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What is Laser Welding and How Does it Work?

   

Laser welding is a process used to join together metals or thermoplastics using a laser beam to form a weld. Being such a concentrated heat source, in thin materials laser welding can be carried out at high welding speeds of metres per minute, and in thicker materials can produce narrow, deep welds between square-edged parts.

Laser welding operates in two fundamentally different modes: conduction limited welding and keyhole welding. The mode in which the laser beam will interact with the material it is welding will depend on the power density across the beam hitting the workpiece.

Conduction limited welding occurs when the power density is typically less than 105W/cm2. The laser beam is absorbed only at the surface of the material and does not penetrate it. Conduction limited welds often then 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 to a small enough spot to produce a power density typically > 106-107 W/cm2, the material in the path of the beam not only melts but also vaporises, before significant quantities of heat can be removed by conduction. The focused laser beam then penetrates in to the workpiece forming a cavity called a 'keyhole', filled with metal vapour (which in some cases can even be ionised, forming a plasma).

This expanding vapour or plasma contributes to the prevention of the collapse of the molten walls of the keyhole in to this cavity.

Furthermore, the coupling of the laser beam in to the workpiece is improved dramatically by the formation of this keyhole. Deep penetration welding is then achieved by traversing the keyhole along the joint to be welded or moving the joint with respect to the laser beam. This results in welds with a high depth to width ratio.

Under the action of surface tension, some of the molten material at the leading edge of the keyhole flows around the keyhole cavity to the back, then cooling and solidifying to form the weld. This leaves the weld cap with a chevron pattern, pointing backwards towards the start point of the weld.

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