From Multispot Welding to Limiting Soot: Tips for Optimal Laser Joins
Manufacturing Engineering Magazine October 2018
By Geoff Giordano
New technologies can produce the ‘holy grail’ of laser welding: spatter-free, no porosity joins
In our May webinar titled “Lasers in Manufacturing: State of the Art in 2018,” we noted the emergence of some novel technologies to produce the “holy grail” of laser welding: spatter-free joins with no porosity and, when required, highly aesthetic outcomes.
To expand on those tools, we asked laser suppliers to speak in-depth about a particular aspect of their systems or process development and provide real-world tips for users—or at least illustrate the power of current laser welding systems for those not yet using them.
Tom Kugler, fiber systems manager for Laser Mechanisms Inc. (Novi, MI), begins our “self-help” section on laser welding by offering a handful of real-world tips for—and a little of the science behind—producing consistent welds with more penetration, little to no porosity and better surface finish:
Understand keyhole process: Laser processing in this regime creates a keyhole—a deep and narrow zone where the beam vaporizes metal off the liquid walls of the keyhole’s sides. The keyhole moves with the beam through the weld path like a hot wire through butter. “Keeping the keyhole open and stable is vital to weld quality,” said Kugler. “A collapsing keyhole can trap gas vapor in the metal, creating porosity and preventing fusion. Anything changing the intensity, focus spot size or position of the beam relative to the keyhole can affect weld quality.”
Limit soot near the weld: Vaporizing metal from the keyhole shoots out and cools to form a fine soot of metal particles that can scatter the laser beam on its way to focus. This scatter will cause the beam to change intensity and focus spot size on the workpiece and can cause the focus spot to move erratically on the surface, he said. The result can almost be a complete loss of weld penetration if the soot is not directed immediately away from the weld zone by gas jets and exhaust systems.
“Soot effects can be an issue if removal is not part of the system design,” said Kugler. “Always move air across the weld zone from a clean air source to the exhaust. This can be as simple as a fan or might involve purpose-built plenums in the machine.” If welding over a large area, start closest to the exhaust system and move away so soot is always moving away from the weld zone and toward the exhaust, he added.
Cross jets are important: Another zone where gas flow is important is the cross jet or air-knife above the weld zone near the laser optics. An air-knife deflects debris coming up from the weld zone toward the exhaust system. An adjustable 360° rotating air knife is often ideal for this purpose.
Focus spot size matters: Producing a smaller focus spot creates a more stable keyhole due to the high power density at focus, with more tolerance to slight changes in intensity, according to Kugler. A change of 2X in focus spot size reduces power density 4X. “Small changes in spot size from variations in soot level and focus position still produce sufficient power density for a stable keyhole,” he said. “Choose spot sizes in the 100-300 µm range if part fit-up allows.”
High-power optics: High average power lasers can stress transmissive lenses. Laser energy passing through a lens will cause some heating in the lens, creating an effect called “thermal focus shift.” This shift can take two to three minutes to reach equilibrium. Meanwhile, the focus position of the system is moving closer and closer to the focusing optics. If too pronounced, this change in focus during welding will degrade the process. Contamination on the optics worsens this effect.
“Reflective focusing mirrors with direct water-cooling just below the surface of the mirror are an alternative,” said Kugler. “Swapping lenses for focusing mirrors creates an optical system with about 10% of the thermal focus shift of transmissive optics—and, they are immune to slight contamination.” These mirrors have their small focus shift in a fraction of a second, he added. Reflective systems are recommended for very high-power welding applications or high duty cycle applications in the 6+ kW range.
Welding vs. Bending Sheet
The primary appeal of laser welding in sheet metal manufacturing, besides high welding speed and design freedom, is a significant reduction in—sometimes even elimination of—rework, said Brett Thompson, sales engineer, Trumpf Inc.(Farmington, CT).
“It can be easy to overlook how impactful element design freedom is when looking at the bigger picture,” he explains. “A bend has always been cheaper than a weld. With high-precision press brakes capable of making complex bending simple, it has been a no-brainer to replace welds with bends wherever possible.”
But, he said, “things get interesting” when a part must still be welded.
“Laser welding has a higher cost per hour than a press brake,” he said, “but if the part is tooled up to weld four seams already, making it five by swapping a bend for a weld can make a lot of sense when considering the weld could even be a faster solution. Thinking more broadly, you apply this same principle to machine capacity. Certain features restrict tooling flexibility on a press brake.”
Consider this scenario in which a job shop has two press brakes, he continued, one with an extended opening and 10′ (3.048 m) of bed length, the other a small electric press brake with an 8″ (203.2 mm) opening height and 3′-long (0.914 m) bend. Producing a fairly simple part (see illustrations) consisting of a square and a rectangle is generally straightforward, although considering the part’s geometry, size and number and height of bends required, a setup requiring tall upper tools with extensions is necessary.
However, what if the larger press brake is booked for another job?
“Does the owner decline the work?” Thompson asks. “Sub it out? Risk losing the job by pushing out delivery until the larger press brake—needed due to the smaller press brake’s inability to accommodate such a large part—becomes available? If you design with laser welding’s flexibility in mind, there’s no need to decide.”
Thompson explained how laser welding’s speed and seams that often require no rework make the solution easy. “It makes sense both from a cost perspective—it’s a lot cheaper to weld this part with a laser than to MIG weld it—and from a machine-capacity perspective to change the design to increase flexibility.
“When we take the sides off this part and weld them to a bend profile, it becomes possible to run the part on whichever press brake is available—no need to wait for the larger machine,” he continued. “From a practical perspective, it’s a lot more likely that the parts will remain consistent, too.”
Overlap Part Fit-Up
As illustrated in this scenario by Erik Miller, business development manager for the laser group of Miller Electric Manufacturing Co. (Appleton, WI), laser welding offers opportunities to not only improve aesthetics but also cut process time vs. TIG or MIG welding.
In precision stainless steel welding applications that require outside corner welds, such as those found in commercial food equipment manufacturing or electrical boxes, laser welding can save significant time compared to traditional welding processes—especially when an overlap part fit-up is used instead of corner-to-corner part fit-up.
In outside corner laser welding, an overlap fit-up provides more welding tolerance in the case of slight over- or under-bend during forming. Laser welding still produces high-quality welds even when fit-up isn’t completely flush.
When these outside corner welds are TIG or MIG welded, a corner-to-corner part fit-up is often used. This means the inside corners of the materials touch and leave a recessed area, similar to a V-groove, that must be filled in with weld metal. A gap in the fit-up can occur more easily in a poorly formed corner-to-corner profile, according to Miller.
In addition, a corner weld that is TIG welded typically must be finished with post-weld grinding to smooth the corners, remove excess weld reinforcements and achieve the necessary aesthetic requirements. “This is also true when corner welds on thicker sheet metals are completed with MIG welding. This post-weld grinding adds significant time to the process,” he said.
Switching to an overlap part fit-up when laser welding outside corners not only delivers a wider operating tolerance for welders, it can also significantly improve productivity, he added.
“Laser welding is up to 10 times faster than TIG welding, thanks to the faster travel speeds. But in some precision stainless steel welding applications, the even greater productivity benefit lies in the ability to reduce the time and money spent on post-weld grinding, letting welders spend less time grinding and more time welding,” said Miller. “This is because laser welding creates a smooth surface and reduces—and in most cases eliminates—distortion, which greatly reduces the need for post-weld grinding. In fact, it can eliminate post-weld grinding altogether in some applications.”
Replacing TIG for Aerospace
Further illustrating laser’s ability to replace established methods, an aerospace manufacturer recently asked Prima Power Laserdyne LLC (Champlin, MN) for assistance in understanding if their TIG welded parts were a candidate for laser welding, explained Mark Barry, vice president sales and marketing.
“We suggested a design modification in fit up that assisted with quality and robustness of the process,” he explained. “The initial process parameters governed by material type were known to the applications team. The changing of power and how the power was delivered affected the weld profile.” A simple metallurgical evaluation showed the resulting profile as a function of feed rate and power. The user was able to identify the best parameter for their requirements.
“Once process parameters were established through the use of the simple matrix, a marked improvement in weld quality and cost was obtained by utilizing nitrogen as an assist or cover gas,” Barry added. “This is not a common practice with conventional welding processes, but has demonstrated a quality and cost advantage when laser welding.” Process development and optimization took less than three days in the lab.
“These drilling, cutting, and welding process parameters we developed will be used again, both as final products and start points for future process development,” he concluded. “The lesson is clear: Don’t settle on the first parameters that work if the results justify a simple evaluation. This is a technique taught for over 35 years to laser system users around the world and helps to obtain the unique processes that only laser machining can accomplish. Our process engineers are well versed in all of these processes and use these techniques to help our customers achieve the desired results.”
When Two Beams Are Better
To improve automotive aluminum welding with its high-brightness direct diode lasers, Laserline GmbH (Mülheim-Kärlich, Germany and Plymouth, MI) used its Multi-Spot Module to create a spot-in-spot process combining the benefits of heat-conduction and keyhole welding.
By superimposing a circular center beam on a square outer beam, the process team of Dr. Axel Luft achieved smooth surfaces, straight edges and high penetration depth while maintaining fast welding speeds and reducing spatter.
Thanks to the smoothing effect of the larger spot, the process allows the use of aluminum magnesium (AlMg) wire to improve outer skin quality. AlMg wire is stiffer than commonly used aluminum silicon wire, producing a stronger joint while allowing faster weld speed thanks to its higher absorption.
To tailor the process further, the module lets users shift power between the inner and outer spots and even move the inner spot to the front or rear of the larger spot. In its process development, Luft’s team improved smoothing by moving the inner spot toward the front of the outer spot. For other applications, the outer spot can be designed for optimal effect. The company began offering motorized versions of the module early this year that are slightly larger than its manual units.
While the triple-spot modules were initially developed for a special brazing process with a new coating for Volkswagen, Luft said, with the spot-in-spot method “we can do a lot of different welding processes. If you have a process where you would like to use a smaller spot but you cannot because you get a robustness problem, we can improve (on that).”
The motorized module allows users to easily switch from thick welds to thinner welds and materials and vary power to the inner or outer spot by up to 80%; a custom fabricator can “go from weld to weld” much easier.
“I think it’s a good addition if you have a job shop” and need to change process parameters quickly from project to project, said Luft. “It is interesting for critical processes and different kinds of materials, like aluminum, steel and copper and all kind of welds” for tailored blanks, power-train components and batteries.
Laserline currently integrates the module into Scansonic optics with filler wire and without, “but we are very open to customizing it if a customer has a process and he wants to improve it.”