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An enhanced spray transfer MIG process yields faster welding travel speeds than conventional solutions

To increase MIG deposition rates, intuition says to use a larger diameter wire. Physics, however, overrules instinct. Welding research at ESAB’s Gothenburg, Sweden, location proved that increasing current density – by using smaller diameter solid wire at very fast wire feed speeds – can increase deposition rates by 44 percent.

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Figure 1. By using a smaller diameter wire and faster wire feed speeds, the SAT process offers the highest deposition rates possible for solid wire MIG welding.

As Figure 1 shows, a 1.2-mm-dia. ER70S-6 wire provides a deposition rate of 8.147 kg per hour when welding at 400 amps (about the maximum 1.0- and 1.2-mm wire can handle). By substituting a 1.0-mm wire and increasing the wire feed speed from 15.6 to 32 m per min., users could increase deposition rates up to 11.76 kg per hour (if they have a wire feeder that runs that quickly; 30 m per min. is the limit on many feeders). That 3.6-kg-per-hour difference equates to a 44 percent improvement. For those currently welding with a 1.6-mm wire, the improvements are even greater.

As a result of researching fast wire feed speeds, ESAB introduced the Swift Arc Transfer (SAT) process approximately four years ago. This enhanced spray transfer MIG process uses typical wire feed speeds of 16 to 25 m per min. It yields welding travel speeds that range from 20 to 100 percent faster than conventional solutions, and it is at least 10 percent faster – in an actual production setting – than any other advanced MIG process ESAB has encountered (see Figure 2).

The process is suitable for welding 2- to 15-mm-thick plate in the flat or horizontal positions; works with fillet, lap and butt joints; and can use carbon, stainless and low alloy filler wire. Typical industrial applications include transportation segments (automotive, railway, shipbuilding, off-highway machinery), structural steel, tank and vessel building, and container construction.

The quest for productivity

Successful high-volume manufacturers relentlessly pursue productivity improvements and cost reduction. Wherever possible, they use mechanized and robotic welding. As good as human operators can be, they simply cannot sustain the pace and consistent quality of a robot, and robotic SAT further increases productivity. For example, robotic SAT produces travel speeds up to 90 cm per min. when making a fillet weld on 10-mm plate (1.0-mm-dia. ER70S-6 wire). For comparison, a typical robotic MIG application has travel speeds of about 50 cm per min. (35 cm per min. is good for a human).

In high-strength steel applications (such as those using AWS A5.28 ER120S-G or AWS A5.28 ER110S-G filler wire), SAT can increase travel speeds by up to 75 percent. For example, conventional spray transfer MIG welding speeds are 40 cm per min. for welding 12-mm plate (throat size is 5 mm). For the same joint, switching to SAT increases travel speed to 70 cm per min. using a 1.0-mm electrode and 92 percent argon/8 percent CO2 shielding gas.

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Figure 2. A comparison of the voltage and wire feed speeds used for solid wire MIG processes.

At Fabtech 2015, ESAB provided a demonstration of pulsed MIG versus SAT, showing how SAT offered travel speeds of 116 cm per min., which is twice as fast as pulsed spray transfer. While the SAT process appeared to create a lot of sparks, the weld bead close-up in the video showed very little spatter.

Intuitively, SAT’s faster travel speeds also reduce heat input. Figure 3 compares welding parameters for SAT, conventional spray transfer and short arc MIG as set for making a fillet weld with a 3-mm throat on 5-mm steel plate. Test results show that the SAT process is 0.5 kilojoules “cooler” than short circuit transfer yet nearly four times faster. Compared to spray transfer, SAT reduces heat input by more than one-third while increasing travel speed by more than one-third.

In addition to faster travel speeds, the SAT process also reduces spatter, provides a consistent penetration profile for a better first-pass yield rate and reduces heat input to minimize and prevent distortion. Note that a 0.9-mm wire works well for welding thin plate, and this diameter (0.035 in.) is one of the most common solid wire diameters used in North America.

Process requirements

The SAT process uses an inverter, or chopper-based, power source, such as ESAB’s Aristo MIG 5000i or Origo MIG 650c and a wire feed system capable of sustaining fast wire feed speeds for a prolonged period of time, such as the Robofeed 3004L or Robofeed 3004 HW “hollow wrist” encapsulated wire feeders.

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Figure 3. Because SAT lowers heat input, it can help combat issues related to warping and poor fit-up of welded components.

So that companies can implement the correct SAT welding parameters and arc start routine without extensive engineering effort, ESAB offers preprogrammed synergic lines for steel (ER70S-6), stainless steel (430Lnb, 430Ti, 308Lsi, 316Lsi) and the previously mentioned low alloy filler wire. The lines are only available through the U82 controller (see Figure 4), which also enables fine tuning preprogrammed lines and development of new SAT lines if required.

Outside of the synergic lines, the most critical component of SAT is ESAB’s OK AristoRod, a non-copper coated MIG wire with advanced surface characteristics. OK AristoRod provides consistent welding performance, a stable arc with low feeding force, trouble-free feedability, excellent arc ignition and an extremely low spatter level.

Traditional ER70S-6 wire uses a copper coating to improve feeding. Contrary to popular belief, the coating does not improve current transfer, reduce contact tip wear or protect against rust. The weakness of a copper coated wire is that particles chip off and contaminate the feeding system, gradually clogging the liner and contact tip. As resistance builds up due to clogging, the wire eventually burns back to the contact tip – which causes unplanned downtime, an undesirable occurrence in a robotic application.

Figure 4
Figure 4. To simplify process installation, ESAB provides preprogrammed synergic lines via its U82 controller.

Because OK AristoRod is a non-copper coated wire, it generates far fewer particles and enables a longer run time between scheduled maintenance and liner and contact tip replacement. To confirm performance, ESAB worked with ISF Aachen, a prestigious German welding research authority, to conduct a benchmark survey comparing OK AristoRod with a wide selection of copper coated wire from leading suppliers.

Figure 5 shows that OK AristoRod runs longer without burning back to the contact tip (more than 330 min. of continuous welding compared to 220 min. or less for copper coated wire), requires less force to feed, increases arc stability and reduces spatter. The ESAB video on the SAT process provides further evidence (at the 0:38 sec. mark), showing how OK AristoRod produces a more stable arc and more controlled metal transfer. As a more controlled process, SAT enhances productivity in other ways beyond increased welding travel speeds.

Other productivity improvements

Since its introduction, ESAB Value Added Engineering has worked with several European companies to implement SAT as part of working with end users to improve productivity and profitability. Some of the highlights include:

  • In a truck compressor tank application using ER70S-6 wire, SAT increased total productivity by 8 to 10 percent through faster travel speeds (from 145 to 192 cm per min.) and reduced undercuts, spatter and distortion.
  • In an automotive exhaust pipe application using 430Lnb wire (0.9 mm dia.), SAT reduced welding cycle time from 24 to 11 sec. for welding a 63.5-mm-dia. pipe with a 1.6-mm wall thickness. Spatter and burn through were also reduced.
  • In a tow bar cross member application, SAT increased deposition rates from 5.56 kg per hour to 7.73 kg per hour and decreased welding cycle time from 400 to 330 sec., a 21 percent improvement.
  • For welding an exhaust manifold, ESAB recommended switching from 1.0-mm- to 0.9-mm-dia. wire and changing welding parameters from a wire feed speed of 13.0 m per min. and 21.4 V to 20 m per min. and 28 V. As a result, welding speed increased from 80 to 100 cm per min., a 25 percent improvement.
  • For welding a box beam in a crane application using 1.2-mm ER70S-6 wire, SAT improved welding speeds from 70 to 100 cm per min., up from previous speeds of 30 to 35 cm per min. Welding defects were greatly reduced, as well.
  • In I-profiles 3.5 to 6 mm thick, SAT doubled welding speed from 40 to 80 cm per min.
Figure 5
Figure 5. Non-copper coated wire reduces wire feed speed variations, which in turn allow a longer run time before a burnback occurs. The vertical axis shows the number of measurements. Source: ISF, Aachen.

Dick Skarin, global value added engineering manager, notes that, “Even customers with experienced welding engineers may not have an experienced robotic welding engineer on staff. In robotic applications, we can almost always improve results by helping customers set the proper torch angle with respect to the joint and the direction of travel, as well as adjust wire stickout, wire feed speed and voltage. Our objective is not to sell customers a particular process or filler metal, but to work with customers and show their true costs of welding and suggest methods for improvement. In applications using larger diameter solid wire, shifting to SAT and using a smaller diameter almost always improves productivity and reduces cost.”

The tow bar application noted above provides a perfect example. This was an existing customer using a premium 1.2-mm copper coated wire. By shifting to SAT, a 1-mm non-copper coated wire and doubling wire feed speed to 22 m per min., the customer increased travel speed from 61.6  to 89.9 cm per min. SAT reduced total production cost by 16 percent and saved the customer more than $60,000 per year.

figure 6 and figure 7

Note that previous attempts to weld with very high wire feed speeds showed inconsistent penetration and weld bead profiles. As Figures 6 and 7 show, SAT provides consistently good penetration and a consistent bead profile. In short, not only does SAT weld it fast, it welds it right the first time.

ESAB