Recently, laser cutting is used in many industries. Generally, in laser cutting of metallic materials, suitable assist gas and its nozzle are needed to remove the molten metal. However, because of the gas nozzle should be set closer to the surface of a workpiece, existence of the nozzle seems to prevent laser cutting from being used flexible. Therefore, the new cutting process, Assist Gas Free laser cutting or AGF laser cutting, has been developed. In this process, the pressure at the bottom side of a workpiece is reduced by a vacuum pump, and the molten metal can be removed by the air flow caused by the pressure difference between both sides of the specimen. In this study, cutting properties of austenitic stainless steel by using AGF laser cutting with 2 kW CO2 laser were investigated. Laser power and cutting speed were varied in order to study the effect of these parameters on cutting properties. As a result, austenitic stainless steel could be cut with dross-free by AGF laser cutting. When laser power was 2.0 kW, cutting speed could be increased up to 100 mm/s, and kerf width at specimen surface was 0.28 mm.
Laser cutting is one of the thermal cutting processes such as gas cutting and plasma cutting [
In laser cutting of metallic materials, molten metal is usually removed by using suitable assist gas at high pressure as shown in Figure
Schematic of conventional laser cutting process of metallic materials with assist gas.
For this reason, the new cutting process, Assist Gas Free laser cutting, hereafter, called as AGF laser cutting, has been developed and investigated about cutting properties in our laboratory [
Schematic of Assist Gas Free laser cutting process.
In the present study, cutting properties of austenitic stainless steel by using AGF laser cutting were investigated. At first, laser power and cutting speed were varied in order to study the effect of these parameters on cutting properties. Moreover, effect of cutting condition on pressure in chamber during cutting process was discussed.
As materials, austenitic stainless steel, JIS SUS304, was used for AGF laser cutting. Thickness of the SUS304 plate was 1 mm in this study. Width of the specimen was 39 mm, and length was 300 mm. Chemical compositions of the material used were shown in Table
Chemical compositions of JIS SUS304, standard value, mass %.
C | Si | Mn | P | S | Ni | Cr |
---|---|---|---|---|---|---|
≦0.08 | ≦1.00 | ≦2.00 | ≦0.045 | ≦0.030 |
|
|
A schematic of AGF laser cutting equipment using a 2 kW CO2 laser was shown in Figure
Schematic of AGF laser cutting equipment.
A chamber connected to a vacuum pump was mounted on an NC work table. After a sheet material was set on top side of the chamber, the pressure in the chamber was reduced by the vacuum pump. When inside of the chamber became near vacuum state, AGF laser cutting experiment was carried out. Laser beam melts the material locally; the molten metal can be removed by the pressure difference between both sides of the sheet.
Process parameters for AGF laser cutting were shown in Table
Process parameters for AGF laser cutting.
Laser type |
CO2 laser (CW) |
Polarization of laser |
Circular polarization |
Laser power |
|
Focal length of lens |
127 mm |
Defocused distance |
±0 mm |
Piercing time | 1 s |
Cutting speed |
|
Shielding gas | N2 : 15 L/min |
Cut length | 100 mm |
A 2 kW CO2 laser of continuous wave with circular polarization was used. In this study, laser power was varied from 1.0 to 2.0 kW. Focal length of lens was 127 mm, and focal position was set at the surface of the SUS304 plate. Piercing time was 1 s every time. Cutting speed was varied from 10 to 110 mm/s. Shielding gas of nitrogen was used to protect the focusing lens. Cut length 100 mm was controlled by the NC program.
Process window of AGF laser cutting is shown in Figure
Process window of AGF laser cutting with different laser power and cutting speed.
The condition which could be cut with all length of 100 mm is shown as circles, and that which could not be cut is shown as crosses. When laser power was increased, the range of cutting speed which could be cut was expanded. This is because heat input to the SUS304 plate is augmented by the increment of laser power at higher cutting speeds.
From this result, critical cutting speed
Critical cutting speed
Laser power |
|
---|---|
1.0 kW | 50 mm/s |
1.5 kW | 80 mm/s |
2.0 kW | 100 mm/s |
Macro cross-section of kerf with laser power of 1.0 kW and cutting speed of 50 mm/s is shown in Figure
Macro cross-section of specimen after AGF laser cutting with laser power of 1.0 kW and cutting speed of 50 mm/s.
Cut kerf was formed and seen at the center of the picture. In order to discuss cross-sectional shapes of kerf, its profiles were measured with all cut specimens. The cross-sectional shapes of kerf at different laser power and cutting speed are shown in Figure
Cross-sectional shapes of kerf at different laser power and cutting speed.
In this figure, the upper side is corresponded to the laser irradiated surface in each graph. In all cutting conditions, cut kerfs are formed perpendicular to the surface of the plate. Furthermore, the kerf shapes are almost the same though laser power or cutting speed is varied. These facts were confirmed by measuring kerf taper. In this study, the taper was defined as one-half of the difference between upper and lower width of kerf, as mentioned in the literature [
Figure
Effect of cutting speed on kerf taper at different laser power.
However, as shown in Figure
Figure
Effect of cutting speed on kerf width at different laser power.
At same cutting speed, when laser power was increased, kerf width was also increased, since the heat input was augmented. By contrast, when cutting speed was increased, kerf width was decreased, as the heat input was also decreased. Maximum kerf width was 0.56 mm at laser power 2.0 kW and cutting speed 10 mm/s. Minimum was 0.25 mm at laser power 1.0 kW and cutting speed 50 mm/s.
Likewise, Figure
Effect of cutting speed on removed area of kerf at different laser power.
At same cutting speed, when laser power was increased, removed area of kerf was also increased. By contrast, when cutting speed was increased, removed area of kerf was decreased. As mentioned above, this is because the influence of increasing and decreasing of the heat input. Maximum removed kerf area was 0.37 mm2 at laser power 2.0 kW and cutting speed 10 mm/s. Minimum was 0.11 mm2 at laser power 1.0 kW and cutting speed 50 mm/s.
Laser power and cutting speed can be treated as one parameter of the heat input. Thus, the kerf width and the removed area of kerf were organized by the heat input. Figure
Effect of heat input on kerf shapes.
Kerf width
Removed area of Kerf
When heat input was increased, both kerf width and removed kerf area were also increased. As shown in Figure
As shown in Figure
In order to evaluate cutting quality, the appearance of cut surface was observed. Figure
Appearance of cut surface at different laser power and cutting speed.
With low cutting speed such as 10 mm/s, the formation of dross was confirmed. When cutting speed was decreased, because the amount of the molten metal was increased, part of the molten metal was not removed from the kerf and remained at the bottom surface of the plate as the dross. However, when cutting speed was increased, dross-free cutting was achieved. In the high cutting speed, since the amount of molten metal is few, the metal was ejected smoothly. Therefore, it’s thought that high-speed cutting is effective in dross-free cutting.
In addition, cut surface was observed in detail after etching the kerf cross-section with hydrochloric acid. Figure
Microstructure of cut surface with laser power of 2.0 kW and cutting speed of 100 mm/s.
An approximately 10
As mentioned above, in AGF laser cutting, the molten metal can be removed by the air flow caused by the pressure difference between both sides of the specimen. However, as cutting progresses, air flows into the chamber from the cut kerf. In other words, the pressure difference decreases. Thus, the pressure in the chamber was measured during cutting process by using a pressure sensor. As an example, the change of the pressure during AGF laser cutting with laser power of 2.0 kW and cutting speed of 40 mm/s was shown in Figure
Change of pressure in chamber during AGF laser cutting with laser power of 2.0 kW and cutting speed of 40 mm/s.
In order to discuss pressure change during AGF laser cutting process, between
Effect of cutting speed on rate of pressure rise during AGF laser cutting at different laser power.
At same cutting speed, when laser power was increased, the rate of pressure rise was also increased, since the kerf width was expanded as shown in Figure
Therefore, AGF laser cutting by low heat input is desirable, because narrow kerf width and dross-free cutting can be obtained as above. In addition, since the rate of pressure rise during cutting is low, longer cutting length can be ensured under low-heat-input cutting.
The present study is focused on cutting properties of austenitic stainless steel by using AGF laser cutting process. The following conclusions can be drawn. Increase in the laser power led to increase of the critical cutting speed. With the laser power of 2.0 kW, the critical cutting speed was 100 mm/s. The cross-sectional shapes of kerf were almost the same though the laser power or the cutting speed was varied. The kerf taper was below 0.06 mm in all cutting conditions. When the heat input was increased, the kerf width and the removed area of kerf were also increased. When the cutting speed was high, the dross-free cutting was achieved. The thin recast layer, about 10 When the laser power was decreased or the cutting speed was increased, the rate of pressure rise in the chamber during cutting process was decreased.