Effect of Solution and Aging Treatment on the Microstructural Evolution and Mechanical Properties of Cold-Rolled 2024 Aluminum Alloy Sheets

Abstract

The cold-rolled 2024 aluminum alloy sheets were subjected to solution treatments at different temperatures followed by artificial aging. The microstructure and mechanical properties were investigated using Vickers microhardness testing, tensile testing, optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicate that as the solution temperature increases, the coarse particles gradually dissolved into the matrix. At a solution temperature of 500 °C, the grains become nearly equiaxed with an average size of ~16.47 μm, and no significant grain growth is observed compared to the as-rolled condition. The refined microstructure contributes to excellent mechanical properties. In contrast, when the solution temperature increases to 550 °C, the microstructure shows severe grain coarsening (up to ~61.39 μm), which indicates that overburning occurs, resulting in a drastic deterioration in mechanical performance. As the aging time increases, precipitates become more uniformly and densely distributed throughout the matrix, and the hardness initially increases and reaches a peak after approximately 6 h of aging at 180 °C. The optimal mechanical performance, characterized by a favorable combination of strength and ductility, is achieved at an aging time of 6 h. In summary, the optimal heat treatment condition for the cold-rolled 2024 aluminum alloy sheet is solution treatment at 500 °C for 1 h followed by aging at 180 °C for 6 h, resulting in a hardness of 154 HV, a tensile strength of 465 MPa and an elongation of 13%.

1. Introduction

Aluminum alloys are widely used in high-performance sectors such as aerospace and automotive industries due to their high specific strength, excellent impact resistance, and good corrosion resistance [1,2,3]. This study focuses on the 2024 aluminum alloy, a member of the Al-Cu-Mg system that possesses high strength, good machinability, high fatigue resistance, and moderate corrosion resistance [4]. It is extensively applied in aircraft skin, frames, ribs, and structural components of vehicles and ships that demand high strength and fatigue resistance. Common heat treatment methods for 2024 aluminum alloy include artificial aging after solution treatment (T6) or natural aging after solution treatment (T4) [5,6]. Despite its widespread use, the appropriate combination of solution and aging parameters for cold-rolled 2024 remains unclear, motivating the present study. We aim to optimize the solution treatment and aging process parameters to enhance the comprehensive properties of cold-rolled 2024 aluminum sheets.

One area of research focuses on the effect of heat treatment on the mechanical properties of 2024 aluminum alloy. Liang et al. [7] performed solution and aging treatment on cold-rolled 2024 aluminum alloy sheets and found that mechanical properties improved significantly with increasing solution temperature up to 510 °C. However, further increase to 520 °C led to a notable reduction in elongation, indicating degradation of plasticity at excessively high temperatures. Sun et al. [8] reported that optimal mechanical properties were achieved through a solution treatment at 500 °C for 70 min followed by aging at 180 °C for 16 h, yielding a yield strength and ultimate tensile strength (UTS) of 411 MPa and 527 MPa, respectively. Mu et al. [9] revealed that T6 treatment of SiC/AA2024 nanocomposites induced a synergistic strengthening mechanism involving Sʹ precipitates and geometrically necessary dislocations, enhancing strength (YS from 413 to 496 MPa; UTS from 501 to 572 MPa) while maintaining elongation (5.4%→6.7%). These studies collectively demonstrate that appropriate heat treatment can significantly enhance the mechanical properties of 2024 aluminum alloys by optimizing microstructural features such as precipitate distribution and dislocation behavior.

Additionally, other studies have demonstrated a strong correlation between mechanical properties and microstructural evolution. Staszczyk et al. [10] examined 500 °C solution-treated AA2024 samples followed by various 180 °C aging protocols (T6, T6I6, dual-stage T-DA), observing the precipitation behaviors of θ-Al₂Cu and S-Al₂CuMg phases. Results showed that dual-stage aging reduced precipitate volume fraction while achieving peak hardness in Alloy A, whereas Alloys B and C formed coarse, non-strengthening intermetallic phases, leading to distinctly different aging responses and providing direct experimental evidence for structure-property relationships. Radutoiu et al. [11] studied the effects of different artificial aging temperatures (150 °C, 175 °C, and 190 °C) after solution treatment (495 ± 5 °C, 1 h, water quench) on AA2024, finding that higher aging temperatures accelerated precipitate coarsening beyond peak strength, leading to rapid hardness decline. Overaging phenomena were particularly pronounced between 175 and 190 °C. Xie et al. [12] constructed a gradient microstructure in AA2024, ranging from ultrafine-grained to coarse-grained regions, observing more uniform distributions of Sʹ and θʹ phases in fine-grained zones. This configuration significantly enhanced hardness and strength while maintaining ductility and corrosion resistance, illustrating the effectiveness of gradient microstructure–precipitation synergy. These findings underscore that the mechanical performance of 2024 aluminum alloys is governed by microstructural evolution, particularly precipitate coarsening, volume fraction, and spatial distribution, which are influenced by aging temperature, time, and grain structure. Moreover, most existing studies on 2024 aluminum alloys have focused on the effect of individual factors on their properties. In contrast, this work combines cold rolling, solution treatment, and aging to clarify their integrated influence on microstructural evolution and mechanical performance.

In summary, given the diverse service environments and processing methods of aluminum alloys, the performance requirements vary accordingly. The solution and aging process significantly influence the mechanical properties and microstructure of 2024 alloy [13,14,15,16]. To meet the high-performance demands of 2024 aluminum alloy in aerospace structures and transportation applications, and given the limited studies on the effect of solution-aging treatment on microstructure and mechanical properties of cold-rolled 2024 sheets, this work investigates the evolution of microstructure and mechanical behavior under various heat treatment conditions. The aim is to clarify the strengthening mechanisms and provide theoretical guidance for optimizing the post-processing of 2024 aluminum sheets.

2. Materials and Methods

The test material used in this study is a 2 mm thick cold-rolled 2024 aluminum alloy sheet. Its chemical composition was analyzed using inductively coupled plasma atomic emission spectrometry (ICP-AES, Agilent Technologies, Santa Clara, CA, USA), as shown in

Table 1. Chemical composition of 2024 aluminum sheet (mass fraction, %).

Element	Cu	Mg	Mn	Fe	Si	Zn	Ti	Al
wt%	3.95	1.33	0.61	0.19	0.091	0.032	0.035	Bal.

. Solution treatments were conducted in an AS-SX2-6-12TP tube furnace, and artificial aging was carried out in a DF-101S thermostatic chamber. The detailed heat treatment route, including solution temperatures and aging durations, is schematically presented in Figure 1. Solution treatments were conducted at 480 °C, 500 °C, 530 °C, and 550 °C for 1 h, followed by immediate water quenching at room temperature with a cooling rate of 100 °C/s. Subsequently, aging treatments were performed at 180 °C for durations ranging from 0 to 8 h, followed by air cooling with a cooling rate of 10 °C/s. After solution treatment, the specimens were sequentially ground with silicon carbide papers from 180 to 2000 grit to ensure consistent scratch orientation and a flat surface without distortion, followed by mechanical polishing until a smooth and scratch-free finish was obtained. The polished specimens were anodized in a 5% HBF₄ solution at 20 V. Systematic trials with varying anodizing times indicated that 50–60 s yielded the most distinct microstructural features, and this duration was therefore adopted. The microstructures were subsequently observed under polarized light using an optical microscope (AX10, Zeiss, Oberkochen, Baden-Württemberg, Germany). Vickers microhardness was measured with a load of 300 g and a dwell time of 10 s. Each sample was tested at a minimum of 10 locations, and average values were calculated for reliability. To further optimize aging parameters, samples aged at 180 °C for various times under selected solution conditions were hardness tested to determine the optimal aging duration. Tensile specimens were machined along the rolling direction using wire cutting (dimensions shown in Figure 2), then heat-treated and tensile tested at room temperature in accordance with ISO 6892-1:201919 [17], using an Instron 3369 universal testing machine (Instron, Norwood, MA, USA) at a loading rate of 1.8 mm/min.

Samples of approximately 10 mm in length were sectioned near the fracture surface using wire electrical discharge machining (EDM) for microstructural observation. After grinding with abrasive papers and mechanical polishing, anodic oxidation was performed. The electrolyte used was a 4 vol% HBF₄ solution, with an anodizing duration of 1 min. The microstructure was observed under polarized light using an optical microscope. Fracture surface morphology was examined using an EVO18 scanning electron microscope (EVO18, Zeiss, Oberkochen, Baden-Württemberg, Germany). The locations for microstructure sampling and fracture surface scanning are illustrated in Figure 3. Thin foil TEM samples were cut out from the cross-section of the original sample. The disks were ground to a thickness of about 100 um, followed by electro-polishing in a double-jet unit operating at 10 V and −25 °C using a 30% nitric acid and 70% methanol solution. The electrolytic time is about 15 s. TEM investigations were performed on Tecnai G2 20 (FEI, Hillsboro, OR, USA) at an operating voltage of 300 kV.

3. Experimental Results and Discussion

3.1. Microstructure

Figure 4 shows the optical micrographs of the samples subjected to solution treatment at different temperatures, with (a–e) showing low magnification and (f–j) showing high magnification. The as-received sample exhibits elongated grains along the rolling direction, characteristic of a deformed structure. After solution treatment at temperatures ranging from 480 °C to 550 °C for 1 h, several dark particles are observed, identified as coarse second-phase or impurity particles [18]. At 480 °C, the matrix grains remain relatively fine. The grains are elongated along the rolling direction, showing a typical deformed microstructure. When the solution temperature increases to 500 °C, the degree of grain equiaxation improves, with clearer and more continuous grain boundaries. At 530 °C, further grain coarsening occurs. Incipient melting may occur in the sheet under this condition. After solution treatment at 550 °C, the grains become significantly coarser. Moreover, typical remelted spherical features were observed, accompanied by a degradation in material performance [19].

Figure 5 presents the relative frequency distribution of grain sizes for the alloy in both the as-received state and after solution treatments at various temperatures, quantified from high-magnification micrographs to evaluate the trend shown in Figure 4. The as-received sample has an average grain size of about 15.35 µm; after treatment at 480 °C, the average size is about 16.07 µm; at 500 °C it slightly increases to about 16.47 µm; at 530 °C it further increases to about 18.27 µm; and at 550 °C it markedly increases to 61.39 µm.

3.2. Vickers Microhardness

As shown in Figure 6, samples aged at 180 °C for 0–8 h after different solution treatments show a rapid hardness increase before gradually decreasing, with a peak at 6 h. This conclusion is consistent with previous studies [10,20]. This trend reflects typical precipitation strengthening: from a supersaturated solid solution to metastable phase precipitation, followed by overaging softening. With increasing solution temperature, hardness first rises and then declines. At 480 °C, the solution treatment is insufficient, and the second-phase particles are not fully dissolved. At 530 °C, the excessively high temperature leads to signs of incipient melting (overburning). The overall hardness of the sample solution-treated at 500 °C is the highest among the three curves. In summary, the optimal heat treatment condition is identified as a solution temperature of 500 °C for 1 h followed by aging at 180 °C for 6 h, under which the material achieves a maximum hardness of 154 HV.

3.3. Stress–Strain Behavior

Figure 7 illustrates the variation in tensile strength and elongation of 2024 aluminum alloy samples under different heat treatment conditions. The as-received sample exhibits a tensile strength of approximately 435 MPa and an elongation of about 15%, indicating a moderate strength level with good ductility. After undergoing solution treatment at 500 °C for 1 h followed by aging at 180 °C for 6 h, the alloy reaches its optimal mechanical condition. The tensile strength further increases to approximately 465 MPa, while the elongation slightly decreases to around 13%. Similar trends were reported by Sun et al. [8] for solution treatment at 500 °C for 70 min followed by aging at 180 °C for 16 h, and by Zhu et al. [21] under optimized solution–aging conditions, both showing increased strength accompanied by reduced ductility.

This behavior can be explained by the precipitation strengthening mechanism, particularly the Orowan strengthening mechanism. When the precipitates are small, such as fine Sʹ phases or GPB zones, they maintain a coherent or semi-coherent relationship with the matrix, allowing dislocations to shear through them directly—this stage is governed by the shearing mechanism. As aging time increases, precipitates grow in size and dislocations bypass them by forming Orowan loops, thus transitioning to a bypass mechanism. This shift enhances the strength of the alloy while also increasing strain localization, resulting in a slight reduction in elongation. The strengthening stress associated with this mechanism is closely related to the size and spacing of precipitates and can be quantitatively described by well-established analytical formulas [22,23,24]:

$$\Delta \sigma ={\displaystyle \frac{Gb}{\lambda }}\ln (r/b)$$

(1)

As can be seen from Equation (1), the increase in yield strength induced by precipitation strengthening can be expressed as a function of several key parameters: G is the shear modulus of the alloy matrix, b is the magnitude of the Burgers vector (representing the fundamental displacement unit for dislocation slip), λ is the average spacing between precipitates, and r is the average radius of the precipitates. The equation essentially quantifies how dislocations interact with precipitates through the Orowan looping mechanism, thereby linking microstructural features to macroscopic strength. Specifically, a larger precipitate radius (r) provides stronger obstacles to dislocation motion, while a shorter inter-precipitate spacing (λ) increases the frequency of Orowan loops. Both effects enhance precipitation strengthening and lead to higher alloy strength. This formulation therefore provides a clear framework for understanding how heat treatment—by controlling precipitate size and distribution—directly influences mechanical properties. This equation describes the specific influence of precipitate size (r) and spacing (λ) on the alloy strength when dislocations form Orowan loops around precipitates. Specifically, a larger precipitate radius (r) provides stronger obstacles to dislocation motion, while a shorter inter-precipitate spacing (λ) increases the frequency of Orowan loops. Both effects enhance precipitation strengthening and lead to higher alloy strength.

When the solution treatment temperature increases to 530 °C, incipient overburning occurs, leading to a drop in tensile strength to approximately 325 MPa and a significant reduction in elongation to below 5%. Upon further increase to 550 °C, the tensile strength drastically decreases to only about 65 MPa, and the elongation approaches zero, indicating complete loss of plasticity. As shown in Figure 4d, this degradation is attributed to severe grain coarsening and overburning in the material.

3.4. Room Temperature Tensile Fracture Behavior

Figure 8 illustrates the fracture morphologies of 2024 aluminum alloy samples under different solution-aging treatments. As shown in Figure 8a, the as-received sample exhibits a relatively small fracture cross-sectional area with an uneven surface, and necking is expected to occur, indicating a ductile fracture mode [25]. From Figure 8c, it can be observed that as the solution temperature increases, the extent of necking decreases. When the solution temperature reaches 500 °C, the cross-sectional area becomes comparable to that of the as-received state, and necking is likewise expected, indicating a ductile fracture mode. At 530 °C, the fracture surface becomes noticeably flatter, with only slight necking, indicating a transition towards a mixed fracture mode involving both ductile and brittle characteristics.

Figure 9 shows the high-magnification fracture morphologies of 2024 aluminum alloy under different solution-aging treatments. In the as-received state (Figure 9a), the fracture surface is densely covered with deep and uniformly distributed dimples, indicative of ductile fracture [26]. After solution treatment at 500 °C and aging at 180 °C (Figure 9b), dimples remain evident but become smaller and fewer, although the fracture is still distinctly ductile. Figure 9c shows fewer dimples with uneven distribution, and the edges of tearing ridges appear blurred, indicating reduced ductility and the onset of a ductile–brittle mixed fracture mode.

Figure 10 presents histograms of equivalent dimple diameters corresponding to the fracture morphologies in Figure 9. In the as-received sample (Figure 10a), the dimple population is the highest, with a histogram heavily weighted toward the smaller-size bins and a relatively narrow spread—consistent with the dense and deep dimples in the SEM image. After solution treatment at 500 °C followed by aging at 180 °C (Figure 10b), the total dimple count decreases and the histogram becomes flatter, indicating a lower dimple density, in agreement with the sparser dimpling observed in Figure 9b. With further thermal treatment (Figure 10c), the dimple count drops further and the distribution broadens toward larger bins, yielding more widely spaced and shallower dimples. This evolution corresponds to the blurred tearing ridges and the ductile–brittle mixed fracture mode seen in Figure 9c.

Figure 11 shows the microstructures near the fracture surfaces of 2024 aluminum alloy samples under different solution-aging treatments. In Figure 11a,b, the fracture regions exhibit typical 45°shear fracture characteristics. The grains near the fracture are mostly banded or elongated, with their major axes aligned with the shear fracture plane, indicating that both the as-received sample and the sample treated at 500 °C solution + 180 °C aging show clear ductile fracture characteristics. As the solution treatment temperature increases to 530 °C and 550 °C (Figure 11c,d), the inclination angle of the fracture surface decreases, and the grains become significantly coarser with well-defined grain boundaries. In Figure 11c, the fracture mode begins to shift from ductile to brittle. In Figure 11d, the grains are markedly coarsened, and remelted particles (fusion balls) appear, indicating a complete loss of ductility and a fully brittle fracture behavior.

Figure 12 presents the TEM images of the as-received sample and the sample subjected to solution treatment at 500 °C, followed by aging at 180 °C. During cold working, a large number of dislocations are generated in the matrix. As shown in Figure 12a,b, a dense network of intersecting dislocation structures can be observed in the matrix, with the formation of dislocation cells and dislocation walls in localized regions, indicating significant work hardening. In addition, coarse rod-like second-phase particles are visible. Due to their large size and inhomogeneous distribution, these particles are ineffective in obstructing dislocation motion and tend to induce stress concentrations and crack initiation [27]. Under tensile or fatigue loading, tearing may occur between these coarse particles and the matrix, leading to premature failure [28] and resulting in a combined degradation of both strength and ductility. As shown in Figure 12c,d, the dislocation structure becomes more regular, and the coarse second-phase particles are significantly dissolved and refined. Meanwhile, uniformly distributed fine rod-like or plate-like precipitates are observed, with sizes mainly ranging from 50 to 150 nm. These are metastable phases formed during artificial aging [29,30,31]. These precipitates are primarily located at dislocation intersections, where they begin to pin dislocation motion, contributing to a limited precipitation strengthening effect [32]. According to existing studies, uniformly dispersed nanoscale S″/S′ precipitates increase microhardness by impeding dislocation motion, whereas subsequent coarsening and transformation toward the equilibrium S phase reduce this effect. Zhu et al. [33] identified semi-coherent S′ as the principal strengthening precipitate closely correlated with microhardness, while Alexopoulos et al. [34] showed that at 160–190 °C microhardness rises with the nucleation/refinement of S″/S′ and decreases upon over-aging as precipitates coarsen, lose coherency, and evolve toward S. Such precipitates can effectively impede dislocation motion, thereby enhancing the strength of the material while reducing its elongation, which is consistent with the strength and elongation trends shown in Figure 7.

4. Conclusions

Solution and aging treatments were performed on cold-rolled 2024 aluminum alloy sheets to investigate the effects of process parameters on the microstructure and mechanical properties. The main conclusions are as follows:

(1)

The hardness and strength peak at a solution temperature of 500 °C and then decrease at 530 °C due to overburning, while elongation declines continuously. The optimal heat treatment condition is solution at 500 °C for 1 h followed by aging at 180 °C for 6 h, yielding a favorable balance of strength and ductility.

(2)

The cold-rolled 2024 aluminum alloy sheet exhibits ductile fracture at room temperature with numerous dimples, which become shallower and fewer as the solution temperature increases, indicating a ductile-to-brittle transition.

(3)

During solution treatment, coarse particles dissolve and grains grow, with higher temperatures leading to larger grain sizes and more complete dissolution. Subsequent aging at 500 °C/1 h + 180 °C/6 h produces fine rod- or plate-like precipitates, strengthening the alloy via the Orowan mechanism. This enhances strength but slightly reduces ductility, offering practical guidance for aerospace applications where both properties are critical.