Laser welding is a modern method for joining stainless steel and other metals in the automotive, aerospace, and electronics industries. It is effective because it can overcome the challenges of the physical properties of these metals, such as stainless steel’s high melting temperature and low thermal conductivity.
Laser welding is a popular production method for stainless steel structural profiles by welding individual components together, such as flat laser cut strips, prefabricated solids, hollows, or other shapes. Despite the advantages in precision, speed and material protection, laser cutting and welding were used in niche applications for a long time. A notable spread of systems for laser cutting and laser beam welding did not take place until the mid-1980s. Before that, these systems were only used in research institutes and specialist companies.
Firing up fusion
Today, laser welding remains a non-standard welding process. In South Africa sheet metal processors are increasingly relying on this high-tech process, which continues to impress with its efficiency, quality, and precision. Laser welding (also known as laser beam welding) is a process that uses a concentrated heat source, in the form of a laser, to melt the materials. They then fuse as they cool down. It is a versatile process as it can thin materials at rapid welding speeds while running narrow and deep welds for thicker materials. The main advantages of laser welding are its high precision, since the laser beam focuses on a small area, resulting in minimal distortion and a high-quality weld.
Of importance is the fact that the focused laser beam delivers high energy density to the workpiece, allowing for rapid melting and solidification. The laser beam penetrates deeper into the medium compared to other welding methods and therefore limits the potential for cracking after autogenous welding. The process also features disadvantages such as the high cost of entry-level laser welders. The laser beam also has limited penetration depth and suffers from limited weld joint access. Data is limited, but a reduced weld strength has been reported when using laser welding. Inert welding gases that are suitable for laser welding include helium, argon, and argon/helium mixtures. Argon is the preferred shield gas for welding certain grades of stainless steel. It’s inert, has high thermal conductivity, and offers excellent arc stability. Argon suppresses plasma, which prevents unwanted sparks and ensures a stable welding arc. Argon’s density is also larger, which is favourable for sinking above the weld pool and better protecting the weld pool.
A range of options
There are three primary types of laser welders used for the welding process:
- Gas laser (CO2): A CO2 laser source is a mixture of gases with CO2 being the main component alongside nitrogen and helium. These lasers can operate in a continuous or pulsed mode at a low current and high voltage to excite the gas molecules. Carbon dioxide lasers are also used in special circumstances, such as in dual-beam laser welding, wherein two beams are produced and arranged either in tandem or side-by-side.
- Solid-state laser: These lasers use Diode Pumped Solid State (DPSS) technology to pump ore such as ruby, glass or yttrium, aluminium, and garnet (YAG), with a laser diode to produce laser rays. They are operated in either continuous wave or pulsed beam mode. The pulsed mode produces joints similar to spot welds but with complete penetration. These lasers have their fair share of disadvantages when compared to modern fibre lasers, but still have excellent beam stability and quality along with high efficiency.
- Fibre laser: Fibre lasers are the newer type of solidstate lasers that offer more power, better quality, and safer operation. The laser beam is created when the fibre absorbs raw light from the pump laser diodes. To achieve this transformation, the optical fibre is doped with a rare-earth element. By using different elements, laser beams with a wide range of wavelengths can be created. This makes fibre lasers perfect for a variety of applications. It is worth noting that a standard laser cutting head cannot be used for welding and a laser welding head cannot meet the cutting speeds and quality demanded in most industrial applications
There are two types of laser beam welding, both with unique operating principles to suit specific applications. How the material interacts depends on the laser beam’s power density.
- Heat Conduction Welding Heat Conduction Welding is where a focused laser beam is used to melt the surface of the base materials. When the joint solidifies, a precise and smooth weld seam is produced. Welds created using the head conduction method don’t need additional finishing. The energy enters the weld zone only by heat conduction. This limits the welding depth and thus the process is therefore ideal for joining thin materials. Heat conduction welding is often used for visible weld seams which need to be aesthetically pleasing.
- Deep Penetration or Keyhole Welding uses keyhole welding (deep penetration) mode to create deep, narrow welds with uniform structure. For metals, power densities of about one megawatt per square centimetre are applied. This not only melts the metal but vaporises it, creating a narrow vapour-filled cavity. This keyhole cavity is filled with molten metal as the laser beam advances through the workpiece. Keyhole welding is a high-speed process and thus, the distortion and the formation of a heat-affected zone are kept to a minimum. Many laser welding applications are conducted without the need for additional filler material. This is called homogeneous welding. However, some challenging materials and applications require filler material to produce satisfactory welds. Adding filler material improves the weld profile, reduces solidification cracking, gives the weld better mechanical properties, and allows for more precise joint fit-up.
Laser-hybrid welding combines the concepts of electric arc and laser beam. The two simultaneously act in the same welding zone, complimenting each other and creating a unique welding process. Although laser welding can be used in conjunction with any arc welding process, these are used more commonly.
- MIG augmented welding (often synonymous with laser-hybrid welding)
- TIG augmented welding
- Plasma-arc augmented welding
Cleaning up
Lasers can also be used to clean material. Laser cleaning is a non-contact method that removes welding scale, corrosion, stains, metal black, and non-ferrous lubricants from stainless steel. Laser cleaning can be used before and after welding to improve the quality of the weld.
Pre-weld laser cleaning can prevent contamination that could interfere with the weld. Post-weld lasers can remove discolouration due to oxidation, which improves the corrosion resistance of stainless steel welds. At this stage, it is not clear if laser cleaning can restore the passivity of laser welded surfaces and it is advised that standard methods of passivation are used on welded areas. Laser cleaning can reach cleaning speeds of
1 to 1.5 meters per minute, which matches common welding speeds.
However, laser cleaning can provide a smooth, high quality weld and remove void-free soldering and brazing. Since it does not involve chemicals, it can improve the health and safety of operators. Since the need for wet chemical washing processes is removed, laser cleaning offers lower production space requirements, lower running costs and is still environmentally friendly.