TWI Frequently asked questions
Laser welding operates in two fundamental modes: conduction limited welding or keyhole welding. The mode in which the laser will operate depends directly on the continuous power density of the laser spot on the workpiece.
Conduction limited welding occurs when the continuous power density is less than 0.5x106W/cm2. The laser radiation is absorbed at the surface of the material and does not penetrate into the material. Therefore, conduction limited welds exhibit a high width to depth ratio.
High power laser welding is accomplished by a keyhole mechanism. When the laser beam is focused to a small enough spot to produce a power density of about 0.5x106-107 W/cm2, the workpiece surface vaporises before significant quantities of heat can be removed by conduction. The focused laser beam penetrates the workpiece and forms a cavity called 'keyhole', which is filled with metal vapour or ionised metal vapour (plasma) and which contributes to the prevention of the collapse of the molten walls collapsing. The coupling of the laser beam to the workpiece is thus dramatically improved and only a small proportion of the laser energy is reflected. Deep penetration welding is achieved by traversing the keyhole along the joint to be made 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 beam to the back and solidifies to form the weld. This action leaves, on close fitting joints, a slightly protruding top surface weld bead with a chevron pattern which points towards the start of the weld. The top weld beads are generally neat and smooth.
