Precipitation behavior of an Al7SiMg alloy processed by laser powder bed fusion during non-isothermal and isothermal heat treatments

Laser powder bed fusion (LPBF) is drawing great interest in industry due to its ability of producing complex-shaped components with superior mechanical properties and unique microstructures. Al7SiMg alloy is one of the most used aluminum alloys in LPBF due to its easy processing window. Direct T5 aging is often applied to LPBF-processed Al7SiMg alloy to obtain better mechanical performances over T6 heat treatment. However, after T5 heat treatment, a comprehensive understanding of the precipitation sequence of the strengthening phase in LPBF-processed Al7SiMg alloy is still lacking. In the present study, a high-resolution TEM examination of the precipitation behavior of Al7SiMg alloy processed by LPBF during heating without and with isothermal holding was conducted to further understand the precipitation sequence of Si precipitates and the associated microstructure evolution. During the heating process, irregular and needle-shaped pure Si precipitates were observed which shared the same [011] zone axis with the aluminum matrix. After analyzing the phase transformation process during heating, the following precipitation sequence is proposed: Si in supersaturated solid solution → Si-rich clusters → Si-rich intermediate phases → pure Si precipitates. During isothermal holding at 160 ℃ (T5), nano-scale Si-rich clusters were observed at the peak aging condition. When the holding time extended to 24 h, larger Si-rich clusters with reduced coherence were found. Strain fields caused by these Si-rich clusters are believed to strengthen the LPBF-processed Al7SiMg alloy after T5.