Laser Light: It travels at 299.792.458 meters per sec. Its visible spectrum ranges from 400 to 700 nanometers. Its smallest unit is a packet of energy, a photon. It is light, and its use is growing in tube and pipe production and fabrication facilities throughout the world.
In most of its natural and artificial forms, light has little power. However, a groundbreaking invention in the latter half of the 1950s increased its power and concentrated it in a small area.
Thus was born a modern and revolutionary concept:
Light Amplification by Stimulated Emission of Radiation, or laser.
Fiber lasers are "the" recognized powerhouse in the manufacturing sector of numerous industries because of the throughput, reliability, and low cost of operation they make possible for machines that cut, weld, mark, and micromachine materials. Specific design elements distinguish fiber lasers from other industrial laser sources, and their unique attributes are enabling breakthrough manufacturing process capabilities. Specifically, high-power single-mode lasers for remote welding and widely flexible pulsed fiber lasers for cutting can address different process challenges by the complete electronic control of all operating parameters. The fiber laser is exceptionally efficient at converting relatively low-brightness pump light from laser diodes into high-brightness output, where the output beam quality is often the only spatial mode allowed by the physics of the fiber design.
Fiber lasers are unique among all other industrial laser types because of two attributes:
> a sealed optical cavity
> a single-mode, guided-wave medium.
Modern fiber lasers, by design, have a fully sealed optical path that is immune to environmental contamination and remains optically aligned without the need for adjustment. All internal components are either in-fiber or hermetically fiber-coupled, and the only free-space interfaces occur at the beam delivery optic, which includes a fused beam spreader to reduce the intensity at the first free-space interface. The combination of the single-mode waveguiding and the fully sealed optical cavity provides a robust laser design that is fixed and measured at the time of manufacture and has minimal variation over time and temperature. Sealed pump diodes and non-darkening fiber technology result in lasers that can be used continuously in production for years without adjustment or degradation.
1. When compared to CO2 laser beam, it provides simplified beam delivery using fiber-optic cables without having to deal with alignment of mirrors.
2. It is absorbed more by metals, especially good conductors, and is less absorbed by plasma vapours formed above the weld pool.
3. Fiber lasers give increased power intensity owing to its ability to be focussed to smaller sizes.
4. Light can be easily transferred to a movable workpiece because it is already present in a flexible fibre. This fact is extremely useful for laser cutting and welding.
5. Optical fibers can be several kilometers long, hence fiber lasers are capable of providing extremely high power output.
6. Large surface area to volume ratio accounts for continuous power output due to efficient cooling.
7. Fiber lasers are very stable with respect to temperature and vibrations. Fibers protects the optical path from thermal distortion.
8. Better beam quality also give cleaner welds.
9. They have very low ownership and maintenance costs and also use low electricity. These factors allow deeper penetration and faster welding speeds in comparison to other welding processes
Power density is one of the most critical parameters in laser processing. With a higher power density, the surface layer can be heated to the boiling point in the microsecond time range, resulting in a large amount of vaporization.
Therefore, high power density is advantageous for material removal processing such as punching, cutting, and engraving. For lower power densities, the surface temperature reaches the boiling point and takes several milliseconds. Before the surface layer is vaporized, the bottom layer reaches the melting point, and it is easy to form a good fusion weld.
Therefore, in conduction laser welding, the power density is in the range of 10^4~10^6W/CM^2.
Laser pulse waveform
Laser pulse waveforms are an important issue in laser welding, especially for sheet welding. When a high-intensity laser beam is incident on the surface of the material, the metal surface will be reflected by 60 to 98% of the laser energy and the reflectivity will vary with the surface temperature. During a laser pulse action, the metal reflectivity changes greatly.
Laser pulse width
Pulse width is one of the important parameters of pulse laser welding. It is an important parameter that is different from material removal and material melting, and is also a key parameter that determines the cost and volume of processing equipment.
The effect of the defocus amount on the weld quality
Laser welding usually requires a certain amount of defocus because the power density at the center of the spot at the laser focus is too high and it is easy to evaporate into holes. The power density distribution is relatively uniform across the planes exiting the laser focus.
There are two ways of defocusing: positive defocusing and negative defocusing.
The focal plane is located above the workpiece for positive defocusing, and vice versa for negative defocus. According to the theory of geometric optics, when the distance between the positive and negative defocus planes and the welding plane are equal, the power density on the corresponding plane is approximately the same, but the shape of the molten pool obtained is actually different. In the case of negative defocusing, a greater penetration can be obtained, which is related to the formation of the molten pool.
Experiments have shown that the laser heating 50~200us material begins to melt, forming liquid phase metal and partially vaporizing, forming high pressure steam, and spraying at a very high speed, emitting dazzling white light. At the same time, the high concentration vapor moves the liquid metal to the edge of the bath and forms a depression in the center of the bath.
When negative defocusing, the internal power density of the material is higher than the surface, and it is easy to form a stronger melting and vaporization, so that the light energy is transmitted to the deeper part of the material. Therefore, in practical applications, when the penetration depth is required to be large, negative defocusing is used; when welding thin materials, positive defocusing is preferred.
The speed of the welding speed will affect the heat input per unit time. If the welding speed is too slow, the heat input is too large, causing the workpiece to burn through. If the welding speed is too fast, the heat input amount is too small, causing the workpiece can’t be welded well.