Achieving thermally stable fine-grain strengthened copper surface via friction stir surface compositing with hybrid micro-nano TiC particles

Thanks to its excellent thermal conductivity, copper material is widely used in industrial plate heat exchangers within the nuclear energy and power generation industries. However, due to its relatively poor mechanical strength, the copper components requires surfaces strengthening. One viable approach is to form a fine-grained layer with enhanced mechanical properties through surface mechanical treatment. However, the fine-grained structures are prone to grain coarsening and even abnormal grain growth (AGG), leading to rapid deterioration of strengthening. In this study, we successfully implemented friction stir surface compositing (FSSC) on pure copper with hybrid micro/nano-TiC particles under ultralow heat-input, achieving a low processing peak temperature of 367.3°C. The TiC particles provided effective Zener pinning on dislocation migration and grain boundary coalescence, resulting in a strengthened fine-grained surface with remarkable microstructural and mechanical thermal stability. After 700°C-30min thermal exposure, FSSC specimens maintained 83 HV surface hardness (50.9% increase over base material) and 203.2 MPa tensile strength (54.5% enhancement). Notably, the size of TiC particles performed a temperature-adaptive relationship with the thermal stability of the FSSC-processed surface. At intermediate temperatures (400 °C), 40 nm TiC particles provided superior thermal stable effect, while at high temperatures (700 °C), 5 μm TiC particles exhibited better thermal stable improvement. Furthermore, in addition to fine grain strengthening and dislocation strengthening mechanisms, the FSSC specimens benefited from Orowan strengthening and twinning-induced plasticity (TWIP) effects induced by TiC particles, achieving a strength-ductility synergy compared to the FSSP. This study provides a practical example for surface strengthening of copper alloys in high-temperature service environments.