Artem Komarov clarified that a lot has changed since laser welding first hit the scene three decades ago. It was a relatively unknown process at the time and a bit of a mystery to most, with unique characteristics for welding stainless steel pipes for a variety of applications. These days, new laser processes, sensor-based seam tracking and CNC systems are making laser welding easier to use and therefore desirable and applicable to many manufacturers.
Despite all the advances, the laser welding process and benefits remain fundamentally the same. This is a fusion welding process that creates a pool of molten material between two metal surfaces that solidifies within milliseconds to form a weld. It can be used in conductive welding mode, as in the case of gas tungsten arc welding (GTAW, also known as tungsten inert gas TIG) and plasma processes. However, most laser welding applications, and those with the greatest benefits, involve a technique known as keyhole welding.
As the material evaporates, a channel (keyhole) is formed in the molten metal. The welding beam is now able to penetrate deeper into the melting zone, rather than transfer energy only to the surface, as with conduction welding. The vapor pressure of the plasma generated inside the hole keeps it open during welding, and the keyhole allows laser energy to create a deep weld with a high aspect ratio.
As a result, laser welding has proven to be a superior process as it provides low average heat generation resulting in a narrow heat affected zone (HAZ), less alloy segregation resulting in improved corrosion resistance, and faster times at anneal temperatures compared to GTAW and plasma welding. welding to ensure diffusion and homogenization of granular structures. In addition, welding speeds can be three to five times faster than GTAW, allowing fabricators to weld thousands of feet of rolled products without stopping to straighten or grind electrodes.
These characteristics are especially important in the production of austenitic, ferritic, and duplex stainless-steel pipes, as well as non-ferrous metal pipes used in the automotive, aerospace, food, medical, oil and gas and chemical industries.
Influence of laser type
Four main factors influence the choice of the optimal laser: the reflectivity of the workpiece, the way the beam is delivered, the shaping capabilities of the machine, and safety requirements.
Reflectivity and energy absorption. The reflectivity of the metal is a factor in choosing the optimal laser for a given application. The shorter wavelengths generated by disk, fiber and diode lasers have better absorption values, especially when welding highly reflective metals such as aluminium, copper and brass. However, when creating a keyhole, the CO2 laser generates plasma, which contributes to higher absorption. This effect does not occur with shorter wavelength lasers.
When welding at high power levels or speeds, high brightness disk and fiber lasers generate more spatter than CO2 lasers. The reasons are complex and not fully understood, but the wavelength and keyhole geometry, in particular the angle of inclination, are determining factors due to their effect on absorption. Whether the amount of spatter is significant depends on the material to be welded. Regardless of its significance, its presence can reduce the corrosion resistance of the material and increase the service life of the welding rolls, as it can accumulate.
beam delivery system. Since the wavelength of a solid-state laser beam is shorter, it can easily pass through transparent materials such as quartz and glass. This characteristic allows the use of fiber optic cables to transmit the beam from the laser to the focusing optics. Because the fiber optic delivery system does not require alignment, except perhaps for the initial connection to the laser, this allows for greater flexibility in system layout.
Two caveats: although fiber optic cables are reliable, they are prone to mechanical and thermal damage. Mechanical damage occurs when the minimum bending radius is exceeded, which can lead to cable breakage. Thermal damage occurs when either end of the cable is contaminated or when the laser beam is reflected.
The long wavelength of the CO2 laser requires the use of mirrors to guide the beam from the resonator to the welding head. Maintaining mirror position and cleanliness are constant challenges. Misalignment is most often the result of a poorly designed design in which the mirrors, resonator, and focusing optics are mounted on different platforms. A well-designed system integrates these elements into a common frame. This virtually eliminates the need for conventional mirror alignment. In addition, supplying clean, dry, low-pressure air through a properly sealed beam delivery system greatly reduces the chance of smoke or dirt building up on the beam delivery mirrors.
Safety. The wavelengths of all lasers are potentially hazardous, requiring precautions to prevent skin burns or permanent eye injury from direct or reflected beams.
Some lasers are so powerful that even diffuse reflection from a surface can be dangerous to the eyes. For this reason, it is highly recommended to use a protective fence around the welding chamber. For solid-state lasers with shorter wavelengths, goggles and lenses with a coating that matches the wavelength and optical density are sufficient. When using CO2 lasers, standard industrial safety goggles are sufficient. Solid state lasers also have more stringent requirements for protective housings to reduce operator access to the welding site. Please note that additional adaptations are necessary if the casing is to withstand large differences in pipe diameters. In addition, light barriers are needed around the production area when the hood needs to be removed to set up the welding chamber tool while the laser is on.
Choosing the Right Tools
It is important to consider suppliers who can offer all available laser technologies, such as seam tracking devices and a fully integrated beam delivery system, as well as provide experience and technological know-how. These factors can help maximize uptime and quality, summed up Artem Komarov.