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Abstract
70/30 (Cu/Zn) brass plates with a 2 mm thickness were jointed by repeated rapid cooling friction stir welding. The joints from each FSW cycle showed the typical construction of a microstructure which includes the stir zone and the thermo-mechanically affected zone, but the morphology and boundary characteristics in these zones changed with the different cycles. In the stir zone, the grain size decreased and the number fraction of the high angle boundaries increased with the increasing number of FSW cycles. The texture analysis suggested that the post-annealing effect, which frequently occurred after the FSW process, was remarkably restricted by the liquid CO2 cooling, which accelerated the refinement of the microstructure. As a result, a joint with an ultrafine grained structure (0·8 μm) and an excellent strength ductility matching (548 MPa and 34% respectively) can be achieved by multi-pass rapid cooling FSW process.
Acknowledgements
The authors are thankful for the financial support of the project for heterostructure control from the Japan Science and Technology Agency (JST), the Priority Assistance for the Formation of Worldwide Renowned Centers of Research–The Global COE Program (Project: Center of Excellence for Advanced Structural and Functional Materials Design) and the Cooperative Research Project of Nationwide Joint from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, and a Grant-in-Aid for Science Research from the Japan Society for Promotion of Science.