PART II: THE PUBLICATIONS
5.4 Publication 4
Optimization of parameters for fibre laser cutting of a 10 mm stainless steel plate The purpose of Publication 4 was to demonstrate the procedure of optimization of processing parameters in order to obtain a high cut edge quality in the cutting of 10 mm thick austenitic stainless steel AISI 304 (EN 1.4301) workpiece using the ytterbium fibre laser. The processing parameters that were considered for optimization in this experimental investigation included the focal point position, the cutting speed, and the focal length of the focusing lens.
The work reported in Publication 4 has demonstrated that the defocus focal point positions - i.e. focus located closer to the bottom surface of the workpiece - are essential for thick-section metal cutting using the fibre laser with an inert assist gas jet as long as the power intensity at the workpiece is sufficient to obtain complete penetration of the workpiece.
The dross attachment on the lower cut edge and cut surface roughness are influenced by the melt removal mechanism in the narrow thick-section laser cut kerf so that resolidification of dross on the cut edge results in a higher surface roughness. The lower section of the cut edge is rougher than the upper section due to the melt build-up at the lower cut section resulting in inefficient melt removal. Efficient melt removal is obtained when the focal point position is located closer to the workpiece bottom surface because of the wider cut kerf that is created with these focal point positions which enhances a high melt removal rate.
Furthermore, the conduction power loss from the cutting zone to the substrate material reduces with increase in cutting speed so that more efficient beam coupling at the cutting front occurs. However, the cutting speed has to be optimized for the cutting of a given workpiece thickness with a particular laser power so that the high temperature gradient at high cutting speed increases the melt fluidity for better melt removal resulting in dross-free cut edges. Very high cutting speed may result in low fluidity of the melt due to insufficient absorbed laser power to cut at the high cutting speed resulting in the recurrence of dross adherence on the lower cut edge.
It was also shown in this work that dross-free cut edges could be obtained when the longer focal length optics was used for focusing of the laser beam thus demonstrating the influence of the cut kerf size on the cut edge quality.
6 CONCLUSIONS AND RECOMMENDATIONS
The performance of the high power ytterbium fibre laser in the cutting of thick-section steel and medium-section aluminium has been experimentally investigated in this study.
Theoretical models of the laser power requirement and the melt removal rate during laser cutting of a metal workpiece have also been developed in this study and compared with the experimental results.
The following important conclusions on the performance of the high power ytterbium fibre laser can be drawn from the results of this study.
1. The required incident laser power for cutting of 10 mm stainless steel and 15 mm mild steel at a given cutting speed using the ytterbium fibre laser is lower than that for the CO2 laser showing a higher absorption of the fibre laser beam by the workpiece. The higher absorptivity of the fibre laser beam by the workpiece results in higher melting efficiency of the fibre laser beam than for the CO2 laser beam.
Consequently, cutting at a particular cutting speed can be achieved with much lower incident laser power when the ytterbium fibre laser is used than when the CO2 laser is used. The conduction power losses from the cutting zone are high at slow cutting speeds but reduce with increase in cutting speed. Therefore, the efficiency of the cutting process increases with increase in cutting speed.
2. The difficulty in achieving an efficient melt removal during high speed cutting of the 15 mm mild steel workpiece with oxygen assist gas using the ytterbium fibre laser can be attributed to the high melting efficiency of the ytterbium fibre laser where the absorptivity was estimated to be greater than 60%. Additionally, the processing parameters that influence the rate of the oxidation reaction and consequently the amount of reaction power addition to the cutting process - namely oxygen pressure, nozzle diameter, and cutting speed - were found to critically affect the resulting cut edge quality. High oxygen pressure, large nozzle diameter and low cutting speed were found to favour the exothermic oxidation reaction and result in deterioration of the cut edge quality. The used oxygen pressure in mild steel laser cutting is reduced with increase in workpiece thickness. However, the partially oxidized melt at the maximum cutting speeds has a high viscosity and results in dross attachment on the lower cut edge and in the worst cases resolidified melt in the cut kerf due to poor melt removal at the bottom section of the cut kerf.
3. The processing parameters that have a significant influence on the rate of melt removal from the cut kerf during thick-section stainless steel laser cutting using an inert assist gas include the assist gas pressure, nozzle diameter, focal point position, and cutting speed. The modelled melt flow velocity and melt film thickness correlated well with the location of the boundary layer separation point (BLS) on the cut edge in the cutting experiments using the ytterbium fibre laser. An increase in the melt flow velocity and reduction in the melt film thickness - favoured by a
lower cut edge without the occurrence of flow separation and with minimal dross attachment on the lower cut edge. The cut kerf width strongly depends on the focal point position, larger cut kerf widths are achieved with defocus focal point positions. The cutting speed affects the fluidity of the melt for efficient melt removal from the cut kerf. High melt fluidity is required for prevention of dross attachment on the lower cut edge. Low cutting speed results in high conduction losses from the cutting zone and reduced melt fluidity while extremely high cutting speed results in insufficient power to maintain high melt fluidity.
4. It was possible to define processing parameter windows - in terms of laser power and cutting speed - for achievement of clean dross-free cut edges in 10 mm stainless steel and 4 mm aluminium workpieces. Different regions of cut edge quality were defined for different combinations of cutting speed and laser power.
The cut edge quality was limited at low cutting speeds due to high heat conduction from the cutting zone which resulted in dross attachment on the lower cut edge.
The dross attachment on the lower cut edge was cleared with increase in cutting speed but recurred at very high cutting speeds where the melt fluidity was reduced due to the high power requirement for cutting at high cutting speeds.
5. High quality dross-free cut edges with low surface roughness and low cut edge squareness deviation could be achieved in 10 mm thick austenitic stainless steel cutting using nitrogen with the following cutting conditions: high assist gas pressure (> 16 bar), large nozzle diameter (e.g. 2.5 mm nozzle), focal point position located close to the bottom workpiece surface in order to create a larger cut kerf width, longer focal length, and optimum cutting speed to ensure high melt fluidity for prevention of dross attachment on the lower cut edge. These conditions enhance a high melt removal rate from the cut kerf resulting in a high cut edge quality.
Basically, the cutting speed has to be reduced from the maximum achievable cutting speed for the used laser power in order to obtain a high cut edge quality.
During inert gas cutting of a metal workpiece, the assist gas pressure is increased with increase in workpiece thickness.
6. This study has also shown the limitations of the ytterbium fibre laser in the area of section metal cutting. The optimized cut edge quality obtained in thick-section stainless steel using the ytterbium fibre laser is still worse than that achieved using the CO2 laser. The interaction of the fibre laser beam with the metal material is not optimum for cutting of thick-section metal workpieces. The intensity of the fibre laser beam on the workpiece reduces greatly after a shorter depth than in CO2 laser cutting. The CO2 laser beam propagates to longer depths before significant reduction in intensity.
7. There are some major problems relating to the cut edge quality in laser cutting of the 15 mm thick mild steel workpiece using an active assist gas jet. The high melting efficiency of the ytterbium fibre laser produces an increased amount of melt resulting in an erratic oxidation reaction. The excess reaction power produced
by the erratic oxidation reaction causes excessive melting and deterioration of the cut edge quality.
8. The efficiency of melt removal from the cut kerf plays a very important role on the cutting performance and the resulting cut edge quality. The rate of melt removal from the cut kerf may be a potential factor limiting the maximum workpiece thickness that can be cut rather than the required laser power.
The results reported in this study have provided an insight into the performance of the ytterbium fibre laser in the area of thick-section metal cutting. The conclusions drawn from this study can be applied to larger workpiece thicknesses than those investigated in this study.
The effects of processing parameters on the cut edge quality during cutting of thick-section mild steel using oxygen assist gas has been investigated in this study. Future work could be directed to the optimization of processing parameters for achievement of good cut edge quality in mild steel during laser oxygen cutting using the ytterbium fibre laser.
Additionally, future work could cover a theoretical and experimental investigation of the absorption coefficient of the fibre laser radiation during laser cutting of a metal workpiece.
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