The laser is a high energy density process that provides a unique welding capability – maximum penetration with minimal heat input. There are three basic weld modes, which correspond to the level of peak power density contained within the focus spot size: conduction mode, transition keyhole mode, and penetration or keyhole mode. Figure 1 is a graphic illustration of the three weld modes.
Conduction mode – Conduction 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 and when particulates are a concern, such as certain battery sealing applications.
Transition mode – occurs at medium power density, around 1 MW/cm2, and results in more penetration than conduction mode. The keyhole is present but has shallow penetration and provides a typical weld aspect ratio (depth/width) of around 1. This mode is used almost exclusively by pulsed Nd:YAG laser, for many spot and seam welding applications
Keyhole or penetration mode – Increasing the peak power density beyond around 1.5MW/cm2 shifts the weld to keyhole mode, which is characterized by deep narrow welds with an aspect ratio greater than 1.5. Figure 2 shows how increasing the peak power density beyond 1 MW/cm2 moves the weld from conduction to penetration or keyhole welding.
In this keyhole welding mode the weld can be completed at either very high speeds in excess of 20”/s with a small weld depth shallow welds, or very deep welds, up to 0.5”. The high power density laser light forms a filament of vaporized material, known as a keyhole, which extends into the material and provides a conduit for the laser light to be efficiently delivered into the material. This direct delivery of energy into the material maximizes weld depth and minimizes the heat into the material, reducing the heat affected zone and part distortion. This type of welding is used in the manufacture of many automotive power train components such as gearboxes and torque converters that require penetration of up to 0.25”, and also in high speed battery tab welding.
The keyhole is surrounded by molten material that acts to close the keyhole. Under steady state and optimized welding conditions, the vapor pressure contained within the keyhole effectively prevents the molten material from permanently collapsing in on itself, which would stop the welding. However, local and short time period collapses of the keyhole may occur even during an optimized weld.
Figure 3 illustrates what happens during penetration or keyhole welding. The arrows in the lighter element indicate the vapor pressure created at the leading edge of the laser, and the arrows in the darker part indicate the direction of fluid flow.
The speed and power penetration characteristics of a 3 and 5kW are shown in Figure 4. These weld cross sections clearly demonstrate the keyhole schematic shown in Figure 1, with narrow deep welds. It is interesting to note that increasing the speed only reduces penetration a little, but really affects the width of the weld as there is less time for conduction outwards from the keyhole.