Laser machining

Laser machining uses a focused beam as a tool that never wears, touches or pushes the workpiece. The laser deposits energy in a small spot, and depending on the irradiance and the time over which it is delivered, the material heats, melts, vaporizes or is ablated directly. A motion system or a galvanometer scanner moves the spot along the path.

Processes

Cutting uses a continuous or long-pulse beam to melt or vaporize a kerf through the material, with a coaxial assist gas blowing the melt out; oxygen adds exothermic reaction energy for steel, nitrogen gives oxide-free edges. Welding melts the joint, either by conduction at moderate irradiance or, above roughly 10⁶ W/cm², in keyhole mode, where the vapour pressure opens a deep narrow cavity that traps the beam and gives welds much deeper than they are wide. Drilling uses repeated pulses; marking and engraving change the surface colour, texture or remove a coating; and micromachining with short pulses scribes, cuts and structures thin films, glass, semiconductors and medical devices.

Lasers used

Multi-kilowatt fiber lasers near 1070 nm now do most thin and medium sheet-metal cutting and much welding, having displaced CO₂ lasers at 10.6 µm there because of their efficiency, fiber delivery and smaller achievable spot. CO₂ lasers remain common for wood, acrylic, textiles and other non-metals, which absorb 10.6 µm strongly. Q-switched nanosecond fiber and solid-state lasers dominate marking. Picosecond and femtosecond lasers are used where heat effects must be minimal. Green and blue lasers are increasingly used for copper and gold, which absorb near-infrared light poorly.

What sets the result

The focused spot size follows from the beam parameter product: a beam of BPP $B$ focused to a half-angle $\theta$ reaches a waist radius of about $B/\theta$, so at a fixed focusing angle a lower BPP gives a smaller spot and higher irradiance, or, for the same spot size, a longer depth of focus. The rest of the process window is the power or pulse energy, the fluence at the workpiece, the pulse duration and repetition rate, the scan speed, the focus position relative to the surface, and the assist gas.

Measurement and control

Process setup relies on measuring the beam at the work plane: power with a high-power thermal sensor, the caustic and focal spot with a beam analyzer designed for kilowatt beams, and the focus position, which drifts with thermal lensing in the optics as they heat. Quality is judged by kerf width, edge roughness, dross, heat-affected zone depth and taper, measured on sectioned or microscope-inspected samples.

References: W. M. Steen, J. Mazumder, Laser Material Processing, 4th ed. (Springer, 2010); ISO 11146-1:2021, Lasers and laser-related equipment: Test methods for laser beam widths, divergence angles and beam propagation ratios.