Adjustment Techniques and Formulas for Laser Beams in Laser Welding Machines
Focus Size Formula
The focusing lens is the optical component closest to the workpiece surface. Correct adjustment of this lens is critical for determining the laser spot size. The following empirical formulas are used to understand the relationships between key parameters:
- Focal Length Formula: do = fθ (Equation 4-6)
- θ = 1.44λ / D (Equation 4-7)
- bo = 16(f/D)^2λ (Equation 4-8)
Where:
- do is the spot diameter
- θ is the divergence angle
- λ is the wavelength of the laser
- D is the lens diameter
- f is the focal length
- bo is the depth of focus
A shorter focal length results in a smaller spot diameter but also reduces the usable distance between the processing head and the workpiece. This can complicate workpiece clamping and increase the risk of lens damage from molten metal or metal vapors.
Focal Number
The ratio of focal length to numerical aperture is known as the focal number. For 10kW CO₂ laser lenses, the optimal range for focal number is 6–9. A lower focal number (around 3) is suitable for high welding speeds, though it can cause lens spherical aberration, impacting focusing performance. For Nd:YAG lasers, a focal number of 4 is ideal.
Depth of Focus and Spot Diameter
A large depth of focus allows the workpiece to receive high laser energy over a greater depth, facilitating deeper laser fusion welding. The depth of focus allows for better adjustability of the workpiece along the laser’s direction.
During operation, thermal distortion can alter the spot size and depth of focus. Thus, these parameters need to be frequently checked and adjusted to ensure the spot remains at the correct working location.
The focused laser beam exhibits a waist, characterized by a small diameter and length. The waist diameter is the spot diameter, and the focal point is where the intensity is maximum. Laser intensity decreases slightly (around 5%) within the depth of focus on either side of the focal point.
Estimating Spot Diameter and Depth of Focus
While the spot diameter is difficult to measure precisely, it can be estimated by multiplying the focal length (f) by the divergence angle (θ). The spot diameter do determines the lens focal length f, but the exact spot size can vary depending on processing conditions. The depth of focus q is interrelated with the spot diameter and must be adjusted accordingly for accurate operation.
Practical Adjustments During Operation
During the welding process, thermal distortion from laser transmission can alter the spot size and depth of focus. It is essential to frequently check and adjust the workpiece position to maintain the correct spot size and ensure that the beam remains at the optimal working location. Continuous monitoring and adjustment help to keep the welding process stable and effective.
Conclusion: Optimizing Laser Beam Parameters for Welding
Proper adjustment of laser beam parameters is crucial for achieving high-quality welding results in laser welding machines. By understanding and applying the relevant formulas for focus size, spot diameter, and depth of focus, operators can improve focusing accuracy and welding efficiency.