Perhaps nothing frustrates companies more than advanced technology that promises to improve productivity yet remains too complex to implement. When it comes to welding, the pulsed GMAW process has, until recently, not been able to achieve the widespread popularity it deserves because it has been too complex to implement, especially for small- to medium-sized companies.
Fortunately, advances in welding technology have put more intelligence into the welding power source and simplified process controls. Today, companies can choose from several inverter-based power sources that feature built-in pulsing controls, factory-set programs and a control panel that is easily understood by average operators. This includes industrial power sources paired with a bench-top or suitcase-style feeder and "all-in-one" machines that combine a power source and wire feeder into a single unit.
These power sources perform both pulsed GMAW and "regular" GMAW (short circuit and spray transfer GMAW); some also perform SMAW.
Are You a Candidate?
Good candidates for pulsed GMAW include applications currently preformed with SMAW, DC-GTAW, short circuit GMAW or spray transfer GMAW on metal 1/8-in. or thicker. The benefits of switching to pulsed GMAW are highlighted below. So that companies can determine if this process is right for their application, a short description of the process and its benefits is in order.
Potential Benefits of Pulsed GMAW
- Faster travel speeds (up to 35 percent faster than short circuit GMAW)
- Higher deposition rates and deposition efficiency
- Reduced spatter for less clean-up time
- Improved weld bead appearance (it looks almost like GTAW)
- Minimized distortion compared to spray transfer
- Flexibility to weld thick or thin sections
- Lower fume emissions (less smoke)
- Energy saving (especially because new inverters are much more efficient than older power sources and because pulsing uses less energy)
- All-position welding (unlike spray transfer GMAW)
Pulsed GMAW is technically a modified spray transfer process. With spray transfer, drops of molten metal are continuously being transferred across the arc. In pulsed spray transfer, the power source rapidly switches the welding output from high peak current to low background current. The peak current pinches off a spray-transfer droplet and propels it toward the weldment for good fusion. The background current maintains the arc, but is too low for metal transfer to occur. Because there is no metal transfer, the weld puddle gets a chance to cool and freeze slightly.
Because the heat input is lower, pulsed GMAW eliminates or minimizes burn-through, distortion, heat-affected zone size and loss of mechanical properties. A faster-freezing weld puddle also provides better control on overhead and vertical welds so the puddle doesn't "roll out" of the joint when welding out of position.
Pulsed GMAW produces little, if any spatter (unlike SMAW or short circuit GMAW). No spatter coupled with a faster freezing weld puddle makes pulsed GMAW desirable when cosmetics are important. A pulsed GMAW bead can resemble a GTAW bead. As a result, finishing costs may be reduced or eliminated.
By using larger wire diameters and faster wire feed speeds, pulsed GMAW also is suitable for heavy weldments. In addition, the peak of pulsed current ensures good penetration and sidewall fusion. Because of its good fusion, pulsed GMAW meets welding code standards (ASME, API, ABS, Lloyds Register, etc.) for pressure vessels, petroleum applications and ship building.
Given the wide flexibility of pulsed GMAW, it can often take the place of SMAW, GTAW and GMAW short circuit/spray transfer in many applications.
Each pulsed GMAW "wave" is composed of four variables - peak current, background current, pulse width and pulse frequency - plus wire feed speed and trim (arc length). The optimum settings for these variables change for a given wire size, alloy and shielding gas type, not to mention joint design, welding position and gun technique.
How, then, does the end user know which settings will provide the optimum arc for a given application? And what does changing one of the variables really do to the bead profile? While no problem for a welding engineer or a very experienced operator dedicated to learning a new process, many small- and mid-sized fabrication and manufacturing companies do not have the luxury of such personnel on staff. As a result, they often shy away from pulsed GMAW.
So Advanced, It's Simple
Recognizing this problem, welding equipment manufacturers began to change the way they designed pulsed GMAW equipment. Previously, they poured a lot of intelligence into a programmable feeder or software package that, through a PC, connected to the power source. Programming capability was necessary because a welding power source needs instructions on how to pulse the current. Simpler "pendant" pulsing controls also are available, but their added cables create a hassle and present one more piece of equipment that could get damaged or stolen.
To eliminate the need to manually set pulsing variables and - perhaps most importantly - the need to understand more sophisticated controls, manufacturers have now developed:
- Power sources with built-in pulsing controls. This eliminates the need for programmable feeders, software/PCs that cost thousands of dollars and add-on pulsing pendant s.
- Factory-set programs for pulsed GMAW with the most common wire types and sizes. Typically, one additional set of pulsing variables can be manually programmed to meet a special application.
- Easy-to-understand operator interfaces; they have just one or two more controls than a regular power source.
To begin pulsed GMAW, the operator simply selects the program that matches the wire type and size, and the power source automatically sets the pulse parameters. To weld a different thickness of metal, the operator adjusts wire feed speed, just as with regular GMAW.
So that the pulsing parameters stay at optimum levels, power sources with built-in pulsing capability feature "synergic" control. Synergic refers to the unit's ability to automatically shift several variables (peak and background amperage, pulse frequency and width) so that they track with the change of a single variable (wire feed speed; see Figure 5). This eliminates the frustration associated with fine tuning numerous variables to find the "sweet spot" that produces a good arc.
Recognizing that welding operators need to be able to tweak the arc to suit their particular welding style or joint configuration, these power source also feature trim control. Adjusting trim changes the arc voltage, which allows operators to control the size and motion of the molten weld pool. Again, this control is synergic, adjusting other variables to maintain arc performance.
Easy to Integrate
Power sources with built-in pulsing controls are even easier to use and provide better results than most people realize.
Anco International, Inc., a 38-person manufacturer of hose couplings in San Bernardino, Calif., experienced slow cycle times and x-ray failures when using the gas tungsten arc welding (GTAW) process on 12-in. diameter, 0.375-in. wall carbon steel hose coupling that required 100 percent penetration.
All three of the company's GTAW machines were exceeding their duty cycle rating (300 amps at 60 percent) because the coupling took approximately 60 minutes to weld. When the machines approached their duty cycle, they "started acting up" and subsequent output fluctuations produced x-ray failures.
In addition to couplings for hoses in the aircraft refueling (including military), concrete pumping, and petroleum markets, Anco makes a 4- and 4-1/2-in. diameter carbon steel ball which functions as part of the ball valve in concrete pumps.
The steel ball is two halves welded together. Anco was using "regular" GMAW and taking the ball to one of its seven CNC machines, a lathe, to machine the weld flush with the ball.
Bill Hosier, vice president of the company, contacted his local welding supply representative and explained his problems. In addition to good bead appearance and higher metal deposition rates, the representative recommended pulsed GMAW as a solution because the pulse of peak amperage would ensure good side wall fusion, yet the low background current would cool the arc so it wouldn't blow through the root pass (made on the inside diameter of the pipe). So that Anco could test the process on its application, the representative let Anco try out a Miller Electric Mfg. Co. Invision™ 456MP, which is an inverter featuring built-in pulsing controls, factory-set pulsing programs and a 5 to 600 amp output. It was paired with Miller's 74-DX wire feeder, a full-function digital feeder.
"We bought that pulsed GMAW system and two others because we cut our welding procedure down to 15 minutes and consistently passed x-ray tests to meet ASME IX standards," states Hosier. The new systems have eliminated most of Anco's GTAW work because of the weld quality and bead appearance.
Anco cut labor costs by 75 percent to $15 per 12-in. coupling, and it makes thousands of couplings per year. The labor reduction alone will allow Anco to receive a return on its investment after manufacturing approximately 600 couplings - if that were the only savings. But, they are now using the equipment to weld that carbon steel ball, also.
"With pulsed GMAW, we're welding the ball in one pass and eliminating the CNC work. We save a dollar a ball, and we make 10,000 balls a year," says Hosier. "I know we also save on our electric bill, which is currently $3,000 a month, because I went from a 100-amp circuit breaker to a 50-amp breaker for the three pulsed GMAW units."
Most importantly, Anco easily learned to use new pulsed GMAW systems.
"The factory set programs for .035 and .045 steel wire work excellently," says Hosier. "In fact, the new system was even easier to learn than a conventional machine. An added benefit was that the inverter's digital readout can be set to display in Spanish, and all our welders are Hispanic."