Instrucciones para la preparación de los trabajos a presentar en el XV congreso Nacional de Ingeniería Mecánica

1.1 Laser cutting process parameters 
Laser cutting is a thermal process. The material is heated to its melting or vaporisation 
temperature . Heating is achieved by concentrating the energy in a very small spot. This 
allows the cutting of almost all types of materials with thicknesses of up to 20 mm in 
the case of steel sheets without the need for very high levels of energy [10]. 
There are different laser generators depending on the type of medium being used. Each 
type of laser creates a different laser beam at a given wavelength. The CO
2 
generators 
are the most used for cutting with a wavelength 10.6 
µm, whilst the Nd:YAG lasers 
generate a beam with a wavelength of 1.06 
µm. In general, the Nd:YAG lasers 
wavelength is better absorbed by most of materials (copper, aluminium, precious 
metals...). Steel, however, has acceptable absorption levels for the beam generated by 
CO
2
.
This, added to the fact that the CO
2
generators are more powerful and cheaper, 
explains why their use in industry is much more widespread. 
Once the beam has been generated, a lens system focuses the beam on a point with 
diameters of around 0.2 mm. The focusing of the beam results in high energy density at 
the irradiated spot surface, a typical value being in the order of 1.4·10
10
W/m
2
. The high 
power density concentrated on the spot vaporises almost all types of material (as long as 
enough beam energy being absorbed) [11]. 
A gas jet is injected coaxially with the laser beam during cutting. The assisting gas jet 
has two effects, depending on the material that is being cut: 
• Inert gas is usually used in the case of composites to protect the heated area from 
the oxidations reactions and to sweep away the vaporised material from the back 
of the cutting surface.  
• On the other hand, oxygen is used to cut steel. This gas generates exothermal 
reactions that favour cutting as well as sweeping away the material that has been 
cut away. Figure 1 shows a diagram of a typical cutting head. 
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Cutting direction 
Evacuated material (Dross) 
Cutting head 
Gas inlet
Lens 
Laser beam
Figure 1. Experimental set-up used and diagram of a laser cutting head. 
Material cutting is achieved by the translation of the spot along the desired cutting path. 
The most common method is the use of 3 axes machines, which move the steel sheet 
underneath the laser beam. There are also machines capable of interpolating up to 5 
axes simultaneously. These type machines are designed for cutting complex forms and 
their programming is much more complex than the 2D case, although there are no 
differences as far as the cutting process is concerned. The main parameters defining the 
process are [12, 13]: 
ƒ Properties and thickness of the material 
ƒ Power of the beam emitted by the generator. 
ƒ Cutting speed: speed of the spot movement along the steel sheet 
ƒ Pressure, flow rate and properties of the cutting assisting gas. 
ƒ Distance between laser head and sheet.
A correct choice of parameters requires the minimum quantity of heat contribution in 
the part during the cutting. In this way, the part will be cut with the minimum amount of 
thermal damage. As the contribution of heat depends on the cutting power and the 
speed, the cutting speed should be maximised and the power minimised in order to 
minimise the thermal damage.