Fracture toughness of laser powder bed fusion-produced AlSi10Mg: Microstructural effects of heat treatment and build orientation

Laser powder bed fusion (LPBF) of AlSi10Mg produces an ultrafine cellular α-Al microstructure with an interconnected Si network, conferring high strength but raising questions about anisotropic fracture resistance after heat treatment. This work quantifies how post-build heat treatments and build orientation control fracture toughness in LPBF AlSi10Mg and links these changes to microstructure. Compact-tension specimens were produced in three orientations (XY, XZ and ZX) and tested in the as-built (AB), direct ageing (DA), and two T6-type conditions. In the AB state, the alloy exhibits high ultimate tensile strength (UTS 415–426 MPa) and modest ductility (8–13%), together with strongly orientation-dependent JR-curves, with initiation toughness 𝐾 𝐽 ⁢ 𝐼 ⁢ 𝑐 ranging from 27 to 39 MPa√m between ZX and XY. DA markedly increases both toughness and isotropy, giving a 𝐾 𝐽 ⁢ 𝐼 ⁢ 𝑐 of 45–49 MPa√m, at the expense of reduced strength ( 𝜎 𝑦 of 142–150 MPa) but enhanced ductility (18–19%). Short and long T6 treatments provide intermediate strength–ductility combinations but significantly lower fracture toughness ( 𝐾 𝐽 ⁢ 𝐼 ⁢ 𝑐 typically 23–34 MPa√m) and flatter JR-curves. Microstructural characterisation shows that all states retain elongated columnar α-Al grains and a high fraction of high-angle boundaries; fracture behaviour is instead governed by precipitation mechanisms and dislocation recovery. This systematic, orientation-resolved dataset helps to separate the roles of intrinsic (crack-tip plasticity) and extrinsic (crack-path tortuosity) toughening in LPBF AlSi10Mg and suggests that, for the present alloy and processing route, there is a practical limit to fracture resistance improvements achievable by post-build heat treatment alone.