Transition from unstable defect-dominated to stable precipitation-controlled behavior in laser powder bed fused AlSi7Mg via direct hot isostatic pressing

Abstract

This study investigates how direct hot isostatic pressing (HIP) post treatment influences the density, microstructure, electrical, thermal conductivity, and mechanical properties of AlSi7Mg components fabricated by laser-based powder fusion (PBF-LB). PBF-LB samples were produced and subsequently subjected to a thermo-mechanical HIP process (180-480 °C, 2000 bar, 2 h). The As-built specimens exhibited a relative density of 99.97 %, tensile strength of 418.7 MPa, yield strength of 267.7 MPa, and elongation of 13.7 %, with electrical and thermal conductivities of 28.1 % IACS and 130 W/m·K, respectively. Increasing HIP temperature effectively eliminated internal pores and relaxed residual stress without inducing recrystallization, while promoting Si precipitation and coarsening within the α-Al matrix. At the HIP temperature of 180 °C, partial stress relaxation occurs however, the Si cellular network and dislocation structures are largely preserved, enabling the microstructure to remain controlled and resulting in strength comparable to the As-built condition while maintaining a relatively high specific strength. After the HIP process at 480 °C, the electrical conductivity and thermal conductivity increased to 47.2 % IACS and 194 W/m·K, respectively, with a relative density of 99.999 %, while elongation increased to 31.4 %. Overall, the direct HIP provides an efficient route to achieve nearly pore free, stress relieved, and thermally stable AlSi7Mg components with excellent conductivity and ductility. These results reveal a clear transition from a defect dominated unstable strengthening mechanism in the As-built condition to a densification, precipitation and recovery controlled stable strengthening mechanism by applying direct HIP process