Optimization of Microstructure and Mechanical Properties in Al-Zn-Mg-Cu Alloys Through Multiple Remelting and Heat Treatment Cycles

Abstract

This study explores the influence of multiple remelting and repeated T6 heat treatment on the microstructure and mechanical properties of Al-Zn-Mg-Cu alloys. With the increase in remelting cycles, the alloy experiences structural alterations due to recovery, recrystallization, and grain growth during remelting, which enhances its mechanical properties. However, the continuous transformation of alloy phases during remelting leads to the deterioration of the morphology and size of α-Al dendrites and MgZn₂ phase, causing a decline in mechanical properties. The results indicate that after three remelting cycles, the grain size of the alloy is significantly reduced, and the α-Al dendrites and MgZn₂ phase maintain favorable morphology and size, thereby achieving the effects of grain refinement and strength enhancement. These findings offer a theoretical foundation for optimizing the properties of Al-Zn-Mg-Cu alloys.

1. Introduction

Al-Zn-Mg-Cu alloys are widely used in structural fields because of their high strength–density ratio and good fracture toughness [1,2]; as the structural material in the aerospace field, the Al-Zn-Mg-Cu alloy is a high-toughness aluminum alloy, due to its high hardness, strength, wear resistance, and other excellent mechanical properties [3]. The Al-Zn-Mg-Cu alloy is one of the most widely used aluminum alloy types at present, with very high strength, but its corrosion performance is poor, will affect its service life. In the reprocessing process of aluminum alloy, remelting will bring some problems, such as the increase of internal stress, which will lead to warping or distortion of processed parts and even cracking. After reaching a certain degree, cracks or even fractures will occur. The processes of remelting and heat treatment are known to induce alterations in the micro-structural characteristics and mechanical properties of metallic alloys. These alterations necessitate further investigation to elucidate the underlying mechanisms and their implications on the alloy’s performance.

Compared with industrial primary aluminum, waste aluminum alloy has lower melting temperature, shorter melting time, lower metal burn loss, and lower energy consumption [4]. More important, secondary aluminum production energy consumption is only 3–5% of the original aluminum production, which can not only reduce the production cost but also help reduce the emission of carbon dioxide and sulfur dioxide pollutants in the industrial process [5,6], playing an important role in energy conservation and environmental protection [7]. Zhou et al. [6] studied the melting, extrusion, and heat treatment processes of the secondary Al-Zn-Mg-Cu alloy, and the results showed that the composition distribution of secondary Al-Zn-Mg-Cu alloy ingots was more uniform, and the microstructure was more optimized. Lin et al. [8] prepared four kinds of secondary Al-Zn-Mg-Cu alloy profiles by melting, composition adjustment, refining, and casting the secondary AA7075 ingot, and finally by extrusion and T6/T73 heat treatment. The mechanical properties of all the secondary profiles were superior to the requirements of ASTM B209-14 and B221M-13. The utilization of scrap aluminum products in the production of secondary aluminum not only facilitates resource conservation but also significantly contributes to the promotion of green manufacturing and sustainable development [9,10]. At the end of the product life cycle, parts made from Al-Zn-Mg-Cu alloys need to be replaced or secondary. Therefore, the recovery or remelting of Al-Zn-Mg-Cu alloys is essential due to the huge use, demand, and technological advances. The remelting process of other alloys can be used for reference in the study of Al-Zn-Mg-Cu alloy remelting [11,12], so it needs to be further explored.

The T6 heat treatment is a pivotal process for optimizing the mechanical attributes of 7xxx series aluminum alloys, which are celebrated for their superior strength in critical industries, such as aerospace and transportation. This treatment excels in inducing the precipitation of strengthening phases, refining grain structures, and directing microstructural development [13]. J.-Y. Choi et al. [14] studied the effects of multiple heat treatment on the microstructure and mechanical properties of the alloy and found that repeated heat treatment would affect the precipitation of the second phase in the alloy, thus causing changes in the microstructure and mechanical properties. Zhao et al. [15] reported significant alterations in the microstructure of TB15 titanium alloy with an increasing number of solution-aging treatment cycles. They observed the coalescence and growth of the α phase, along with a thickening of the grain boundaries of the original β grains. Concurrently, there was a noted deterioration in the mechanical properties of the alloy as the frequency of the solution aging treatments escalated.

In comparison, research on multiple remelting is still relatively limited. Existing studies have shown that the remelting process can alter the grain structure, precipitate phase behavior, and mechanical properties of aluminum alloys [16]. Barot and Sutaria [17] investigated the multiple remelting behavior of Al-Si-Cu alloy and found that with the increase in the number of remelting cycles, the grain size first decreased and then increased, while the precipitation-strengthening effect gradually weakened. However, there has not yet been a systematic study on the effects of multiple remelting + T6 treatment on the micro-structure and mechanical properties of Al-Zn-Mg-Cu alloys.

Although many researchers confirm the differences in microstructure between heat treatment aluminum and raw aluminum remelting, few researchers have systematically explored the mechanism of microstructure and properties of heat-treated alloys after repeated remelting. Therefore, in this study, the changes in microstructure and mechanical features of Al-Zn-Mg-Cu alloys after multiple remelting and repeated heat treatment were conducted to investigate the melting process, utilizing optical microscopy (OM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) with times as the independent variable. This study reveals a strategy for simultaneously enhancing strength and ductility without changing the manufacturing process and provides the possibility of producing high-performance Al-Zn-Mg-Cu alloys for industrial applications.

2. Materials and Methods

The Al-Zn-Mg-Cu alloy used in this study was provided by ALG Aluminium Inc. (Nanning, China), with its chemical composition listed in

Table 1. Chemical composition of the experiment Al-Zn-Mg-Cu alloy.

Alloys	Elements (wt.%)
	Al	Zn	Mg	Cu	Cr	Fe	Si	Mn
A1	89.66	5.58	2.61	1.40	0.03	0.18	0.08	0.15
A2	89.36	5.87	2.48	1.50	0.02	0.18	0.09	0.13
B1	89.37	5.85	2.48	1.56	0.02	0.18	0.09	0.13
B2	89.06	6.12	2.43	1.66	0.02	0.20	0.09	0.13
C1	90.04	5.87	1.83	1.49	0.02	0.18	0.08	0.14
C2	89.94	6.01	1.78	1.55	0.02	0.20	0.09	0.14
D1	90.40	5.73	1.70	1.46	0.02	0.18	0.08	0.14
D2	90.67	5.93	1.20	1.48	0.02	0.21	0.09	0.14

. To ensure consistent composition across different remelting cycles, inductively coupled plasma optical emission spectrometry (ICP-OES) was used after each remelting to adjust alloying elements accordingly.

The experimental flowchart is shown in Figure 1. In the process of remelting experiment, the samples were subjected to T6 heat treatment before each remelting. The aging treatment was carried out in a DG450C drying oven. Before melting, the experimental alloys were preheated to 200 °C. The crucible with raw materials is put into the resistance furnace, the temperature is raised to 720~740 °C, and the heat is kept for a certain time until the alloy is completely melted, followed by the covering agent and the refining agent, respectively. Then, the heat is raised to 800 °C, and the casting is finished after a certain time. These metal melts were picked at 800 °C cast and cooled to room temperature with an iron mold.

The processing steps of alloy samples are as follows: the rolled Al-Zn-Mg-Cu (T6) aluminum alloy is sample A, and sample a is obtained after remelting and T6 heat treatment, and so on. Detailed sample information is shown in

Table 2. The multiple remelting and heat treatment procedures parameters of the Al-Zn-Mg-Cu alloy.

Step	T6 Heat Treatment	Multiple Remelting + Repeated T6	Remelting Cycles
A1	470 °C × 1 h + 120 °C × 24 h	1st T6 (Rolled condition)	0
A2	470 °C × 1 h + 120 °C × 24 h	Sample A (T6 + remelting)	1
B1	470 °C × 1 h + 120 °C × 24 h	Sample A2 (T6)	1
B2	470 °C × 1 h + 120 °C × 24 h	Sample B (T6 + remelting)	2
C1	470 °C × 1 h + 120 °C × 24 h	Sample B2 (T6)	2
C2	470 °C × 1 h + 120 °C × 24 h	Sample C (T6 + remelting)	3
D1	470 °C × 1 h + 120 °C × 24 h	Sample C2 (T6)	3
D2	470 °C × 1 h + 120 °C × 24 h	Sample D (T6 + remelting)	4

.

Initial rolled sample (A), first remelting + T6 treated sample (a), second remelting + T6 treated sample (b), and so forth up to the fourth remelting (d). The metallographic sample was uniformly taken at the bottom 20 mm of the bar sample (φ25 mm × 130 mm), polished step by step with abrasive paper according to the standard, and then polished with a diamond-polishing paste, corroded with Keller’s reagent. The microstructures were characterized using optical metallographic microscope (OM, Zeiss, Oberkochen, Germany) and desktop scanning electron microscope (SEM, TM400PLUS, Hitachi, Japan) equipped with an energy-dispersive spectrometer (EDS, IXRF5500, Tokyo, Japan). Samples for microstructure observation were polished and etched by Keller’s reagent for a certain time. The metallographic analysis was carried out on an optical microscope (OM, Zeiss/Observer. A1, Oberkochen, Germany). In order to analyze and compare phase morphology more accurately, ten OM images of different annealed samples were taken at 100× magnification. In SEM and OM tests, a section of 1 cm height is cut from a cylindrical ingot as a test sample. The samples of XRD and thermal analysis are a small fraction of the samples clipped after tensile test. And the mechanical properties were characterized using a universal testing machine (8801, INSTRON, Buckinghamshire, UK). Tensile testing was conducted on a computerized testing machine (Shimadzu/AG-X, Kyoto, Japan) at room temperature. Parameters of yield strength (YS, $\mathsf{\sigma }$_0.2), ultimate tensile strength (UTS), and ductility were obtained at a strain rate of 1 mm/min. According to ASTM E8M-04 standard [18] (sub-size sample), 21 totally different samples (3/per ingot) were machined out for tensile properties assessment. The Vickers hardness was characterized using an automatic turret microhardness tester (HV, AVH-5L, Shanghai Hengyi Precision Instruments, Shanghai, China). The electron backscatter diffraction (EBSD) was characterized via a NORDLYS electron backscatter diffraction microscope.

3. Results and Discussion

3.1. Grain Structure Evolution

Figure 2 show the metallographic images of the Al-Zn-Mg-Cu alloy after electrolytic polishing. Figure 2A–D depict the metallographic images before remelting, while Figure 2a–d show the corresponding images after remelting. Prior to remelting, Figure 2A reveals significant grain size variation, along with the presence of small dark brown particles. As remelting progresses, Figure 2B–D illustrate a gradual transformation in grain morphology, with the phase distribution within the matrix becoming increasingly homogeneous. The grains evolve from irregular, fractured polygons to more complete, predominantly polygonal shapes, accompanied by a denser distribution of the α-Al phase.

Under identical experimental conditions, the remelting process (Figure 2a–c) leads to a progressive reduction in grain sizes, with more pronounced dendrite structures, indicating effective grain refinement and an enhancement in the alloy’s strength. However, after the fourth remelting cycle (Figure 2d), grain coarsening occurs, characterized by an abrupt increase in grain size and the formation of intermittent grain boundaries.

At this stage, the alloy’s performance begins to deteriorate. To investigate the effects of remelting iterations, an in-depth analysis was conducted on Al-Zn-Mg-Cu alloy specimens, as shown in Figure 3. This study focused on examining the microstructure and grain orientation distribution under different remelting conditions. Crystal diffraction data of the polished samples were obtained using a scanning electron microscope (SEM) equipped with electron backscatter diffraction (EBSD) accessories. The results were processed with Channel 5 software to analyze grain orientation and microscopic texture variations in the Al-Zn-Mg-Cu alloy across different remelting iterations. Figure 3a represents the alloy after a single remelting, while Figure 3b corresponds to three remelting cycles. As observed in Figure 3a, although the grain size is relatively small, dendrite growth is insufficient, and the grain orientation appears disordered without a clear pattern. The texture is distributed across different crystallographic directions (X₀, Y₀, and Z₀). In contrast, Figure 3b shows that after three remelting cycles, the grains are larger, but the dendrites are fully developed, exhibiting a strong preferred orientation. Most textures converge toward the <001> crystal direction along X₀, indicating a more defined structural alignment.

An in-depth analysis was conducted on the Al-Zn-Mg-Cu alloy specimens subjected to varying remelting iterations, as depicted in Figure 4.

3.2. Impact on Mechanical Properties

Figure 5 shows the effects of different remelting times and repeated T6 heat treatment times on the mechanical properties of the alloy. In the absence of remelting, the alloy exhibits a trend in initial decline followed by a subsequent increase in fracture strength, yield strength, and hardness. Conversely, under the state of remelting, the alloy demonstrates an initial increase followed by a decrease in both fracture strength and yield strength. Comparative analysis reveals that, relative to the original sample, the ultimate tensile strength undergoes a significant enhancement of 88.7%, rising from 115.4 MPa to 217.7 MPa. Similarly, the yield strength increases by 84.5%, from 115.1 MPa to 212.5 MPa. However, at the fourth remelting iteration, both tensile and yield strengths undergo a notable decrease, with ultimate tensile strength dropping to 147.2 MPa and yield strength to 147.5 MPa.

A concurrent examination of the metallographic diagram in Figure 2c reveals that the grain distribution attains a more uniform state at the third remelting cycle, coinciding with the optimal fracture strength and ultimate tensile strength observed at this point. As the number of remelting cycles increases, α-Al dendrites and MgZn₂ phase maintain their morphological and dimensional integrity, thereby preserving high mechanical properties. However, the continuous phase transformation during remelting eventually leads to the degradation of the morphology and size of α-Al dendrites and MgZn₂ phase, ultimately resulting in a decrement in the mechanical properties of the alloy.

Upon analysis of Figure 6a–c, it becomes evident that with increasing remelting iterations, the fracture surface gradually exhibits cleavage features, concurrent with an enhancement in tensile properties. Specifically, upon the third remelting, the cleavage steps become more pronounced, marking the optimal tensile performance. However, subsequent to the fourth remelting, excessive remelting leads to repeated recrystallization of grains, resulting in the appearance of fine cracks within the alloy. This ultimately leads to a deterioration in the overall properties of the alloy.

A comparative analysis of the fracture morphology between non-remelted and remelted alloys, under identical remelting conditions, reveals intriguing differences. As illustrated in Figure 6C,c, in the absence of remelting, a cleavage surface emerges at the fracture, indicating a correlation with the structural morphology and tensile properties. Upon introducing remelting and with subsequent increases in remelting iterations, the alloy’s fracture surface gradually transitions towards a smoother and flatter appearance. However, upon reaching the fourth remelting cycle, small cracks begin to manifest at the alloy’s fracture, indicating a transition towards a typical cleavage fracture mechanism. Based on these observations, it can be concluded that the alloy attains its optimal performance at the third remelting iteration.

3.3. Microstructure and Precipitation Phases

Based on the SEM mapping image in Figure 4, following the initial remelting iteration, the Al element is predominantly concentrated within the crystalline structure, while a small amount of Mg is present in the second phase. In contrast, the Cu element is more concentrated in the second phase, with only trace amounts detectable in the matrix. This observation suggests that the black particles correspond to the Al₂Cu phase. As the number of remelting iterations increases, the grain size undergoes a gradual refinement process. This phenomenon can be attributed to the reduction in size and enhancement in density of the precipitated phases within the alloy during the remelting process, which collectively contribute to additional strengthening effects [19]. Additionally, subsequent heat treatments result in the formation of numerous small, high-density η’ (MgZn₂) phase, necessitating a higher consumption of Mg and Zn elements.

Consequently, the coarse second-phase particles that persist after solid solution treatment undergo partial dissolution. As evident in Figure 2a–d, the coarse second-phase particles in the alloy undergo a significant reduction following the third remelting iteration. It is apparent that the primary precipitated phase in the heat-treated alloy is the η’ phase. Given the finer and more uniform grain boundary distribution observed in the alloy after the third remelting iteration, its hardness and strength are superior to other iterations. Furthermore, the small size and high density of the η’ phase at this stage contribute to the alloy’s peak hardness and strength.

In face-centered cubic crystalline materials, the cube texture is typically aligned close to the <001> direction and resides within the soft orientation region. Brass and Goss textures occupy the mid-orientation region, whereas the copper texture, proximate to the <111> direction, resides within the hard orientation region [20]. Grains situated in the soft orientation region are predisposed to slipping. As the Schmidt factor value increases, the Young’s modulus decreases, facilitating material yield and consequently reducing its strength. In general, in aluminum alloys, the cube texture is usually arranged near the <001> direction, within the soft orientation region. The brass and Gaussian textures are located in the medium-oriented region, while the copper textures near the <111> direction are located in the hard-oriented region. In the third remelting, due to the preferred orientation of dendrites, their texture gathers in the soft orientation region <001>, resulting in grain slip easily. With the increase in Schmidt coefficient, Young’s modulus decreases, effectively reducing the internal stress of the material and improving the tensile property of the material. This also explains the phenomenon that fractures in Figure 6a–c gradually show cleavage characteristics.

The influence of varying remelting times on the strength and hardness of the Al-Zn-Mg-Cu aluminum alloy exhibits a distinct trend in initial decrease followed by a subsequent increase. Initially, prior to any remelting, the microstructure assumes an equiaxial grain structure, characterized by a preponderance of bulk grains and a near absence of subgrain boundaries.

During the first remelting cycle, a synergistic effect of material recrystallization and precipitated phase comes into play. This process potentially negates prior work-hardening phenomena, yet the emergence of regenerated grain boundaries enhances the pinning effect of the strengthening phase within the material. Consequently, a renewed work-hardening phenomenon manifests, leading to an elevation in the alloy’s strength and hardness.

However, the recrystallization rate decreased with the increase in the number of remelting cycles. At the same time, the diffusion ability of atoms and the migration rate of grain boundaries are enhanced, which leads to the rapid growth of grains [21]. It is worth noting that the strength of the alloy decreases with the increase in the number of remelts. Apparently, at the fourth remelting, the alloy showed signs of softening, the most obvious manifestation being a decrease in its strength and hardness. Recrystallization particularly facilitates the reformation of deformed grains into novel equiaxed crystals, thereby mitigating residual deformation structures. Furthermore, the purification of grain boundaries during recrystallization reduces the presence of impurity particles, while accelerated grain growth further diminishes deformation structures and residual stress, ultimately enhancing the overall performance of the alloy material.

3.4. Phase Composition and Grain Structure

Figure 7 presents the XRD pattern of the alloy, exhibiting distinct variations based on the varying remelting durations. To quantitatively assess the MgZn₂ content within the Al-Zn-Mg-Cu alloy, following the third and fourth remelting processes, the adiabatic method was employed. This methodology ensures precise and reliable estimations.

To ensure the accuracy of measurements, the X’PERT HighScore Plus 3.0 software was utilized to meticulously determine the areas of the most prominent peaks associated with Al and MgZn₂. The software’s precision and reliability are paramount in ensuring the integrity of our data. Additionally, the research referenced the PDF card for accurate K values. The RIR values for Al and MgZn₂ at the third and fourth peaks are as follows: RIRAl(3) = 4.29, RIRMgZn₂(3) = 3.67, RIRAl(4) = 4.61, and RIRMgZn₂(4) = 3.67, respectively. These values are crucial in the subsequent calculations, as they directly influence the estimation of MgZn₂ content.

Subsequently, all these values were substituted into Formula (1) [6] to calculate the MgZn₂ content. This approach ensures that calculations are rigorous, accurate, and in line with established scientific methodologies. The results obtained from this analysis provide valuable insights into the compositional changes in the alloy during the remelting process.

$${\mathrm{W}}_{\mathrm{M}\mathrm{g}\mathrm{Z}{\mathrm{n}}_2}={\displaystyle \frac{{\mathrm{I}}_{\mathrm{M}\mathrm{g}\mathrm{Z}{\mathrm{n}}_2}}{{\mathrm{K}}_{\mathrm{A}\mathrm{l}}^{\mathrm{M}\mathrm{g}\mathrm{Z}{\mathrm{n}}_2}(\frac{{\mathrm{I}}_{\mathrm{A}\mathrm{l}}}{{\mathrm{K}}_{\mathrm{A}\mathrm{l}}^{\mathrm{A}\mathrm{l}}}+\frac{{\mathrm{I}}_{\mathrm{M}\mathrm{g}\mathrm{Z}{\mathrm{n}}_2}}{{\mathrm{K}}_{\mathrm{A}\mathrm{l}}^{\mathrm{M}\mathrm{g}\mathrm{Z}{\mathrm{n}}_2}})}}$$

(1)

${\mathrm{W}}_{\mathrm{M}\mathrm{g}\mathrm{Z}{\mathrm{n}}_2}$ is the mass fraction of MgZn₂, ${\mathrm{I}}_{\mathrm{M}\mathrm{g}\mathrm{Z}{\mathrm{n}}_2}$ is the strongest peak area of MgZn₂, ${\mathrm{K}}_{\mathrm{A}\mathrm{l}}^{\mathrm{M}\mathrm{g}\mathrm{Z}{\mathrm{n}}_2}$ is the ratio of ${\mathrm{K}}_{\mathrm{M}\mathrm{g}\mathrm{Z}{\mathrm{n}}_2}$, and ${\mathrm{K}}_{\mathrm{A}\mathrm{l}},{\mathrm{K}}_{\mathrm{A}\mathrm{l}}^{\mathrm{A}\mathrm{l}}$ is the ratio of ${\mathrm{K}}_{\mathrm{A}\mathrm{l}}$ and ${\mathrm{K}}_{\mathrm{A}\mathrm{l}}$.

According to the calculation, the contents of MgZn₂ in the Al-Zn-Mg-Cu alloy at the third and fourth remelting are 2.32% and 2.18%, respectively. The results show that the semi-fused η’ phase is transformed into non-fused η phase in the remelted state. Therefore, based on the above analysis, the distribution of a large number of small dispersed GP region and η’ phase in the grain makes it have a high strength at the third remelting.

From Figure 7a, it is evident that following the initial T6 treatment, the diffraction peak of MgZn is particularly prominent. However, subsequent multiple T6 treatments combined with remelting result in a gradual decrease in the diffraction peak intensity of MgZn, indicating a reduction in the MgZn phase. These observations suggest that the remelting process has a significant impact on the phase composition of the alloy. Additionally, it is conceivable that a portion of Mg and Zn within the matrix may react with other elements, leading to the formation of Al₂Cu and S(Al₂CuMg) phases. This reaction could contribute to the depletion of Mg and Zn in the matrix.

Figure 7b reveals that during the third remelting, the diffraction peak intensity of MgZn₂ attains its peak value. This phenomenon can be attributed to the fine-grained structure of MgZn₂, which is intimately interlocked with the matrix. The smaller grain size of MgZn₂ enhances its strengthening effect. Alternatively, it is possible that with continued remelting, MgZn and Al_xCu phases within the alloy become more prevalent than MgZn₂, resulting in a morphological shift. When the number of remelting exceeds a certain threshold, the MgZn₂ within the alloy transforms entirely into MgZn.

Figure 8 illustrates the impact of remelting cycles on the grain structure evolution of the Al-Zn-Mg-Cu aluminum alloy. Following a T6 heat treatment, the ordered atomic groups and dispersed phases within the melt serve as nucleation sites during alloy casting [22]. The characteristics of these nucleation sites, including their structure and size, significantly influence the microstructure of the alloy upon solidification. When the die-casting of the alloy results in a fine grain structure, it is indicative of a higher number of crystal nuclei and grain boundaries within the material.

During the remelting process, the atomic groups within the melt become more ordered, leading to an increased number of potential nucleation embryos. Consequently, even after remelting, the grain structure remains relatively fine. Conversely, if the initial casting exhibits a coarser grain structure, it suggests the presence of fewer ordered atomic groups within the aluminum alloy melt following remelting. This, in turn, results in a reduced number of nucleation embryos, leading to a persistence of the coarser grain structure even after the remelting process. These observations highlight the intricate relationship between the thermal treatment, melt composition, and the resulting grain structure of 7075 aluminum alloy. Understanding and controlling these variables is crucial for optimizing the mechanical properties and performance of the alloy.

It is evident from observations that as the number of remelting iterations increases, the atomic clusters within the alloy’s microstructure undergo gradual melting, particularly those of smaller sizes. Consequently, there is a decrease in the overall count of atomic clusters with each subsequent remelting process. This decrease, in turn, translates to a reduced nucleation substrate during solidification, leading to a smaller quantity of grains formed. It is noteworthy that each atomic cluster possesses the capability to serve as a germination site for novel grains. Therefore, a decrement in the number of crystal nuclei during solidification necessitates an elevation in grain size. As the grain size enlarges, there is a corresponding decrement in the grain boundary area and, consequently, a reduction in grain boundary defects. However, this enlargement in grain size is indicative of a gradual decline in the microstructural properties of the alloy. Excessive remelting results in grain coarsening and a progressive deterioration of microstructural properties.

Dendritic textures, along with some unmelted particles and portions of the matrix surface, can serve as nucleation sites for crystal formation, resulting in a fine and uniform microstructure in the alloy after the third remelting. In general, the function of yield strength and grain size can be expressed by the Hall–Petch equation [23]:

$$\sigma ={\sigma }_0+\mathrm{K}{d}^{-1/2}$$

(2)

where $\sigma$ is the required yield strength, ${\sigma }_0$ is the yield stress of a single crystal, and K is a constant, representing the influence of grain boundaries on strength; d is the average size of each grain. Upon undergoing three iterations of remelting, the grain size of the alloy is significantly reduced. This refinement in grain size leads to an enhancement in both the yield stress and toughness of the alloy, thereby achieving the desired effects of grain refinement and strength augmentation.

4. Conclusions

In this study, the effects of multiple remelting and repeated heat treatment on Al-Zn-Mg-Cu alloys were investigated to analyze property changes during the remelting process and determine the optimal number of remelting cycles. The key conclusions are as follows:

(1)

The properties of the alloy were studied after multiple heat treatment and four times remelting. It can be seen from the metallographic structure that under the same conditions, the grain size becomes smaller and smaller and the dendrites are developed during remelting, that is, the grains are refined during the remelting process. With the increase in the number of remelting times, the alloy underwent different structural changes due to the recovery, recrystallization and grain growth during the remelting process, and the mechanical properties of the alloy were improved. Under the state of remelting, the alloy demonstrates an initial increase followed by a decrease in both fracture strength and yield strength. Comparative analysis reveals that, relative to the A2 sample, the ultimate tensile strength undergoes a significant enhancement of 88.7%, rising from 115.4 MPa to 217.7 MPa. Similarly, the yield strength increases by 84.5%, from 115.1 MPa to 212.5 MPa. However, at the fourth remelting iteration, both tensile and yield strengths undergo a notable decrease, with ultimate tensile strength dropping to 147.2 MPa and yield strength to 147.5 MPa. The yield strength (YS, σ0.2) and ultimate tensile strength (UTS) are the parameters obtained when the strain rate is 1 mm/min.

(2)

Under the same conditions, it is found that the fracture strength and yield strength of the alloy without remelting are better than those in the melting state. With the increase in remelting times, α-Al dendrites and MgZn₂ phase continue to maintain favorable grain shape and size, and their mechanical properties are still high. However, due to the continuous transformation of the alloy phase during remelting, the morphology and size of α-Al dendrites and MgZn₂ phase begin to deteriorate, leading to the decrease in mechanical properties.

(3)

Upon successive remelting, the MgZn₂ phase within the alloy undergoes a transformation to MgZn, which, along with Al_xCu, exhibits a more favorable morphology relative to MgZn₂. With each iteration of heat treatment, the MgZn₂ phase that precipitates become increasingly coherent with the aluminum matrix, with a diminutive size correlating positively with strengthening effects.

(4)

In the context of the remelting process, there is a progressive ordering of atomic groups within the melt, which augments the population of potential nucleation embryos. This ordering leads to a refinement of the grain structure, which is maintained even post-remelting. In contrast, an initial coarser grain structure in the casting implies a scarcity of ordered atomic groups within the aluminum alloy melt subsequent to remelting. This deficiency in ordered atomic groups correlates with a diminished number of nucleation embryos, perpetuating the coarser grain structure through the remelting process. These findings underscore the complex interplay between thermal treatments, melt composition, and the resultant grain structure in the 7075 aluminum alloy, emphasizing the significance of these factors in dictating material properties.