Detailed overview

The study of laser-material interactions is a fundamental research topic with critical applications in aerospace, automotive, and electronics industries for permanent marking, functional surface structuring, and precision micromachining. Our research group develops and applies an integrated methodology combining high-precision experiments and advanced computational models to study thermomechanical phenomena from the thermoelastic regime through plastic deformation to melting and material removal.

Experimental Approach

We employ a high-power continuous wave (CW) fiber laser system (TRUMPF TruFiber 2000 P compact, 2 kW, 1075 nm) integrated with a 3-axis CNC milling machine for precision processing. For pulsed laser studies, a 6 ns, 532 nm laser source is used. We developed multi-diagnostic techniques for simultaneous recording of thermal and mechanical response:

• Thermal Imaging (Optris PI-1M): Temperature distribution monitoring with 80 Hz frame rate and long-pass filter (750 nm cut-on) to capture thermal transients.

• Thermocouples (K-type): Localized temperature measurements at specific distances (0.5-2 mm) from the laser spot with high response time.

• White Light Interferometry (WLI): Non-contact surface topography characterization with nanometer-scale resolution for measuring deformation depth, width, and surface roughness.

• Strain Gauges (FLGB-02-11): Real-time measurement of surface deformation using quarter-bridge Wheatstone circuit for strain validation.

Computational Approach

To understand and predict material behavior, we develop advanced multiphase, multiphysics computational models using LS-DYNA FEM software:

3D Coupled Thermo-Structural FEM Model: Simulation of transient thermal and mechanical response from solid to melting regime, considering:

• 3D transient heat conduction equation with moving Gaussian heat source

• Mechanical equation of motion with thermal stress coupling

• Constitutive model for elastoplastic behavior (strain, strain rate, temperature dependence)

• Damage/fracture model for material removal

• Temperature-dependent material properties (thermal conductivity, specific heat, thermal expansion)

Applications and Key Findings

CW Fiber Laser Processing

We demonstrated that the high-power CW fiber laser can be effectively limited to controlled surface modification regimes (plastic deformation, etching, shallow engraving) by exploiting modulated power control capability. For Al 1050 H14:

• Marking (200 W, 1200 mm/min): Maximum temperature ~600°C (below melting), plastic strain 0.023, surface displacement 0.25 μm

• Etching (200 W, 75 mm/min): Temperature exceeds 660°C, groove depth 10-20 μm, width 250 μm

• Cutting (450-1000 W): Kerf width ~300 μm for 1 mm thick Al sheets

Pulsed Laser Micromachining

We developed a downscaled FEM modeling approach incorporating experimentally measured surface roughness profiles through step-like linear approximations. Key findings:

• Laser fluence threshold for melting: 1.5 J/cm² for Al 6061

• Surface roughness influences absorptivity, with enhanced absorption for roughness greater than laser wavelength

• Downscaling to 0.5 μm step accuracy enables precise identification of fracture and permanent deformation zones (fractured volume ~340 μm³)

• Temperature-dependent Johnson-Cook model accurately captures transition from elastic to plastic to melting regimes

Heat Affected Zone (HAZ) Characterization

We characterized HAZ for AISI H13, AISI 1045 steel, and Ti6Al4V alloy under CW laser irradiation (1-4 W, 2-100 mm/min scanning speed):

• Plastic regime: Maximum surface temperature 980°C (AISI H13), HAZ width 120 μm, depth 50 μm

• Melting regime: Maximum surface temperature 1476°C, HAZ width 170 μm, depth 70 μm

• Surface roughness reduces after laser irradiation in plastic regime (e.g., AISI H13 Rt from 3.33 μm to 2.76 μm)

Laser-Assisted Machining (LAM)

We developed orthogonal cutting models with moving Gaussian heat source to evaluate LAM of AISI H13 steel:

• Cutting force reduction up to 7.7%, thrust force reduction up to 15%

• Higher reduction achieved with increased laser heat flux (2.44 kW/mm²) and beam diameter (300 μm)

• Improved chip formation and surface finish compared to conventional machining

• Optimal temperature for strength reduction: ~850-900°C at 1 mm distance from cutting tool

Examples