Laser welding requires that the laser raise the temperature of the material to be welded. The fiber laser light must be absorbed by the material to induce a temperature rise. In effect, the fiber laser light beam is focused onto the material similar to the way the sun can be focused by a magnifying glass. The difference is that the laser’s power density is many orders of magnitude higher.
Laser light photons, packets of light energy that make up the laser, impinge onto the material and are partly or wholly absorbed. The energy of the photon is absorbed in the metal material and causes a heat waves within the metal. Repeated absorption of photons eventually leads to metal surface breakup and melting.
Even for metals that absorb well, such as steel, the laser is initially reflected. A small percentage of the laser is absorbed, heating the metal surface. The increased surface temperature increases the absorption of the fiber laser light photons. This creates a snowball effect, in which the material is rapidly heated by the laser, leading to melting and formation of the weld.
Fiber laser welding is a high power density process that provides a unique welding capability to maximize penetration with minimal heat input. The weld is formed as the intense laser light rapidly heats the material – typically in fractions of milliseconds. There are three types of welds, based on the power density contained within the focus spot size: conduction mode, transition keyhole mode, and penetration/keyhole mode.
Conduction mode welding is performed at low energy density, typically around 0.5 MW/cm², forming a weld nugget that is shallow and wide. The heat to create the weld into the material occurs by conduction from the surface. Typically this can be used for applications that require an aesthetic weld or when particulates are a concern, such as certain battery sealing applications.
Transition mode laser welding occurs at medium power density, around 1 MW/cm2, and results in more penetration than conduction mode due to the creation of what is known as the “keyhole.” The keyhole is a column of vaporized metal that extends into the material; its diameter is much smaller than the weld width and is sustained against the forces of the surrounding molten material by vapor pressure. The depth of the keyhole into the material is controlled by power density and time. Because the optical density of the keyhole is low it acts as a conduit to deliver the laser power into the material.
Keyhole or penetration mode welding – Increasing the peak power density beyond 1.5MW/cm2 shifts the weld to keyhole mode, which is characterized by deep narrow welds with an aspect ratio greater than 1.5. The penetration depth rapidly increases when the peak power density is beyond 1 MW/cm2, transitioning the weld mode from conduction to keyhole/penetration welding.
Penetration or keyhole mode welding is characterized by narrow welds. This direct delivery of laser power into the material maximizes weld depth and minimizes the heat into the material, reducing the heat affected zone and part distortion. In this keyhole mode, the weld can be either completed at very high speeds – in excess of 500mm per second with small penetration typically under 0.5 mm – or at lower speed, with deep penetration up to 12 mm.
If conduction welding can be thought of as a point source heating from the surface, then the keyhole can be thought of as a line source heating from within the metal providing a more efficient welding source.
In transition mode the time or power density is just sufficient to create but not extend the keyhole deep into the part. Therefore, the welds exhibit shallow penetration with a typical weld aspect ratio (depth/width) of around 1. This mode of welding is used almost exclusively for many spot and low heat input seam welding applications.