Laser cutting of metals is one of the most advanced thermal processing methods used in modern industry. It combines optical, thermodynamic, hydrodynamic, and metallurgical phenomena occurring over very short timescales and at a microscopic scale. Understanding the physics of the process enables optimization of technological parameters, improvement of edge quality, and increased production efficiency and stability.
1. Laser Beam Interaction with the Metal Surface
Absorption and Reflection of Radiation
When a laser beam strikes a metal surface, part of the energy is reflected and part is absorbed by the material. The degree of absorption depends on:
- the wavelength of the radiation,
- the temperature of the material,
- the surface condition (oxidation, roughness),
- the angle of beam incidence.
Metals at room temperature reflect a significant portion of radiation, so initial absorption is limited. As temperature increases, absorption rises, creating a positive feedback loop — the heated material absorbs energy ever more efficiently.
Absorption can be expressed as:
A = 1 − R
where:
- A – absorptance,
- R – reflectance.
In the vapor state, energy absorption is very high, which stabilizes the process.
2. Energy Concentration and Beam Power Density
A key physical parameter is the laser beam power density, which reaches values of:
- 10⁶ – 10⁸ W/cm² – the material melting range,
- above 10⁸ W/cm² – evaporation and formation of a vapor channel.
Strong energy focusing causes a rapid temperature rise in a very small area, enabling local melting or evaporation of the metal.
3. Mechanisms of Material Melting and Evaporation
Once the melting point is exceeded, the material transitions to a liquid state. Continued energy input leads to:
- superheating of the liquid,
- intense evaporation,
- buildup of high metal vapor pressure.
The liquid-to-vapor transition weakens intermolecular bonds and causes material particles to be ejected from the surface.
4. Formation of the Vapor Channel (Keyhole)
At high power densities, a vapor channel known as a keyhole forms within the material. It is a narrow, deep cavity:
- filled with metal vapors,
- with walls coated by a thin layer of liquid metal,
- stabilized by surface tension and vapor pressure.
The vapor channel increases energy absorption through multiple reflections of radiation inside the cavity, improving process efficiency.
5. Dynamics of the Liquid Metal Pool
A liquid metal pool forms in the beam interaction zone. Its behavior is governed by multiple physical phenomena.
Forces Acting on the Liquid Metal
- surface tension,
- liquid viscosity,
- metal vapor pressure,
- Marangoni forces (arising from temperature gradients),
- interaction with the process gas jet.
Flow Phenomena
Complex liquid metal flows occur within the cutting gap, determining:
- the quality of the cut surface,
- process stability,
- the formation of defects.
6. Role of the Process Gas
The assist gas plays a critical role in the process.
Removal of Liquid Metal
The gas jet expels molten material from the cutting gap.
Protection Against Chemical Reactions
Inert gases such as nitrogen or argon prevent oxidation.
Support of Exothermic Reactions
Oxygen reacts with the metal, releasing additional thermal energy and increasing cutting speed.
7. Material Removal Mechanisms
Depending on process parameters, three main mechanisms are distinguished:
- Cutting by melting and expulsion of liquid metal.
- Reactive cutting with oxygen (metal combustion).
- Cutting by direct material evaporation.
8. Formation of the Cutting Gap and Edge
The cutting gap is a dynamic system in which:
- the laser acts as a concentrated energy source,
- liquid metal is removed by the gas,
- the cutting front advances with the movement of the cutting head.
The geometry of the gap depends on laser power, cutting speed, gas pressure, and material properties.
9. Formation of Dross and Its Control
Dross forms when molten metal is not completely expelled and solidifies on the bottom edge of the workpiece.
Its formation is influenced by:
- liquid metal viscosity,
- gas pressure and type,
- cutting speed,
- linear energy input of the process.
10. Heat-Affected Zone (HAZ)
Characteristics
The heat-affected zone (HAZ) is the region where the material was not melted but its structure was altered due to elevated temperatures.
Microstructural Changes
The HAZ may exhibit:
- phase transformations,
- changes in hardness,
- increased brittleness,
- residual stresses.
The size of this zone depends on process parameters and the thermal properties of the material.
11. Heat Conduction and Thermal Phenomena
The temperature distribution during cutting depends on:
- the thermal conductivity of the metal,
- material thickness,
- beam interaction time,
- laser operating mode (continuous or pulsed).
Materials with high thermal conductivity dissipate energy more rapidly, affecting the width of the heat-affected zone.
12. Parameters Influencing Process Stability
Beam Parameters
- wavelength,
- beam quality (M²),
- focal position,
- operating mode.
Technological Parameters
- laser power,
- cutting speed,
- gas type and pressure,
- nozzle standoff height.
Material Properties
- absorptivity,
- thermal conductivity,
- melting point,
- liquid metal viscosity.
13. Process Instability Phenomena
The process may become destabilized due to:
- oscillations of the vapor channel,
- turbulence in liquid metal flow,
- unstable energy absorption,
- surface contamination,
- incorrect beam focusing.
Keyhole stability and the balance between evaporation and liquid metal expulsion are critical to cut quality.
14. Current Research Directions
Current research on laser cutting focuses on:
- modeling of coupled thermal and hydrodynamic phenomena,
- analysis of dynamic energy absorption,
- real-time process monitoring,
- use of artificial intelligence for parameter optimization,
- minimization of the heat-affected zone.
Summary
The physics of laser cutting of metals encompasses the complex interactions of laser radiation with material, melting and evaporation processes, liquid metal hydrodynamics, and thermal and metallurgical phenomena. Of key importance are energy absorption, vapor channel formation, liquid metal pool flow, and effective material removal by the process gas. Control of these phenomena enables high edge quality, minimization of the heat-affected zone, and process stability — making laser cutting one of the most precise and efficient metal processing technologies available.
