Laser welding operates in two fundamentally different modes: conduction limited welding and keyhole welding. The mode in the laser beam will interact with the material it is welding will depend on the power density of the focused laser spot on the workpiece.
Conduction limited welding occurs when the power density is typically less than 105W/cm2. The laser radiation is absorbed only at the surface of the material and does not penetrate into the material. Therefore, conduction limited welds exhibit a high width to depth ratio.
Laser welding is more usually accomplished using higher power densities by a keyhole mechanism. When the laser beam is focused on a small enough spot to produce a power density typically > 106-107 W/cm2, the workpiece surface vaporises before quantities of heat can be removed by conduction. The focused laser beam penetrates the workpiece and forms a cavity called a ‘keyhole’, which is filled with metal vapour or ionised metal vapour (plasma). This expanding vapour or plasma contributes to the prevention of the collapse of the molten walls of the keyhole into this cavity. Furthermore, the coupling of the laser beam to the workpiece is improved dramatically by the formation of the keyhole. Deep penetration welding is achieved by traversing the keyhole along the joint to be made (or moving the joint with respect to the laser beam), and results in welds with a high depth to width. Under the action of vapour pressure and surface tension, the molten material at the leading edge of the keyhole flows around the cavity created by the beam to the back, and solidifies to form the weld. This action leaves a top bead with a chevron pattern, which points to the start of the weld.