Lasers create welds by outputting either discrete packets of energy known as pulses or extended output known as a continuous wave. A pulsed laser produces a series of pulses at a certain pulse width and frequency until stopped. Continuous wave (CW) simply means that the laser remains on continuously until stopped. Pulsed Nd:YAG lasers operate in pulsed mode only, diode lasers operate in continuous wave, and fiber lasers can operate in either pulsed or CW mode.
A pulsed laser can produce peak power greater than its average power. For example, a pulsed Nd:YAG laser with 25W of average power can produce a peak power of 5kW, two hundred times greater than average. Therefore, pulsed lasers offer very efficient spot welding capability; they can provide penetration with a large spot diameter for weld strength and better fit-up forgiveness. Pulsed lasers are used for spot welding and for seam sealing when heat input is a factor, for example, for implantable medical devices or batteries.
In contrast, a 25W CW laser produces discrete pulses of laser light by a modulation or “gated” output with a peak power of 25W. The term “gated” originated in early generation lasers, which could not provide a modulated output. A mechanical opening was rotated in the beam path to provide the gated output.
CW lasers can penetrate at high processing speeds. For example, a 500W multi-mode laser can provide 0.02-inch penetration at 5 inches per second. These lasers are a good option for high speed seam welding applications, for example, airbags initiators, battery tabs to terminals, and pressure sensors. CW lasers are used in gated mode when lap welding very thin materials (< 0.002-inches thick). Figure 1 shows the use of pulsed and continuous wave lasers for several applications.
How to choose among pulsed, continuous, or modulated output
Choosing when to use pulsed, continuous wave or modulated output is based on the application. Pulsed operation is typically used for spot welding. For seam welding, the choice is made based on heat input and cycle time. For instance, when seam welding an implantable device, a pulsed laser is used to minimize heat input and maintain a uniform weld around a complex geometry. In contrast, the fast cycle time offered by high speed CW operation is favored for welding airbag initiators. Figure 2 compares power and time for pulsed and CW laser options.