The effect of macro and microstructure on the mechanical and corrosion behaviour of an innovative LPBF-processed AlSi9Cu3 alloy

The rapid solidification associated with laser powder bed fusion (LPBF) promotes oversaturation of the α-Al matrix with alloying elements and redistribution of second phases, resulting in a fine-grained melt pool microstructure that strongly affects corrosion behaviour. This work investigates the corrosion response of LPBF-processed AlSi9Cu3 alloy under different heat treatments, focusing on the correlation between melt pool features, microstructure, and corrosion mechanisms. In the as-built condition, the alloy exhibited a well-defined melt pool structure with Si- and Cu-rich precipitates concentrated along the melt pool boundaries. These precipitates acted as efficient cathodic sites, driving micro-galvanic interactions and leading to an inter-melt pool corrosion morphology. Low-temperature heat treatments preserved the melt pool structure while promoting the precipitation of additional Cu- and Si-rich phases. This increased the galvanic interactions and exacerbated selective corrosion, producing a more severe inter-melt pool attack. Solution treatments dissolved and redistributed the precipitates, producing a more homogeneous microstructure. The resulting reduction of galvanic coupling significantly improved corrosion resistance and promoted a more uniform corrosion morphology. Conversely, artificial aging promoted the precipitation and coarsening of Cu-rich phases, which increased intergranular corrosion susceptibility. These findings emphasize the pivotal role of melt pool structure control and tailored heat treatments in designing LPBF aluminium alloys with improved corrosion resistance. • Corrosion morphology is linked to the melt pool macrostructure in LPBF AlSi9Cu3. • Direct aging increases selective attack due to fine Cu- and Si-rich precipitates. • Solution treatments dissolve Si- and Cu-rich phases improving corrosion resistance. • SKPFM reveals high Volta potential difference at melt pool boundary–HAZ interface.