Microstructures and mechanical properties in friction stir repaired cracks of Cu crystallizer

Objective Aiming at the problems of grain boundary fatigue crack and wear of Cu-Cr-Zr plate during service in steel continuous casting mold fields, solid state repairing process of surficial cracks is essential. Methods A method of friction stir repairing (FSR) for copper crack was proposed. 3 mm thick Cu-Cr-Zr plates were selected as research object. The cracks with cross section size of 1 mm × 1 mm and 2 mm × 2 mm were chosen as the examples, while FSR with and without a pin were used respectively. Results After FSR, 1 mm × 1 mm and 2 mm × 2 mm surface preset cracks completely disappeared and no defects appeared in the repair area. After repair, hardness distribution of cross section presented roughly W shape, and heat input of pinless FSR was small. The hardness of the repair area could reach more than 80% that of base material, tensile strength reached 233 MPa, up to 80% that of base material. The heat input of FSR with a pin was high, and tensile strength of the repair area reached 223 MPa, which was 76% that of base material. FSR with a pin introduced mechanical stirring action, promoting the flow of materials. The repair area expanded from a narrow arc to a bowl without a pin, which was suitable for the repair of large size cracks. Conclusion FSR without a pin could repair small size crack, FSR with a pin could further broaden the repair size of cracks, which confirmed that FSR was an effective method to realize repair of high thermal conductivity copper components. Aiming at the future larger size components, additive FSR could be considered.