Laser Welding is a non-contact process which requires access to the weld zone from only one side of the parts being welded. The laser weld is formed as the intense laser light rapidly heats the material - typically calculated in milliseconds.
Three type of laser welds can be achieved with laser welding: conduction, conduction/penetration and penetration or 'keyhole.' Conduction welds are performed at low energy, resulting in wide, shallow weld nuggets. Conduction/penetration welds utilize a medium energy density and result in a deeper weld nugget. Penetration or keyhole welds are resultant of direct energy delivery into the material being welded resulting in deep, narrow nuggets.
As laser welding is a non-contact process, there are many joint geometries that can be welded, but there must be a close fit-up at the joint interfaces.
Laser welders can join a wide range of steels, nickel alloys, titanium, aluminum, and copper. As with other joining technologies, some materials are difficult to laser weld unless they meet specific characteristics, namely, reflectivity, the effect of high thermal cycling, and the vaporization of volatile alloying elements.
To learn more read Fundamentals of Laser Welding and Nd:YAG or Fiber Laser for Micro Welding.