Additive manufacturing of metallic heterostructured materials: from 1D to 4D

Additive manufacturing (AM) offers unique opportunities for the design and fabrication of metallic heterostructured materials (HMs) with complex geometries and programable spatial heterogeneity. Unlike traditional HMs fabrication processes, AM progresses from the melt pool to the layer and finally to 3D objects. Thus, the melt pool and layer are unique units for forming HMs, and AM-processed HMs have unique categories based on hetero-zone distributions at different melt pools and layers. This review summarizes research progress in AM-processed HMs with categories across 1D to 4D frameworks according to the spatial distribution and functional evolution of heterogeneous zones. Specifically, 1D HMs refer to linear-format multi-materials with one immediate interface or gradient region either parallel or perpendicular to the build direction, while 2D HMs are multi-material featured with layer-wise multiple interfaces along the build direction or transverse direction. 3D HMs are spatially distributed multiple materials with multiple interfaces along both horizontal and vertical directions. 4D HMs extend 1D-3D HMs by introducing stimulus-responsive functions, enabling time-dependent changes in shape, properties or functions under external stimuli. Representative AM routes, interfacial defects, microstructural evolution, deformation and fracture behaviors and strengthening mechanisms are discussed, followed by prospectives for future development of AM HMs.