34th INTERNATIONAL CONFERENCE ON
PRODUCTION ENGINEERING
28. - 30. September 2011, Niš, Serbia
University of Niš, Faculty of Mechanical Engineering
THE COMPLEXITY OF DEFINING THE QUALITY OF LASER CUTTING
Bogdan NEDIĆ, Jelena BARALIĆ, Miroslav RADOVANOVIĆ
Faculty of mechanical engineering, Kragujevac, Technical faculty, Čačak, Faculty of mechanical engineering, Niš
nedic@kg.ac.rs, jbaralic@tfc.kg.ac.rs, mirado@masfak.ni.ac.rs
Abstract: Laser cutting is thermal cutting process based on melting or evaporation of workpiece material
in zone of cut. Cut quality is very important characteristic of laser cutting that ensures the advantage in
regard on other contour cutting processes. The paper presents results of research on the effects of laser
cutting parameters on surface quality by laser cutting. In laser cutting, the edges of the workpiece have a
characteristic grooved pattern. Groove lag refers to the greatest distance between two drag lines in the
direction of the cut. This paper gives mathematical model to define drag line depending on the laser
cutting parameters.
Key words: Laser, laser cutting, quality of laser cut, modeling of drag line
1. INTRODUCTION
Laser cutting is an attractive process for contour cutting
of plate. It is the most accurate and cost-effective process
and for some the only way to create new products. Laser
cutting is thermal cutting process. Laser is generator of
light beam. Laser beam is a high intensity beam of light.
It can be focused into a very small spot (0.1-0.2 mm in
diameter) on the workpiece surface by a lens or focusing
mirror. The intensity of the focused laser beam typically
is 107-108 W/cm2. High concentrated light energy in the
spot of the focused laser beam melts or evaporates almost
any material in a fraction of a second. In laser cutting,
highly concentrated light energy obtained by laser
radiation is used for cutting of workpiece material in zone
of cut. The laser beam is a new universal cutting tool that
can cut almost all known materials. Laser cutting is an
economical alternative to many other methods of cutting.
Speed, flexibility and precision, which are the main
features of laser cutting, reduce production costs
significantly and quickly return investments. ([2], [3], [4])
distance 0.1-0.2 mm, and the lower zone, in the area of
output laser beam, in which the cut surface roughened
with grooves relating whether due to solvents and slag
from the cutting zone. For this reason was adopted to
measure the surface roughness of laser cut at a distance of
one third of the thickness of the upper edge of the cut.
The topography of the cut grade includes roughness,
waviness and shape variations. It differs from the
roughness of the cut in the direction of the laser beam axis
and the roughness of the laser beam in the direction
normal to the axis of the laser beam and the direction of
the workpiece movement. Physical and chemical
characteristics of the material in the surface layer section
refer to the surface layer formed in the laser cutting and
thermal effect of laser beam on the workpiece material. In
doing so, we observe the microstructure of materials,
hardness, appearance and size of residual stresses the
oxide layer [1].
2. LASER CUT QUALITY
The quality of the cutting laser treatment is determined by
the size and shape accuracy and surface quality of cut.
Assuming that the accuracy of the shape and dimensions
of the features desk axis CNC laser cutting machines,
quality control unit and a precision engineered parts, we
can conclude that the quality of treatment reduces the
quality of the cut. The quality of the cut refers to the
geometry section, the parameters of the topography and
physical-chemical characteristics of the material in the
surface layer. The geometry of laser cut is defined by the
cut width (sr), the cut slope (), the curve of the cutting
edges (r) and the appearance of slag (hs), figure 1 [1].
Visual observation of the laser cut surface may be noticed
two zones: upper, in the area of the input laser beam,
which is surface-treated with a fine regular grooves at the
Fig. 1. Schematic view of the laser cut [1]
In laser cutting, surfaces of cut have a characteristic
grooved pattern. At low cutting speeds, the grooves run
almost parallel to the laser beam. As the cutting speed
increases, the grooves bend away from the direction of
cutting. Groove lag refers to the greatest distance between
two drag lines in the direction of the cut. The groove lag
is evaluated visually. The evaluation is carried out on a
picture of cut with the aid of a magnifying glass or
microscope. These grooves or furrows are result from the
regular shape forms a focused laser beam, the speed of
movement of the workpiece and the process of creation,
of relating and curing the incisional solvents.
The quality of processing in laser cutting process is
influenced by many factors. All these factors can be
grouped into effects:
1. The influence of laser cutting machines:
 the influence of the laser source (wavelength,
laser power, mode, polarization, stability and
power mode)
 the influence of optical transmission systems for
laser beam (quality of mirrors)
 the influence of cutting head:
- The focusing system (lens quality, focal
length, diameter of the focused beam, the
depth of the focus);
- Nozzle (shape, diameter hole, position
relative to the workpiece);
- Sensor positioning;
 The influence of coordinate work table (accuracy
of positioning, static and dynamic stiffness);
 the influence of power supply systems (electrical
power, gas supply, cooling system);
 the influence of control units (increments of
management, reliability).
2. The influence of laser beam (beam diameter,
divergence, monochromatic, coherence, polarization).
3. The influence of workpiece:
 influence of workpiece material (type,
absorption, diffusion coefficient, coefficient of
thermal conductivity, melting temperature,
temperature of evaporation);
 influence of workpiece geometry (shape,
dimensions, tolerances);
 surface quality and surface coatings;
4. The influence of the assist gas (type, purity)
5. The influence of processing parameters (laser power,
cutting speed, assist gas pressure).
From these numerous factors have the greatest impact:
laser power, material properties of the workpiece, cutting
speed, type, purity and pressure of assist gas, laser beam
characteristics and time stability of laser power, laser
beam adjusting processing parameters, process of cutting
and others. The most important characteristic which
makes estimates of the machine is power laser radiation.
Since the laser cutting process is heat, the amount of heat
produced an influential factor which depends on the
ability of the laser cutting. Having that other things being
equal, increasing power of laser radiation allows cutting
thicker materials and increased cutting speeds. Stable
power, fashion and concentrating the energy of laser
radiation (spatial and temporal) is a key factor for the
application of lasers in materials processing technology.
Cutting speed is the speed of the workpiece or the laser
beam on the surface of objects. Can be determined
experimentally or by using the known dependence in
which represented laser power density and properties of
the material is cut. More parameters affect the speed of
cutting: laser power, mode, size, light stains, the type and
thickness of the workpiece, the initial melting and
evaporation energy and so on. Taking into account the
quality of cut and cutting speed by observing the function
of the thickness of the workpiece can be useful to define
the area of the cutting speed is limited by upper and lower
curve. Outside of this area is an area of incomplete
cutting. Within the area of usable cutting speed by
changing the speed of cutting is done changing the cut
quality (appearance and size of the slag, roughness, etc.).
A variation of the focus position relative to the surface of
the workpiece shows different characteristics on the
quality of the cut. Depending on the type of material and
thickness of the work focus is positioned at or slightly
below the surface of the workpiece.
Assist gas in the area leads through the machining process
in which the head has to be coaxial with the focused laser
beam. Using assist gas to blow the melted and vaporized
material of the work more easily drained from the cutting
zone, which directly affects the purity and quality of cut.
In addition, the melted material can not be re-cut and prehardened workpiece weld. Using assist gas to blow also
prevents binding of slag on the back of the cut, and
cutting speed can be increased to 40%. The task of the
assist gas is in the process of cutting to protect the lens
from the vaporized and melted material of the workpiece.
For some metals is used reactive gas, oxygen, because it
helps the exothermic reaction. Using oxygen as assist gas
increases the total energy used in the process 40% of
cutting so that it is possible to increase cutting speed by
25% compared to the cutting speed at which the air is
assist gas. Besides the type of assist gas and purity of the
assist gas have very significant impact on the processing
parameters and quality of cut. Assist gas pressure affects
the cutting speed and the speed of relating melted and
vaporized material to the appearance of slag at the edges
of origin section. Pressure decreases when increasing the
thickness of the workpiece or the cutting speed is reduced.
In the action of laser radiation on the workpiece in the
formation of various vapors that are in the ground state or
united with the ingredients of atmospheric air can be
deposited on optical surface of the focusing element. This
reduces laser radiation. In cases where the deposited layer
of large, laser radiation can act on it as the material of the
workpiece, and how it is on the lens, will cause damage.
Larger particles are vaporized or molten material that can
cause uncontrollable burst permanent damage to optical
elements. A stream of assist gas which is directly in front
of the lens blow protects it from damage. ([5], [6], [7])
3. EXPERIMENTAL TESTS
Experimental study was aimed to determine the influence
of processing parameters on the quality of laser cut. It was
examined the effects of laser power, cutting speed, assist
gas pressure, and focus position on the surface quality,
especially to grooves and drag lines.
Tests were performed at Bystronic laser cutting machine BYSTAR 3015, in company "Metal Systems - Process
Processing" of Kragujevac.
The material of the work is S235 JRG2 (Č0361), 15 mm
of thickness. The material properties of samples are
shown in Table 1.
Table 1. The material properties of samples
Type of
material
Chemical composition
C, %
Mn, %
Si, %
P, S, %
S235 JRG2
0.03 -0.30
≤ 0.17 ≤ 1.40
(Č0361)
≤ 0.045
Mechanical
properties
Re , N/mm2
≥ 235
Testing refers to the quality of the optimal values of
cutting parameters for material processing S235 JRG2
(Č0361) 15 mm of thickness, with varying processing
parameters. Varying one parameter is taken by its
maximum and minimum recommended value while other
parameters have optimum values recommended by the
manufacturer of laser machines for a given material.
Roughness measurement was carried out on the
measuring device Talysurf-6. Using this measurement
system parameter values were obtained by the surface
topography of processed material.
This study was conducted with the following parameters:
- Laser power (kW): P = 4.3, 4.5, 5.1
- Cutting speed (mm/min): Vp = 1200, 1400, 1700
- Assist gas: O2
- Pressure of assist gas (bar): p = 0.5, 0.7, 0.8
- Focus position (mm): f = 0.4, 0.5, 0.6
When testing was done cutting the panels 15 mm thick
sections measuring 50 x 50 mm.
2.
Increase speed of movement of support slightly
affects the quality of treatment within certain limits,
provided that a significant increase in speed leads to
weld site incision and drainage of molten material
impossibility,
3. Increasing the power of the laser beam affecting the
surface quality,
The influence of assist gas pressure is negligible.
After measurement, surface topography, carried out the
measurement deviations from the line of cut a straight
line, as figure 3. These measurements were aimed at
determining the influence of laser cutting regimes on
grooves and the drag lines and to determining the
equations of drag lines.
Deviation y, mm
Depth
of cut
x, mm
Fig. 3. The position of the coordinate system for
measuring deviations of drag lines
To determine the equation, was determined by measuring
variations in each 3 mm. As the best shape that represents
the wrong line of cut adopted in equation
y = a x + b x2
(1)
In Figures 4, 5 and 6 are shown the surface obtained by
laser cutting and cut the curve represented mathematically
depending on the parameters of laser cutting regimes.
-2
-1
0
1
0
Deviation, mm
2
y
3
Sample 11:
f=0,4 mm
y  0,221  x  0.0165  x 2
Fig. 2. Form of samples
Optimal values of the parameters regime, recommended
by the manufacturer of machines, material thickness 15
mm are:
 Laser power (kW): 4.5
 Cutting speed (mm/min): 1400
 Assist gas O2 pressure (bar): 0.7
 Focus position (mm): 0.5
Sample 1 was processed with these regimes.
The analysis of test results show that influence of laser
cutting parameters on surface quality can conclude the
following:
1. Increasing the distance from the focus of the work
negatively affect the surface roughness, or you get
worse surface quality,
Depth, mm
6
Sample 1:
f=0,5 mm
y  0 ,152  x  0.0129  x 2
9
12
Sample 8:
f=0,6 mm
y  0 ,148  x  0.0171  x 2
15
x
Fig. 4. Drag lines in dependence of focus position
-2
-1
0
1
0
Deviation, mm
2
y
3
Sample 5:
p=0,59 bar
y  0 ,116  x  0.0136  x 2
Depth, mm
6
Sample 1:
p=0,7 bar
y  0 ,152  x  0.0129  x 2
9
12
Sample 6:
p=0,86 bar
y  0 ,169  x  0.0160  x 2
15
x
Fig. 5. Drag lines in dependence of pressure of assist
gas O2
-2
-1
0
1
0
Deviation, mm
2
y
4. CONCLUSION
Technological problems related to the application of laser
machines for cutting materials have insufficient
knowledge of the application of laser techniques and nonexistence of a lack of reliable information and practical
knowledge of the influential parameters on the process of
treatment. Knowledge of the laser cutting process, and its
dependence on various factors, make it possible to
achieve the required quality of cut and at the same time
achieve maximum productivity. By establishing the
legality of the processing quality and treatment process of
influential factors is necessary to build the appropriate
database. Integrating knowledge and data in computersupported preparation of production achieved maximum
technical and economic effects.
Experimental study was aimed to determine the influence
of processing parameters on the quality of laser cut.
Experimental researches related to surface quality of
carbon steel S235 JRG2 (Č0361) and were performed at
Bystronic laser cutting machine - BYSTAR 3015. It was
examined the effects of focus position, pressure of assist
gas and cutting speed on the grooves and drag lines.
Applied experimental studies have shown that the
curvature of drag lines can be represented by a
mathematical curve in the form of y = a x + b x2.
The analysis of test results shows that the parameters of
laser cutting have influence to form of curvature of drag
lines. It can be concluded that some parameters have
significant influence. Drag line may well be present by
mathematical second-order polynomial model. To
establish the drag line equations depending on the regime
of laser cutting parameters it is needed one extensive
investigation with the exact definition of the beginning of
the defined parameters.
ACKNOWLEDGEMENT
This paper is part of project TR35034 The research of
modern non-conventional technologies application in
manufacturing companies with the aim of increase
efficiency of use, product quality, reduce of costs and
save energy and materials, funded by the Ministry of
Education and Science of Republic of Serbia.
3
Depth, mm
6
REFERENCES:
Sample 1: Vp=1400 mm/min
y  0 ,152  x  0.0129  x 2
9
12
Sample 2: Vp=1200 mm/min
y  0,093  x  0.0119  x 2
15
x
Fig. 6. Drag lines in dependence of cutting speed
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