Process And Mechanism Research Of Laser Welding Hastelloy C-276/304 Dissimilar Metals

CAP1400 is a high-power nuclear power technology with independent intellectual property rights in China.The canned motor was used in the nuclear reactor coolant pump of CAP 1400.The high end seal welding quality of pump can is essential to guarantee the safe service of nuclear coolant pump.At present the arc welding process introduced in AP1000 of USA was adopted in the end seal welding technology.In order to further improve the welding quality and at the same time accelerate the independent manufracturing progress of high-power nuclear coolant pump in China,the pulsed laser was employed to realize the seal welding of nuclear pump can and end cap/flange dissimilar metals(Hastelloy C-276 and austenitic stainless steel 304).Via the studies of weld morphology,microstructure and property,the evaluation analysis of the laser dissimilar welding process for pump can end sealing was conducted,the formation mechanism of weld metal and the evolution mechanism of micro structure were revealed,and the controlling of mechanical and corrosion resistance properties of weld joint was realized.For some defects of weld microstructure and property,the mew process that follow-up ultrasonic vibration assisted laser welding of dissimilar metals was proposed,and the further optimization of weld microstructure and ptoperty was realized.This paper will provide theoretical foundation and new process reference for pump can end seal welding manufacturing.Specific research contents and conclusions are as follows:(1)Influence of laser welding parameters on weld morphology was analyzed,optimized welding parameter scope based fine weld morphology was determined,the optimization criterion of welding parameters was proposed,and the fine welding formation of Hastelloy C-276 and 304 dissimilar metals for nuclear coolant pump can end seal welding was realized.Based on temperature field analysis by finite element,the prediction model for weld geometric features was built up,and relation of welding parameters and temperature evolution characteristic was explored.The real-time observation of welding process by high speed camera was carried out,it was found that the solidification time of welding molten pool diring single pulse cycle was 25 ms,which was shorter than the pulse cycle.In addition,the metal vapours above molten pool expanded and contracted periodically,and the cycle was 3 ms.(2)Using the SEM,TEM,EPMA and XRD,the characteristics of microstructure,element distribution and phase structure of weld metal were analyzed,the results indicated that,there was no obvious HAZ in the weld metal,and the grain in weld metal was significantly smaller than that in base metal.The unmixed zone defect was found at the interface between weld metal and 304 base metal.There were macro and miceo segregations of elements in weld metal.The major phase structure in weld metal kept as austenitic phase,however,the Mo-riched precipitated phase precipitated in grain boundary,and the precipitation of which led to the formation of Mo-lacked zone in grain boundary with the width of about 30 nm.The nominal element composition and major phase structure prediction model was obtioned.With the increase of 304 base metal dilution level,the width of unmixed zone was shortened,the nominal content of element Mo was decreased,the segregation degree of element Mo was increased,and the content of precipitated was reduced.Finally,the relation of dilution level and microstructure in weld metal was built up,and the controlling of microstructure based on dilution level was realized.(3)The mechanical and corrosion resistance properties of weld joint was evaluated by tensile-shear,mechanical tear,micro-hardness and electrochemical corrosion tests.By analyzing the influence of dilution level on the properties of weld joint,it indicated that,with the increase of dilution level,the strength and corrosion resistance property was weakened.The synergistic effect of weld morphology and microstructure on weld properties was studied,and the most optimized weld property and the corresponding welding parameter was obtained.At this welding parameter,the tensile-shear and anti-tear properties of weld joints reached maximum,the yield strength was the 124%of Hastelloy C-276 base metal,the value of micro-hardness was not lower than that of the dissimilar metals,and the holistic corrosion resistance property was close to the Hastelloy C-276 base metal.However,the tensile strength was about 83%of Hastelloy C-276 base metal,and the intergranular corrosion resistance property of weld metal was weak than dissimilar metals.(4)The new process that follow-up ultrasonic field assisted laser wedling of dissimilar metals was proposed,and the influence mechanism of ultrasonic field on the fusion and solidification behaviors and the elements diffusion rule was explored.It was found that ultrasonic field could improve the peak temperature and average cooling rate and accelerate the element diffusion rate.By analyzing the influence of ultraonsic field on the microstructure and properties of weld metal,it indicated that,with the assistance of ultrasonic filed,the width of unmixed zone was reduced;the average grain radius was reduced to 0.55 ?m,which was about half of the grain radius in pure laser welding;the segregation degree of element Mo was suppressed,and the precipitated phase content was reduced to 0.62%;the element distribution was homogenized.With the help of ultrasonic field,the mechanical and intergranular corrosion resistance properties of weld joint were improved,finally,the further improvement of microstructure and weld property was realized.