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Laser Welding Equipment Selection Basics - Laser Performance Parameters
Laser Types:
Nd
Laser: A solid-state laser commonly used for medium to small power laser welding applications, such as in microelectronics and jewelry industries.
CO2 Laser: A gas laser suitable for high-power applications like metal cutting and large sheet metal welding.
Average Power:
The output power of the laser averaged over time, typically measured in watts (W). Average power determines welding speed and penetration depth, with higher power enabling faster welding speeds and deeper penetration.
Peak Power:
The maximum power that the laser can achieve during pulsed operation. Peak power determines the instantaneous energy density during welding, crucial for welding quality and control of the heat-affected zone.
Wavelength:
The wavelength of the laser determines its absorption characteristics on different materials. Common wavelengths include 1.06 micrometers (Nd
laser) and 10.6 micrometers (CO2 laser), each suited to specific applications and material types.
Pulse Energy:
The energy released by the laser per pulse in a pulsed laser system. Pulse energy dictates the heat input per pulse, crucial for high-precision welding of small components.
Spot Size:
The diameter of the laser beam at the focal point. Spot size directly influences welding resolution and the size of the weld spot, determined by the combination of focusing optics and laser parameters.
Repetition Rate:
The number of pulses emitted per second by a pulsed laser. Repetition rate affects the laser's operational efficiency and welding speed, with higher rates suitable for high-volume production.
Beam Quality:
Indicates the laser beam's ability to focus and the shape of the beam during propagation. Excellent beam quality ensures efficient energy transfer and stable focusing during welding.
Fiber Delivery:
The method of delivering the laser beam output to the welding head via optical fibers. Fiber delivery allows for flexible workstation layouts and high-quality beam transmission.
1. Laser Performance Parameters
1.1 Continuous Wave Laser (Using IPG-YLR-2000-SM as an Example)
As a user of lasers, the main parameters to consider are typically as follows:
Output Power: Generally indicated by maximum output power, common values include 2000W, 3000W, 4000W, 6000W, etc. Laser designs usually incorporate redundancy, so the maximum output power may exceed the nominal output power by tens or even hundreds of watts. Power stability is assessed by the range of power fluctuations, typically between 1% to 3%. Some manufacturers may overstate this figure, necessitating testing with a power meter under gradient power emission.
Laser Wavelength: Typically between 1060-1080nm. This parameter is chosen based on the laser absorption characteristics of different materials. For instance, blue light (450nm) or green light (532nm) lasers are used for specific applications depending on the material's absorption characteristics.
1.2 Definition and Calculation of Beam Divergence Angle
The beam divergence angle measures the rate at which a beam diverges outward from its waist. In applications like free-space optical communication, very low beam divergence angles are crucial. Beams with extremely small divergence angles, where the beam radius remains nearly constant over long transmission distances, are referred to as collimated beams.
Due to diffraction, some divergence in the beam is inevitable (assuming the light propagates in an isotropic medium). Highly focused beams exhibit larger divergence angles. If a beam's divergence angle is significantly larger than what is physically determined, the beam is considered to have poor beam quality.
Given the beam radius, the half divergence angle θ\thetaθ is related to the beam quality factor MMM as follows:
θ=M⋅θ0\theta = M \cdot \theta_0θ=M⋅θ0
where:
θ\thetaθ is the half divergence angle,
MMM is the beam quality factor,
θ0\theta_0θ0 is the fundamental half divergence angle determined by the beam's initial conditions.
To calculate the half divergence angle θ\thetaθ for a laser beam, especially for Nd
lasers emitting at 1064 nm with ideal beam quality, you can use the formula involving the beam quality factor M2M^2M2 and the Rayleigh length ZRZ_RZR. Here’s how it can be expressed:
θ=λπω0⋅M2\theta = \frac{\lambda}{\pi \omega_0} \cdot M^2θ=πω0λ⋅M2
Where:
θ\thetaθ is the half divergence angle in radians (mrad),
λ\lambdaλ is the wavelength of the laser (typically between 1060-1080 nm for Nd
),
ω0\omega_0ω0 is the beam waist radius (beam radius) at the focus,
M2M^2M2 is the beam quality factor,
π\piπ is the mathematical constant pi.
The Rayleigh length ZRZ_RZR is related to the beam waist radius ω0\omega_0ω0 by:
ZR=πω02λZ_R = \frac{\pi \omega_0^2}{\lambda}ZR=λπω02
The half divergence angle θ\thetaθ is crucial for determining the size of the beam spot when the laser is defocused. It's typically measured using a beam profiler or derived from the complex amplitude distribution on a certain plane.
For the example of a Nd
laser with a 1064 nm wavelength and a beam radius (beam waist) of 1 mm, with a half divergence angle of 0.34 mrad (0.019°), you would compute M2M^2M2 based on these parameters to understand the beam quality and its divergence characteristics.
1.3 M2 Factor
For laser beams, the beam quality is typically characterized using the beam parameter product M2. The M2 factor compares the actual shape of the beam with that of an ideal Gaussian beam.
In the above equation, ω represents the waist radius of the actual beam, θ is the divergence half-angle of the actual beam; ω0 is the waist radius of the ideal Gaussian beam, θ0 is the divergence half-angle of the ideal Gaussian beam. Essentially, M2 determines how well the actual beam matches the perfect Gaussian beam. The divergence angle θR-real of a real laser beam will be greater than the divergence angle θG-Gaussian of an ideal Gaussian beam. The wavefront of the beam is flat near the waist in the near field and curved in the far field, as shown in the diagram below.
ISO Standard 11146 defines the M2 factor as:
M2=4λπω0θ0M^2 = \frac{4 \lambda}{\pi \omega_0 \theta_0}M2=πω0θ04λ
In Equation (1), ω0\omega_0ω0 is the beam waist radius, θ\thetaθ is the divergence half-angle of the laser, and λ\lambdaλ is the laser wavelength.
The divergence half-angle of a Gaussian beam is determined by Equation (2):
θ0=λπω0\theta_0 = \frac{\lambda}{\pi \omega_0}θ0=πω0λ
Substituting the derived divergence angle into Equation (1) simplifies the calculation of the M2 factor for a Gaussian beam:
M2=θθ0M^2 = \frac{\theta}{\theta_0}M2=θ0θ
An M2 factor of 1 corresponds to a diffraction-limited Gaussian beam, while M2 factors greater than 1 indicate beams deviating from the ideal Gaussian profile. M2 can only be equal to or greater than 1; values less than 1 are not achievable.
M2 is a dimensionless parameter and has no units.
For helium-neon gas lasers, the M2 factor typically ranges between 1 and 1.1.
For asymmetric beams, coefficients for M2 can differ for directions orthogonal to the beam axis and each other, especially for laser diode outputs.
The Rayleigh range can also decrease with an increase in the M2 factor:
ZR=πω02λZ_R = \frac{\pi \omega_0^2}{\lambda}ZR=λπω02
For general laser applications where a laser QBH head is attached to a welding or scanning mirror head, the focal spot size DDD can be quickly and easily calculated using the following formula: DDD is the diameter and fff is the focal length.
1.5 Depth of Focus
Depth of focus, also known as the Rayleigh length, is the distance zzz at which the waist radius ω0\omega_0ω0 of a laser beam increases to 2ω0\sqrt{2} \omega_02ω0. The Rayleigh length is defined as twice the depth of focus. A longer Rayleigh length indicates that the beam's spot area changes less and the energy density variation is smaller when welding workpieces. Within the coverage area of the spot, the power density is more stable. A longer Rayleigh length generally means a longer depth of focus. This means that after finding the focus point, slight variations in the height of the workpiece and the welding of highly reflective materials are less likely to cause virtual welding. This is because minor height changes do not significantly affect the laser energy density, thereby reducing the precision required in finding the focus point.