A review of laser weld formation and beam-material interactions

Laser beam welds are generally categorized as resulting in two distinct modes, known as conduction and keyhole. Conduction mode welds are relatively wide and shallow with a low height-to-width aspect ratio, and keyhole mode welds are narrow and deep with a high height-to-width aspect ratio. The difference between these regions is a combinatory result between process parameters and material properties. High power and/or small beam diameters result in high power densities that enable metal vapor to impart a back pressure into the molten weld pool. It is this vaporization and the ability to couple the energy deeper into the workpiece that results in deep penetration. The energy absorption into the material is based on the photon wavelength, angle of beam incidence, power density, atmospheric conditions, plume formation, and material properties. These physical beam-matter interactions change the overall shape of the weld pool and influence defect formation such as porosity, root spiking, and/or solidification cracking. In addition, the solidification rate within the weld pool is strongly influenced by the weld pool geometry and changes the resultant microstructure. The transition between conduction and keyhole is not distinct and varies based on the laser parameters, material composition, shielding gas, and atmospheric pressure. This chapter elucidates the parameters that develop distinct weld pool geometries and the physics associated with laser beam welding and weld pool development.