Understanding Penetration Depth
Penetration depth is the vertical distance from the top surface to the deepest point of fusion. It directly impacts joint strength and is controlled by laser power, welding speed, focus position, beam quality, and material properties.
Keyhole vs. Conduction Mode
Conduction Mode (Power Density < 1 MW/mm²):
- Heat conducted from surface into material
- Shallow, wide weld profile (aspect ratio < 0.5)
- Stable, predictable process
- Suitable for thin materials (<2mm)
Keyhole Mode (Power Density > 1 MW/mm²):
- Vapor cavity (keyhole) forms in weld pool
- Deep, narrow weld profile (aspect ratio > 1)
- 10× more efficient penetration than conduction
- Required for thick materials (>3mm)
- Risk: Porosity from keyhole collapse - use trailing gas
Focus Position Optimization
Focus position critically affects power density and penetration. The optimization curve shows:
- At Surface (0mm): Maximum power density, but shallow penetration due to surface reflection
- Below Surface (-0.5 to -2mm): Optimal for most applications
- Best penetration efficiency
- Keyhole stabilized below surface
- Reduced spatter
- Deep Below (-3 to -5mm): Reduced penetration, wider beam
- Above Surface (+1 to +5mm): Defocused beam, poor penetration
Beam Quality (M²) Impact
M² (beam parameter product) measures beam quality:
| M² Value | Quality | Penetration Impact |
|---|---|---|
| 1.0 | Perfect (TEM₀₀) | 100% (theoretical max) |
| 1.05-1.1 | Excellent (fiber lasers) | ~95-98% |
| 1.2-1.5 | Good (disk lasers) | ~80-90% |
| >2.0 | Poor (needs maintenance) | <70% |
Lower M² allows smaller spot diameter → higher power density → deeper penetration.
Aspect Ratio (Depth/Width)
Aspect ratio indicates weld profile:
- < 0.5: Shallow wide (conduction mode)
- 0.5-1.0: Moderate (transition zone)
- 1.0-3.0: Deep narrow (keyhole, ideal)
- > 3.0: Very deep (porosity risk, use He trailing gas)
Power Density Calculation
Power Density = Laser Power / Spot Area
For spot diameter d (mm):
I = P / (π × (d/2)²) MW/mm²
Critical thresholds:
- 0.1-1 MW/mm²: Conduction welding
- 1-5 MW/mm²: Keyhole welding
- >10 MW/mm²: Deep keyhole (potential for defects)
Material-Specific Considerations
High Thermal Conductivity (Al, Cu)
- Heat dissipates quickly → reduced penetration
- Require higher power or slower speed
- Preheat recommended for thick sections
Low Absorptivity (Al @1064nm ~8%)
- Most laser energy reflected
- Use green lasers (515nm) for better absorption (40-60%)
- Oxide layer on surface improves absorption
Stainless Steel (Ideal for Laser)
- Good absorption (~30-40%)
- Moderate thermal conductivity
- Optimal penetration characteristics
Optimization Strategy
- Select Target Penetration: 50-80% of thickness (full penetration risky)
- Calculate Required Power Density: Aim for 1-5 MW/mm² for keyhole
- Optimize Focus Position: Use -0.5 to -2mm for deep penetration
- Adjust Speed: Balance penetration with weld quality
- Monitor M²: Poor beam quality → penetration loss, check optics
Common Issues
Insufficient Penetration:
- Increase power or reduce speed
- Move focus below surface (-1mm)
- Check beam quality (M² < 1.2)
- Verify material absorption (clean surface)
Burn-Through:
- Reduce power or increase speed
- Move focus away from optimal (-2mm → -1mm)
- Use backing gas to control backside oxidation
Porosity in Deep Welds:
- Add trailing gas (Ar or He) to stabilize keyhole
- Reduce speed slightly to allow gas escape
- Tilt workpiece 3-5° to promote gas exit