Fatigue Behaviors of Newly Developed Eco-7175-v3 Extruded Aluminum Alloy

Abstract

In this study, the fatigue characteristics are evaluated for a new extruded aluminum 7175 with an experimental composition that uses a magnesium-calcium alloy during the alloying process instead of the standard pure magnesium. Specimens of ECO-7175-v3 were fabricated and subjected to fatigue life testing, fatigue life data analysis, and observation of their fracture characteristics through optical microscopy and scanning electron microscopy (SEM), and metallography to study their grains and surface characteristics. The S-N curve shows that the fatigue life for the new fabricated ECO-7175-v3 aluminum can exceed 5×107 cycles, with a fatigue strength of approximately 220 MPa or less, about 40% of its tensile strength. The fatigue strength of ECO-7175-v3 is an improvement over ECO-7175-v1, which is also shown by its higher fatigue strength coefficient, σF′, of 1,589.7 MPa. When ECO-7175-v3 specimens were subjected to stresses of up to 67% of the ultimate tensile strength (UTS), the fracture surfaces are shown generally to have softly defined fracture features and propagation bands. These fracture surface characteristics are unique to ECO-7175-v3 and are attributed to its larger average grain size. Irrespective of the stress amplitude, like ECO-7175-v1, all crack initiation points of all specimens are seen at the surface and no inclusions to act as stress concentrators are seen. Reliability analysis shows the hazard rates for ECO-7175-v3 remain relatively constant most of the time before increasing towards the end. The trend of the hazard rates indicates failures are due to wear-outs and not due to defects. Reliability analysis also shows that at any given fatigue life cycle and stress level, ECO-7175-v3 has a lower probability of failure compared to ECO-7175-v1.