Negative strain rate sensitivity of yield strength of Al-Si alloy additive-manufactured using laser powder bed fusion

The present article reports an unexpected finding of negative strain rate sensitivity of strength (at lower strain than approximately 3 %) of the Al-12%Si alloy manufactured by laser powder bed fusion (L-PBF) process. The post heat-treated specimens exhibited positive strain rate sensitivity (higher strength at higher strain rate), whereas all specimens exhibited no serrated stress-strain curve, indicating a different mechanism from the Portevin-LeChatelier effect. Transmission electron microscopy revealed the dynamic precipitation of nano-sized Si-phase inside the supersaturated solid solution of the α-Al matrix in the as-manufactured specimen deformed at ambient temperature. The dynamically precipitated Si-phase could interact with the introduced dislocations and enhance the flow stress at an early stage of plastic deformation (in particular, at a lower strain rate). The dynamic precipitation would contribute to the negative strain rate sensitivity of strength. The present results provided new insights to control the unique strain-rate dependent strength of the L-PBF manufactured Al-Si alloys, in terms of the degree of supersaturation (concentrated solute element) controlled via the L-PBF process.