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Traditional Versus Laser Welding

Ed Sinkora
By Ed Sinkora Contributing Editor, SME Media

Laser and traditional welding are still competing

With much faster processing speeds and higher quality, you might think laser welding would quickly take over the field. But traditional welding hangs on. And depending on who you ask and what applications you consider, it may never go away. So what are the pros and cons of each method that continue to result in a mixed market?

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Trumpf’s Fusion Line features a laser assisted with wire to introduce more mass into the weld, bridging gaps up to 1 mm wide.

Traditional methods of welding remain popular. Broadly speaking, three types of traditional welding used in industry are MIG (metal inert gas), TIG (tungsten inert gas), and resistance spot. In resistance spot welding, two electrodes press the parts to be joined between them, large current is forced through that spot, and the electrical resistance of the part material generates the heat that welds the pieces together. It’s a fast method and according to Erik Miller, business development manager for Miller Electric Mfg LLC’s laser group in Appleton, Wis., it’s been the dominant method used in automotive, especially for bodies. But, he added, the biggest market for laser welding has been in replacing resistance spot welding. Conversely, Miller hasn’t seen “any sort of avalanche” in laser replacing TIG or MIG. And even within the company’s automation group, roughly 90 percent of the projects are in MIG.

Big on MIG

What accounts for MIG’s enduring popularity? “The consumable is a continuously fed wire,” Miller said. “So it is adding material and reinforcing the weld, making it perfect for a fillet weld [in which the pieces are perpendicular].” Autogenous laser fuses the two parent materials together. A laser can make a fillet weld, but the accuracy and precision of the parts and everything else have to be an order of magnitude tighter, according to Miller.

“With a MIG weld on a fillet, the tolerance is at least plus or minus half a wire diameter, and generally even more,” he said. Likewise MIG’s process window for other types of welds is much bigger than laser. In other words, parts do not have to be as accurate and fixtures do not have to ensure a nearly perfect fit, like they do with autogenous laser.

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The dark area under the weld at left shows the deep penetration and complete fusion of a good MIG weld. The image at right shows a laser weld with complete fusion but very shallow penetration that reduces mixing between the filler and substrate.

MIG welding is also easier to automate. As Miller put it, the only factors you need to control are travel speed, voltage, amperage, torch angle, and work angle, and “if you do five out of the ten things right, you’ll still get a good weld.” Automating laser welding requires a robot with excellent path accuracy and repeatability, and there are more factors in the welding process to control. TIG is similar in this respect.

That’s not to say that automating MIG welding is so easy anyone can do it. It still requires an expert to do the programming and diagnose problems. Ed Hansen director of global product management, flexible automation, for ESAB Welding & Cutting Products, Denton, Texas, said that’s another plus for MIG.

“After many years of empirical and scientific evidence, traditional welding is well understood. We know what it takes to make a predictable result that delivers the joint the structure requires. And even though we talk about the scarcity of skilled labor, which is a real problem for the industry, there is still a large pool of experienced welders, technicians, and engineers that are all familiar with managing those traditional processes.” For most products, it is a simple, inexpensive solution that provides acceptable results.

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The dark area under the weld at left shows the deep penetration and complete fusion of a good MIG weld. The image at right shows a laser weld with complete fusion but very shallow penetration that reduces mixing between the filler and substrate.

It is the case that the upfront cost of either a MIG or TIG system is less than a laser system. However, the cost of lasers have been coming down and will continue to do so. “The laser is somewhere between a third and half the cost of a laser welding system,” Hansen said, “and the cost as a function of welding capability is dropping 10-15 percent per year.”

Miller also noted that “the laser process head is more expensive than traditional heads, the delivery fiber is expensive, and safeguarding a laser cell is more expensive as well.” For example, a laser cell must be “light tight,” with 4" (101.6 mm) thick walls to withstand a direct hit for 10 minutes without burning through. (The laser would not be in focus over a 4" [101.6 mm] thick depth.) TIG and MIG systems can be shielded by inexpensive sheet metal that allows for gaps.

On the other hand, when accounting for differences in throughput and cost per part, laser often wins, as we’ll see. That’s particularly true for TIG, which is a very slow process requiring a high degree of skill, making it expensive to use. For that reason Miller said TIG is largely limited to industrial food equipment and appliance manufacturing, plus some precision components. “People choose TIG for food equipment applications because the weld doesn’t have a porous surface—it’s very smooth,” he said. But if those parts need to be produced in volume, the ROI on a laser system will “blow the doors off” TIG, so naturally it’s taking over in those cases.

Masoud Harooni, product manager in laser welding for Trumpf Inc., Hoffman Estates, Illinois, said that even TIG cannot produce a fully satisfactory surface for food processing and other applications where appearance is critical. “It’s not as bad as MIG, but a TIG surface definitely requires post-process grinding, which isn’t needed with laser,” said Harooni. “Also, the speed of laser welding for visible welds is two to three times faster than TIG. If you see a nice radius on a refrigerator or similar part, it was either ground or laser welded.”

One last vote for traditional welding: With the exception of a few specialized cases, laser welding must be automated, given safety concerns. And that leaves plenty of work for human welders, as Hansen explained. “You can’t have a robot go up scaffolding or climb into the bilge of a ship. We can dream about such super robots, but in practical terms, they won’t be here for the near future.”

Trends Driving Laser Adoption

As Miller sees it, U.S. manufacturing tends to be conservative, and “if there’s not a problem to solve, the lowest cost, most robust, most vetted solution will be chosen. So people only start looking towards laser when MIG welding doesn’t work or TIG welding is too slow.”

Volume TIG welding has either already moved overseas or been taken over by laser, so where is laser challenging MIG?

One key concern is the damage—either metallurgical or structural—potentially caused by the MIG’s relatively lengthy and widespread heat transfer into the part, followed by a long cooling cycle. Conversely, laser transmits heat energy in a very small beam, melting only a localized area. The total heat input is much less than MIG and the part cools very fast, minimizing distortion and metallurgical effects.

Harooni offered a useful analogy: “Imagine a bottle of water on a sandy beach, compared to a needle. If you put a five pound weight on the bottle it’s not going to penetrate the sand. But if you put just a few ounces on the needle, it will. Think of the weight you apply as heat, the bottle as MIG, and the needle as laser.”

ESAB’s Hansen said that laser reduces the heat input by roughly 85 percent compared to MIG and “residual stress in a weld is directly proportional to heat input. The more heat that you put into it, the more residual stress you induce. And that means buckling and distortion and shrinkage and all these things that cause a nightmare when you take that part and make an assembly out of it or fit it into a structure or a vehicle.”

The bigger the part, the more small individual residual stresses become macro deflections that are very costly and difficult to fix later, he added. And that’s a major consideration for customers trying to “lightweight” their products. What’s more, he said, “some alloys segregate or change properties when you heat them, or grain structures grow in undesirable ways. In many of these materials the grain structure and microstructures are different if you melt and then cool the weld.”

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With its heat source standing off at a distance and a highly concentrated melt area below, laser welding seems like magic, as shown here in a Miller Electric system.

Miller of Miller Electric pointed out that the latest generation of high-strength steels “get a lot of their strength by sophisticated heat treatment processes. When you melt and solidify them under a low cooling rate [as in MIG welding], all of those strengths go away. Laser can help maintain the parent strength of the material.”

In another example, Miller said that MIG welding titanium is difficult due to “a floating cathode problem. The arc isn’t stable. So laser is a perfect choice.” With 6000 series aluminum, the problem is hot cracking. “Hot cracking is a function of the magnesium silicide migrating to the grain boundary. So if you can heat the material, melt it, and cool it before the magnesium silicide migrates, then you can create a crack-free weld,” he said. “Laser can do that using the latest scanning techniques, in which we move the beam back and forth with a mirror.”

Laser’s Awesome Throughput

From Miller’s perspective, the majority of laser applications are in difficult-to-weld materials. From Harooni’s perspective, laser is so much faster that even sheet metal projects are moving to laser. How much faster? Trumpf’s Harooni said MIG welding normally proceeds at 20-30" (508-762 mm) per minute—at most 40" (1,016 mm) per minute. Laser, according to Harooni, can weld at almost 200" (508 cm) per minute, so the joining process alone is already much faster. The second benefit is the reduction in post-processing. Harooni observed that if the appearance of the weld is important, you would have to follow a MIG weld with a lengthy grinding cycle, which wouldn’t be necessary after laser welding.

“That’s why,” he added, “it’s typically the case that a part built with MIG welding at a cost of $25 would cost only $15 to laser weld, even considering the higher initial investment in laser welding.” For example, Harooni recounted a recent project in which Trumpf cut cycle time on welding a large door from ten hours to 35 minutes. Another customer had difficulty MIG welding an aluminum electrical enclosure. Blowholes were a frequent problem and the total cycle time was four hours. Harooni said Trumpf cut that to 18 minutes with laser welding.

Hansen added that laser’s ability to penetrate deeply into material multiplies its advantage over traditional welding. Because not only is laser three to ten times faster than MIG (and even faster compared to TIG), it can weld relatively thick joints that would require multiple passes with MIG or TIG.

“The traditional techniques also require clean-up and grinding between the passes, further adding to the overall cycle time,” Hansen explained. “Laser can do single pass welding up to about a half an inch, versus about five passes for MIG welding, depending on the processor you use. Above half an inch, laser welding would require cutting or grinding a bevel to the edge beforehand, but it’s a much smaller bevel than the whole joint bevels needed for MIG welding.”

So for half-inch-thick material, laser welding would be 15-50 times faster than MIG, just in welding speed—and even faster when you also consider the extra post-processing required for MIG.

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Comparing techniques: An un-welded joint at bottom, a MAG weld just above it, a Trumpf Fusion Line weld (which combines wire and laser) above that, and at top an autogenous laser weld after redesigning the joint for that process.

Of course, with such high production rates, you need a lot of weld work to feed a laser system and maximize your ROI. As Hansen put it, “typically, laser can produce as much as three to five sub arc welding systems on plate welding, for example. You need a lot of work to feed five sub arc systems.”

New Tech Combined with Old

Because autogenous laser welding requires a tight fit between the parts to be joined, in many cases it is best to redesign the joint locations to present overlapping surfaces to the laser (to use its piercing capability). More manufacturers are willing to invest in better upstream processes and tooling in order to take advantage of laser’s higher throughput.

But for those that are resistant to such change, or in situations where gaps are unavoidable, there are hybrid systems that combine laser and wire feed technology and other new developments that broaden laser’s applicability. One simple concept, (mentioned earlier with reference to solving the hot cracking problem) is wobbling the laser spot. Miller said it’s an old concept that has recently become much more economical. He offered the example of moving a 1.2-mm diameter spot back and forth over a 3-mm area at high speed, effectively capturing the larger area and still making a good weld.

Hansen said hybrid systems combine the MIG process and a laser beam. “We’re really using the laser to achieve penetration. Normally, if you want to affect penetration in a MIG weld, you have to add more amperage. By using the laser to do the penetrating, we can dial back the amperage on the MIG and use the smallest weld that our structure will allow for engineering purposes. So the laser allows us to optimize the MIG.” There is also synergy between the processes due to the laser beam stabilizing the arc. “We can travel with the arc much faster than we could if we didn’t have a laser beam. That’s how we’re able to go so fast with the hybrid process,” he said.

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A set of large six-axis ESAB laser gantries for welding passenger rail cars.

Trumpf’s Fusion Line, which Harooni described as “a process laser assisted with wire to introduce more mass into the gaps,” can bridge gaps up to 1-mm wide.

For their part, ESAB developed adaptive welding technology that senses part conditions and changes the process parameters to suit them. The system uses a camera that “paints a laser stripe on the part and then looks at it from a parallax angle to see the shape of the joint, about 20-40 mm ahead of the process,” said Hansen. Laser coherent imaging is used to measure the keyhole being cut into the metal by the laser. “We can measure the depth of penetration and the shape of the keyhole and use that information either as a quality measure or in a closed-loop to control the process,” he said.

The system automatically adapts the laser penetration, laser power, gas metal arc parameters, wire feed speed, voltage, gas flow, and travel speed as the weld head processes through the part. The goal, which was driven by U.S. Navy requirements, is to bring the benefits of low heat input laser welding to “conventionally prepared parts” (i.e. parts that weren’t machined to tight tolerances for standard laser welding). Hansen reported that this broadens the process window for hybrid welding by a factor of five over what would have been possible with steady-state controls.

Laser welding remains relatively new to many users, and Harooni stressed Trumpf’s commitment to training and support from the beginning, plus the benefits of offline programming of their systems once installed.

Trumpf also offers TeachLine, a new camera-based sensing system that detects the location of the seam to be welded. “Customers don’t want to interrupt production to program a new part, or make changes to their programming, so they can use this offline programming and upload the part, program it, and take it to the cell. With TeachLine they don’t need to tweak it. TeachLine would see the part and adjust the program you made offline. The combination of offline programming and TeachLine helps our customers make production changes quickly.”

ESAB is also rolling out a new “digital solution” suite that combines a huge amount of information covering the entire welding process, to include the filler material, base material, and gas, to make systems easier to use. As Hansen put it, “It’s easy to make a complicated system. It’s very difficult to make a complicated system seem simple. And that’s where we’re going with our digital solutions. We’re capitalizing on our knowledge of the process to make smart decisions about process control so that the operator doesn’t have to be as experienced or as knowledgeable as they did in the past.”

ESAB is also working on ways to make its equipment capable of assessing the quality of the weld it is producing, and ideally prevent itself from creating a defect or discontinuity.

Finally, traditional welding has seen improvements as well, such as advanced waveforms and Miller Electric’s ActiveWire concept, which feeds the MIG wire forward and backwards continuously to reduce spatter and heat input. The approach broadens the MIG applications that can be automated and makes MIG a viable solution for even some ultra-thin material welding.

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