Microstructure and mechanical properties of laser welded dissimilar materials joints between CLF-1 and ITER-grade 316LN steels for nuclear fusion reactor

The structural integrity of the Test Blanket Module (TBM) in the International Thermonuclear Experimental Reactor (ITER) depends on robust dissimilar joints between China low-activation ferritic (CLF-1) steel and ITER-grade 316LN (316LN-IG) austenitic stainless steel. In this study, CLF-1/316LN-IG dissimilar steel joints were fabricated by laser welding, and the microstructural evolution in the weld metal (WM) and heat-affected zone (HAZ) was systematically investigated by multiscale characterization combined with mechanical testing, with particular emphasis on correlating microstructure with mechanical performance. The WM consists of columnar austenitic dendrites and lath martensite, while the HAZ of CLF-1 is distinguished by the presence of intragranular TaC carbides and intergranular (Cr, W)23C6 carbides. The welded joint exhibited excellent overall tensile performance, with an ultimate tensile strength of 619.0 MPa, comparable to that of 316LN-IG, a yield strength of 365.8 MPa, 11.5% higher than 316LN-IG, and the total elongation reaches 45.5%, 70.4% higher than CLF-1. Tensile fracture occurs in the base material far from the weld. The impact absorbed energy of the weld is intermediate between those of CLF-1 and 316LN-IG steel. The enhancement of strength is attributed to the high dislocation density in the lath martensite within the WM, and to the strengthening effect of nanoscale carbides in the CLF-1 HAZ. Moreover, first-principles and thermodynamic calculations elucidate the phase stability of TaC and (Cr, W)23C6, providing a fundamental basis for the structural and materials design of TBM components.