**10. Variables affecting laser welding**

Achieving optimal conditions is a necessary and sufficient condition to achieve an ideal weld with a suitable appearance. Therefore, it is necessary to study the variables and parameters affecting LBW [3–7, 13, 17, 19–21].

### **10.1 Parameters**

*10.1.1 Parameters related to the laser source*


*10.1.2 Parameters related to the operation of the system and the welding process*


### **10.2 Effect of laser source type**

The function basis of a laser source is depended on the state of matter of the source active medium (gas or solid-state). The most well-known gas lasers are CO2 lasers, employing a combination of helium, nitrogen, and carbon dioxide gases with a ratio of 1:1:18. The sources operate based on the molecular energy levels of the gas. Hence, they are considered molecular lasers. The most important factor in optimizing the performance of a CO2 laser is efficient cooling of the gas and prevention of decomposition and failure of the gas molecules. The new generation industrial CO2 lasers are fabricated with low-power ranges (10-20 watts) to high-power ranges (about 6000 watts), using radio frequency (RF) waves as the pumping source called RF-excited lasers. The advantage of these lasers over other gas lasers is the possibility of operating in high-frequency pulse mode and the extended life span of the sealed-off tubes. Another type of gas laser is the gas dynamic laser. Since the function of these lasers is based on the sudden decrease of the gas pressure, the gas type of the source is very determinative during the pressure reduction and the sudden cooling mechanism of the active medium. This type of source is widely used for military purposes.

Nd:YAG lasers are the most widely used solid-state sources, having higher optical and physical properties, and greater efficiency compared to gas lasers. In Nd:YAG lasers, the active medium is a Y3Al5O12 crystal, where some of the Y3+ ions have been replaced by Nd3+ ions, resulting in the formation of Nd:YAG crystal and providing a promising active medium with several high-intensity wavelengths in the infrared region. Another solid-state source is used in diode lasers (semiconductor lasers),

resulting in the fabrication of sources with ultrahigh efficiency and great tunability. However, the main limitation of these lasers is their high divergence.

Nd:YAG laser welding can be used more than CO2 due to its shorter wavelength (1.06 mm) which allows displacement using optical fibers and also reduces the reflection of metal surfaces [3–7, 13, 17, 19, 20].

#### **10.3 Pulse shape**

In most cases, LBW uses a square pulse shape (**Figure 15**). But two other types of pulse shapes are also used in special welds. The first type (Spike pulse) is used for light-reflecting materials such as copper and aluminum (**Figure 15**). The second type (Annealing pulse) is used to minimize the radiant heat cycle during welding for cracksensitive materials (**Figure 15**) [3–7, 13, 17, 19, 20].

It should be noted that the capability of pulse shaping in some lasers such as Nd: YAG sources can facilitate operations such as drilling and cutting. So that a chain of very short pulses with a higher peak power than the main pulse is descended immediately after the main pulse to form a quick coupling by the laser beam or numerous pre-pulses with a lower peak power before the main pulse (a pre-pulse should have lower power than the main pulse) are descended on thin foils to prevent the welding area from being punctured.

#### **10.4 Peak power**

The peak power of a laser source is the maximum power that the source can provide in either continuous welding (CW) or pulsed welding modes. It is measured in watts (W) or kilowatts (kW).

One of the important parameters of pulsed laser welding is pulse peak power. In fact, with this peak power, penetration welding can be created with a low-power laser. The peak power of a square pulse (Pp, (J/ms)) is equal to the pulse energy (J) divided by the pulse time or width (Pulse duration (ms)) (Eq. (3)) [3–7, 13, 17, 19, 20].

$$P\_p = \frac{Pulse\ energy}{Pulse\ duration} \tag{3}$$

## *Laser Welding DOI: http://dx.doi.org/10.5772/intechopen.102456*

Obviously, by significantly reducing the pulse time in a low-power laser, a high pulse peak power can be achieved to create penetration welding.

An optimum peak power creates the deepest penetration in the given energy without the expulsion of materials. Welded joints, which are made with high peak power and short pulse widths are narrow and deep and require a high heat cycle.
