Extraordinary specimen-size effect on long-life fatigue of additively manufactured AlSi10Mg

It is well known that the mechanical properties of materials are strongly influenced by the size of the testing specimens, especially for the defect-induced high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF) of high-strength alloys with failure cycles beyond 10⁷. Typically, this phenomenon of specimen-size effect becomes more pronounced as the fatigue life extends, and the fatal fatigue-crack initiates within a risk domain of control volume V₉₀ where the applied stress is equal or larger than 90 % of the nominal one. Here, for the first time, we report a newly observed specimen-size effect on fatigue behavior of an additively manufactured aluminium alloy (AlSi10Mg) produced by powder bed fusion - laser beam. Fatigue tests were precisely conducted by ultrasonic cycling at a resonant frequency of 20 ± 0.5 kHz under a stress ratio R = –1 at room temperature and in ambient air. Five types of specimens with minimal diameters of 3.5, 5.3, 8.1, 12.25 and 18.6 mm were tested. As specimen size increases, the HCF resistance drops sharply and exhibits a large scatter, whereas the VHCF limit degrades only slightly. For small-sized types, fatal crack initiates at specimen surface, subsurface or interior with the increasing failure cycles. But for large-sized types, fatal crack nucleates frequently in specimen interior even with “fish eye” morphology in HCF regime. Furthermore, the crack initiation site gradually tends to shift beyond the control volume part with the increase of specimen size, making the concept of V₉₀ challenging for large-sized specimens, regardless of fatigue lives.