| Nickel-base alloy is one of the most important engineering materials in the 21 st century.Because of its excellent high temperature performance and corrosion resistance,it has been widely used in aerospace,petrochemical,nuclear power and other fields.Ultra-thin nickel-base superalloy is mainly used in bellows,steam condensing pipes and other precision structures with harsh working environment,which puts forward higher requirements for welding process.The results show that the thermal conductivity of nickel-base alloy is small,and it tends to overheat,which easily leads to the coarseness of weld grain,the high viscosity of liquid nickel-base alloy and the poor weld formation.When the ultra-thin plate is welded,it is easy to burn through and deform.Therefore,when welding the ultra-thin nickel-base alloy plate,the welding method with strong controllable heat input and high energy density should be selected.Microplasma arc welding is characterized by fast welding speed,narrow weld,small heat affected zone and small welding deformation.Under the pulse current,the process parameters can be adjusted well,so the heat input can be controlled more precisely.Therefore,it is of great significance to study the characteristics of weld formation and microstructure changes under different pulse parameters by using microplasma arc welding with pulsed current(P-MPAW)ultra-thin nickel-base alloy.In this paper,P-MPAW was used to weld 100μm thick Inconel-718 alloy ultra-thin plate.In order to study the influence of pulse current on weld formation and microstructure,36 sets of pulse parameters were selected,and the average line energy(average current is 2A)was kept constant.Furthermore,the qualitative and quantitative analysis of the weld formation and microstructure distribution of the joint is carried out.Aiming at the weld formation of 100μm ultra-thin nickel-base alloy P-MPAW,the effects of weld formation morphology,weld formation coefficient and pulse parameters on it were studied.The results show that five kinds of weld morphology are obtained by 100μm ultra-thin nickel-base alloy P-MPAW,which are weld pool discontinuity,weld pool continuous unequal width,weld pool remelting,fish scale weld and continuous weld pool.With the increase of pulse frequency,base current or duty cycle,the weld morphology shows a trend of changing from weld pool discontinuity to weld pool continuity,and the width of the weld decreases,and the thickness of the weld increases,which reduces the weld forming coefficient and improves the weld forming.According to the research of the duty cycle of the welding seam forming,the duty cycle of the welding seam forming is much larger than that of the pulse current.And the width of the front side of the welding seam is slightly larger than that of the back side.In this paper,five kinds of weld morphology based on 100μm ultra-thin nickel-base alloy P-MPAW are studied.The weld is divided into three areas: non remelting area,remelting area and continuous area.The microstructure of HAZ,PMZ and WZ centers in these three areas is analyzed based on superfield microscope.The distribution area and average diameter of equiaxed grains are measured and analyzed by Image-J,and then the influence of pulse parameters on the distribution area and average diameter of equiaxed grains is discussed.The results show that in the non remelting zone of the discontinuous weld pool,with the decrease of duty cycle,the area of HAZ,the average grain size and the area of PMZ decrease,while the average grain size of PMZ,the area of equiaxed zone and the average grain size of WZ increase.In the non remelting zone with continuous weld pool and unequal width,the high base current or duty cycle makes the HAZ area larger,while the area and average grain size of PMZ are smaller,and the change of the area and average grain size of the equiaxed zone of WZ is more complex.In the non remelting zone of weld pool remelting,reducing the base value current and duty cycle is conducive to the reduction of the area of HAZ and PMZ and the average grain size,and the corresponding area of WZ equiaxed crystal zone is also reduced,while the average size of equiaxed crystal has little change.In the remelting zone of the remelting weld pool,the area of HAZ and the average grain size of PMZ are the smallest under the smaller duty cycle and base current,and the average grain size of HAZ will decrease when the base current is properly increased,while the area of PMZ is the smallest under the 70% duty cycle and 1.20 A base current,and the area and average grain size of the equiaxed zone of WZ are larger when the duty cycle is high.In the remelting zone of fish scale weld,the area of HAZ is smaller when the pulse parameter is low,while the average grain size of HAZ increases;when the duty cycle and base current are small,the area of PMZ decreases when the frequency is increased,which is not conducive to the refinement of PMZ grain;when the duty cycle is 30%,the frequency is 15 Hz,and the base current is 1.60 A,the equiaxed grain area of WZ is the largest and the grain size is the smallest.In the continuous zone of continuous weld pool,increasing the base current and pulse frequency at low duty cycle is conducive to the reduction of HAZ area and average grain size;when the duty cycle is 30% and the frequency is 500 Hz,the PMZ area and average grain size are the smallest;at low duty cycle,high frequency and base current,the area of equiaxed region is larger,but the average size of equiaxed region is not affected by pulse parameters;In addition,the area and average grain size of the weld under the pulse current are obviously better than that under the constant current,which shows that the reasonable pulse combination can effectively improve the structure of the weld under the condition of the constant average line energy.In addition,the area and average grain size of the weld under the pulse current are obviously better than that under the constant current,which shows that the reasonable pulse combination can effectively improve the structure of the weld under the condition of the constant average line energy.Through the study of the area and size of the microstructure and the macro forming of the weld under different pulse parameters,the theoretical basis for improving the welding process can be provided. |
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