How do lasers weld? When laser welding metal, one must first raise the temperature of the metal to a point where the laser's energy can be absorbed by the material. To do this, the laser is focused on the material much like the sun might be focused by a magnifying glass for a science experiment, only the laser’s power density is many orders of magnitude higher, around 106 Watts per square centimeter (W/cm2).
Laser light photons - packets of light energy that make up the laser beam - impinge onto the material, and are partly or wholly absorbed in the material lattice causing heat waves within it. These packets are also called "phonons," defined as "elementary vibrational motion in which a lattice of atoms or molecules uniformly oscillates at a single frequency." Repeated absorption of these photons eventually leads to the lattice breakup and melting. Figure 1 shows how this happens:
Some of that laser light, of course, is reflected - even in metals that readily absorb laser light, like steel. As the metal heats, however, it absorbs more and reflects less, leading to a snowball effect of light energy absorption. The laser’s peak power density will then cause either conduction or keyhole laser welding – but that’s another blog post in this back to basics series! Figure 2 is a time-based schematic of laser absorption for welding showing how laser light energy absorption increases over time: