**7. Laser welding procedure**

The principles of the LBW process are not complicated. The procedure schematic is presented in **Figure 9**. (1) A pump, which is the energy source provides the energy required for the process. The pump stimulates the laser to such an extent that the electrons held by the atoms are moved to higher energy levels. (2) Electrons reduce their energy levels dramatically, releasing photons. The spontaneous emission of photons is what leads to the production of the laser beam. (3) Spontaneously emitted photons collide with the ones having higher energy levels. The collision reduces the energy levels of the electrons, leading to the emission of another group of photons. Both groups are now having the same wavelength and moving at the same speed. (4) Photons are emitted in all directions. However, they are all limited to travel in the same medium and hit the resonator before reflecting from the medium. The intensifying mirror then determines the level and direction of emission. To perform any type of amplification, the fraction of atoms must be higher than that of low-energy photons. (5) The laser beam is targeted and focused on the workpieces to be welded. Highly-focused light energy is converted to heat energy at the workpiece surface. (6) During a process known as surface conductivity, the generated heat melts the material surface. The generated heat is controlled to be below the boiling point of the parent material. This technique is an ideal solution when welding materials that have high thermal conductivity. Apart from welding, other procedures such as drilling, cutting, and stripping can be carried out using laser beam energy [3, 13–16].

By combining the LBW and GMAW techniques, the laser-GMAW hybrid welding is developed. This combination is an attractive tool with a great potential for welding lightweight structures, especially aluminum alloys. This hybrid welding technique is generally acknowledged for its efficiency, robustness, and flexibility. By combining a

deep-penetrating laser beam with high filler feeding of GMAW the primary applications of LBW and GMAW can be improved significantly. The main benefits of this technique are high gap-bridging ability, deep and stable weld penetration, facile addition of the filler metal, and low distortion. This hybrid method allows much wider groove tolerance in comparison with LBW of specific alloys such as aluminum alloys. Furthermore, the distortion reduction decreases the required post-welding treatments and facilitates the assembling process because the hybrid-welded components are more dimensionally precise. Moreover, if very accurate metallurgical factors are needed, the hybrid process can be easily balanced with the filler metal, which declines the susceptibility to hot cracking, especially for specific aluminum alloys. This combined process can also enhance the weld bead shape appearance and quality (e.g. by elimination of undercut), reducing the porosity and increasing the welding speed.

Since hybrid laser arc welding (HLAW) apparatuses are influenced by each of the two processes, the weld geometry of HLAW is controlled by the heat input of each process as presented in **Figure 10**. For instance, by increasing the power of GMAW, the width to depth ratio of the weld is increased. Nevertheless, due to the contribution of both techniques, the HLAW-welded joints are usually similar to LBW at the bottom and similar to GMAW on the top of the joints.

In addition, due to the involvement of a high-density laser beam, keyhole formation is a characteristic of the HLAW process in most cases. On the other side, a

**Figure 10.** *Hybrid laser GMAW welding process.*

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

conduction-like process without the formation of the keyhole is obtained if the beam is not focused or its power is insufficient. A previous study regarding the aluminum LBW showed that initially, the Nd:YAG laser beam absorption by the base metal surface could be as low as 10%. But, when the base metal was molten, the beam absorption greatly increased up to almost 100%, especially when the keyhole was formed. Interestingly, it has been reported that the arc stability of GMAW is increased when it is coupled with a laser beam. This enhancement is achieved when the arc is close enough to the beam and they share the same melting pool. For example, since the aluminum melt has a lower electrical resistance than that of the solid-state or oxide layer, the arc favors the path with the lower resistance. Besides, the interaction between the keyhole and arc plasmas increases the arc stability. The energy from the formed keyhole creates a metal plasma, ionizing the shielding gas of the GMAW process that facilitates the strike and stabilizes the arc. Furthermore, the HLAW arc has a finer geometry, a higher electrical conductivity, and a higher current density (up to 500% of the GMAW arc). On the other side, since in HLAW, the metal plasma is originated from both the base metal and the filler metal, more metal vapor is produced than that of the LBW. Consequently, the keyhole formation is much easier and process failure is prevented. The penetration of this technique is higher than the LBW due to the higher plasma pressure. Since the molten pool is larger during this process, the weld pool is in the liquid state for a longer time compared to the LBW. This is beneficial in the case of welding aluminum alloys due to the high hydrogen solubility in the molten aluminum. Hence, a larger melting pool gives more time to hydrogen bubbles to escape from the weld, resulting in the formation of fewer gas pores.

Because of the interaction between the two processes, the advantages are more than the drawbacks. For welding aluminum alloys, these advantages depend on the welding parameters, the alloy composition, and the joint type. A majority of the authors stated that the welding speed is increased by using the hybrid technique. Moreover, it improves the penetration of the weld seam, which is 10-20% and 20-50% higher than the LBW and GMAW, respectively. Additionally, many studies have expressed that the stability during aluminum welding is higher in comparison with LBW or GMAW processes. Additionally, the applied heat input is lower due to the elevated speed and high energy density of HLAW. By lowering the heat input the distortion of the welded components is directly decreased. Since the GMAW process generates a large welding seam, gap bridging is improved during HLAW compared to the LBW. It has been reported that the HLAW can increase the gap bridging from 1.05 to 1.19 mm compared to the GMAW, while the maximum gap tolerance of the autogenous LBW is around 0.3 mm. Another benefit of this process is its higher wire feed alignment compared to the LBW. Since in the HLAW process the feeding wire does not have to intersect with the laser beam and the weld pool, the addition of filler metal is more facile than that of the cold wire fed LBW. The reduction in the component distortion, high gap bridging, filler application, and wire misalignment tolerance are the main important aspects of automated HLAW that increase the robustness of this process for industrial applications compared to the primary original processes of LBW and GMAW.
