Research On Laser-TIG Hybrid Welding Of Chinese Low Activated Ferritic/Martensitic (CLAM) Steel

Reduced Activation Ferritic/Martensitic（RAFM）steel is considered as the structural material in the International Thermonuclear Experimental Reactor（ITER）Test Blanket Module（TBM）.Numerous steels（9Cr-2WVTa,F82H,JLF-1,ARAA and India RAFM steel）have been classified as RAFM steels.Simultaneously,CLAM steel is selected as the primary promising candidate of structural material for the Chinese TBM for ITER,several issues such as Pb-Li corrosion,tritium permeation,tritium leakage and magnetohydrodynamics effects need to be addressed.Fusion welding is considered as a key technology to obtain good quality of cladding member.Various welding process such as hot isostatic pressing（HIP）,electron beam welding（EBW）,tungsten inert gas（TIG）welding and laser beam welding（LBW）have been used to join CLAM steel.However,in the public domain,the report on Laser-TIG hybrid welding of CLAM steel is very limited.In this paper,the Laser-TIG hybrid weld joints of China low activation martensite（CLAM）steel were produced,and then experienced different post weld heat treatment（PWHT）to achieve a good combination of strength and toughness.The microstructure and mechanical properties of the joints were investigated.Meanwhile,the performance of weld joints before and after PWHT subjected to transient heat loads were studied by using a laser beam thermal load test to simulate the transient events in the future fusion reactors.The results indicate that the microstructure of the as-deposited weld metal（WM）was composed of coarse lath martensite and a small amount of hard delta-ferrite,leading to relatively high tensile strength,but low impact toughness（only 16.5%of parent metal）.After PWHT,M₂₃C₆ type carbides were precipitated along lath martensite and prior austenite grain boundaries.With the prolonging of PWHT,the amount and size of precipitates were increased.After PWHT at 760°C for 2 h,the precipitates were aggregated.After PWHT,The strength of the weld joints was slightly weakened but acceptable（⁵46 MPa-604 MPa）,while the toughness of the weld joints significantly increased,with evidence of many dimples in the ductile fracture surface.After the material undergoes laser transient thermal loading,surface damage such as cracks,pits,re-solidified metal and oxidation were observed.With the increase of transient thermal loading power density,the surface morphology response of CLAM steel base metal and weld seam changed accordingly,and the diameter of damaged annular zone increased.The number and width of cracks in the center region became large and dense.The non-uniformity of the microstructures of the WM leaded to different response morphologies.The metal burns at the intersection of fusion zone（FZ）and completely quenched zone（CQZ）where the microstructure was not uniform had a greater hardness was more severe than that of the WM,and the performance is poorer against high-power thermal shock.After transient heat loads with laser power densities up to 205.7 MW/m²,the WM subjected to PWHT at 760°C for 0.5 h appeared better performance of resistance to oxidation.Hence,PWHT at 760°C for 0.5 h was suggested for the Laser-TIG hybrid weld joints of CLAM steel,which gave rise to sufficient strength,high toughness and excellent resistance to oxidation.