No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Phase transformation and microstructure evolution of an ultra-high strength Al-Zn-Mg-Cu alloy during homogenization
Abstract
Homogenization treatment of an ultra-high strength Al-Zn-Mg-Cu alloy was studied by SEM, TEM, DSC, hardness and electrical conductivity test. The second phases in as-cast 7A56 alloy were AlZnMgCu, Al2Cu and Al7Cu2Fe. AlZnMgCu phase has a similar structure of MgZn2. After 380 °C homogenization treatment, transformation from AlZnMgCu to Al2CuMg occurred; after 470 °C homogenization treatment, AlZnMgCu were mainly directly dissolved into the matrix. That phenomenon was caused by the diffusion behavior of Zn and Cu at two different temperatures that Zn diffuses rapidly even at 380 °C while Cu shares a much lower diffusion rate. Much of the AlZnMgCu phase can be dissolved at 470 °C for a short time while at 380 °C it was seldom eliminated. After homogenized at 470 °C for 24 h, the dissolution process gradually fall into a balance with more than 95% had been eliminated and prolonging time imposes no evident influence on promoting the dissolution. Hardness and electrical conductivity are highly consistent with the vibration of AlZnMgCu phase. With the dissolution of which the hardness increases while electrical conductivity increases.
... AA7075, AA7050, AA7055, AA7085, are commonly applied in the transportation industry [1][2][3] with long-term objective of better integrated properties such as high strength and fracture toughness, good corrosion resistance and high fatigue resistance [4][5][6][7][8][9][10][11][12][13][14][15]. These properties are highly impacted by the strengthening precipitates, dispersoids and coarse constituents, i. e., insoluble Cu-/Fe-rich intermetallics and the partially soluble S (Al 2 CuMg) phase [7][8][9], as well as serious macro-/micro-segregation [12][13][14][15][16], all of which highly depend on the composition and processing [17][18][19][20][21][22][23][24][25][26][27][28][29][30]. Specifically, the soluble g (MgZn 2 ) phase or its precursors such as metastable g' and GP zones are mainly responsible for the alloy strength [7], while the workability and fracture behavior are highly affected by the soluble T (Al 2 Mg 3 Zn 3 or Al 6 Mg 11 Zn 11 ) phase, partially soluble S phase, insoluble Al 7 Cu 2 Fe and Mg 2 Si phases [8,14,[31][32][33][34], besides processing parameters. ...
... Fig. 9 shows the variation of the Zn:Mg ratio and (Mg + Cu) content as well as related phase components in the as-cast alloys. It reveals that the alloys with high Zn:Mg ratio and low (Mg + Cu) content (such as AA7085 [63,64], AA7037 [63,65] and AA7056 [21,43,65], AA7097 [65] alloys) mainly contain a-Al and M phase but the alloys with low Zn:Mg ratio and high (Mg + Cu) content (such as AA7x50 alloy [47,66] and the present alloys) mainly contain a-Al, M and S or Cu-enriched phases. However, the as-cast alloys only contain a-Al and M phases for low Zn:Mg ratio and low (Mg + Cu) content (such as AA7040/7010 [54], AA7449 [43] alloy), or for high (Mg + Cu) content and high Zn:Mg ratio (such as AA7136 [43], AA7095 [65] alloys). ...
... The alloys with high Zn:Mg ratios do not contain the Cu-enriched phase or S phase for equal (Mg + Cu) content, such as the alloys 2-2 and 3-1, AA7095 [65] (Al-9.1Zn-2.0 Mg-2.2Cu, wt%) and AA 7A56 [21] (Al-9.0Zn-2.1 Mg-2.0Cu, wt%) alloys. It can be found that the as-cast alloys mainly contain a-Al and M phases except for the blue area that may contain additional Cu-enriched or S phase (Fig. 9). ...
... AA7075, AA7050, AA7055, AA7085, are commonly applied in the transportation industry [1][2][3] with long-term objective of better integrated properties such as high strength and fracture toughness, good corrosion resistance and high fatigue resistance [4][5][6][7][8][9][10][11][12][13][14][15]. These properties are highly impacted by the strengthening precipitates, dispersoids and coarse constituents, i. e., insoluble Cu-/Fe-rich intermetallics and the partially soluble S (Al 2 CuMg) phase [7][8][9], as well as serious macro-/micro-segregation [12][13][14][15][16], all of which highly depend on the composition and processing [17][18][19][20][21][22][23][24][25][26][27][28][29][30]. Specifically, the soluble g (MgZn 2 ) phase or its precursors such as metastable g' and GP zones are mainly responsible for the alloy strength [7], while the workability and fracture behavior are highly affected by the soluble T (Al 2 Mg 3 Zn 3 or Al 6 Mg 11 Zn 11 ) phase, partially soluble S phase, insoluble Al 7 Cu 2 Fe and Mg 2 Si phases [8,14,[31][32][33][34], besides processing parameters. ...
... Fig. 9 shows the variation of the Zn:Mg ratio and (Mg + Cu) content as well as related phase components in the as-cast alloys. It reveals that the alloys with high Zn:Mg ratio and low (Mg + Cu) content (such as AA7085 [63,64], AA7037 [63,65] and AA7056 [21,43,65], AA7097 [65] alloys) mainly contain a-Al and M phase but the alloys with low Zn:Mg ratio and high (Mg + Cu) content (such as AA7x50 alloy [47,66] and the present alloys) mainly contain a-Al, M and S or Cu-enriched phases. However, the as-cast alloys only contain a-Al and M phases for low Zn:Mg ratio and low (Mg + Cu) content (such as AA7040/7010 [54], AA7449 [43] alloy), or for high (Mg + Cu) content and high Zn:Mg ratio (such as AA7136 [43], AA7095 [65] alloys). ...
... The alloys with high Zn:Mg ratios do not contain the Cu-enriched phase or S phase for equal (Mg + Cu) content, such as the alloys 2-2 and 3-1, AA7095 [65] (Al-9.1Zn-2.0 Mg-2.2Cu, wt%) and AA 7A56 [21] (Al-9.0Zn-2.1 Mg-2.0Cu, wt%) alloys. It can be found that the as-cast alloys mainly contain a-Al and M phases except for the blue area that may contain additional Cu-enriched or S phase (Fig. 9). ...
The alloy design and homogenization processes are intimately associated with the microstructure, phase composition and performance for Al-Zn-Mg-Cu alloys. The microstructures and phase composition of a series of Al-Zn-Mg-Cu alloys before and after the homogenization treatments were investigated along with thermodynamic calculation to understand the underlying relationship. The eutectic microstructures (α-Al+M (Mg(ZnAlCu)2)) are dominating with Cu-enriched [AlCuMgZn] particles, both depending on the Zn:Mg ratio and (Cu+Mg) content, in addition to minor constituent θ (Al2Cu) and Al7Cu2Fe phases in the as-cast alloys. The optimal homogenization process was suggested based on the analysis of the residual phases (i.e., the S (Al2CuMg) phase) since all (for low/mediate-(Cu+Mg) alloys) or partially (for high-(Cu+Mg) alloys (∼>4.24 wt%)) S (Al2CuMg) particles were dissolved during the homogenization. This residual S phase may be transformed from the primary M and/or Cu-enriched [AlCuMgZn] phases. The homogenization kinetics calculation results agreed well with above experimental results. A critical (Cu+Mg) level and a linear correlation between Cu and Mg concentrations were revealed based on the thermodynamically modelling, which can be conductive to determine the optimal homogenization process. Furthermore, the solubility limit and stoichiometric balance principles based on controlling the homogenized microstructures can guide the composition design for advanced high-strength aluminum alloys.
... The transportation of alloy elements is calculated via the scalar transportation equation. Necessary transportation properties of the main alloy elements Zn, Mg, Cu, and Sc can be found in [27,28]. In Case 11, the 5B71 and 7055 wires were fed in the same XOZ plane with an included angle of 30°, where 5B71 was the upper wire, and 7055 was the lower wire. ...
... The transportation of alloy elements is calculated via the scalar transportation equation. Necessary transportation properties of the main alloy elements Zn, Mg, Cu, and Sc can be found in [27,28]. In Case 11, the 5B71 and 7055 wires were fed in the same XOZ plane with an included angle of 30 • , where 5B71 was the upper wire, and 7055 was the lower wire. ...
Double-wire arc welding involves simultaneously feeding two wires into a molten pool, improving the efficiency and flexibility of traditional welding techniques. However, the interactions between the two wires and the molten pools are complex, which increases the difficulties in process and composition control. This work focuses on the weld pool flow characteristics in double-wire TIG arc welding. A CFD model incorporating a liquid bridge transfer model was developed to simulate the fluid flow phenomenon. Results show that the bead-forming appearances and flow characteristics of double-wire arc welding show no significant differences from single-wire arc welding. Welding current and welding speed have significant effects on the weld bead dimensions, while only welding current has effects on the flow characteristics. Wire feed XOZ angles show no significant influences on weld bead forming appearances and molten pool flow characteristics. Wire feed XOY angles influence the symmetry of the weld bead and the fluid flow. In 5B71/7055 heterogeneous double-wire arc welding, achieving a uniform distribution of alloy elements is difficult due to the complex convection patterns within the molten pool.
... However, Chen did not distinguish the primary T B eutectic phase from the later precipitated T B phase, and neither of them found that the S phase transformed into the θ phase during homogenization. Some researchers have found the η-S phase transformation during the homogenization of the 7-series alloy (Al-Zn-Mg-Cu) by calculating the atomic diffusion velocity, SEM observation, TEM observation and other means [48,49], the S-θ phase transformation in this work could have similar behaviors. In further research, we will perform a more in-depth study on the S-θ phase transition, including the kinetics of such transformation and the effects of alloying elements and pretreatment parameters on this process, and apply computational simulations to study the microstructural evolution of Al-Cu-Li alloys. ...
... However, Chen did not distinguish the primary TB eutectic phase from the later precipitated TB phase, and neither of them found that the S phase transformed into the θ phase during homogenization. Some researchers have found the η-S phase transformation during the homogenization of the 7-series alloy (Al-Zn-Mg-Cu) by calculating the atomic diffusion velocity, SEM observation, TEM observation and other means [48,49], the S-θ phase transformation in this work could have similar behaviors. In further research, we will perform a more in-depth study on the S-θ phase transition, including the kinetics of such transformation and the effects of alloying elements and pretreatment parameters on this process, and apply computational simulations to study the microstructural evolution of Al-Cu-Li alloys. ...
An optimized homogenization process for Al alloy ingots is key to subsequent material manufacturing, as it largely reduces metallurgical defects, such as segregation and secondary phases. However, studies on their exact microstructural evolution at different homogenization temperatures are scarce, especially for complex systems, such as the 2195 Al-Cu-Li alloy. The present work aims to elucidate the microstructural evolution of the 2195 Al-Cu-Li alloy during homogenization, including the dissolution and precipitation behavior of the TB (Al7Cu4Li) phase and S (Al2CuMg) phase at different homogenization temperatures. The results show that there are Cu segregation zones (Cu-SZ) at the dendrite boundaries with θ (Al2Cu) and S eutectic phases. When the temperature rises from 300 °C to 400 °C, fine TB phases precipitate at the Cu-SZ, and the Mg and Ag in the S phases gradually diffuse into the matrix. Upon further increasing the temperature to 450 °C, TB and θ phases at the grain boundaries are coarsened, and an S-θ phase transition is observed. Finally, at 500 °C, all TB and S phases are dissolved, leaving only θ phases at triangular grain boundaries. This work provides guidance for optimizing the homogenization procedure in 2195 alloys.
... The composition of intermetallic phases in Al-Zn-Mg-Cu alloy is complicated due to the variety of elements and the high main alloying element content. According to previous research reports, numerous intermetallic phases formed in as-cast alloy, including T(AlZnMgCu), θ(Al2Cu), S(Al2CuMg), η(MgZn2) and Al7Cu2Fe, etc [9][10][11][12][13]. The second phases with different types in as-cast alloy are not the same with the change of main alloying elements content. ...
... °C, 473.5 °C, and 471.1 °C, respectively. According to the research of other researchers [10], the endothermic peak at this temperature corresponds to the dissolution of the nonequilibrium eutectic Mg(Zn, Cu, Al)2 phase. It can be seen that the melting temperature of the eutectic Mg(Zn, Cu, Al)2 phase tends to decrease with the increase of Zn/Mg ratio. ...
The effect of Zn/Mg ratio on the as-cast microstructure and its evolution during homogenization of Al-Zn-Mg-Cu alloys was investigated by optical microscopy (OM), differential scanning calorimetry (DSC), scanning electron microscope (SEM) and X-ray diffraction (XRD). Experimental results showed that serious dendritic segregation existed in the as-cast microstructures while the second phases were mainly AlZnMgCu phase and Al2Cu phase. With the Zn/Mg ratio increasing from 1.5 to 2.0, the area fraction of AlZnMgCu phase decreased from 2.85% to 2.53%, which was attributed to the content of Mg element. Non-equilibrium eutectic phases dissolved into the matrix during homogenization and phase transformation from AlZnMgCu phase to Al2CuMg phase (S phase) was observed in low-Zn/Mg ratio alloy and mid-Zn/Mg ratio alloy. In the high Zn/Mg ratio alloy, the eutectic AlZnMgCu phase directly dissolved into the matrix during the homogenization, and no transformation from AlZnMgCu phase to S phase was found. A higher number of S phases appeared in low-Zn/Mg ratio alloy during homogenization treatment compared with mid-Zn/Mg ratio alloy with a regime of 465°C/24h. It could be inferred that low-Zn/Mg ratio alloys had a stronger phase transformation tendency from AlZnMgCu phase to S phase. Increasing the homogenization treatment temperature could impair the transition tendency from AlZnMgCu phase to S phase.
... These brittle precipitates along the grain boundaries may accelerate crack initiation and propagation, thereby deteriorating the mechanical properties [3]. As reported by Ref. [51,52], the main types of precipitates in the Al-Zn-Mg-Cu alloy were η(Mg(Zn,Cu,Al) 2 ), S(Al 2 CuMg) and T(Al 2 Mg 3 Zn 3 ) phases under the equilibrium solidification condition. The molten pool solidification during the deposition process was non-equilibrium [53], so the formation and distribution of precipitation phases were different. ...
The pore defects and grain coarsening are the main reasons for performance deterioration in wire-arc directed energy deposition (DED) of ultra-high-strength aluminium alloy. In this study, experimentation and numerical simulation were performed to investigate the Zn element loss, bubble behaviours, microstructure and performance and effects of wire feeding rate (WFR) in wire-arc DED of 7075 aluminium alloy. The results suggested that both the Zn-bubble and H-bubble did not flow upward directly, but flowed with the molten pool convection. A low WFR decreased the sources of Zn vapour and hydrogen instead of promoting the bubble escape, thus the porosity was decreased (up to 95.8% decrease with WFR = 4 m/min). Meanwhile, it reduced the grain morphology difference, and promoted the dispersed distribution of precipitated phases. After the heat treatment, the optimal performance reached 527.80 MPa of ultimate tensile strength and 7.57% of elongation, approaching the performance level of a forged 7075-T6 plate.
... Spectrum 5 corresponds to the Al 13 Fe 4 phase. Although some Cu has dissolved in this phase, this is not sufficient to form the Al 7 Cu 2 Fe phase, as the Cu content should be higher than the Fe content 50 . With longer annealing times (as in samples A6 and A9), the amount of dissolved Cu increases. ...
The high-strength 7xxx Al alloys are frequently used due to their excellent properties. To achieve these properties, heat treatment is crucial. In this study, the influence of La on the microstructure evolution of Al–Zn–Mg–Cu alloys during solution annealing, the first step of heat treatment, was investigated. The results showed that La additions significantly affect the microstructure by increasing in the eutectic solidus temperature, influencing the melting enthalpy of Mg2Si/LaAlSi, promoting the formation of LaAlSi and reducing Mg2Si, increasing the melting enthalpy of α-Al, shortening the time to reach the eutectic melting peak and preventing a significant increase in grain size. A 10 h solution heat treatment was recommended, with minimal benefits after 12 h. 0.15 wt% La was the minimum to prevent an increase in grain size. La altered the Al45Cr7 phase and formed a new Al20Cr2La phase with larger dimensions and sharper edges. Faster cooling rates refine the grain size due to precipitation along the grain boundaries. The content of the T-phase (Mg32(Al, Cu, Zn)49) decreased with increasing annealing time. Prolonged annealing promoted the diffusion of Mg and Zn from the eutectic phases, which led to their dissolution in the matrix. Zn diffused out first, followed by Mg. Prolonged annealing favored the formation of the Al7Cu2Fe phase over the Al13Fe4 phase.
... After the hightemperature holding, as the nucleation sites, the T phase dissolves and only the homogeneous Al 3 Zr is left. A significant improvement of the intergranular corrosion (IGC) resistance can be obtained finally [60]. ...
Both preliminary heat treatment and final heat treatment play an irreplaceable role in the production of aluminum alloy products. The homogenization degree of compositions, the size of the dispersoid and its coherent relationship with the matrix, the size and aspect ratio of recrystallized grains, the supersaturation of solutes, and the characteristics of precipitates directly determine the mechanical properties, corrosion resistance, and the workability of materials. In engineering applications, non-isothermal phenomena involving heating and/or cooling processes are inevitable due to the impact of product scales. Therefore, novel heat treatment technology with higher engineering applicability is a necessary extension of the solid-state phase transformation theory. Based on the typical wrought aluminum alloys such as AlZnMg(Cu), AlMgSi, and AlCu(Li), the current researches on engineeringable homogenization, solid solution, and aging technologies were summarized. The novel technologies focused on multistage heat treatments and non-isothermal heat treatments.
... Some researchers were concentrated on developing new types of dispersoids by adding different rare earth elements [9,11,12] while some investigated the match of homogenization and aging treatments to obtain good property [13]. Still, there were investigations focusing on the dissolution of eutectic phases with or without chemical composition variations [14][15][16]. But second phase dissolution conditions during different homogenization stages still needs further investigations. ...
The dissolution of second phase with relatively high melting point in as-cast Al-Zn-Mg-Cu alloys was closely related to Mg and Cu contents. In present work, second phases in three Al-Zn-Mg-Cu alloys with simultaneously enhanced Mg and Cu contents (named by LMC alloy, MMC alloy and HMC alloy as Mg and Cu contents progressively enhanced) were analyzed and the correlated dissolution during homogenization was investigated. The results showed that both Mg(Zn,Cu,Al)2 phase and Cu-rich phase existed in as-cast alloys while HMC alloy possessed more eutectic phases. As homogenized by 470°C/24h, Mg(Zn,Cu,Al)2 phase had dissolved completely, LMC alloy contained little Al2CuMg phase and the amount of it for the three alloys was arranged as LMC alloy < MMC alloy < HMC alloy. As furtherly homogenized by a second stage at 480°C for 12h, no endothermic peak for Al2CuMg phase was observed for LMC alloy and only Fe-rich phase existed. Meanwhile, Al2CuMg phase still remained in MMC and HMC alloy. As the homogenization time prolonging to 36h, Al2CuMg phase in MMC alloy dissolved completely while that still existed in HMC alloy. Adding a third stage at 490°C for HMC alloy, no Al2CuMg phase could be observed for 24h. This gave rise to a method by incrementally grading homogenization temperature combined with prolonging soaking time to fulfill the dissolution of second phase for Al-Zn-Mg-Cu alloys with enhanced Mg and Cu contents
... X-ray diffraction patterns of as-deformed alloys are shown in Fig. 2. The analysis of the XRD diffraction peaks shows that the phase structure consists of α(Al) and η(MgZn2) in as-deformed alloys. The study of Xu [14] declared that the lattice structure of the AlZnMgCu phase is similar to MgZn2 phase. They all exhibit a hexagonal structure. ...
The main alloying elements have a decisive influence on the type and quantity of the second phase of the Al-Zn-Mg-Cu alloy, and even on the dissolution of the second phase during solution treatment. The effect of Zn/Mg ratio on second phase dissolution during solution treatment of Al-Zn-Mg-Cu alloys was investigated by means of differential scanning calorimetry (DSC), scanning electron microscope (SEM) and electrical conductivity testing. The results showed that Mg (Zn,Cu,Al)2 phase and Fe-rich phase existed in the as-deformed alloys. In addition, a small amount of Al2CuMg phase was found in the low and medium Zn/Mg ratio alloy. The number of Mg (Zn,Cu,Al)2 phases as the major second phase in the alloys was inversely proportional to the Zn/Mg ratio. Mg (Zn,Cu,Al)2 phase essentially dissolved into the matrix after solution treatment at 465°C/2h. Increasing the solution temperature and time were both beneficial to the dissolution of the Al2CuMg phase. With the increase of the solution temperature from 465°C to 475°C, the conductivity of the alloy showed a decreasing trend initially and then increased. As the solution time increased at 470°C, the electrical conductivity of the low-Zn/Mg ratio alloy decreased and then increased due to the more secondary phase. After the second phase was fully dissolved in the alloy, the electrical conductivity gradually increased with the increase of the solution time.
... In summary, due to the pressure of the development cycle and the pursuit of rapid achievement, some difficult basic theories with a long research cycle have been increasingly ignored by domestic and foreign scholars, mainly including the basic theory of heat treatment and the strengthening mechanisms [63], plastic deformation mechanisms [64], and fracture mechanisms of HSAA [65]. ...
Due to their high strength, high toughness, and corrosion resistance, high-strength aluminum alloys have attracted great scientific and technological attention in the fields of aerospace, navigation, high-speed railways, and automobiles. However, the fracture toughness and impact toughness of high-strength aluminum alloys decrease when their strength increases. In order to solve the above contradiction, there are currently three main control strategies: adjusting the alloying elements, developing new heat treatment processes, and using different deformation methods. This paper first analyzes the existing problems in the preparation of high-strength aluminum alloys, summarizes the strengthening and toughening mechanisms in high-strength aluminum alloys, and analyzes the feasibility of matching high-strength aluminum alloys in strength and toughness. Then, this paper summarizes the research progress towards adjusting the technology of high-strength aluminum alloys based on theoretical analysis and experimental verification, including the adjustment of process parameters and the resulting mechanical properties, as well as new ideas for research on high-strength aluminum alloys. Finally, the main unsolved problems, challenges, and future research directions for the strengthening and toughening of high-strength aluminum alloys are systematically emphasized. It is expected that this work could provide feasible new ideas for the development of high-strength and high-toughness aluminum alloys with high reliability and long service life.
... Several studies have identified the major secondary phases in Al-Zn-Mg-Cu alloy: η(Mg(Zn, Cu, Al) 2 ), T(Al 2 Mg 3 Zn 3 ), S(Al 2-CuMg), and θ(Al 2 Cu) [24][25][26]. Based on the EDS mapping and XRD analysis, the triangular-shaped secondary phases could be η(Mg(Zn, Cu, Al) 2 ). ...
... The composition of the phase wrapping Fe-rich intermetallics, represented by P2 measurement, contains Al, Zn, Mg and Cu, with the absence of Fe. The eutectic grey phases (P3) show enrichments of Zn, Mg and Cu, indicating the presence of T (AlZnMgCu) phases (Wang et al. 2018;Xu et al. 2017). ...
It remains challenging to achieve attractive mechanical strength and ductility in commercial 7xxx alloys fabricated by wire-arc additive manufacturing (WAAM). Here, we combine WAAM processing with solution and artificial aging treatments to functionalise their inherent age-hardening capabilities via microstructure controlling. The as-deposit WAAM 7055 alloy shows a heterogeneous microstructure with a bimodal grain size distribution, and the primary metallurgical defects are spherical gas pores with an inter-layer distribution. Upon the optimised T6 temper (470°C/2 h for solution treatment and 120°C/65 h for artificial aging), nano-sized platelike η’ precipitates are generated, and the tensile strength boosts from ∼226 MPa to ∼562 MPa. Evident yielding with a uniform plastic deformation is also mediated by the T6 temper, with the yield strength > 500 MPa and the fracture strain > 5%, which is comparable to conventional wrought 7xxx alloys. This work provides insight into the microstructure tuning for commercial 7xxx alloys via WAAM for achieving excellent mechanical properties. © 2022 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
... Summary of alloy elements on relevant properties of Al-Zn-Mg-Cu[3,6,[8][9][10][11][12][13][14][15][16][17][18]. ...
Notably, 7xxx series aluminum alloy has become the most popular nonferrous alloy, extensively used in industry, construction and transportation trades, due to its high comprehensive properties such as high static strength, high strength, heat resistance, high toughness, damage resistance, low density, low quenching sensitivity and rich resource. The biggest challenge for aluminum alloy today is to greatly improve the corrosion resistance of the alloy, while maintaining its strength. The preparation method of 7xxx series aluminum alloy requires controlling time lapses in the process of heating, holding and cooling, and there are many species precipitates in the crystal, but the precipitated strengthening phase is a single type of equilibrium η′ phase. Therefore, more attention should be paid to how to increase the volume fraction of η′ precipitates and modify the comprehensive performance of the material and focus more on the microstructure of the precipitates. This article reviews the progress of 7xxx series aluminum alloy materials in micro-alloying, aging precipitation sequence, the strengthening-toughening mechanism and the preparation method. The effect of adding trace elements to the microstructure and properties of 7xxx series aluminum alloy and the problems existing in aging precipitation characteristics and the reinforcement mechanism are discussed. The future development direction of 7xxx series aluminum alloy is predicted by developing a method for the process-microstructure-property correlation of materials to explore the characteristic microstructure, micro-alloying, controlling alloy microstructure and optimizing heat-treatment technology.
This study examines the effect of homogenization and Zn/Mg ratio on the mechanical properties, stress corrosion cracking resistance, and quench sensitivity of Al–Zn–Mg–2.5Cu (AA7095) alloys. The alloys are processed using one‐stage and two‐stage homogenization followed by high‐temperature solution treatment and T6 aging. Mechanical properties, including hardness, tensile strength, and ductility, are evaluated. Quench sensitivity is assessed by measuring the loss of hardness at varying distances from the water‐quenched end. The results indicate that alloy B (5.4) exhibits the highest quench sensitivity due to higher precipitation kinetics. Additionally, the study reveals that the alloy with a higher Zn/Mg ratio (5.4) displays better precipitation strengthening but shows a more significant loss in hardness during quenching. SCC resistance is evaluated in both air and NaCl solution environments, showing that the alloy with a lower Zn/Mg ratio demonstrated better SCC resistance. These findings provide insights into optimizing the mechanical properties and resistance to SCC of AA7095 alloys through homogenization and Zn/Mg ratio control.
The microstructural evolution and mechanical properties of semi-continuous cast 7A62 alloy during single and double-stage homogenizations were studied, and the diffusion behavior was analyzed. The results show that the secondary phases are effectively dissolved back during homogenization, and the alloy contains the minimum residual phases when homogenized at 465 °C. The hardness reaches the maximum value after 465 °C/24 h treatment and then decreases with the elevating of homogenization temperature. By comparison with the as-cast alloy, both the tensile strength and elongation of the 465 °C/24 h sample are significantly improved. Thus, the most appropriate single-stage homogenization for the 7A62 alloy is determined to be 465 °C/24 h. After the double-stage homogenization at 420 °C/4 h + 465 °C/28 h, the residual phases are fewer and the alloy elongation is further improved in comparison with the single-stage treatment. The diffusion kinetics of the alloying element atoms are investigated concerning the size and morphology of secondary phases. The strengthening mechanisms are also analyzed and related to the microstructure.
To increase Al-Zn-Mg-Cu alloys’ resistance to corrosion, an intermittent variable rate non-isothermal aging is proposed to solve this problem. Through the use of polarization curves, electrochemical impedance spectroscopy, intergranular corrosion (IGC), exfoliation corrosion (EXCO), and transmission electron microscopy, researchers examined the effects of intermittent variable rate non-isothermal aging on the corrosion resistance and microstructure morphology of Al-Zn-Mg-Cu alloy. The results show that when the heating rate before and after intermittent aging is 10 °C/h and 30 °C/h, respectively, the minimum IGC depth of the alloy is 18.9 μm, the slowest corrosion rate is 0.0065 mm/a, and the EXCO grade is P. In addition, the grain boundary precipitation phase of the alloy is intermittently distributed and independently rounded, and the PFZ width is 75.9 nm. It effectively obstructs the anodic corrosion channel, and slows down the corrosion progression. The highest content of Cu ions increases the electrode potential of GBPs and enhances the corrosion resistance of the alloy.
The research study explored the impact of magnesium on the microstructural characteristics and mechanical properties of Al-Mg-Mn alloy in both its as-cast and homogenized states. Various analytical techniques including optical microscopy, x-ray diffraction, scanning electron microscopy, energy-dispersive x-ray spectroscopy, Vickers microhardness testing, and electrical conductivity measurements were employed for the analysis of the Al-Mg-Mn alloy. The findings reveal that the primary phases present in the as-cast alloy consist of α-Al, β (), and . As the magnesium content increases, notable alterations are observed in the diffraction angle θ, inter-planar spacing d, lattice constant a, and the solute magnesium concentration in the α-Al matrix. In instances where the Mg content is low, the grain size and distribution in both the as-cast and homogenized alloys appear coarse and uneven. However, as the magnesium content rises, there is a reduction in the primary dendrite length and columnar crystal zone in the as-cast alloy, alongside enhanced development of secondary dendrites and gradual diminishment of the grain size. This reduction in grain size is particularly pronounced in the homogenized alloy. Additionally, the hardness of both the as-cast and homogenized alloys experiences an increase, while the electrical conductivity undergoes a decrease with escalating magnesium content. Notably, in comparison with the as-cast alloy, the hardness and electrical conductivity of the homogenized alloy exhibit enhancements. The outcomes of this study offer valuable insights for the subsequent processing and deformation applications of the Al-Mg-Mn alloy.
Wire-arc directed energy deposition is promising for manufacturing ultra-high-strength Al-Zn-Mg-Cu alloy components. However, the undesired microstructure and performance of as-deposited alloys make the heat treatment a mandatory post-process, which still lacks systematic investigation. This work aims to establish a comprehensive understanding of it. An interplay between second-phase dissolution and defect formation in the single-stage solution of as-deposited Al-Zn-Mg-Cu alloy was found: as solution temperatures increase, the equilibrium proportion of residual phases decreases, but the proportion of pore defects increases significantly. Based on a systematic full-factor solution test, a 430°C/4 h + 450°C/4 h step solution process was proposed, which suppressed the pore defects by about 54.8% while keeping the effect of the residual phase dissolution unchanged. After the step solution and artificial aging, the average yield strength of the alloy reached 475 MPa, and the elongation reached 6.5%, showing 201.6% and 85.7% improvement compared with the as-deposited state, respectively.
7075 aluminum alloy is widely used in the aerospace field because of its low density, high specific strength, high fracture toughness, and good machinability. Herein, the evolution of the microstructure and properties of 7075 aluminum alloy at different temperatures is systematically studied. Based on the rolling and heat treatment process, a short process of rolling and heat treatment is proposed, which can significantly improve the properties. The study finds that the nanophases are distributed in a “streamline‐like” or “lip‐like” shape after rolling. When the rolling temperature is below 550 °C, the phases are mainly Al7Cu2Fe, the grain size does not change significantly, and shear bands appear inside the grains. When rolled at 600 °C, the grains coarsen significantly and the shear bands become longer, resulting in overburning and cracking. The broken phases are smoothed when rolled at 550 °C. It is observed that the nanophases becomes larger. Due to the strain‐strengthening effect, the ultimate tensile strength increases by 6% with little change in elongation. In industrial production, direct aging treatment after high‐temperature rolling can reduce energy consumption and improve production efficiency.
A significant constraint of aluminum alloys in aircraft and automobile industrial applications is their poor mechanical properties as well as low tribological behavior. In the present study, the correlation between microstructure, hardness, and wear behavior of high Zn (10 wt.%) alloys containing Al-Zn-Mg-Cu was studied. Homogenization treatment at 450°C in the range of 0 to 40 h, followed by an aging process at 120°C/24 h, was applied to the alloy samples. The results show that the alloy homogenized for 16 h exhibits peak hardness due to the presence of precipitates at the grain boundary region as well as in the matrix, which act as intense points for pinning in the Al-matrix to slow the dislocation movement. The wear behavior of the alloy was investigated by a dry sliding wear test. Worn surfaces were investigated to identify the wear mechanism. Archard’s and Fleischer’s wear models were analyzed to correlate with the wear test results of the alloy during homogenization time.
To enhance the formability, production efficiency and mechanical properties of Al–Mg–Si alloy, the 6063 Al alloy was extruded from as-cast, furnace-cooling and air-cooling homogenized billets and followed by online quenching, respectively. The effects of initial billet states, extrusion parameters and online quenching process on the formability, microstructure and mechanical properties of the profile were investigated. It was found that the second phases in as-cast and furnace-cooling homogenized billets not only brought no brittle crack during extrusion, but also refined the grain size. The online quenching of the 6063 Al alloy profiles with complex sections prevented the abnormal grain growth occurring in offline solid solution treatment, so as to improve production efficiency. The strengths of the profiles extruded from as-cast and air-cooling homogenized billets were equivalent. Therefore, most of 6063 Al alloy profiles could be directly extruded from as-cast billet without homogenization treatment, which improved the production efficiency. The furnace-cooling homogenized billet had the lowest flow stress, indicating lower break-through force to activate extrusion. The strength of the profile extruded from furnace-cooling homogenized billet was higher than that of air-cooling homogenized billet. Therefore, the furnace-cooling homogenized billet was recommended for the profile with complex section or higher performance requirement.
Cu-Zn-Al alloy is one type of brass, which has high strength and high corrosion resistant. It has been applied on ship propellers and marine equipment. In this research, the addition of aluminum (Al) with variation of 1, 2, 3, 4% aluminum to know the effect on mechanical properties and micro structure at casting process using a copper chill and without copper chill. This alloy is melted using furnace in 1100°C without holding. Then, the molten metal is poured into the mold with copper chill and without copper chill. The speciment of Cu-Zn-Al alloy were chracterized by using Optical Emission Spectroscopy (OES), Metallography Test, X-Ray Diffraction (XRD), Hardness Test of Rockwell B and Charpy Impact Test. The result is the addition of aluminum and the use of copper chill on the molds can reduce the grain size, increases the value of hardness and impact.
In this paper, the effect of two-stage homogenisation treatment on the evolution of the second phase was investigated. The microstructure of the extruded 7055 aluminium alloy was composed of α(Al) matrix and other second phases, including MgZn2, Al2CuMg, and Al7Cu2Fe phases. The segregation of second phase was serious and the content was 27.9%. During the 460°C × 24 h homogenisation, MgZn2 phases dissolved quickly, but there was still a significant amount of Al2CuMg phases present. After two-stage homogenisation (460°C × 24 h + 480°C × 10 h), the content of Al2CuMg phase decreased from 15% to 3% and the degree of segregation was reduced. In addition, the mechanical properties of the extruded 7055 alloy were effectively improved.
An Al-Mg-Sc-Zr alloy was additively manufactured by laser direct energy deposition (DED) under different laser powers, and the microstructures and mechanical properties of the as-deposited samples were investigated. The samples showed a fully equiaxed grain structure with grain sizes of 2–30 μm. Most of the blocky primary Al3(Sc, Zr)-precipitated phases (<5 μm) were arranged along the grain boundaries. A small amount of fine granular secondary Al3(Sc, Zr) phases (<0.5 μm) were precipitated owing to the cyclic heat treatment during the DED forming process. According to the EBSD(Electron backscatter diffraction) results, the texture index and strength of the sample were only slightly greater than 1, indicating that the material structure exhibited a certain but not obvious anisotropy. The sample in the horizontal direction had better yield strength, tensile strength, and elogation properties (399.87 MPa, 220.96 MPa, 9.13%) than that in the building direction (385.40 MPa, 219.40 MPa, 8.24%), although the sample in the plane had the finest equiaxed grains. The ductility of the sample deteriorated as the number of pores increased.
Although in-situ alloying is a flourishing technique for developing novel metal alloys for Laser Powder Bed Fusion (LPBF), the mixed alloying elements regularly segregate, causing a heterogeneous chemical composition enriched with new metastable phases. Hence, the present study focuses on the homogenization of an in-situ alloyed AlSi10Mg4Cu metal system by pursuing various heat-treatment pathways. We firstly investigated the alloy homogenization behavior upon conducting single-step heat-treatments at 485, 500, and 515 °C for set times. Experimental findings revealed that the alloy was challenging to homogenize in one-go. Heat-treating at 485 °C did not enable the complete dissolution of Cu-rich phases. By increasing temperature at 500 °C, the alloy was neither homogenized nor near-fully dense due to the untimely incipient melting of metastable Q-Al4Cu2Mg8Si7 phase which caused pores. Lastly, homogenization at 515 °C led to the enhanced dissolution of θ-Al2Cu phase and its precursors but simultaneously exacerbated the incipient melting process of Q-Al4Cu2Mg8Si7 phase observed at 500 °C. Based on the analyses of single-step homogenization behaviors, we designed a multi-step homogenization heat-treatment to dissolve the low-melting Q- phase selectively avoiding its deleterious incipient melting. The material underwent then a homogenization treatment with three consecutives steps at 495 °C for 4 h, 505 °C for 6 h and 515 °C for 6 h. The homogenization response was satisfying this time: incipient melting of Q-Al4Cu2Mg8Si7 phase was annihilated, and Cu solubilization was greatly enhanced. The supersaturation level was comparable with that of the as-built condition, i.e., optimal for following age-hardening treatments.
High-strength Al-Zn-Mg-Cu alloys such as AA7075 have drawn considerable attention and interest from both industry and academia owing to their high specific strengths and good fatigue resistance. Wire-arc directed energy deposition (DED), an emerging near-net-shape manufacturing technology, faces significant challenges in printing AA7075 due to its hot cracking susceptibility. In this study, we use nano-treated AA7075 wire as feedstock to additively manufacture a crack-free deposition of the high-performance alloy. After T6 heat treatment, the nano-treated AA7075 achieves exceptional yield strength (510.3 MPa), ultimate tensile strength (606.0 MPa), and elongation (12.6%). In addition, nanoparticles homogenize the microstructure upon solidification and inhibit grain growth from cyclic thermal exposure, yielding refined, equiaxed grains throughout the deposition and enabling isotropic mechanical properties in both as-built and T6-treated conditions. Thus, this study highlights a promising intersection of nano-treatment and wire-arc directed energy deposition for printing traditionally unprintable materials.
The recrystallization behavior of cold-rolled Al–Zn–Mg–Cu-based alloy sheets was systematically investigated by varying the Cu and Mg contents. With increasing Cu and Mg contents, the recrystallized grain size was reduced from 54.9 to 23.3 μm owing to the reduced ratio of the size to volume fraction of microscale particles. Interestingly, the sensitivity of different particles to particle-stimulated nucleation (PSN), which is reflected by the linear experimental coefficient (k), was dependent on their morphologies and relative fractions. The k value was considerably small (0.07) in the Cu-free alloys, implying that the Zn and Mg containing particles have an insignificant effect on the PSN. However, the k value increased significantly to 0.61 with the increase in Cu content to 1.3 wt%, because the rod-shaped Al7Cu2Fe particles formed a large plastic deformation zone around them. The addition of 2.0 wt% Mg led to the increase in the relative fraction of round-shaped Al2CuMg particles, increasing the k value from 0.46 to 0.86. The Al7Cu2Fe particles were a stronger source of PSN than the Al2CuMg particles, forming more recrystallized grains. Although Mg addition could increase the yield strength of the aged sheets, a strength–ductility trade-off arose. However, the addition of Cu simultaneously increased the yield strength and elongation because the uniform distribution of grain boundary precipitates caused by grain refinement could reduce the stress concentration during plastic deformation.
The effects of Mg content on the microstructures and second phases of the as-cast Al–Zn–Mg–Cu alloys were investigated. The results show that the nonequilibrium eutectic structure consists of α(Al), Al7Cu2Fe, η(MgZn2) and T(AlCuMgZn) intermetallic compounds. The alloys with the highest Mg content generate nonequilibrium eutectic structures. The maximum value of nonequilibrium eutectic structures is 10.13±0.62%. As the Mg content increases, the number of tertiary dendrites in the α(Al) matrix increases significantly, and more Mg can be dissolved into the Al- matrix. In addition, as the Mg content increases, the crystallization temperature range decreases from 169.8 K to 157.3 K. When the Mg content is higher than 2.6 wt.%, the microstructure evolution of the Al–Zn–Mg–Cu aluminum alloy is as follows: Liq. → Liq. + α(Al) → Liq. + α(Al) + Al7Cu2Fe → α(Al) + Al7Cu2Fe + η(MgZn2) + T(AlCuMgZn). These results play a certain role in promoting the basic research of Al–Zn–Mg–Cu aluminum alloys with high Mg content.
To improve the low compactness and limited size of powder metallurgy (P/M) aluminum alloy materials, cold rolling deformation on P/M aluminum alloy was carried out in this work. And the microstructure and mechanical properties of P/M aluminum alloy during cold rolling were investigated from multi-dimensional and multi-scale perspectives. The results indicate that the microstructure gradually evolved from the original equiaxial structure to the fiber structure after cold rolling. Also, the grain size decreased with an increase in cumulative reductions. When the cumulative reduction was increased from 0 to 80%, the relative density and tensile strength of the samples increased from 98.31% and 221.94 MPa to 99.04% and 302.88 MPa, respectively. On this basis, the synergetic effect of microstructure densification and deformation strengthening of the P/M 2024 Al alloy during cold rolling was investigated. The improved mechanical properties mainly resulted from the combined effects of the increased microstructural compactness and deformation strengthening.
The present study reports the dissolution behavior of the secondary alloy phases of an Al–8.3Zn–2Mg–2.4Cu–0.15Zr alloy (AA7055) during in-situ heating within a scanning electron microscope operating under highly evacuated condition. It has been shown that the dissolution of the major secondary alloy phases (i.e., η (MgZn2)-base and T (Al2Mg3Zn3)-base phases present in the inter-dendritic channels) is initiated at a substantially low temperature (~300 °C) under SEM operating condition. It is to be contrasted with the fact that the typical homogenization temperature of 7xxx series Al alloys is always greater than 450 °C under atmospheric pressure. Characterization of morphological changes during the in-situ heating to different predetermined temperatures reveals that the dissolution process occurs by the thinning, discontinuation,, and spheroidization (TDS) mechanism which encompasses spheroidization and thinning processes. In addition, the dissolution kinetics of the fine eutectic phases have been found to be significantly accelerated compared to that under atmospheric pressure. The results are further augmented by conventional ex-situ homogenization experiments. The compositional variations of secondary phases have been investigated as a function of time and temperature. The result shows that elemental compositions of Fe-bearing phases vary marginally, whereas that of Zn, Cu-rich phases move from non-equilibrium to equilibrium values.
In this study, the microstructural evolution of an as-cast Al-Zn-Mg-Cu alloy (AA7085) during various homogenization schemes is investigated. It is found that in a single-stage homogenization scheme, some of the primary eutectic gets transformed into the Al2CuMg phase at 400 degrees C, and the primary eutectic and Al2Cu phase gradually dissolve into the alloy matrix at 450 degrees C. The Al3Zr particles are mainly precipitated at the center of the grain because Zr is peritectic. However, the homogeneous distribution of the Al3Zr particles improves and the fraction of Al3Zr particles increases in two-stage homogenization scheme. At the first low-temperature (e. g., 400 degrees C) stage, the Al3Zr particles are homogeneously precipitated at the center of the grain by homogeneous nucleation and may be heterogeneously nucleated on the residual second-phase particles at the grain boundary regions. At the second elevated-temperature (e. g., 470 degrees C) stage, the Al3Zr nuclei become larger. A suitable two-stage homogenization scheme for the present 7085-type Al alloy is 400 degrees C/12 h + 470 degrees C/12 h.
In this research work, the influence of higher surface hardness on the fretting fatigue life of hard anodized aerospace AL7075-T6 alloy was investigated. An optimization of the parameters of hard anodized Al7075-T6 alloy to obtain higher surface hardness was presented. Confirmation test was carried out to show the improvement after using the best parameter combination attained from the optimization process. The result shows that hard anodized coating (surface with hardness of 360. HV and thickness of around 17. μm) can significantly improve fretting fatigue life of specimens at low stress, while at high stress, the extent of fretting fatigue life decreases. Hard anodized coating with higher thickness (29. mm) and high surface hardness (393. HV) decreased the fretting fatigue life of specimens at stresses beyond 200. MPa, this mainly attributed to the coating brittleness and micro-cracking.
This article investigates the phase transitions of complex quaternary Al-Zn-Mg-Cu alloys with Zr addition at overaged conditions.
Differential scanning calorimetry (DSC) is employed to quantitatively analyze the phase transformation phenomena of a wide
range of 7xxx series alloys through endothermic and exothermic reactions. The DSC observations detailing heat effect peaks
and thermal parameters of η′ dissolution contain valuable information on the presence of equilibrium phases and the optimum
alloying element contents. Based on DSC experimental data and phase diagrams, the balance of critical properties such as strength
and electrical conductivity of Al-Zn-Cu-Mg 7xxx series alloys has been studied by considering the formation, dissolution,
and incipient melting of S and T phase, dissolution of η′ phase as well as the formation of η phase. Nine Al-Zn-Mg-Cu alloys
have been studied through microstructural examination and detailed DSC analysis. The correlation between the properties and
the DSC data of the selected alloys has been analyzed.
KeywordsAl-Zn-Mg-Cu alloys–correlation–differential scanning calorimetry–phase transitions–properties
Precipitate redistribution and texture evolution are usually two concurrent aspects accompanying grain refinement induced by various surface treatment. However, the detailed precipitate redistribution characteristics and process, as well as crystallographic texture in the surface refined grain layer, are still far from full understanding. In this study, we focused on the microstructural and crystallographic features of the sliding friction treatment (SFT) induced surface deformation layer in a 7050 aluminum alloy. With the combination of transmission electron microscopy (TEM) and high angle angular dark field scanning TEM (HAADF-STEM) observations, a surface ultrafine grain (UFG) layer composed of both equiaxed and lamellar ultrafine grains and decorated by high density of coarse grain boundary precipitates (GBPs) were revealed. Further precession electron diffraction (PED) assisted orientation mapping unraveled that high angle grain boundaries rather than low angle grain boundaries are the most favorable nucleation sites for GBPs. The prominent precipitate redistribution can be divided into three successive and interrelated stages, i.e. the mechanically induced precipitate dissolution, solute diffusion and reprecipitation. The quantitative prediction based on pipe diffusion along dislocations and grain boundary diffusion proved the distribution feasibility of GBPs around UFGs. Based on PED and electron backscatter diffraction (EBSD) analyses, the crystallographic texture of the surface UFG layer was identified as a shear texture composed of major rotated cube texture {001} 〈110〉 and minor {111} 〈112〉, while that of the adjoining lamellar coarse grained matrix was pure brass. The SFT induced surface severe shear deformation is responsible for texture evolution.
The effect of three-stage homogenization on mechanical properties and stress corrosion cracking of Al-Zn-Mg-Zr alloys has been investigated. The results reveal that the interface of η phases provide the favorable sites for Al3Zr dispersoids under the first-stage homogenization at 350 °C, and therefore three-stage homogenization can reduce size and improve density and homogeneity of Al3Zr dispersoids. Uniform, fine and dense Al3Zr dispersoids inhibit recrystallization effectively and improve the uniformity of the grains. Under the same T74 aging temper, the main strengthening mechanisms of the material are η' phases and dispersion strengthening, and as a result the strength of the material increases by 11.02 MPa and 3.11 MPa when compared with one-stage and two-stage homogenization, respectively. Al3Zr dispersoids can pin dislocations and reduce the accumulation of dislocations at grain boundaries significantly, besides, uniform grains contribute a lot to the elongation, leading to a higher elongation and impact toughness of the material. The improved corrosion resistance is due to a decrease in large-angle grain boundaries which are favorable channels for corrosion, and SCC susceptibility (ISSRT) of the material is only 2.6%.
In the present work, the influence of one-step and two-step aging treatments on hardness, electrical conductivity and mechanical properties of a high Zn-containing Al-Zn-Mg-Cu alloy is investigated and detailed aging parameters subjected to various aging tempers, i.e., T6, T79, T76, T74 and T73, are proposed. The nanoscale precipitates under different tempers are qualitatively investigated by means of transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HREM) techniques. Based on the precipitate observations, precipitate size distributions and neighbor precipitates distances are extracted from bright-field TEM images projected along 〈110〉Al orientation with the aid of an imaging analysis. The results show that with the deepening of aging degree, the conductivity of one-step and two-step aging increase continuously while the hardness increases for one-step aging and decreases for two-step aging at the second preservation. The tensile strength decreases as the aging degree deepens and the yield strength shows a similar trend. In addition, as the degree of over-aging deepens, the precipitate size distribution interval becomes broader, the average precipitate size turns larger and the average distance of neighbor precipitates also becomes greater. The influence of precipitates on mechanical properties is discussed.
Three Al–Zn–Mg–Cu alloys used for oil drill pipes (Alloy A: Al–6.9Zn–2.3Mg–1.7Cu–0.3Mn–0.17Cr; Alloy B: Al–8.0Zn–2.3Mg–2.6Cu–0.2Zr, Alloy C: Al–8.0Zn–2.3Mg–1.8Cu–0.18Zr) were studied by hardness tests, tensile tests and transmission electron microscopy (TEM). The results show that the ultimate tensile strength, yield strength and elongation for Alloys A, B and C are 736 MPa, 695.5 MPa and 7%; 711 MPa, 674 MPa and 12.5%; 740.5 MPa, 707.5 MPa and 13%, respectively after solid solution treatment ((450 °C, 2 h)+(470 °C, 1 h)) followed by aging at 120 °C for 12 h. The dominant strengthening phases in Alloy A are GPII zone and η′ phase, the main precipitate in Alloy B is η′ phase, and the main precipitates in Alloy C are GPI zone, GPII zone and η′ phase, which are the reason for better comprehensive properties of Alloy C. The increase of zinc content leads to the improvement of the strength. The increase of copper content improves the elongation but slightly decreases the strength. Large second-phase particles formed by the increase in the manganese content induce a decrease in the elongation of alloys.
7050 aluminum alloy ingot with fine microstructure was prepared by low frequency electromagnetic casting (LFEC) process. The homogenization treatment showed that the LFEC ingot has a better homogenization effect than conventional DC ingot. Under the same homogenization condition, the LFEC ingot has a small amount of low melting point phases among grains and more amount of solute elements inside grains. After homogenizing for 12 h the solute concentrations in the LFEC ingot can reach the average content of the alloy, but those in the DC ingots haven't reached until 48 h homogenizing.
Aluminium alloys have been the primary material for the structural parts of aircraft for more than 80 years because of their well known performance, well established design methods, manufacturing and reliable inspection techniques. Nearly for a decade composites have started to be used more widely in large commercial jet airliners for the fuselage, wing as well as other structural components in place of aluminium alloys due their high specific properties, reduced weight, fatigue performance and corrosion resistance. Although the increased use of composite materials reduced the role of aluminium up to some extent, high strength aluminium alloys remain important in airframe construction. Aluminium is a relatively low cost, light weight metal that can be heat treated and loaded to relatively high level of stresses, and it is one of the most easily produced of the high performance materials, which results in lower manufacturing and maintenance costs. There have been important recent advances in aluminium aircraft alloys that can effectively compete with modern composite materials. This study covers latest developments in enhanced mechanical properties of aluminium alloys, and high performance joining techniques. The mechanical properties on newly developed 2000, 7000 series aluminium alloys and new generation Al-Li alloys are compared with the traditional aluminium alloys. The advantages and disadvantages of the joining methods, laser beam welding and friction stir welding, are also discussed.
The evolution of the eutectic structures in the as-cast and homogenized 7X50 aluminum alloys was studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectrometer (EDS), differential scanning calorimetry(DSC), X-ray diffraction(XRD) and tensile test. The results show that the main phases are S(Al2CuMg), T(Al2Mg3Zn3) and MgZn2, with a small amount of Al7Cu2Fe and Al3Zr in the as-cast 7X50 alloy. The volume fraction of the dendritic-network structure and residual phase decreases gradually during the homogenization. After homogenization at 470 °C for 24 h and then 482 °C for 12 h, the T(Al2Mg3Zn3) phase disappears and minimal S(Al2CuMg) phase remains, while almost no change has happened for Al7Cu2Fe. There is a strong endothermic peak at 477.8 °C in the DSC curve of as-cast alloy. A new endothermic peak appears at 487.5 °C for the sample homogenized at 470 °C for 1 h. However, this endothermic peak disappears after being homogenized at 482 °C for 24 h. The T(Al2Mg3Zn3) phase cannot be observed by XRD, which is consistent with that T phase is the associated one of S(Al2CuMg) phase and MgZn2 phase. The volume fraction of recrystallized grains is substantially less in the plate with pre-homogenization treatment. The strength and fracture toughness of the plate with pre-homogenization treatment are about 15 MPa and 3.3 MPa·m½ higher than those of the material with conventional homogenization treatment.
The aim of the investigation is to assess the microstructural features and associated physical, mechanical and electrochemical properties of a 7017 Al-Zn-Mg alloy of various tempers. A 7017 Al-Zn-Mg alloy was subjected to different ageing schedules to produce under-(T4), peak-(T6), over-(T7) and highly over-aged tempers. Optical microscopy, hardness measurement, electrical conductivity measurement, tensile testing and SEM fractographs, differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and electrochemical polarization studies have been used to characterize the alloy tempers. Hardness measurement and tensile testing showed the characteristic age hardening phenomenon of aluminium alloys. Optical and TEM micrographs have revealed the variation in size of matrix strengthening η' (MgZn2) and also the size and distribution of grain boundary η (MgZn2) precipitates with ageing time. DSC thermograms exhibiting exothermic and endothermic peaks indicated the characteristic solid state reactions sequence of the 7017 alloy. Potentiodynamic polarisation study of the 7017 alloy of various tempers in 3.5 wt. % NaCl solution at near neutral pH showed typical active metal dissolution behavior, but at pH 12 an active-passive-transpassive transition behavior has been observed.
The microstructures and mechanical properties evolution from the surface to the center of Al-Zn-Mg-Cu alloy ingots obtained by direct casting were revealed. Homogenization treatment research status for Al-Zn- Mg-Cu alloy has been reported in the paper which indicated that it was very necessary to optimize the homogenization parameter further. Based on the consideration of low energy consumption, easy operation, and good effect, a new homogenization parameter applying to large size DC ingots of Al-Zn- Mg-Cu alloy was determined. A new homogenization kinetic equation deriving from Gauss diffusion mathematical model was established and applied to verify the optimum homogenization parameter. In addition, the microstructure evolution, Brinell hardness, thermal analysis of the ingots treated at different parameters was studied. The final result shows that the microstructures and mechanical properties from the surface to the center of the ingots are not uniform and the optimum homogenization parameter is 470 degrees C x 12 h + 480 degrees C x 8 h which is also appropriate according to the new homogenization kinetic model.
An altered surface layer forms on an Al–Zn–Mg–Cu alloy during surface preparation by abrasion with grinding paper. Strain-induced dissolution of η′/η precipitates and formation of nano-sized subgrains were observed in the surface layer with thickness of several hundred nanometers. The segregation of solute elements along dislocations and subgrain boundaries and the precipitation of Al2Cu phase at the sub-boundaries and the free surface were related to enhanced diffusion accelerated by deformation-induced vacancies, dislocations and subgrain boundaries. The microstructure evolution in this layer is mainly attributed to the shear strain and is modified by temperature rise during surface abrasion. The unique surface microstructural changes produced by abrasion might alter the surface properties.
a b s t r a c t The evolutions of mechanical properties and the morphologies of secondary phases in homogenized 7050 alloy were studied in detail by conductivity measurement, tensile test, scanning electron microscope (SEM), energy dispersive spectrometry (EDS), electron probe microanalysis (EPMA) and X-ray diffraction (XRD). Both σ b and δ of the 7050 alloy increase with increasing homogenized temperature, and reach the saturation values at 440 1C/2 h. The secondary phases in as-cast 7050 alloy consist of T (Al 4 Mg 2 CuZn or Al 3 Mg 3 CuZn 2), S (Al 2 CuMg) and Al 7 Cu 2 Fe. During homogenization, the region mainly situated at the margin of T phase having (wt%) 2.3–5.8 Mg, 72.2–82 Al, 5.4–14.8 Cu and 6.3–7.2 Zn sluggishly dissolves into the α(Al) matrix; at the same time, an elemental diffusion cell network appears within T phase, composed of the white cell wall having (wt%) 12. Zn. The network fades gradually when the light gray cell interior transforms to the gray phase and then to the dark gray phase, and disappears finally when the composition of the equilibrium S phase approaches. The diffusion of Zn happens even at 380 1C, and while the obvious diffusions of Mg and Cu begin at 440 1C. S phase contains 77% of the residual phases in 7050 alloy homogenized at 450 1C/2 h. & 2014 Elsevier B.V. All rights reserved.
In the present work, the microstructure evolution of A1-8.35Zn-2.5Mg-2.25Cu alloy during homogenization treatments was investigated. The main grain boundary phases in Al-Zn-Mg-Cu alloy were Mg(Zn,Cu,A1)(2), which had the structure similar to MgZn2 containing Al and Cu elements, and Al7Cu2Fe. The content of Mg(Zn,Cu,A1)(2) phase firstly decreased sharply within the initial 1 h, and then the mesh-like Mg(Zn,Cu,A1)(2) structure was gradually evolved to be discontinuous particles. The ratio of Al, Zn, Mg and Cu elements in Mg(Zn,Cu, A1)(2) phase was slightly affected by the homogenization treatment. Combining with other literature results, it could be concluded that the phase transition from Mg(Zn,A1,Cu)(2) to Al2CuMg phases was very difficult when Zn content was higher than 8 wt.%. It is suggested that the significant effect of Zn might be explained by the diffusion behavior of Zn element.
In the present work, Al–Zn–Mg–Cu alloy was aged by non-isothermal cooling aging treatment (CAT). At high initial aging temperature (IAT), the hardness was decreased with the decreased cooling rate. However, when IAT was lower than 180 °C, the hardness was increased with the decreased cooling rate. Conductivity was increased with the decreased cooling rate regardless of IAT. The tensile strength, yield strength and conductivity of Al alloy after (200–100 °C, 80 °C/h) CAT were increased 2.9%, 8.1% and 8.3% than that after T6 treatment, respectively. With an increase of IAT and decrease of cooling rate, the fine GP zone and η′ phase were transformed to be larger η′ and η precipitates. Moreover, continuous η phase at grain boundary was also grown to be individual large precipitates. Cooling aging time was decreased about 90% than that for T6 treatment, indicating cooling aging could improve the mechanical properties, corrosion resistance and production efficiency with less energy consumption.
The tilt pour gravity casting process coupled with the Controlled Diffusion Solidification (CDS) process technology was employed to demonstrate the ability to cast AA7050 wrought alloy into high integrity near net shaped components with high strength and ductility. The CDS technology involves mixing two precursor alloys at different thermal mass and subsequently casting the resultant mixture into near net shaped cast components. The process enables casting of the high performance Al wrought alloys into near net shaped components by circumventing the problem of hot tearing by obtaining a non-dendritic morphology of the primary Al phase. This study presents the process and alloy parameters necessary for the casting of 7050 Al wrought alloy (Al-Zn-Mg-Cu) using the CDS process technology. The uniaxial tensile properties after various heat treatment conditions such as as-cast, solutionizing and annealing mandatory to the development of the artificial ageing were investigated and presented along with in-depth and quantitative microstructural analyses.
The microstructure of the as-cast 7A55 aluminum alloy and its evolution during homogenization were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and differential scanning calorimetry (DSC) analysis. The results indicate that the microstructure of the as-cast 7A55 aluminum alloy mainly consists of the dendritic network of aluminum solid solution, Al/AlZnMgCu eutectic phases, and intermetallic compounds MgZn2, Al2CuMg, Al7Cu2Fe, and Al23CuFe4. After homogenization at 470°C for 48 h, Al/AlZnMgCu eutectic phases are dissolved into the matrix, and a small amount of high melting-point secondary phases were formed, which results in an increasing of the starting melting temperature of 7A55 aluminum alloy. The high melting-point secondary phases were eliminated mostly when the homogenization time achieved to 72 h. Therefore, the reasonable homogenization heat treatment process for 7A55 aluminum alloy ingots was chosen as 470°C/72 h.
The microstructural factors such as type, area fraction, morphology, distribution, and size of second phases in as-cast and homogenized 7055 aluminum alloy and the influence of impurity content variations have been investigated by using optical microscope (OM), scanning electron microscope (SEM), energy dispersive X-ray analysis (EDS), and X-ray diffraction (XRD). In as-cast microstructures, the dominant second phases of η [Mg(Al, Cu, Zn)2] with extended solubility of Cu and Al, a small amount of impurity phases of Al7Cu2Fe and Al3Fe with a little solubility of Cu and Si, and trace Mg2Si are identified. The variations of Fe and Si contents have no significant influence on the area fraction of η phases, but the area fraction of Fe-rich phase decreases from 0.231 to 0.102 pct with Fe content decreasing from 0.080 to 0.038 wt pct. Decreasing Fe contents reduces the size parameters of Fe-rich phases and refines their morphology correspondingly. After being homogenized at 753 K (480 °C) for 24 hours, η phases are largely dissolved, but the coarse impurity phases are insoluble. Compared with as-cast microstructures, the area fraction and composition of Fe-rich phases change a little but their morphologies are slightly coarsened.
The effect of the solution treatment on the tensile property and fracture toughness of aluminum alloy 7050 were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), tensile test and the plane-strain fracture toughness test. The results show that with increasing single-stage solution temperature, the volume fraction of the residual phase decreases, but the volume fraction of the recrystallized grains and the size of the sub-grains increase. Thus, the strength and fracture toughness of the single-stage solution treated samples increase first and then decrease. The enhanced solution treated samples result in an improved dissolution of the residual phase, a lower recrystallized grains fraction and smaller sub-grains, which leads to a higher strength and fracture toughness than that of the single-stage solution treated samples. The grain structure of the high temperature pre-precipitation treated samples is similar to that of the enhanced solution treated samples. However, the high temperature pre-precipitation treated samples exhibit a lower strength and fracture toughness, due to a mass of AlZnMgCu phase precipitating from the matrix.
In this work, improved solution treatments and their effects on the microstructure and mechanical properties of as-rolled Al–6.22Zn–2.11Mg–2.39Cu alloy have been investigated. By exploring the dissolution process of soluble constituent particles and monitoring changes in the grain structure, a solution treatment sequence with a good balance between a minimum volume fraction of remaining constituents and a partially recrystallised grain structure has been obtained. The results indicate that MgZn2 (η) particles can be completely dissolved after holding at 475 °C for only 5 min, whereas the dissolution of Al2CuMg (S) particles is relatively difficult and their complete dissolution requires a stepped solution treatment. By conducting a final solution treatment step at an increased temperature of 495 °C, all the S-phase particles can be dissolved, whereas the recrystallised fraction can still be controlled to be less than 50%. After retrogression and re-ageing (RRA), samples subjected to a final-stage solution treatment at 495 °C have a relatively higher volume fraction of η′ and η precipitates and can therefore have better mechanical properties.
The microstructural evolution and composition distribution of an Al-Zn-Cu-Mg-Sc-Zr alloy during homogenization were investigated by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The results show that severe dendritic segregation exists in Al-Zn-Cu-Mg-Sc-Zr alloy ingot. There are a lot of eutectic phases at grain boundary and the distribution of the main elements varies periodically along interdendritic region. The main eutectic phases at grain boundary are Al7Cu2Fe phase and T (Al2Mg3Zn3). The residual phases are dissolved into the matrix gradually during homogenization with increasing temperature and prolonging holding time, which can be described by a constitutive equation in exponential function. The overburnt temperature of the alloy is 473.9 °C. The optimum parameters of homogenization are 470 °C and 24 h, which is consistent with the result of homogenization kinetic analysis.
The evolution of the microstructure and phases of the direct chill semicontinuous casting ingot of 7B04 super-high strength aluminum alloy during homogenization treatment was studied with metallographic analysis, scanning electron microscopy(SEM), energy spectroscopy and differential scanning calorimetry(DSC). The results show that a considerable amount of non-equilibrium eutectics containing Al, Zn, Cu and Mg exist in the direct chill semicontinuous casting ingot of 7B04 super-high strength aluminum alloy, and their melting point is 478 °C. During homogenization treatment at 470 °C, these eutectics dissolve into the matrix partly, coarsen and also transform into Al2CuMg phase whose equilibrium melting point is 490 °C in the alloy. Moreover, the homogenization treatment at 470 °C for 72 h results in the disappearance of the non-equilibrium eutectics though Al2CuMg phase can not dissolve completely.
In this study, a large billet of Al–12Zn–2.4Mg–1.1Cu–0.5Ni–0.2Zr alloy was prepared by spray deposition technique, the billet was subsequent processed by hot extrusion, solid solution treatment at 758K and aging at 393K for 20h. The aged alloy was then subjected to tensile testing, it was shown that yield strength, tensile strength and elongation rate of the alloy reached 689MPa, 750MPa and 11%, respectively. The composition, microstructure and fracture characteristics of the tensile sample were explored by EDS (energy dispersive spectrometer), optical microscopy (OM), transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. A spherical morphology of the primary α-phase was observed in the spray-deposited Al alloy, dispersed and fine-scale L12–Al3Zr phases, GPII zones and η′ precipitates, were also evident in the microstructure of the aged alloy. Fracture analysis of the tensile sample exhibited transgranular fractographic features. We suggest that precipitate-phase hardening and solid solution strengthening were responsible for reinforcing of the spray-deposited alloy.
Using a combination of experimental techniques, including anomalous small-angle scattering and atom-probe tomography, the evolution of precipitate microstructures during the different steps of retrogression and re-ageing (RRA) heat treatments of an Al–Zn–Mg–Cu alloy has been systematically evaluated. Quantitative information on the morphology, scale and chemistry of the precipitates provide new insight into the mechanisms at work during this process. It is shown that both the final chemistry and precipitate size distribution are different in the final RRA temper compared to classical heat treatments, with the presence of small clusters nucleated during the re-ageing step, and an average precipitate composition richer in Cu, together with a matrix enrichment in Zn, related to the difference in diffusivity between the two solute atoms. The mechanisms of precipitate evolution during the reversion and re-ageing steps are discussed in light of the influence of the process parameters.
The structure of ingots of aluminum alloy 7050 after casting and homogenization is studied by the methods of differential
scanning calorimetry, light and scanning electron microscopy, and x-ray diffractometry.
Keywordsaluminum alloy-DC casting of ingots-homogenization-phase composition
Intergranular sustained-load cracking of Al-Zn-Mg-Cu (AA7xxx series) aluminum alloys exposed to moist air or distilled water at temperatures in the range 283 K to 353 K (10 °C to 80 °C) has been reviewed in detail, paying particular attention to local processes occurring in the crack-tip region during crack propagation. Distinct crack-arrest markings formed on intergranular fracture faces generated under fixed-displacement loading conditions are not generated under monotonic rising-load conditions, but can form under cyclic-loading conditions if loading frequencies are sufficiently low. The observed crack-arrest markings are insensitive to applied stress intensity factor, alloy copper content and temper, but are temperature sensitive, increasing from ~150 nm at room temperature to ~400 nm at 313 K (40 °C). A re-evaluation of published data reveals the apparent activation energy, E
a
for crack propagation in Al-Zn-Mg(-Cu) alloys is consistently ~35 kJ/mol for temperatures above ~313 K (40 °C), independent of copper content or the applied stress intensity factor, unless the alloy contains a significant volume fraction of S-phase, Al2CuMg where E
a
is ~80 kJ/mol. For temperatures below ~313 K (40 °C) E
a
is independent of copper content for stress intensity factors below ~14 MNm−3/2, with a value ~80 kJ/mol but is sensitive to copper content for stress intensity factors above ~14 MNm−3/2, with E
a
, ranging from ~35 kJ/mol for copper-free alloys to ~80 kJ/mol for alloys containing 1.5 pct Cu. The apparent activation energy for intergranular sustained-load crack initiation is consistently ~110 kJ/mol for both notched and un-notched samples. Mechanistic implications are discussed and processes controlling crack growth, as a function of temperature, alloy copper content, and loading conditions are proposed that are consistent with the calculated apparent activation energies and known characteristics of intergranular sustained-load cracking. It is suggested, depending on the circumstances, that intergranular crack propagation in humid air and distilled water can be enhanced by the generation of aluminum hydride, AlH3, ahead of a propagating crack and/or its decomposition after formation within the confines of the nanoscale volumes available after increments of crack growth, defined by the crack arrest markings on intergranular fracture surfaces.
An Al 7050 alloy containing 11 components was directionally solidified with two cooling rates of 0.45 and 0.9 K s−1. The microstructure and microsegregation in the solidified alloy were experimentally determined by metallography, scanning electron microscopy, image analysis, X-ray map and EPMA. The microstructures of the alloy solidified with the two cooling rates exhibited dendritic structures. Al, Sigma, S, θ, Mg2Si, Al7Cu2Fe and Al13Fe4 were found in the solidified structure. The volume fractions of solids and solute concentration gradients in the dendrites were studied experimentally by image analysis and an area scan technique, and numerically by using a modified Scheil model, respectively. The model predictions were coupled with robust phase diagram calculation package panengine. The calculated results from the modified Scheil model were in good agreement with the measured data. The PanAluminum–Aluminum Database was used to obtain the needed Gibbs free energies of all phases during the modeling.
The influence of two novel aging treatments, T6I6 (130 °C, 80 min + 65 °C, 240 h+130 °C, 18 h) and high-temperature pre-precipitation(HTPP) aging (445 °C, 30 min+120 °C, 24 h) on the tensile properties, intergranular corrosion, exfoliation corrosion behaviors and microstructures of 7075 Al alloy was studied, which were compared with the T6, T73 and RRA treatments. Fine η′ precipitate with high density was obtained in the alloy with the T6 and RRA treatments. The η′ precipitate density in the HTPP aged alloy is decreased due to the formation of coarse particles during the pre-precipitation process at high temperature of 445 °C. The 7075-T6I6 alloy possesses higher precipitate density and whole precipitate volume fraction within the grain than the 7075-T73 alloy, and its whole precipitate volume fraction is even greater than that of the 7075-T6 alloy. Compared with T6 treatment, the RRA, T73, T6I6 and HTPP aging treatments cause the discontinuous distribution of the η precipitates at the grain boundary, which decreases the intergranular corrosion and exfoliation corrosion susceptibility of the alloy. Meanwhile, the T6I6 and RRA treatments can keep the high strength of the 7075 Al alloy, but the studied HTPP aging and T73 treatments lower its strength.