Short-Crack Growth Behavior in Additively Manufactured AlSi10Mg Alloy

The very high cycle fatigue (VHCF) behavior of an additively manufactured aluminum alloy, AlSi10Mg-T6, has been studied to better understand the role of process-related defects on fatigue life and, in particular, initiation and small crack propagation behavior. Ultrasonic fatigue methodologies were used for the determination of an operative fatigue strength for lifetimes in the range of 10 8 -10 9 cycles. Fatigue crack growth rates were determined for small fatigue cracks that initiated and grew from artificially produced (FIB) surface defects in specimens subjected to ultrasonic fatigue. X-ray computed tomography (CT) was used in an effort to detect possible crack initiation and growth in specimens cycled for fractions of the nominal VHCF fatigue strength. From the baseline fatigue experiments using fully reversed loading ( R = − 1), a nominal fatigue strength of 90 MPa was established for lifetimes in the 10 8 -10 9 range, and this maximum stress value was used in subsequent crack growth and CT studies. Using ultrasonic fatigue allowed very slow average crack growth rates on the order of 10 −11 m/cycle at a °ΔK of 1.5 Mpa m 1/2 . Small crack growth from FIB-produced micronotches appeared to be transgranular and non-crystallographic. While modest increase in size of some features identified by x-ray CT for interrupted fatigue tests, particularly at defects characterized as flat or oblate spheroids, no definitive evidence of the emergence of a fatal crack from a single defect was observed. This contrasts with the fatigue behavior of as-built AM AlSi10Mg alloy, where porosity-dominated fatigue crack initiation and the size and type of defects can be correlated with fatigue lifetimes.