Effect of Cu Addition on the Precipitation Hardening and Mechanical Properties of Al–Mg Based Cast Alloys

Abstract

This study examines the formation of different phases of Al-6 mass% Mg–xCu (x = 1 and 3 mass%) alloys in as-cast condition. Further, it investigates the dissolution of these phases upon solution heat treatment (SHT) and studies the precipitation behavior of these ternary alloys. Scanning electron microscopy with energy-dispersive spectrometry and high resolution X-ray diffraction analyses show the presence of the second phases of Al3Mg2 (β), Al6CuMg4 (T), and Al2CuMg (S) in Alloy I (Al–6Mg–1Cu), whereas Alloy II (Al–6Mg–3Cu) had only T and S second phases (with a much higher number of S phases). Upon SHT, a significant number of eutectic phases were dissolved in Alloy I, whereas in Alloy II, the number of undissolved S phases was relatively high. A differential scanning calorimetry (DSC) analysis of experimental alloys in as-quenched states reveals two exothermic peaks related to the formation of nanoclusters and S″ or S′ metastable phases. Both alloys undergo a rapid hardening stage during the aging process, in which approximately 50%–60% of total hardness was achieved. This is attributed to the formation of nanoclusters. The maximum yield strength achieved at the peak hardness condition was approximately 200 MPa for Alloy I, whereas it was approximately 160 MPa for alloy II. Alloy I took a long time to reach peak hardness, which is correlated with the stability of nanoclusters for a longer time. Earlier peak hardness in Alloy II, despite having nanoclusters, is correlated with undissolved eutectic phases acting as heterogeneous nucleation sites for the formation of S″ or S′ metastable phases.

Microstructure Evolution and Mechanical Properties of Al–Cu–Mg Alloys with Si Addition

“…Peak #2 may be linked to the formation of GPB zones or solute clusters. Peak #3 observed between 250 and 280 °C could be associated with the precipitation of S″ or S′ phases, as suggested in prior research [ 20 , 21 , 22 , 23 ].…”

“…It divides the diagram into two sections on the Al-rich corner: α-Al + Al 2 Cu (θ) + S and α-Al + Mg 32 (Al, Cu) 49 (T) + S. A quasi-binary eutectic reaction (L => α-Al + S) occurs at a Cu/Mg ratio of 2.40, resulting in various non-variant reactions in the Al-rich corner of Al–Cu–Mg ternary alloys [ 2 ]. Low-Mg-containing alloys in the α-Al + θ + S section of the diagram are primarily composed of a primary α-Al matrix and eutectic α-Al-θ phases, along with a small amount of eutectic α-Al-S phases [ 3 , 5 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 ]. High-Mg-containing Al–Cu–Mg commercial alloys have compositions lying in the α-Al + S + θ and α-Al + S sections of the diagram [ 1 , 2 , 19 , 20 , 21 ].…”

“…Low-Mg-containing alloys in the α-Al + θ + S section of the diagram are primarily composed of a primary α-Al matrix and eutectic α-Al-θ phases, along with a small amount of eutectic α-Al-S phases [ 3 , 5 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 ]. High-Mg-containing Al–Cu–Mg commercial alloys have compositions lying in the α-Al + S + θ and α-Al + S sections of the diagram [ 1 , 2 , 19 , 20 , 21 ]. However, due to their higher Mg content, these alloys have a significantly greater amount of eutectic α-Al-S phases than low-Mg-containing alloys.…”

“…However, the development of Al–Cu–Mg alloys with compositions in the α-Al + S + T section of the Al–Cu–Mg ternary phase diagram has received very little attention so far, according to the authors’ knowledge [ 19 , 20 , 21 ]. High-Mg-containing Al–Cu–Mg alloys exhibit significantly higher tensile strength compared to Al–Si-based commercial alloys in the as-cast condition.…”

“…However, compared to other heat-treated Al alloys such as A356 and A206 alloys, their response to heat treatment is less effective [ 1 ]. This is because the high Mg content leads to the formation of a higher amount of S phases in the as-cast microstructure, which do not dissolve during solution heat treatment, thus decreasing the heat treatment response of these alloys [ 20 ]. However, selecting a composition within the α-Al + S + T section of the Al–Mg–Cu ternary phase diagram results in the formation of T precipitates that can be dissolved during solution heat treatment, thus improving the precipitation hardening of high-Mg-containing alloys.…”