This study investigates the influence of solution and aging treatments on the mechanical properties and microstructure of an Al-Cu-Mn-Er cast alloy. Through systematic experimentation and analysis, optimal heat treatment parameters were identified to achieve superior strength, hardness, and ductility. The results highlight the critical role of solution temperature, time, and aging conditions in tailoring the alloy’s performance for aerospace and automotive applications.
1. Introduction
Al-Cu-Mn cast alloys are widely recognized for their high strength-to-weight ratio, corrosion resistance, and suitability for high-temperature applications. The addition of rare earth elements like Er enhances grain refinement, suppresses hot cracking, and improves mechanical properties. However, the interplay between heat treatment parameters and the resulting microstructure-property relationships in Er-modified Al-Cu-Mn cast alloys remains underexplored. This work addresses this gap by optimizing solution and aging treatments to maximize the alloy’s hardness, tensile strength, and elongation.
2. Experimental Methodology
2.1 Material Composition
The chemical composition of the cast alloy, determined via X-ray fluorescence and ICP analysis, is summarized in Table 1.
Table 1: Chemical composition of the Al-Cu-Mn-Er cast alloy (mass fraction, %)
| Cu | Mn | Mg | Er | Ti | Fe | Al |
|---|---|---|---|---|---|---|
| 5.05 | 0.35 | 0.25 | 0.15 | 0.10 | 0.25 | Balance |
2.2 Heat Treatment Parameters
- Solution Treatment: Conducted at 520–550°C for 2–14 h, followed by water quenching.
- Aging Treatment: Performed at 165–185°C for 0–24 h.
2.3 Characterization Techniques
- Microstructural Analysis: Optical microscopy (OM), scanning electron microscopy (SEM), and X-ray diffraction (XRD).
- Thermal Analysis: Differential scanning calorimetry (DSC) at 15°C/min.
- Mechanical Testing: Vickers hardness (HV₀.₁), tensile strength (Rm), yield strength (Rp₀.₂), and elongation (A).
3. Results and Discussion
3.1 Thermal Behavior and Phase Evolution
DSC analysis revealed critical phase transformations during heating (Figure 1):
- Low-Temperature Peak (480–520°C): Corresponds to the dissolution of θ (Al₂Cu) and ternary α-Al + θ + T (Al₁₂CuMn₂) phases.
- High-Temperature Peak (570–600°C): Represents the melting of Al₈Cu₄Er intermetallics.
The absence of Al₃Er peaks confirmed Er’s interaction with Al and Cu to form low-melting-point Al₈Cu₄Er, which persists post-solution treatment.
3.2 Optimization of Solution Treatment
Hardness and tensile properties were maximized at 540°C for 12 h (Table 2). Prolonged heating (>12 h) caused grain coarsening, while temperatures >540°C induced overburning.
Table 2: Effect of solution temperature on hardness (HV₀.₁)
| Temperature (°C) | 520 | 530 | 540 | 550 |
|---|---|---|---|---|
| Hardness (HV₀.₁) | 128.4 | 132.7 | 137.3 | 121.9 |
The dissolution kinetics of θ phase (Al₂Cu) followed Fick’s first law:
J=−D∂x∂C
where J = diffusion flux, D = temperature-dependent diffusion coefficient, and ∂x∂C = concentration gradient. Higher temperatures (e.g., 540°C) accelerated Cu diffusion, promoting θ-phase dissolution without overburning.
3.3 Aging Behavior and Precipitation Strengthening
Aging at 185°C for 6 h yielded peak hardness (142.3 HV₀.₁) and tensile properties (Table 3). The precipitation sequence aligned with classical Al-Cu alloys:
SSS→GP Zones→θ′′→θ′→θ
Table 3: Mechanical properties under optimal aging conditions
| Property | Rm (MPa) | Rp₀.₂ (MPa) | A (%) | Hardness (HV₀.₁) |
|---|---|---|---|---|
| As-Cast | 210.5 | 165.8 | 2.1 | 102.5 |
| 540°C/12 h + 185°C/6 h | 370.4 | 300.3 | 6.5 | 142.3 |
Elevated aging temperatures increased vacancy concentration (Cv), accelerating GP zone formation:
Cv=C0exp(−kTQv)
where C0 = pre-exponential factor, Qv = activation energy for vacancy formation, k = Boltzmann constant, and T = temperature.
3.4 Microstructural Evolution
- As-Cast State: Coarse dendritic structures with Al₈Cu₄Er and AlCuMnFe intermetallics (Figure 2a).
- Post-Solution Treatment: Dendritic segregation reduced; θ phase dissolved into α-Al matrix (Figure 2b).
- Post-Aging Treatment: Fine θ’ precipitates (5–20 nm) uniformly distributed, enhancing dislocation pinning (Figure 2c).
XRD analysis (Figure 3) confirmed the absence of θ-phase peaks post-solution treatment, indicating near-complete dissolution. Residual Al₈Cu₄Er and AlCuMnFe phases occupied <2% area fraction (Table 4).
Table 4: Residual phase area fraction after solution treatment
| Solution Time (h) | 2 | 6 | 12 | 14 |
|---|---|---|---|---|
| Area Fraction (%) | 3.10 | 2.70 | 1.95 | 1.92 |
4. Conclusions
- The Al-Cu-Mn-Er cast alloy achieves optimal properties under 540°C/12 h solution treatment + 185°C/6 h aging, with Rm = 370.4 MPa, Rp₀.₂ = 300.3 MPa, A = 6.5%, and HV₀.₁ = 142.3.
- Er forms thermally stable Al₈Cu₄Er phases, which refine grains but remain undissolved during heat treatment.
- Elevated aging temperatures accelerate θ’ precipitation, governed by vacancy-mediated diffusion.
This work provides a framework for designing high-performance Al-Cu-Mn-Er cast alloys through tailored heat treatment protocols.