The Analysis Of Gas Flow Field In Oxygen-Assisted Laser Cutting Of Thick Mild Steel

Laser cutting is one of the most important application technologiesin laser machining industry. Compared with the traditional flame cuttingand plasma cutting, laser cutting has plenty of remarkable advantages, suchas excellent cutting quality, rapid cutting velocity, high automation, largerange of cutting materials and so on. Due to the limitation of laser power,only the laser radiation itself is not sufficient to cut through thick metalplates. In oxygen-assisted laser cutting, the continuous oxidation betweenthe base metal and oxygen will provide large amount of heat to melt themetal up, and then the melt is blew out of the cutting kerf by the shearingforce of assisting gas, thus oxygen-assisted is a efficient way to improvethe laser cutting ability.The paper discusses the research status and development of theassisting gas flow field analysis in laser cutting, as well as oxygen-assistedlaser cutting. A 3-D model of impinging gas flow in oxygen assisted lasercutting is established, and the influences of oxidation to the flow structureand features are discussed. Also the effects of changing cutting parameters such as oxygen pressure and standoff distance on the flow field andcharacters are studied.At first the structure of cutting area and kerf are described, and thefluid dynamics of assisting gas in the kerf is analyzed. Based oncompressible Reynolds-averaged N-S equations, SST k- turbulence modeland heat transfer equations in cutting area, the 3-D symmetric impinginggas flow model is established and FVM is adopted to numerically simulatethe gas flow of converging nozzle.The symmetric impinging gas flow process is studied, and themechanism of the interaction between gas and plate is analyzed.Investigation into the flow field structure carries out the distribution ofoxygen pressure, gas velocity, shearing stress and mass flow rate. Throughthe comparisons between if oxidation effect is included in the simulationmodel, the results reveal that oxidation will cause disturbance to the flowfield at the kerf bottom, nevertheless it will reduce the pressure, gasvelocity and shearing stress, causing deterioration to the cutting ability ofassisting gas. This indicates that while modeling reaction involved cuttingprocess, oxidation is an essential factor. The paper also examined thechanges in flow field structure and characters due to various oxygenpressure and standoff distances. It's found out that while increasing oxygen pressure, inclinations and curvatures of streamlines appear, while theshearing stress is not increased accordingly. Gas velocity and shearingstress will decrease due to the enlargement of standoff distance, and theflow field is more dispersed.On the base of theoretical research, the paper carried out a series ofexperiments. The impacts of oxygen pressure and cutting velocity onstriation wavelength and kerf width are shown. By the mean time,influences of standoff distance and oxygen pressure on the cutting surfaceprofile are examined, and experiments show coincidence with simulationsIn conclusion, the research in this dissertation enriches the theory andmethod of the analysis of assist gas jet in oxygen assisted laser cutting, andprovides theoretical foundation and important reference to high-pressureand high-velocity laser cutting in thick metal plate.Finally, the paper summarizes all the done work and views the studydirection in future.