TWI Knowledge Summary
Cutting with a carbon dioxide laser
by Paul Hilton
Principles of laser cutting
Whether laser cutting with CO 2 or Nd:YAG lasers, the principles employed are basically the same. The beam from the laser is focused on to the surface of the material being cut by means of a lens.The focused laser beam heats the material surface and a very local melt capillary is quickly established throughout the depth of the material. The diameter of this capillary is usually just slightly greater than the diameter of the focused laser beam. The great majority of CO 2 laser cutting is performed using an assist gas. The significant feature of gas assisted laser cutting is that the molten material is ejected from the base of the capillary by a jet of gas coaxial with the laser beam. For some materials this gas can further assist the process by chemical (exothermic) reaction as well as physical work. The cut is generated by either moving the focused laser beam across the surface of the stationary material or by keeping the laser beam stationary and moving the workpiece. Hybrids of these two options are also possible. In this way simple and complex linear cuts or two dimensional parts can be produced. More complex systems are required for three dimensional processing.
Most laser cutting with CO 2 lasers is performed in the power range 1 - 1.5kW. However, over the last few years higher beam quality lasers with powers up to 6kW have become available and these lasers have been able to extend the thickness cut, on steel for example, to 20 - 25mm.
Status of the process
The first ever 'gas assisted' laser cuts were made in 1967 in a joint experiment between TWI and SERL (Services Electronics Research Laboratory). SERL had, by the end of 1966, developed a 300W pulsed slow flow CO 2 laser (for military reasons) but were also looking for potential industrial uses. This development was only two years after the first reporting of lasing action from the CO 2 molecule, and probably marks the start of the laser materials processing industry as we know it today.
Sheet metal cutting has since become, by far, the dominant industrial use of lasers in materials processing. It is believed that approximately 12 000 industrial laser cutting systems have been installed world-wide, with a total market value over 5 billion US dollars. Over 60% of this equipment is installed in Japan.
Today, laser cutting is used extensively for producing profiled flat plate and sheet, for diverse applications in the engineering industry sectors. For three dimensional components, multi-axis gantry laser beam manipulators have extended laser cutting to the automotive sector, this type of equipment being used for trimming pre-production body panels at all leading car manufacturers. More recently laser cutting has also found its way, very successfully, into other industry sectors such as shipbuilding, traditionally seen as fairly slow to adopt high technology processes.
Metals, ceramics, polymers and natural materials such as wood and rubber can all be cut using CO 2 lasers. For steels the dominant process utilises an oxygen assist gas, which provides exothermic energy to the cutting process. As a result, thick sections (up to 20 - 25mm with the most advanced equipment) can be cut commercially and the cut quality and speed are generally considered high when compared with other thermal cutting processes. Laser cutting is also generally regarded as a 'low distortion' process, compared with other thermal cutting options. Stainless steel, aluminium and titanium are also cut using CO 2 lasers, this time using a high pressure (up to 15 bars) inert assist gas to aid the process and blow material from the cut kerf. Less cutting of thermoplastic materials is performed currently because of the nature of the fume generated when some plastics are vaporised.
Current issues
Current research in CO 2 laser cutting involves, on one hand, use of very high beam quality lasers (capable of being focused to small spots of high power density) for high speed cutting of thin metals (over 140m/min for 0.25mm thick material). On the other hand, research is developing techniques to increase the thickness of material that can be cut. In this respect steel up to 60mm thick has been cut with moderate (<5kW) laser power.
Commercial development of CO 2 laser cutting systems involves use of special focusing lenses, employing twin foci, to improve cut quality on non-ferrous metals, and engineering improvements to the machines to improve acceleration and speed, to maintain development of the process itself.
Application issues involve: cut quality, use of laser cuttable steels, thick section cutting and component nesting technology.
Precautions
The CO 2 lasers used for laser cutting are 'class 4' and as such, current production level machinery is configured to prevent human access to laser beams during normal operation. The CO 2 laser beams can cause serious burns to human tissue and permanent damage to the eyes. CO 2 lasers also employ high voltage power supplies and there is an associated electrical hazard, with a subsequent risk of electric shock, during service and maintenance. Other hazards involve those of moving machinery and fume products from the cutting process.
Further information
You can use the Weldasearch literature database to supplement what you find in JoinIT.
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