ISSUE 114 AUTUMN 2024 LASER CUTTING
THE LASER USER
LOW-PRESSURE LASER CUTTING TO
MANAGE HAZARDOUS LASER FUME JACOB LAVIN ET AL.*
Conventional inert laser cutting processes typically employ high gas pressures (10–16 Bar) to ensure clean and efficient cuts. However, this approach generates a substantial volume of hazardous by- products, such as fine particles and fumes, which can pose serious risks to human health and the environment. In the context of nuclear decommissioning, where the materials being cut can be radioactive, managing these hazardous by-products is of utmost importance. The containment and filtration systems required to handle these by-products are complex, costly, and limit processing speeds [1,2]. Therefore, there is a critical need to develop laser cutting techniques that minimise the generation of hazardous by-products while maintaining cutting efficiency.
This study investigates the potential of low- pressure (3–6 Bar) laser cutting techniques to achieve this goal, with a particular focus on the dynamics of melt ejection, gas flow, and gas cooling. It is key to note that cut quality is not an important factor for the application of nuclear decommissioning, hence the feasibility of low pressure cutting.
Gas flow through kerf-like geometries
A previous study by our group at Loughborough University demonstrated the criticality of gas jet–molten layer interactions inside the cut on laser fume formation [3]. The results showed that moving the location of key gas dynamic features inside the cut could be used to influence the size and volume of particles ejected during the cutting process, and that gas pressure was the dominant factor in controlling gas dynamic interactions.
Figure 1 shows a diagram of the imaging system used to assess key gas jet interaction inside an idealised laser cut along with an image of the typical gas jet flow structure present inside a laser cut. To influence the composition of the fume, the changes in gas dynamic interactions must be affecting the melt breakdown process. One aspect that the idealised imaging cannot assess is the influence of gas composition. Hence, the present study assesses how melt breakdown is influenced by changes in gas dynamic interactions and gas composition.
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Figure 1 (a) Schlieren imaging system used to assess key gas dynamic features in idealised laser cut geometry. (b) Schlieren image of gas jet inside an idealised laser cut geometry.
Melt ejection imaging
High-speed imaging of melt ejection during laser cutting showed that at lower pressures, the ejected particles were fewer and larger, indicating a reduction the total number of aerosolized by-products. In addition, melt ejections from cut conditions that used nitrogen, Figure 2 (c, d), were much more stable than those that used air, Figure 2 (a, b). During high pressure cutting with air the ejection mechanisms where violent and explosive leading to many small, high velocity particles being produced. Cutting at lower pressures did lead to more stable cutting conditions, noted by comparison of Figure 2 (a) and Figure 2 (b). In all cases, analysis of the <10 μm particles indicated they have velocities in the range of 40–80 m/s, much faster than the velocities noted by other studies looking at larger droplets.
A phenomenon not noted by previous studies, likely due to the uniqueness of low pressure cutting, was the formation of large dross balls that existed for several milliseconds. The balls were observed to capture ejected material, preventing it from breaking down into fine particles. This eject condition was most prevalent with the 3 Bar nitrogen cutting, Figure 2 (d). Such cutting conditions correspond to large dross adherence to the underside of the cuts as presented in Figure 2 (e). Cuts 1-3 in Figure 2 (e) show the underside of plates cut with air
at increasing supply pressure. Lots of dross is noted in cut 1 with little in cut 3.
Gas boundary layer modelling
To further understand the results recorded during melt ejection imaging, boundary layer theory was applied to the conditions of this study to predict the shear stresses on the molten cut front. It is widely accepted that the gas jet boundary layer will become turbulent at some distance along the cut front, however, the exact location of this transition point is difficult to determine. For simplicity, only the turbulent model is presented here, however, a laminar and turbulent model is presented in [4]. Figure 3 demonstrates that shear stress along the cut front increases with increasing supply pressure and, for a given pressure the shear stresses applied by air are slightly higher than that of nitrogen. Included in this model is the influence of change in melt temperature, for details on how this was incorporated consult [4]. It is key to note that the temperature of the melt during air cutting was estimated to be 412°C hotter than that of the nitrogen.
The increase in shear stress with increasing supply pressure supports the results observed during the melt ejection imaging, indicating that the change in breakdown mechanism was due to increasing shear stresses. The difference in shear stresses between different gas compositions of
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