| Magnetic field-assisted laser-MIG hybrid welding combines the advantages of high laser energy density,good arc bridging capability,and magnetic field-regulating microstructures.Comparing with arc welding,it can improve welding speed;comparing with laser welding,it can improve bridging capabilities,and change microstructures.However,there are few studies on the effect of magnetic-assisted laser-MIG hybrid welding on the microstructure and properties of welds at home and abroad.In this paper,the SUS316 L austenitic stainless steel has been welded using magnetic-field assisted laser-MIG hybrid welding method,then the following researches has been carried out: Firstly,the microstructures of the welds were characterized.Combining numerical simulation with theoretical analysis,the influence of magnetic-field assisted laser-MIG hybrid welding on microstructure of welds have been revealed.Then,the fatigue crack initiation lifetime and crack propagation rate of the welds have been tested.The failure behavior has been analyzed to establish the correlation between microstructure and corrosion fatigue performance.Finally,based on the study of the microstructure formation mechanism of magnetic-field assisted laser-MIG hybrid welding,the mechanisms of the welding method's corrosion fatigue behavior of the welds have been revealed.During the research process,the following results have been achieved:1.Firstly,it has been found that ferrite content in the weld is decreased,and the morphology is in a skeleton shape;Then,combined with simulation and high-speed imaging,it was found that the fusion pool was expanded under 16 mT-24 mT magnetic field via the interaction among mangentic field and the arc current and induced current,that is,the "magnetic pool expansion effect".Finally,the magnetic field-assisted laser-MIG hybrid welding has an expansion effect on molten pool,which prolongs the time of solid transformation and enhances ?-? phase transformation,that is,"Magnetic-field induces 316 L welds ?-? phase transition enhancement".2.Firstly,it has been found that the grain orientation diversified,and the skeletal ferrite and austenite is in non-K-S orientation relationship;Then,the state of liquid metal flow at the solidification front has been qualitative analyzed,and the thermo-electromagnetic effect has been put forward.Finally,the magnetic field-assisted laser-MIG hybrid welding has a thermoselectromagnetic effect at solidification front of molten pool,which enhances 316 L grain orientations diversity,that is,"Magnetic-field induces 316 L grain orientations diversity".3.The relationship between grain orientations and corrosion fatigue crack initiation has been established:(1)Based on the theory that {100} plane is the most slippage in the double system slip during plastic deformation process of 316 L,and the decrease of grains in the <100> direction can suppress local slip band cracking;(2)The effect of the distribution of ferrite orientations on the local corrosion in 316 L welds has been found.That is,the diversification of ferrite orientations can promote the uniformity of weld surface potential,reduce local corrosion,and inhibit the formation of corrosion fatigue crack sources.(1)and(2)take synergistic effect,so as to inhibit corrosion fatigue crack initiation.4.The relationship between ferrite and corrosion fatigue crack propagation has been established:(1)Basing on the "shielding effect" of ferrite on austenite,that is,the reduction of ferrite can promote the strain-induced transformation from austenite to martensite at fatigue crack tip,increasing the crack tip closure effect.(2)The effect of the two-phase orientation relationship on the crack propagation path has been found,that is,the ?/? phases in the nonK-S orientation relationship has a semi-coherent interface,increases the interfacial energy of the two phases,hinders dislocation movement,and promotes crack propagation path deflection.(1)and(2)take synergistic effect,so as to decrease corrosion fatigue crack propagation.Based on the "Magnetic-field induces 316 L grain orientations diversification" and "Magnetic-field induces 316 L phase transition enhancement" mechanisms,the use of magnetic-field-assisted laser-MIG hybrid welding has improved the corrosion fatigue performance of welds.The use of magnetic-field assisted laser-MIG hybrid welding has improved the corrosion fatigue performance of welds.The best intensity of magnetic field is 16 mT and 24 mT.Weakly oriented weld grains of welds has been reduced by 19%,ferrite content in welds has been reduced by 66%.Fatigue crack initiation life in air has been prolonged 62.2%,corrosion fatigue crack initiation life in simulated seawater environment(3.5% NaCl solution)has been prolonged by 10%;fatigue crack growth growth rate has been reduced 33% in air,corrosion fatigue crack growth rate in simulated seawater environment(3.5% NaCl solution)has been reduced by 20%. |
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