**8. Types of laser welding based on laser sources**

Several types of lasers can be utilized for LBW. These include fiber lasers, Nd:YAG pulsed lasers, and Nd:YAG continuous-wave lasers. However, one should know that the type of employed laser source for LBW depends on the application [3, 6, 7, 13, 17].


#### **Table 1.**

*Solid-sate, semiconductor, gas, and liquid dye lasers.*

Fiber lasers can be used for a wide range of applications, from joining very small components used by medical engineering and electronics industries to welding thick components in the automotive and aerospace industries. Fiber lasers are versatile and inexpensive, which are suitable for achieving high-quality spot welds [3, 6, 7, 13, 17].

Laser sources are generally classified based on the state of matter of their active medium and their temporal modes. Hence, according to physics, lasers are categorized into solid-state, semiconductor, gas, and liquid dye lasers [4]. According to the temporal modes, they are classified into two modes of continuous-wave (CW) and pulsed mode. In the CW mode, the beam is continuously irradiated without interruption, whereas in the pulsed mode the beam is irradiated periodically. **Table 1** displays the laser types and their wavelengths.

These medium-power lasers use continuous-wave mode during the welding process. Therefore, the average power of the source must be higher than a certain limit. Continuous-wave lasers are ideal for high-speed and deep penetration welding. They produce joints with a very low heat input rate. Hence, they create a smaller HAZ [3, 6, 7, 13, 17]. Nd:YAG pulsed lasers generate discrete pulses of controllable energy that can be used for ideal welds. Although the average power of these lasers is often low, they employ high-power peaks for welding. They can be properly utilized for joining large spot welds as well as deep spot and seam welds.

The laser active medium is doped by a few numbers of impurity ions in solid-state layers. Among the solid-state lasers, Nd:YAG lasers are mostly employed for LBM applications. The solid-state sources (e.g. Nd:YAG, ruby, and Nd-glass lasers are vastly utilized for the machining of metals. However, Nd:YAG lasers can also be used for ceramic materials. Gas lasers are categorized into three types according to their composition (i.e. neutral atom, ion, and molecule). In general, gas lasers can be used in either CW or pulsed modes. They are also used with the transverse flow, axial flow, and folded axial flow for construction applications. Among them, CO2 lasers are most commonly employed for machining ceramics, plastics, nonmetals, and even organic materials.

Although semiconductor lasers are made of solid materials, their functioning principles are different from solid-state sources. The function is based on the radiative recombination of charge carriers. These sources can produce wide beam divergence angles (around 40°).

In comparison with other lasers, liquid-state sources are easier to fabricate. Their main advantages are the simple cooling procedure and replacement of the laser cavities. Unique properties of liquid organic molecules allow the liquid dye lasers to be adjusted over a wide range of wavelengths (200-1000 nm).
