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

This study examines the formation of different phases of Al-6 mass% Mg– x Cu ( 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 Al 3 Mg 2 (β), Al 6 CuMg 4 (T), and Al 2 CuMg (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.