Influence of cooling rate on the microstructure and mechanical properties of Al–Cu–Li–Mg–Zn alloy

Abstract

This work investigated the influence of cooling rate (0.04, 0.87, 15.2 and 156.2 K/s) on the microstructure and compressive properties of Al–4Cu–3Li-0.7 Mg–1Zn alloys fabricated by several casting molds. The experiment results revealed that the microstructure cooled at low cooling rates of 0.04, 0.87 and 15.2 K/s is composed of α-Al, α(Al)+T2, θ (Al2Cu), T1 phases and core-shell configuration (Al13Fe4/Al7Cu2Fe phase). This core-shell structure is observed for the first time in an Al–Cu–Li alloy. However, T1 and core-shell configuration vanish at the high cooling rate of 156.2 K/s due to the insufficient diffusion time of Cu, Li and Fe elements. As the cooling rate increases, the average secondary dendrite arm spacing (SDAS) decreases significantly from 110.3 to 7.5 μm and the relationship between SDAS and cooling rate has been established. The average diameter/thickness of the α(Al)+T2, Al2Cu and Al13Fe4/Al7Cu2Fe phases decreases dramatically from 16.9 to 1.0 μm, 8.1 to 0.8 μm and 7.5 to 0.7 μm with the increase of cooling rate, respectively. Fine spherical Al13Fe4/Al7Cu2Fe and Al2Cu phases are beneficial to the improvement of comprehensive compressive properties. Additionally, the solute concentration in the matrix decreases, while the hot-tear resistance and volume fraction of secondary phases increase with increasing cooling rate. The empirical equations are established between (compressive properties, SDAS) and cooling rate. The fracture failure is responsible for the initial hot tearing at low cooling rates, the α(Al)+T2 or Al2Cu or Al13Fe4/Al7Cu2Fe phases at the moderate cooling rate, and the α(Al)+T2 phase at the high cooling rate.