Research On Microstructure And Performance Of High Power Laser Welded Joints For CLF-1 Steel Used In Nuclear Fusion

The Test Blanket Module（TBM）is one of the three major projects in the International Thermonuclear Experimental Reactor（ITER）program.It provides important theoretical and applied support for the design and development of future demonstration fusion reactors and commercial reactor blankets.The domestically developed China Low Activation Ferritic/Martensitic steel（CLF-1 steel）exhibits good mechanical properties in the harsh neutron irradiation environment and is an important candidate structural material for TBM.The microstructure of CLF-1 steel consists of tempered martensite and dispersed carbides,with high impact toughness（237 J）and a significant amount of alloying elements.However,conventional welding of CLF-1 steel presents significant challenges due to large variations in mechanical properties and differences in microstructure and chemical composition across different regions of the welded joints.In this study,we focus on the urgent need for TBM welding and investigate the microstructure and properties of 10 mm thick CLF-1 steel welded by high power laser welding（HPLW）.A combined heat source model,which includes a Gaussian-distributed surface heat source and a conical heat source,has been employed.This model was used to establish a finite element simulation for 10 mm thick CLF-1 steel subjected to HPLW.It was utilized to simulate and calculate the temperature distribution within the weld and the residual stresses for welding heat input values（Q）of 2727 J/cm,3000 J/cm,3563 J/cm,and 4500 J/cm,respectively.The results indicate that the simulated temperature distribution closely matches the actual temperature distribution during the welding process.The simulated cross-sections of the weld and the heat-affected zone closely resemble the actual joint cross-sections,with a maximum error of≤0.04mm.The Von Mises stresses within the joints exhibit an irregular distribution for different Q values,with a maximum value of<200 MPa.The transverse residual stresses in the center of the weld are consistent with the longitudinal residual stress distribution and are in good agreement with the experimental data obtained at lower Q values.As the heat input（Q）decreases,there is a trend of decreasing peak temperature,Von Mises stresses,transverse residual stresses,longitudinal residual stresses,and deformation within the weld.The finite element simulation of HPLW provides precise predictions for residual stresses and deformation during the actual welding process of the TBM structure.As Q gradually increases,the average width of lath martensite in the weld metal gradually increases,and both the average volume fraction and width ofδ-Fe show clear growth trends.However,within the martensite plates,there is no precipitation of Ta-rich MX dispersion-strengthening phases.This leads to a tendency towards brittleness in the microstructure,a reduction in high-density dislocations,and is the fundamental reason for the lower impact toughness of the weld metal.When Q is 2727 J/cm,the impact toughness of the weld metal reaches its highest at 40 J.For the low-Q weld metal specimens,post-weld heat treatments were conducted,including 710°C/2 h（referred to as PWDT）,980°C/1 h+710°C/2 h（referred to as PWNT）,PWDT secondary cycle（referred to as PWDT-SC）,and PWNT secondary cycle（referred to as PWNT-SC）.In the PWDT state,δ-Fe still remains,and the size of lath martensite slightly decreases.Large elliptical-shaped M₂₃C₆ carbides precipitate along the original austenite grain boundaries and the boundaries of lath martensite,and a significant amount of Ta-rich MX-type carbides with a dispersed circular distribution precipitates within the lath martensite.Calculations indicate that the pinning force of these two types of carbides within the grains reaches its maximum,significantly enhancing the dispersion strengthening effect.This results in the weld metal achieving an impact performance（236 J）equivalent to that of the base metal,and the joint exhibits good creep resistance.In the PWNT state,the lath martensite undergoes complete phase transformation recrystallization,δ-Fe disappears,and the microstructure of the weld metal and the heat-affected zone becomes similar to the base metal.However,there is a slight increase in size.Due to the intensified solute segregation,grain growth,and an increased density of lattice vacancies,these factors induce the precipitation and coarsening of M₂₃C₆ carbides.This disrupts the intergranular bonding strength in the microstructure,reduces the pinning force of the microstructure,and subsequently decreases the dispersion strengthening effect.This leads to an impact toughness of 156 J in the weld metal.In the PWDT-SC state,the impact toughness significantly decreases to 112 J.This is caused by the increased size and volume fraction of M₂₃C₆ and MX carbides in the weld metal,which are attributed to microstructure inheritance and the re-precipitation of carbides.The PWNT-SC state exhibits similar microstructural characteristics and impact toughness to the PWNT state.Under low Q conditions（2727 J/cm）,the impact of tantalum（Ta）content（0%,0.26%,0.54%,and 0.81%by mass）on the microstructure and mechanical properties of the weld metal reveals that as the Ta content increases,the quantity and size of lath martensite andδ-Fe in the weld metal gradually decrease.Meanwhile,the quantity and size of co-precipitated ferrite（F）and pearlite（P）increase gradually.When the Ta content goes from 0%to 0.26%,the weld metal is composed of a significant amount of lath martensite,approximately 0.47%δ-Fe,about 13.1%F,and roughly 6.6%P.The appearance of F and P reduces the weld’s susceptibility to brittleness.Furthermore,within the martensite plates,the quantity of nanoscale MX carbides enriched with Ta gradually increases,enhancing the dispersion strengthening effect.This leads to an increase in the weld metal’s impact toughness from the original 40 J to approximately 82.3 J,an improvement of about 2.06 times.When the Ta content increases from 0.26%to 0.81%,the lath martensite andδ-Fe in the weld metal gradually disappear.Conversely,the quantities of F and P increase progressively,along with their sizes.Simultaneously,coarse Ta C carbides（approximately 100 nm）precipitate within the weld metal.This results in a significant decrease in impact toughness,down to 8 J.During the high power vacuum laser welding（HPVLW）process,as the vacuum level increases,the oxygen content in the weld decreases.This effectively suppresses the keyhole’s area,height,and brightness near the weld,enhancing the weld’s absorption of the laser beam.Consequently,this further reduces the heat input（Q）and diminishes theδ-Fe content in the weld.It also reduces the width of the martensite plates,increases the density of high-density dislocations,and results in a more uniform distribution and smaller size of M₂₃C₆ carbides in the PWDT-state weld.Additionally,there is an increase in the density of MX carbides,with a significant rise in the Ta content,going from 3.53 wt.%to 31.35 wt.%.This enhancement leads to an increase in the impact toughness of the PWDT-state weld,surpassing that of the base material with a value of 280 J.The impact toughness of the as-welded state（162 J）also sees a substantial improvement.The microstructure and mechanical properties of HPVLW PWNT-state welds are essentially similar to those of HPLW PWNT-state welds.