1. Laser Cutting University of Texas at Austin
ME392Q – Manufacturing Processes: Unit Process By
Riko Tantra
Date: April 22, 2003

2. Presentation Overview
Laser & Laser Cutting Fundamentals
Material Removal Types
Equipment
Different Type of Lasers for Laser Cutting
Laser Parameters for different materials
Cutting Speed & Depth of Cut
Comparison of Laser cutting to other method
Costs
Advantages & Disadvantage of Laser Cutting
References

3. Laser Fundamentals
Acronym of Light Amplification Stimulated Emission of Radiation
Basic:
Atoms initially at the Ground State
The atoms go to Excited State when a high energy is applied (called ‘pumping’)
When atoms moves back to the ground state, photons (particle of light) are released
Laser Beam Characteristics:
Monochromaticity
Coherence
Very Limited Diffraction
Extremely high Radiance

4. Laser Beam Formation Example (Ruby Laser)
1. Laser in OFF state
4. Photons runs parallel to the rod direction & reflect back and forth and stimulate emission on more atoms
2. Flash Tube excite atoms in the Ruby Rod
5. Laser light passes through partially-reflective mirror
3. Some Atoms emit Photons

5. Laser Machining
Laser Cutting

6. Material Removal Types of Laser Cutting
Vaporization: low vaporization temperature materials
Fusion: Material is melted & ejected (by an inert gas jet)
Reactive Fusion: dross is no longer a metal, but an oxide
Thermal stress cracking or controlled fracturing: for brittle materials
Scribing: Mechanical snapping along scribed line
Ablation (Excimer laser): breaking organic material bonds
Burning in reactive gas

7. Equipment Laser-beam generator
Beam delivery: Circular polarizers, mirrors, beam splitters, focusing lenses and fiber optic couplings
Workpiece positioning
Auxiliary devices: Laser head, safety equipment, etc.
In addition, assist gases also required

8. Smart Laser Cutting System
Picture from [2]

9. Different Type of Laser for Laser Cutting
CO2 laser (most commonly used for laser cutting):
a. Have the highest Continuous Wave (CW) power
b. Capable to extract as much as 10kW/m of discharge tube (with traverse flow laser)
c. Have a high energy efficiency (up to 10%)
d. Capable of both CW and Pulsed operation (5kHz)
CO2 Laser Schematic [8]

10. Nd:YAG:
a. has the highest peak power for pulsed operation
b. May be operated in either CW or pulsed (200Hz) temporal modes
Nd: Glass: more economical but has lower thermal conductivity. Used for low pulse repetition rates (1Hz; due to its poor thermal properties) & high pulse energies. Ideal for drilling.
Nd: Ruby: low energy efficiency & power, Limited to pulsed laser operation

11. 5. Excimer:
a. High power (Average power over 100W) pulsed beams (1kHz)
b. Laser length limited to 2-3 m due to the absorption coefficient  Material narrower materials that can be processed vs. that of CO2 laser
c. Used to machine solid polymer pieces, remove polymer films, micromachine ceramics, medical applications
d. Ablation material removal process
e. Higher precision & less heat affected zone vs. CO2 & Nd:YAG lasers
f. Produces large area beams  use mask to produce series of holes holes in a polymide sheet in 3 sec vs 50 sec using CO2 or Nd:YAG lasers.

12. Laser Beam Temporal Modes
Continuous Wave (CW) commonly results in the highest cutting speed & better surface finish. Roughness is determined by thickness, alloy content, etc. [52]
Pulsed beam results in the fewest thermal effects & least distortion of workpiece. With drilling overlapping holes (see right), it’s possible to cut with smoother surface.

13. Comparison of Major Material Machining Lasers

14. Cutting Considerations for Different Materials
Ferrous Metals:
High efficiency due to easy-to-remove oxide creation
One approximate rule:1.5kW laser power will cut
a. 1mm thick mild steel at approx 10m/min
b. 10mm thick mild steel at approx 1m/min
Non-Ferrous Metals:
Mostly less efficient than cutting steel, due to the higher reflectivity, thermal conductivity & less efficient oxidation reaction
Similar edge qualities to SS

15. Non-Metal: Most non-metallic materials are highly absorptive at CO2 laser wavelength. Cutting process:
i. Melt Shearing (mostly for thermoplastic): cut
very quickly & high quality edges
ii. Vaporization: usually only for acrylic
iii. Chemical degradation: slow cutting, high temperature, but flat & smooth result

16. Cutting Speed on Mild Steel
Cutting Speed on Stainless Steel

17. Cutting Speed on Aluminum
Cutting Speed on Acrylic
Max Cutting Speed for Polymer: V=PQt-B
P = Laser Power (W) t = material thickness (mm)
Q = an experimentally derived constant for the polymer
B = an experimentally derived constant for the material

18. Power setting for different cutting applications [9]
Application Requirement
Recommended Laser Power
Cutting consideration
Thin materials: Non- metals
150 Watt Average, 450 Watt peak
Up to 0.04” thick can be cut at full speed of 1200in/min with 150 watt
Thicker materials: Non-metals
250 watt to 500 watt average - up to 1500 watt peak
Up to 1”: Power  Cutting Speed  , cleaner result & lower HAZ
Metals
150 watt to 500 watt average - up to 1500 watt peak
Al, Brass, SS use 500 W due to its reflectivity. As thickness , also power need to be 

19. Laser Cutting Analysis
Cutting depth, s
s = 2.a.P/(1/2..v.d.(cp.(Ts-To)+L))
a = absorbtivity of the material
P = Beam power
 = density
v = scanning velocity
d = spot diameter (=2.R)
cp = specific heat
Ts = surface temperature
To = ambient temperature
L = latent heat of fusion

20. Typical CO2 Laser Cutting Parameters

21. Characteristics of cuts by Laser Cutting
Kerf Width: CO2 laser range from 0.1-1mm
Roughness: 0.8mm material  1 m
10 mm material  10 m
Dross: undesirable; removed by extremely high assist gas or by applying antisplatter coatings (i.e. graphite)
Dimensional Accuracy: main problem is thermal effect (distortion)

22. Comparison of Laser cutting to other methods

23. Cutting Cost example CAPITAL COST:
Laser Generation: $ several hundred thousand
Cooling system, power supply, multi-axis robot: exceed cost of laser
OPERATING COST:
CO2 lasers cost $70-$100/watt (Nd:YAG costs 10-20%more)
Safety devices
Skilled operator
Example CO2 system operating at 1500W
Electricity at 7cent/kW-hr $2.10/hr
Internal laser optics $2.06/hr
(lifetimes per manufacturer)
Focusing lens (500hr lifetime) $1.10/hr
Laser gas $1.03/hr
Assist gas $3.60/hr
(based on 10ga. Carbon steel w/ O2 assist)
TOTAL: $9.89/hr

24. Advantages of Laser Cutting
Laser machining is a thermal process: depends on thermal and optical rather than the mechanical properties
Laser machining is a non-contact process: No cutting forces generated
Laser machining is a flexible process
Laser machining produces a higher precision and smaller kerf widths results (as small as 0.005mm dia hole)
(cont’d…)

25. Advantages of Laser Cutting (Cont’d)
For most industrial materials up to 10mm thick, laser cutting has a significantly higher MRR
Laser Cutting has ability to cut from curved workpieces
For cutting fibrous material (wood, paper, etc.) laser cutting eliminates residue and debris

26. Disadvantage of Laser Cutting
Low energy efficiency
Material damage: Heat affected zone (HAZ)
Laser cutting effectiveness reduces as the workpiece thickness increases
Laser cutting produces a tapered kerf shape (due to divergence)

27. References
Chryssolouris, G., Laser Machining Theory and Practice, Springer-Verlag, New York City,NY 1991
Steen, W M., Laser Material Processing 2nd ed., Springer-Verlag, London 1998
Migliore, L., Laser Materials Processing, Marcel Dekker, Inc, New York City, NY 1996
How Laser Works. Maschler, M. Howstuffworks homepage April <
Wang F.F.Y, Laser Materials Processing, North-Holland, New York City, NY 1983
Benedict, G.F., Nontraditional Manufacturing Processes, Marcel Dekker, New York City, NY 1987
Kalpakjian, S., Manufacturing Processes for Engineering Materials, Addison Wesley Longman, Menlo Park, CA 1997
Fast Axial Flow Lasers – Theory of Operation. April PRC Laser Homepage March 14, 2000 <
Advance Laser Cutting Technology. April Beam Dynamics Homepage <