**14. Conclusions**

This chapter is focused on the fundamentals, parameters, and applications of laser beam welding. Currently, laser sources have many applications in the field of material processing. Laser beam welding as a new technology in recent years has found wide applications in various industries such as automotive, military, aerospace, shipbuilding, electronics, etc. An energy source provides the energy required for laser production. This source stimulates the electrons held by the atoms to move to higher energy levels. Electrons reduce their energy levels dramatically, releasing photons. The spontaneous emission of photons is what leads to the production of the laser beam. In LBW, a thin and deep joint is achieved, and the heat input applied to the workpieces is so much lower than the conventional welding methods. This property allows LBW to be widely used in certain applications in which a high ratio of penetration depth to joint width is required. LBW has a great power density (in the range of megawatts per cubic centimeter), which offers a very small HAZ due to its high heating/cooling rate. The weld pool size may vary between 0.2 and 13 mm. though only smaller sizes are used for welding. Different sources include fiber lasers, Nd:YAG pulsed lasers, and Nd:YAG continuous-wave lasers are used for LBW based on the application. LBW employs three types of modes including conduction mode, conduction/penetration mode, and penetration or keyhole mode to join the materials. The main difference between these modes is in the type of heating mode, weld pool filling, depth of penetration, and shape of the weld pool. Different parameters affect the LBW process such as the chemical composition of the parent material, welding gap, welding speed, gas shielding type, beam shape (geometry), joint type, etc. Many equations are suggested to determine the LBW parameters, pulse energy, frequency, power and power density, and beam focusing adjustments, which are described in detail in the chapter.
