Conduction vs. Keyhole Mode Welding
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Conduction Mode Welding
At low levels of energy density, the laser beam interacts with only the surface of the parts being welded. As the energy is transferred to atoms on the surface, they heat up and in turn transfer heat to the atoms below the surface. This mode of welding where the heat is transferred to the interior by conduction is called conduction mode welding. Typically conduction mode welds form when the energy density is relatively low and the spot is relatively large. Conduction mode welds are common with low energy pulsed welding. Cross sections of conduction mode welds are have a bowl shaped profile and relatively shallow. The size and shape of the section profile depends upon the material properties and rate of energy input. The second and third section shown at the bottom of this page are good examples of conduction mode welding. The second section is an example of a butt weld where as the third section is an example of lap weld.
Keyhole Mode Welding
As the rate of energy delivery is increased by increasing the energy density, the fusion zone rapidly heats up to a point where the molten metal starts to vaporize at the center of the weld beam spot and opens up a blind hole (keyhole) in the molten metal. The weld beam now is able to reach deeper into the fusion zone and rather than transferring energy only at the surface as with the conduction mode welds. Vapor pressure from the hot metal keeps the hole open during the weld. Presence of a keyhole allows the laser energy to reach deeper into the fusion zone and consequently is able to produce a deeper aspect ratio weld. Keyhole mode is common during high energy CW (continuous wave) laser welding but can also be found with high energy pulsed welding as can be seen in the cross section on the far right. Note the higher aspect ratio of the weld section and and the presence of a porosity trapped at the bottom of the weld; both are indication of the formation of a keyhole during welding.
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