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Microstructure and mechanical properties of an Al-Zn-Cu-Mg alloy processed by hot forming processes followed by heat treatments
Materials Characterization 157 (2019) 109901
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Microstructure and mechanical properties of an Al-Zn-Cu-Mg alloy processed by hot forming processes followed by heat treatments
T
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Muhammad Abubaker Khana, Yangwei Wanga,b, , Mohamed A. Afifia, Abdul Malika, Faisal Nazeera, Ghulam Yasinc, Bao Jiaweia, Hao Zhangd a
School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China National Key Laboratory of Science and Technology on Material under Shock and Impact, Beijing 100081, China c State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China d Haoran Spray Forming Alloy CO., LTD, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Al-Zn-Cu-Mg alloy Spray formed Hot forming Heat treatments Precipitates Strength
An investigation was conducted to determine the microstructure and tensile properties of an Al-Zn-Cu-Mg alloy processed by spray formed then hot forming processes (forging and extrusion at 703 K). Hot formed samples were solution treatment (743 K/1.5 h) and followed by different heat treatments; T6 aging treatment (393 K/ 24 h), T74 tempering treatment (393 K for 6 h followed by 433 K for 12 h) and T73 tempering treatment (393 K for 8 h followed by 433 K for 24 h) conditions. The results show that during the T6 aging treatment the number of precipitates is increased which are mainly of η′ and η. The high number of fine percipitates is leading to an enhancement of the tensile strengths and the ductility. The tensile yield strength of T6 and T74 heat treatments are decreasing gradually to different degrees with increasing the testing temperatures up to 473 K and drops dramatically after 473 K. The tensile yield strength (YS) of the T73 tempering treatment sample decreases dramatically after temperature 373 K. The coarse precipitates after T73 tempering treatment were unable to pin grain boundary and dislocation movement during the tensile testing at 373 K. The Precipitates start to dissolve during T73 tempering treatment forming new fine η′ and G.P. zones phases inside the η phase.
1. Introduction Al-Zn-Cu-Mg alloy is extensively used in aircraft, aerospace and automobiles applications due to their high strength to weight ratio [1–3]. The high volume fraction of Zn and Mg contents in the aluminium (Al) alloy are responsible for improving the mechanical properties by forming fine precipitates through pinning dislocation motion [4,5]. However, Al alloys processed by the conventional forming methods consist of cracks, macro segregations and coarse grains due to the lower cooling rate [6–8]. The novel spray forming have improved to solve the conventional forming problems [9,10]. Spray forming is presenting outstanding mechanical properties compared with the other conventional forming processes like casting, powder metallurgy and ingot metallurgy [9,11–15]. This owes to the refined grains and homogeneity of the microstructure. The combined effect of forging and extrusion after spray forming further enhance the solid solubility of the α-Al matrix and increase in the number of fine precipitates [6,16–19]. A lot of studies have been reported the microstructure and
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mechanical properties of the hot formed Al–Zn–Mg–Cu alloy and the influence of post-hot forming Al alloy heat treatments [8,20–24]. Experimental investigations showed that Al-Zn-Mg-Cu-Zr alloy processed by spray forming followed by two passes of hot extrusion improved the tensile properties significantly due to the refined grains and the existence of homogeneously distributed precipitates [23]. A further reaging and regression treatment after hot extrusion achieved the best ultimate tensile strength due to strengthening by precipitates [23,25]. Another study showed that the presence of grain boundary (GB) precipitates in the spray deposited Al-Zn-Mg-Cu alloy followed by T73 treatment reduces the ultimate strength of the alloy by comparison with the spray deposited Al alloy followed by peak aged condition [22,26]. Consequently, it is suggested that spray forming followed by aging treatments increases the mechanical response of the Al alloys. Conversely, to date the information about the precipitates evolution of the spray formed Al-Zn-Cu-Mg alloy processed by a combination of hot forming processes like extrusion and forging and further heat treatments has limited information. In addition, tensile behaviour at room temperature (RT) and elevated temperature of the hot formed and post
Corresponding author at: School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China. E-mail address: [email protected] (Y. Wang).
https://doi.org/10.1016/j.matchar.2019.109901 Received 20 May 2019; Received in revised form 1 August 2019; Accepted 29 August 2019 Available online 30 August 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.
Materials Characterization 157 (2019) 109901
M.A. Khan, et al.
Fig. 1. Flow process diagram of Al-Zn-Cu-Mg alloy.
Fig. 2. True stress-true strain of Al-Zn-Cu-Mg alloy (a) spray formed and hot forming, spray formed and hot forming followed by (b) T6 aging treatment (c) T74 tempering treatment and (d) T73 tempering treatment.
2. Experimental material and procedure
hot formed heat treatment has limited information. However, this information is an important prerequisite for proper use of these alloys. Accordingly, the aim of this research was to investigate the microstructure and the tensile properties over temperature ranging from RT to 673 K of a spray formed and hot forming processes Al-Zn-Cu-Mg alloy followed by different heat treatments.
The experiments were carried out on spray formed Al-Zn-Cu-Mg alloy (AA 7055 Al alloy). The spray-formed ingots were produced in the Haoran Co. Ltd., Nanjing, China by using a SFZD-5000 type chamber. During spray forming, the molten metal was atomized by N2 gas at 700 °C–750 °C to prevent it from contamination. The distance of atomizing deposition was kept constant by 650 mm. In addition, the spray 2
Materials Characterization 157 (2019) 109901
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Table 1 Tensile properties of Al alloy processed by hot forming and after further heat treatments at different temperatures. As deposited YS RT K 373 K 473 K 573 K 673 K a b
a
190 ± 10 – – – –
Hot forming UTS
b
224 ± 10 – – – –
T6 aging treatment
T74 tempering treatment
T73 tempering treatment
YS
UTS
YS
UTS
YS
UTS
YS
UTS
270 ± 5 250 ± 4 145 ± 10 55 ± 3 25 ± 5
415 ± 5 350 ± 10 210 ± 7 60 ± 4 30 ± 5
700 ± 10 570 ± 12 410 ± 5 120 ± 5 30 ± 5
784 ± 10 700 ± 12 445 ± 5 125 ± 5 35 ± 5
640 ± 6 470 ± 2 410 ± 10 110 ± 2 35 ± 5
700 ± 6 540 ± 2 435 ± 10 115 ± 2 40 ± 5
560 ± 12 490 ± 10 145 ± 9 55 ± 10 30 ± 5
640 ± 2 540 ± 10 200 ± 9 60 ± 10 35 ± 5
YS: Yield strength (MPa) UTS: Ultimate tensile strength (MPa).
Fig. 3. Microstructure of the Al-Zn-Cu-Mg alloy processed by (a) spray forming and (b) spray forming followed by hot forming processes.
Fig. 4. Microstructure of the Al-Zn-Cu-Mg alloy processed by spray forming followed by hot forming processes and further different heat treatments; (a) T6, (b) T74 and (c) T73.
were performed for each condition. The microstructures features were investigated with optical microscopy (OM) and transmission electron microscopy (TEM). For OM, specimens were ground, polishing and etched with killer's solution to reveal the microstructure. Specimens for TEM were ready through polishing and then double-jet electrolytic polishing with a solution of methanol and nitric acid. The average grains size was calculated by considering ~500 particles in all conditions with the help of the linear intercept method following ASTM E112-12 standard [28].
pressure, substrate rotation speed and withdraw speed was about 0.6–1.0 MPa, 45–55 revolution per min and 2–4 mm/min, respectively. The chemical composition of the spray formed Al alloy is in wt%, of Al7.9% Zn-2.4% Cu-1.8% Mg-0.13% Zr-0.8% Fe-0.3% Si. The ingots were further heated in muffle furnace for 2 h by using stirring fan. After heating in the furnace, the samples were hot forming by forging at 703 K followed by hot extrusion at 703 K to avoid cracking problems at lower temperatures The extrusion speed was 0.2 mm/s and the extrusion ratio was 24:1 [27]. The hot forming samples were subjected to different heat treatments by solution treatment at 743 K for 1.5 h and then quenched in water. Samples are further subjected to T6 tempering at 393 K for 24 h, T74 tempering treatment at 393 K for 6 h followed by 433 K for 12 h and T73 tempering treatment at 393 K for 8 h followed by 433 K for 24 h, respectively. The flow diagram of the complete process of the hot forming Al alloy is presented in Fig. 1. The tensile testing was carried out under strain rate of 1.0 × 10−3 s−1 with Instron 5985 testing instrument at temperatures from RT to 673 K. Samples were heated in a heat resistance furnace and held for 10 min to confirm a uniform temperature. For accuracy, three samples
3. Experimental results 3.1. Mechanical properties The true stress-true strain curves of the spray formed followed by hot forming and after heat treatments are depicted in Fig. 2 of temperatures ranges from RT-673 K under tensile testing. The tensile properties obtained from the curves with standard deviations are concise in Table 1. The curves are after testing (a) spray formed and hot forming Al alloy, and after subsequent heat treatments through (b) T6 3
Materials Characterization 157 (2019) 109901
M.A. Khan, et al.
Fig. 5. TEM micrograph of Al-Zn-Cu-Mg alloy (a) after spray forming, (b) magnified view and corresponding SAED along ⟨110⟩Al after spray forming (c) after spray forming and hot forming and (d) magnified view and corresponding SAED along ⟨112⟩Al after spray forming and hot forming.
followed by hot forming (hot forging + hot extrusion). The microstructure of the spray formed samples in Fig. 3(a) has equiaxed grains by an average grain size of ~60 μm. After hot forming processes, the microstructure showed elongated grains having a size of ~25 μm as revealed in Fig. 3(b). Fig. 4 displays the microstructure of the post-hot forming heat treatments. Fig. 4(a) displays the microstructure analysis of the T6 aging treatment contains elongated grains with average size ~30 μm. The sizes are slightly increased in Fig. 4(b) after T74-tempering treatment with the grain size of ~40 μm which are further increased after T73-tempering treatment as demonstrated in Fig. 4(c). TEM micrographs of the spray formed and spray formed followed by hot forming processes of the Al-Zn-Cu-Mg alloy are presented in Fig. 5. Fig. 5(a) displays the microstructure after spray forming which contains low dislocation density with presence of coarse irregular and platelet particles along the dislocations and the Al matrix. The average particles size in the spray formed Al alloy is ~700 nm. The selected area electron diffraction (SAED) pattern along 〈110〉Al illustrated the presence of η close to {111}Al as depicted in Fig. 5(b) [14,29]. Dislocation density is increased after hot forming with presence of recrystallized sub grains as presented in Fig. 5(c). Moreover, coarse precipitates are detected along the GB as shown by arrows in Fig. 5(c) with presence of various fine precipitates that are homogeneously dispersed in the microstructure which are presented in Fig. 5(d) mainly of G.P. zones and η′ precipitates along 〈112〉Al diffraction. Typical images of the Al-Zn-Cu-Mg alloy processed by hot forming followed by T6, T74 and T73 tempering treatments are displayed in Fig. 6. Fig. 6(a) displays the microstructure of hot forming followed by T6 aging treatment, where coarse precipitates are presented along the GB. Fig. 6(b) presents high number of fine precipitates distributed
aging treatment, (c) T74 tempering treatment and (d) T73 tempering treatment. It was found that the yield and ultimate tensile strengths decrease gradually with increasing the testing temperature to 473 K in the hot forming Al alloy, and after further T6 aging treatment and T74 tempering treatment as shown in Fig. 2. In contrast, the strengths exhibits a dramatic drop after 473 K in the T73 tempering treatment of the Al alloy. Increasing the temperature to 573 K results in further dramatic drop in the strength in all conditions. The reduction in the YS of the hot forming between RT and 673 K is shown in Fig. 2 (a) is ~240 MPa whereas after T6 treatment in Fig. 2 (b) ~660 MPa, ~600 MPa after T74 displayed in Fig. 2(c) and ~530 MPa after T73 treatment as represented in Fig. 2(d). The hot forming Al alloy followed by T6 aging treatment displays the best strengths with good ductilities in comparison of other tested specimens. The yield and ultimate strengths at RT are higher by ~160% and 88% over hot forming Al alloy samples. The hot forming Al alloy followed by T73 tempering treatment has the lowest strengths by comparison with the other heat treatment conditions with approximately equal ductility. For example, the yield and ultimate strengths are lower by ~14% and ~9%, and ~25% and ~23% when tested at RT by comparison with the T74 tempering and T6 aging treatments, respectively. In addition, the hot forming Al alloy followed by T74 tempering treatment has the highest work hardening comparing to the other tested samples at all tested temperatures. 3.2. Microstructure characterization Typical microstructure micrographs are displayed in Fig. 3 of the AA7055 alloy processed by (a) spray forming and (b) the spray forming 4
Materials Characterization 157 (2019) 109901
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Fig. 6. TEM micrograph of hot-formed Al-Zn-Cu-Mg alloy: (a, b) after T6 aging treatment, (c, d) after T74 tempering treatment and (e, f) after T73 tempering treatment; (b), (d) and (f) are in higher magnification with corresponding SAED showing precipitates and precipitation free zone (PFZ). Table 2 Microstructural features tested in the spray formed, hot forming Al alloy and hot forming Al alloy followed by T6, T74 and T73 heat treatments.
Grain size Precipitate type (size nm)
Spray formed
Spray formed and hot forming
Spray formed and hot forming followed by T6
Spray formed and hot forming followed by T74
Spray formed and hot forming followed by T73
~60 μm Agglomerated mainly of η (~700 nm)
~25 μm G.P. zones, η′ (~6 nm)
~30 μm η′, η (~11 nm)
~40 μm η′, η (~15 nm)
~55 μm η′, η (~20 nm)
5
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Fig. 7. (a) HAADF-STEM of Al alloy processed by spray forming (b) EDX map showing the contents of irregular particles agglomeration.
4. Discussions
homogenously as well as precipitation free zone (PFZ) of ~200 nm in width along the GB. The precipitates are predominantly of η′ with spots of η having a size of ~11 nm mainly spherical with some few platelets along the GB. Similarly, Fig. 6(c) shows many coarse platelets precipitates along the GB in the T74 tempering treatment. Indexing along ⟨110⟩Al depicts the presence of η′ and η precipitates with average size of ~15 nm which are homogeneously dispersed and separated by a PFZ with width of ~70 nm. Precipitates are increased in numbers with a slight increase in size after T73 tempering treatment which are homogeneously distributed as displayed in Fig. 6(e). PFZ is observed in Fig. 6(f) which became narrower to ~60 nm. The precipitates observed in the matrix and across the GB are mainly of η′ and η. For convenience, the microstructural features of the spray formed, hot forming Al alloy with further T6, T74 and T73 treat treatments are concise in Table 2. Fig. 7 shows an EDX map of the spray formed Al alloy in which a coarse particle contains Mg, Zn, Cu, Fe and Si with presence of Al. A GB precipitates are observed in the hot forming followed by T73 tempering treatment sample as presented in Fig. 8(a). The HRTEM of a selected platelet precipitate shows that it consists of two overlapped precipitates as verified by the mismatch of the lattice planes as highlighted with the arrow in Fig. 8(b). The Fast-Fourier-Transform (FFT) pattern in Figs. 8(c) and 8(d) verify that these two precipitates are η′ and η, respectively. The EDX mapping in Fig. 9 also displays that the selected precipitates are η′ and η where η has Mg:Zn in the ratio of ~2:1 with presence of Cu [30]. High amount of coarse platelets precipitates were detected along the GB as observed in Fig. 10(a) in the hot forming Al alloy followed by T73 tempering treatment. A selected particle in Fig. 10(b) shows the presence of η′ within a coarse η phase as confirmed by the FFT pattern. The existence of two fine G.P. zones within the η phase are also represented in Fig. 10(c) which have platelet and spherical morphologies.
4.1. Precipitates evolution during spray forming, hot forming, and post-hot forming heat treatments In the present investigation, the spray formed Al-Zn-Cu-Mg alloy consists of coarse particles along dislocation lines, which are rich with Al, Mg, Zn, Fe, and Si as displayed in Fig. 7. Because of straining at high temperatures during hot forming processes (forging + extrusion), precipitates are refined and some precipitates are evolved from the solutes in the Al matrix [31–34]. During the hot forming processes, the solutes transform into fine G.P. zones and η′ precipitates which are uniformly distributed as depicted in Fig. 5(d). Post-hot forming heat treatments of the Al alloy leads to an increase in the number of the fine precipitates as presented in Fig. 6. The precipitates in the T6 aging treatment were mainly of metastable η′ and η precipitates which are following the precipitation sequence in the AA7055 alloy [35–37]. The number of precipitates is increased after T74 and T73 tempering treatments as displayed in Fig. 6 but with an increase in the average size of precipitates. This is due to the longer heat treatment duration which leads to the coarsening of precipitates [38]. The presence of η′ inside coarse η and two fine G.P. zones within the η phase after T73 tempering treatment can be ascribed to the start of dissolution of precipitates after the long tempering treatment duration. Precipitate dissolution during postplastic deformation heat treatments were identified in the binary Al alloys such as AleCu and AleMg alloys [39,40]. Furthermore, it was observed that precipitates along the GB were coarser in size having platelet morphology and this may result in some overlapping. According to Gibbs-driven equilibrium GB segregation [41], solute segregates to the adjacent GB area accompanied with equilibrium increased solute concentration along the boundaries. This local increase in the solute concentration along the boundaries causes higher compositional levels and leads to the formation of GP zones, 6
Materials Characterization 157 (2019) 109901
M.A. Khan, et al.
Fig. 8. TEM images of Al-Zn-Cu-Mg alloy spray formed followed by hot forming and subsequent T73 tempering treatment showing (a) overlapping of two precipitates along the grain boundaries, (b) HRTEM showing the overlapping precipitates of η′ and η and boundary between the two phases, (c) the corresponding FFT pattern of η′ phase and (d) the corresponding FFT pattern of η phase.
which later with hot forming processes transforms into η′ then stable η with subsequent growth during the heat treatments. In addition, during T6 aging treatment solutes continue to segregate forming a flux of solutes along the GB and leads to the presence of PFZ. Increasing duration in the T74 and T73 tempering treatments results in the growth of precipitates and overlapping which narrows the PFZ width as presented in Fig. 6.
contribution to the strength enhancement of the material. Besides, the results show the presence of fine precipitates after hot forming processes which can impede dislocation motion and increase the strength of the material [52]. The results of tensile testing display that the post-hot forming heat treatments are very advantageous in improving the tensile strengths at RT. Inspection of Fig. 6 shows a significant increase in the number of fine precipitates which are uniformly distributed along with the matrix. The precipitates were able to pin the dislocation motion by either the Orowan bypass or shearing mechanisms and enhance the strength of the material [42,47]. The size of precipitates is increased with increasing the heat treatments duration through T74 and T73 tempering treatments which leads to the decrease of strengths when tested at RT by comparison with the T6 aging treatment samples. In addition, trapping dislocations by large volume fraction of fine precipitates in T6 aging treatment samples increase the uniform elongation and leads to an enhancement in the overall ductility [49]. Consequently, the hot forming AA7055 Al alloy followed by T6 heat aging treatment displayed the optimum balance between strength and ductility in comparison of all other post-heat treatment specimens.
4.2. The strengthening mechanisms In order to determine the influence of different strengthening mechanisms of the hot forming Al alloy and post-hot forming heat treatments, it is essential to investigate the different strengthening mechanisms. These mechanisms consist of (i) interactions of dislocations that provide work-hardening (ii) GB strengthening (iii) solid solution hardening and (iv) precipitation strengthening [27,42–44]. In the current research, the effect of solid solution hardening can be neglected as the material alloy was hot forming followed by further heat treatments [27,45]. Following the Hall-Petch relationship [46,47], it was determined that grain refinement can result in the enhanced strength of the material alloy. According to the results, the hot forming samples have refined grains comparing to the spray formed an alloy which is due to the recrystallization process during processing. However, as a result of the dislocation tangles and networks within the grains which impede dislocation motion [48], the increased numbers of dislocations after hot forming processes are expected to make a substantial
4.3. Temperature dependence during tensile testing The temperature dependence of the YS in the Al alloy processed by hot forming followed by heat treatments is illustrated in Fig. 11. Therefore, an increase in the tensile heating temperature from RT to 7
Materials Characterization 157 (2019) 109901
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Fig. 9. (a) HAADF-STEM along {111} of the overlapped precipitates showing the PFZ and (b) EDX map of the overlapping particles.
Fig. 10. TEM images of hot forming Al-Zn-Cu-Mg alloy and subsequent T73 tempering treatment showing (a) grain boundary precipitation and the PFZ, (b) HRTEM presence of fine η′ nucleated within the η precipitate and (c) HRTEM of two G.P. zones within the η precipitate.
strength with further heating [40]. By inspection of Fig. 11, a substantial drop of the YS in the T73 tempering treatment samples starts earlier at 373 K which ascribes to the coarser precipitates observed in Fig. 6 which provide a lower drag force to obstacle the GB and dislocation movement [50]. According to the results presented, the hot forming Al alloy followed by T6 aging treatment can be used in the structural application with thermal environments up to 473 K.
473 K leads to a gradual decrease of the YS followed by step drop after testing at 473 K to 673 K in the hot forming alloy and in the post-hot forming T6 aging and T74 tempering treatments samples. Increasing the heating temperature escalate the annihilation of dislocation and leading to the decrease the YS [50]. The presence of GB precipitates mainly of η′ and η can be effective in pinning GB during testing up to 393 K. However, precipitates lead to growth which is unable to further pin GB migration or dislocation motion results in a significant drop of 8
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Fig. 11. Temperature dependence of the yield strength of the hot forming AlZn-Cu-Mg alloy and hot forming Al alloy followed by T6, T74 and T73 heat treatments.
5. Summary and conclusions 1. Spray formed Al alloy consists of coarse precipitates which fragmented during hot forming to fine spherical precipitates. The precipitates evolution leads to the presence of large number of η′ and η precipitates during T6 tempering treatment. The size of precipitates increased during the T74 and T73 tempering treatments which are mainly of η′ and η. Precipitates starts to dissolve during T73 tempering treatment forming new η′ and two fine G.P. phases inside a coarse η phase. 2. Hot forming Al alloy followed by T6 aging treatment enhanced the yield strength of Al alloy with good ductility. The large number of fine precipitates after the tempering treatment improves the tensile strength and ductility of the material up to temperature 300 K. 3. Grain boundary precipitates are identified in the hot forming followed by T6 aging treatment samples with presence of overlapped particles. PFZ is detected which is decreasing with increasing the heat treatment duration during T74 and T73 tempering treatments. 4. The tensile yield strength decreases gradually to different degrees with increasing temperature up to 473 K while drops dramatically in strength under temperatures 473 K to 673 K for the hot forming, after T6 aging treatment and after T74 tempering treatment. The yield strength drops severally after 373 K in the T73 K tempering treatment samples. Acknowledgments The authors would like to express special thanks of Haoran Spray Forming Alloy CO., LTD., Jiangsu, China, for their support in the manufacturing of spray formed Al alloys. References [1] F. Wang, B. Xiong, H. Liu, X. He, Microstructural development of spray-deposited Al–Zn–Mg–Cu alloy during subsequent processing, J. Alloys Compd. 477 (2009) 616–621, https://doi.org/10.1016/j.jallcom.2008.10.115. [2] V. Balasubramanian, V. Ravisankar, G.M. Reddy, Effect of pulsed current welding on fatigue behaviour of high strength aluminium alloy joints, Mater. Des. 29 (2008) 492–500, https://doi.org/10.1016/j.matdes.2006.12.015. [3] M. Jeyakumar, S. Kumar, G. Gupta, Microstructure and properties of the sprayformed and extruded 7075 Al alloy, Mater. Manuf. Process. 25 (2010) 777–785, https://doi.org/10.1080/10426910903447253. [4] M.X. Guo, J.Q. Du, C.H. Zheng, J.S. Zhang, L.Z. Zhuang, Influence of Zn contents on precipitation and corrosion of Al-Mg-Si-Cu-Zn alloys for automotive applications, J. Alloys Compd. 778 (2019) 256–270, https://doi.org/10.1016/j.jallcom.2018.11.
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[41] J. Gibbs, The collected works, Thermodynamics, 1 Yale. University Press, 1948, , https://doi.org/10.2307/3609900. [42] K. Ma, H. Wen, T. Hu, T.D. Topping, D. Isheim, D.N. Seidman, E.J. Lavernia, J.M. Schoenung, Mechanical behavior and strengthening mechanisms in ultrafine grain precipitation-strengthened aluminum alloy, Acta Mater. 62 (2014) 141–155, https://doi.org/10.1016/j.actamat.2013.09.042. [43] K. Kendig, D. Miracle, Strengthening mechanisms of an Al-Mg-Sc-Zr alloy, Acta Mater. 50 (2002) 4165–4175, https://doi.org/10.1016/S1359-6454(02)00258-6. [44] M. Cabibbo, Microstructure strengthening mechanisms in different equal channel angular pressed aluminum alloys, Mater. Sci. Eng. A 560 (2013) 413–432, https:// doi.org/10.1016/j.msea.2012.09.086. [45] M. Raghavan, Microanalysis of precipitate free zones (PFZ) in Al-Zn-Mg and Cu-NiNb alloys, Met. Trans. A 11 (1980) 993–999, https://doi.org/10.1007/ BF02654713. [46] P.H.R. Pereira, Y.C. Wang, Y. Huang, T.G. Langdon, Influence of grain size on the flow properties of an Al-Mg-Sc alloy over seven orders of magnitude of strain rate, Mater. Sci. Eng. A 685 (2017) 367–376, https://doi.org/10.1016/j.msea.2017.01. 020. [47] M.A. Afifi, Y.C. Wang, P.H.R. Pereira, Y. Huang, Y. Wang, X. Cheng, S. Li, T.G. Langdon, Effect of heat treatments on the microstructures and tensile properties of an ultrafine-grained Al-Zn-Mg alloy processed by ECAP, J. Alloys Compd. 749 (2018) 567–574, https://doi.org/10.1016/j.jallcom.2018.03.206. [48] W. Kim, J. Kim, T. Park, S. Hong, D. Kim, Y. Kim, J. Lee, Enhancement of strength and superplasticity in a 6061 Al alloy processed by equal-channel-angular-pressing, Metall Mater Trans A 33 (2002) 3155–3164, https://doi.org/10.1007/s11661-0020301-4. [49] S. Cheng, Y. Zhao, Y. Zhu, E. Ma, Optimizing the strength and ductility of fine structured 2024 Al alloy by nano-precipitation, Acta Mater. 55 (2007) 5822–5832, https://doi.org/10.1016/j.actamat.2007.06.043. [50] M.A. Afifi, P.H.R. Pereira, Y.C. Wang, Y. Wang, S. Li, T.G. Langdon, Effect of ECAP processing on microstructure evolution and dynamic compressive behavior at different temperatures in an Al-Zn-Mg alloy, Mater. Sci. Eng. A 684 (2017) 617–625, https://doi.org/10.1016/j.msea.2016.12.099.
[30] S. Maloney, K. Hono, I. Polmear, S. Ringer, The chemistry of precipitates in an aged Al-2.1 Zn-1.7 Mg at.% alloy, Scr. Mater. 41 (1999), https://doi.org/10.1016/ S1359-6462(99)00253-5. [31] D.-W. Suh, S.-Y. Lee, K.-H. Lee, S.-K. Lim, K.H. Oh, Microstructural evolution of Al–Zn–Mg–Cu–(Sc) alloy during hot extrusion and heat treatments, J Mater Process Tech 155 (2004) 1330–1336, https://doi.org/10.1016/j.jmatprotec.2004.04.195. [32] H. Zhang, N.-p. Jin, J.-h. Chen, Hot deformation behavior of Al-Zn-Mg-Cu-Zr aluminum alloys during compression at elevated temperature, Trans. Nonferrous Met. Soc. China. 21 (2011) 437–442, https://doi.org/10.1016/S1003-6326(11)60733-4. [33] C. Shi, J. Lai, X. Chen, Microstructural evolution and dynamic softening mechanisms of Al-Zn-Mg-Cu alloy during hot compressive deformation, Materials 7 (2014) 244–264, https://doi.org/10.3390/ma7010244. [34] M.A. Afifi, Y.C. Wang, X. Cheng, S. Li, T.G. Langdon, Strain rate dependence of compressive behavior in an Al-Zn-Mg alloy processed by ECAP, J. Alloys Compd. (2019) 1079–1087, https://doi.org/10.1016/j.jallcom.2019.03.390. [35] G. Sha, A. Cerezo, Early-stage precipitation in Al–Zn–Mg–Cu alloy (7050), Acta Mater. 57 (2009) 3123–3132, https://doi.org/10.1016/j.actamat.2004.06.025. [36] A. Deschamps, F. Livet, Y. Brechet, Influence of predeformation on ageing in an Al–Zn–Mg alloy—I. Microstructure evolution and mechanical properties, Acta Mater. 47 (1998) 281–292, https://doi.org/10.1016/S1359-6454(98)00293-6. [37] M. Liu, B. Klobes, K. Maier, On the age-hardening of an Al–Zn–Mg–Cu alloy: a vacancy perspective, Scr. Mater. 64 (2011) 21–24, https://doi.org/10.1016/j. scriptamat.2010.08.054. [38] T. Engdahl, V. Hansen, P. Warren, K. Stiller, Investigation of fine scale precipitates in Al–Zn–Mg alloys after various heat treatments, Mater. Sci. Eng. A 327 (2002) 59–64, https://doi.org/10.1016/S0921-5093(01)01876-7. [39] W. Huang, Z. Liu, M. Lin, X. Zhou, L. Zhao, A. Ning, S. Zeng, Reprecipitation behavior in Al–Cu binary alloy after severe plastic deformation-induced dissolution of θ’ particles, Mater. Sci. Eng. A 546 (2012) 26–33, https://doi.org/10.1016/j.msea. 2012.03.010. [40] M. Munoz-Morris, C.G. Oca, D.G. Morris, Mechanical behaviour of dilute Al–Mg alloy processed by equal channel angular pressing, Scr. Mater. 48 (2003) 213–218, https://doi.org/10.1016/S1359-6462(02)00501-8.
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Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Microstructure and mechanical properties of an Al-Zn-Cu-Mg alloy processed by hot forming processes followed by heat treatments
T
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Muhammad Abubaker Khana, Yangwei Wanga,b, , Mohamed A. Afifia, Abdul Malika, Faisal Nazeera, Ghulam Yasinc, Bao Jiaweia, Hao Zhangd a
School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China National Key Laboratory of Science and Technology on Material under Shock and Impact, Beijing 100081, China c State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China d Haoran Spray Forming Alloy CO., LTD, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Al-Zn-Cu-Mg alloy Spray formed Hot forming Heat treatments Precipitates Strength
An investigation was conducted to determine the microstructure and tensile properties of an Al-Zn-Cu-Mg alloy processed by spray formed then hot forming processes (forging and extrusion at 703 K). Hot formed samples were solution treatment (743 K/1.5 h) and followed by different heat treatments; T6 aging treatment (393 K/ 24 h), T74 tempering treatment (393 K for 6 h followed by 433 K for 12 h) and T73 tempering treatment (393 K for 8 h followed by 433 K for 24 h) conditions. The results show that during the T6 aging treatment the number of precipitates is increased which are mainly of η′ and η. The high number of fine percipitates is leading to an enhancement of the tensile strengths and the ductility. The tensile yield strength of T6 and T74 heat treatments are decreasing gradually to different degrees with increasing the testing temperatures up to 473 K and drops dramatically after 473 K. The tensile yield strength (YS) of the T73 tempering treatment sample decreases dramatically after temperature 373 K. The coarse precipitates after T73 tempering treatment were unable to pin grain boundary and dislocation movement during the tensile testing at 373 K. The Precipitates start to dissolve during T73 tempering treatment forming new fine η′ and G.P. zones phases inside the η phase.
1. Introduction Al-Zn-Cu-Mg alloy is extensively used in aircraft, aerospace and automobiles applications due to their high strength to weight ratio [1–3]. The high volume fraction of Zn and Mg contents in the aluminium (Al) alloy are responsible for improving the mechanical properties by forming fine precipitates through pinning dislocation motion [4,5]. However, Al alloys processed by the conventional forming methods consist of cracks, macro segregations and coarse grains due to the lower cooling rate [6–8]. The novel spray forming have improved to solve the conventional forming problems [9,10]. Spray forming is presenting outstanding mechanical properties compared with the other conventional forming processes like casting, powder metallurgy and ingot metallurgy [9,11–15]. This owes to the refined grains and homogeneity of the microstructure. The combined effect of forging and extrusion after spray forming further enhance the solid solubility of the α-Al matrix and increase in the number of fine precipitates [6,16–19]. A lot of studies have been reported the microstructure and
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mechanical properties of the hot formed Al–Zn–Mg–Cu alloy and the influence of post-hot forming Al alloy heat treatments [8,20–24]. Experimental investigations showed that Al-Zn-Mg-Cu-Zr alloy processed by spray forming followed by two passes of hot extrusion improved the tensile properties significantly due to the refined grains and the existence of homogeneously distributed precipitates [23]. A further reaging and regression treatment after hot extrusion achieved the best ultimate tensile strength due to strengthening by precipitates [23,25]. Another study showed that the presence of grain boundary (GB) precipitates in the spray deposited Al-Zn-Mg-Cu alloy followed by T73 treatment reduces the ultimate strength of the alloy by comparison with the spray deposited Al alloy followed by peak aged condition [22,26]. Consequently, it is suggested that spray forming followed by aging treatments increases the mechanical response of the Al alloys. Conversely, to date the information about the precipitates evolution of the spray formed Al-Zn-Cu-Mg alloy processed by a combination of hot forming processes like extrusion and forging and further heat treatments has limited information. In addition, tensile behaviour at room temperature (RT) and elevated temperature of the hot formed and post
Corresponding author at: School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China. E-mail address: [email protected] (Y. Wang).
https://doi.org/10.1016/j.matchar.2019.109901 Received 20 May 2019; Received in revised form 1 August 2019; Accepted 29 August 2019 Available online 30 August 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.
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Fig. 1. Flow process diagram of Al-Zn-Cu-Mg alloy.
Fig. 2. True stress-true strain of Al-Zn-Cu-Mg alloy (a) spray formed and hot forming, spray formed and hot forming followed by (b) T6 aging treatment (c) T74 tempering treatment and (d) T73 tempering treatment.
2. Experimental material and procedure
hot formed heat treatment has limited information. However, this information is an important prerequisite for proper use of these alloys. Accordingly, the aim of this research was to investigate the microstructure and the tensile properties over temperature ranging from RT to 673 K of a spray formed and hot forming processes Al-Zn-Cu-Mg alloy followed by different heat treatments.
The experiments were carried out on spray formed Al-Zn-Cu-Mg alloy (AA 7055 Al alloy). The spray-formed ingots were produced in the Haoran Co. Ltd., Nanjing, China by using a SFZD-5000 type chamber. During spray forming, the molten metal was atomized by N2 gas at 700 °C–750 °C to prevent it from contamination. The distance of atomizing deposition was kept constant by 650 mm. In addition, the spray 2
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Table 1 Tensile properties of Al alloy processed by hot forming and after further heat treatments at different temperatures. As deposited YS RT K 373 K 473 K 573 K 673 K a b
a
190 ± 10 – – – –
Hot forming UTS
b
224 ± 10 – – – –
T6 aging treatment
T74 tempering treatment
T73 tempering treatment
YS
UTS
YS
UTS
YS
UTS
YS
UTS
270 ± 5 250 ± 4 145 ± 10 55 ± 3 25 ± 5
415 ± 5 350 ± 10 210 ± 7 60 ± 4 30 ± 5
700 ± 10 570 ± 12 410 ± 5 120 ± 5 30 ± 5
784 ± 10 700 ± 12 445 ± 5 125 ± 5 35 ± 5
640 ± 6 470 ± 2 410 ± 10 110 ± 2 35 ± 5
700 ± 6 540 ± 2 435 ± 10 115 ± 2 40 ± 5
560 ± 12 490 ± 10 145 ± 9 55 ± 10 30 ± 5
640 ± 2 540 ± 10 200 ± 9 60 ± 10 35 ± 5
YS: Yield strength (MPa) UTS: Ultimate tensile strength (MPa).
Fig. 3. Microstructure of the Al-Zn-Cu-Mg alloy processed by (a) spray forming and (b) spray forming followed by hot forming processes.
Fig. 4. Microstructure of the Al-Zn-Cu-Mg alloy processed by spray forming followed by hot forming processes and further different heat treatments; (a) T6, (b) T74 and (c) T73.
were performed for each condition. The microstructures features were investigated with optical microscopy (OM) and transmission electron microscopy (TEM). For OM, specimens were ground, polishing and etched with killer's solution to reveal the microstructure. Specimens for TEM were ready through polishing and then double-jet electrolytic polishing with a solution of methanol and nitric acid. The average grains size was calculated by considering ~500 particles in all conditions with the help of the linear intercept method following ASTM E112-12 standard [28].
pressure, substrate rotation speed and withdraw speed was about 0.6–1.0 MPa, 45–55 revolution per min and 2–4 mm/min, respectively. The chemical composition of the spray formed Al alloy is in wt%, of Al7.9% Zn-2.4% Cu-1.8% Mg-0.13% Zr-0.8% Fe-0.3% Si. The ingots were further heated in muffle furnace for 2 h by using stirring fan. After heating in the furnace, the samples were hot forming by forging at 703 K followed by hot extrusion at 703 K to avoid cracking problems at lower temperatures The extrusion speed was 0.2 mm/s and the extrusion ratio was 24:1 [27]. The hot forming samples were subjected to different heat treatments by solution treatment at 743 K for 1.5 h and then quenched in water. Samples are further subjected to T6 tempering at 393 K for 24 h, T74 tempering treatment at 393 K for 6 h followed by 433 K for 12 h and T73 tempering treatment at 393 K for 8 h followed by 433 K for 24 h, respectively. The flow diagram of the complete process of the hot forming Al alloy is presented in Fig. 1. The tensile testing was carried out under strain rate of 1.0 × 10−3 s−1 with Instron 5985 testing instrument at temperatures from RT to 673 K. Samples were heated in a heat resistance furnace and held for 10 min to confirm a uniform temperature. For accuracy, three samples
3. Experimental results 3.1. Mechanical properties The true stress-true strain curves of the spray formed followed by hot forming and after heat treatments are depicted in Fig. 2 of temperatures ranges from RT-673 K under tensile testing. The tensile properties obtained from the curves with standard deviations are concise in Table 1. The curves are after testing (a) spray formed and hot forming Al alloy, and after subsequent heat treatments through (b) T6 3
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Fig. 5. TEM micrograph of Al-Zn-Cu-Mg alloy (a) after spray forming, (b) magnified view and corresponding SAED along ⟨110⟩Al after spray forming (c) after spray forming and hot forming and (d) magnified view and corresponding SAED along ⟨112⟩Al after spray forming and hot forming.
followed by hot forming (hot forging + hot extrusion). The microstructure of the spray formed samples in Fig. 3(a) has equiaxed grains by an average grain size of ~60 μm. After hot forming processes, the microstructure showed elongated grains having a size of ~25 μm as revealed in Fig. 3(b). Fig. 4 displays the microstructure of the post-hot forming heat treatments. Fig. 4(a) displays the microstructure analysis of the T6 aging treatment contains elongated grains with average size ~30 μm. The sizes are slightly increased in Fig. 4(b) after T74-tempering treatment with the grain size of ~40 μm which are further increased after T73-tempering treatment as demonstrated in Fig. 4(c). TEM micrographs of the spray formed and spray formed followed by hot forming processes of the Al-Zn-Cu-Mg alloy are presented in Fig. 5. Fig. 5(a) displays the microstructure after spray forming which contains low dislocation density with presence of coarse irregular and platelet particles along the dislocations and the Al matrix. The average particles size in the spray formed Al alloy is ~700 nm. The selected area electron diffraction (SAED) pattern along 〈110〉Al illustrated the presence of η close to {111}Al as depicted in Fig. 5(b) [14,29]. Dislocation density is increased after hot forming with presence of recrystallized sub grains as presented in Fig. 5(c). Moreover, coarse precipitates are detected along the GB as shown by arrows in Fig. 5(c) with presence of various fine precipitates that are homogeneously dispersed in the microstructure which are presented in Fig. 5(d) mainly of G.P. zones and η′ precipitates along 〈112〉Al diffraction. Typical images of the Al-Zn-Cu-Mg alloy processed by hot forming followed by T6, T74 and T73 tempering treatments are displayed in Fig. 6. Fig. 6(a) displays the microstructure of hot forming followed by T6 aging treatment, where coarse precipitates are presented along the GB. Fig. 6(b) presents high number of fine precipitates distributed
aging treatment, (c) T74 tempering treatment and (d) T73 tempering treatment. It was found that the yield and ultimate tensile strengths decrease gradually with increasing the testing temperature to 473 K in the hot forming Al alloy, and after further T6 aging treatment and T74 tempering treatment as shown in Fig. 2. In contrast, the strengths exhibits a dramatic drop after 473 K in the T73 tempering treatment of the Al alloy. Increasing the temperature to 573 K results in further dramatic drop in the strength in all conditions. The reduction in the YS of the hot forming between RT and 673 K is shown in Fig. 2 (a) is ~240 MPa whereas after T6 treatment in Fig. 2 (b) ~660 MPa, ~600 MPa after T74 displayed in Fig. 2(c) and ~530 MPa after T73 treatment as represented in Fig. 2(d). The hot forming Al alloy followed by T6 aging treatment displays the best strengths with good ductilities in comparison of other tested specimens. The yield and ultimate strengths at RT are higher by ~160% and 88% over hot forming Al alloy samples. The hot forming Al alloy followed by T73 tempering treatment has the lowest strengths by comparison with the other heat treatment conditions with approximately equal ductility. For example, the yield and ultimate strengths are lower by ~14% and ~9%, and ~25% and ~23% when tested at RT by comparison with the T74 tempering and T6 aging treatments, respectively. In addition, the hot forming Al alloy followed by T74 tempering treatment has the highest work hardening comparing to the other tested samples at all tested temperatures. 3.2. Microstructure characterization Typical microstructure micrographs are displayed in Fig. 3 of the AA7055 alloy processed by (a) spray forming and (b) the spray forming 4
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Fig. 6. TEM micrograph of hot-formed Al-Zn-Cu-Mg alloy: (a, b) after T6 aging treatment, (c, d) after T74 tempering treatment and (e, f) after T73 tempering treatment; (b), (d) and (f) are in higher magnification with corresponding SAED showing precipitates and precipitation free zone (PFZ). Table 2 Microstructural features tested in the spray formed, hot forming Al alloy and hot forming Al alloy followed by T6, T74 and T73 heat treatments.
Grain size Precipitate type (size nm)
Spray formed
Spray formed and hot forming
Spray formed and hot forming followed by T6
Spray formed and hot forming followed by T74
Spray formed and hot forming followed by T73
~60 μm Agglomerated mainly of η (~700 nm)
~25 μm G.P. zones, η′ (~6 nm)
~30 μm η′, η (~11 nm)
~40 μm η′, η (~15 nm)
~55 μm η′, η (~20 nm)
5
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Fig. 7. (a) HAADF-STEM of Al alloy processed by spray forming (b) EDX map showing the contents of irregular particles agglomeration.
4. Discussions
homogenously as well as precipitation free zone (PFZ) of ~200 nm in width along the GB. The precipitates are predominantly of η′ with spots of η having a size of ~11 nm mainly spherical with some few platelets along the GB. Similarly, Fig. 6(c) shows many coarse platelets precipitates along the GB in the T74 tempering treatment. Indexing along ⟨110⟩Al depicts the presence of η′ and η precipitates with average size of ~15 nm which are homogeneously dispersed and separated by a PFZ with width of ~70 nm. Precipitates are increased in numbers with a slight increase in size after T73 tempering treatment which are homogeneously distributed as displayed in Fig. 6(e). PFZ is observed in Fig. 6(f) which became narrower to ~60 nm. The precipitates observed in the matrix and across the GB are mainly of η′ and η. For convenience, the microstructural features of the spray formed, hot forming Al alloy with further T6, T74 and T73 treat treatments are concise in Table 2. Fig. 7 shows an EDX map of the spray formed Al alloy in which a coarse particle contains Mg, Zn, Cu, Fe and Si with presence of Al. A GB precipitates are observed in the hot forming followed by T73 tempering treatment sample as presented in Fig. 8(a). The HRTEM of a selected platelet precipitate shows that it consists of two overlapped precipitates as verified by the mismatch of the lattice planes as highlighted with the arrow in Fig. 8(b). The Fast-Fourier-Transform (FFT) pattern in Figs. 8(c) and 8(d) verify that these two precipitates are η′ and η, respectively. The EDX mapping in Fig. 9 also displays that the selected precipitates are η′ and η where η has Mg:Zn in the ratio of ~2:1 with presence of Cu [30]. High amount of coarse platelets precipitates were detected along the GB as observed in Fig. 10(a) in the hot forming Al alloy followed by T73 tempering treatment. A selected particle in Fig. 10(b) shows the presence of η′ within a coarse η phase as confirmed by the FFT pattern. The existence of two fine G.P. zones within the η phase are also represented in Fig. 10(c) which have platelet and spherical morphologies.
4.1. Precipitates evolution during spray forming, hot forming, and post-hot forming heat treatments In the present investigation, the spray formed Al-Zn-Cu-Mg alloy consists of coarse particles along dislocation lines, which are rich with Al, Mg, Zn, Fe, and Si as displayed in Fig. 7. Because of straining at high temperatures during hot forming processes (forging + extrusion), precipitates are refined and some precipitates are evolved from the solutes in the Al matrix [31–34]. During the hot forming processes, the solutes transform into fine G.P. zones and η′ precipitates which are uniformly distributed as depicted in Fig. 5(d). Post-hot forming heat treatments of the Al alloy leads to an increase in the number of the fine precipitates as presented in Fig. 6. The precipitates in the T6 aging treatment were mainly of metastable η′ and η precipitates which are following the precipitation sequence in the AA7055 alloy [35–37]. The number of precipitates is increased after T74 and T73 tempering treatments as displayed in Fig. 6 but with an increase in the average size of precipitates. This is due to the longer heat treatment duration which leads to the coarsening of precipitates [38]. The presence of η′ inside coarse η and two fine G.P. zones within the η phase after T73 tempering treatment can be ascribed to the start of dissolution of precipitates after the long tempering treatment duration. Precipitate dissolution during postplastic deformation heat treatments were identified in the binary Al alloys such as AleCu and AleMg alloys [39,40]. Furthermore, it was observed that precipitates along the GB were coarser in size having platelet morphology and this may result in some overlapping. According to Gibbs-driven equilibrium GB segregation [41], solute segregates to the adjacent GB area accompanied with equilibrium increased solute concentration along the boundaries. This local increase in the solute concentration along the boundaries causes higher compositional levels and leads to the formation of GP zones, 6
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Fig. 8. TEM images of Al-Zn-Cu-Mg alloy spray formed followed by hot forming and subsequent T73 tempering treatment showing (a) overlapping of two precipitates along the grain boundaries, (b) HRTEM showing the overlapping precipitates of η′ and η and boundary between the two phases, (c) the corresponding FFT pattern of η′ phase and (d) the corresponding FFT pattern of η phase.
which later with hot forming processes transforms into η′ then stable η with subsequent growth during the heat treatments. In addition, during T6 aging treatment solutes continue to segregate forming a flux of solutes along the GB and leads to the presence of PFZ. Increasing duration in the T74 and T73 tempering treatments results in the growth of precipitates and overlapping which narrows the PFZ width as presented in Fig. 6.
contribution to the strength enhancement of the material. Besides, the results show the presence of fine precipitates after hot forming processes which can impede dislocation motion and increase the strength of the material [52]. The results of tensile testing display that the post-hot forming heat treatments are very advantageous in improving the tensile strengths at RT. Inspection of Fig. 6 shows a significant increase in the number of fine precipitates which are uniformly distributed along with the matrix. The precipitates were able to pin the dislocation motion by either the Orowan bypass or shearing mechanisms and enhance the strength of the material [42,47]. The size of precipitates is increased with increasing the heat treatments duration through T74 and T73 tempering treatments which leads to the decrease of strengths when tested at RT by comparison with the T6 aging treatment samples. In addition, trapping dislocations by large volume fraction of fine precipitates in T6 aging treatment samples increase the uniform elongation and leads to an enhancement in the overall ductility [49]. Consequently, the hot forming AA7055 Al alloy followed by T6 heat aging treatment displayed the optimum balance between strength and ductility in comparison of all other post-heat treatment specimens.
4.2. The strengthening mechanisms In order to determine the influence of different strengthening mechanisms of the hot forming Al alloy and post-hot forming heat treatments, it is essential to investigate the different strengthening mechanisms. These mechanisms consist of (i) interactions of dislocations that provide work-hardening (ii) GB strengthening (iii) solid solution hardening and (iv) precipitation strengthening [27,42–44]. In the current research, the effect of solid solution hardening can be neglected as the material alloy was hot forming followed by further heat treatments [27,45]. Following the Hall-Petch relationship [46,47], it was determined that grain refinement can result in the enhanced strength of the material alloy. According to the results, the hot forming samples have refined grains comparing to the spray formed an alloy which is due to the recrystallization process during processing. However, as a result of the dislocation tangles and networks within the grains which impede dislocation motion [48], the increased numbers of dislocations after hot forming processes are expected to make a substantial
4.3. Temperature dependence during tensile testing The temperature dependence of the YS in the Al alloy processed by hot forming followed by heat treatments is illustrated in Fig. 11. Therefore, an increase in the tensile heating temperature from RT to 7
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Fig. 9. (a) HAADF-STEM along {111} of the overlapped precipitates showing the PFZ and (b) EDX map of the overlapping particles.
Fig. 10. TEM images of hot forming Al-Zn-Cu-Mg alloy and subsequent T73 tempering treatment showing (a) grain boundary precipitation and the PFZ, (b) HRTEM presence of fine η′ nucleated within the η precipitate and (c) HRTEM of two G.P. zones within the η precipitate.
strength with further heating [40]. By inspection of Fig. 11, a substantial drop of the YS in the T73 tempering treatment samples starts earlier at 373 K which ascribes to the coarser precipitates observed in Fig. 6 which provide a lower drag force to obstacle the GB and dislocation movement [50]. According to the results presented, the hot forming Al alloy followed by T6 aging treatment can be used in the structural application with thermal environments up to 473 K.
473 K leads to a gradual decrease of the YS followed by step drop after testing at 473 K to 673 K in the hot forming alloy and in the post-hot forming T6 aging and T74 tempering treatments samples. Increasing the heating temperature escalate the annihilation of dislocation and leading to the decrease the YS [50]. The presence of GB precipitates mainly of η′ and η can be effective in pinning GB during testing up to 393 K. However, precipitates lead to growth which is unable to further pin GB migration or dislocation motion results in a significant drop of 8
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Fig. 11. Temperature dependence of the yield strength of the hot forming AlZn-Cu-Mg alloy and hot forming Al alloy followed by T6, T74 and T73 heat treatments.
5. Summary and conclusions 1. Spray formed Al alloy consists of coarse precipitates which fragmented during hot forming to fine spherical precipitates. The precipitates evolution leads to the presence of large number of η′ and η precipitates during T6 tempering treatment. The size of precipitates increased during the T74 and T73 tempering treatments which are mainly of η′ and η. Precipitates starts to dissolve during T73 tempering treatment forming new η′ and two fine G.P. phases inside a coarse η phase. 2. Hot forming Al alloy followed by T6 aging treatment enhanced the yield strength of Al alloy with good ductility. The large number of fine precipitates after the tempering treatment improves the tensile strength and ductility of the material up to temperature 300 K. 3. Grain boundary precipitates are identified in the hot forming followed by T6 aging treatment samples with presence of overlapped particles. PFZ is detected which is decreasing with increasing the heat treatment duration during T74 and T73 tempering treatments. 4. The tensile yield strength decreases gradually to different degrees with increasing temperature up to 473 K while drops dramatically in strength under temperatures 473 K to 673 K for the hot forming, after T6 aging treatment and after T74 tempering treatment. The yield strength drops severally after 373 K in the T73 K tempering treatment samples. Acknowledgments The authors would like to express special thanks of Haoran Spray Forming Alloy CO., LTD., Jiangsu, China, for their support in the manufacturing of spray formed Al alloys. References [1] F. Wang, B. Xiong, H. Liu, X. He, Microstructural development of spray-deposited Al–Zn–Mg–Cu alloy during subsequent processing, J. Alloys Compd. 477 (2009) 616–621, https://doi.org/10.1016/j.jallcom.2008.10.115. [2] V. Balasubramanian, V. Ravisankar, G.M. Reddy, Effect of pulsed current welding on fatigue behaviour of high strength aluminium alloy joints, Mater. Des. 29 (2008) 492–500, https://doi.org/10.1016/j.matdes.2006.12.015. [3] M. Jeyakumar, S. Kumar, G. Gupta, Microstructure and properties of the sprayformed and extruded 7075 Al alloy, Mater. Manuf. Process. 25 (2010) 777–785, https://doi.org/10.1080/10426910903447253. [4] M.X. Guo, J.Q. Du, C.H. Zheng, J.S. Zhang, L.Z. Zhuang, Influence of Zn contents on precipitation and corrosion of Al-Mg-Si-Cu-Zn alloys for automotive applications, J. Alloys Compd. 778 (2019) 256–270, https://doi.org/10.1016/j.jallcom.2018.11.
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