Effects of Heterogenization Treatment on the Hot-Working Temperature and Mechanical Properties of Al-Cu-Mg-Mn-(Zr) Alloys

Abstract

This study investigated the effects of a minor Zr addition (0.15 wt%) and heterogenization treatment (one-stage/two-stage) on the hot-working temperature and mechanical properties in Al-4.9Cu-1.2Mg-0.9Mn alloy. The results indicated that the eutectic phases (α-Al + θ-Al₂Cu + S-Al₂CuMg) dissolved after heterogenization, retaining θ-Al₂Cu and τ₁-Al₂₉Cu₄Mn₆ phases, while the onset melting temperature increased to approximately 17 °C. A change in the onset melting temperature and evolution of the microstructure is used to assess an improvement in hot-working behavior. With the minor Zr addition, the alloy exhibited enhanced mechanical properties due to grain growth inhibition. Zr-added alloys show 490 ± 3 MPa ultimate tensile strength and 77.5 ± 0.7 HRB hardness after T4 tempering, compared to 460 ± 2.2 MPa and 73.7 ± 0.4 HRB for un-added alloys. Additionally, combining minor Zr addition and two-stage heterogenization resulted in finer Al₃Zr dispersoids. Two-stage heterogenized alloys had an average Al₃Zr size of 15 ± 5 nm, while one-stage heterogenized alloys had an average size of 25 ± 8 nm. A partial decrease in the mechanical properties of the Zr-free alloy was observed after two-stage heterogenization. The one-stage heterogenized alloy had 75.4 ± 0.4 HRB hardness after being T4-tempered, whereas the two-stage heterogenized alloy had 73.7 ± 0.4 HRB hardness after being T4-tempered.

Keywords: Al-Cu-Mg-Mn alloy; Al3Zr; heterogenization; melting temperature; zirconium.

Figure 1

Heat treatment cycle schedule of the as-cast alloys.

Figure 2

Microstructure observation by SEM-BEI and EPMA analysis of: (a) as-cast Alloy A; (b) as-cast Alloy B; (c) 1-Het. Alloy A; (d) 1-Het. Alloy B. (Arrow 1: eutectic θ-Al₂Cu, 2: eutectic τ₁-Al₂₉Cu₄Mn₆, 3: ternary eutectic α + θ-Al₂Cu + S-Al₂CuMg).

Figure 3

Microstructure observation showing T-phase (rod-like) and Al₃Zr (spherical-like) by TEM analysis of: (a) 1-Het. Alloy A; (b) 2-Het. Alloy A; (c) 1-Het. Alloy B; (d) 2-Het. Alloy B, and EDS analysis results of: (e) T-phase, and (f) Al₃Zr dispersoids, and also: (g) HR-TEM image along with the inset showing fast Fourier transform (FFT) pattern of the red line region.

Figure 3

Microstructure observation showing T-phase (rod-like) and Al₃Zr (spherical-like) by TEM analysis of: (a) 1-Het. Alloy A; (b) 2-Het. Alloy A; (c) 1-Het. Alloy B; (d) 2-Het. Alloy B, and EDS analysis results of: (e) T-phase, and (f) Al₃Zr dispersoids, and also: (g) HR-TEM image along with the inset showing fast Fourier transform (FFT) pattern of the red line region.

Figure 4

Three-dimensional optical microstructure of the Zr-free Alloy A: (a) 1-Het. and then cold-rolled; (b) 2-Het. and then cold-rolled; (c) 1-Het. and then T4-treated; (d) 2-Het. and then T4-treated.

Figure 5

Three-dimensional optical microstructure of the Zr-added Alloy B: (a) 1-Het. and then cold-rolled; (b) 2-Het. and then cold-rolled; (c) 1-Het. and then T4-treated; (d) 2-Het. and then T4-treated.

Figure 6

Euler angle colored EBSD maps and misorientation angle distribution of T4 state of: (a) 1-Het. Alloy A; (b) 2-Het. Alloy A; (c) 1-Het. Alloy B; (d) 2-Het. Alloy B; (e) IPF figure.

Figure 7

DSC traces of the as-cast and one-stage heterogenization Alloy A(0Zr) and Alloy B(0.15Zr). (a) DSC curve; (b) Gaussian fitting of the as-cast Alloy B.

Figure 8

Representative engineering stress-strain curve and fracture micrographs of the experimental alloys after T4 heat treatment. (a) Alloy A (1-Het.); (b) Alloy A (2-Het.); (c) Alloy B (1-Het.); (d) Alloy B (2-Het.).