employs continuously fed wire, and to transmit current to this wire, the welding gun must make electrical contact with the wire. This electrical contact occurs through a contact tip that the wire feeds through.
The contact tip's size corresponds to the wire diameter. For example, a 0.035-inch contact tip corresponds to an 0.035-in. wire. However, this is not the only consideration. The contact tip size you choose also depends on the application. Industrial applications require large contact tips, which have more mass and help to keep tip temperatures lower than small tips do.
Tapered and nontapered contact tips are available. Tapered tips typically are long and used with tapered nozzles in applications with restricted access and limited space, such as pipeline welding. Non-tapered tips have more mass in the front and tend to hold up to heat better and last longer.
Contact tips also come in threaded and nonthreaded varieties. Some styles of contact tips tend to seize the diffuser, making them difficult to remove and replace. Seizing is less likely to occur with nonthreaded contact tips. Nonthreaded designs typically have more area and conduct heat and electricity better than threaded contact tips, and often take less time to change after a burnback.
Threaded tips have traditionally been the most common-welders are accustomed to using them, and removal and installation is intuitive.
Because non-threaded contact tips do not require tools to change, they are quicker to replace after burnback. Some fabrication shops use tips that have just four settings, such as flush with the end of the nozzle, 1/8-inch recess, 1/4-in. recess, or 1/8 in. stick out. Fewer choices improve consistency among welders.
When contact tips start to wear, an oval hole can start to develop. This phenomenon, keyholing, can lead to irregularities in the arc from poor electrical pickup. Irregularities in the arc can increase spatter, which in turn leads to more postweld grinding. This is an inefficient use of a welder's labor-every additional minute of grinding time is one less minute of valuable production time.
The nozzle directs the shielding gas to the weld. Like contact tips, nozzles are either threaded or nonthreaded and come in many different shapes and sizes for various applications. Nonthreaded nozzles are easier to change than threaded nozzles, but because they have no threads, they don't fit as tightly as threaded nozzles do. Threaded nozzles are more secure, yet many require more effort to change.
In applications in which welds are visible on the finished product, look for consumables that require the least post-weld cleanup. For example, components that improve arc starts and thereby reduce spatter result in less cleaning.
Threaded nozzles generally stay on the gun better and offer better tip-nozzle concentricity. Slip-on nozzles generally are a bit quicker to change and clean.
For spray arc transfer or pulse spray of solid wire, the nozzle should extend past the contact tip, which helps deliver the shielding gas closer to the arc when longer electrode stickout is required. It also helps the contact tip run cooler.
In short-circuit transfer mode, the contact tip should be flush or extend only slightly past the nozzle, allowing for the short electrode stickout required. Since a certain amount of spatter with this type of transfer occurs, extending the contact tip can help to decrease the spatter buildup on the nozzle. This in turn allows the gas to flow unrestricted.
The size of the nozzle opening depends on the size of the weld puddle, the volume of shielding gas required, and the difficulty in reaching the area that needs welding. For example, for welding deep V-groove butt joints, a small, tapered nozzle may be necessary to get the contact tip close enough to the weld puddle. In contrast, high-voltage, high-amperage applications usually require high gas flow rates, so a nozzle with a large inside diameter gives better shielding gas coverage of the larger weld pool.
A good rule of thumb is to use the largest nozzle that fits the application. For an application that has restricted clearance or limited visibility, a small nozzle might be the best choice. But generally, bigger tends to be better because the larger the nozzle, the more gas coverage it provides. Some nozzles have built-in orifices that help direct gas, smoothing the gas flow and covering the weld better.
Nozzles typically are made from brass or copper. Copper nozzles are well-suited for use in heavy industrial applications because of their ability to withstand intense heat. Brass nozzles resist spatter better than copper, but tend to melt or burn when exposed to extreme heat.
The electrode wire is fed to the welding gun through a cable liner, and typically you need spring steel coiled liner for steel applications. Because these liners are steel, they are rigid, resist buckling, and have a long life.
Aluminum applications typically require liners made from nylon, Teflon(r), or some type of plastic, because these materials have lower friction than steel and they help keep contamination out of the weld. When aluminum wire is pushed through a steel liner, the wire can pick up little bits of steel that can contaminate the weld.
Liners need to be replaced because they wear out from continuous use or become kinked from improper use. Decreasing the friction between the liner and the wire and preventing improper use both help to minimize liner replacement and maximize welder uptime. Kinks in the liner cause the wire to catch and can result in erratic arc performance, which can lead to more operator downtime caused by more spatter cleanup.
To maximize liner life and reduce postweld cleanup, you can blow out the liner with compressed air. The liner can be blown out from the contact tip end or from the feeder end to remove dirt or copper flaking.
Liners usually need to be changed when you switch electrode size. Although some liners may be used for more than one wire size, if they are either too big or too small, they can cause poor feeding. One of the biggest causes of poor feeding is an improperly trimmed liner, such as one that is (trimmed too short or trimmed so a burr on the liner impedes electrode feeding). Patented jump liner technology allows a toolless change of the most common liner wear point, which is at the radius of a GMAW gun's body tube, instead of the entire liner.
The liner size needs to match wire size, usually within a specific range. For example, a 0.035-in. wire typically is used in a liner capable of running 0.035-in. to 0.045-in. wire.
The optimal consumable is one that provides the best quality and the longest life. The better the consumable quality is, the less postweld work is needed, and the less downtime is incurred. Also, the longer the consumable lasts, the less time you spend replacing it. Time is money, and whether you are choosing a large power source or a small consumable, choosing the right equipment is essential to maximizing productivity.
Article provided by Bernard®, an Illinois Tool Works Company, 449 W. Corning Road, P.O. Box 667, Beecher, IL 60401, 708-946-2281, www.bernardwelds.com.