LASER WELDING offers many advantages over traditional welding techniques - consistent, reliable joints with minimal distortion due to heating, a small heat affected zone (HAZ), a narrow weld profile with excellent appearance (especially if gas shielding is used), and considerably faster weld rates. These qualities are a result of using a small focus spot size, 0.008" to .04" (0.2 - 1 mm), appropriate gas shielding, well-engineered joint design, and ensuring repeatable fit up of parts. Reliable fixturing is also key - inconsistencies often give poor results.

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 welded. Low carbon and low alloy steels, stainless steels, nickel based alloys, titanium and refractory alloys, with some restrictions, are all weldable. Aluminum and copper, together with their alloys, are difficult to weld due to the high reflectivity and thermal conductivity. However, with the advent of higher power lasers, these materials are being welded. 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.

Power is the determining factor for calculating penetration and feed rate obtainable. CO₂ lasers produce higher average power than Nd:YAG lasers. Therefore, they can achieve deeper penetration welds at faster rates. Welding is performed in CW or pulsed mode operation depending on metal composition, weld type, and process speed.

Pulse Length is selected to achieve melting and to control the cooling rate. CO₂ laser welding pulse lengths vary from ‹0.5 milliseconds to 5 milliseconds; pulse lengths for Nd:YAG lasers vary from 2 msec to 10 msec. Longer pulse lengths are selected to reduce solidification stresses in weldments of crack sensitive metals by slowing the cooling rate.

Pulse repetition rate (frequency) in welding determines process speed, percent each pulse overlaps the preceding pulse, and cooling rate. Pulse overlap varies from 40% up to 70% in hermetic welds. Cooling rate is a function of material composition and mass.

Pulse energy for welding is lower than for laser cutting and drilling processes. CO₂ laser pulse energy used for welding usually range in fractions of joules, while Nd:YAG and glass laser pulse energy may exceed several joules. Higher pulse energy levels will disturb the weld pool.

Lens considerations include power density to achieve the needed penetration, joint type, and part fit-up. Joint design with poor fit-up or critical alignment can be accommodated using longer focal length lenses. When welding metals that create spatter, longer focal length lenses or reflective focusing optics are recommended for increased lens life. For CO₂ lasers, sub-kilowatt power levels 2.5' f.l. and 5.0" f.l. lenses can be used. Multi-kilowatt CO₂ lasers primarily use 5" or longer f.l. lenses. Nd:YAG and glass lasers generally use focal lengths of 2.5" to 8" for welding.

A shroud can be used to protect the focusing lens from spatter and to direct a diffuse flow of gas to protect the weld pool from the atmosphere. Some welds require a trailing gas shield to completely protect the weldment from contamination. Shroud exit orifice diameters are commonly 0.100" to 0.250". Shroud standoff distances vary from 0.125" to 0.375". Shielding gasses are predominantly a single inert gas or inert gas mixtures. Shield gas pressure is kept low (5 to 15 psi) to prevent weld pool disruption. Back side shielding techniques are similar to those employed in arc welding.