Penetration Depth Calculator

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)
• Often easier to stabilize in thin-section work
• 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
• Often used for thicker sections (>3mm), depending on the joint and power density
• Risk: Porosity from keyhole collapse - use trailing gas

Focus Position Checks

Focus position critically affects power density and penetration. The reference curve shows:

• At Surface (0mm): Maximum power density, but shallow penetration due to surface reflection
• Below Surface (-0.5 to -2mm): Common starting range for many applications
  • Often improves penetration efficiency
  • May help stabilize the keyhole below the surface
  • Can reduce spatter in some setups
• 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:

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
• Generally favorable penetration response

Parameter Adjustment Strategy

1. Select Target Penetration: 50-80% of thickness (full penetration risky)
2. Calculate Required Power Density: Aim for 1-5 MW/mm² for keyhole
3. Focus Position: Start around -0.5 to -2 mm when deeper penetration is the target
4. Adjust Speed: Balance penetration with weld quality
5. 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 the model peak (-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

Related Resources

• Energy & Heat Calculator
• Weld Width Calculator
• Material Absorption Data