Goodbye to Burn-Through in Welding: Pulsed Welding Technology Solves Sheet Metal Problems | MillerWelds

Goodbye to Burn-Through in Welding: Pulsed Welding Technology Solves Sheet Metal Problems

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Learn how pulsed MIG and TIG technology improve first-pass yield rates, lower cycle times and reduce piece costs.
Welder welding aluminum

Address causes of burn-through

If you weld sheet metal and use conventional welding equipment, you know the challenges of welding this material — obtaining good fusion while simultaneously controlling:
  • Heat input

  • Weld bead profile

  • Arc starts and stops

  • Arc performance while welding

  • Activities that do not add value, such as grinding and rework
     

The key word here is control. While thicker sections of metal might not need advanced control, sheet metal offers little room for error, but lots of room for improvement. Take welding aluminum, for example.

Causes of burn-through in welding aluminum

Selecting a weld process for thin aluminum is like the story of Goldilocks tasting porridge. Short-circuit MIG is too cool and subsequently prone to cold lap (aluminum’s excellent thermal conductivity transfers heat away from the weld area before good fusion can occur). Spray transfer MIG is too hot and prone to burn-through, especially on the back of the weldment or when gaps exist. AC TIG, the process traditionally selected to weld thin aluminum, has the slowest travel speeds; this increases cycle time and heat input, which makes the weldment prone to warping.

How to prevent burn-through when MIG welding

Fortunately, pulsed MIG technology is just right for welding thin aluminum. Pulsed MIG gives users:

  • The ability to control heat input. The pulse of peak current provides the good fusion associated with spray transfer, while the low background current cools the weld puddle and lets it freeze slightly.
  • Good travel speeds. Switching from AC TIG to pulsed MIG can increase travel speeds significantly while cutting heat input.
  • The ability to control bead profile. Using a simple function called arc control, operators can adjust the characteristics of the arc cone, which lets them tailor the arc to the application. A softer arc cone can help tie in both sides of the joint or the outside corner, while a stiffer arc provides good fusion at the root of the joint. 
As a practical example of how pulsed MIG can benefit welding thin aluminum, consider the case of Alum-Line, an Iowa trailer manufacturer. This company addressed production issues on two different components by switching to pulsed MIG technology. 
 
In the first area, which involved welding .125-inch wall aluminum tubing using spray transfer MIG, burn-through often occurred when part fit-up was less than optimal. Cycle time was about 60 minutes. Switching to pulsed MIG reduced cycle time to approximately 30 minutes, largely by eliminating rework.
 

Image of an AC TIG weld bead

Profile Pulse bead

Fig. 1 — Bead comparison: Fabricators can substitute pulsed MIG for AC TIG on aluminum because it maintains bead aesthetics while increasing travel speeds and lowering heat input. The top image shows an AC TIG bead, while the bottom image shows a Profile Pulse™ bead. The Profile Pulse feature is available on select Miller® equipment, including AlumaFeed®, Continuum™ and MPa+ feeder systems. 

In another area, Alum-Line made long TIG welds on .080-, .100- and 1/8-inch aluminum diamond plate. Here, the operator fought warping. After switching to pulsed MIG, the operator increased travel speed by 30 percent, maintained good bead appearance (see Fig. 1, Bead Comparison) and solved warping issues by reducing heat input. Most importantly, Alum-Line operators in both areas learned to weld with the new pulsed MIG technology after just a few hours of training.

Manufacturers developed new pulsed MIG welding systems to overcome operator training and acceptance issues. Pulsed MIG always held great promise, but older technology forced operators to hold a long arc length to avoid unintentional short circuits and arc re-strikes. Unfortunately, holding a long arc makes it difficult to control the weld puddle, and becoming proficient can require weeks of training time. Old technology forced the operator to adapt to the machine — and many operators refused to adapt.

Conversely, with new technology, arc lengths can be run closer to the puddle, giving the operator more control than what was available with older welding equipment. Secondly, this new technology allows the operator to vary stickout without varying arc length. This means operators can hold a longer electrode stickout to weld in deep corners and the system will maintain the arc length.

Newer technology eliminates the hassle of programming pulsing variables in most applications. Operators set arc length and wire feed speed they are comfortable with. Once an arc length is set, the system’s synergic control allows that arc length to be maintained across the range of wire feed speeds, so the operator doesn’t have to change the arc length again. Even if the operator goes from 200 inches per minute to 600 inches per minute, arc length stays the same. The only variables operators have to adjust are arc length (voltage), wire feed speed (amperage) and arc control (should operators choose to fine-tune arc characteristics), and these are adjusted by simple control knob and digital display.

TIG welding for stainless

As good as pulsed MIG is, TIG will continue to play a leading role in thin-gauge stainless steel appliance applications because it provides greater control over several variables.
Stainless steel does not adequately dissipate heat when welded. Instead, it holds heat in the area where the welding arc is concentrated, which can lead to warping and rust. Manufacturers scrap warped parts. This wastes time and materials, and it can be the biggest barrier to meeting production goals.
 
Sensitization can occur in stainless steel parts. Sensitization occurs at heat levels and time durations that allow the carbon to bond with the chromium, pulling the chromium from the grain structure. These then form chrome carbides that migrate to the grain boundaries, allowing stress corrosion cracking and depleting the chrome heat-affected zone — allowing corrosion to take place. 
 
Manufactures welding thin stainless know that pulsed TIG welding improves results. However, they may not be aware that conventional TIG technology cannot pulse faster than 10 or 20 times a second. Conversely, the newest generation of TIG inverters can pulse as fast as 5,000 pulses per second (PPS).

Higher pulsing rates increase puddle agitation, which in turn produces a better molecular grain structure (strength) within the weld. Pulsing the current at higher speeds also constricts and focuses the arc. This increases arc stability, penetration and travel speeds. It can also reduce scrap rates and post-weld grinding.

Pulsed TIG Waveforms
Fig. 2 — Pulsed TIG Waveforms: Operators set four variables when programming a pulsed TIG output: peak amperage, background amperage, pulses per second (PPS) and peak time.

As with pulsed MIG, pulsed TIG technology pulses the arc between a peak and a background current (see Fig. 2, Pulsed TIG Waveforms). Increasing the number of pulses per second:

  • Produces a smoother ripple effect in the weld bead
  • Narrows the weld bead
  • Reduces heat input
  • Increases travel speed 

H.L. Lyons Company, a Kentucky manufacturer of stainless steel appliance components, switched from conventional TIG welders to TIG inverters for their high speed pulsing capabilities. By increasing pulsing rates from 10 PPS up to 175 PPS, the company cut welding time by up to 50 percent and reduced finishing time (post-weld grinding) by one-third. Combined, these benefits enabled each welder to finish almost twice as many parts per shift.

Photo of straight TIG weldPhoto of pulsed TIG weld

Fig. 3 — Straight TIG, Pulsed TIG : The pulsed TIG weld bead took 30 percent less time to weld and it requires almost no clean-up. It also clearly indicates the reduced heat-affected zone.

To confirm these results, Miller developed the sample images shown in Fig. 3 (straight TIG, pulsed TIG). The samples show an outside corner weld on 22-gauge 304 stainless steel made without filler metal. The straight DC TIG weld bead took 45 seconds to complete, while the pulsed DC TIG sample took 30 seconds to complete. Pulse settings were: 40 amps peak amperage, background current 20 percent of peak, 175 PPS and 75 percent peak arc-on time.

Benefits of pulsed MIG welding

Many sheet metal welding and forming companies in the automotive, appliance and other high-end industries face constant pressure to meet quality standards, pricing pressures, warranty and productivity goals. Advanced welding technologies can help operations reach these goals.

There are many different pulsed MIG and pulsed TIG systems available, as well as other advanced MIG processes that address the issues (such as spatter) inherent with conventional short-circuit MIG welding. Each system offers varying degrees of sophistication, simplicity, output and automation capabilities. To find the system that best matches your application, ask your welding supply representative to demonstrate a variety of systems.

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