Theoretical And Experimental Study On Laser Welding Of Invar Alloys For LNG Ships

Invar alloy(invar 36)is used in precision instrumentation,LNG storage and transportation equipment,space remote sensors and artificial satellites due to its low coefficient of thermal expansion and excellent mechanical properties at low temperatures.The conventional welding method,defects such as pores,cracks,slag inclusions,undercuts,etc.are easily generated,which affects the low-temperature performance of the weld,and the conventional welding method has the disadvantages of low welding efficiency and high cost.In this thesis,the simulation and experimental study on the temperature field of laser self-fluxing lap sealing welding of LNG marine invar alloy sheet are carried out.(1)Numerical simulation of the temperature field in laser self-welding Invar alloy.The heat transfer model for laser welding of invar alloy was established,the numerical simulation of welding temperature field was carried out,the influence law of welding process parameters on the weld morphology was studied,and a reasonable range of welding process parameters was determined: laser power 2600W-3600 W,welding speed 1100mm/min-1700mm/min,energy distribution ratio is between 1:0~1:1.(2)The mechanism of formation and suppression method of welding porosity was studied.The EDS spectrum was used to analyze the compositio n and content of elements inside and outside the pore.The result shows that the contents of O and Si in the inner wall of the pore was higher than that outside the pore.According to the analysis,the main reason for the generation of Welding porosity is that the segregation of Si and the external oxygen enter the molten pool and cannot escape.Welding porosity can be effectively suppressed by designing and manufacturing soldering seam argon protection device.(3)Single factor and orthogonal welding experimental analysis was carried out.The single factor experiment was used to analyzes the influence law of welding process parameters on the weld macroscopic morphology;the orthogonal experimental results was used to optimize the process parameters and de termine the optimum parameters: laser power is 3400 W,welding speed is 1300mm/min,defocusing The amount is +20 mm,the incident angle is 5°,and the energy distribution is 2:1.(4)Analysis of the results of the laser welding experiment of invar alloy.Th e weld quality is analyzed under the optimum parameters.The surface of the weld is of good quality;the heat affected zone is narrow and coarse austenite crystal;the weld is coarse columnar and equiaxed grain,in which there are fine dendrites and cell structures.The average microhardness of the weld is slightly lower than that of the base metal.The tensile strength of the weldment reached 94.8% of the base metal,and the maximum elongation reached 61.95% of the base metal.The fracture is located in the heat affected zone of the lower plate and is a typical ductile fracture.Finally,the cross-section morphology and size of the weld obtained by the experiment and simulation are compared and analyzed: the error of penetration and melting width are less than 0.05 mm,which indicates that the welding simulation is correct and reasonable.In this thesis,the research of the theory and process of the laser welding of the invar alloy is carried out by combining the simulation with the experiment,and the theoretical basis for laser welding of the invar alloy of the LNG ship is provided.