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Laser welding glossary

Autogeneous process – No filler material is added during the welding process.
Beam quality – Also called the M2 number, refers to how efficiently the laser can be focused and defines the power density cross section distribution through the beam. The M2 value is a comparison of the laser to that of a perfect laser with a pure Gaussian mode.
Butt joint – a joint geometry in which two pieces of material are placed side by side and the weld is made at the interface between the two materials.
Collimating lens – An optic in the focus head, which transforms the diverging laser into near parallel/ collimated light.
Conduction mode – Welding 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.
Continuous wave (CW) laser – A laser that produces extended output – the laser remains on continuously until stopped.
Diverging beam – Defined as a laser that is increasing in size with distance; occurs when using a connectorized fiber, either QBH, LLK-Q, or another type.
Fillet joint – A joint geometry in which the materials are placed one on top of another. The weld penetrates the top layer into the second layer, creating the weld.
Focus head – Directs and focuses the output from the laser delivery fiber onto the work piece.
Gallium arsenide (GaAs) – Medium most commonly selected for high power diode laser.
Gated or modulated output – A CW laser that produces pulses of light output. The peak power does not exceed the laser’s rated average power.
Gaussian mode – Defines the power distribution in the laser cross section that has a maximum at the beam center which drops away similarly to a bell curve to the edges.
Keyhole or penetration mode – Occurs when peak power density is increased beyond 1.5MW/cm2. Characterized by deep narrow welds with aspect ratio greater than 1.5.
Lap joint – A joint geometry in which the parts are placed one on top of the other and the beam is directed at an angle to produce a weld along the surface where the pieces meet.
Laser resonator – Controls how the laser is generated in the laser medium. Consists of a rear 100 percent reflector and a front partial reflector.
Multi-mode fiber lasers – Lasers that use core diameter fibers greater than 50 microns.
Optical coupler – Used to de-couple the lasing fiber from the process/delivery fiber.
Optical spot size – Diameter of the focused laser spot on the work piece. Note that this is not the same as the weld width, which in the majority of cases is larger than the optical spot size.
Optics – The general term that covers all the optical elements in the focus head, and includes the collimator lens, beam expander, dichroic mirror, focus lens and cover slide.
Optimum peak power – Peak power that creates the deepest penetration at a given energy without material expulsion.
Peak power – Parameter that directly controls weld penetration that can be controlled and programmed on the laser. The units of peak power are watts (W).
Photons (Laser light photons) – The laser is comprised of many small packets of light energy that are known as photons traveling in the same direction and in phase.
Plume – When a keyhole is formed, metal vapor is ejected above the melt pool. This acts to ionize the surrounding gas, which can be observed as a white plume above the weld.
Power ramping – Increasing or decreasing the laser power at the start of the weld and at the end of a seam weld.
Pulse energy – The energy contained within a pulse. Product of peak power (Pp) and pulse width.
Pulse width – Duration of the laser pulse, measured in milliseconds.
Pulsed laser – A laser that can provide a higher peak power than its average power.
Seam welding – The length of weld produced by keeping the laser turned on in CW mode or by placing a series of spot welds on a part.
Sequential firing – Also known as time share firing, refers to multiple laser outputs fired one at a time.
Simultaneous firing – Also known as energy share firing, refers to multiple outputs from a laser that all fire at the same time.
Single mode fiber laser – Refers to small core diameter fibers, between 10-20 microns, that restrict the laser to a single transverse mode that results in a power density cross section through the laser that has a high central maximum that falls off sharply.
Spot to spot overlap percentage – In spot welding, percentage of the previous spot that is covered by the subsequent one.
Square pulse – Natural ramp up and down of the laser. Typically, this is a step input.
Transition keyhole mode – Occurs at medium power density, around 1 MW/cm2 and higher, resulting in more penetration than conduction mode. This mode is used almost exclusively by the pulsed Nd:YAG or fiber laser, for many spot and seam welding applications