**9.2 Conduction/penetration welding**

This mode occurs at medium energy density per unit area (1 MW/cm2 ) and leads to greater penetration than the conduction state (**Figure 11**) [3, 5–7, 13, 17]. In this case, the keyhole exists with shallow penetration that provides a characteristic aspect ratio (depth/width) of 1. This mode is carried out almost exclusively via pulsed Nd:YAG laser for various spot and seam welding applications.

**Figure 11.** *Different modes of laser welding.*

**Figure 12.**

*The effect of thermal conductivity on the weld pool shape (a) heating, (b) surface melting, (c) surface vaporization, (d) plasma formation, and (e) ablation.*

#### **9.3 Keyhole welding**

By increasing the peak power density (>1.5 MW/cm2 ) the welding mode is shifted to the keyhole, i.e. deep narrow welds with an aspect ratio higher than 1.5. The laser beam heats up and melts the material quickly upon irradiation. If the intensity is high enough, a key-shaped cavity filled with the base metal vapor is formed, reflecting the generated heat into the material bulk that is sealed by the molten material behind the laser beam (**Figure 13**). In this case, welding is often performed by high-energy laser sources to join thick workpieces or fill cavities. This mode is called keyhole welding and it is accompanied by a muffled sound [3, 5–7, 13, 17]. Keyhole formation improves the laser heat absorption via two major mechanisms; first, through Fresnel absorption mechanism, absorbing the beam by successive reflections of the beam on the keyhole walls (**Figure 13**), and second, through the absorption of the laser energy into a vapor-filled cavity caused by a phenomenon called inverse Bremsstrahlung process [5, 17].

In the keyhole mode, the weld can be accomplished at either very high travel speeds (up to 20″/s with short depth welds, or very deep welds (i.e. up to 0.5″). The high-power density of the laser beam forms a thread of vaporized material, called a keyhole, extending into the bulk and providing a channel for the beam to be efficiently delivered into the joint. This direct energy delivery into the workpiece maximizes the weld depth and minimizes the heat input to the base metal, minimizing the HAZ and workpiece distortion. This mode is used for the production of many automotive and train components such as torque converters and gearboxes, which require up to 0.25″ penetration. The keyhole is surrounded by the melt, which tries to close it. Under steady-state conditions (optimized welding), the vapor pressure confined

#### **Figure 13.**

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Absorption of the laser beam due to successive radiations from the wall of the keyhole.
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within the keyhole prevents the melt from collapsing in on itself permanently, which would interrupt the welding. But even during an optimized weld localized collapses of the keyhole may take place.

The main difference between these modes of LBW is that in the first mode the surface of the weld pool is not broken, but in the case of penetration welding, the surface of the weld is opened so that the laser beam penetrates the molten pool. Conduction welding is not prone to gas absorption during the process, which is due to the lack of penetration of the laser beam into the material bulk. In penetration welding, discontinuous closure of the keyhole increases the susceptibility to porosity formation in the weld pool (**Figure 14**) [3, 5–7, 13, 17].

#### **Figure 14.**

*The schematic of different modes of laser welding (a) conduction mode, (b) transition keyhole mode, and (c) keyhole/penetration mode.*

Since LBW is a high energy density method, it does not require thermal conduction to achieve a deep penetration, which is in contrast to the conventional methods of arc welding and gas welding that obtain penetration via increasing the heat input. In conduction LBW, the weld width is often greater than the depth and the heat input is greater than the amount required for penetration. Moreover, in penetration or keyhole welding, the laser heat is transferred from the surface into the joint and creates a deep and thin weld pool (**Figure 14**) [4, 18].
