| Aiming at the control strategy of non-fusion defects and combination properties optimization of 301L-MT austenitic stainless steels(SS)welded by ultra-narrow gap laser welding(UNGLW),the causes and inhibition mechanism of non-fusion defects,and the relationship between melting performances of welding wire and fusion quality of welded joint have been studied,which revealed the interaction effect between laser energy and materials.Based on the response surface analysis and multi-objective optimization method,quality optimization model of welded joints was established,which has clarified the influence rules of process parameters on the combination properties.This research has high theoretical value and engineering application significance.In this paper,8mm thick austenitic SS was chosen as the research object and the following studies have been carried out.First,based on the theoretical analysis and experiment,the causes and preventive measures of non-fusion defects are studied from ultra-narrow gap groove design,laser power screening and melting performances of welding wire.Then the relationships between the welding parameters and the quality parameters of the welded joints were analyzed by the RSM(response surface method),and the process of the UNGLW is further explored.Finally,the optimal process parameters were obtained by multi-objective optimization,which has been verified by experiments.The following conclusions have been obtained in this paper.The non-fusion defects were restrained from the design of groove types.Considering the design principle of ultra-narrow gap,adaptability of the laser beam,shrinkage of the groove and the effect of protecting gas,the ultra-narrow gap groove with 2mm depth,1.4mm width of root face and 3°bevel angle was most suitable for experiments.The control strategy to prevent non-fusion defects from melting performances of welding wire was obtained.The side-wall fusion of welded joints mainly depends on the heat conduction of molten pool and thermal radiation.Under the big-droplet transfer,the heat conduction effect of the molten pool will be seriously affected by the complex counterforce,the barrier of droplet to laser power and the intense disturbance of droplet on the molten pool,which will lead to non-fusion defects.Moreover,interlayer non-fusion defects were caused due to the lack of droplet transfer driving force.However,under liquid-bridge transfer mode,the melting and transfer process of welding wire is very stable,which had little effect on the heat conduction effect of molten pool,so the excellent fusions between filler metals and base materials can be obtained under proper parameters.The mathematical models between input factors(welding speed,laser power,wire feeding speed)and responses(tensile strength,weld integrity,weld width,overlap factor,welding layer number)were established based on the RSM,which can reveal the change rules of combination properties of welded joints under various parameters.The line energy(laser power and welding speed)had the most obvious effect on different responses.With the increase of line energy,the tensile strength,integrity,width and overlap shape factors will be increased,while the number of passes decreased sharply.Larger wire feeding speed can improve efficiency and reduce the overlap shape factor,but it will seriously affect the fusion quality and reduce the tensile strength.Therefore,the wire feeding speed should be properly matched according to the power and welding speed.Besides,there were significant multi-factors interactions in tensile strength,integrity and overlap shape factor.The selection of smaller wire feed rate under high welding speed can ensure good fusions for each layer of weld seam,improving tensile strength.Relative high laser power can obtain ideal weld integrity and reduce the negative influence of welding speed.Under the premise of ensuring fusion quality,the selection of moderate welding speed and wire feeding speed can reduce the overlap shape factor and ensure the beautiful appearance of the weld seam.On the basis of the above mathematical models and influence rules of parameters,the process parameters were optimized by the multi-objective optimization theory in order to further restrain non-fusion defects and improve combination properties of the welded joints.The optimum welding parameters were as follows: 9.62mm/s for welding speed,1.45 kW for laser power and 1.07m/min for wire feed rate.The final optimum average expectation was 0.81.Besides,the verification experiments indicated that the average accuracy of optimized model was 96.68%(763.28 MPa for tensile strength,99.75% for weld integrity,2106.38μm for weld width,202.13μm for overlap factor and 3 layers for welding layer number).Under the optimized process parameters,the efficiency was high,and the welded joints had excellent strength(24.72% higher than that of MAG welded joint)without non-fusion defects.Meanwhile,the proper weld width and overlap shape factor can ensure the good appearance of the weld,reducing the range of heat affected zone and eliminating stress concentration.Microcosmic characteristics of welded joints under optimized parameters have shown that the weld zone mainly consisted of austenite columnar crystals and lath ferrites,and there were a large number of equiaxed grains produced by spontaneous nucleation.The average number of grain size in observed area was about 24.94μm.UNGLW has the advantages of high energy density,small heat input and short residence time at high temperature,which can help to reduce the width of heat affected zone,refine grains and improve strength.In summary,UNGLW can achieve good fusions under small heat input and improve the combination properties of welded joints. |
PDF Full Text Request