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Place Amphithéâtre V, CentraleSupelec
Very high cycle fatigue durability of additively manufactured aerospace alloys
As a new industrial revolutionary manufacturing technology, additive manufacturing (AM) plays a crucial role in the aerospace field. AM technology is increasingly used in key equipment innovations, such as advanced aeroengines (GE9X, GE) and aerospace engines (Raptor 3, SpaceX). It is expected that with the maturity of AM technology, it will be widely used in the future because of its significant advantages in near-net forming etc. Due to inevitable AM defects, the need for ultra-long-life durability of critical components in harsh service environment is increasingly evident. Focusing on challenges of very high cycle fatigue (VHCF) mechanics of AM aerospace alloys, this talk covers: Ultrasonic fatigue testing with the loading frequency of about 20 kHz was employed to research the VHCF durability of typically AM structural metals for the aerospace, such as the nickel-based superalloy (IN718 and IN939), titanium alloy (Ti6Al4V) under elevated temperatures and various stress ratios, and aluminum alloy (AlSi7Mg), which were fabricated by the laser powder bed fusion (LPBF, one of AM technique). As a result, compared with those conventional alloys, the AM alloys show lower fatigue strength. AM defects (such as gas pore and lack of fusion), matrix (columnar grains), and inclusions can act as initially fatal fatigue microcracks, and the fatigue sensitivity level depends on the location, size, and type of these defects. Fatigue design equations (K-T diagram) for both surface and interior crack initiation are successfully constructed according to the characteristic region of fracture surface after constant amplitude and two-step variable amplitude fatigue loading. The K-T diagram was utilized to identify a critical defect size associated with fatigue failure. Defect-based modified fatigue life prediction formulas were developed, which accurately predict fatigue life within a broad life range. The fine granular area (FGA) region (one of the key characteristic regions for VHCF) induced by grain refinement is composed of many nanograins, which result from dislocation movement within martensite laths. Dislocation pile-up and rearrangement in martensitic laths form dislocation cells, which further develop into low-angle grain boundaries and nanograins.
Acknowledgement: Financial support of Key Program of the National Natural Science Foundation of China (Grant No. 12332012) is acknowledged.