July 19, 2013
Tube and pipe producers have relied on GTAW and plasma for many decades, and CO2 lasers for more than a decade, but recently another choice has emerged: Fiber lasers. Relying on a solid fiber rather than a gas to generate the laser beam isn’t ideal for every material and wall thickness, but it will change the industry in a profound way.
When metal fabricators were first introduced to laser cutting as a viable thermal cutting option, they understood the word laser to mean just one type, the CO2 laser. Of the many other laser types that were available, none were suitable for metal fabrication, so the CO22 laser carved out a stable niche as the only laser used in this industry, complementing oxyfuel and plasma. However, nothing lasts forever, and in recent years fiber lasers have emerged as a contender for cutting metal parts, and this technology has been gaining ground rapidly among fabricators.
Likewise, some stainless steel tube and pipe producers switched from conventional welding processes to lasers. CO2 lasers were first on the tube and pipe scene, but fiber has begun to emerge as a viable option for this application too. A primary driver for the technology’s emergence is that it doesn’t require helium, whereas CO2 lasers do. The price for crude helium has increased about 70 percent since 2000, and the price for grade A helium has quadrupled, so it represents a much greater cost than it used to. Furthermore, helium is in short supply.
Stainless steel tube- and pipemakers have relied on plasma and gas tungsten arc welding (GTAW, also known as tungsten inert gas [TIG] welding) for many years because the processes are robust and reliable. Although lasers are fundamentally different from conventional welding processes because they use a light beam instead of an electric arc to produce the weld heat, all of these processes have one thing in common: To produce good results, the strip edge presentation must be optimized and stable. This means maximizing edge quality and minimizing edge wave, edge mismatch, edge gap, and seam wandering.
In a perfect situation, one with a new mill and perfect coil that was properly slit and handled in accordance with best practices, the production process would be predictable and stable. In reality, tube and pipe operations aren’t perfect. An aging mill, irregular maintenance, varying coil edge quality, and improper unwinding or winding can result in a process with a lot of variability. On a TIG or plasma mill, increasing the weld power to compensate is tempting, but this is akin to using a hammer to drive a screw: It might work, but the results aren’t pretty. It’s the wrong tool for the job.
As critical as edge presentation and seam location consistency are on a conventional mill, these factors are literally 10 times as important on a laser mill because a laser beam spot is about 10 percent the size of a TIG or plasma arc. For tube or pipe mill owners who have control over the edge presentation variables, or who are willing to invest some time and money to regain control, the switch to a fiber laser can prove beneficial in a number of ways.
While the forming processes don’t change when switching from conventional welding to laser, the equipment and welding processes differ greatly.
Weld Profile and HAZ. The biggest difference between conventional welding techniques and a laser system is the power density. A laser system concentrates the heat into such a small area (for example, 10 kW focused to circle between 0.010 and 0.025 in. in diameter) that it creates a keyhole weld, a weld so deep that in most cases it goes through the entire thickness (see Figure 1). The small spot size also results in a narrow heat-affected zone, which in turn provides a stronger weld. In some cases, a laser weld is 50 percent stronger than a conventional weld.
It’s worth noting that when using the TIG process, the weld profile can vary from operator to operator and from the start of the shift to the end. This is because the tungsten electrode often is ground by hand, and the potential exists for each operator to grind the electrode in a unique way. Also, the electrode changes shape slightly throughout the shift, altering the arc’s characteristics and subsequently the weld.
Seam Tracking. Although the laser keyhole process creates a stronger weld, the small spot size is a drawback because it’s difficult to keep it on the seam as the seam wanders from left to right. To compensate for a small amount of seam wandering, some mills use two or three laser beams. Unlike dual- or tricathode TIG systems that have one power supply per cathode, a dual- or triple-spot laser system relies on a single power source to create a single laser beam and a splitter to divide the beam into two or three. The beam power density isn’t a rigid fraction, fixed at halves or thirds, but can be changed by turning an adjustment screw on the laser head. This provides some versatility in finding the optimal power ratio among the various spots.
Another way to compensate for seam wandering is to use a seam tracking unit. A typical tracker uses a separate laser with triangulation to follow the weld seam, directing the laser head appropriately. This isn’t to say that a weld seam tracker module can compensate for infrequent maintenance or a badly worn mill. Seam tracking typically moves the spot laterally ± 0.080 in. For a laser spot size, typically a maximum of 0.020 in. dia., the tracker allows the laser head to cover an area 0.100 in. wide. On a neglected mill, the seam can wander ± 0.125 in. A seam tracker can help to compensate for normal wear on an aging mill, but it can’t solve the severe problems that come from years of neglect.
Speed. When all else is optimized and the laser is appropriately sized for the application, fiber welding can increase the mill’s productivity by 50 percent compared to conventional welding. In an extreme case, productivity can increase 300 percent.
Depending on the application, the speed limit is a phenomenon called humping. In layman’s terms, it’s similar to a boat making a wake. As the boat moves along, it creates a wake, an area from which water is displaced. After the boat passes, the wake fills in. A laser makes a keyhole weld, which is similar, creating a slot in the material. As the tube or pipe passes the laser, molten material fills the slot, just as water rushes in to fill the wake. However, if the line speed is too fast, the material can’t fill in the void before it begins to resolidify. Splitting the beam into two or three spots, which can be inline, side by side, or staggered, distributes the heat over a larger area and helps to reduce the humping phenomenon.
Helium Use. CO2 laser welding uses helium for two reasons. First, it’s one component of the resonator gas (although it’s called a CO2 laser, the resonator actually uses a mixture of carbon dioxide, nitrogen, and helium). Second, helium is used as a shielding gas, also called a plasma suppression gas, for the welding process. The use of a fiber laser, which creates the laser beam with laser diodes that pump a fiber doped with rare-earth material, eliminates the first use. The second use is optional; when a fiber laser uses helium as a shielding gas, it’s mainly for cosmetic reasons, and argon is a suitable substitute.
Fiber lasers achieve a shielding effect without using a separate gas. As the heat from the laser melts the metal and creates a keyhole, the metal generates vapors that act as a shield, preventing oxygen from reacting with the steel—in other words, the process creates its own shielding gas.
Eliminating helium consumption on a mill running 24/5 saves more than $75,000 annually at the current price, which is likely to rise.
Sensors. The sensor package laser systems use for process monitoring typically includes a seam tracker to keep the laser beam centered on the seam; an in-process weld monitor, which provides feedback about the weld parameters such as infrared and plasma energy; and a trailing camera fixed to the weld head that evaluates the geometry of the weld profile. After the system is taught the parameters of a good weld, it monitors the weld area and alerts the operator when the process changes. The sensors are in the beam path and are integral components of the laser head; they aren’t add-ons.
This is more advanced than TIG, which doesn’t make use of such sensors. For the most part, the operator relies on a voltage reading to determine the status of the arc. The downside is that, as the electrode changes shape over the course of a shift, the voltage monitor shows a decrease. The displayed voltage can fall by as much as 0.5 volts over an 8-hour shift even if the input voltage is stable. TIG mill operators know this and compensate by mentally adding a few tenths of a volt as the day wears on, so it’s not a big problem, but it is a minor headache that laser mill operators don’t have to deal with.
Fume Extraction. Fume extraction is more important with the laser welding process than with conventional welding. The sensors must have a clear view of the weld area to sense the infrared and plasma energies, laser backscatter, and other parameters. Any smoke that obscures the sensor’s view inhibits the sensor package’s ability to monitor the process.
Beam Delivery. A CO2 laser system uses a series of mirrors to direct the laser beam from the laser resonator to the laser head. Proper mirror alignment is critical to maximizing the system’s efficiency, so it’s important to minimize anything that can disturb the alignment. The base’s thermal expansion can be enough to throw off the alignment, so the laser resonator and the weld portion of the mill must be set up on one concrete pad, not two separate pads, to minimize the distortion caused by this. Unfortunately, a heavy-duty tube shear can be enough to upset the alignment. At the same time, mirrors must be kept clean for maximum efficiency.
The other beam delivery method is a fiber-optic cable. Because it is a glass fiber, it is subject to a minimum bend radius, so due care must be taken when routing the fiber through equipment enclosures and on robotic arms. Also it can be subject to contamination at the input and output ends. If the laser light hits a contaminant, the absorbed heat creates a thermal stress raiser, causing the fiber to fail.
Guarding. The bright light emitted by TIG, plasma, and laser can cause tissue damage to eyes and skin, so proper guarding is required. The American National Standards Institute’s specification ANSI Z49 discusses safety for welding processes; ANSI Z136 covers the use of lasers.
Efficiency. One more element in the cost/benefit analysis is electricity consumption. A fiber laser runs more efficiently than a CO2 (30 percent versus 5 to 10 percent). At $0.08 per kWh and keeping all variables equal, the fiber laser saves $45,000 per year.
This isn’t to say that laser welding, whether done by a fiber or CO2 laser, is suitable for every stainless steel tube or pipe. For many wall thicknesses and materials, conventional processes provide the best results, so there is no need to consider switching technologies. For some fringe applications, two or more of these might provide excellent results, so related factors influence the decision, such as the capital, running, and maintenance costs; available floor space; operator training time and cost; and process familiarity. And in a few cases, a laser provides the best results.
It’s also important to remember that a laser can’t cure a problem. In fact, laser welding is a much more demanding process than conventional welding. Keeping in mind the principles of mill operation, especially the various welding processes’ capabilities and challenges, is crucial before considering a change to fiber laser welding.
Laser technologies usually are described by the medium used to generate the laser beam, which happens in the resonator. The lasing medium can be any form of matter: gas, liquid, solid, or plasma. Five main laser types—one gas and four solid-state— have proven useful for cutting and welding metal:
The beam delivery system uses either a series of mirrors or a fiber-optic cable (not to be confused with the fiber that generates the laser beam in the resonator).
Lasers also are designated by the duration of the laser beam and power classification. Laser duration can be continuous wave (CW), single pulse, single-pulse Q-switched, repetitively pulsed, or mode locked. Laser classifications are 1 to 4, rated for their potential to cause injury to the eyes or skin.
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