Nickel-aluminum bronze (NAB) alloy design under two-steps casting and submerged friction stir processing

Abstract

This research aimed to investigate the impact of friction stir processing (FSP) as a modifying technique on the microstructure and mechanical properties of Cu–10Al, Cu–10Al–5Ni, and Cu–10Al–5Ni–5Fe nickel-aluminum bronze (NAB) alloys under two different conditions of ambient and underwater submerged conditions concerning the as-cast state of these materials. Adding nickel as an alloying element to the Cu–10Al composition resulted in the formation of NiAl intermetallic compound and a 3 % increase in the volume percentage of secondary phases. Furthermore, incorporating iron into the Cu–10Al–5Ni composition prevented the formation of detrimental γ2 phase and instead created κ intermetallic compounds within the structure, leading to an overall increase of approximately 14 % in the volume percentage of secondary phases compared to the Cu–10Al composition. The initial coarse, rough, and heterogeneous structure of the cast samples was transformed into a fine, equiaxed, and homogeneous structure through the friction stirring modification conducted in both air and underwater environments for all alloy compositions, primarily due to action of dynamic recrystallization (DRX). Notably, a finer structure was achieved when the FSP was performed underwater in the submerged state. Regarding developments in mechanical properties, FSP treatment performed in air demonstrated an average enhancement of 15 % in strength and hardness across all three compounds. In contrast, the flexibility of Cu–10Al, Cu–10Al–5Ni, and Cu–10Al–5Ni–5Fe alloys in terms of elongation to failure was reduced by 70 %, 80 %, and 20 %, respectively, in comparison to the cast samples. Conversely, the underwater submerged FSP led to a notable increase of 55 % in both strength and hardness for all three compounds. However, the ductility of the alloys was reduced by 80 % compared to the cast states.