Laser Cutting Processes

Laser cutting encompasses several distinct physical mechanisms, each optimized for specific materials and applications. Understanding these processes is crucial for parameter selection and quality optimization.

Process Classification

Primary Cutting Mechanisms

1. Melt and Blow - Most common for metals
2. Vaporization - High precision, thin materials
3. Oxidation (Flame) Cutting - Thick steel sections
4. Controlled Fracture - Brittle materials

Melt and Blow Cutting

Physical Mechanism

The most widely used laser cutting process involves:

1. Laser Heating: Material heated above melting point
2. Melt Pool Formation: Localized molten zone creation
3. Gas Ejection: High-pressure assist gas removes molten material
4. Kerf Formation: Continuous cut through material thickness

Heat Transfer Analysis

Energy Balance Equation: ∇·(k∇T) + Q_laser = ρc_p(∂T/∂t) + L_m(∂f_m/∂t)

Where:

• k = thermal conductivity
• T = temperature
• Q_laser = laser heat source
• ρ = density
• c_p = specific heat
• L_m = latent heat of melting
• f_m = melt fraction

Melt Pool Dynamics

Melt Pool Geometry:

• Width: Typically 1.2-2× kerf width
• Depth: Function of power and speed
• Shape: Influenced by surface tension and gas flow

Marangoni Flow: Surface tension gradients drive fluid flow in melt pool:

• Temperature-dependent surface tension
• Affects mixing and heat transfer
• Influences cut quality

Assist Gas Functions

Primary Functions:

1. Melt Removal: Momentum transfer to molten material
2. Oxidation Control: Inert gas prevents oxidation
3. Cooling: Convective heat removal
4. Plasma Suppression: Prevents plasma formation

Gas Selection Criteria:

Process Parameters

Power Density Requirements:

• Threshold: 10⁴ - 10⁵ W/cm²
• Optimal: 10⁵ - 10⁶ W/cm²
• Material dependent: Higher for high conductivity metals

Cutting Speed Relationships:

Where:

• v
  = cutting speed
  (\unit{mm/min})
• P
  = laser power
  (\unit{W})
• \rho
  = material density
  (\unit{kg/m^3})
• t
  = material thickness
  (\unit{mm})
• L_m
  = latent heat of melting
  (\unit{J/kg})

Interactive Process Flow

Real-Time Process Monitoring

Interactive Parameter Optimization

Vaporization Cutting

Mechanism

Direct material vaporization without significant melting:

1. Rapid Heating: Temperature exceeds boiling point
2. Phase Change: Solid → vapor transition
3. Vapor Ejection: High-pressure vapor removal
4. Minimal HAZ: Limited heat conduction

Thermodynamics

Energy Requirements: Q_total = Q_sensible + Q_latent

Q_sensible = m·c_p·(T_boiling - T_ambient) Q_latent = m·L_v

Where L_v = latent heat of vaporization

Vapor Pressure Effects: Vapor pressure creates recoil pressure: P_recoil = 0.54 × P_vapor

Applications

Optimal Conditions:

• Thin materials (< 1 mm)
• High precision requirements
• Minimal HAZ tolerance
• Non-metallic materials

Material Examples:

• Thin metals (< 0.5 mm)
• Polymers and plastics
• Paper and textiles
• Biological tissues

Process Characteristics

Advantages:

• Narrow kerf width (< 0.1 mm)
• Minimal heat-affected zone
• High precision
• Clean cuts

Limitations:

• High power requirements
• Slow cutting speeds
• Limited thickness capability
• High energy consumption

Oxidation Cutting

Chemical Process

Combines laser heating with exothermic oxidation:

Reaction for Iron: Fe + ½O₂ → FeO + 272 kJ/mol 3Fe + 2O₂ → Fe₃O₄ + 1118 kJ/mol

Energy Contributions

Total Energy: E_total = E_laser + E_combustion

Combustion Energy:

• Can exceed laser energy by 2-3×
• Enables cutting of thick sections
• Self-sustaining process possible

Process Control

Critical Parameters:

1. Oxygen Purity: >99.5% for optimal performance
2. Gas Pressure: 0.5-6 bar depending on thickness
3. Nozzle Design: Affects gas flow and mixing
4. Cutting Speed: Must match oxidation rate

Oxidation Rate: Controlled by:

• Oxygen diffusion to reaction zone
• Heat removal from reaction products
• Iron oxide removal efficiency

Applications

Optimal Materials:

• Carbon steel (primary application)
• Low-alloy steels
• Cast iron

Thickness Capabilities:

• Up to 300 mm with high-power lasers
• Economic for thick sections (> 20 mm)
• Faster than plasma cutting for many applications

Quality Considerations

Edge Characteristics:

• Oxide layer formation
• Slightly rough surface
• Heat-affected zone present
• Dimensional accuracy good

Post-Processing:

• Oxide removal may be required
• Grinding or machining for critical surfaces
• Heat treatment for stress relief

Controlled Fracture Cutting

Mechanism

Thermal stress-induced fracture for brittle materials:

1. Localized Heating: Laser creates thermal gradient
2. Stress Development: Thermal expansion mismatch
3. Crack Initiation: Stress exceeds fracture strength
4. Crack Propagation: Controlled fracture path

Stress Analysis

Thermal Stress: σ_thermal = E·α·ΔT/(1-ν)

Where:

• E = Young’s modulus
• α = thermal expansion coefficient
• ΔT = temperature difference
• ν = Poisson’s ratio

Applications

Suitable Materials:

• Glass and ceramics
• Silicon wafers
• Brittle composites
• Some crystalline materials

Process Variants:

• Scribe and Break: Score line followed by mechanical breaking
• Thermal Shock: Rapid heating/cooling cycles
• Controlled Crack Growth: Continuous laser-guided fracture

Process Selection Guidelines

Material-Based Selection

Metals:

• Thin (< 3 mm): Melt and blow with nitrogen
• Medium (3-20 mm): Melt and blow or oxidation
• Thick (> 20 mm): Oxidation cutting preferred

Non-Metals:

• Polymers: Vaporization or melt and blow
• Ceramics: Controlled fracture or vaporization
• Composites: Process depends on matrix/fiber combination

Quality Requirements

High Precision:

• Vaporization cutting
• Pulsed laser operation
• Optimized beam quality

High Speed:

• Oxidation cutting (steel)
• High-power continuous wave
• Optimized gas flow

Minimal HAZ:

• Vaporization cutting
• Short pulse duration
• Rapid traverse speeds

Advanced Process Variants

Pulsed Laser Cutting

Advantages:

• Reduced heat input
• Better control of melt pool
• Improved edge quality
• Suitable for thin materials

Parameter Control:

• Pulse duration: μs to ms range
• Pulse frequency: Hz to kHz
• Peak power: Higher than CW average
• Duty cycle: Typically 10-90%

Multi-Pass Cutting

Applications:

• Very thick materials
• High-quality requirements
• Difficult-to-cut materials

Strategy:

• First pass: Rough cutting
• Subsequent passes: Quality improvement
• Progressive parameter optimization

Hybrid Processes

Laser-Waterjet:

• Laser preheating + waterjet cutting
• Reduced cutting forces
• Improved edge quality

Laser-Plasma:

• Laser piercing + plasma cutting
• Fast thick section cutting
• Economic for large parts

Process Monitoring and Control

Real-Time Monitoring

Sensor Technologies:

• Photodiodes: Plasma emission monitoring
• Pyrometers: Temperature measurement
• Acoustic: Process sound analysis
• Vision Systems: Melt pool observation

Adaptive Control

Feedback Parameters:

• Cutting speed adjustment
• Power modulation
• Gas pressure control
• Focus position optimization

Quality Indicators

Process Signatures:

• Stable plasma emission
• Consistent acoustic signature
• Uniform melt pool geometry
• Minimal spatter generation

Related Topics

• Material Properties - How material characteristics affect process selection
• Quality Control - Measuring and optimizing cut quality
• Parameter Optimization - Detailed parameter selection
• Advanced Applications - Specialized cutting techniques

Next: Explore Quality Control to understand how to measure and optimize cutting performance across all process types.