March 17, 2014
Using laser welding for making tube or pipe on a mill is a proven technology, but and advancements in tracking systems and controls have made lasers easier to use than when they were first introduced. However, this doesn’t mean that swapping gas tungsten arc welding (GTAW) or plasma welding for laser is simple. Thinking about changing over to laser requires an understanding of how the laser welding process differs from GTAW and plasma welding, and how the various lasers differ from each other.
For all the advancements, the laser welding process and benefits remain fundamentally unchanged. It is a fusion welding process in which a molten puddle is generated between two metal surfaces and solidifies in a matter of milliseconds to form a weldment. It can be used in a conduction welding mode, as is the case with gas tungsten arc welding (GTAW, also known as tungsten inert gas [TIG]) and plasma processes. The vast majority of laser welding applications, however, and those in which the greatest advantages are derived, involve a technique known as keyhole welding.
In this mode, the laser beam is focused down to a spot diameter ranging from 0.010 to 0.020 inch. This creates an extremely intense energy density, or power density, which causes the fusion zone to rapidly heat to a point where the molten metal starts to vaporize at the center of the weld beam spot. As the material evaporates, it opens up a channel (keyhole) in the molten metal. The weld beam is now able to reach deeper into the fusion zone rather than transferring energy only at the surface as with conduction welds. Vapor pressure from the plasma produced inside the hole keeps it open during the weld, and the keyhole allows the laser energy to produce a deep, high-aspect-ratio weld.
As a result, laser welding has proven to be a superior process as it provides low average heat input, resulting in a narrow heat-affected zone (HAZ), less segregation of alloys leading to higher corrosion resistance, and less time at annealing temperatures compared to GTAW and plasma welding to effect diffusion and homogenization of the grain structures. In addition, welding speeds can be three to five times faster than GTAW, thereby enabling a fabricator to weld thousands of feet of coiled products without stopping to dress or grind electrodes.
These attributes are particularly important in the production of austenitic, ferritic, and duplex stainless steel and nonferrous tubes used in the automotive, aerospace, food processing, medical, oil and gas, and chemical industries.
All laser systems used for tube and pipe welding applications comprise the following elements:
CO2 lasers have been, and continue to be, the technology of choice for welding high-value tubes and pipes, evidenced by the hundreds of installations worldwide. However, the CO2, laser isn’t the only one available and affordable these days. In recent years solid-state lasers (disk, fiber, and direct-diode) have begun to receive consideration. Chief attributes include higher wall plug efficiency (WPE), a simpler beam delivery process, and no helium requirement.
More About CO2. CO2 lasers use a combination of helium (He), nitrogen (N), and carbon dioxide (CO2) gases, which are energized by an electrical discharge, typically a radio frequency (RF), to create a laser beam with a wavelength of 10.6 µm. Because the power source uses electrical power to create the electrical discharge, and the discharge powers the laser beam, this process uses a secondary interaction to make the laser work. The two major arguments against the use of CO2 lasers are the WPE, which is 10 to 12 percent, and their use of helium. A cause for concern is the price, which has risen over the past few years because of the possible closure of the Federal Helium Reserve in Amarillo, Texas.
However, the Helium Stewardship Act of 2013 passed by Congress has forestalled the closure because it is a critical element to a number of technologies and industries, including medical scanners, LCD screens, welding tools, and computer chips. This decision was meant to ensure a secure, reliable, and continuous supply of helium for end users and provide the time for commercial producers in the U.S., the Middle East, South America, and Russia to develop the necessary production facilities. According to market researcher Cryogas International, the reserves in these parts of the world are sufficient to sustain the industry for the foreseeable future.
Solid-state Characteristics. Disk and fiber lasers use electrically excited, light-emitting diodes (LEDs) to provide the optical energy needed to react with a medium in the laser resonator. This reaction produces a laser beam with a wavelength of approximately 1 µm and with a WPE of 25 to 30 percent.
Direct-diode lasers generate the laser beam directly from the LEDs. The wavelength typically is from 920 to 1,040 nm, and this type achieves a WPE of 40 percent or higher.
Four main factors contribute to choosing the optimal laser: the workpiece’s reflectivity, the beam delivery method, the mill’s forming capability, and safety compliance.
Reflectivity and Power Absorption. The metal’s reflectivity is a factor in choosing the optimal laser for the application. The shorter wavelengths generated by disk, fiber, and diode lasers have better absorption rates, especially when welding highly reflective metals such as aluminum, copper, and brass. However, when creating a keyhole, a CO2 laser generates plasma, which contributes to higher absorption. This effect does not occur with the shorter-wavelength lasers.
When welding at high power levels or speeds, high-brightness disk and fiber lasers generate more spatter (expulsion) than CO2 lasers. The reasons are complicated and not fully understood, but wavelength and keyhole geometry, specifically the incidence of angle, are contributing factors because of their influence on absorption. Whether the amount of spatter is significant depends on the material being welded. Regardless of its significance, its presence can decrease the material’s corrosion resistance and increase weld roll maintenance because it can accumulate.
Beam Delivery System. Because its wavelength is shorter, a solid-state laser beam passes through transparent materials such as quartz and glass easily. This characteristic enables the use of fiber-optic cables to transmit the beam from the laser to the focusing optics. Since the fiber-optic delivery system requires no alignment, perhaps with the exception of the initial connection to the laser, this grants greater flexibility in system layout.
Two caveats: Although fiber-optic cables are robust, they are susceptible to mechanical and thermal damage. Mechanical damage comes from exceeding the minimum bend radius, which can fracture or break the cable. Thermal damage occurs if either end of the cable becomes contaminated or if back reflections of the laser beam occur.
The longer wavelength of the CO2 laser requires the use of mirrors to guide the beam from the resonator to the weld head. Maintaining mirror alignment and cleanness are ongoing challenges. Misalignment most often results from a poorly designed structure, one in which the mirrors, resonator, and focusing optics are installed on different platforms. A well-engineered system integrates these elements onto a common frame (see Figure 1 and Figure 2). This virtually eliminates the need for routine mirror alignment. In addition, when a low-pressure supply of clean, dry air flows through a properly sealed beam delivery system, it greatly reduces the potential for smoke or dirt to accumulate on the beam delivery mirrors.
Forming Requirements. Although the tube forming process is the same whether using arc or laser technology, a new mill designed for laser welding is more productive than a mill designed for arc welding and upgraded to laser welding. However, 50 to 60 percent of laser welding systems are integrated into an existing tube mill. While an older mill may be perfectly acceptable for welding at 15 feet per minute, increasing weld speeds to 30 FPM or higher may reveal limitations. This raises questions such as “Can the mill consistently provide good edge presentation?” “What welding speed does the flying cutoff allow?” and “Will other downstream processes limit the maximum output of the tube mill?”
While arc welding is more forgiving, edge condition, excessive weld seam gap, edge mismatch, and seam wandering can be problematic. Since the focused laser beam is about 1⁄10 the size of a GTAW or plasma arc, these concerns become more critical. The gap width between the strip edges has to be sufficiently smaller (less than 0.004 in.) than the diameter of the focused laser beam (approximately 0.012 in.). For maximum welding speeds as well as maximum consistency, the gap width should be technically zero.
Likewise, the height mismatch of the strip edges, which affects weld strength, should be less than 15 percent depending on the wall thickness, since the
volume of the laser-generated melt pool will not compensate for excessive height differences between the two edges. This is especially critical for the weld bead on the ID, which cannot be planished by grinding, squeezing, or hammering.
Safety. The wavelengths of all the lasers discussed in this article are potentially hazardous, requiring precautions to prevent laser light from causing skin burns or permanent eye injuries from direct or reflected beams. In the U.S., the American National Standards Institute (ANSI) Z136 series provides guidance for the use of protective eyewear and other elements of safe laser use.
Some lasers are so powerful that even the diffuse reflection from a surface can be hazardous to the eye. For this reason, an enclosure around the weld box is highly recommended. For the shorter wavelengths of solid-state lasers, goggles and glasses with coatings specific to the wavelength and optical density are sufficient. When using CO2 lasers, the standard industrial safety glasses are adequate. Solid-state lasers also have more stringent requirements for safety enclosures to reduce operator access to the welding point. Note that additional provisions are necessary if the enclosure must accommodate wide variations in tube diameters. In addition, light barriers are necessary around the production area when the enclosure must be removed for adjustments to the weld box tooling while the laser is on.
With the variety of laser technologies available today, choosing the right laser source may seem a bit baffling. For highly reflective materials or when employing conduction welding for thin-wall tubes, the shorter wavelength of disk, fiber, and direct-diode lasers are worth consideration. For welding heavier walls, or welding at high speeds, CO2 lasers provide greater process stability with less spatter.
The available technologies also aid in the laser welding process. Optical seam tracking systems provide active adjustment and real-time display of the laser beam’s position relative to the weld seam centerline as well as the gap width and edge mismatch. Today beam delivery systems are available with a CNC, which allows programs to be saved. The programs store the power levels and shield gas flow rates and control the position of the focusing optic, seam tracking device, and shield gas nozzles using motorized X and Z axes, thereby decreasing the setup time when changing from one job to another.
Finally, it’s important to consider suppliers that can offer all of the available laser technologies, such as seam tracking devices and a fully integrated beam delivery system, and provide the experience and process know-how. These factors can help to maximize uptime and quality.
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