| Traditionally, steel plates with a thickness of more than 20 mm are extensively welded by narrow-gap SAW/GMAW welding method. Compared with traditional arc welding, laser welding of thick plate has a huge advantage. The energy density of laser beam is focused, which causes small heat input, narrow heat-affected zone, small welding distortion, high welding efficiency and excellent mechanical properties of welded joints. In this study, 10Ni3CrMoV steel with a thickness of 36 mm was efficiently welded by laser welding and laser hybrid welding using multi-pass welding techniques.The stabilities of the laser single arc hybrid welding and laser double arc hybrid welding arc are studied from the point of the force and energy. For the laser single arc hybrid welding, the influence of the welding current, laser power and the distance between the laser induced plasma and the arc on the coupling mechanism of laser induced plasma and the arc is focused on through testing. The results show that there is an optimum distance range between the laser induced plasma and the arc for the different laser power and welding current. For the laser double arc hybrid welding, when the electromagnetic force of the leading arc on the laser induced plasma is generally equal to that of the trailing arc on the laser induced plasma at the same arc voltages, the laser induced plasma can play a guiding and stability effects on the two arcs, resulting in smooth metal transfer. In order to reveal the influence of groove form and size on the stability of laser welding, the behaviour of laser induced plasma inside the groove is studied using high-speed camera system. The results indicate that the laser induced plasma can be suppressed and the stability of the welding process can be improved using the appropriate size of the U-groove, which can fully use the laser energy and increase weld penetration. The optimum sizes of U-shaped groove are groove width with a range from 3 to 4 mm and groove depth with a range from 4 to 5 mm. By collecting current and voltage signals and observing the interaction between the laser induced plasma and the arc inside a groove, V form with the smallest angle of 20°is determined. The laser induced plasma can attract and maintain the arc, making the welding process stability.Based on computational fluid dynamics, three-dimensional numerical model including a nozzle, air and workpiece is established to simulate the changes of side-blown gas static pressure, gas flow velocity and mole fraction of He in the entire flow field under the U-groove and no groove conditions. The simulation results shows that gas static pressure, gas flow velocity and mole fraction of He increase in the appropriate sizes of the U-groove, which can enhance the cooling of the plasma and increase the electron recombination rate. Therefore it can effectively inhibit laser induced plasma, resulting greater penetration and stability of the welding process.Thermal cycling curves are obtained under different heat input in pure laser welding and laser hybrid welding. SHCCT diagram of 10Ni3CrMoV adapted to the laser welding is established, which can provide an important reference for the determination of welding process parameters of thick plate. The TTT curves and CCT curves of 10Ni3CrMoV steel are obtained using JMatPro software. The microstructures in heat-affected zone of 10Ni3CrMoV steel and mechanical properties of welded joints are obtained by simulating. The results show that the microstructures transform from martensite + bainite + ferrite + austenite to bainite + martensite+ austenite to martensite+ austenite as the cooling rate increases. Microstructures of the first thermal cycle and the second thermal cycle in HAZ are simulated using Gleeble 3500. The martensite lath width and the size of lath cluster in coarse grain zone obtained by the second thermal cycle are greater than that obtained by the first thermal cycle. With the increase of t8/5, microstructures obtained by the first thermal cycle transform from lath martensite+carbide to martensite+carbide+granular bainite. The yield strength and tensile strength of coarse-grained region are the largest, the impact toughness is the lowest, and the hardness is the highest. However, the yield strength and tensile strength of the incomplete recrystallization region are the smallest. The hardness of the tempering zone is the lowest. The microstructures of the fine grained region at the peak temperature of 900?in the second thermal cycle mainly consist of martensite, granular bainite and carbide. The granular bainite cause the embrittlement of the microstructure, which makes that the yield and tensile strength is the smallest and the impact toughness is generally lower. The region with the peak temperature of 900?in the second thermal cycle becomes the weak part in the whole welded joint.Microstructure evolution in the weld and heat affected zone of 10Ni3CrMoV steel under different heat input is studied. With the heat input of 11.2kJ/cm, the weld metal is mainly composed of granular bainite. With the heat input of 8kJ/cm, the weld metal primarily consists of acicular ferrite. And with the heat input of 6.7 kJ/ cm, the weld metal is mainly composed of martensite. High-temperature liquid metal of welded joints has relatively short residence time for low heat input. During the cooling process, the second phase particles has small tend to gather and grew up, making the acicular ferrite nuclei increased. Therefore acicular ferrite in the weld metal is increased.The mechanical properties of welded joints and weld metal are comprehensively tested. The results show that for the heat input of 8kJ/cm, the impact toughness of weld metal is the highest, the tensile strength and yield strength of the weld metal are much larger than that of the base metal, and the elongation of welded joints is slightly less than the base metal. The results also reveal that the maximum hardness of welded joints is less than 360HV, the tensile strength and yield strength of welded joints are greater than that of the base metal, and the elongation of welded joints is slightly less than that of the base metal. Bending performance can meet the requirements. |
PDF Full Text Request