The Characteristics And Mechanism Of High-power Solid State Laser Cutting Of Thick Steel Platewith Oxygen

With the rapid development in shipbuilding, heavy machinery, large pressure vessel, bridge engineering, nuclear engineering and military fields, laser-oxygen cutting technology for thick steel plate was required urgently. However, with the increase of thickness of thick steel plate, some cutting defects occurred, such as deep striation, dross attachment and side overburn. Additionally, when the thickness of steel plate exceeds 25 mm, the laser-oxygen cutting speed decreased remarkably and the repeatability of the kerfs was unreliablealso. Therefore, it is necessary to study the characteristics oflaser-oxygen cutting of thick steel plate and its exothermic reaction mechanism of iron oxidation, and to find out the fundamental factors influencing the cutting efficiency. It would be a theoretical foundation for improving laser-oxygen cutting technique and development of advanced cutting method for thick steel plates.An experimental system for laser-oxygen cutting thick steel plate was established firstly. In order to cutting thick steel plate, a special laser-oxygen cutting head with long focal distance was developed which can be used both in atmospheric and underwater environments. A high speed camera system and a spectrometer were also prepared in order to capture the dynamic physical characteristics during laser-oxygen cutting process. Laser-oxygen cutting experiments, aimed at 30 mm and thicker mild steels, were conducted to find out the influences of cutting parameters on cutting quality and kerf morphology. The results showed that a good kerf without dross attachment could be obtained with the cutting speed from 0.5m/min to 0.8m/min. The maximum cutting speed reached 1.2 m/min for 30 mm thick steel plate.However, former researchers reported the laser-oxygen cutting speed was only 0.25 m/min for cutting 32 mm thick steel plate. A kind of dross-free coating was developed to solve the dross attaching for cutting thick steel plate.Two kinds of cutting modes were found, narrow-kerf mode and wide-kerf mode, for laser-oxygen cutting of thick plate. The pressure of oxygen and laser defocusing distance were important factors affecting on the transition of two kind of cutting modes. When the pressure of oxygen was higher than 0.20 MPa, the cutting mode was always wide-kerf cutting mode; a lower pressure of oxygen and small laser spot size resulted to narrow-kerf cutting mode.The dynamic behaviors of high temperature liquid zone in kerf both in narrow-kerf and wide-kerf cutting modes were studied in detail using high speed camera system. The results showed that there existed three typical zones in cutting kerf of thick steel plate:(1) “High luminance zone”: high temperature iron violent oxidation reaction zone with direct laser irradiation;(2) “Low luminance zone”: low temperature iron oxidation reaction zone without direct laser irradiation;(3) “Dark zone”: without iron oxidation reaction and no laser irradiation.The heat transfer mechanisms of the two cutting modes were also analyzed based on the dynamic behaviors of the three typical zones.The process of transformation from narrow-kerf cutting mode into wide-kerf cutting mode was captured with high speed camera system, and the result indicated that the oxygen pressure influenced the process remarkably. The enthalpy of per mole iron oxidation reaction during laser-oxygen cutting thick steel plate process was measured through an experiment. It was found that the value was lower than that in laser cutting thin sheet, which indicated higher energy efficiency in laser cutting thick steel plate process.The mechanism of iron oxygen reaction was analyzed in laser-oxygen cutting process of thick mild steel from the view of oxygen molecule excitation and dissociation by laser plasma. The emission spectrum of “High luminance zone” in kerf was obtained using spectrometer. The results showed that laser induced plasma plume covered the“High luminance zone”, and a massive high energy level oxygen atoms and +1 valence ionization oxygen atoms were generated due to the violent collision between laser induced plasma and oxygen molecules. It could be assumed that the oxygen molecules in laser induced plasma could be excited and dissociated and not just oxygen molecules but also high energy level oxygen atoms and ions will participate in the iron oxygen reaction.The evidence of oxygen molecules excitation and dissociation was revealed through mass spectrometer and related theoretical analysis. The results demonstrated that the laser induced plasma played a very important role on excitation and dissociation of oxygen molecules at the initial stage of laser-oxygen cutting. During steady laser cutting process, laser induced plasma is also very important, moreover, the high temperature surrounding initiated by laser induced plasma is also helpful to the process.The influences of high energy level oxygen particles on iron oxidation reaction were listed as follows:(1) While high energy level oxygen particles participating in, more enthalpy of oxidation reaction during laser-oxygen cutting process was generated compare with only oxygen molecules participating in;(2) Reducing the ignition temperature of iron oxidation reaction remarkably due to that they had smaller energy-barrier and made the reaction started more easily;(3) Accelerating the speed of the iron oxidation reaction significantly, which allowed more iron participating in the reaction atunit time and increased laser-oxygen cutting speed.According to the phenomenon that the high energy level oxygen particles could improve cutting efficiency, a new cutting method, laser-flame hybrid cutting technology, was developed. The flame could develop a high temperature channel in kerf, and maintained a dynamic stability of laser induced plasma, which increased the cutting efficiency and speed effectively. With a laser power of 6 k W, 140 mm thick mild steel was cutted successfully at a speed of 0.3m/min by the new cutting method, which was superior to that of conventional technical level.According to the characteristics of laser oxygen cutting of thick steel plate, a new hypothesis was proposed that laser induced plasma plays a very important and unneglectable role in laser oxygen cutting process of thick steel plate. That is very different from laser welding and laser cutting of thin plate, in which laser induced plasma should be removed as much as possible to increase the laser absorption efficiency. The proposed new hypothesis in this paper should be studied further in future.