| With the progress of industrial production and the development of national defence construction,the application of large thick-walled components is becoming more and more widespread.The demand for efficient,high-quality thick plate welding is becoming more and more urgent in the fields of shipping,rail transportation,nuclear power and aerospace.In order to solve the problem of non-fusion in the process of narrow gap welding of thick plates and to improve welding efficiency,some researchers have proposed a oscillating laser-TIG hybrid welding method.The oscillating laser has the advantages of reducing porosity,refining grain size and improving the uniformity of the weld seam,which is expected to refine the grain and improve energy utilisation and welding efficiency when combined with TIG welding.This paper investigates the oscillating laser-TIG hybrid welding process and mechanism of nuclear grade 316 LN austenitic stainless steel to provide theoretical and technical support for understanding the interaction between the oscillating laser and the TIG arc and to expand the application of this method in nuclear power.First,this paper uses the cross-sectional sandwich method to study the melt pool flow,keyhole morphology and keyhole dynamic behaviour of oscillating laser welding.The results show that the keyhole shape of oscillating laser welding is quite different from that of conventional laser welding.Conventional laser welding keyhole is deep conical,oscillating laser welding keyhole transitions from deep conical gradually to shallow Vshaped with the increase in frequency and amplitude.When the frequency is greater than50 Hz and the amplitude is greater than 2 mm,the depth of the keyhole varies periodically with the frequency.The flow state of the oscillating laser welding melt pool was analysed by high-speed camera.And on this basis,the oscillating laser-TIG hybrid welding process was further studied on the impact of laser oscillating parameters on the melt pool flow,and it was found that the oscillating laser can significantly promote the melt pool flow.Then,the heat source characteristics of oscillating laser and oscillating laser-TIG hybrid welding were studied by theoretical calculations and spectral analysis.The oscillating laser heat source energy deposition state on the surface of the workpiece was clarified by theoretical calculations.The results show that as the frequency increases,the laser energy deposition density peak in the welding direction gradually decreases and eventually plateaus.As the amplitude increases,the peak laser energy deposition density also gradually decreases.The weld seam shape of oscillating laser welding and oscillating laser-TIG hybrid welding was observed respectively.It found that both of theirs weld seams are presented "double melt pool" characteristics.The forming characteristics under different welding process parameters were obtained.It has a certain guiding role in optimizing welding process.The plasma properties of oscillating laser-TIG hybrid welding were studied and the coupling mechanism between the laser and the arc was described.Finally,the weld microstructure and properties of the oscillating laser-TIG hybrid welding elaboration and experimental verification of the oscillating laser-TIG hybrid welding to suppress porosity and non-fusion defects mechanism,the application of the method in the nuclear power stainless steel narrow gap welding prospects explored.By comparing with TIG welding,it was found that the weld strength of hybrid welding is comparable with TIG welding,and both reach more than 95% of the strength of the base material.But the hybrid welding welding speed is faster,is 2.5 times the TIG welding,welding efficiency is higher.hybrid welding heat input is smaller,56.6% of TIG welding,weld grain more refined.Fracture morphology analysis shows that the key to further improving the strength and post-break elongation of the hybrid welded seam is to optimise the process and reduce the porosity in the weld.The sandwich method of observation of the keyhole morphology and the stress analysis of the keyhole shows that the hybrid welding suppression of porosity defects is mainly achieved by enhancing the stability of the keyhole;through the coaxial high-speed camera observation of the narrow gap filling welding process,it was found that the hybrid welding suppression of unfused defects in the sidewall mainly relies on laser to promote the flow of the melt pool and enhance the wetting effect of molten metal on the sidewall.Finally,the weld microstructure and properties of the oscillating laser-TIG hybrid welding were analyzed,and the mechanism of the oscillating laser-TIG hybrid welding to suppress the porosity and non-fusion defects was described and experimentally verified,and the application prospect of this method in the narrow gap welding of nuclear stainless steel was explored.The weld strength of the oscillating laser-TIG hybrid welding is similar to that of TIG welding,and both of them reach more than 95% of the strength of base metal.Hybrid welding is 1.78 times more efficient than TIG welding.The heat input of hybrid welding is smaller,56.6% of TIG welding,and the weld grain is more refined.The fracture morphology analysis shows that the key to further improve the weld strength and elongation is to optimize the process and reduce the porosity in the weld.The observation of the keyhole morphology and the force analysis of the keyhole by sandwich method show that the porosity defects are mainly suppressed by improving the stability of the keyhole.By observing the process of narrow gap filling welding with high speed camera,it is found that the hybrid welding mainly relies on laser scanning to directly heat the side wall and promote the flow of molten pool to enhance the wetting effect of molten metal on the side wall to suppress the side wall non-fusion defects. |
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