Journal of Materials Research and Technology (Jul 2022)
High cycle fatigue behavior of bimetallic Al 7025/CP-Mg rods produced by rotary swaging
Abstract
This study aims to investigate the monotonic uniaxial tensile/compressive properties and the high cycle fatigue behavior of aluminum alloy 7025 (Al 7025)/commercially pure magnesium (CP–Mg) bimetallic rods produced via rotary swaging method at ambient temperature. In these lightweight bimetallic rods, the outer layer of aluminum can protect the magnesium core from corrosion and improve the rods' mechanical properties. The microhardness and microstructure were studied in the cross-section of the rods to investigate the manufacturing process effects on the starting materials. Based on the obtained results, under the compressive loading, the ultimate strength of the Al/Mg sample increased by 34% compared with the pure magnesium sample. Furthermore, while only 31% of the bimetallic cross-section is composed of aluminum, the tensile and compressive yield strengths of Al/Mg specimens are closer to those of the aluminum specimens. Microhardness results and the microstructural evaluation indicate that no significant chemical interface is formed between the layers. According to the fatigue test results, adding the outer layer of aluminum can increase the fatigue life more than ten times, compared with the monolithic magnesium specimens. The aluminum layer acts as a deterrent agent, delaying crack initiation and causing such substantial life improvement. The aluminum layer plays a more significant role at the lower stress levels, where most of the fatigue life is spent on crack initiation. Based on the finite element analysis, compared to the monolithic magnesium specimen at the same position, adding an aluminum layer around the magnesium core causes a 46.8% reduction in the maximum stress at the surface of the magnesium core in the bimetallic specimen. The fractography shows that the cracks initiated at the surface and propagated through the aluminum sleeve are barricaded by Al/Mg interface, resulting in new crack initiation sites on the magnesium core.
