With an increased focus on efficiency and cost of manufacturing products, the use of Laser Beam Welding (LBW) makes sense for industries like the automotive, electronics and aerospace. Laser Beam Welding combined with a fiber optic delivery system allows for an efficient and consistent product. LBW is the joining of two or more pieces of metal together using laser beam. The process leaves a narrow weld and heat affected zone. There are five elements of a LBW system.
- Beam Delivery
- Focus Head
- Process (weld)
The two most common lasers used in Laser Beam Welding are CO2 (Carbon Dioxide) and Nd:YAG (Neodymium-Doped Yttrium-Aluminum-Garnet). Between the two Nd:YAG is used with a fiber optic beam delivery system because of its shorter wavelength.
The Nd:YAG laser’s active substance is a Neodymium in the form of a dopant in a transparent rod of Yttrium Aluminum Garnet. The wavelength when energy is supplied by a flash tube is 1.06 microns. The shielding gas is either Argon or a Argon / CO2 mixture, but at low power or for spot welds not using shielding gas will result in acceptable welds.
A beam delivery system is designed to transfer the output of the laser to the focusing head. There are two types of fibers available for a fiber optic beam delivery system, step-index and gradient fiber.
A step-index fiber has a constant refractive index with an abrupt step transition to a different refractive index in the cladding. The output is a “top hat” distribution.
The uniform output profile is 86% radius of the fiber, produces more uniform intensity profile (than gradient) and a wider spot.
The gradient fiber has a variable refractive index in the core. This means the output profile will be a parabolic function of the radius.
The focus head is used to create the final focus spot from the output of the fiber cable, shown in the below figure. This is done using two lenses, a collimator lens and focus optic.
The final focus spot diameter size is determined with the following equation:
Final Focus Spot = Fiber Core Diameter (F2 Focal Length / F1 Focal Length)
The image below is an example of a focus head with an on axis CMOS camera. Cover glass is used to protect the optics from spatter.
During the welding process the focus point is adjusted to the surface or just below the workpiece. The workpiece material is melted. During this phase some material is converted to plasma, this is not so much a problem with Nd:YAG as it is with CO2. A shielding gas is optionally used to prevent the material from reacting with the air and protect the lens. As soon as the beam moves, the heat is interrupted and the metal solidifies fast. This leaves a small heat affected zone. Figure 5 shows an cross section view of the weld process.
Unlike a hard optic delivery system, where the laser is fixed and the workpiece must move, the fiber optic delivery system allows for the beam to be moved into the correct position and angle. A two handling facility can also be achieved, where both the workpiece and optics move. This allows for a more flexible production system than what can be accomplished with a hard optic delivery.