In-situ observation of microporosity clustering upon internal stress redistribution in wire arc additively manufactured Al-Li alloys

Microporosity is one of the most typical defects in Al-Li alloys fabricated by Wire Arc Additive Manufacturing (WAAM), and it leads to cracks under tensile loading. However, no studies have been focusing on the complex coupling phenomenon of microporosity evolution and internal stress/strain re-distribution under tensile loading and thus critical damage mechanism (delamination or transgranular propagation) in such new materials as WAAM Al-Li alloys is still un-known. To clarify this, X-ray Computed Tomography (XCT) is used to track the entire crack initiation and propagation process as a result of microporosity characteristics, including its growth, aggregation, and strain re-distribution in WAAM Al-Li alloys under tensile loading. It has been found that with increasing strain, the aggregation of large-sized micropores as a result of strain concentrations has been observed. Cracks initiate from the large-sized micropores and expand rapidly along the clusters of microporosity instead of non-weldable chain of the inter-layer region and the elongated micropores or longitudinal clusters of microporosity are responsible for the intergranular propagation of cracks instead of the transgranular small-sized porosity path. The analysis of stress redistribution and nearest neighboring of large-sized micropores during the tensile test identifies the correlation between the fracture path of WAAM components and large-sized porosity aggregation zone. Therefore, critical damage characteristics of WAAM Al-Li alloys have been found and their mechanical properties can be improved by minimizing the clustering of microporosity with a size greater than 50 μm.