Laser welding is the method of using a high-powered laser beam to melt and bond materials such as metals or thermoplastics to create a weld. Thin materials can be welded with lasers because it uses such a concentrated heat source. The concentrated heat source enables thin materials to experience a high welding speed of meters per minute. It also enables denser materials to yield narrow, deep welds between square-edged segments.

There are two different modes used to describe the result of the melted metal during laser welding. These modes include conduction limited welding and keyhole welding. The mode that the laser beam will interact with the segment it is welding varies based on the power density of the beam striking the work piece.

In conduction limited welding the power density is large enough to cause the metal to melt resulting in a wide, shallow weld. This weld happens when the power density is less than 105W/cm2. The laser beam gets absorbed into the exterior of the material and does not infiltrate it.

Generally, laser welding uses higher power densities through keyhole apparatuses. Keyhole welding vaporizes the metal creating a deep and narrow weld. The laser beams power of about 106-107 W/cm2 is forced into a tiny area. This focused laser beam infiltrates deep into the work piece creating a narrow and deep cavity or keyhole, which gets filled with metal vapor. In some cases, this metal vapor can be ionized creating plasma. The swelling plasma plays a key role in preventing the collapse of the melted walls in the keyhole.

Additionally, this deep penetration weld is accomplished by moving the joint in tandem with the laser beam, resulting in a weld with an excellent depth to width ratio. Further, as long as the laser power is strong enough and the travel speed not too high, the hole will stay open.

Surface tension causes some of the molten material at the front of the keyhole to flow around the keyhole cavity to the back, cool, and then solidify creating the weld. This method produces a weld cap that has a chevron pattern that points towards the start point of the weld in the back.