Laser welding simplifies fabrication of cosmetically important joints
November 9, 2012
Estes Design and Manufacturing, Indianapolis, finds that laser welding produces a joint that is cosmetically appealing to customers. “We’re looking for an appearance that matches almost exactly the outside of a formed corner,” said Jay Reddick, the company's laser welding development manager.
Five years ago Estes Design and Manufacturing adopted laser welding not just because of its eye-popping speed. The Indianapolis fabricator is a high-mix, low-volume operation. Fast welding may be a bonus, but the contract fabricator isn’t producing long runs of any one product.
For many jobs, fit-up tolerance is about ±0.005 in. Precise cutting and panel bending upstream certainly helps meet these tolerances, but the team still must use special fixtures to ensure the joint is exactly where it should be. The company didn’t use the process initially to perform work that couldn’t be joined with any other method, nor did customers demand that the fabricator bring in laser welding. Managers weren’t planning to use the process to move into new markets, either, at least not initially.
So why exactly bring in this precise, fast, though challenging welding method? Ron Estes, vice president of operations, pointed to the postweld grinding cells.
“We’re in a corner of the market where we’re dealing with highly cosmetic products made with light-gauge sheet metal,” he said. “Our fabricating processes are highly automated and very accurate. We were dealing with a lot of products for several industries that required welding, and we saw that whether it was MIG or TIG welding, it typically required a lot of secondary operations to make that welded area blend with the surrounding material.”
As a constraint process, grinding not only increased labor costs, but it also slowed overall part flow. The shop uses highly automated cutting and bending centers, but such speed doesn’t mean so much with a big bottleneck in grinding. The laser welding system has allowed the fabricator to reduce the grinding and graining time greatly and, for some jobs, eliminate it entirely. This, sources said, has made the entire laser welding endeavor well worth it.
“Our thought was that if we could create a surgically precise weld and eliminate the need for secondary operations, including grinding and graining, we could give the customer a superior product for a lot less money,” Estes said.
About 15 years ago personnel witnessed efficiency and accuracy in upstream processes like cutting and bending, but they also saw that grinding bottleneck. So managers began considering options and experimenting. Refocusing the head of a 1980s-vintage, flat-bed laser cutting machine, technicians laser welded a few test workpieces. The machine had no modern level of control, so they had to do a lot of finessing to make it work, but they eventually welded 20-gauge cold-rolled steel. They liked what they saw.
“We did some simple butt welds and were encouraged by what we were seeing—good penetration and very flat, nice-looking welds,” Estes said. “In some instances, the welds almost looked like an ornamental feature. It gave us the encouragement that there might be some merit to pursuing this process.”
About a dozen years ago the fabricator began welding some initial samples for customers, demonstrating the process’s potential. One of the first samples was for a stainless steel oven door that required a lot of welding and a massive amount of finishing. At the time workers ground the corner weld joints and then grained the surface of the door, so that the entire component looked uniform.
As Estes explained, “We didn’t have a laser welding machine at the time, but in cooperation with one of the laser OEMs, we were able to run the parts and achieve very good results. It would be several years before we brought in a machine, but we at least validated the concept.”
Five years ago the company invested in a Prima Power CO2 laser machine. It can act as a flat-sheet cutter, a 3-D cutting system, as well as a 3-D welding machine. Switching between laser cutting and laser welding doesn’t take long. “It allowed us to generate revenue through traditional laser cutting, which basically supported our laser welding R&D,” Estes said.
As laser welding development manager, Jay Reddick has an unusual title in the contract fabrication world. He spearheaded the company’s R&D effort, which took almost two years. The president of the company insisted that the new technology be developed to make legacy processes better and less costly. To that end, Reddick and his team took existing products that were welded with traditional processes and experimented with laser welding.
A laser beam emerging from a focusing head looks like two cones, one on top of the other. Where those cones meet at the point in the middle is the focal point. The weld spot diameter changes depending on where that focal point is located in relation to the metal surface being welded. By moving the focal point away from the metal, the more out of focus the beam is and, subsequently, the wider the area affected at the point where the beam contacts the material. The more out of focus the beam is at the point of weld, the more energy it takes to maintain the weld.
The focal point is adjusted based on the type of weld needed. A higher focal point produces a more defocused beam for slower, more cosmetic welds; a lower focal point produces a small spot size for faster welds that aren’t appearance-critical.
Speed makes the process attractive not only for processing efficiency but also for reducing stresses on the material. As Reddick explained, “You’re moving so fast that the residual stresses imparted into the metal, especially in stainless steel, almost go away.”
When Reddick took on his new position, he found he was tackling a process that had been developed primarily for the automotive industry—a low-mix, high-volume environment very unlike Estes Design and Manufacturing. “We were starting out in a new territory.”
Conventional laser welding doesn’t use a filler wire, so many of the rules of traditional wire welding don’t apply. Reddick’s years of welding cosmetically important joints did come in handy, though, especially when analyzing assist gas requirements. “Getting coverage with an inert atmosphere, without blowing impurities into the inert atmosphere, is an art unto itself,” he said.
He pointed out one recent application that required a full-penetration weld on a stainless steel corner joint with a clean root below and a cosmetically important surface on top. This required argon shielding below to cover the root, and nitrogen above to provide an inert atmosphere to shield the surface from impurities and help cool the metal. The heavy argon underneath supported the molten metal at the root. “These are the kind of things you don’t learn in school,” Reddick said. “We just had to determine what each gas did and how each gas affected the laser weld.”
When presented with a cosmetically critical joint, technicians first consider what exactly the customer wants the joint to look like and then work backward from there (see Figures 1 and 2). Reddick pointed to a corner weld, formed by two bent flanges on a panel. “We’re looking for an appearance that matches almost exactly the outside of a formed corner,” Reddick said. “Consider the outside of a bend line on a part. That is exactly what we want that welded outside corner to look like. This takes experimentation; trial and error with various joint configurations, including overlaps, leads us to configurations that give us that ‘full-radius’ weld.”
In other words, they wanted a joint that really doesn’t look like a joint at all. In this situation, Reddick explained that a half-overlap configuration, with one edge covering only half of the abutting edge, often works best, because the laser weld tends to fuse the adjacent edges to create the desired smooth corner. But again, the exact joint configuration depends on the workpiece at hand.
A half overlap between two perpendicular sheets may seem like it gives some wiggle room on fit-up, but this is only 22-gauge material. “We’re only talking about a 0.015-in. overlap,” Reddick said. “That means our forming tolerance has to be tighter than that.”
Consider a rectangular panel with 90-degree flanges, welded on all four corners. “On our very best day with our very best equipment, we can create a tight corner on all four sides, but the position in space of those four corners really has to be within half the width of a laser beam diameter,” Estes said. “That’s very challenging.”
Advanced equipment can bend flanges so that they mate perfectly to adjacent flanges, but the absolute positions of all four corners of the panel may not be exactly the same (again, less than the width of a laser beam) from part to part. This, sources said, comes from the unavoidable variability of sheet metal forming.
“We can form the corner precisely enough with our panel benders so that it will be extremely tight, and [laser welding] can fuse both edges of the material,” Estes said. “But to position all four corners in a point in space that’s accurate enough for a precise CNC positioning system to bring a laser beam there, to weld at a specific point in space, that’s where the clever fixturing design come into play.”
“We term this, ‘Bringing the corner to the laser,’” Reddick said.
How exactly does the fabricator accomplish this? That, sources said, is proprietary, but generally speaking, the shop’s fixturing technology can hold joints accurately enough for the 4-kW, gantry-style laser system, which can position the processing head to within ±0.003 in. in the X, Y, and Z dimensions. For each job, the fixture requires every bit as much thought as the parameters for the laser welding process itself.
During the past few years the company has determined which of its existing products represent good candidates for laser welding. Technicians then ran samples, showed them to customers, and generated price comparisons. Sources conceded that acceptance of laser welding has occurred gradually, but at this point the level of acceptance has ramped up significantly, to the point where the fabricator soon may be installing a new laser welding system to handle the demand.
“The cost-reduction benefits speak louder than anything,” Estes said.
The more the company laser welds, the greater the knowledge base it builds. In the beginning, Reddick recalled working with customers to alter designs slightly, to take advantage of the process. Now, however, the company attempts to work with customers early in the design phases.
So what makes an ideal candidate for laser welding? As Reddick explained, “It depends on the part geometry. It depends on the manufacturing process that goes into that part, and it depends on the material type and thickness, what the customer’s requirements are aesthetically for the joint, what the requirements are structurally. Many factors go into the decision, and a lot of them are based on our experience over the past five years.”
If a part has a lot of straight-line joints, the results from laser welding can raise some eyebrows. Some straight-line geometries in 20-gauge steel now are being welded at up to 200 in. per minute. That’s 20 to 40 times faster than conventional welding. For some cosmetic stainless applications, the company has achieved welding speeds between 60 and 120 IPM. Of course, specific speeds depend on the application.
Combine these speed benefits with the fact that such welds may require less grinding or eliminate it altogether, and one can see why the fabricator spent so much time and effort preparing its laser welding process. Such dramatic improvements are worth years of effort.