Effect of as-cast microstructure on precipitation behavior and thermal conductivity of T5-treated Al[sbnd]7Si[sbnd]0.35Mg alloy

Abstract

This study examines the precipitation evolution during T5 heat treatment (casting and direct aging) of an Al7Si0.35Mg alloy component with varying thicknesses. We employ a step mold casting technique to develop an Al7Si0.35Mg alloy component with cavity thickness ranging from 4 to 40 mm. Our experimental investigations unveil varying solidification times (from 5.0 to 152.2 s) and cooling rates (ranging from 2 to 50 K/s) due to the step mold casting. The thermal conductivity of component sections increase from 174.1 to 178.9 W/mK as the cooling rate surged to 50 K/s. Microstructure analysis of 50 K/s alloy reveals a large number density of coarse Mg-Si-rich pre-precipitates near dendritic cell boundaries, alongside fine GPII and β″ pre-precipitates within the cell interior. The pre-precipitation behavior in 50 K/s alloy is attributed to the retention in the mold until thick sections fully solidify, leading to low solute supersaturation and higher conductivity. Contrarily, 2 K/s alloy exhibits the formation of fine dot-like β″ and a few coarse Mg-Si-rich pre-precipitates along the cell boundaries. This alloy demonstrates superior age-hardening behavior and its low thermal conductivity increases during aging. This increment is associated with a small lattice misfit between precipitates and matrix in the as-cast state, inducing significant lattice distortion which lowers the thermal conductivity. Upon peak aging, these distortions are replaced by dislocations at the precipitate-matrix interface due to large lattice misfits. Finally, we offer an insight into how the solidification microstructure affects T5 treatment considering hardness and conductivity synergy.