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• The EBW test of different weld penetration and scale FW with W/Cu sheets was realized. • When the weld penetration is small, the surface focus welding is the best. Otherwise, the lower focus welding is the best. • Reduced scale FW welding with weld width less than 4 mm and good surface forming is realized. Water- cooled ceramic breeder (WCCB) blanket is one of the candidate blanket for China Fusion Engineering Test Reactor (CFETR) independently developed by China Institute of plasma physics. The first wall (FW) is a component of WCCB, which directly faces the plasma. FW contains flow channels and has complex shape structure, therefore, it is difficult to complete the full-scale manufacturing by one kind of welding process. Hence, in the development process, the components of FW are first connected by hot isostatic pressing (HIP), and then connected by electron beam welding (EBW), so as to form the full-scaled FW connection. In this paper, the EBW process of FW was studied. The results showed that when the electron beam current was 50 mA and the focus was on the surface, the welded joint with 20 mm penetration and good shape can be formed. After the one time welding and the twice welding (overlapping first welding) of the weld, the tensile test fracture occurred in the base metal. The impact energy of the one time welding was 13 MPa, 18 MPa, and 21 MPa, respectively, and the impact energy of the twice welding was 19 MPa, 20 MPa, and 51 MPa, respectively. The microstructure of the joint was composed of δ-ferrite and lath martensite. When the weld penetration was increased to 50 mm, the lower the focus is, the smaller the porosity at the bottom of the weld is. Similarly, when the lower focus was used, the porosity at the bottom of the weld disappeared basically; The EBW test on the scaled FW with W/Cu sheets showed that when the we took the test pieces with the W-Cu sheet spacing is 4 mm, the welded joint with good surface shape and no internal defects can be formed without scanning deflection electron beam welding, and ultrasonic testing showed that EBW does not affect on the HIP interface. This provides a favorable basis for the future test of equal ratio FW test piece and the welding of actual FW. [ABSTRACT FROM AUTHOR]