Effects of Residual Strain on Grain Growth of Extruded Aluminum Micro-channel Tubes During Heat Treatment

Post-extrusion cold deformation followed by brazing has a significant effect on mechanical properties of micro-channel tubes used in heat exchangers. To understand this relationship, cold rolling with different thickness reductions (0–15%) and a following brazing are conducted on extruded tubes, and the microstructure and strength are examined. The results show that with increasing rolling reduction, tensile strength and burst strength of the as-rolled tubes increase monotonously. While after brazing, both strengths decline obviously, and them decrease first and then increase with increasing rolling reduction. The change is attributed to recrystallization and grain growth observed in the post-braze tubes. Driven by different levels of strain from the rolling, recrystallized grain size increases first and then decreases. At 5% thickness reduction, abnormal large grains form in the web and result in the minimum post-braze strength. Finally, the ductile fraction mode is observed in the web, which shear strength determines the burst strength.