Microstructural, mechanical and electrical properties of aluminum-copper butt joints produced by high-speed friction stir welding

Driven by the accelerating trends for lightweighting and electrification of vehicles, friction stir welding (FSW) of aluminum with copper is gaining increasing attraction in the automotive industry. Therefore, this work investigated FSW of 1.6 mm-thick oxygen-free high conductivity (OFHC) copper (Cu99.95) and 2.0 mm-thick EN AW-1050A aluminum (Al99.5) sheets. The microstructure of the butt joints and the mechanical and electrical properties of the welded blanks were analyzed. The experimental methods included scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD), micro-computed tomography (μCT), hardness mapping, quasi-static uniaxial tensile testing, three-point bending and resistivity/conductivity measurements. Even at the high welding speed of about 1 m/min force-controlled FSW was successful with a tool offset towards the aluminum sheet. Sound joints with finely dispersed copper particles in the weld and with a 0.5 μm-thin intermetallic layer between the weld and the copper sheet were obtained. Tensile strength, fracture strain and hardness of the defect-free joints were about a quarter lower, but their electrical conductivity was slightly greater than those of the aluminum sheet. The decrease of the mechanical properties was mainly due to dynamic recrystallization (DRX), whereas the increase of the conductivity was due to the dispersion of copper particles in the weld. However, decreasing the forge depth caused sporadic seizure and release of the weld metal by the tool shoulder and reduced the forge pressure applied on the weld metal. Accordingly, the material flow became unstable, open voids formed due to lack of refill on the weld surface, and lack of bonding between the rings of the stir pattern occurred. These defects impaired the tensile properties of the blanks, but their influence on the electrical properties was less significant. • Friction stir butt welding of thin Al-Cu blanks was successful at the high welding speed of 1 m/min. • Aligning the pin exactly to the butt between the sheets was crucial for achieving defect-free joints. • Neither excessive mixing of Al with Cu nor notable formation of brittle intermetallic phases occurred. • The thin intermetallic layer between weld and Cu sheet did not affect the mechanical/electrical properties. • The electrical conductivities of the Al-based weld and of the Al sheet did not significantly differ.