CW CO2 laser cutting of tungsten alloy using O2 assist gas.
Begic, Derzija ; Kulenovic, Malik ; Cekic, Ahmet 等
1. INTRODUCTION
Laser cutting is one of the important applications of lasers in
industry, especially for machining the difficult to cut materials.
Compared with other conventional mechanical processes, laser cutting
removes little material, involves highly localized heat input to the
work piece, minimizes distortion, and offers no tool wear (Ready, 1997).
Of particular interest to manufacturers using laser cutting are the
productivity and the quality of components made by laser cutting
process. Both aspects are managed by the selection of appropriate laser
process parameters, which are unique for each material and thickness.
Consequently, investigation into the affecting parameters in laser
cutting process is necessary to improve the final product quality. Most
work reviewed in the literature considers only one or two characteristic
properties of the laser cut surface to describe quality. Kerf width,
surface roughness and size of heat affected zone are often used to
describe laser cut quality. (Yilbas, 2008) found that laser output power
and oxygen assisting gas pressure have significant effect on the
percentage of kerf width variation in laser cutting of thick sheet
metals. Some researchers (Hanadi et al., 2008) showed that the size of
heat affected zone increases with increasing the laser power and
decreases with increasing cutting speed and gas pressure. The surface
roughness increases by increasing the material thickness at keeping
constant others process parameters during CW C[O.sub.2] laser cutting of
high alloy steels (Cekic et al., 2008). Oxygen gas as assist gas
produces better surface roughness compared to air and nitrogen during
laser cutting of tungsten composite materials using pulsed Nd: YAG (Uebel et al., 2008). They also observed that the nitrogen assist gas
has developed an oxide free surface and a low discoloration, while the
oxygen assist gas surface is strongly oxidized and discoloured.
However, we did not find in the literature many studies that
consider the C[O.sub.2] laser cutting of refractory materials.
Accordingly, the aim of this paper is to study the effect of the
operating parameters such as cutting speed, laser power and assist gas
pressure on the kerf width, surface roughness and size of heat affected
zone in CW C[O.sub.2] laser cutting of tungsten alloy, and hence obtain
the optimum ranges of laser power, cutting speed and assist gas
pressure.
2. EXPERIMENTAL SETUP
In order to achieve the stated objective, laser cutting experiments
were carried out using 1 mm tungsten alloy sheets to investigate the
effect of laser cutting parameters on the cut quality. The products
manufactured of the tungsten alloy sheets find new possibilities for the
application in different industrial areas, e.g. in medical application,
the automobile sectors and aircraft industry. Experimental
investigations were conducted at the University of Applied Science Jena,
Germany. The laser used in the experiment is a ROFIN DC020 C[O.sub.2]
laser system with a nominal output power of 2000 W. The laser beam was
focused using a 127 mm focal length lens. The possible influence of
different process gas pressures on the cutting process quality was
investigated using oxygen with commercial purity as assist gas. Oxygen
assist gas was used coaxially with the laser beam via a 2 mm exit
diameter nozzle. Three main parameters have been selected for the
present study. These are laser power, cutting speed and assist gas
pressure. The laser power was varied within the range of 1500-2000 W,
the cutting speed varied within the range of 3000-6000 mm/min, and the
assist oxygen gas pressure range was 5-17.5 bar. Testing the effect of
one parameter on the cut quality requires the variation of one parameter
while keeping the other two parameters at the pre-selected values.
The controlled parameters have been the top surface kerf width, the
size of heat affected zone and the surface roughness. A visual
inspection of each cut was carried out to ensure that no pitting and
burrs are present in the cut area. Fig. 1 shows examples of the
measurements taken. Surface roughness on the cut edge was measured in
terms of the average roughness Ra, using a Taylor-Hobson stylus instrument. Roughness was measured along the length of cut at
approximately the middle of the thickness. The kerf width was measured
using a Stemi microscope fitted with a video camera and a zoom lens. It
was also used for measuring size of heat affected zone as indicated by a
distinct blue band.
[FIGURE 1 OMITTED]
3. RESULTS AND DISCUSSION
The effect of the laser power and cutting speed on the heat
affected zone and surface roughness is illustrated in figure 2, and 3,
respectively. Experiments show that the size of heat affected zone and
kerf width depend of the cutting speed and laser power, while the effect
of power on the surface roughness is secondary. This is more pronounced
for higher cutting speed. Generally, an increasing in cutting speed and
a decreasing in power results in a decreasing in the size of heat
affected zone for the power range from 1500 to 2000 W.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
This can be explained in terms of the material ability to conduct
heat. As the cutting speed increases, the time for heat conduction is
lowered and the spread of heat damage is reduced. At the maximum cutting
speed the size of heat affected zone slightly changes, while it is
observed that kerf width increased at the power of 1750 W and speed of
5500 mm/min. The maximum cutting speed for obtaining a through cut is
dependent on power.
From figure 4, it is clear that the kerf width increases and the
size of heat affected zone decreases with increasing of assist gas
pressure. It can be observed that, at pressure-values higher than 12.5
bar, the size of heat affected zone and kerf width retain approximately
constant. Form the economical point of view, it is better to use the
pressure of 12.5 bars as the optimum gas pressure. Figure 5 shows an
increase in roughness values with increasing gas pressure. The obtained
results can be explained on this way; by increasing the oxygen pressure
increases the exothermal reaction which produced the undesired increase
in the kerf width and the mechanical force removes deeper grooves and
causes higher roughness.
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
4. CONCLUSION
The effects of laser power, cutting speed and oxygen assist gas
pressure on the quality characteristics of laser cut tungsten alloy
specimens studied in this paper. Based on the conducted investigations,
the following could be concluded:
* Size of heat affected zone increases with increasing the laser
power and decreases with increasing cutting speed and gas pressure.
* Kerf width increases with increasing gas pressure and decreases
with increasing cutting speed and decreasing laser power.
* Surface roughness increases with increasing gas pressure and
decreases with increasing cutting speed. On the other hand, laser power
has no great influence on the surface roughness.
* Based on the above conclusions, for laser cutting of tungsten
alloy of 1mm thick, it is recommended to use the laser power of 2000 W
and high cutting speed within 5500-6000 mm/min when oxygen is used as
assist gas at 12.5 bar. Influence of assist gas kind on the cut quality
in laser cutting of tungsten alloy, it is recommended to further
investigations and a comparison with these results.
Acknowledgement
The authors gratefully acknowledge the support of the Department of
Laser and Opto-Technologies at the University of Applied Science Jena,
Germany for this work.
5. REFERENCES
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