Simulation And Experimental Research On Fiber Laser Underwater Cutting Stainless Steel

The 21 st century is the era of ocean. There are many problems including high cost of repair or demolition, severe operation environment, low-security, unfriendly ocean environment etc. when the offshore engineering structures such as ocean platform, submarine pipeline, oil depots etc. are built, maintained and dismantled. Fiber laser underwater cutting technology has become a promising technology for the advantages of being high-efficiency, convenient, inexpensive and environment-friendly. Fiber laser is more suitable for underwater operation compared with CO2 laser, because it is light and stable, costs less for maintenance and easy to realize high power and long distance transmission and so on.(1) The paper includes simulation and experimental research on fiber laser underwater cutting stainless steel. It simulated the temperature field of laser underwater cutting stainless steel in ANSYS. Study on the effects of fiber laser cutting temperature field by different laser power and cutting speed. The thermal physical parameters of material and convective heat transfer coefficient of water was linear simplified to describe boundary conditions of materials and the environment accurately. The position of the highest temperature point was determined by cutting speed rather than laser power at the same time point.(2) In the experiment of fiber laser underwater cutting of 1 mm thick 304 stainless steel with the assistance of high-pressure argon gas, the influence regularity of changing the condition of laser power, cutting speed, thickness of the water layer, salinity and temperature of the water can be studied by measuring the average width of cutting kerf. The best cutting quality can be reach with the conditions of 1500 W laser power and 300 mm/min cutting speed. Macroscopically, kerf shape is affected by the surface tension of fusion pool, the gravity of liquid metal and the assistant gas pressure. Lower laser power, faster cutting speed and thicker water layer decrease laser cutting efficiency and quality. Meanwhile, too thick water layer and low water temperature can accelerate cooling of the steel plate, which leads to the loss of laser energy, and the large number of ions in brine will absorb laser energy as well.(3) This paper has done a lot for further research on the effects of laser parameters and environmental parameters on the efficiency and quality of underwater laser cutting, and the forming law of cutting seam profile during cutting process. It comprehensively assessed the microstructure morphology of the fusion zone and HAZ (Heat affected zone) from stainless steel plate joint-cutting cross section in the views of solidification mode, phase constitution, grain size and micro hardness. On a micro level, metallographic structure and micro hardness of the fusion zone, HAZ and substrate are different. The surface nucleation occurred on the edge of the fusion zone, and the unit cell size increased as the laser energy density went up. High laser power density promoted crystal grain growth. In addition, the microstructure in HAZ was coarse because of the thermal cycle. The micro hardness of HAZ is lower than that of the substrate and fusion zone. Micro hardness of the outer surface of the fusion zone was 242.8 HV with some oxidized zone showing a hardness of 963 HV,4.3 times than that of the substrate.This dissertation is focused on the thermal transmission and phase transformation between water and cutting piece in fiber laser underwater cutting. In addition, temperature field and kerf shape formation of laser underwater cutting were investigated. It provides theoretical support for the efficient demolition of waste oil platforms. Besides, it is strategic significance in construction of powerful marine economic nation.