September 6, 2009
Gas tungsten arc welding is easier than ever to automate. At the same time, robotic GMAW technology now can produce welds that are close to GTAW quality
Robotic systems manufacturers and integrators have a confession when it comes to TIG, or gas tungsten arc welding: They've sometimes steered people away from it.
"I've worked with arc welding robots since 1982, and for most of my career, if someone told me he wanted to buy a robot that TIG-welded, the first thing I did was try to talk him out of it," said Brian Doyle, group manager for welding and robots at Panasonic Factory Solutions Co. of America, Buffalo Grove, Ill.
He added that he doesn't steer customers away as often, and there's a reason for it. GTAW automation has gotten better.
Historically a bear to automate, GTAW requires precise joint fit-up and high part repeatability. If a badly out-of-tolerance part makes it into the welding cell, and the tungsten unexpectedly touches the base metal or dips in the weld pool, problems mount. A scrapped part is one thing; but a contaminated tungsten welding (and perhaps ruining) 50 more parts before being noticed, that's something else.
Thanks partly to precise upstream processes, robotic GTAW has evolved as a practical option. A tab-and-slot joint design made possible by laser cutting, for instance, makes for a joint that a GTAW robot could easily work with. But choosing a robotic arc welding process isn't straightforward, considering the options now on the market. Robotic GTAW is more practical in more places (see Figure 1). At the same time, some gas metal arc welding variants can produce weld quality close to that offered by GTAW.
People choose GTAW when a joint requires superior cosmetic or structural properties (see Figure 2). A GTAW robot mimics a good welder, at least ideally. The operator sets gas preflow, usually pure argon, at a low flow rate. He starts the arc and then ramps up the current using the foot pedal; near the end he ramps down the current to the crater (weld end) and continues the postflow gas to ensure neither the weld nor the hot tungsten electrode is contaminated.
Put a GTAW torch on a robot arm and challenges abound. Consider tungsten life. As the tungsten tip starts to erode, the weld's heat characteristics change the input. Tungsten life varies greatly. A tungsten performing the occasional, autogenous outside corner or lap weld will outlast one performing continuous groove welds requiring filler metal. Automatic tungsten changing helps matters, Doyle said, using a collet system that allows the robot to remove the tungsten and snap another preground tungsten in its place.
"With GTAW, changing the contact-to-work distance also changes the heat," explained Jarrod Bichon, vice president of integrator RobotWorx, Marion, Ohio.
For this reason the process requires a steady tip-to-workpiece distance. It's a hallmark of a good gas tungsten arc welder to be able to adapt on-the-fly for joint changes. Without specialized sensors or cameras, a conventional GTAW robot will stay on its preprogrammed path.
Another challenge is the high-frequency arc starts in which a jolt of energy initiates the arc between the tungsten and workpiece. This eliminates touching the electrode to the workpiece, as is required by a lift-start. But HF also can wreak havoc on nearby electrical equipment.
"The problem with high frequency is that if other electronics are in series with that high freq, the noise will cause problems, even if you have [the robot cell] grounded directly," Bichon said.
So components must have appropriate shielding. Some systems offer case-hardened robot arms, and most have shielded cables, which should be neatly organized and separated with trays to avoid any HF issues.
Some robotic systems avoid HF and actually offer lift-starts. One robot system, designed for both GMAW and GTAW, has a system in which "the electrode [wire for GMAW, tungsten for GTAW] touches the workpiece, causing the robot arm to jerk back a preset distance, drawing the arc, and beginning the upslope of current," Doyle said.
The upslope begins the waveform integral to GTAW, and tweaks to it have made the process a bit more forgiving in automated environments. This includes synchronizing welding parameters with pulsing technology. As Doyle described, certain systems have servo filler metal feeders that—similar to some GMAW setups—synchronize the speed of the filler metal with the pulse, so the wire feeds faster during the pulse peak and slower (or not at all) during the pulse trough. This motion emulates the moves of a skilled TIG welder dabbing his filler rod into the pool.
Pulse has another advantage. "The pulse itself is hot enough that it can burn away a lot of impurities," said Matt Brooks, a welding engineer at OTC Daihen Inc., Tipp City, Ohio.
Robotic AC GTAW is available as an option, too, as well as AC/DC GTAW. Because AC welding alternates between electrode positive (for oxide cleaning) and negative (for penetration), it's the process of choice for welding aluminum prone to oxide buildup. But aspects of AC also tend to wear the electrode more, making it "ball up" and create detrimental arc characteristics. Hence the need for more frequent tungsten changeouts, Brooks said, which can damper the productivity of an automated setup.
The AC/DC GTAW mode adds DCEN into the mix to increase penetration by concentrating heat at the workpiece and away from the tungsten, and commonly is used for materials such as steel, stainless steel, chrome-moly, and nickel-based alloys.
"You run AC for a short period of time, then DC [electrode negative]," Brooks said. "It doesn't help with steel or stainless, but it really helps on aluminum. The big advantage is that your tungsten lasts two to three times longer, depending on the application."
Manufacturers also have tweaked the position of filler metal feed. A GTAW artisan positions his body just right so he can feed the filler metal at the desired angle under the torch. A robot can't do this, and because filler wire must be fed through a feeder tube separated from the tungsten electrode, a GTAW robotic torch is bulky. Recent design improvements help matters, Doyle said. For instance, certain models integrate a seventh axis on the filler tube, so it can rotate around the tool-center point for those hard-to-reach welds (see Figure 3).
Other models introduce the filler at a steeper angle to the torch, with the wire a little closer to parallel with the tungsten electrode. This makes the end-of-arm assembly less bulky so that the torch can reach into tight spaces (see Figure 4).
This setup has an added benefit. "We [informally] call it 'warm wire,'" Doyle said.
Cold-wire GTAW is the conventional setup; the filler is fed at an angle to the weld pool. Hot-wire GTAW involves a separate heat source that preheats the filler metal before it enters the pool. In Doyle's "warm-wire" setup, the filler metal is fed at a steep angle and enters the arc for a longer period, preheating the wire before it melts into the weld pool. The setup adds flexibility as well, he said, because the steep angle allows for more precise wire targeting. The filler doesn't have to "lead" the weld pool. If welding around, say, a corner, the filler metal can enter slightly behind the pool's leading edge, or to the side.
The filler wire preheating allows for much faster travel speeds. The technology has been tested on lap welds in stainless steel at up to 30 inches a minute, Doyle said, "which is pretty good travel speed for MIG, and almost unheard of for TIG. But you have to choose your joint design carefully. To use the same technology in a fillet, you'll be going back to normal TIG speeds, about 10 to 15 inches a minute."
This brings up some challenges in GTAW that are hard to circumvent. Select circumstances aside, GTAW is slow, though automation is faster than the manual alternative. Also, fillet and open-root joint designs remain difficult, even with all of the advancements. In fillets, the arc tends to wander from one side to the other, giving inconsistent results. Lap, outside corner, and similar joint designs are ideal, though good joint repeatability is still paramount.
People choose GTAW for several reasons. If the welding code requires it or the design calls for an autogenous or extremely high-quality weld (a high-pressure pipe weld root pass, for instance), the application likely will go the way of the tungsten torch. But often people choose GTAW because they're dealing with either very thin or cosmetically important joints, many requiring that "stacked-dime" appearance.
Thanks to recent advancements, some GMAW process variations now can produce joints that are identical, or at least close, to those produced with GTAW.
GMAW's simplicity and speed are driving developments.
"A vertical-down weld [using GMAW] on thin sheet could travel 70 inches a minute," said Geoff Lipnevicius, manager, product development, automation division, at The Lincoln Electric Co. in Cleveland.
"[In GMAW] the entire electrode is consumed," RobotWorx' Bichon said. "There's less going on at the end of the torch, so everything becomes a little easier."
Industry is embracing GMAW advancements that tweak the pulse, wire feed, and electrical controls. GMAW process enhancements alter short-circuit transfer, which operates at lower amps and voltages and, therefore, works well with thin material. Each welding equipment-maker offers its own solutions, all with a common goal: control heat to fill the joint with minimal spatter and an attractive appearance, be it a seamless or stacked-dime look, and avoid melt-through.
OTC Daihen offers its Control Bridge Transfer, which controls the parameters of a short-circuit gas metal arc weld both electronically and physically, Brooks said. "The electronic method controls the welding parameters through the duration of the arc period and cuts down the parameters during sections where the forces are greatest and that cause spatter," he explained. "The physical method involves controlling the wire feed, or in the case of robotic applications, manipulation of the robot's arm. Here, the system creates a physical pull on the wire during the periods that cause spatter, thereby reducing the arc forces." To attain a stacked-dime look on aluminum, OTC Daihen offers its GMAW Wave Pulse, a superimposed low-frequency waveform.
Lincoln Electric offers its Surface Tension Transfer®, or STT®, pulsed GMAW variant. "In simplest terms, the peak current controls the arc height and the background controls arc width and overall heat input," Lipnevicius explained. Most significant, the STT technology separates wire feed and current, allowing each parameter to be dialed in separately.
For that stacked-dime look, FANUC Robotics offers HeatWave (see Figure 5), which "allows you to oscillate the procedures as you oscillate the torch," Lipnevicius said. "That ability to coordinate weaving with your welding procedures has helped expand the number of applications that can be welded with steel, stainless steel, and aluminum."
This GMAW technology emulates the TIG welder sticking the rod into the puddle. The robot changes welding parameters in real time. On thinner materials with a gap, the robot rocks back and forth along the joint, stitching the gap like a sewing machine, dipping the wire into the joint with each oscillation. On thicker materials, the robot can move in a weave pattern across the joint, adjusting parameters as it moves.
Lipnevicius added that the technology can help in welding dissimilar thicknesses, with the process producing more amperage and penetration on the thick side to avoid cold lap, then immediately reducing the amperage and penetration on the thin side to avoid burn-through.
Panasonic offers similar benefits in its TAWERS™ robot system, which is able to control GMAW parameters at different areas of the joint. "As it weaves, it can change not only the amperage, but also the voltage and wire feed speed to make sure you dig into [the thicker material] and then have it cool off for the [thinner material]," Doyle said.
Still others offer alternatives. Fronius USA has its cold metal transfer process, a GMAW variant that alters the way metal transfer occurs. CMT oscillates the wire feed back and forth to control heat and spatter, among other things, synchronizing wire motion with the GMAW short circuit. As Gerald Obritzberger, director of sales and marketing for the Brighton, Mich., company, put it during his presentation during the 2008 FABTECH® Intl. & AWS Welding Show, "The wire is moved toward the workpiece until a short circuit occurs. At that moment the wire speed is reversed and the wire pulled back. When the short circuit opens again, the wire speed is again reversed, with wire moves toward the workpiece, and the process begins again," allowing the process to weld various sensitive materials, including extremely thin aluminum.
Miller Electric, Appleton, Wis., has its Regulated Metal Deposition Process (RMD™), a modified short-circuit-transfer process that, again, can replace GTAW in some manual and robotic environments. As Kevin Summers, Miller's manager of automation business development, explained, "Early prediction of the short clearing enables quick reduction of the available current, minimizing the arc force and reducing the amount and size of spatter. The molten puddle is calm, reducing splash and cold lap effects normally associated with short-circuit transfer."
Florence, S.C.-based ESAB Welding & Cutting Products offers its Aristo SuperPulse™ which again allows for controlled GMAW with low heat input that can replace GTAW in certain applications. It is currently integrated into the FlexPendant for ABB robots. Its "process pulse/short-arc" option is designed to weld thin sheet.Weighing the Options
The list of GMAW variants grows ever longer. Most welding equipment-makers offer them, and many can tackle applications that before only GTAW could handle cost-effectively.
Nevertheless, certain applications surely will use GTAW automation for the foreseeable future, particularly for extremely high-quality joints in which high speed isn't always so desirable.
"GMAW has a high quench effect, with high heat applied for a short period of time," Lipnevicius said. "So while the travel speed is enviable, sometimes it isn't always so good. GTAW slows this process down, reducing the quench effect, which ensures that certain mechanical properties remain within the base material and weld mixture."
On the horizon the industry may see more multiprocess robots, Doyle said, offering both GTAW and GMAW. "Manufacturers, particularly contract fabricators, will have GTAW as another tool in their tool belt."
Still, he said, nothing in the foreseeable future will change GTAW's need for joint repeatability. But if the right applications are chosen, and the upstream processes are repeatable, robotic GTAW makes good business sense.
"If you can squeeze enough variation out of the part," Doyle said, "then it may be worthwhile to attain those huge productivity gains by going to robotic GTAW."
Although certain gas metal arc welding processes are producing welds close to gas tungsten arc quality, the two processes' gas flow requirements couldn't be more different. Robots capable of both GMAW and GTAW should have separate gas line feeds, and the GTAW line pressure should be much lower. While GMAW usually uses 35 to 40 cubic feet per hour, 10 to 15 CFH should be plenty for many GTAW applications.
Preflow and postflow times are important. "You need to watch your gas flow after the arc is extinguished," said Lincoln Electric's Geoff Lipnevicius. If the gas shuts off too quickly while the tungsten is still red-hot (a temptation because TIG welds often solidify quickly), "you get oxidation [on the tungsten consumable], which shortens tungsten life."
When setting up a welding cell, it's best to set up the lines and flowmeters so they're calibrated at the appropriate lower pressures. Panasonic's Brian Doyle said that some unfamiliar with gas requirements have set the gas lines to 90 PSI, the same as the shop's air lines. This causes hoses to build up gas to 90 PSI when idle; when the gas solenoid opens, a surge of gas emerges, which can wreak havoc on arc starts.
For GTAW, he said, "you should size your pipeline to 20 PSI and then have your flowmeters calibrated to 20 PSI." He also suggested moving the metering point closer to the gas solenoid valve, to reduce pressure buildup in the hoses.
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