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Vol. 8. Issue 5.
Pages 3891-3907 (September - October 2019)
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Vol. 8. Issue 5.
Pages 3891-3907 (September - October 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.06.052
Open Access
Effect of inter-annealing between two stages of extrusion on the microstructure and mechanical property for spray deposited Al–Cu–Li alloy 2195
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Yongxiao Wang, Guoqun Zhao
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zhaogq@sdu.edu.cn

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, Xiaoxue Chen, Xiao Xu, Liang Chen, Cunsheng Zhang
Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan, Shandong 250061, PR China
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Abstract

The spray deposited alloys need to be first extruded to reduce porosity, and then the extrusion is conducted again for final profile forming. It is required to carry out an inter-annealing between the two stages of extrusion. In this study, billets after the first extrusion were annealed with different parameters, and the effect of annealing process on the microstructure was investigated. Then the annealed billets were extruded again to obtain the extruded plates. The role of inter-annealing processes on the properties of extruded plates was investigated through microstructure characterization and mechanical properties testing. The results show that the inter-annealing processes increase the width of extrusion fibers and lead to static recrystallization in the extruded billets. The <100>//ED texture in extruded billets increases obviously after the inter-annealing due to the static recrystallization. The solubility of the secondary particles can be increased with the inter-annealing temperature and holding time, which thus enhances the effect of solution strengthening. The billets annealed with higher temperature and longer time exhibit stronger deformation resistance owing to the high solution strengthening effect. After the second extrusion, the texture of <211>//ED is introduced into the extruded plates, which causes apparent anisotropy of mechanical properties. The plates extruded by the billets with different inter-annealing parameters have similar microstructures, which thus results in similar mechanical properties. However, after solution and aging treatments, the extruded plate with inter-annealing at high temperature for a long time shows significant performance degradation, which results from the severe grains coarsening.

Keywords:
Extrusion
Inter-annealing
Al–Li alloy
Spray deposit forming
Microstructure
Mechanical properties
Full Text
1Introduction

Al–Cu–Li alloy 2195, as a typical third-generation Al–Li alloy, exhibits low density, high specific strength and modulus, excellent corrosion resistance, and good weldability, therefore are of real interest to the aerospace industry [1,2]. Successful applications of this alloy to the structural components have brought significant weight savings to aerospace vehicles such as Discovery space shuttle and Orion spaceship [3,4]. Spray deposit forming is an important way to produce aeronautic alloys with superior performance. In recent years, several kinds of high strength aluminum alloys have been developed using spray deposit forming technology and exhibited excellent mechanical properties [5–7]. However, some pores are inevitable to exist in the spray deposited billets due to the melt atomization by the inert gas. Reddy et al. [8] found the as-spray formed UL40 Al–Li alloy has porosity of about 4.5 vol.% and the maximum size of the pore is about 100 μm. Therefore, the porosity in spray deposited billet needs to be reduced via compaction processes, and the hot extrusion is one of the common compaction methods. Jeyakumar et al. [9] and Bai et al. [10] eliminated almost the pores in the deposited billet of 7000 series aluminum alloys by means of the hot extrusion method.

The extruded billet, after reducing porosity, has to be deformed again at elevated temperature to form the final components. Before the second deformation, the preheating and temperature holding (i.e., inter-annealing processing) is necessarily carried out on the as-extruded billet for microstructural homogenization. The inter-annealing process has impacts on the microstructure of the extruded billet in three ways. Firstly, during the annealing at high temperature, stored energy in the extruded billet can prompt the movement of dislocations and the transformation of substructures, which thus results in static recovery (SRV), static recrystallization (SRX) and grain growth [11,12]. The degree of SRV and SRX, the rate of grain growth, and the final grain size are closely related to the heating parameters [13,14]. Secondly, the annealing process has also influence on the secondary particles in the matrix. Some large-sized particles have low solution temperature and they gradually break up and dissolve into matrix during heating process. The solubility of the large particles is dominated by the annealing parameters [15]. The finely dispersed phases such as Al3Zr and Al3Sc often have a significant effect on recrystallization and grain growth, whose size and distribution are also related to the annealing process [16,17]. Thirdly, texture components in the extruded billet may be changed by the recrystallization during the annealing process. The recrystallization grains in deformed aluminum alloys usually nucleate and grow with specific orientation and the recrystallization textures differing from the deformation ones can be formed as a result [11,18]. Thus, it can be recognized that the annealing process would determine the texture components in the billet.

The changes of microstructure in the billets annealed with different conditions result in different initial states for the next deformation [19]. As a result, the microstructural evolution in the secondary deformation processes and the final states of the formed parts are probably affected by the initial states of the billets. Firstly, the finer initial grains could accelerate the dynamic recrystallization (DRX) process by providing more nucleation sites at grain boundaries [20–22] or making the misorientation of low angle grain boundaries increase faster [23]. Secondly, the solubility of particles in the billets is critical for solution strengthening effect that determines the deformation resistance [24]. Moreover, the morphology and the distribution of the particles in the billets also have a significant influence on the recrystallization behavior. Zhang et al. [25] studied the effect of secondary particles on the recrystallization nucleation utilizing 3D serial sectioning method and found that the recrystallization grains prefer to nucleate around the large particles. The hindrance effect on the grain boundaries provided by the fine particles has also been discussed in detail by Huang and Logé in their extensive review paper [26].

The inter-annealing process between the two stages extrusion has an essential effect on the microstructure of the billet and thereby plays a key role in the microstructural evolution during the second extrusion. The final components may exhibit differences in microstructure and properties. However, no study has focused on the inter-annealing between two hot extrusions of spray alloys until now. In the present study, the spray deposited billet of alloy 2195 is first extruded, and then the extruded billets are annealed with different parameters. The microstructures are characterized, and the effects of inter-annealing processes on the microstructure of the extruded billet are revealed. Subsequently, the annealed billets are extruded again, and extruded plates are obtained. Through the microstructure characterization and mechanical properties testing, the influence of inter-annealing on the extruded plates is studied in detail.

2Experimental material and method2.1Experimental material

The spray deposited alloy 2195 used in this study is produced by Haoran Co., Ltd., Jiangsu, P.R. China. The chemical composition of the alloy is 3.72 Cu, 1.06Li, 0.44Mg, 0.31Ag, 0.12Zr (wt.%) and balance Al. Spray deposited billet was first extruded to reduce its porosity. The extrusion temperature and ram speed were set at 470 °C and 1.8 mm/s, respectively. The extrusion ratio is 9:1. After extrusion, the round bars with a diameter of 160 mm were obtained and then cooled in air. The microstructure of the deposited billet and extruded bar are shown in Fig. 1. The microstructure of the deposited alloy exhibits typical equiaxed grains with a size of 50–80 μm, as shown in Fig. 1(a). After extrusion, the grains are elongated along the extrusion direction and exhibit visible extrusion fiber structure, as shown in Fig. 1(b). (The sample for the microstructural observation was taken from about 10 mm away from the surface of the extruded bar.)

Fig. 1.

Euler color maps derived by EBSD data for presenting the grain structure of (a) spray deposited alloy and (b) extruded alloy.

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2.2Inter-annealing and extrusion experiments

Several cylinders with a diameter of 50 mm and a height of 30 mm were taken from the same radius of the large-scale extruded bar for the following experiments. The cylindrical billets were firstly annealed with different conditions and then air-cooled down to ambient temperature. Subsequently, the annealed billets were extruded again. The extrusion temperature and ram speed were set at 480 °C and 0.1 mm/s, respectively. The extrusion ratio is about 11.5. The plates with a cross-section size of 34 mm × 5 mm were obtained after the extrusion. The entire processes are shown in Fig. 2. The annealing processes of the billets were carried out in a box resistance furnace. The extrusion experiments were carried out on a hydraulic press with the maximum pressure of 200 T. An external heater was employed to heat the extrusion die, container and billet, and the temperature was monitored and controlled by a temperature control system. Fig. 3 shows the photographs of the apparatus and die used in the extrusion experiments. After extrusion, the extruded plate and die were quenched together in cooling water.

Fig. 2.

The route for the entire processes.

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Fig. 3.

Photographs of the apparatus and dies (a) extrusion press and control system (b) structure of extrusion module (c) extrusion dies.

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2.3Sampling and characterization

In order to study the microstructure changes of the extruded billets during the inter-annealing processes, the samples for microstructure observation were taken from the annealed billets, as shown in Fig. 4(a). The Electron Probe Micro-Analyzer (EPMA) and the Electron Backscatter Diffraction (EBSD) were employed to observe the microstructure, including secondary particles, grain structure, and texture. In order to study the effects of inter-annealing on the microstructure and mechanical properties of the extruded plates, the samples for microstructure observation and tensile testing were cut from the extruded plates. Fig. 4(b) shows the sampling positions and sample sizes. The distribution of secondary particles, the grain morphology, and the texture were observed by using EPMA and EBSD. In order to investigate the mechanical properties and its anisotropy, the tensile samples were tested along 0°, 45° and 90° directions, respectively. However, the sample sizes are not compliant with the relevant standards due to the limited width of the extruded plate. Even so, the effect of different inter-annealing processes on the tensile properties still can be investigated by comparison of the test results. The tensile specimens are tested on an electrical testing machine. All the tensile tests were performed at a constant speed of 0.5 mm/s. The observation of secondary particles and elemental analysis were carried out on the EPMA of JXA-8530F. The SEM of JEM-7800F with an Oxford EBSD system was used for the acquisition of orientation data. For the EBSD test, the mechanically polished sample was further electro-polished in a solution containing 15 vol.% perchloric acid and 85 vol.% ethanol at 28 V for 10 s to remove the plastically deformed layer. The extrusion direction, transverse direction and thickness direction of the extruded plate were defined as ED, TD and ND directions, respectively, as shown in Fig. 4(b). The sample coordinate in the EBSD system consists of the x-axis, y-axis and z-axis, which are respectively parallel to the ED, TD and ND directions. The EBSD orientation data were analyzed using the commercial software HKL CHANNEL 5.

Fig. 4.

Sampling positions and sample sizes of (a) annealed billet and (b) extruded plate.

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3Results and discussion3.1Initial microstructure of billet

Fig. 5 shows the microstructure of the billet after the first extrusion. The IPF-Mapping shown in Fig. 5(a) exhibits the grain structure of the billet, in which the black lines represent the high angle grain boundaries (HAGBs) with misorientation angle larger than 15°, and the white lines indicate the low angle grain boundaries (LAGBs) with misorientation angle range from 2° to 15°. As can be seen from this figure, the microstructure of the billet has a mixed grains structure consisting of the extrusion fibers and fine recrystallization grains. The average width of fibers is about 5 μm, and the fine grains are formed by the incomplete DRX during the hot extrusion process. The misorientation distribution is shown in Fig. 5(b), in which the black dashed line represents the theoretical distribution for a randomly oriented assembly of grains according to the results reported by Mackenzie [27]. There is a big difference between the experimental misorientation distribution and Mackenzie plot, which suggests that the grains in the extruded billet have an obvious preferential orientation. According to the inverse pole figure (IPF) of ED direction, as shown in Fig. 5(c), two kinds of extrusion textures, i.e. strong <111>//ED and weak <001>//ED are found in the billet. Besides, a weak peak corresponding to high angle misorientations of around 50°–60° can be found in Fig. 5(b), which may be ascribed to the recrystallized grains [23]. The secondary particle in the billet was also observed by Backscattered Electrons (BSE) image, as shown in Fig. 5(d). There are many particles in the extruded billet, which are uniformly dispersed in the matrix and exhibit very different morphology and size. Meanwhile, it is evident that the particles distribute along the extrusion direction. The proportion of secondary particles in the extruded billet was about 10.0% according to the count by the image analysis software.

Fig. 5.

Microstructure of the extruded billet (a) IPF-Mapping (b) misorientation distribution (c) inverse pole figure of ED direction (d) BSE image for observing the secondary particles.

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Fig. 6 shows an SEM image with high magnification and the main elements mappings of the extruded billet, in which the distribution of elements Al, Cu, Mg, Ag, Zr, Si, and Fe is presented. Element Li is unable to be detected due to its small atomic number. It can be observed that most of the particles in the matrix contain element Cu. Element Mg is enriched in the particles with large size, while the content of Mg in the small particles is very less. The distribution of Ag is highly consistent with that of Mg. Murayama and Hono [28] and Reich et al. [29] found that the co-clusters of Mg–Ag can be prompted to form once a trace of Ag was added into the Al alloys with high Cu/Mg ratio. The alloy 2195 used in this study has a high Cu/Mg ratio and a small amount of Ag, which provides sufficient conditions for the formation of Mg–Ag co-clusters. The element Zr distributes uniformly in the matrix. Elements Si and Fe have concentration at a few positions, which are probably corresponding to the insoluble compounds in the matrix.

Fig. 6.

SEM image and mappings of main elements in the extruded billet.

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3.2Effect of inter-annealing on the microstructure of extruded billet

Fig. 7 shows the distribution of particles in the billets annealed with different conditions. The fraction of the particles under each annealing condition is counted, and the result is provided in Fig. 7. As can be seen, the second particles are dissolved into the matrix after the inter-annealing processes, which results in a significant reduction in the proportion of particles compared with that of the extruded billet. With the increase of annealing temperature, the proportion of particles decreases gradually. But it no longer changes significantly once the temperature is higher than 510 °C, which indicates the most of the soluble particles have been dissolved into the matrix at 510 °C. The remaining insoluble phases are difficult to be dissolved even at a higher temperature. Furthermore, by comparing the proportion of particles in the billets with different inter-annealing time at the same temperature, it can be found that at the low annealing temperature of 430 °C, the proportion of the particles decreases gradually as the annealing time extends. However, at the higher annealing temperature, the particle proportion exhibits little change with the holding time. This phenomenon may be resulting from the fact that the soluble particles, at the high temperature, have been almost dissolved through 2 h of holding, and the extended annealing time cannot increase the solubility of particles.

Fig. 7.

Distribution and proportion of particles in the billets annealed with different conditions (a) 430 °C/2 h (b) 430 °C/4 h (c) 430 °C/8 h (d) 470 °C/2 h (e) 470 °C/4 h (f) 470 °C/8 h (g) 510 °C/2 h (h) 510 °C/4 h (i) 510 °C/8 h (j) 550 °C/2 h (k) 550 °C/4 h (l) 550 °C/8 h (the numerals denote the proportion of second-phase particles).

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After the inter-annealing processes, the distribution of the elements in Cu, Mg, and Ag is obtained by using EPMA, as shown in Fig. 8. It can be found that at the low annealing temperature of 430 °C, there is still apparent concentration for all the elements, which indicates the elements are unable to diffuse uniformly at this temperature. When the annealing temperature rises to 470 °C, it is evident that the distribution of Mg and Ag becomes very uniform, which indicates that the co-clusters of Mg–Ag can be dissolved into the matrix at this temperature. However, some Cu-rich particles with large size still exist in the matrix even though the small particles have disappeared. As the temperature increases to 510 °C, the large-sized Cu-rich particles have also been dissolved almost. The concentrations of Cu dispersing in the matrix mainly result from the precipitation during the air-cooling of the billet and a period of natural aging.

Fig. 8.

Element mappings and BSE images of the billets annealed with (a) 430 °C/2 h (b) 470 °C/2 h (c) 510 °C/2 h (element mappings with the same inter-annealing condition are listed in the same column).

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In order to study the effects of inter-annealing on the grain structure and micro-texture, the samples with different annealing parameters were observed using EBSD technology. The IPF-Mappings are shown in Fig. 9. The fraction of LAGBs for each sample is obtained and presented in Fig. 9. The microstructure after the inter-annealing still exhibits obvious extrusion fibers, but the average width of the fibers is very different for the various inter-annealing parameters. The average width increases gradually with the increase of inter-annealing temperature and holding time, and the effect of temperature is more significant than time. Besides, the fiber structure at high inter-annealing temperature breaks into many fragmented grains that distribute among the fibers. It is especially obvious at the temperature of 550 °C (as shown in Fig. 9(e) and (f)). At the low temperature, the grain boundaries have reduced mobility, and the massive particles in the matrix have a strong pinning effect on the grain boundaries. The microstructure changes mainly by way of SRV at the low temperature. During the process of SRV, the dislocations are annihilated and rearranged to form lots of sub-structures, while the HAGBs cannot be changed significantly [11]. Therefore, the fiber structure is still evident at the low inter-annealing temperature and its width is relatively narrow due to the weak mobility of the HAGBs. The fraction of LAGBs at the low temperature also becomes very high due to the formation of sub-structures, as shown in Fig. 9(a). As the annealing temperature increases, the misorientation of LAGBs increases gradually and they finally transform into HAGBs. The large fibrous grains break into many fragmented grains by the transformation of boundaries, and the mixed microstructure of fibrous and fragmented grains are consequently formed. Meanwhile, the particles in the matrix are reduced significantly at the high temperature, as shown in Fig. 7, which results in a decrease of the pinning effect. The reducing pinning effect accelerates the transition from LAGBs to HAGBs, which further promotes the progress of grain fragmentation. The massive transformation of LAGBs to HAGBs at high temperature also results in the decrease of LAGBs fraction, as shown in Fig. 9(e) and (f). The above microstructural evolutions at the high temperature are commonly referred to as continuous static recrystallization (CSRX) or extended recovery [26].

Fig. 9.

IPF-mappings and LAGBs fractions of the billets annealed at different conditions (a) 430 °C/4 h (b) 470 °C/2 h (c) 470 °C/8 h (d) 510 °C/4 h (e) 550 °C/4 h (f) 550 °C/8 h.

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The billets annealed with different conditions contain similar texture components, i.e. strong <111>//ED and <100>//ED fiber textures. The inter-annealing parameters have little effect on the texture types. However, the content of <100> component increases significantly after the inter-annealing processes comparing with texture content in the extruded billet. Hales et al. [18] studied textures evolution in deformed alloy 2195 and found that the texture components of Cube {001} <100> and R-Cube {013} <100> can be formed in the recrystallization process. Therefore, the strong <100> component after the inter-annealing probably results from the SRX process.

3.3Effect of inter-annealing on extrusion load

Fig. 10 shows the peak loads and load curves in the extrusion processes for the billets annealed with different annealing conditions. (Since the load curves at low inter-annealing temperatures show no obvious peaks, the stable loads just after the initial fast rising stage are considered as the peak loads for the convenience of description.) As can be seen, the peak load increases with the rising of annealing temperature, which is mainly caused by the increased effect of solution strengthening. In the previous study, the authors have investigated the hot deformation behavior of deposited alloy 2195 and found this alloy exhibits a noticeable effect of solution strengthening [30]. As shown in Fig. 7, more secondary particles are dissolved into the matrix as the temperature increases, which results in an increase of solutes in the matrix and thus leading to the rising of extrusion load [31]. The similar solution strengthening effect during hot deformation has also been reported in other references [24,32]. As shown in Fig. 10(b), the extrusion load curves of the billets annealed with 510 °C and 550 °C show prominent peaks, while the load curves for other annealing conditions have not evident peaks and always increase in the whole extrusion processes. The increase is more noticeable for the load curves of the extruded billets without any annealing. For the billets annealed with a lower temperature than 480 °C, the remaining particles in the matrix continue to be dissolved during the hot extrusion, which increases deformation resistance. However, the solutes in the matrix will not increase or even decrease during the extrusion for the billets annealed at higher temperatures than 480 °C. Therefore, the extrusion curves for high annealing temperatures exhibit peaks when the material breaks through the die outlet.

Fig. 10.

Peak loads and load curves of the extrusion processes for the billets annealed with different inter-annealing conditions (a) peak loads (b) extrusion load curves.

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Further observation of Fig. 10(a) reveals that the peak load increases obviously when the inter-annealing temperature rises from 470 °C to 510 °C. However, the peak loads are similar for the billets annealed with 430 °C and 470 °C, as well as for the billets annealed with 510 °C and 550 °C. This phenomenon is likely to be caused by the change of solubility during the preheating and hot extrusion. The remaining particles in the billet annealed at 430 °C can be further dissolved during the heating and hot extrusion processes, which increases deformation resistance. For the billet annealed with 470 °C, the remaining particles in the matrix are not in a position to be dissolved obviously, because the extrusion temperature of 480 °C is very close to the inter-annealing temperature. Therefore, the solution strengthening effect caused by the particles dissolution during the extrusion process is similar to each other for the billets annealed with 430 °C and 470 °C, which results in similar extrusion loads. At the high extrusion temperature, the solution strengthening effect will not be reduced obviously during the extrusion process owing to the small kinetics of precipitation. Therefore, the extrusion load of the billet annealed with 510 °C is larger than that of the billet annealed with 470 °C. It can be easily found from Fig. 7 that the effect of solution strengthening for the billets annealed with 510 °C and 550 °C is close to each other, which results in similar extrusion load. Moreover, it can also be found in Fig. 10(a) that the billets annealed for 2 h and 4 h exhibit roughly same peak load, while the loads for 8 h annealing increases obviously. Two reasons may cause this phenomenon. Firstly, the particles will be further dissolved into the matrix as the annealing time extends, thus leading to the enhancement of solution strengthening. Secondly, the longer holding time make the solutes diffuse more fully and form some atom atmospheres with large size, which have a stronger hindrance to mobile dislocations.

3.4Effect of inter-annealing on the microstructure of extruded plate

Fig. 11 shows the distribution of the secondary particles in the extruded plate without any inter-annealing process. The proportion of the particles in the plate is about 8.9%, which is less than that in the extruded billet. The reduction of particles is mainly attributed to the preheating and the hot extrusion processes, in which the particles can be dissolved significantly. Moreover, by comparison of Figs. 11 and 5(d), it can be seen that the number of large-sized particles in the matrix after the second extrusion is remarkably reduced, and the average particle size decreases. The large-sized particles are broken during the second extrusion deformation, and thus forming the finer dispersed particles.

Fig. 11.

Distribution and proportion of the particles in the extruded plate without inter-annealing (the numeral denotes the proportion of second-phase particles).

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Fig. 12 shows the distribution and proportion of particles in the extruded plates with different inter-annealing processes. Compared with the billet before extrusion (as shown in Fig. 7), the proportion of particles in the extruded plates is generally higher, which mainly results from the precipitation after the extrusion process. There is a time interval from the plate being extruded out to the quenching. During this time, the extruded plate was cooled in the air with a slow rate, thus resulting in precipitation of the secondary particles in the matrix. In addition, it can be further observed that the size of particles after the extrusion becomes dispersive and fine, which is probably resulted from three reasons. Firstly, some soluble particles remaining in the billet continue to be dissolved during the extrusion process, which reduces the particle size. This occurs primarily in the billets with low annealing temperature, i.e. inter-annealing of 430 °C and 470 °C. Secondly, the large-sized particles are broken during the extrusion and distribute uniformly in the matrix. Thirdly, the slow cooling before quenching causes uniform precipitation in the matrix. For the billets annealed at higher temperatures, the supersaturation of the matrix is higher, which enhances the precipitation kinetics. Therefore, the extruded plates with the high annealing temperature of 510 °C and 550 °C contain more fine precipitates. According to Fig. 12, the inter-annealing time has little effect on the morphology and distribution of the secondary particles in the extruded plate.

Fig. 12.

Distribution and proportion of particles in the extruded plates with different inter-annealing (a) 430 °C/2 h (b) 430 °C/4 h (c) 430 °C/8 h (d) 470 °C/2 h (e) 470 °C/4 h (f) 470 °C/8 h (g) 510 °C/2 h (h) 510 °C/4 h (i) 510 °C/8 h (j) 550 °C/2 h (k) 550 °C/4 h (k) 550 °C/8 h (the numerals denote the proportion of second-phase particles).

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Fig. 13 shows IPF-Mapping along ED direction and IPF of the extruded plate without any inter-annealing process. After the second extrusion, the grain structure of the plate is still similar to that of the extruded billet, as shown in Fig. 5(a), i.e. extrusion fibers accompanied by recrystallized grains. The average fiber width and recrystallized grain size, however, increase slightly, which may be attributed to the high extrusion temperatures and slow ram speed. As shown in Fig. 13(b), the texture components in the extruded plate are very different from those in the billet. The <111>//ED texture disappears after the extrusion, while the texture of <211>//ED becomes very strong. This change in texture components is owing to the different deformation state. Tempus et al. [33,34] suggested that the plane strain deformation often leads to the strong texture of Bs {011}<211>, while the textures of <111> and <100> are usually introduced by the axisymmetric deformation. The cross-section of the extruded plate has a large aspect ratio, and its deformation state is close to plane strain. The strong texture of <211> is therefore introduced to the extruded plate. However, the original billet is taken from an extruded rod with large-size which is deformed in an axisymmetric manner, thus leading to the <111> and <100> textures.

Fig. 13.

IPF-mapping and IPF of the extruded plate without inter-annealing (a) IPF-mapping along ED direction (b) IPF.

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Fig. 14 shows IPF-Mappings of the extruded plates with different inter-annealing processes. While the grains in the billets annealed with various parameters exhibit different morphology and size, the grain structure of the plates after the second extrusion tends to be the same. Previous studies have proved that the same recrystallized grain size can be obtained if the samples of different initial grain size are deformed at the same temperature and strain rate [26,35,36]. Besides, Humphreys and Hatherly [14] reviewed a series of studies on microstructure evolution from low strain to high strain deformation, and suggested that the substructure size at the low strain level has no longer changed significantly as the strain increases and many permanent microstructures can be formed and are not easy to be changed at the high strain of 1.5. In this study, the billets processed with different inter-annealing are extruded by the same condition, i.e. the temperature, strain rate, and deformation degree are quite similar for each extrusion. Therefore, the microstructures of the extruded plates are very similar. Further observation of Fig. 14 reveals that the grain size of the extruded plates with inter-annealing of 510 °C and 550 °C is slightly larger than that of the others. This phenomenon is more evident for the annealing condition of 550 °C, as shown in Fig. 14(j)–(l). As shown in Fig. 7, the billets with higher annealing temperature contain fewer secondary particles in the matrix; therefore, the migration of grain boundary is more easily. Meanwhile, the high temperature on the extruded plates can be maintained for a period before quenching. The rapid movement of the grain boundaries results in grain growth at the remaining highly temperature. The textures of the extruded plates with different inter-annealing conditions are also similar, which are close to the textures shown in Fig. 13, viz. <211> and <100> textures.

Fig. 14.

IPF-mappings of the extruded plates with different inter-annealing processes (a) 430 °C/2 h (b) 430 °C/4 h (c) 430 °C/8 h (d) 470 °C/2 h (e) 470 °C/4 h (f) 470 °C/8 h (g) 510 °C/2 h (h) 510 °C/4 h (i) 510 °C/8 h (j) 550 °C/2 h (k) 550 °C/4 h (l) 550 °C/8 h.

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Fig. 15 shows the GOS distribution of the extruded plates with different inter-annealing processes. The dark grains in the distribution maps have low GOS value, which suggests these grains are the recrystallized grains with a low strain level. As can be seen, the plates with higher inter-annealing temperature contain less recrystallized grains, which may be caused by two reasons. Firstly, it may be caused by the finer grains in the billets annealed at low temperature, as shown in Fig. 9. Wu et al. [37] indicated that the fine grains could provide more nucleation sites for DRX at the grain boundaries and accelerate the DRX process. Belyakov et al. [23] also suggested that smaller grain size makes the misorientation of LAGBs increase faster and accelerates the kinetics of grain refinement significantly. Therefore, the small grain size in the billets annealed at the low temperature is favorable for forming new recrystallized grains. Secondly, the billets annealed with low temperature contain more large-sized particles (as shown in Fig. 7) around which the strain accumulation is easy to be produced. The recrystallization nucleation is therefore prompted by the way of particle stimulated nucleation (PSN), as reported by Zhang et al. [25] and Huang et al. [38].

Fig. 15.

GOS distribution of the extruded plates with different inter-annealing processes (a) 430 °C/2 h (b) 470 °C/2 h (c) 510 °C/2 h (d) 550 °C/2 h.

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3.5Effect of inter-annealing on mechanical properties of extruded plate

The tensile properties of the extruded plates with different inter-annealing were tested along ED, TD and 45° directions, and the results are presented in Fig. 16. As can be seen, the mechanical properties in the three directions are very different. The properties along ED direction exhibit the highest ultimate tensile strength (UTS) and the moderate elongation. The elongation in 45° direction is larger than that in other directions, but the UTS is the lowest. The UTS and elongation in the direction of TD, overall, are slightly lower than those in the direction of ED. The anisotropy in mechanical properties mainly results from the crystallographic texture in the extruded plates. Fig. 16(d) shows the Schmid factor distribution of the extruded plate without inter-annealing when the tensile directions parallel to the ED, TD and 45° directions, respectively. (Schmid factor is calculated by means of the CHANNEL 5 software based on the EBSD data.) It can be seen that the Schmid factor in 45° direction is obviously higher than those in the other two directions, which is owing to the apparent orientation of grains in the extruded plate. The Schmid factor reflects the difficulty of plastic deformation under external load, and the plastic deformation occurs easier if the factor is larger [39,40]. Therefore, the tensile strength along the 45° direction is the lowest and the elongation is highest. The average Schmid factor for the load direction along ED is slightly smaller than that of TD load direction, and more grains are having low Schmid factor of less than 0.4. Therefore, the tensile strength in the ED direction is slightly higher than that in the TD direction.

Fig. 16.

Tensile properties of the extruded plates with different inter-annealing processes along (a) ED (b) 45° direction and (c) TD, and (d) Schmid factor distributions when the tensile direction parallels to the ED, TD and 45° direction (taking the extruded plate without inter-annealing as an example).

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As shown in Fig. 16, the properties of the extruded plate without inter-annealing exhibit a lower value of UTS and a higher elongation compared with others. It can be observed from Fig. 11 that a large number of undissolved particles are present in the extruded plate without inter-annealing. Therefore, the solution strengthening effect is weak. The low solubility in the matrix is also detrimental to the precipitation of strengthening phases during cooling and natural aging. Thus, the extruded plate without inter-annealing has lower UTS. The low tensile strength means that dislocations are easy to slip in the matrix during the tensile deformation, which results in the high elongation.

As shown in Fig. 16, the extruded plates with different inter-annealing processes exhibit similar mechanical properties without noticeable regular changes, which suggests that the inter-annealing between two stages hot extrusion has little impact on the properties of the final parts. However, it is still noteworthy that the extruded plates with the high inter-annealing temperature of 510 °C and 550 °C exhibit high values of UTS, while the elongations show rising first and then declining. As mentioned earlier, the higher inter-annealing temperature can result in a more obvious solution strengthening effect and cause some precipitates in the matrix, which is beneficial to the increase of strength. In addition, during the tensile deformation, the dynamic precipitation can occur for the samples containing sufficient solutes, which is commonly termed as Portevin-Le Chatelier (PLC) effect. This effect has been reported by Zhang et al. [41], and they found that the PLC effect occurs only in the alloys with sufficient solute elements in which the effective solution atmospheres can lock mobile dislocations. Fig. 17 shows the stress–strain curves of the extruded plates with different inter-annealing. As can be seen, the curves for the low inter-annealing temperatures (430 °C and 470 °C) show a weak PLC effect. However, the PLC effect becomes obvious for the high inter-annealing temperatures (above 510 °C) due to the increase of solutes in the matrix. The PLC effect can lock the mobile dislocations by forming the effective solution atmospheres, and the high solute content in the matrix can aggravate the dislocation tangle, which is beneficial to maintaining the strain-hardening rate. Glazer et al. [42] and Deschamps et al. [43] have demonstrated that the elongation is considerably affected by the strain-hardening rate, and the high strain-hardening rate usually leads to large tensile elongation. Therefore, the large elongation at high inter-annealing temperatures is mainly attributed to the high strain-hardening rate associating with the strong PLC effect.

Fig. 17.

Stress–strain curves for the extruded plates with different inter-annealing (along ED direction).

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The extruded plates with different inter-annealing processes are further solution treated with 490 °C/40 min and 520 °C/40 min and then aged at 170 °C for 24 h. Fig. 18 shows the tensile properties along ED direction for the plates after solution and aging treatments. As can be seen, most of the plates still exhibit similar tensile strength and elongation after the thermal treatments. However, the properties of plates with inter-annealing of 550 °C/8 h exhibit severe decline. Fig. 19 shows IPF-Mappings of the extruded plates with different inter-annealing processes after the solution and aging treatments. As can be seen, the grain size in the plates after the thermal treatments increases obviously. Especially for the plates with inter-annealing of 510 °C/8 h and 550 °C/8 h, the grains grow abnormally after thermal treatments, which probably results from the coarsening of the dispersoids. Many studies have shown that the dispersed phases will coarsen and decrease in number if the homogenization is carried out at a high temperature and for a long time, which can reduce the pinning on the grain boundaries, thus causing recrystallization and rapid growth of grains [3,16,44,45]. Therefore, the poor performance of the plate with inter-annealing of 550 °C/8 h can be attributed to the coarse grain structure associating with the coarsening dispersoids. The tensile properties of the plate with inter-annealing of 510 °C/8 h show no decline after the thermal treatments, which may result from partially fine grains in the microstructure as shown on the right side of Fig. 19(c). Even so, the inter-annealing process should avoid being performed at 510 °C for a long time due to the possible risks in microstructure and mechanical properties.

Fig. 18.

Tensile properties after solution and aging treatments for the plates with different inter-annealing processes (along ED direction).

(0.14MB).
Fig. 19.

IPF-Mappings of extruded plates with different inter-annealing processes after solution and aging treatments (a) 430 °C/8 h (b) 470 °C/8 h (c) 510 °C/8 h (d) 550 °C/8 h.

(0.85MB).
4Conclusions

In this paper, the inter-annealing process between two stages extrusion of the spray deposited alloy 2195 was studied. The effect of the inter-annealing on the microstructure of the extruded billet was investigated through the characterization of secondary particles, grain structure, and micro-texture. The annealed billets were extruded again, and the influences of the inter-annealing on the extruded plates were revealed by microstructure characterization and mechanical properties testing. Some important conclusions were drawn as follows:

  • 1

    The secondary particles contained in the extruded billet are dissolved into the matrix during the inter-annealing process, and the solubility is mainly affected by the annealing temperature. The microstructure of the billets can be changed in the process of the inter-annealing by the ways of SRV, SRX and the fiber width increase. After inter-annealing, the content of <100> texture increases owing to the occurrence of SRX.

  • 2

    During the inter-annealing process of the extruded billet, the dissolution of particles leads to a noticeable effect of solution strengthening. The extrusion load for the billet annealed with different conditions is therefore very different. The billet annealed with higher temperature and longer time exhibits greater deformation resistance.

  • 3

    The proportion and the morphology of particles in the extruded plates mostly inherit the characteristics of particles in the annealed billets. That is, the plate with lower inter-annealing temperature contains more particles with a larger size, while the particles in the extruded plate with higher inter-annealing temperature are less and finer. The grain structure of the extruded plates with different inter-annealing processes is very similar, but the grain size of the plates with a high inter-annealing temperature is slightly larger. After the second extrusion, a strong texture of <211>//ED is introduced into the plates, which causes obvious anisotropy of mechanical properties.

  • 4

    The extruded plates with different inter-annealing processes exhibit similar mechanical properties without obvious regular changes. This phenomenon suggests that the inter-annealing between two stages hot extrusion has little influence on the properties of the final parts.

  • 5

    After solution and aging treatments, the plates extruded by the billets annealed at high temperature for a long time exhibit serious grain coarsening, which causes a noticeable decline in mechanical properties. After general consideration, for the extruded billet of alloy 2195, it is reasonable to perform the preheating process at a relatively low temperature for a short time before the second extrusion. Excessive preheating temperature and holding time will not only increase the deformation resistance but lead to serious grain coarsening during the thermal treatments.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgments

This project is supported by National Natural Science Foundation of China (Grant No. 51735008) and Science Fund for Distinguished Young Scholars of Shandong Province (Grant No. JQ201810).

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