There are two heating modes used to describe the melting of metals in laser welding: "conduction mode" and "keyhole mode". These modes are created by different power densities of the beam striking the workpiece and lead to varying results. Power density is determined by dividing the laser beam power by the area of the focused laser spot.

1. Conduction Mode Welding
2. Keyhole Welding
3. Conduction/Penetration Mode Welding

Conduction Mode Welding

Conduction mode welding is a technique where laser energy is transferred into the workpiece through heat conduction, and the maximum weld depth is limited by the heat conductivity of the materials. Typically ranging from a few tenths of a millimeter to 1 millimeter, the weld depth is constrained. It occurs at power densities lower than 0.5 MW/cm², ensuring minimal vaporization of the workpiece. 

Penetration is primarily controlled by conduction from the point of contact - the workpiece's surface - which heats a small area above the melting point. Convection is also a factor once a weld pool forms. The resulting weld is shallower and possesses a broader heat-affected zone compared to keyhole welding. This method is more prevalent with low-power lasers, typically below 1 kW. The shape of the weld pool in conduction mode welding is influenced by the flow within the pool and the presence of surface-active elements.

In conduction welding, the width of the weld is always wider than the depth, making it ideal for joining thin-walled materials and spot welding. This method is suitable for applications requiring aesthetically pleasing welds, as laser conduction welds produce rounded, smooth beads that often do not require additional grinding or finishing. Conduction mode welds are commonly used to create corner welds on the visible surfaces of electronics packages. Another prevalent application of conduction mode welding is in battery sealing, especially in situations where particulates might be a concern.

To analyze heat flow in conduction mode welding, an approximate assessment can be made using point heat source analysis. Additionally, understanding the material properties aids in analyzing the flow within the molten pool.

Keyhole Welding

Keyhole welding is a laser welding technique that uses exceptionally high power densities, exceeding 1.5 MW/cm². A portion of the work material undergoes vaporization, forming a cavity known as the keyhole. This keyhole is enveloped by molten metal, which, in turn, is encircled by solid material. As the laser beam moves along the joint, the molten material surrounding the keyhole fills the cavity. The keyhole cavity contains vapor, plasma, or both.

When laser energy penetrates the keyhole, it becomes trapped and travels deeper into the material, achieving extremely high absorption rates, surpassing 90% once the keyhole is formed. The keyhole mode allows for the creation of welds that are both narrow and deeply penetrating. This characteristic is particularly appealing for machined components and aerospace engine parts because it minimizes distortion.

Conduction/Penetration Mode Welding

Conduction/Penetration mode welding takes place at medium power density, approximately 1 MW/cm², offering deeper penetration than conduction mode. When the laser energy supplied to a conduction weld doesn't dissipate rapidly, the processing temperature can exceed the vaporization point. As the material vaporizes, the weld depth significantly deepens, leading to penetration welding. Although the keyhole is present, it has shallow penetration, resulting in a typical weld aspect ratio (depth/width) of about 1. This mode is predominantly utilized with pulsed Nd:YAG lasers and finds extensive application in spot and seam welding applications.