laser welding

Laser welding offers many advantages over traditional welding techniques:

• Consistent, reliable joints with minimal distortion due to heating
• A small heat affected zone (HAZ)
• Deep penetration of precise narrow welds
• A narrow weld profile with excellent appearance (especially if gas shielding is used)
• Low heat input
• Minimum part distortion
• No secondary processing
• High repeatability
• Considerably faster weld rates

These qualities are a result of using a small focus spot size, 0.001" to .04", appropriate gas shielding, well-engineered joint design, and ensuring repeatable fit up of parts. Successful laser welding is also dependent on controlling process variables such as selecting the correct type of laser for the material and thickness. Reflectivity and conductivity will affect weld quality. Part preparation and cleanliness is also very important as any contamination in the joint can produce voids and porosity in the weld. The correct combination of these elements is critical for achieving optimum laser welds.

Lasers typically used for welding are CO2 and Nd:YAG. The choice of laser is determined by material type and thickness, cycle time and weld penetration requirements. Welding usually requires an inert shielding gas to protect the weld against oxidation and contamination. The shielding gas also suppresses plasma created by welding. The most frequently used shields are helium, argon or nitrogen. Many ferrous and non-ferrous metals can be laser welded. Low carbon and low alloy steels, stainless steels, nickel based alloys, titanium and refractory alloys, with some restrictions, are all ideal for laser welding. Aluminum and copper, together with their alloys, are difficult to weld due to the high reflectivity but can be welded with Yag lasers because of their shorter wavelength that couple better with these highly reflective materials. Dissimilar metals can also be welded but the metallurgy must be compatible.

There are two types of beam delivery options commonly used when welding: conventional beam delivery and fiber optic cable. Conventional beam delivery, suitable for both Nd:YAG and CO2 lasers, is ideal for 5-axis positioning systems where flexibility and movement of the machine axes cannot be hindered or constrained. This type of beam delivery allows the laser machine tool to be positioned very quickly. Fiber optic delivery is suitable only for welding with Nd:YAG lasers. The CO2 beam is the wrong wavelength to be transmitted through a fiber. An advantage of using a fiber for welding is its ability to mount an end effector on a robot, creating a very flexible, low cost platform for welding large complex 3D parts.

For welding with conventional beam delivery, the spot size of the focused beam at the weld is a function of the collimated beam diameter from the laser, the beam divergence of the laser, and focus lens. If welding is being carried out using fiber optic cables, the spot size is a function of the fiber diameter, recollimating lens, and focus lens. The optimum spot size for the required weld is usually achieved by varying the focus lens, which in turn varies the power density delivered to the weld joint. If the power density is too great, the material will vaporize, creating a very poor weld. If the power density is too low, the weld will either lack penetration or no welding will occur. Welding speed can also be adjusted to effect penetration, heat input to the material, and/or bead size.