Metallurgical and mechanical property investigation of ETP Cu similar hybrid friction welds for nuclear fusion applications

The evolution of microstructure and mechanical properties in electrolytic tough-pitch (ETP) copper subjected to hybrid friction welding (HFW) was investigated. The effect of introducing a programmable brake on the flywheel’s rotation while maintaining constant friction and forging pressures was explored. The welded joints were examined through a series of investigations, including optical and scanning electron microscopy with electron backscatter diffraction, tensile and microhardness testing, followed by fractography. The optimised weld achieved the highest ultimate tensile strength (≈274 MPa), a 7.87% increase over the base material, and the greatest elongation (~22%), with ductile fractures and visible necking. The measured hardness distribution depicted higher values for the weld zone compared to the thermo-mechanically affected zone, both expressing a significant decrease from the heat-affected zone and base material due to pre-welding strain hardening existing within the copper alloy. EBSD analysis on the weld zone and thermo-mechanically affected zones revealed the presence of refined grains, with low grain orientation spread (GOS) and kernel average misorientation (KAM) values, which indicated the occurrence of dynamic recrystallisation (DRX) mechanisms during HFW. HFW was able to successfully produce ETP Cu-similar welds with 98.1% joint efficiency.