Solid-state laser technology has matured, leading to development of new, cost-effective welding applications, such as hybrid welding
By Bruce Morey
Lasers for welding have proven their worth in the last 35 years in select applications. Despite their high capital cost, the precise, intense heat source makes lasers the right choice where there are tight tolerances, close fit-ups, and thin materials. These are applications where lasers produce less distortion, with their smaller heat-affected zone (HAZ) around the weld. That lack of distortion is critical in applications from medical devices to sheetmetal lap joints on automobile body parts, according to Ed Hansen, global product manager for ESAB (Florence, SC), the international welding company.
He said that the laser has become more popular as solid-state lasers have come to dominate the field. Because of their operating wavelength in the near infrared, solid-state lasers deliver their beams along flexible fiber optics instead of the optics and mirrors required with older CO2 technology.
“Fiber-delivered lasers are now useful on larger and higher-volume parts,” he said, noting that automated welding applications that do not currently use lasers should consider them given the advances. “Most of the things you need to do for automated welding,” he said, “you will need to do to be successful at using a laser.”
However, as an autogenous process with no filler material, laser-only welding has been limited to those thin-walled and tight-tolerance applications. Laser-only welding is currently limited to applications where joint tolerances allow no more than about 0.1 mm of gap variation. In a number of ways that is changing. One way is with hybrid welding.
Companies like ESAB are using their expertise in gas metal arc welding (GMAW)—think MIG—combined with fiber-delivered laser energy to create the best of both in these hybrid systems. The addition of GMAW means using an arc to add-in filler material from a wire. Welds in thicker materials benefit from the precise, deep penetration heat source of lasers, and the combined system is faster and more forgiving. “With a filler metal added, this allows you to start applying laser welding to joint fit-ups and joint designs that are not optimal,” Hansen said. Modest gaps can be bridged. Certain amounts of surface contamination are tolerable and weld chemistry and mechanical properties can be manipulated. Designers can also add fillets and bead reinforcements for greater strength and to resist fatigue failures. “This means laser welding can be applied in more conventional applications,” he said.
ESAB’s Hybrio is such a hybrid system, combining a solid-state laser with GMAW. It welds at 3–10 times the speed of conventional processes, with 80–90% less heat input, according to the company. Its wider bead bridges gaps that are four times wider than a conventional laser-only process. Just as importantly, an adaptive control system monitors the weld joint in real time, adjusting for joint gaps and mismatches and further broadening the process window to handle gaps up to 1.5 mm. New applications now opened to laser welding include shipbuilding, construction, pipeline supplies (such as oil-country tubular goods), heavy equipment/off-highway, and railroad equipment.
In terms of hybrid welding, Jim Hurley, southeast regional sales manager for Trumpf Inc. Laser Technology Center (Plymouth, MI), also pointed out that the laser not only saves time, but material as well. Many weld joints prepared for welding are V-grooves, and a wide joint is needed for traditional GMAW to get heat energy to the bottom. With laser’s deep penetration, a smaller included angle is needed and hence less filler material.
Solid-State Lasers—Power and Multiplexing
Hansen from ESAB noted that while solid-state and fiber lasers now run up to many 10’s of kW in power, the practical limit of what they would use in hybrid welding is around 12 kW. Beam quality in terms of beam parameter product (BPP) need not be finer than 10–12 mm-mrads, in most welding applications. In fact, for “high power” welding applications, from hybrid to remote scanning, 75% or more of most applications require lasers that provide power between 4–6 kW, according to Hurley from Trumpf, with a BPP around 8 mm-mrad or better. For example, a common laser for welding is the Trumpf TruDisk 6002. It provides a near-IR beam at 6 kW with a BPP of 8 mm-mrad. Another plus is that some models deliver their energy through up to six individual fibers, enabling a single laser to power a number of independent workcells, reducing capital cost.
As important as the advent of hybrid welding is, Hurley also noted that remote laser welding remains important. Remote welding uses the unique standoff capability of lasers and scanning optics. Remote welding systems rapidly direct a laser beam over large parts like automotive doors and closures. They weld a number of spots and short seams separated by distance, saving time over traditional spot-welding methods. In many cases, it produces a better weld, according to Hurley. Remote welding started with far-IR beams from CO2 lasers delivered in flying optics. Today, he noted that solid-state lasers are now the choice. This means that laser heads mounted on common six-axis articulated arm robots provide unprecedented mobility, combining motion of the head with directed motion of the beam.
With advancements like solid-state remote welding and hybrid welding, Hurley believes there is plenty of room for growth for laser welding, especially in North America. “The Europeans are leaders in developing and deploying it,” he said. They are more comfortable with the technology, according to Hurley. “They are seeing the benefits,” he said. Those benefits will grow on manufacturers in North America, he believes.
The Foundation Elements for Growth
“Laser cutting is like a divorce, but laser welding is like a marriage,” said Paul Denney, senior laser applications engineer of Lincoln Electric (Cleveland, OH). “For a cut getting separation cleanly and quickly is what you worry about. However, for a successful weld you do not only worry about getting things to ‘stick’ together but also what has to be done so that ‘union’ will last in the long term. For laser welds you have to be concerned about the chemistry of the base and weld metal, the resulting microstructure of the weld and the HAZ, and the size.” He sees laser-welding growing, from remote welding to creating tailor-welded blanks. Lincoln Electric supplies laser-welding systems, hybrid laser systems that combine laser and GMAW, and hot wire cladding laser systems.
The key, as Denney sees it, is to think of laser welding as a revolutionary, not an evolutionary, process, especially for the newer hybrid approaches. “You do not want to try and replace a resistance or arc-welding process one-for-one. For example, lasers want to give you a high aspect, deep penetration weld, but if you look at the drawings from most companies they might specify an edge-lap joint or fillet-type joint,” he explained. Simply using a laser to weld the fillet they specify, you cannot get high enough deposition rates to justify the laser. “What you want to do is use a joint that works to the strength of the laser like a butt joint,” he said.
Anecdotes like these speak to the need to introduce laser welding early in the design process, with design engineers involved as well as the manufacturing engineers on the factory floor. At that point, it is vital to explain the process in terms of economics. “You almost need to talk to the finance guys,” he said, explaining that with a rapid process like lasers, if the part is designed to the laser process, you can actually reduce the cost per part.
At the same time, manufacturers are necessarily cautious about adopting novel approaches to mainline products. “You need to ease into some novel areas and then, once it has proven itself, [manufacturers] can trust it and use it more broadly,” he said. “That has happened in transmissions and in car seats. Eight years ago, no one was really using laser remote welding for car seats and now almost all suppliers are doing it. Why? Because they trust it,” he said.
He also pointed out that for select industries and applications, laser welding has reached a certain maturity, like tailor-welded blanks, drive train, and medical components. Costs for lasers have decreased, but he sees that cost-curve flattening in the last couple of years. Nevertheless, he predicts both hybrid and remote welding applications will expand, replacing other welding methods. “Sweeping a laser beam from spot to spot is much faster than moving a resistance weld gun between spots even as the welds themselves take about the same,” he said. Therefore, for the same number of spot welds, that means faster cycle times and higher throughput.
Machine Tool Approach to Laser Systems
“Welding once was background noise compared to laser cutting, which is where the majority of systems were sold,” said Mark Barry, vice president sales & marketing of Prima Power Laserdyne (Champlin, MN). “But about five years ago, we began to see an expansion in laser welding applications.”
From a negligible slice of their business, he has seen welding grow to about 25%. Laserdyne specializes in delivering turnkey class 1 laser installations that provide all of the stable movement and control of an accurate machine tool. Customers include turbine, high-precision medical device, and electronics device manufacturers. He attributes two reasons for the growth of laser welding: the nature of the parts and the availability of efficient and economical fiber lasers. “The parts we deal with today are more often near-net shape. We are joining finished parts together,” he said. These highly machined, high-value parts already have the tight fit-up needed for an autogenous process. They also need a process that minimizes distortion—ideal for laser welding.
The fiber lasers Laserdyne incorporates into its machines are providing an ideal and convenient laser source to meet the needs for precision welding. The fiber delivery means an output beam source that is evenly distributed as a top-hat (as opposed to a peak Gaussian distribution), with the advantage of low divergence.
Even with the advent of fiber lasers, laser welding capital expense costs can be relatively high but are offset by many advantages. As Barry found out from an extensive review with existing customers, capital cost was not the most important buying factor. The factors that matter include good process control; high-quality welds; robust operation with high uptime; and ease of use by operators not expert in lasers.
“Fiber lasers when employed correctly are easy to use and easy to teach people to use,” he said. “They provide a large window of acceptable parameters.” Some customers prefer a lower average power with many pulses; others prefer high power with few pulses. Customers can obtain a variety of results from the same basic system. “The simple processes actually mean we can do more interesting welding,” he added.
Another key development Barry is seeing is a single laser system tasked to do multiple operations, such as cutting, drilling, and welding. He attributes this to the newer quasi-continuous wave (QCW) fiber laser. “Before, manufacturers would cluster lasers in the same area, now they are distributing them into work cells because they can perform multiple operations and they do not need specialized operators who are experts with lasers,” he explained.
The next big jump in development may very well be high-brightness direct diode lasers, according to Hansen. Attributes he likes include lower cost, higher electrical efficiency and the small footprint or form factor. In fact, they are similar in size to current welding power.
What are direct-diode lasers? Many solid-state lasers use diodes to excite a lasing material. Therefore, disk-lasers or fiber-lasers use diode lasers as an intermediate power source. The direct diode laser skips the intermediate process. The trade-off is poorer beam quality but higher efficiency. Laserline (Santa Clara, CA) is a supplier of high-power direct diodes used to create the laser beam used in welding, cutting, or brazing. “The wall plug efficiency of a direct-diode system is between 40–48%,” said Wolfgang Todt of Laserline.
Laserline’s LDF series direct-diode lasers range from 3-20 kW power, though beam quality decreases as power increases. For example, the Laserline’s LDF 3-kW version is 20 mm-mrad, the 6-kW version is only 40 mm-mrads, with standoff distances of 150 mm.
These are better than they used to be and today there are a number of applications where this is sufficient beam quality, especially in aluminum welding, Todt said. Welding with direct diode lasers is not always autogenous. Audi uses LDF diodes to weld aluminum with aluminum-silicon wire filler, using 2–6kW LDF lasers.
In other applications requiring higher quality, Laserline introduced a beam converter device for its LDF line of laser diodes. “For up to 4 kW, the beam converter provides 8 mm-mrads of beam quality with a standoff distance of 500–650 mm,” Todt said. There are trade-offs for the enhanced quality. Wall-plug efficiency is reduced and the beam converter adds capital expense. Said Todt: “It is our answer to fiber lasers in applications requiring higher-quality fiber-optic delivery of a beam.” ME
This article was first published in the March 2014 edition of Manufacturing Engineering magazine. Click here for PDF.
Published Date : 3/1/2014