Resumo

We report results of experiments investigating the feasibility of cutting thick (> 15 mm) mild steel plate with a fiber coupled Nd: YAG laser. The experiments  were performed with a continuous wave 2.5 kW Nd: YAG laser delivered to the workpiece through a 0.6mm diameter silica core optical fiber. The mild steel samples range in thickness from 10 to 50 mm. The effects of a range of operating parameters such as focal spot and cutting nozzle position relative to steel surface, assist gas pressure, power and process speed, on cut surface quality are presented and discussed. Results to date show that it is possible to cut up to 50 mm thick mild steel plate at speeds up to 200 mm/min with as low as 500 W of Nd: YAG laser power. The cut surface is smooth and there is no dross. These results show promise for the application of Nd: YAG laser technology for the cutting of thick steel plate.

1. Introdução

Laser cutting represents approximately a 1/4 of the laser materials processing industry [1]. In more than 30 years since the first gas assisted laser cut was produced [2] little has changed to the laser cutting method. For cutting mild steels, a laser beam is focused at or near the workpiece surface and surrounded by a broader co-axial stream of oxygen assist gas. Typically laser powers up to 3 kW are used to cut mild steels to 12 –15 mm thickness with thicker plates cut mainly with plasma or oxy-fuel systems. Though it is possible to cut metals with the CO2 laser up to 40 mm thickness, there is a significant decline in cut quality and reproducibility [3].

One solution to cut thicker mild steel plate is to increase the laser power. While there  are a number of advantages to this approach there are also significant challenges. At higher powers (3.5 kW and higher) the beam quality becomes unstable, lifetime of optical components  is  reduced,  equipment  and  running  costs  are  high  and  cutting precision deteriorates. It was demonstrated in [4] that for a given quality of surface finish, though the thickness of the kerf remains approximately constant, the cutting speed does not reduce proportionately to it, indicating a reduction in cut efficiency with increasing material thickness. Reduction in cut efficiency as the material becomes thicker is attributed to a reduction in the assist gas’s ability to shear the melt. With the cutting of thicker materials the pressure must increase to allow the removal of molten material. However, when using oxygen assist gas, the exothermic nature of the reaction means that the oxygen pressure must be reduced with increasing thickness to stop an overreaction taking place within the kerf. Close control of oxygen pressure is essential to prevent burning uncontrollably away from the heated area. This represents a contradiction in requirements for successful cutting of thick mild steel. It limits maximum cut thickness, despite the ability to extend cut performance by increasing laser power. To overcome this limitation and to extend the thickness capability of reactive fusion cutting, alternative and novel methods are required.

Numerous techniques have been developed to overcome reduction in cutting performance as mild steel thickness increases. Included in these are: laser flame cutting[5], dual focus lenses [6], beam sawing with adaptive optics [7], laser cutting using a co-axial (annular) nozzle [8], dual-beam CO2 laser cutting [9], spinning laser beam [13, 14] and Laser Assisted Oxygen Cutting (Lasox©)[10 – 12].

We have reported previously [14] cutting thicker mild steel plate using the spinning Nd:YAG laser beam. Reported here are the results of cutting thick mild steel plate (>15 mm) using a fibre delivered Nd:YAG laser by wobbling the laser beam (a method analogous to spinning the beam) and by the method of oxygen dominated laser cutting such as that of Lasox cutting [10, 11, 12]. Oxygen dominated Nd:YAG laser cutting trials were conducted using firstly low and then high oxygen assist gas pressures.

2. Wobbling the laser beam

2.1 Experimental Details

Wobbling the laser beam was produced by partial rotation (oscillation) of an optical window through an angle as shown in Figure 1(a). This resulted in a maximum focal spot offset of 0.45 mm at a maximum frequency of 20 Hz. A resultant track, here with exaggerated wavelength to show oscillatory motion, is shown in Figure 1(b). It was possible to vary the amplitude of oscillation of the window to effect changes in kerf width in order to study the effect of widening the kerf on the cutting process.

2.2 Cutting steels using laser assisted oxygen dominated approach

The method of laser assisted oxygen dominated cutting was implemented on AS3678 mild steel plates of thickness 16 to 50 mm. Oxygen assist gas pressures were kept to either less than 120 kPa (low pressure oxygen cutting –LoPOx) or to high pressures (high pressure oxygen cutting – HiPOx). Cut results were recorded as a function of cut quality (cut striation, kerf shape, excessive dross) and cut speed.

3. Results

3.1Wobbling the laser beam.

By wobbling the beam on the workpiece the maximum cut thickness was increased from 12 mm, encountered with conventional cutting, to 16 mm. A graph of maximum cut speed for various thicknesses and laser powers, seen in Figure 2, indicates that though the cut thickness was improved with the wobbling beam, the cut speed is similar to that of conventional (CW) cutting, This indicates that the cut process occurring within the kerf remain unchanged during wobble beam cutting. Similar cut speeds were also achieved with the spinning beam [14].

The increased cut thickness can be attributed to the increased kerf width. This is demonstrated by varying the amplitude of the wobble as shown in Figure 3. Here, as the wobble amplitude is sequentially reduced from a maximum amplitude of 0.45 mm to zero, the kerf width is reduced, corresponding to a reduction in the ability to clear the melt. This clearly demonstrates the necessity to have adequate kerf width to allow dross to clear. This view is also expressed by others [12], where it is suggested that both fluid dynamics and thermodynamics are constrained by narrow kerfs.

3.2 Cutting steels using laser assisted oxygen dominated approach

3.2.1 Low Pressure Oxygen Dominated Cutting – LoPOx

The LoPOx cutting process uses the same larger diameter laser beam and  narrow imposing oxygen jet at the top of the workpiece as seen in the Lasox process, however with assist gas pressures below 120 kPa. Cut surfaces shown in Figure 4 using the LoPOx process demonstrate that low incident laser powers do not hinder laser cutting as long as primary and ongoing initiation of the cut is able to take place. Indeed as cut speed increases, incident laser power may contribute too much energy and therefore cause excessive striation to take place. This is demonstrated in the figure by observing the 450 mm/min cut speed, where a better surface was generated by 533 W incident laser power than was achieved at 1420 

W. Here, the rate of exothermic reaction is determined by the cut speed. Incident laser power is only required to heat the top surface to greater than 1000C [11] and initiate the reactive fusion process. Excessive incident laser power reduces the cut quality. This demonstrates that issues of oxygen – iron interaction, not the incident laser power, now primarily govern the cut quality. Hence this is an oxygen dominated laser cutting process.

In Figure 4 as the power is reduced for each cut speed first indications of minimal incident power is the poor start of cut as seen at the right hand end. This demonstrates that power requirements at cut initiation are higher than those of the ongoing cutting process and the power required for quick establishment of a steady cut process and not power for the ongoing process is the essential criteria.

When LoPOx cutting using a smaller co-axial nozzle diameter for the same thickness material, the same cut speeds are obtained but with a narrower kerf width and consequently reduced oxygen flow. However the high quality cuts were unable to be achieved at the lower laser powers with the larger nozzle diameter used in Figure 4. This is in spite of a more intense laser spot as a result of passing through a smaller diameter nozzle. This demonstrates that the requirement of a sufficiently wide kerf to allow the dross to clear applies equally for the oxygen dominated cutting process.

The sides of the cut are tapered more than those encountered in conventional (laser dominated) cutting. The oxygen dominated nature of the cut process means that the kerf is influenced by the shape of the imposing oxygen jet with the top of the kerf being the same width as the co-axial nozzle used.

The clearance between the nozzle and workpiece was varied with typical results of this variation shown in Figure 5. For various nozzle diameters, cut quality was reduced significantly with clearances greater than 25% of the nozzle diameter. Increases in nozzle– workpiece clearance exposed more of the flow from the nozzle to the ambient atmospheric gasses before entering the kerf [8]. The change in clearance was  undertaken without corresponding changes in laser spot diameter with similar results. This further demonstrates that changes to assist gas and not incident laser power intensity was the factor affecting laser cut quality over the range tested. Figure 5 also shows the effect of a too small a clearance (0.1 mm) where the converging beam is not yet exceeding the gas jet diameter therefore not permitting the oxygen dominated laser cutting process to operate.

A maximum cutting thickness of 32mm was achieved using Nd:YAG LoPOx cutting. Cutting beyond this thicknesses with the nozzle diameters used caused the formation of excessive dross within the kerf and a loss of perpendicularity of the cut. This further demonstrates the relationship between kerf width and cut thickness when low (conventional) cutting pressures are used.

3.2.2High Pressure Oxygen Dominated Nd:YAG Laser Cutting – HiPOx

Using a much higher supply pressures and smaller diameter nozzles, it was found to be possible to cut steels thicker than those previously obtained by the LoPOx process. Cut capacity was shown to be between 32 and 50 mm thickness using AS 3679 steel plate. Typical cut speeds with respect to material thickness and laser power are shown in Figure

6. The figure shows a continuation of cut processes from the low pressure region used for thinner materials.

The effect of using high delivery pressures mean that the flow of the gas is complex and can give rise to internal shock features. Evidence of the interaction of shock structures during cutting can be seen as “ridges” or lesser marks in the cut surface and seen as lines running perpendicular to the stria. Further, the shifting of these ridges with nozzle- workpiece clearance results from reinforcement or annulment of assist gas internal shocks and the characteristic shock appearing at the start of the kerf in the shape of an “X” [15]. Work [16, 17] also indicates a complex and sometimes oscillatory interaction of shocks with the kerf walls. Evidence of the oscillatory nature of the cut is in the steady “buzz” that can be heard under some cutting conditions.

Using a 1.5 mm diameter co-axial nozzle, cut capacity was shown to be satisfactory for 32 and 40 mm plate with results of cutting 40 mm plate shown in Figure 7. Nozzle- workpiece clearance was significantly increased with the high assist gas pressures and the shape of the kerf was far less tapered than that seen in LoPOx as a result of the less divergent high velocity gas stream. Such kerfs can be seen in Figure 8.

Profile cutting using the fibre delivered Nd:YAG LoPOx technique is feasible with examples shown in Figure 9. Here temperature increases on the inside of corners result in increased tapering at these points. This is seen on the circular cut of Figure 9 (a) and undercutting of corners in Figure 9(b). Undercutting of sharp corners is best overcome by the use of reduced cutting speeds as shown in the figure.

High pressure oxygen dominated cutting using the Nd:YAG laser like that used with the CO2 [12] also shows itself to be excellent in piercing with less than one second required to pierce 32 mm AS3679 plate. The removal of dross ejected upwards remains an issue, with its presence on the plate surface in the cut path detrimental to cut quality

4. Discussion

Despite the novel laser cutting processes and the increase in cut thickness, the cutting process itself remains unchanged. This is evidenced by the reduction in cut speed with cut thickness and similarity in the cut speed for conventional, spinning beam and wobbling beam cuts. Consequently, despite the changes in approach the fundamental factors that govern cutting of thick steel plate by reactive fusion, such as losses to conduction and restriction of melt flow removal due to viscosity and surface tension still remain.

The larger and varying kerf widths produced by wobbling the beam as well as the various kerf widths generated by using oxygen dominated laser cutting with the Nd:YAG laser demonstrate the need for appropriately wide kerfs as cut thickness increases. However at moderate thicknesses (~32 mm) increasing the kerf beyond that produced by the largest LoPOx nozzle, becomes impracticable as oxygen consumption becomes prohibitive. To this end, the use of HiPOx comes into its own. Using high pressure and consequently high velocity assist gas stream allows oxygen to be less combined with atmospheric gasses  and  so  be  more  easily  available  for  reactive  fusion.  Further  it  provides a significantly increased shear forces on the melt face to overcome resistance to its clearance from the kerf. An added characteristic of the HiPOx process is the large nozzle- workpiece clearances obtained. This ensures the reliability of the high pressure nozzles.

Oxygen dominated cuts rely only on the incident laser power to initiate and then sustain the cut. Results show that these powers are far lower than those required for equivalent conventional cutting. However, higher powers are required for the initiation of a steady cut than those needed for the overall cut process to be maintained. Consequently an increased power could be used at start of cut only to maximize power efficiency.

Profile cutting has been shown to be feasible with the drawback of undercutting the inside of cut corners. This can be overcome by appropriate programming of cut speed at these positions. Piercing of thick plate is shown to be feasible but there are issues of the upwardly ejected dross later interfering with the delivery of the assist gas during subsequent cutting. This could be dealt with by either the presence of an outwards facing annular air jet surrounding the nozzle or operator cleaning by the use of a CNC wait command after all piercing is initially performed.

5. Conclusion

The use of oxygen dominated laser cutting along with the use of wider cut kerfs demonstrates the feasibility of using the moderately powered, fibre delivered Nd:YAG laser to cut thick mild steel plate. This may be done using low pressure delivery for mild steel plate up to 32 mm thickness. High pressure gas delivery has shown that cut thicknesses to 50 mm are easily achievable along with the ability to rapidly pierce the material. There are ongoing issues of cut quality associated with shock artifacts and also issues regarding the undercutting of corners that require careful CNC programming. To successfully pierce requires subsequent removal of upward ejected dross from the cut path to ensure  that the cut quality of the underlying workpiece is maintained.

6. Acknowledgements

The authors wish to thank the CRC for Intelligent Manufacturing Systems and Technologies Limited for their funding of the Spinning Beam project without which the above research and results could not be assembled.