Laser fusion cutting using supersonic nozzles

Abstract

This research takes a systematic approach to study the flow patterns of gas jets from the conventional conical (subsonic) nozzle and the newly designed supersonic nozzle under a high-pressure gas regime by a three-dimensional mathematical model. Computer simulation was then confirmed by experimental shadowgraphic technique. The relationships between the type of nozzles, nozzle dimensions, stand-off distance, inlet stagnation pressure and the flow field distribution, incident shock and normal shock have been established. Compared with that of the subsonic nozzle, the gas jet from a supersonic nozzle possesses the desired dynamic characteristics such as uniform distribution, maximum and even momentum thrust and parallel jet boundary under the condition of the designed pressures. Then, a model calculating the three-dimensional stationary geometric shape of the cutting front obtained in the high-pressure laser fusion cutting regime was developed using numerical solution of energy balance. In this model, the energy absorbed by the workpiece includes not only the energy from the laser beam but also the energy from the multiple reflections generated by the beam impinging at the cutting front. The effects of the laser power, cutting speed, focus position, multiple reflections and inert gas pressure on the geometric shape of the cutting front have been analyzed systemically. The geometric shape of the cutting front was then used as boundary conditions in subsequent calculation of the gas flow field distribution inside a laser cut kerf. Finally, a third mathematical model was developed to calculate the distribution of the gas flow field at the entrance of the cut kerf, inside the cut kerf and at the exit of a cut kerf under the condition of a gas jet from a supersonic nozzle and the inlet stagnation pressure >= 5 bar. A two-dimensional analytical method was adopted to locate approximately the position and shape of the detached shock above the cut kerf. A method of two-dimensional characteristics was applied in calculating the gas flow field distribution along the cutting front. The effects of the inlet pressure, the exit diameter of the nozzle and the displacement of the beam/nozzle axis upon the distribution of the gas flow field at different locations inside the cut kerf as well as the cut edge quality were analyzed systematically. The relationships between the roughness of the cut edge surface, the dross formation at the bottom of the cut kerf, the cutting parameters, and the flow field distribution along the cutting front were established. The three models developed can explain some peculiar process phenomena and predict the optimum process parameters used in high-pressure gas laser fusion cutting. High-pressure gas-assisted laser fusion cutting experiments on stainless steels and double layer mild steel have been carried out to confirm the theoretical modelling work and the results were in good agreement with each other. The theoretical analysis was used to explain the common problems and phenomena found in high-pressure gas-assisted laser fusion cutting. The experimental results showed that supersonic nozzle has far better adaptability than the subsonic nozzle in high-pressure gas-assisted laser cutting process.