| In order to meet the lightweight requirements of high-speed train body manufacturing with higher speed level,it is imperative to improve the welding quality of aluminum alloy medium-thick plate structure.Ultra-narrow gap laser wire-filling welding has great application potential in medium-thick plate welding.However,due to the introduction of filler wire in the welding process,the relationship between the welding process parameter interaction and the mechanical properties of welded joint has become particularly complex.Moreover,for the multi-pass thermal cycling problem involved in the medium-thick plate structure welding,the influence on the microstructure evolution and mechanical properties of the joint is still not clear.Therefore,this paper optimized the process parameters of ultra-narrow gap laser wirefilling welding for 20 mm medium-thick plate aluminum alloy.The evolution law of joint microstructure under multi-pass thermal cycles was analyzed.The relationship between weld microstructure and mechanical properties and its strengthening mechanism was revealed.In this paper,the optimization experiments of ultra-narrow gap laser wire-filling welding was designed firstly.And the response surface models between the welding process parameters and the comprehensive properties of joint was established by central composite design method.All the models are significant and have great fitting degree.According to the response surface model analysis,when the laser power is moderate,the comprehensive performance of joint will be enhanced with the increase of welding speed and wire feed rate.The optimal process parameters were obtained by multi-objective optimization as follows: laser power 4.57 k W,welding speed 14.54 mm/s,wire feed rate 4.00 m/min.Based on above parameters,the welded joint has excellent comprehensive properties: the average tensile strength 215.16 MPa,the weld integrity 98.82 %,and the weld width 3.9456 mm.The multi-pass welding was implemented based on the optimal welding process.The microstructure evolution of the welded joint under multi-pass thermal cycles was analyzed.In the first thermal cycle,the weld microstructure produces a banded segregated layer due to solute segregation.And the weld solidifies from the liquid to form supersaturated solid solution with a small amount of ? phase precipitation in the fast-cooling condition of laser welding.The peak temperature of the second thermal cycle is higher than the melting point,and the welding directly acts on the previous weld to form the remelting zone,which makes the weld microstructure form uniform fine grains with more ? phase precipitation.During the third to fourth thermal cycles,since the peak temperature is only higher than solid phase transition,the growth of ? phase is accompanied by the increase of ? phase precipitation and aggregation.The peak temperature of the fifth to ninth thermal cycles does not reach the temperature point of solid phase transition.The growth of ? phase has obvious directivity,and the ? phase precipitated from ? phase keeps gathering and growing to form the coarse ? phase.Fine equiaxed grains appear in the heat-affected zone under thermal cycles,and then gradually grows and coarsens under the multi-pass thermal cycles.Finally,the relationship between the weld microstructure and mechanical properties under the multi-pass thermal cycles was clarified.In the weld,the increase of fine grains and large angle grain boundary proportion produce obvious strengthening effect,which improves the mechanical properties together with solid solution strengthening effect.And then with the increase of weld texture intensity and the second phase precipitation proportion,the deformation resistance and precipitation strengthening effect of the weld are strengthened.Therefore,with the comprehensive effect,the average hardness of the weld center is constantly increased,and the average tensile strength is increased by 22 MPa as a whole.The mechanical properties of the weld are strengthened. |
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