Key factors of laser welding

Characteristics of Laser Welding Process and Influencing Factors :

1. Energy density of laser input. The main methods to adjust laser energy density include:

• Adjust the laser output energy (by modifying excitation voltage)

• Adjust the spot size (by changing the output focal length)

• lChange the energy distribution in the spot (by changing fiber types: peak-output type—GI fiber; trapezoidal-output type—SI fiber)

• lChange the width and waveform of the output pulse

2. Material reflectivity:

Most metals will reflect most of the laser energy when the laser starts to irradiate, so the power of the beam should be increased accordingly at the beginning of the welding process. When using pulsed laser welding technology, the quality consistency of the entire welding section can be ensured by connecting the arc starter plate. When the metal surface begins to melt or vaporize, its reflectivity decreases rapidly.

Main factors affecting material absorption of laser beams:

1. Temperature:

• At room temperature (20°C), metals typically absorb less than 20% of laser energy.

• Absorption rises to 40–50% when melting or vaporization occurs.

• Near the boiling point, absorption can reach up to 90%.

2. DC resistivity of materials:

  Absorption rate is proportional to the square root of the material’s DC resistivity and inversely proportional to the square root of the laser wavelength.

3. Incident angle of laser beam:

  Larger incident angles reduce absorption. Maximum absorption occurs when the laser is perpendicular to the metal surface. In order to protect the laser output lens, a certain incident angle needs to be maintained.

4. Surface state of materials:

  In order to reduce reflectivity, a thin layer of metal powder can be applied to the metal surface, but the two must be able to form an alloy. For example,copper, gold, and silver can be covered with a thin sharp layer. At this time, under the same melting depth, the energy required for welding is about one-fourth of that required for copper, gold, and silver.

5. Focusing Quality and Defocus Amount:

  The focusing (spot size) of a high-quality YAG laser welding device is guaranteed by the coaxial accuracy of the optical path of the device itself, the imaging ratio of the output optical fiber and the output head, etc. The position when the laser emission focus falls exactly on the workpiece is zero. The defocus refers to the distance the focus is away from this zero point. When the focus position exceeds the zero point, it is called negative defocus (the focus goes deep into the workpiece), and its distance value is negative defocus. Conversely, the distance value when the focus is less than zero is positive defocus. To obtain a larger melting depth, the focus position can be selected at a certain position inside the workpiece, negative defocus is used for welding.

6. Welding penetration depth:

  When pulse laser welding, it is mainly carried out by heat transfer melting. The direct penetration depth of the laser beam itself into the metal is limited, which mainly depends on the thermal conductivity of the material (the larger the thermal conductivity, the greater the penetration depth), rather than the power of the laser.

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