Laser Welding Parameters
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Ability of a laser to produce desired results greatly depends on several laser parameters and often their interdependence. It is important for engineers to fully understand these relationships prior to choosing a particular laser wavelength or machine.
Energy delivered in a single pulse is used as a starting point for most calculations. With the latest generation power supplies, energy in each pulse can be programmed to be delivered in a desired profile allowing ramp up at the beginning and gradual cool down the end of the pulse. Pulse shaping helps in improving process control.
Power density is essentially a measure of number of laser photons impinging on the material. Power density is measured in terms of Watts/cm2 and is calculated as Pulse energy divided by spot size. Even in a single spot, the power density can vary significantly depending on the beam quality of the laser, and is discussed below.
Read as M-squared, M2 is a measure energy distribution in the beam. A perfect beam with M2 of 1 essentially has a sharp peak at the center with a Gaussian distribution away from the center and towards the edge. A low M2, close to 1, is preferred for micro-machining while a high M2 is preferred for welding and heat treating.
Pulse Duration and Repetition Rate
The length of the time that the laser energy pulse is on is defined as the pulse duration. Majority of laser processing applications are operated in the pulsed mode where the laser emits energy in pulses of specified duration at a defined repetition rate.There are applications where the laser is on continuously, referred to as CW or Continuous Wave mode, and are used for welding, soldering, and heat treating applications.
Even though a laser may have a fairly low average power rating, the peak power in each pulse can be very high. For example a typical 10W laser can have peak power of the order of 5 kW. This is possible because the laser energy is delivered in a very short pulse. Peak power can be calculated as Pulse energy divided by pulse width. A pulse that has 1 Joule of energy delivered in 1 msec, will have a peak power of 1 kW. Keep in mind that the calculated value is the average for the pulse duration and actual peak power could be even greater since energy is not uniformly delivered over the duration of the pulse.
Spot size or diameter depends on focal length, wavelength, M2, and beam diameter by the following relation.
Spot Diameter = 2fλ M2/D, where f = focal length of focusing lens
λ = wavelength
M2 = beam quality metric
D = beam diameter
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