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Effect of microstructure on hot tensile deformation behavior of 7075 alloy sheet fabricated by twin roll casting
Materials Science & Engineering A 652 (2016) 221–230
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of microstructure on hot tensile deformation behavior of 7075 alloy sheet fabricated by twin roll casting Lei Wang a,b, Huashun Yu a, Yun-Soo Lee b, Min-Seok Kim b, Hyoung-Wook Kim b,n a Key Laboratory for Liquid–Solid Structural Evolution and Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jinan 250061, China b Metallic Materials Division, Korea Institute of Materials Science, Changwon 642-831, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 18 September 2015 Received in revised form 22 November 2015 Accepted 24 November 2015 Available online 28 November 2015
7075 aluminum alloy sheet with a thickness of 1 mm was successfully fabricated by twin roll casting and subsequent rolling process. The hot tensile deformation behavior of 7075 alloy was evaluated and large elongation over 200% was obtained at 450 °C under the high strain rate of 1 10 1 s 1. In order to clarify the reason for the large elongation obtained at high strain rate, the effects of microstructure on hot tensile deformation behavior were investigated. The results show that high solidification rate during TRC casting process induced small particles ( 1 μm) in homogeneous distribution. The relatively high fraction of particles over 1 μm in size attributed the homogeneous recrystallized microstructure with fine grains induced by particle simulated nucleation (PSN). This fully recrystallized fine-grained microstructure of TRC alloy sheet contributed to high ductility and formability of 7075 alloy sheet under high strain rate. & 2015 Elsevier B.V. All rights reserved.
Keywords: 7075 aluminum alloy Twin roll casting Hot tensile test High strain rate Elongation Particles
1. Introduction There has been recently a growing demand for producing lightweight vehicles in order to reduce the energy consumption and CO2 emission. High strength-weight ratio, good formability, good corrosion resistance are considered as general characteristics of aluminum alloys applied to automotive industry [1]. Although, 5XXX and 6XXX series aluminum alloys have been widely used for automobile application, more high strength aluminum alloys are required for light weight of automobile body structure. Nowadays, 7XXX series of alloys which have been developed for aircraft and space applications [2] are considered as a good candidate for the automobile body structure. Twin roll casting process (TRC) is considered as a cost-efficient process to fabricate aluminum alloy sheets with good mechanical properties, wherein molten metal is fed onto water-cooled rolls and solidifies with a high cooling rate [3]. Due to the high solidification rate achieved in twin roll casting, the microstructure of TRC alloys differs significantly from that of conventional casting alloys. Traditional twin roll casting process is difficult to produce high strength aluminum alloy strip, because of thermal fragmentation and segregation induced by high contents of alloying elements [4]. However, we successfully fabricated 7075 alloy sheets n
Corresponding author. E-mail address: [email protected] (H.-W. Kim).
http://dx.doi.org/10.1016/j.msea.2015.11.079 0921-5093/& 2015 Elsevier B.V. All rights reserved.
by twin roll casting and subsequent rolling process, the sheets showed good tensile properties for automobile body application [3]. Moreover, it presented large elongation over 200% when deformed at elevated temperature under high strain rate [5]. Small and homogeneous particles were induced during TRC process, which was considered to have a great effect on the alloy formability [6]. It is reported that micro-cracks were easily initiated by the debonding of the interface between matrix and particles [7] or by inner fracture of large particles [8,9]. Tewari et al. [6] reported the influence of the particle distribution of 5754 aluminum alloys on their localization in uniaxial strain. It is presented that large particles lowered the localization strain. In this study, the effect of particles on the microstructural evolution was investigated to clarify the reason for large elongation obtained at high strain rate. For comparison of different particle distribution, the alloy sheet fabricated by permanent mold casting (PMC) process was also prepared.
2. Experimental procedures The material in the present study is 7075 aluminum alloy. The chemical composition is Al–5.2Zn–2.3Mg–1.5Cu (wt%) shown in Table 1. For comparison of different particle distribution, test samples are produced by two different casting processes; twin roll casting (TRC) and permanent mold casting (PMC).
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Table 1 Chemical composition of the alloys studied (wt%). Alloy
Zn
Mg
Cu
Mn
Cr
Fe
Si
Ti
Al
7075
5.18
2.27
1.49
0.045
0.22
0.23
0.11
0.05
Bal.
The TRC sample with 4.5 mm in thickness was fabricated by a horizontal type twin roll caster with water-cooled Cu–Cr rolls with the diameter of 300 mm [3,5]. At this time, the molten alloy heated up to 680 °C was transferred into the preheated tundish and nozzle. The roll gap was 4 mm and casting roll speed was 5-6 rpm. The PMC sample was made by pouring the molten alloy heated to 680 °C into a rectangular block steel mold in the size of 195 180 25 mm3. The cast ingot (t¼25 mm) was homogenizing treated at 465 °C for 24 h followed by hot rolling at 460 °C to 4.5 mm in thickness. As-cast TRC alloy sheet (t ¼4.5 mm) and hot rolled PMC alloy sheet (t¼ 4.5 mm) were annealed at 400 °C for 1 h followed by warm rolling at 250 °C to 2.5 mm in thickness. After re-annealed, the sheets were finally rolled to the thickness of 1 mm at ambient temperature. The tensile test samples measuring 5 mm 1 mm in cross section with a gauge length of 10 mm were machined from the final cold-rolled sheets parallel to the rolling direction. Hot tensile tests were performed on a tensile machine with a three-zone temperature-controlled furnace at a constant crosshead speed. Uniaxial tensile tests were conducted at initial strain rates of 0.001, 0.005, 0.01 and 0.1 s 1 and deformation temperatures of 300, 350, 400 and 450 °C, respectively. The samples were held at desired temperature for 10 min before tensile loading to ensure a
homogeneous temperature distribution through the samples. Microstructural observation was carried out using an optical microscope (OM, ECLPPSE MA200) and electron back-scattered diffraction (EBSD) technique. Samples for optical microscopy were etched by 5% fluoroboric acid (HBF4) for 120 s. Particle observation was performed by scanning electron microscopy (SEM, JSM6610LV) and transmission electron microscope (TEM, JEM-2100F). Energy dispersive spectroscopy (EDS) was used to obtain the composition of the particles. Samples for TEM observation were mechanically ground to a thickness of 80-100 μm and followed by twin-jet electropolishing operated at 20.5 V and 20 °C using a 25% nitric acid and 75% methanol solution. The texture and orientation distribution functions (ODFs) were determined using TSL OIM Analysis software.
3. Results 3.1. Initial microstructure Fig. 1 shows the microstructure of TRC and PMC alloys. The grain structures in as-cast alloys (Fig. 1(a) and (c)) were mostly equiaxed. The grain sizes were 40 μm and 41 μm, respectively, for as-cast TRC and PMC alloys. Moreover, TRC sample exhibited very fine second dendrite arm spacing (DAS) around 6 μm due to the high cooling rate during casting process. It is seen that coldrolled TRC and PMC samples had a deformation structure, where band-liked structures were observed along the rolling direction (Fig. 1(b) and (d)). As to whole thickness reduction from 25 mm to 1 mm during thermo-mechanical process (TMP) for PMC samples,
TRC
PMC
Fig. 1. Optical micrographs of (a) and (c) as-cast alloys and (b) and (d) cold-rolled sheets, respectively, for TRC and PMC alloys.
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223
Fig. 2. Second-phase particles distribution in cold-rolled (a) and (b) TRC and (c) and (d) PMC samples.
TRC samples possessed total thickness reduction from 4.5 mm to 1 mm. Thus, the band thickness seemed thinner in PMC sample due to fewer TMP steps involved in TRC process. Micrographs presenting second-phase particles were shown in Fig. 2. From optical microstructure, it is observed that there was a significant difference of particle size and distribution between TRC (Fig. 2(a)) and PMC samples (Fig. 2(c)). By the high solidification rate during casting process, the size of particles in TRC sample was small and homogeneous, while some large particles (4 5 μm) appeared in PMC alloy. Moreover, the spatial arrangement of particles differed greatly. It is observed that the TRC sample showed a relatively uniform particles distribution throughout the
thickness except of the slight clusters of particles at the center line [6], while coarse particles in sparse distribution were observed in PMC alloy. As is shown in SEM micrographs with high magnification, fine particles in homogeneous distribution were observed in TRC sample (Fig. 2(b)), while much smaller particles and some remarkably large particles with sharp corners simultaneously existed in PMC sample (Fig. 2(d)). Fig. 3 shows the particle size distribution of TRC and PMC alloys, respectively. It is observed that the number fraction of the particles over 1 μm in TRC alloy (Fig. 3(a)) occupied about 6%. However, 99.6% of particles in PMC alloy (Fig. 3(b)) were under 1 μm and only rare fraction of particles over 1 μm were observed.
Fig. 3. Particle size distribution of (a) TRC and (b) PMC alloys.
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300 oC
450 oC
TRC
PMC
Fig. 4. OIM micrographs of TRC and PMC samples heat-treated for 10 min at (a) and (c) T ¼300 °C and (b) and (d) T¼ 450 °C.
In addition, some large particles ( 45 μm) was also presented in PMC alloy. Although they occupied very small proportion, they would have a significant effect on deformation behavior. Fig. 4 shows the microstructure of samples heat-treated for 10 min before hot tensile tests, which is corresponding to the initial microstructure of samples before tensile loading. It is observed that the initial microstructure of TRC sample held at low temperature of 300 °C (Fig. 4(a)) remained partially recrystallized microstructure with elongated grains and the average grain size was 26.91 μm shown in Fig. 5(a), while at high temperature of 450 °C (Fig. 4(b)) fine and recrystallized grains ( 14.74 μm, Fig. 5 (b)) replaced the elongated grains. However, the elongated grains were remained in PMC sample (Fig. 4(c)), even at high temperature (Fig. 4(d)), partially recrystallized microstructure was observed. Moreover, the grain size changed slightly from 23.44 μm at 300 °C (Fig. 5(c)) to 23.26 μm at 450 °C (Fig. 5(d)). The grain size was very similar in these two conditions because of no further recrystallization, even at higher temperature. This microstructural difference was considered to be correlated with the different particle size and distribution in TRC and PMC alloys.
followed by a dramatic decrease to fracture. Moreover, it is clear that the peak flow stress regularly increased with the increasing strain rate and decreasing testing temperature, which was also reported similarly by other literatures [10–12]. Nevertheless, the peak stress of PMC samples was similar to that of TRC samples. 3.3. Fracture elongation Fig. 7 shows the elongation to failure at different temperature and strain rate. It is obvious that TRC and PMC samples represented apparently different ductility. As seen from the figure, the fracture elongation increased regularly with the increasing test temperature in TRC samples (Fig. 7(a)), while in PMC samples (Fig. 7(b)) it firstly increased and then decreased with the temperature except the case at the strain rate of 1 10 1 s 1. It should be emphasized that large elongation over 200% was observed in TRC samples at high temperature of 450 °C and under high strain rate of 1 10 1 s 1. As discussed above, different particle size and distribution was observed in TRC and PMC samples. The effect of the particles on elongation should be considered.
3.2. Hot tensile stress–strain curves 4. Discussion The true stress–strain curves of TRC and PMC alloys deformed at high temperatures were presented in Fig. 6, which shows the effects of deformation temperature and strain rate on flow behavior of the experimental alloys. It is revealed that the stress of both TRC and PMC alloys gradually increased to the peak after the initial strain hardening, and then decreased slowly with increasing strain,
4.1. Particle's effect on microstructure and ductility Fig. 8 shows the particle number density per unit area in TRC and PMC alloys. It is obvious that the total number density of particles in PMC alloy was much higher compared to TRC alloy.
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Fig. 5. Grain size distribution of TRC and PMC 7075 aluminum alloy heat-treated for 10 min at (a) and (c) T ¼ 300 °C and (b) and (d) T ¼ 450 °C.
Especially, the density of particles smaller than 1 μm in PMC alloy was about four times than that in TRC alloy. However, the particles in size over 1 μm in PMC alloy were in much lower density. Moreover, it is noticed that a few of remarkably large particles (4 5 μm) was also observed in PMC alloy, while TRC alloy did not consisted of such large particles. As shown in Fig. 9, bright-field TEM micrographs presented the precipitation distribution. The rodlike MgZn2 and spheroidal
Al12Mg2Cr (or Al18Mg3Cr2) were precipitated with sparse number density in TRC alloy (Fig. 9(a)). Nevertheless, PMC alloy (Fig. 9(b)) showed a large number of the precipitation in the aluminum matrix, which is in accordance with the observed particle distribution in Fig. 8. This is related with the supersaturation in the aluminum matrix of PMC alloy during homogenization process and then the dispersoids were more sufficiently precipitated in the following thermal-mechanical processes. Furthermore, dislocation
Fig. 6. True stress–strain curves of (a) TRC and (b) PMC alloys deformed at various conditions.
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Fig. 7. Elongation to failure as a function of testing temperature for (a) TRC and (b) PMC alloys.
Fig. 8. Particle number density of TRC and PMC alloys.
tangles were clearly observed in PMC alloy. It indicates that the precipitation with the average size of 0.1 μm in PMC sample had a
strong pinning effect on subgrain boundaries which would inhibit the ability for subgrain rotation to lead to coalescence [13]. Hence, dynamic recrystallization during hot deformation was possibly restrained in PMC alloy. As shown in Fig. 10, microstructure of the TRC and PMC samples after hot tensile to fracture was presented. The fracture stain of sample (a)–(d) was 0.42, 1.29, 0.60 and 0.97, respectively, which was corresponding to the tensile elongation shown in Fig. 7. It is observed that this microstructure was similar with the initial microstructure before tensile loading as shown in Fig. 4. Large and elongated grains were observed paralleled to the tensile axis deformed at 300 °C (Fig. 10(a)) in TRC alloy and the average grain size was 35.73 μm as shown in Fig.11(a). When increasing deformation temperature to 450 °C (Fig. 10(b)), small equiaxed grains appeared and large elongated grains were mostly replaced by fine and homogeneous grains ( 16.3 μm, Fig. 11(b)), which is an evidence of recrystallization. As shown in the initial microstructure in Fig. 4 (b), recrystallized fine grains were observed through the whole thickness of samples. This microstructure was considered to be beneficial to grain boundary sliding during hot deformation, which would contribute to large elongation shown in Fig. 7(a). Mikhaylovskaya [14] also studied that small size grains are beneficial to homogeneous deformation, which is corresponding to large
Fig. 9. TEM micrographs displaying precipitation in (a) TRC and (b) PMC alloys.
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ε=0.42
300 oC
ε=1.29
227
450 oC
TRC
ε=0.60
ε=0.97
PMC
Fig. 10. OIM micrographs of TRC and PMC samples after deformed at 1 10 1 s 1 and at temperatures of (a) and (c) T ¼ 300 °C and (b) and (d) T ¼450 °C.
fracture elongation. However, PMC alloys presented elongated large grains and some small grains when deformed at 300 °C (Fig. 10(c)), and the grain size distribution was presented in Fig.11 (c). As similar to the initial microstructure in Fig. 4(d), recrystallization partially took place, showing inhomogeneous microstructure through the sheet thickness, and slight grain growth ( 23.45 μm, Fig. 11(d)) was observed when deformed at high temperature of 450 °C (Fig. 10(d)), which is considered as one possible reason for low fracture elongation. Particles were considered to play a significant role on the microstructural evolution. As discussed above, inhomogeneous particle distribution was observed in PMC alloy (Fig. 2(d) and Fig. 8). Large particles larger than 5 μm and fine particles smaller than 1 μm existed simultaneously. As coarse particles (45 μm) presented sparsely in the PMC alloy, the non-uniform of the strain distribution was induced [15]. Meanwhile, the particles in much smaller size about 0.1 μm were also observed in PMC alloys. They were considered presumably to be η-MgZn2 and Al12Mg2Cr [16] or Al18Mg3Cr2 [17], resulting from precipitation during thermo-mechanical process (TMP). Recrystallization could be hindered by the pinning effect of fine particles, resulting in partial recrystallization. Thus, the inhomogeneous microstructure in both initial microstructure (Fig. 4(d)) and deformed microstructure (Fig. 10(d)) was obtained in PMC alloys, leading to low fracture elongation at high temperature (see Fig. 7(b)). Compared to PMC alloys, however, small particles ( 1 μm) in TRC alloy was considered to contribute to particle simulated nucleation (PSN) of recrystallization because of the high dislocation density around particles [18]. It is shown from Fig. 8 that TRC alloys consisted of higher density of small particles ( 1 μm). This indicated that particles in TRC alloy had a significant effect on PSN to enhance recrystallization. Thus, this fully recrystallized fine grain and homogeneous microstructure was obtained in TRC 7075 aluminum alloy at high temperature,
which was considered as beneficial for high ductility (Fig. 7(a)) by grain boundary sliding and homogeneous deformation. As is shown in Fig. 12, remarkably large particle was observed in PMC alloy. Compared to the TRC sample, the presence of large particles in slower solidified PMC sample is due to the increased diffusion distance [19] during solidification process. Such particle distribution could have an influence on the mechanical properties. The results of EDS analysis showed that the large particles contained aluminum, iron and copper as primary elements with an approximate atomic ratio of 23:4:1 (Fig. 12(c)), indicating particles to be Al23Fe4Cu which is hard brittle and undissolved [20]. Wang [21] studied that the probability of crack inside particles was strongly related to the particle size and large particles were likely to crack when the internal particle stress approached the particle fracture stress during deformation. As shown in Fig. 12(a), cracks inside the large particle were observed. Furthermore, it has been reported [22] that stress concentration created by these sharp corners at large particle ends became the crack generation site at the interface between particles and matrix. As shown in Fig. 12(b), from the observation of fracture tip in PMC sample, cracks was induced at the interface between large particles and matrix, finally resulting in rupture. Thus, this remarkably large particles (45 μm) in PMC could serve as initial sites for early cracks leading to low ductility (see Fig. 7(b)) at high temperature. 4.2. Texture evolution The texture evolution during hot deformation is presented in Fig. 13 and the ODFs show the difference of texture components in TRC and PMC alloys. TRC sample showed a prominent rolling deformation textures (Brass, Copper and S component) and weak Cube texture
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Fig. 11. Grain size distribution of TRC and PMC 7075 aluminum alloy after deformed at 1 10 1 s 1 and at temperatures of (a) and (c) T ¼ 300 °C and (b) and (d) T ¼450 °C.
component when deformed at low temperature (Fig. 13(a)) in accordance with its microstructure discussed above which was unrecrystallized microstructure characterized by elongated grains (Fig. 10(a)). However, when it was deformed at high temperature of 450 oC, the TRC sample fully recrystallized to equiaxed grains (Fig. 10(b)) and possessed a strong recrystallization texture (rotated Cube component) where rolling texture almost disappeared with negligible volume fraction of Brass and Copper texture component (Fig. 13(b)). It is generally accepted that rotated cube texture is related to particles in alloys and it is an evidence of PSN effect [23]. This is in accordance with the observed microstructure in Figs. 4(b) and 10(b), where PSN took place and had a significant effect on recrystallization. It is also observed that PMC sample had a strong deformation textures, especially the Brass component when deformed at 300 oC (Fig. 13(c)), similar component as TRC sample, however, higher rolling texture intensity was presented as a result of larger thickness reduction [6, 18]. It is clear that the intensity of the recrystallization texture components increased slightly while that of the deformation texture components still remained when PMC sample was deformed at 450 oC (Fig. 13(d)). As a result, recrystallization texture and deformation texture existed simultaneously, which is consistent with its partial recrystallized microstructures characterized by concurrence of equiaxed and elongated grains (Fig. 10(d)).
4.3. Tensile strength It is shown from Fig. 5 that the tensile strength of both alloys at elevated temperature increased with the increasing strain rate and decreasing deformation temperature. As the deformation temperature increased, dislocations activity was increased because of the improved atomic average kinetic energy, leading to flow stress decreasing. Moreover, dynamic recovery and dynamic recrystallization is considered to occur at high deformation temperature. As shown in Fig. 10(b) and (d), partially or fully recrystallized microstructure was observed, indicating the occurrence of recrystallization which was the main softening mechanism. With the strain rate increasing, the enhanced deformation storage energy and the work hardening effect raised the flow stress. In addition, negligible difference of the strength was observed when deformed at low temperature and high strain rate. However, it is evident that the strength of PMC alloy was higher than that of TRC when deformed at high temperature and low strain rate. It is considered to be caused by the pinning effect of the fine particles. As shown in Fig. 9(b), large amount of fine particles in PMC alloy were observed and they could act as the barrier to the dislocation movement and grain boundary sliding during hot tensile deformation, which increased the tensile strength. 5. Conclusions 7075 aluminum alloy strip was successfully fabricated by twin
L. Wang et al. / Materials Science & Engineering A 652 (2016) 221–230
10μm
229
40μm
Fig. 12. Backscattered electron image of (a) large particles in PMC sample, OM micrograph of (b) cracks initiated from large particles and (c) EDS spots results on large particles in (a) (Fe-bearing).
300 oC
450 oC
TRC
PMC
Fig. 13. ODFs of TRC and PMC samples deformed at 1 10 1 s 1 and at temperatures of (a) and (c) T ¼ 300 oC and (b) and (d) T ¼450 oC.
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roll casting. Large elongation over 200% was obtained at the temperature of 450 oC under the high strain rate of 1 10 1 s 1. In order to investigate the particle effect on the microstructure and tensile deformation behavior at high temperature, 7075 aluminum alloy strip fabricated by permanent mold casting (PMC) was for comparison in this study. (1) High solidification rate in TRC alloy induced small particles ( 1 μm) in homogeneous distribution. However, inhomogeneous particle distribution was observed in PMC alloy, wherein large fraction of fine particles with the size around 0.1 μm and some remarkably large particles over 5 μm were concurrent. (2) Particles had a significant effect on the microstructure of alloys. The relatively high fraction of particles over 1 μm in TRC alloy attributed the homogeneous recrystallized microstructure with fine grains induced by particle simulated nucleation (PSN), which was characterized by the rotated cube recrystallization texture at high temperature. However, fine size particles ( 0.1 μm) in PMC alloy inhibited recrystallization and resulted in partial recrystallized microstructure which was dominated by mixture of rolling and recrystallization texture. (3) Particles had a significant effect on the ductility of alloys. Particles ( 1 μm) induced fine grained microstructure which was beneficial to high ductility and formability of TRC alloy sheets during hot tensile deformation. However, fine particles (0.1 μm) in PMC alloy inducing inhomogeneous microstructure decreased elongation at high temperature. Meanwhile, remarkably large particles (45 μm) initiated the cracks at the particle-matrix interface and also the cracks inside particles, leading to low fracture elongation.
Acknowledgments The authors are grateful for financial support from the
Fundamental Research Program of Korea Institute of Materials Science (KIMS, No. PNK4240). Lei Wang is also grateful for financial support from the China Scholarship Council (CSC, No. 201306220124). Mr. W.J. Kim and Mrs. Y.M. Oh for TEM and EBSD test operation, respectively, are also appreciated.
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of microstructure on hot tensile deformation behavior of 7075 alloy sheet fabricated by twin roll casting Lei Wang a,b, Huashun Yu a, Yun-Soo Lee b, Min-Seok Kim b, Hyoung-Wook Kim b,n a Key Laboratory for Liquid–Solid Structural Evolution and Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jinan 250061, China b Metallic Materials Division, Korea Institute of Materials Science, Changwon 642-831, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 18 September 2015 Received in revised form 22 November 2015 Accepted 24 November 2015 Available online 28 November 2015
7075 aluminum alloy sheet with a thickness of 1 mm was successfully fabricated by twin roll casting and subsequent rolling process. The hot tensile deformation behavior of 7075 alloy was evaluated and large elongation over 200% was obtained at 450 °C under the high strain rate of 1 10 1 s 1. In order to clarify the reason for the large elongation obtained at high strain rate, the effects of microstructure on hot tensile deformation behavior were investigated. The results show that high solidification rate during TRC casting process induced small particles ( 1 μm) in homogeneous distribution. The relatively high fraction of particles over 1 μm in size attributed the homogeneous recrystallized microstructure with fine grains induced by particle simulated nucleation (PSN). This fully recrystallized fine-grained microstructure of TRC alloy sheet contributed to high ductility and formability of 7075 alloy sheet under high strain rate. & 2015 Elsevier B.V. All rights reserved.
Keywords: 7075 aluminum alloy Twin roll casting Hot tensile test High strain rate Elongation Particles
1. Introduction There has been recently a growing demand for producing lightweight vehicles in order to reduce the energy consumption and CO2 emission. High strength-weight ratio, good formability, good corrosion resistance are considered as general characteristics of aluminum alloys applied to automotive industry [1]. Although, 5XXX and 6XXX series aluminum alloys have been widely used for automobile application, more high strength aluminum alloys are required for light weight of automobile body structure. Nowadays, 7XXX series of alloys which have been developed for aircraft and space applications [2] are considered as a good candidate for the automobile body structure. Twin roll casting process (TRC) is considered as a cost-efficient process to fabricate aluminum alloy sheets with good mechanical properties, wherein molten metal is fed onto water-cooled rolls and solidifies with a high cooling rate [3]. Due to the high solidification rate achieved in twin roll casting, the microstructure of TRC alloys differs significantly from that of conventional casting alloys. Traditional twin roll casting process is difficult to produce high strength aluminum alloy strip, because of thermal fragmentation and segregation induced by high contents of alloying elements [4]. However, we successfully fabricated 7075 alloy sheets n
Corresponding author. E-mail address: [email protected] (H.-W. Kim).
http://dx.doi.org/10.1016/j.msea.2015.11.079 0921-5093/& 2015 Elsevier B.V. All rights reserved.
by twin roll casting and subsequent rolling process, the sheets showed good tensile properties for automobile body application [3]. Moreover, it presented large elongation over 200% when deformed at elevated temperature under high strain rate [5]. Small and homogeneous particles were induced during TRC process, which was considered to have a great effect on the alloy formability [6]. It is reported that micro-cracks were easily initiated by the debonding of the interface between matrix and particles [7] or by inner fracture of large particles [8,9]. Tewari et al. [6] reported the influence of the particle distribution of 5754 aluminum alloys on their localization in uniaxial strain. It is presented that large particles lowered the localization strain. In this study, the effect of particles on the microstructural evolution was investigated to clarify the reason for large elongation obtained at high strain rate. For comparison of different particle distribution, the alloy sheet fabricated by permanent mold casting (PMC) process was also prepared.
2. Experimental procedures The material in the present study is 7075 aluminum alloy. The chemical composition is Al–5.2Zn–2.3Mg–1.5Cu (wt%) shown in Table 1. For comparison of different particle distribution, test samples are produced by two different casting processes; twin roll casting (TRC) and permanent mold casting (PMC).
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Table 1 Chemical composition of the alloys studied (wt%). Alloy
Zn
Mg
Cu
Mn
Cr
Fe
Si
Ti
Al
7075
5.18
2.27
1.49
0.045
0.22
0.23
0.11
0.05
Bal.
The TRC sample with 4.5 mm in thickness was fabricated by a horizontal type twin roll caster with water-cooled Cu–Cr rolls with the diameter of 300 mm [3,5]. At this time, the molten alloy heated up to 680 °C was transferred into the preheated tundish and nozzle. The roll gap was 4 mm and casting roll speed was 5-6 rpm. The PMC sample was made by pouring the molten alloy heated to 680 °C into a rectangular block steel mold in the size of 195 180 25 mm3. The cast ingot (t¼25 mm) was homogenizing treated at 465 °C for 24 h followed by hot rolling at 460 °C to 4.5 mm in thickness. As-cast TRC alloy sheet (t ¼4.5 mm) and hot rolled PMC alloy sheet (t¼ 4.5 mm) were annealed at 400 °C for 1 h followed by warm rolling at 250 °C to 2.5 mm in thickness. After re-annealed, the sheets were finally rolled to the thickness of 1 mm at ambient temperature. The tensile test samples measuring 5 mm 1 mm in cross section with a gauge length of 10 mm were machined from the final cold-rolled sheets parallel to the rolling direction. Hot tensile tests were performed on a tensile machine with a three-zone temperature-controlled furnace at a constant crosshead speed. Uniaxial tensile tests were conducted at initial strain rates of 0.001, 0.005, 0.01 and 0.1 s 1 and deformation temperatures of 300, 350, 400 and 450 °C, respectively. The samples were held at desired temperature for 10 min before tensile loading to ensure a
homogeneous temperature distribution through the samples. Microstructural observation was carried out using an optical microscope (OM, ECLPPSE MA200) and electron back-scattered diffraction (EBSD) technique. Samples for optical microscopy were etched by 5% fluoroboric acid (HBF4) for 120 s. Particle observation was performed by scanning electron microscopy (SEM, JSM6610LV) and transmission electron microscope (TEM, JEM-2100F). Energy dispersive spectroscopy (EDS) was used to obtain the composition of the particles. Samples for TEM observation were mechanically ground to a thickness of 80-100 μm and followed by twin-jet electropolishing operated at 20.5 V and 20 °C using a 25% nitric acid and 75% methanol solution. The texture and orientation distribution functions (ODFs) were determined using TSL OIM Analysis software.
3. Results 3.1. Initial microstructure Fig. 1 shows the microstructure of TRC and PMC alloys. The grain structures in as-cast alloys (Fig. 1(a) and (c)) were mostly equiaxed. The grain sizes were 40 μm and 41 μm, respectively, for as-cast TRC and PMC alloys. Moreover, TRC sample exhibited very fine second dendrite arm spacing (DAS) around 6 μm due to the high cooling rate during casting process. It is seen that coldrolled TRC and PMC samples had a deformation structure, where band-liked structures were observed along the rolling direction (Fig. 1(b) and (d)). As to whole thickness reduction from 25 mm to 1 mm during thermo-mechanical process (TMP) for PMC samples,
TRC
PMC
Fig. 1. Optical micrographs of (a) and (c) as-cast alloys and (b) and (d) cold-rolled sheets, respectively, for TRC and PMC alloys.
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Fig. 2. Second-phase particles distribution in cold-rolled (a) and (b) TRC and (c) and (d) PMC samples.
TRC samples possessed total thickness reduction from 4.5 mm to 1 mm. Thus, the band thickness seemed thinner in PMC sample due to fewer TMP steps involved in TRC process. Micrographs presenting second-phase particles were shown in Fig. 2. From optical microstructure, it is observed that there was a significant difference of particle size and distribution between TRC (Fig. 2(a)) and PMC samples (Fig. 2(c)). By the high solidification rate during casting process, the size of particles in TRC sample was small and homogeneous, while some large particles (4 5 μm) appeared in PMC alloy. Moreover, the spatial arrangement of particles differed greatly. It is observed that the TRC sample showed a relatively uniform particles distribution throughout the
thickness except of the slight clusters of particles at the center line [6], while coarse particles in sparse distribution were observed in PMC alloy. As is shown in SEM micrographs with high magnification, fine particles in homogeneous distribution were observed in TRC sample (Fig. 2(b)), while much smaller particles and some remarkably large particles with sharp corners simultaneously existed in PMC sample (Fig. 2(d)). Fig. 3 shows the particle size distribution of TRC and PMC alloys, respectively. It is observed that the number fraction of the particles over 1 μm in TRC alloy (Fig. 3(a)) occupied about 6%. However, 99.6% of particles in PMC alloy (Fig. 3(b)) were under 1 μm and only rare fraction of particles over 1 μm were observed.
Fig. 3. Particle size distribution of (a) TRC and (b) PMC alloys.
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300 oC
450 oC
TRC
PMC
Fig. 4. OIM micrographs of TRC and PMC samples heat-treated for 10 min at (a) and (c) T ¼300 °C and (b) and (d) T¼ 450 °C.
In addition, some large particles ( 45 μm) was also presented in PMC alloy. Although they occupied very small proportion, they would have a significant effect on deformation behavior. Fig. 4 shows the microstructure of samples heat-treated for 10 min before hot tensile tests, which is corresponding to the initial microstructure of samples before tensile loading. It is observed that the initial microstructure of TRC sample held at low temperature of 300 °C (Fig. 4(a)) remained partially recrystallized microstructure with elongated grains and the average grain size was 26.91 μm shown in Fig. 5(a), while at high temperature of 450 °C (Fig. 4(b)) fine and recrystallized grains ( 14.74 μm, Fig. 5 (b)) replaced the elongated grains. However, the elongated grains were remained in PMC sample (Fig. 4(c)), even at high temperature (Fig. 4(d)), partially recrystallized microstructure was observed. Moreover, the grain size changed slightly from 23.44 μm at 300 °C (Fig. 5(c)) to 23.26 μm at 450 °C (Fig. 5(d)). The grain size was very similar in these two conditions because of no further recrystallization, even at higher temperature. This microstructural difference was considered to be correlated with the different particle size and distribution in TRC and PMC alloys.
followed by a dramatic decrease to fracture. Moreover, it is clear that the peak flow stress regularly increased with the increasing strain rate and decreasing testing temperature, which was also reported similarly by other literatures [10–12]. Nevertheless, the peak stress of PMC samples was similar to that of TRC samples. 3.3. Fracture elongation Fig. 7 shows the elongation to failure at different temperature and strain rate. It is obvious that TRC and PMC samples represented apparently different ductility. As seen from the figure, the fracture elongation increased regularly with the increasing test temperature in TRC samples (Fig. 7(a)), while in PMC samples (Fig. 7(b)) it firstly increased and then decreased with the temperature except the case at the strain rate of 1 10 1 s 1. It should be emphasized that large elongation over 200% was observed in TRC samples at high temperature of 450 °C and under high strain rate of 1 10 1 s 1. As discussed above, different particle size and distribution was observed in TRC and PMC samples. The effect of the particles on elongation should be considered.
3.2. Hot tensile stress–strain curves 4. Discussion The true stress–strain curves of TRC and PMC alloys deformed at high temperatures were presented in Fig. 6, which shows the effects of deformation temperature and strain rate on flow behavior of the experimental alloys. It is revealed that the stress of both TRC and PMC alloys gradually increased to the peak after the initial strain hardening, and then decreased slowly with increasing strain,
4.1. Particle's effect on microstructure and ductility Fig. 8 shows the particle number density per unit area in TRC and PMC alloys. It is obvious that the total number density of particles in PMC alloy was much higher compared to TRC alloy.
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Fig. 5. Grain size distribution of TRC and PMC 7075 aluminum alloy heat-treated for 10 min at (a) and (c) T ¼ 300 °C and (b) and (d) T ¼ 450 °C.
Especially, the density of particles smaller than 1 μm in PMC alloy was about four times than that in TRC alloy. However, the particles in size over 1 μm in PMC alloy were in much lower density. Moreover, it is noticed that a few of remarkably large particles (4 5 μm) was also observed in PMC alloy, while TRC alloy did not consisted of such large particles. As shown in Fig. 9, bright-field TEM micrographs presented the precipitation distribution. The rodlike MgZn2 and spheroidal
Al12Mg2Cr (or Al18Mg3Cr2) were precipitated with sparse number density in TRC alloy (Fig. 9(a)). Nevertheless, PMC alloy (Fig. 9(b)) showed a large number of the precipitation in the aluminum matrix, which is in accordance with the observed particle distribution in Fig. 8. This is related with the supersaturation in the aluminum matrix of PMC alloy during homogenization process and then the dispersoids were more sufficiently precipitated in the following thermal-mechanical processes. Furthermore, dislocation
Fig. 6. True stress–strain curves of (a) TRC and (b) PMC alloys deformed at various conditions.
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Fig. 7. Elongation to failure as a function of testing temperature for (a) TRC and (b) PMC alloys.
Fig. 8. Particle number density of TRC and PMC alloys.
tangles were clearly observed in PMC alloy. It indicates that the precipitation with the average size of 0.1 μm in PMC sample had a
strong pinning effect on subgrain boundaries which would inhibit the ability for subgrain rotation to lead to coalescence [13]. Hence, dynamic recrystallization during hot deformation was possibly restrained in PMC alloy. As shown in Fig. 10, microstructure of the TRC and PMC samples after hot tensile to fracture was presented. The fracture stain of sample (a)–(d) was 0.42, 1.29, 0.60 and 0.97, respectively, which was corresponding to the tensile elongation shown in Fig. 7. It is observed that this microstructure was similar with the initial microstructure before tensile loading as shown in Fig. 4. Large and elongated grains were observed paralleled to the tensile axis deformed at 300 °C (Fig. 10(a)) in TRC alloy and the average grain size was 35.73 μm as shown in Fig.11(a). When increasing deformation temperature to 450 °C (Fig. 10(b)), small equiaxed grains appeared and large elongated grains were mostly replaced by fine and homogeneous grains ( 16.3 μm, Fig. 11(b)), which is an evidence of recrystallization. As shown in the initial microstructure in Fig. 4 (b), recrystallized fine grains were observed through the whole thickness of samples. This microstructure was considered to be beneficial to grain boundary sliding during hot deformation, which would contribute to large elongation shown in Fig. 7(a). Mikhaylovskaya [14] also studied that small size grains are beneficial to homogeneous deformation, which is corresponding to large
Fig. 9. TEM micrographs displaying precipitation in (a) TRC and (b) PMC alloys.
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ε=0.42
300 oC
ε=1.29
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450 oC
TRC
ε=0.60
ε=0.97
PMC
Fig. 10. OIM micrographs of TRC and PMC samples after deformed at 1 10 1 s 1 and at temperatures of (a) and (c) T ¼ 300 °C and (b) and (d) T ¼450 °C.
fracture elongation. However, PMC alloys presented elongated large grains and some small grains when deformed at 300 °C (Fig. 10(c)), and the grain size distribution was presented in Fig.11 (c). As similar to the initial microstructure in Fig. 4(d), recrystallization partially took place, showing inhomogeneous microstructure through the sheet thickness, and slight grain growth ( 23.45 μm, Fig. 11(d)) was observed when deformed at high temperature of 450 °C (Fig. 10(d)), which is considered as one possible reason for low fracture elongation. Particles were considered to play a significant role on the microstructural evolution. As discussed above, inhomogeneous particle distribution was observed in PMC alloy (Fig. 2(d) and Fig. 8). Large particles larger than 5 μm and fine particles smaller than 1 μm existed simultaneously. As coarse particles (45 μm) presented sparsely in the PMC alloy, the non-uniform of the strain distribution was induced [15]. Meanwhile, the particles in much smaller size about 0.1 μm were also observed in PMC alloys. They were considered presumably to be η-MgZn2 and Al12Mg2Cr [16] or Al18Mg3Cr2 [17], resulting from precipitation during thermo-mechanical process (TMP). Recrystallization could be hindered by the pinning effect of fine particles, resulting in partial recrystallization. Thus, the inhomogeneous microstructure in both initial microstructure (Fig. 4(d)) and deformed microstructure (Fig. 10(d)) was obtained in PMC alloys, leading to low fracture elongation at high temperature (see Fig. 7(b)). Compared to PMC alloys, however, small particles ( 1 μm) in TRC alloy was considered to contribute to particle simulated nucleation (PSN) of recrystallization because of the high dislocation density around particles [18]. It is shown from Fig. 8 that TRC alloys consisted of higher density of small particles ( 1 μm). This indicated that particles in TRC alloy had a significant effect on PSN to enhance recrystallization. Thus, this fully recrystallized fine grain and homogeneous microstructure was obtained in TRC 7075 aluminum alloy at high temperature,
which was considered as beneficial for high ductility (Fig. 7(a)) by grain boundary sliding and homogeneous deformation. As is shown in Fig. 12, remarkably large particle was observed in PMC alloy. Compared to the TRC sample, the presence of large particles in slower solidified PMC sample is due to the increased diffusion distance [19] during solidification process. Such particle distribution could have an influence on the mechanical properties. The results of EDS analysis showed that the large particles contained aluminum, iron and copper as primary elements with an approximate atomic ratio of 23:4:1 (Fig. 12(c)), indicating particles to be Al23Fe4Cu which is hard brittle and undissolved [20]. Wang [21] studied that the probability of crack inside particles was strongly related to the particle size and large particles were likely to crack when the internal particle stress approached the particle fracture stress during deformation. As shown in Fig. 12(a), cracks inside the large particle were observed. Furthermore, it has been reported [22] that stress concentration created by these sharp corners at large particle ends became the crack generation site at the interface between particles and matrix. As shown in Fig. 12(b), from the observation of fracture tip in PMC sample, cracks was induced at the interface between large particles and matrix, finally resulting in rupture. Thus, this remarkably large particles (45 μm) in PMC could serve as initial sites for early cracks leading to low ductility (see Fig. 7(b)) at high temperature. 4.2. Texture evolution The texture evolution during hot deformation is presented in Fig. 13 and the ODFs show the difference of texture components in TRC and PMC alloys. TRC sample showed a prominent rolling deformation textures (Brass, Copper and S component) and weak Cube texture
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Fig. 11. Grain size distribution of TRC and PMC 7075 aluminum alloy after deformed at 1 10 1 s 1 and at temperatures of (a) and (c) T ¼ 300 °C and (b) and (d) T ¼450 °C.
component when deformed at low temperature (Fig. 13(a)) in accordance with its microstructure discussed above which was unrecrystallized microstructure characterized by elongated grains (Fig. 10(a)). However, when it was deformed at high temperature of 450 oC, the TRC sample fully recrystallized to equiaxed grains (Fig. 10(b)) and possessed a strong recrystallization texture (rotated Cube component) where rolling texture almost disappeared with negligible volume fraction of Brass and Copper texture component (Fig. 13(b)). It is generally accepted that rotated cube texture is related to particles in alloys and it is an evidence of PSN effect [23]. This is in accordance with the observed microstructure in Figs. 4(b) and 10(b), where PSN took place and had a significant effect on recrystallization. It is also observed that PMC sample had a strong deformation textures, especially the Brass component when deformed at 300 oC (Fig. 13(c)), similar component as TRC sample, however, higher rolling texture intensity was presented as a result of larger thickness reduction [6, 18]. It is clear that the intensity of the recrystallization texture components increased slightly while that of the deformation texture components still remained when PMC sample was deformed at 450 oC (Fig. 13(d)). As a result, recrystallization texture and deformation texture existed simultaneously, which is consistent with its partial recrystallized microstructures characterized by concurrence of equiaxed and elongated grains (Fig. 10(d)).
4.3. Tensile strength It is shown from Fig. 5 that the tensile strength of both alloys at elevated temperature increased with the increasing strain rate and decreasing deformation temperature. As the deformation temperature increased, dislocations activity was increased because of the improved atomic average kinetic energy, leading to flow stress decreasing. Moreover, dynamic recovery and dynamic recrystallization is considered to occur at high deformation temperature. As shown in Fig. 10(b) and (d), partially or fully recrystallized microstructure was observed, indicating the occurrence of recrystallization which was the main softening mechanism. With the strain rate increasing, the enhanced deformation storage energy and the work hardening effect raised the flow stress. In addition, negligible difference of the strength was observed when deformed at low temperature and high strain rate. However, it is evident that the strength of PMC alloy was higher than that of TRC when deformed at high temperature and low strain rate. It is considered to be caused by the pinning effect of the fine particles. As shown in Fig. 9(b), large amount of fine particles in PMC alloy were observed and they could act as the barrier to the dislocation movement and grain boundary sliding during hot tensile deformation, which increased the tensile strength. 5. Conclusions 7075 aluminum alloy strip was successfully fabricated by twin
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40μm
Fig. 12. Backscattered electron image of (a) large particles in PMC sample, OM micrograph of (b) cracks initiated from large particles and (c) EDS spots results on large particles in (a) (Fe-bearing).
300 oC
450 oC
TRC
PMC
Fig. 13. ODFs of TRC and PMC samples deformed at 1 10 1 s 1 and at temperatures of (a) and (c) T ¼ 300 oC and (b) and (d) T ¼450 oC.
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roll casting. Large elongation over 200% was obtained at the temperature of 450 oC under the high strain rate of 1 10 1 s 1. In order to investigate the particle effect on the microstructure and tensile deformation behavior at high temperature, 7075 aluminum alloy strip fabricated by permanent mold casting (PMC) was for comparison in this study. (1) High solidification rate in TRC alloy induced small particles ( 1 μm) in homogeneous distribution. However, inhomogeneous particle distribution was observed in PMC alloy, wherein large fraction of fine particles with the size around 0.1 μm and some remarkably large particles over 5 μm were concurrent. (2) Particles had a significant effect on the microstructure of alloys. The relatively high fraction of particles over 1 μm in TRC alloy attributed the homogeneous recrystallized microstructure with fine grains induced by particle simulated nucleation (PSN), which was characterized by the rotated cube recrystallization texture at high temperature. However, fine size particles ( 0.1 μm) in PMC alloy inhibited recrystallization and resulted in partial recrystallized microstructure which was dominated by mixture of rolling and recrystallization texture. (3) Particles had a significant effect on the ductility of alloys. Particles ( 1 μm) induced fine grained microstructure which was beneficial to high ductility and formability of TRC alloy sheets during hot tensile deformation. However, fine particles (0.1 μm) in PMC alloy inducing inhomogeneous microstructure decreased elongation at high temperature. Meanwhile, remarkably large particles (45 μm) initiated the cracks at the particle-matrix interface and also the cracks inside particles, leading to low fracture elongation.
Acknowledgments The authors are grateful for financial support from the
Fundamental Research Program of Korea Institute of Materials Science (KIMS, No. PNK4240). Lei Wang is also grateful for financial support from the China Scholarship Council (CSC, No. 201306220124). Mr. W.J. Kim and Mrs. Y.M. Oh for TEM and EBSD test operation, respectively, are also appreciated.
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