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Quantitative analysis of the microstructure of transitional region under multi-heat isothermal local loading forming of TA15 titanium alloy
Materials and Design 32 (2011) 2012–2020
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Quantitative analysis of the microstructure of transitional region under multi-heat isothermal local loading forming of TA15 titanium alloy P.F. Gao, H. Yang ⇑, X.G. Fan State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, P.O. Box 542, Northwestern Polytechnical University, Xi’an 710072, PR China
a r t i c l e
i n f o
Article history: Received 3 November 2010 Accepted 25 November 2010 Available online 1 December 2010 Keywords: A. Non-ferrous metals and alloys C. Forging F. Microstructure
a b s t r a c t In this paper, an analogue experiment was carried out to study the effect of processing parameters including deformation temperature, deformation degree, cooling mode and loading pass on the microstructure of transitional region under isothermal local loading forming of TA15 titanium alloy. The volume fraction, grain size and aspect ratio of primary a phase of transitional region were quantitatively characterized. It is found that deformation temperature and deformation degree also have interaction on the microstructure evolution of transitional region under isothermal local loading forming. At a certain deformation degree, primary a grain size increases first and then decreases with increasing temperature. However, primary a grain size varies little with deformation degree at higher temperature (in upper two phase region) but increases firstly and then decreases with deformation degree at lower temperature (in lower two phase region). Primary a aspect ratio increases with deformation degree at lower temperature but varies little at higher temperature. The morphology of transformed structure in b matrix is greatly influenced by deformation temperature and less influenced by deformation degree under air-cooling. The precipitated Widmanstatten a phase in b matrix is in lamellar form and arranges in colonies under air-cooling, but it is in thinner acicular form and distributes disorderly under water quenching. Loading pass has little influence on the morphology of microstructure. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Large-scale integral component of titanium alloy with complex shape (such as bulkhead) has increasingly extensive application in aerospace field since whose material and structural characteristics make aircraft light weight and reliability. This kind of components often serves as key load-bearing structure in severe conditions, so not only the quality of macroscopical forming but also the fine microstructure is needed [1–4]. However, forming of these components is difficult due to the complexity of their shape, hard-to-deform properties of titanium alloy and high requirement for forming quality. The isothermal local loading forming technology, which can control the flow of material, reduce the forming load, enhance plasticity, and enlarge the size of component to be formed, provides a feasible method to form these components [3,4]. However, the isothermal local loading forming is a complicated multi-step hot working process with coupling effects of multifields and multi-factors. During isothermal local loading forming, load is applied to part of the billet and the component is formed by changing loading region. In each forming stage, there exist loading region, unloading region, transitional region, and the alternation among them, as illustrated in Fig. 1. The material in ⇑ Corresponding author. Tel./fax: +86 029 8849 5632. E-mail address: [email protected] (H. Yang). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.11.058
different loading regions undergoes different thermal processing paths, which will influence the microstructure as well as the mechanical properties of final product [4]. Transitional region, connecting the loading region with unloading region, undergoes more complex uneven plastic deformation and concomitantly sophisticated microstructure evolution, which makes the transitional region a key factor for the component. On the other hand, microstructure of titanium alloys is very sensitive to processing parameters, and the desired microstructure can be obtained at proper processing parameters to obtain good mechanical properties such as strength, ductility, toughness and fatigue resistance [5]. Therefore, to study and reveal the effect of processing parameters on microstructure of transitional region under isothermal local loading forming of titanium alloy is a key problem urgent to be solved. By now, most work has been done on the deformation mechanism and microstructure evolution of titanium alloys during thermomechanical processing [5–12]. Primary study on the deformation mechanism and microstructure evolution of titanium alloy under local loading forming has also been conducted. Sun and Yang designed an analogue experiment to study the effects of local loading conditions on microstructure of TA15 titanium alloy forgings qualitatively [3]. However, the influence of processing parameters on the microstructure of titanium alloy under isothermal local loading forming needs further quantitative study. Fan et al. [13]
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Fig. 1. Illustration of isothermal local loading forming [3].
developed a set of models describing the microstructure evolution of TA15 alloy using internal state variable method and embedded the models in FE software to investigate the grain size and distribution of primary a grains during local loading forming. Fan et al. [4] carried out an experiment of local loading forming of large-scale rib–web component of TA15 titanium alloy to study the effect of deformation inhomogeneity on microstructure and mechanical properties of large-scale rib–web component under local loading forming. However, investigations are furthermore needed so as to get a general and detailed influence law of processing parameters on the microstructure of transitional region, a key factor for the component, under isothermal local loading forming of titanium alloy. In the present paper, an analogue experiment was conducted and different process schedules were proposed to study the effect of processing parameters (deformation temperature, deformation degree, cooling mode and loading pass) on the microstructure of transitional region under isothermal local loading forming of TA15 titanium alloy. It will provide technological basis for optimizing processing parameters for isothermal local loading forming of large-scale integral component of TA15 titanium alloy.
2. Experimental procedure The experimental material is TA15 titanium alloy in hot rolled plate form with specification 1200 170 80 mm and the b-transus temperature 990 °C. The chemical compositions of this alloy
Fig. 2. Original microstructure of the billet.
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are as follows (wt.%), Al: 6.8; Mo: 1.7; V: 2.2; Zr: 2.0; Fe: 0.3 and Ti balance. The microstructure of as-received material consists of about 60% primary equiaxed a phase (with a standard deviation of 6%) and transformed b matrix, as shown in Fig. 2. The specimens used in test were rectangular solid of 20 16 12 mm. The isothermal local loading forming was implemented by one or two loading passes, two loading steps included in one loading pass, as illustrated in Fig. 3. When the isothermal local loading forming was implemented by two loading passes, the amount of compression in every loading pass was half of the total. In every loading step, the sample was heated to deformation temperature at 12 °C/min, held for 15 min, and pressed under corresponding conditions (Fig. 3a and b) at a constant nominal strain rate and then air-cooled or water quenched. The heat treatment route used after isothermal local loading was as follows: (1) heating to 810 °C and holding for 1 h and (2) air-cooling to room-temperature. The test parameters consists of deformation temperatures of 910, 930, 950 and 970 °C, deformation degrees of 30%, 50% and 70%, loading pass of one and two, cooling modes of air-cooling and water quenching and strain rate of 0.01 s 1. To acquire the deformation behavior of sample during isothermal local loading forming and determine the microstructure image locations of transitional region, corresponding numerical simulations of isothermal local loading forming was carried out through DEFORM-3D software. Fig. 4 shows the sample deformed at temperature of 950 °C to the degree of 50% by one-pass isothermal local loading. It can be seen that the simulated result closely matches the experimental result. Simulation results show that the strain distribution of sample during isothermal local loading forming is mainly determined by deformation degree. Fig. 5 shows the effective strain distribution on cross-section of samples deformed by one-pass isothermal local loading to different deformation degrees. The material in transitional region undergoes complex uneven plastic deformation and different local regions may undergo different thermal processing paths during practical isothermal local loading forming. Choosing a series of points and tracing their strain paths to determine the representative points of transitional region, i.e. the microstructure image locations of transitional region, we found that Zone A and
Fig. 4. Sample formed by isothermal local loading process: (a) experimental result; (b) simulated result.
Fig. 3. Schematic diagram of the two loading steps in one loading pass: (a) the first-loading step; (b) the second-loading step.
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Fig. 5. Effective strain distribution on the cross-section corresponding to that of Fig. 3: (a) 30%, 950 °C; (b) 50%, 950 °C; (c) 70%, 950 °C.
Zone B (illustrated in Fig. 5) have close accumulated strain after processing but different strain paths. After first-loading step, the effective strain at Zone A and Zone B are about 90% and 10% of the total respectively. After second-loading step, two zones have close accumulated strain. Therefore, Zone A and Zone B were selected as the representative zones of transitional region, i.e. the microstructure image locations. The microstructure was observed and analyzed using Olympus optical microscope. The corrosive solution was 13%HNO3 + 7%HF + 80%H2O. The size and morphology of primary a phase have great influence on the performance of TA15 titanium alloy. Therefore, the volume fraction, grain size and aspect ratio of primary a phase were quantitatively measured.
3. Results and discussion 3.1. Effect of deformation temperature and deformation degree It is well known that deformation temperature and deformation degree have considerable influence on the microstructure of titanium alloy respectively. However, hot deformation of titanium alloys is a complicated mutual coupling process of deformation behavior, heat transfer and microstructure evolution. Moreover, the interaction between deformation temperature and deformation degree was studied in the hot compression of titanium alloy [5]. Hence, the interaction between deformation temperature and deformation degree on the microstructure evolution of transitional region under isothermal local loading forming should also be studied. Figs. 6 and 7 show the microstructure of Zone A and Zone B deformed at different temperatures and deformation degrees by one-pass isothermal local loading and air-cooled respectively. As illustrated in Figs. 6 and 7, the microstructure mainly consists
of primary a phase (ap) and transformed b matrix composed of residual b phase and secondary a phase (as) throughout specimens, but little prior lamellar a phase could be found in most deformation conditions, which is different from the result reported in [4]. This is related to the a ? b phase transformation during thermomechanical processing. Ding et al. [7] studied the a ? b phase transformation during thermomechanical processing, and pointed out that most of the prior a lamellar phase inside b grains would transform to b phase firstly at high temperature, but the prior a phase near grain boundary was difficult to transform to b phase because of the relatively high concentration of Al at grain boundary. They also distinguished the phase transformation into three stages: non-transformed, partially transformed and completely transformed. In the work of Fan et al. [4], an amount of as, which is precipitated from b phase during the cooling down after first-loading step, retained and grew thicker in the following second-loading step, there the lamellar a phase were produced. In this work, the as in b matrix were transformed into high-temperature b phase before hot working and then all the high-temperature b phase would transform into transformed b matrix having a lamellar as structure upon post-deformation cooling. Therefore, there was little prior lamellar a phase in microstructure under most deformation conditions, suggesting that the initial microstructure for two loading step were both ap and pure high-temperature b phase in most deformation conditions. An example of microstructure just after the second-loading step deformation obtained at the deformation temperature of 950 °C and deformation degree of 50% is presented in Fig. 8. No lamellar a phase could be found in the microstructure just after deformation, which confirms the conclusion that initial microstructure was ap and pure high-temperature b phase in most deformation conditions. It can be seen from Figs. 6 and 7 that Widmanstatten a phase precipitated in transformed b matrix have different morphology
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Fig. 6. Microstructure of Zone A deformed at different temperatures and deformation degrees by one-pass isothermal local loading and air-cooled: (a) 910 °C, 30%; (b) 930 °C, 30%; (c) 950 °C, 30%; (d) 970 °C, 30%; (e) 910 °C, 50%; (f) 930 °C, 50%; (g) 950 °C, 50%; (h) 970 °C, 50%; (i) 910 °C, 70%; (j) 930 °C, 70%; (k) 950 °C, 70%; (l) 970 °C, 70%.
Fig. 7. Microstructure of Zone B deformed at different temperatures and deformation degrees by one-pass isothermal local loading and air-cooled: (a) 910 °C, 30%; (b) 930 °C, 30%; (c) 950 °C, 30%; (d) 970 °C, 30%; (e) 910 °C, 50%; (f) 930 °C, 50%; (g) 950 °C, 50%; (h) 970 °C, 50%; (i) 910 °C, 70%; (j) 930 °C, 70%; (k) 950 °C, 70%; (l) 970 °C, 70%.
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Fig. 8. Microstructure of two zones deformed to 50% at temperature of 950 °C by one loading pass and water quenched but without annealing: (a) Zone A; (b) Zone B.
under different deformation conditions. Seshacharyulu and Dutta [14] studied the mechanism of b ? a transformation and found that more dislocation density and distortion energy in deformed b phase led to an acceleration in kinetics of b ? a transformation through increasing nucleation locations of as and enhancing the precipitating and coarsening of as by pipe-diffusion through dislocation. Wanjara et al. [15] pointed out that the cooling rate after deformation affected both thickness and orientation of precipitated alpha plates in transformed b matrix. Therefore, morphology of the transformed microstructure in b matrix is considerably influenced by the dislocation density in deformed high-temperature b phase and the cooling rate after deformation. According to the forming process of isothermal local loading, morphology of transformed microstructure in b matrix is determined by the condition of second-loading step in this work. As samples in Figs. 6 and 7 were all cooled in air having close cooling rate, the different morphology of Widmanstatten a phase precipitated in transformed b matrix was mainly caused by the dislocation density and distortion energy in deformed b phase. When the deformation temperature is in upper two phase region, the high-temperature b phase has high volume fraction bearing relatively small deformation and b phase is prone to dynamic recovery, resulting in low dislocation density and distortion energy. The small driving force for b ? a transformation, at higher deformation temperature, achieves selective growth of nucleated as plates. The as first nucleates preferentially at b grain boundaries, leading to more or less discontinuous a layer along b grain boundaries (Fig. 6e, h and l), during continued cooling the a plates nucleate at b grain boundary itself and grow into b grain as parallel plates belonging to the same variant of Burgers relationship (socalled a colony) until they meet other a colonies [16]. Therefore, the precipitated as is in thick lamellar form and straight to bunchiness (Fig. 6e, h and l) at higher temperature. On the contrary, when deformation temperature is in lower two phase region, dislocation density and distortion energy in deformed b phase is higher, resulting in more nucleation location for as and a larger driving force for b ? a transformation. In addition, the growth of as is restrained at lower temperature. As a result, the precipitated as is in fine and acicular form at lower temperature (Figs. 6 and 7). It has been well known that an increase in deformation degree would increase the dislocation density and distortion energy in deformed b phase. According to the analysis above, large deformation would get shorter and disorder lamellar as in transformed b matrix. On the other hand, the produce of as is a heating activation so deformation degree has strong influence on the morphology of precipitated as in transformed b matrix at higher temperature. Therefore, when the samples are deformed at higher temperature and to larger deformation degree (50% and 70%), shorter and disorder lamellar as in transformed b matrix could be found (Figs. 6k, l, 7k and l). For the same reason, by comparing Figs. 6 and 7, it can be found that at the temperature of 970 °C, the lamellar as in transformed b matrix of Zone B is shorter and disorder than that of Zone
A. This is because the deformation degree of Zone A is much less than that of Zone B in second-loading step. Altogether, the dislocation density and distortion energy in deformed b phase is influenced by deformation temperature and deformation degree, and then which would affect the morphology of transformed microstructure within prior-beta grains. It is noted that some fine strip and equiaxed a phase are found, when the samples are deformed at 910 °C to the degree of 70% (Figs. 6i and 7i). In work of Fan et al. [4], analogous microstructure was also been found, which was caused by the break and spheroidization of lamellar a phase in initial microstructure for secondloading step during deformation. However, in this work, one possible explanation for this phenomenon is that the ap is elongated to strip then broken up and spheroidized under large deformation and low deformation temperature. This is consistent with the result that break up of the strip a phase occur to an increasing extent as the strain increases and the processing temperature decreases, which has been reported in Ref. [15]. Besides, the recrystallization of a phase during thermomechanical processing of titanium alloys in two phase region has been reported in previous works [8,17– 20]. Therefore, the recrystallization of a phase may also make a contribution to the produce of fine equiaxed a phase. It can be concluded that the composition and morphology of microstructure are mainly influenced by deformation temperature and deformation degree, and which has little difference in different zones of transitional region. The quantitatively measured microstructure parameters of Zone A and Zone B deformed at different conditions corresponding to that of Figs. 6 and 7 are shown in Fig. 9. It can be seen from Fig. 9a and b that ap content of two zones both decreases with temperature at a certain deformation degree, due to the transformation of a ? b phase. And the ap content decreases slowly with increasing temperature in lower two phase region (<950 °C), but decreases rapidly with temperature in upper two phase region (>950 °C). This agrees with the correlation between volume fraction of ap and heating temperature of TC11 titanium alloy reported in [21]. At a certain temperature, the volume fraction of ap varies little with deformation degree as illustrated in Fig. 9a and b. Fan et al. [4] studied the effect of deformation inhomogeneity on microstructure of TA15 titanium and pointed out that ap content grew sharply with strain. The reason is that more lamellar a phase would be globularized and then merged into primary a at large deformation, resulting in the increase of primary a. However, in this work, the globularization of lamellar a phase would not happen, causing that the volume fraction of ap varies little with deformation degree. By comparing Fig. 9a and b, it can be found that at certain deformation temperature and deformation degree, two representative zones undergo different thermal processing paths but have close ap content, suggesting that thermal processing path has little influence on ap volume fraction under the condition of this work. The grain size of ap under different conditions are presented in Fig. 9c and d. Here it is observed that ap grain size of two zones deformed at different conditions all increase after multi-heat forging, compared to that of initial microstructure (7.56 lm). For all deformation degrees, ap grain size increases first and then decreases with increasing temperature, having the peak value at 930 °C and the least value at 970 °C, as illustrated in Fig. 9c and d. There are two mechanisms responsible for this phenomenon: transformation of a ? b phase and the break up of ap grains. When deformation temperature is in lower two phase region, the transformation extent of a ? b phase is small, which has little influence on the grain size of ap, so that ap grain size is determined mainly by the break up of ap grains. With increasing temperature in lower two phase region, ap content decreases slightly but undergoes less deformation and break up resulting in the increase of ap grain size. This
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Fig. 9. Microstructural features of two zones deformed at different temperatures and deformation degrees by one-pass isothermal local loading and air-cooled: (a) ap content of Zone A; (b) ap content of Zone B; (c) ap grain size of Zone A; (d) ap grain size of Zone B; (e) ap aspect ratio of Zone A; (f) ap aspect ratio of Zone B.
is because b phase is easier to deform than ap during the thermomechanical processing of titanium alloys in two phase region [22]. On the other hand, volume fraction of ap decreases rapidly with temperature in upper two phase region, and decreases to only about 20% at 970 °C. This would minish the grain size of ap markedly so that ap grain size decreases with temperature in upper two phase region and reaches the least value at 970 °C. The effect of deformation temperature on ap grain size is caused by the characteristics of a ? b phase transformation in hot work of titanium alloy. However, deformation degree has little effect on the transformation of a ? b phase in this work. As a result, the deformation temperature has close influence on ap grain size at different deformation degrees. From Fig. 9c and d, we can also find that ap grain size increases firstly and then decreases sharply with increasing deformation degree at a certain deformation temperature except for 970 °C. The influence of deformation degree on ap grain size should be explained from three different changes of ap grains during deforma-
tion as follows: the break up of ap grains, the dynamic coarsening of ap grains, and the recrystallization of a phase. However, deformation temperature not only has influence on the deformation of a phase as explained above, but also has influence on the recrystallization of a phase. Previous investigations [19,20] pointed out that when deformation temperature is in upper two phase region, dynamic recrystallization of a phase is suppressed in TA15 alloy. Therefore, the influence of deformation degree on ap grain size varies with deformation temperature. At lower temperature, the three mechanisms mentioned above are all not obvious at the deformation degree of 30%. With increasing deformation degree to 50%, larger deformation could promote the mergence of two ap grains with higher and lower distortion energy, which is driven primarily by the difference of their distortion energy and partly by the decreasing of total interfacial energy [5]. The dynamic coarsening of ap grains plays a leading role, so the ap grain size increases. However, when the deformation degree increases to 70%, the break up of ap grains and the recrystallization of a phase are both enhanced
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markedly, so the ap grain size decreases again. On the other hand, when the samples are deformed at high temperature (970 °C), the deformation extend of a phase is small and dynamic recrystallization of a phase is restrained but the dynamic coarsening of ap grains is remarkable at high temperature so that the grain size of ap varies little with deformation degree at high temperature, as illustrated in Fig. 9c and d. By comparing Fig. 9c and d, it can be found that ap grain size of Zone A is smaller than that of Zone B at the deformation degree of 70% for all temperatures. This is because Zone A and Zone B undergo different thermal processing paths. As deformation is very small in second-loading step, the second-loading step can be taken as a heat treatment for Zone A. This extra heat treatment at deformation temperature would promote the recrystallization of a phase and decrease ap grain size. Nevertheless, recrystallization of a phase in the extra heat treatment demands lots of distortion energy which can be obtained at larger deformation. Therefore, ap grain size of Zone A is slightly smaller than that of Zone B at the deformation degree of 70%. The aspect ratio of ap under different conditions are presented in Fig. 9e and f. The compression, break up and recrystallization of a phase all affect the aspect ratio of ap. At a certain deformation degree, ap aspect ratio is determined by compression and recrystallization of a phase at lower temperature. While, the compression and recrystallization of a phase have counter effect on the aspect ratio of ap. With increasing temperature, compression and recrystallization of a phase both are weakened at higher temperature. As a result, the ap aspect ratio varies little with temperature for all deformation degrees, as illustrated in Fig. 9e and f. Similar to the influence of deformation degree on ap grain size, the effect of deformation degree on ap aspect ratio varies with
deformation temperature, too. As mentioned above, ap undergoes large deformation and is elongated greatly at lower temperature. The compression of a phase plays a leading role on the morphology of ap grains so that ap aspect ratio increases with deformation degree at lower temperature, as illustrated in Fig. 9e and f. However, it is noticeable that at temperature of 910 °C, the aspect ratio get the least value at deformation degree of 70% (Fig. 9e and f), which does not correspond to the regularity above. This is because a lot of fine ap is produced, when the samples are deformed to 70% at temperature of 910 °C, as shown in Figs. 6i and 7i. In contrast, deformation degree has little influence on ap aspect ratio at higher temperature due to the compression and recrystallization of a phase are both weakened. In comparison of Fig. 9e and f, it can be found that ap aspect ratio of Zone A is smaller than that of Zone B at the deformation degree of 70% for all temperatures. This is also for the reason that Zone A undergoes an extra heat treatment at deformation temperature which would decrease the aspect ratio of a phase. From above analysis, it can be found that the deformation degree has different influence on the morphology of transformed structure in b matrix, ap grain size and ap aspect ratio in different temperature regions, revealing that deformation temperature and deformation degree also have interaction on the microstructure evolution of transitional region in this work. This is because the morphology and parameters of microstructure, which are determined by deformation temperature and deformation degree together, varies through a ? b phase transformation, break up of ap under large deformation, recrystallization of a phase, dynamic coarsening of ap and so on. Moreover, these processes present different characteristics in different temperature regions: a ? b phase transformation and dynamic coarsening of ap are both
Fig. 10. Microstructure of two zones deformed at different deformation temperatures to the degree of 50% by one-pass isothermal local loading and different cooling modes: (a) Zone A, 970 °C, AC; (b) Zone B, 970 °C, AC; (c) Zone A, 950 °C, AC; (d) Zone B, 950 °C, AC; (e) Zone A, 970 °C, WQ; (f) Zone B, 970 °C, WQ; (g) Zone A, 950 °C, WQ; (h) Zone B, 950 °C, WQ.
Table 1 The quantitatively measured microstructure parameters corresponding to the microstructure shown in Fig. 10. Volume fraction of ap (%)
950 °C 950 °C 970 °C 970 °C
AC WQ AC WQ
Size of ap (lm)
Aspect ratio of ap
Zone A
Zone B
Zone A
Zone B
Zone A
Zone B
40.96 40.65 21.38 21.52
39.54 39.49 17.72 19.24
12.07 11.00 9.29 8.74
13.37 10.58 7.85 8.29
2.64 2.44 2.31 2.39
2.88 2.17 2.30 2.41
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thermally activated process, but break up of ap under large deformation and recrystallization of a phase are both suppressed at higher temperature. Therefore, deformation temperature and deformation degree also present interaction on the microstructure evolution of transitional region in this work. 3.2. Effect of cooling mode Fig. 10 shows the microstructure of two zones deformed at different deformation temperatures to the degree of 50% by one-pass isothermal local loading and different cooling modes. The corresponding quantitatively measured microstructure parameters are shown in Table 1. As illustrated in Fig. 10, the microstructure all consists of ap and transformed b matrix at different cooling modes. Considering the morphology of microstructure, it can be found that the most distinction between different cooling modes is that the transformed structure in b matrix has different morphology: the precipitated as in transformed b matrix is in lamellar form and arranges in colonies when the samples are cooled in air, but that is in thinner acicular form and distributes disorderly when the samples are quenched in water. It has been mentioned that the cooling condition affect the morphology, thickness and orientation of precipitated a plates in transformed b matrix markedly. The precipitating process and morphology of precipitated as in transformed b matrix at the cooling mode of air-cooled has been discussed in Section 3.1. When the samples are quenched in water, the cooling rate is so high that there is not enough time for b phase to form equilibrious a phase through regular diffusion of a-stable elements, for static recrystallization and for lamellar a phase precipitating from b phase after deformation. The high-temperature b phase transformed completely into a phase by a collective migration in short range of atoms, leaving behind a fine plate-like, or acicular, mar-
Fig. 11. Microstructure of two zones deformed at 970 °C to the degree of 50% followed by water quenching under different loading passes: (a) Zone A, one loading pass; (b) Zone B, one loading pass; (c) Zone A, two loading passes; (d) Zone B, two loading passes.
tensitic microstructure, as illustrated in Fig. 8. However, this martensite phase is metastable and would decompose into fine and interlacing acicular a phase during annealing [3,16]. Therefore, water quenching leads to a needle-like structure, whereas air-cooling leads to a coarse lamellar colonies structure. It is evident in Table 1 that volume fraction of ap varies little with cooling mode, indicating that cooling mode has little influence on ap content. It can also be found from Table 1 that under otherwise identical conditions, the ap grain size quenched by water was smaller than that cooled in air. Such a phenomenon has also been reported in [3] qualitatively. This is because water quenching process could suppress the spheroidizing growth of primary a phase. Meanwhile water quenching can make lots of defects and distortion energy restored to room-temperature, which would increase crystallization nuclei, thus providing driving force for recrystallization of a phase in the subsequent heating process. Hence, the ap grain size quenched in water is smaller than that cooled in air. On the other hand, it is difficult to determine a simple relationship between ap aspect ratio and cooling mode from Table 1. In comparison of the microstructure parameters at different cooling modes in Table 1, it can be found that microstructure parameters of Zone A are all close to that of Zone B at different cooling modes, indicating that different cooling modes have similar influence on two zones which undergo different thermal processing paths in the conditions of present work. 3.3. Effect of loading pass Fig. 11 shows the microstructure of two zones deformed at 970 °C to the degree of 50% by different loading passes and water quenched. The corresponding quantitatively measured microstructure parameters are shown in Table 2. From Fig. 11 we can find no significant difference on the composition and morphology of microstructure between the microstructure obtained by one-pass isothermal local loading and two-pass isothermal local loading. However, it is obvious that ap grains of samples deformed by two-pass isothermal local loading are coarser than that of samples deformed by one-pass isothermal local loading, as illustrated in Fig. 11. This is also reflected by the quantitatively measured result shown in Table 2. When the samples are deformed by two-pass isothermal local loading, the samples would undergo heat forging four times making ap grains coarse slightly. Besides, the deformation degree is relative small in every loading step when the samples are deformed by two-pass isothermal local loading, which leads to less ap grains broken up, less defects within the crystal produced, and less distortion energy generated, so that the recrystallization of ap is weakened. Therefore, the ap grain size of samples deformed by two-pass isothermal local loading is larger than that of samples deformed by one-pass isothermal local loading. It can be seen from Table 2 that ap content of microstructure obtained at different loading passes are close. This reveals that loading pass have little influence on the volume fraction of ap. The same to cooling mode, it is also difficult to find a simple relationship between loading pass and ap aspect ratio from Table 2. In addition, Zone A and Zone B have the close microstructure parameters at different loading passes, as illustrated in Table 2.
Table 2 The quantitatively measured microstructure parameters corresponding to the microstructure shown in Fig. 11. Volume fraction of ap (%)
One loading pass Two loading passes
Size of ap (lm)
Aspect ratio of ap
Zone A
Zone B
Zone A
Zone B
Zone A
Zone B
21.52 20.94
19.24 20.06
8.74 9.37
8.29 9.02
2.39 2.31
2.41 2.49
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4. Conclusions
References
The microstructure of transitional region under multi-heat isothermal local loading forming of TA15 titanium alloy has been quantitatively investigated. From this work, the following conclusions can be drawn:
[1] Shen Gangshu, Furrer David. Manufacturing of aerospace forgings. J Mater Process Technol 2000;98:189–95. [2] Gorynin IV. Titanium alloys for marine application. Mater Sci Eng A 1999;263:112–6. [3] Sun Zhichao, Yang He. Microstructure and mechanical properties of TA15 titanium alloy under multi-step local loading forming. Mater Sci Eng A 2009;523:184–92. [4] Fan XG, Yang H, Sun ZC, Zhang DW. Effect of deformation inhomogeneity on the microstructure and mechanical properties of large-scale rib–web component of titanium alloy under local loading forming. Mater Sci Eng A 2010;527:5391–9. [5] Yu Weixin, Li Miaoquan, Luo Jiao. Effect of processing parameters on microstructure and mechanical properties in high temperature deformation of Ti–6Al–4V alloy. Rare Met 2009;38:19–24. [6] Lütjerin G. Influence of processing on microstructure and mechanical properties of (a + b) titanium alloys. Mater Sci Eng A 1998;243: 32–45. [7] Ding R, Guo ZX, Wilson A. Microstructural evolution of a Ti–6Al–4V alloy during thermomechanical processing. Mater Sci Eng A 2002;327: 233–45. [8] Zong YY, Shan DB, Xu M, Lv Y. Flow softening and microstructural evolution of TC11 titanium alloy during hot deformation. J Mater Process Technol 2009;209:1988–94. [9] Semiatin SL, Seetharaman V, Weiss I. Flow behavior and globularization kinetics during hot working of Ti–6Al–4V with a colony alpha microstructure. Mater Sci Eng A 1999;263:257–71. [10] Zhang XY, Li MQ, Li H, Luo J, Su SB, Wang H. Deformation behavior in isothermal compression of the TC11 titanium alloy. Mater Des 2010;31:2851–7. [11] Momeni A, Abbasi SM. Effect of hot working on flow behavior of Ti–6Al–4V alloy in single phase and two phase regions. Mater Des 2010;31:3599–604. [12] Li LX, Lou Y, Yang LB, Peng DS, Rao KP. Flow stress behavior and deformation characteristics of Ti–3Al–5V–5Mo compressed at elevated temperatures. Mater Des 2002;23:451–7. [13] Fan XG, Yang H, Sun ZC, Tang Z. Modeling for microstructure evolution in titanium alloy large-scale complex component isothermal forming under local loading. In: Proceedings of the 9th international conference on technology of plasticity, Gyeongju; September 2008. p. 332–3. [14] Seshacharyulu T, Dutta B. Influence of prior deformation rate on the mechanism of a ? a + b transformation in Ti–6Al–4V. Scripta Mater 2002;46:673–8. [15] Wanjara P, Jahazi M, Monajati H, Yue S. Influence of thermomechanical processing on microstructural evolution in near-a alloy IMI834. Mater Sci Eng A 2006;416:300–11. [16] Li Shikai, Xiong Baiqing, Hui Songxiao. Effects of cooling rate on the fracture properties of TA15 ELI alloy plates. Rare Met 2007;26:33–8. [17] Behrang Poorganji, Makoto Yamaguchi, Yoshio Itsumi, Behrang Poorganji, Makoto Yamaguchi, Yoshio Itsumi, et al. Microstructure evolution during deformation of a near-a titanium alloy with different initial structures in the two-phase region. Scripta Mater 2009;61:419–22. [18] Sun ZC, Yang H, Han GJ, Fan XG. A numerical model based on internalstate-variable method for the microstructure evolution during hot-working process of TA15 titanium alloy. Mater Sci Eng A 2010;527: 3464–71. [19] Liu Yong, Zhu Jingchuan, Wang Yang, Zhan Jiajun. Hot compressive deformation behaviors and micro-mechanisms of TA15 alloy. Rare Met 2007;26:162–7. [20] Xu WC, Shan DB, Yang GP, Lu Y. Flow behavior and microstructure evolution during hot compression of TA15 titanium alloy. Trans Nonferrous Met Soc China 2006;16:2066–71. [21] Zhou YG, Zeng WD, Yu HQ. An investigation of a new near-beta forging process for titanium alloys and its application in aviation components. Mater Sci Eng A 2005;393:204–12. [22] Semiatin SL, Montheillet F, Shen G, Jonas JJ. Self-consistent modeling of the flow behavior of wrought alpha/beta titanium alloys under isothermal and nonisothermal hot-working conditions. Metall Mater Trans A 2002;33:2719–27.
(1) For most processing conditions, the transitional region has uniform microstructure morphology and close microstructural features unaffected by the variation of local deformation path. However, when the samples are deformed at high temperature (970 °C) or large deformation degree (70%), primary a grain size and aspect ratio of local zone which undergoes larger deformation in first-loading step are both smaller than that of local zone which undergoes larger deformation in second-loading step slightly. (2) Deformation temperature and deformation degree also have interaction on the microstructure evolution of transitional region under isothermal local loading forming. At the cooling mode of air-cooling, deformation temperature influences the morphology of transformed structure in b matrix greatly but deformation degree poses minor influence on it; volume fraction of primary a phase is determined by deformation temperature only; at a certain deformation degree, primary a grain size increases first and then decreases with increasing temperature. At higher temperature, primary a grain size varies little with deformation degree, but primary a grain size increases firstly and then decreases with deformation degree at lower temperature; Primary a aspect ratio increases with deformation degree at lower temperature, but varies little at higher temperature. (3) Cooling mode has significant effect on the morphology of transformed structure in b matrix. The precipitated secondary a phase in b matrix is in lamellar form and arranges in colonies when the samples are cooled in air, but that is in thinner acicular form and distributes disorderly when the samples are quenched in water. In addition, primary a grain size quenched in water is smaller than that cooled in air. However, cooling mode has little effect on primary a content and primary a aspect ratio. (4) Whether the samples are deformed by one-pass or two-pass isothermal local loading, the close morphology of microstructure are obtained. Primary a grain size of samples deformed by two-pass isothermal local loading are grater than that of samples deformed by one-pass isothermal local loading.
Acknowledgements The authors would like to gratefully acknowledge the support of Natural Science Foundation for Key Program of China (No. 50935007), National Basic Research Program of China (No. 2010CB731701), the 111 Project (B08040) and Fund of the state key Laboratory of Solidification Processing of NWPU (KB201027).
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Quantitative analysis of the microstructure of transitional region under multi-heat isothermal local loading forming of TA15 titanium alloy P.F. Gao, H. Yang ⇑, X.G. Fan State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, P.O. Box 542, Northwestern Polytechnical University, Xi’an 710072, PR China
a r t i c l e
i n f o
Article history: Received 3 November 2010 Accepted 25 November 2010 Available online 1 December 2010 Keywords: A. Non-ferrous metals and alloys C. Forging F. Microstructure
a b s t r a c t In this paper, an analogue experiment was carried out to study the effect of processing parameters including deformation temperature, deformation degree, cooling mode and loading pass on the microstructure of transitional region under isothermal local loading forming of TA15 titanium alloy. The volume fraction, grain size and aspect ratio of primary a phase of transitional region were quantitatively characterized. It is found that deformation temperature and deformation degree also have interaction on the microstructure evolution of transitional region under isothermal local loading forming. At a certain deformation degree, primary a grain size increases first and then decreases with increasing temperature. However, primary a grain size varies little with deformation degree at higher temperature (in upper two phase region) but increases firstly and then decreases with deformation degree at lower temperature (in lower two phase region). Primary a aspect ratio increases with deformation degree at lower temperature but varies little at higher temperature. The morphology of transformed structure in b matrix is greatly influenced by deformation temperature and less influenced by deformation degree under air-cooling. The precipitated Widmanstatten a phase in b matrix is in lamellar form and arranges in colonies under air-cooling, but it is in thinner acicular form and distributes disorderly under water quenching. Loading pass has little influence on the morphology of microstructure. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Large-scale integral component of titanium alloy with complex shape (such as bulkhead) has increasingly extensive application in aerospace field since whose material and structural characteristics make aircraft light weight and reliability. This kind of components often serves as key load-bearing structure in severe conditions, so not only the quality of macroscopical forming but also the fine microstructure is needed [1–4]. However, forming of these components is difficult due to the complexity of their shape, hard-to-deform properties of titanium alloy and high requirement for forming quality. The isothermal local loading forming technology, which can control the flow of material, reduce the forming load, enhance plasticity, and enlarge the size of component to be formed, provides a feasible method to form these components [3,4]. However, the isothermal local loading forming is a complicated multi-step hot working process with coupling effects of multifields and multi-factors. During isothermal local loading forming, load is applied to part of the billet and the component is formed by changing loading region. In each forming stage, there exist loading region, unloading region, transitional region, and the alternation among them, as illustrated in Fig. 1. The material in ⇑ Corresponding author. Tel./fax: +86 029 8849 5632. E-mail address: [email protected] (H. Yang). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.11.058
different loading regions undergoes different thermal processing paths, which will influence the microstructure as well as the mechanical properties of final product [4]. Transitional region, connecting the loading region with unloading region, undergoes more complex uneven plastic deformation and concomitantly sophisticated microstructure evolution, which makes the transitional region a key factor for the component. On the other hand, microstructure of titanium alloys is very sensitive to processing parameters, and the desired microstructure can be obtained at proper processing parameters to obtain good mechanical properties such as strength, ductility, toughness and fatigue resistance [5]. Therefore, to study and reveal the effect of processing parameters on microstructure of transitional region under isothermal local loading forming of titanium alloy is a key problem urgent to be solved. By now, most work has been done on the deformation mechanism and microstructure evolution of titanium alloys during thermomechanical processing [5–12]. Primary study on the deformation mechanism and microstructure evolution of titanium alloy under local loading forming has also been conducted. Sun and Yang designed an analogue experiment to study the effects of local loading conditions on microstructure of TA15 titanium alloy forgings qualitatively [3]. However, the influence of processing parameters on the microstructure of titanium alloy under isothermal local loading forming needs further quantitative study. Fan et al. [13]
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Fig. 1. Illustration of isothermal local loading forming [3].
developed a set of models describing the microstructure evolution of TA15 alloy using internal state variable method and embedded the models in FE software to investigate the grain size and distribution of primary a grains during local loading forming. Fan et al. [4] carried out an experiment of local loading forming of large-scale rib–web component of TA15 titanium alloy to study the effect of deformation inhomogeneity on microstructure and mechanical properties of large-scale rib–web component under local loading forming. However, investigations are furthermore needed so as to get a general and detailed influence law of processing parameters on the microstructure of transitional region, a key factor for the component, under isothermal local loading forming of titanium alloy. In the present paper, an analogue experiment was conducted and different process schedules were proposed to study the effect of processing parameters (deformation temperature, deformation degree, cooling mode and loading pass) on the microstructure of transitional region under isothermal local loading forming of TA15 titanium alloy. It will provide technological basis for optimizing processing parameters for isothermal local loading forming of large-scale integral component of TA15 titanium alloy.
2. Experimental procedure The experimental material is TA15 titanium alloy in hot rolled plate form with specification 1200 170 80 mm and the b-transus temperature 990 °C. The chemical compositions of this alloy
Fig. 2. Original microstructure of the billet.
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are as follows (wt.%), Al: 6.8; Mo: 1.7; V: 2.2; Zr: 2.0; Fe: 0.3 and Ti balance. The microstructure of as-received material consists of about 60% primary equiaxed a phase (with a standard deviation of 6%) and transformed b matrix, as shown in Fig. 2. The specimens used in test were rectangular solid of 20 16 12 mm. The isothermal local loading forming was implemented by one or two loading passes, two loading steps included in one loading pass, as illustrated in Fig. 3. When the isothermal local loading forming was implemented by two loading passes, the amount of compression in every loading pass was half of the total. In every loading step, the sample was heated to deformation temperature at 12 °C/min, held for 15 min, and pressed under corresponding conditions (Fig. 3a and b) at a constant nominal strain rate and then air-cooled or water quenched. The heat treatment route used after isothermal local loading was as follows: (1) heating to 810 °C and holding for 1 h and (2) air-cooling to room-temperature. The test parameters consists of deformation temperatures of 910, 930, 950 and 970 °C, deformation degrees of 30%, 50% and 70%, loading pass of one and two, cooling modes of air-cooling and water quenching and strain rate of 0.01 s 1. To acquire the deformation behavior of sample during isothermal local loading forming and determine the microstructure image locations of transitional region, corresponding numerical simulations of isothermal local loading forming was carried out through DEFORM-3D software. Fig. 4 shows the sample deformed at temperature of 950 °C to the degree of 50% by one-pass isothermal local loading. It can be seen that the simulated result closely matches the experimental result. Simulation results show that the strain distribution of sample during isothermal local loading forming is mainly determined by deformation degree. Fig. 5 shows the effective strain distribution on cross-section of samples deformed by one-pass isothermal local loading to different deformation degrees. The material in transitional region undergoes complex uneven plastic deformation and different local regions may undergo different thermal processing paths during practical isothermal local loading forming. Choosing a series of points and tracing their strain paths to determine the representative points of transitional region, i.e. the microstructure image locations of transitional region, we found that Zone A and
Fig. 4. Sample formed by isothermal local loading process: (a) experimental result; (b) simulated result.
Fig. 3. Schematic diagram of the two loading steps in one loading pass: (a) the first-loading step; (b) the second-loading step.
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Fig. 5. Effective strain distribution on the cross-section corresponding to that of Fig. 3: (a) 30%, 950 °C; (b) 50%, 950 °C; (c) 70%, 950 °C.
Zone B (illustrated in Fig. 5) have close accumulated strain after processing but different strain paths. After first-loading step, the effective strain at Zone A and Zone B are about 90% and 10% of the total respectively. After second-loading step, two zones have close accumulated strain. Therefore, Zone A and Zone B were selected as the representative zones of transitional region, i.e. the microstructure image locations. The microstructure was observed and analyzed using Olympus optical microscope. The corrosive solution was 13%HNO3 + 7%HF + 80%H2O. The size and morphology of primary a phase have great influence on the performance of TA15 titanium alloy. Therefore, the volume fraction, grain size and aspect ratio of primary a phase were quantitatively measured.
3. Results and discussion 3.1. Effect of deformation temperature and deformation degree It is well known that deformation temperature and deformation degree have considerable influence on the microstructure of titanium alloy respectively. However, hot deformation of titanium alloys is a complicated mutual coupling process of deformation behavior, heat transfer and microstructure evolution. Moreover, the interaction between deformation temperature and deformation degree was studied in the hot compression of titanium alloy [5]. Hence, the interaction between deformation temperature and deformation degree on the microstructure evolution of transitional region under isothermal local loading forming should also be studied. Figs. 6 and 7 show the microstructure of Zone A and Zone B deformed at different temperatures and deformation degrees by one-pass isothermal local loading and air-cooled respectively. As illustrated in Figs. 6 and 7, the microstructure mainly consists
of primary a phase (ap) and transformed b matrix composed of residual b phase and secondary a phase (as) throughout specimens, but little prior lamellar a phase could be found in most deformation conditions, which is different from the result reported in [4]. This is related to the a ? b phase transformation during thermomechanical processing. Ding et al. [7] studied the a ? b phase transformation during thermomechanical processing, and pointed out that most of the prior a lamellar phase inside b grains would transform to b phase firstly at high temperature, but the prior a phase near grain boundary was difficult to transform to b phase because of the relatively high concentration of Al at grain boundary. They also distinguished the phase transformation into three stages: non-transformed, partially transformed and completely transformed. In the work of Fan et al. [4], an amount of as, which is precipitated from b phase during the cooling down after first-loading step, retained and grew thicker in the following second-loading step, there the lamellar a phase were produced. In this work, the as in b matrix were transformed into high-temperature b phase before hot working and then all the high-temperature b phase would transform into transformed b matrix having a lamellar as structure upon post-deformation cooling. Therefore, there was little prior lamellar a phase in microstructure under most deformation conditions, suggesting that the initial microstructure for two loading step were both ap and pure high-temperature b phase in most deformation conditions. An example of microstructure just after the second-loading step deformation obtained at the deformation temperature of 950 °C and deformation degree of 50% is presented in Fig. 8. No lamellar a phase could be found in the microstructure just after deformation, which confirms the conclusion that initial microstructure was ap and pure high-temperature b phase in most deformation conditions. It can be seen from Figs. 6 and 7 that Widmanstatten a phase precipitated in transformed b matrix have different morphology
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Fig. 6. Microstructure of Zone A deformed at different temperatures and deformation degrees by one-pass isothermal local loading and air-cooled: (a) 910 °C, 30%; (b) 930 °C, 30%; (c) 950 °C, 30%; (d) 970 °C, 30%; (e) 910 °C, 50%; (f) 930 °C, 50%; (g) 950 °C, 50%; (h) 970 °C, 50%; (i) 910 °C, 70%; (j) 930 °C, 70%; (k) 950 °C, 70%; (l) 970 °C, 70%.
Fig. 7. Microstructure of Zone B deformed at different temperatures and deformation degrees by one-pass isothermal local loading and air-cooled: (a) 910 °C, 30%; (b) 930 °C, 30%; (c) 950 °C, 30%; (d) 970 °C, 30%; (e) 910 °C, 50%; (f) 930 °C, 50%; (g) 950 °C, 50%; (h) 970 °C, 50%; (i) 910 °C, 70%; (j) 930 °C, 70%; (k) 950 °C, 70%; (l) 970 °C, 70%.
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Fig. 8. Microstructure of two zones deformed to 50% at temperature of 950 °C by one loading pass and water quenched but without annealing: (a) Zone A; (b) Zone B.
under different deformation conditions. Seshacharyulu and Dutta [14] studied the mechanism of b ? a transformation and found that more dislocation density and distortion energy in deformed b phase led to an acceleration in kinetics of b ? a transformation through increasing nucleation locations of as and enhancing the precipitating and coarsening of as by pipe-diffusion through dislocation. Wanjara et al. [15] pointed out that the cooling rate after deformation affected both thickness and orientation of precipitated alpha plates in transformed b matrix. Therefore, morphology of the transformed microstructure in b matrix is considerably influenced by the dislocation density in deformed high-temperature b phase and the cooling rate after deformation. According to the forming process of isothermal local loading, morphology of transformed microstructure in b matrix is determined by the condition of second-loading step in this work. As samples in Figs. 6 and 7 were all cooled in air having close cooling rate, the different morphology of Widmanstatten a phase precipitated in transformed b matrix was mainly caused by the dislocation density and distortion energy in deformed b phase. When the deformation temperature is in upper two phase region, the high-temperature b phase has high volume fraction bearing relatively small deformation and b phase is prone to dynamic recovery, resulting in low dislocation density and distortion energy. The small driving force for b ? a transformation, at higher deformation temperature, achieves selective growth of nucleated as plates. The as first nucleates preferentially at b grain boundaries, leading to more or less discontinuous a layer along b grain boundaries (Fig. 6e, h and l), during continued cooling the a plates nucleate at b grain boundary itself and grow into b grain as parallel plates belonging to the same variant of Burgers relationship (socalled a colony) until they meet other a colonies [16]. Therefore, the precipitated as is in thick lamellar form and straight to bunchiness (Fig. 6e, h and l) at higher temperature. On the contrary, when deformation temperature is in lower two phase region, dislocation density and distortion energy in deformed b phase is higher, resulting in more nucleation location for as and a larger driving force for b ? a transformation. In addition, the growth of as is restrained at lower temperature. As a result, the precipitated as is in fine and acicular form at lower temperature (Figs. 6 and 7). It has been well known that an increase in deformation degree would increase the dislocation density and distortion energy in deformed b phase. According to the analysis above, large deformation would get shorter and disorder lamellar as in transformed b matrix. On the other hand, the produce of as is a heating activation so deformation degree has strong influence on the morphology of precipitated as in transformed b matrix at higher temperature. Therefore, when the samples are deformed at higher temperature and to larger deformation degree (50% and 70%), shorter and disorder lamellar as in transformed b matrix could be found (Figs. 6k, l, 7k and l). For the same reason, by comparing Figs. 6 and 7, it can be found that at the temperature of 970 °C, the lamellar as in transformed b matrix of Zone B is shorter and disorder than that of Zone
A. This is because the deformation degree of Zone A is much less than that of Zone B in second-loading step. Altogether, the dislocation density and distortion energy in deformed b phase is influenced by deformation temperature and deformation degree, and then which would affect the morphology of transformed microstructure within prior-beta grains. It is noted that some fine strip and equiaxed a phase are found, when the samples are deformed at 910 °C to the degree of 70% (Figs. 6i and 7i). In work of Fan et al. [4], analogous microstructure was also been found, which was caused by the break and spheroidization of lamellar a phase in initial microstructure for secondloading step during deformation. However, in this work, one possible explanation for this phenomenon is that the ap is elongated to strip then broken up and spheroidized under large deformation and low deformation temperature. This is consistent with the result that break up of the strip a phase occur to an increasing extent as the strain increases and the processing temperature decreases, which has been reported in Ref. [15]. Besides, the recrystallization of a phase during thermomechanical processing of titanium alloys in two phase region has been reported in previous works [8,17– 20]. Therefore, the recrystallization of a phase may also make a contribution to the produce of fine equiaxed a phase. It can be concluded that the composition and morphology of microstructure are mainly influenced by deformation temperature and deformation degree, and which has little difference in different zones of transitional region. The quantitatively measured microstructure parameters of Zone A and Zone B deformed at different conditions corresponding to that of Figs. 6 and 7 are shown in Fig. 9. It can be seen from Fig. 9a and b that ap content of two zones both decreases with temperature at a certain deformation degree, due to the transformation of a ? b phase. And the ap content decreases slowly with increasing temperature in lower two phase region (<950 °C), but decreases rapidly with temperature in upper two phase region (>950 °C). This agrees with the correlation between volume fraction of ap and heating temperature of TC11 titanium alloy reported in [21]. At a certain temperature, the volume fraction of ap varies little with deformation degree as illustrated in Fig. 9a and b. Fan et al. [4] studied the effect of deformation inhomogeneity on microstructure of TA15 titanium and pointed out that ap content grew sharply with strain. The reason is that more lamellar a phase would be globularized and then merged into primary a at large deformation, resulting in the increase of primary a. However, in this work, the globularization of lamellar a phase would not happen, causing that the volume fraction of ap varies little with deformation degree. By comparing Fig. 9a and b, it can be found that at certain deformation temperature and deformation degree, two representative zones undergo different thermal processing paths but have close ap content, suggesting that thermal processing path has little influence on ap volume fraction under the condition of this work. The grain size of ap under different conditions are presented in Fig. 9c and d. Here it is observed that ap grain size of two zones deformed at different conditions all increase after multi-heat forging, compared to that of initial microstructure (7.56 lm). For all deformation degrees, ap grain size increases first and then decreases with increasing temperature, having the peak value at 930 °C and the least value at 970 °C, as illustrated in Fig. 9c and d. There are two mechanisms responsible for this phenomenon: transformation of a ? b phase and the break up of ap grains. When deformation temperature is in lower two phase region, the transformation extent of a ? b phase is small, which has little influence on the grain size of ap, so that ap grain size is determined mainly by the break up of ap grains. With increasing temperature in lower two phase region, ap content decreases slightly but undergoes less deformation and break up resulting in the increase of ap grain size. This
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Fig. 9. Microstructural features of two zones deformed at different temperatures and deformation degrees by one-pass isothermal local loading and air-cooled: (a) ap content of Zone A; (b) ap content of Zone B; (c) ap grain size of Zone A; (d) ap grain size of Zone B; (e) ap aspect ratio of Zone A; (f) ap aspect ratio of Zone B.
is because b phase is easier to deform than ap during the thermomechanical processing of titanium alloys in two phase region [22]. On the other hand, volume fraction of ap decreases rapidly with temperature in upper two phase region, and decreases to only about 20% at 970 °C. This would minish the grain size of ap markedly so that ap grain size decreases with temperature in upper two phase region and reaches the least value at 970 °C. The effect of deformation temperature on ap grain size is caused by the characteristics of a ? b phase transformation in hot work of titanium alloy. However, deformation degree has little effect on the transformation of a ? b phase in this work. As a result, the deformation temperature has close influence on ap grain size at different deformation degrees. From Fig. 9c and d, we can also find that ap grain size increases firstly and then decreases sharply with increasing deformation degree at a certain deformation temperature except for 970 °C. The influence of deformation degree on ap grain size should be explained from three different changes of ap grains during deforma-
tion as follows: the break up of ap grains, the dynamic coarsening of ap grains, and the recrystallization of a phase. However, deformation temperature not only has influence on the deformation of a phase as explained above, but also has influence on the recrystallization of a phase. Previous investigations [19,20] pointed out that when deformation temperature is in upper two phase region, dynamic recrystallization of a phase is suppressed in TA15 alloy. Therefore, the influence of deformation degree on ap grain size varies with deformation temperature. At lower temperature, the three mechanisms mentioned above are all not obvious at the deformation degree of 30%. With increasing deformation degree to 50%, larger deformation could promote the mergence of two ap grains with higher and lower distortion energy, which is driven primarily by the difference of their distortion energy and partly by the decreasing of total interfacial energy [5]. The dynamic coarsening of ap grains plays a leading role, so the ap grain size increases. However, when the deformation degree increases to 70%, the break up of ap grains and the recrystallization of a phase are both enhanced
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markedly, so the ap grain size decreases again. On the other hand, when the samples are deformed at high temperature (970 °C), the deformation extend of a phase is small and dynamic recrystallization of a phase is restrained but the dynamic coarsening of ap grains is remarkable at high temperature so that the grain size of ap varies little with deformation degree at high temperature, as illustrated in Fig. 9c and d. By comparing Fig. 9c and d, it can be found that ap grain size of Zone A is smaller than that of Zone B at the deformation degree of 70% for all temperatures. This is because Zone A and Zone B undergo different thermal processing paths. As deformation is very small in second-loading step, the second-loading step can be taken as a heat treatment for Zone A. This extra heat treatment at deformation temperature would promote the recrystallization of a phase and decrease ap grain size. Nevertheless, recrystallization of a phase in the extra heat treatment demands lots of distortion energy which can be obtained at larger deformation. Therefore, ap grain size of Zone A is slightly smaller than that of Zone B at the deformation degree of 70%. The aspect ratio of ap under different conditions are presented in Fig. 9e and f. The compression, break up and recrystallization of a phase all affect the aspect ratio of ap. At a certain deformation degree, ap aspect ratio is determined by compression and recrystallization of a phase at lower temperature. While, the compression and recrystallization of a phase have counter effect on the aspect ratio of ap. With increasing temperature, compression and recrystallization of a phase both are weakened at higher temperature. As a result, the ap aspect ratio varies little with temperature for all deformation degrees, as illustrated in Fig. 9e and f. Similar to the influence of deformation degree on ap grain size, the effect of deformation degree on ap aspect ratio varies with
deformation temperature, too. As mentioned above, ap undergoes large deformation and is elongated greatly at lower temperature. The compression of a phase plays a leading role on the morphology of ap grains so that ap aspect ratio increases with deformation degree at lower temperature, as illustrated in Fig. 9e and f. However, it is noticeable that at temperature of 910 °C, the aspect ratio get the least value at deformation degree of 70% (Fig. 9e and f), which does not correspond to the regularity above. This is because a lot of fine ap is produced, when the samples are deformed to 70% at temperature of 910 °C, as shown in Figs. 6i and 7i. In contrast, deformation degree has little influence on ap aspect ratio at higher temperature due to the compression and recrystallization of a phase are both weakened. In comparison of Fig. 9e and f, it can be found that ap aspect ratio of Zone A is smaller than that of Zone B at the deformation degree of 70% for all temperatures. This is also for the reason that Zone A undergoes an extra heat treatment at deformation temperature which would decrease the aspect ratio of a phase. From above analysis, it can be found that the deformation degree has different influence on the morphology of transformed structure in b matrix, ap grain size and ap aspect ratio in different temperature regions, revealing that deformation temperature and deformation degree also have interaction on the microstructure evolution of transitional region in this work. This is because the morphology and parameters of microstructure, which are determined by deformation temperature and deformation degree together, varies through a ? b phase transformation, break up of ap under large deformation, recrystallization of a phase, dynamic coarsening of ap and so on. Moreover, these processes present different characteristics in different temperature regions: a ? b phase transformation and dynamic coarsening of ap are both
Fig. 10. Microstructure of two zones deformed at different deformation temperatures to the degree of 50% by one-pass isothermal local loading and different cooling modes: (a) Zone A, 970 °C, AC; (b) Zone B, 970 °C, AC; (c) Zone A, 950 °C, AC; (d) Zone B, 950 °C, AC; (e) Zone A, 970 °C, WQ; (f) Zone B, 970 °C, WQ; (g) Zone A, 950 °C, WQ; (h) Zone B, 950 °C, WQ.
Table 1 The quantitatively measured microstructure parameters corresponding to the microstructure shown in Fig. 10. Volume fraction of ap (%)
950 °C 950 °C 970 °C 970 °C
AC WQ AC WQ
Size of ap (lm)
Aspect ratio of ap
Zone A
Zone B
Zone A
Zone B
Zone A
Zone B
40.96 40.65 21.38 21.52
39.54 39.49 17.72 19.24
12.07 11.00 9.29 8.74
13.37 10.58 7.85 8.29
2.64 2.44 2.31 2.39
2.88 2.17 2.30 2.41
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thermally activated process, but break up of ap under large deformation and recrystallization of a phase are both suppressed at higher temperature. Therefore, deformation temperature and deformation degree also present interaction on the microstructure evolution of transitional region in this work. 3.2. Effect of cooling mode Fig. 10 shows the microstructure of two zones deformed at different deformation temperatures to the degree of 50% by one-pass isothermal local loading and different cooling modes. The corresponding quantitatively measured microstructure parameters are shown in Table 1. As illustrated in Fig. 10, the microstructure all consists of ap and transformed b matrix at different cooling modes. Considering the morphology of microstructure, it can be found that the most distinction between different cooling modes is that the transformed structure in b matrix has different morphology: the precipitated as in transformed b matrix is in lamellar form and arranges in colonies when the samples are cooled in air, but that is in thinner acicular form and distributes disorderly when the samples are quenched in water. It has been mentioned that the cooling condition affect the morphology, thickness and orientation of precipitated a plates in transformed b matrix markedly. The precipitating process and morphology of precipitated as in transformed b matrix at the cooling mode of air-cooled has been discussed in Section 3.1. When the samples are quenched in water, the cooling rate is so high that there is not enough time for b phase to form equilibrious a phase through regular diffusion of a-stable elements, for static recrystallization and for lamellar a phase precipitating from b phase after deformation. The high-temperature b phase transformed completely into a phase by a collective migration in short range of atoms, leaving behind a fine plate-like, or acicular, mar-
Fig. 11. Microstructure of two zones deformed at 970 °C to the degree of 50% followed by water quenching under different loading passes: (a) Zone A, one loading pass; (b) Zone B, one loading pass; (c) Zone A, two loading passes; (d) Zone B, two loading passes.
tensitic microstructure, as illustrated in Fig. 8. However, this martensite phase is metastable and would decompose into fine and interlacing acicular a phase during annealing [3,16]. Therefore, water quenching leads to a needle-like structure, whereas air-cooling leads to a coarse lamellar colonies structure. It is evident in Table 1 that volume fraction of ap varies little with cooling mode, indicating that cooling mode has little influence on ap content. It can also be found from Table 1 that under otherwise identical conditions, the ap grain size quenched by water was smaller than that cooled in air. Such a phenomenon has also been reported in [3] qualitatively. This is because water quenching process could suppress the spheroidizing growth of primary a phase. Meanwhile water quenching can make lots of defects and distortion energy restored to room-temperature, which would increase crystallization nuclei, thus providing driving force for recrystallization of a phase in the subsequent heating process. Hence, the ap grain size quenched in water is smaller than that cooled in air. On the other hand, it is difficult to determine a simple relationship between ap aspect ratio and cooling mode from Table 1. In comparison of the microstructure parameters at different cooling modes in Table 1, it can be found that microstructure parameters of Zone A are all close to that of Zone B at different cooling modes, indicating that different cooling modes have similar influence on two zones which undergo different thermal processing paths in the conditions of present work. 3.3. Effect of loading pass Fig. 11 shows the microstructure of two zones deformed at 970 °C to the degree of 50% by different loading passes and water quenched. The corresponding quantitatively measured microstructure parameters are shown in Table 2. From Fig. 11 we can find no significant difference on the composition and morphology of microstructure between the microstructure obtained by one-pass isothermal local loading and two-pass isothermal local loading. However, it is obvious that ap grains of samples deformed by two-pass isothermal local loading are coarser than that of samples deformed by one-pass isothermal local loading, as illustrated in Fig. 11. This is also reflected by the quantitatively measured result shown in Table 2. When the samples are deformed by two-pass isothermal local loading, the samples would undergo heat forging four times making ap grains coarse slightly. Besides, the deformation degree is relative small in every loading step when the samples are deformed by two-pass isothermal local loading, which leads to less ap grains broken up, less defects within the crystal produced, and less distortion energy generated, so that the recrystallization of ap is weakened. Therefore, the ap grain size of samples deformed by two-pass isothermal local loading is larger than that of samples deformed by one-pass isothermal local loading. It can be seen from Table 2 that ap content of microstructure obtained at different loading passes are close. This reveals that loading pass have little influence on the volume fraction of ap. The same to cooling mode, it is also difficult to find a simple relationship between loading pass and ap aspect ratio from Table 2. In addition, Zone A and Zone B have the close microstructure parameters at different loading passes, as illustrated in Table 2.
Table 2 The quantitatively measured microstructure parameters corresponding to the microstructure shown in Fig. 11. Volume fraction of ap (%)
One loading pass Two loading passes
Size of ap (lm)
Aspect ratio of ap
Zone A
Zone B
Zone A
Zone B
Zone A
Zone B
21.52 20.94
19.24 20.06
8.74 9.37
8.29 9.02
2.39 2.31
2.41 2.49
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4. Conclusions
References
The microstructure of transitional region under multi-heat isothermal local loading forming of TA15 titanium alloy has been quantitatively investigated. From this work, the following conclusions can be drawn:
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(1) For most processing conditions, the transitional region has uniform microstructure morphology and close microstructural features unaffected by the variation of local deformation path. However, when the samples are deformed at high temperature (970 °C) or large deformation degree (70%), primary a grain size and aspect ratio of local zone which undergoes larger deformation in first-loading step are both smaller than that of local zone which undergoes larger deformation in second-loading step slightly. (2) Deformation temperature and deformation degree also have interaction on the microstructure evolution of transitional region under isothermal local loading forming. At the cooling mode of air-cooling, deformation temperature influences the morphology of transformed structure in b matrix greatly but deformation degree poses minor influence on it; volume fraction of primary a phase is determined by deformation temperature only; at a certain deformation degree, primary a grain size increases first and then decreases with increasing temperature. At higher temperature, primary a grain size varies little with deformation degree, but primary a grain size increases firstly and then decreases with deformation degree at lower temperature; Primary a aspect ratio increases with deformation degree at lower temperature, but varies little at higher temperature. (3) Cooling mode has significant effect on the morphology of transformed structure in b matrix. The precipitated secondary a phase in b matrix is in lamellar form and arranges in colonies when the samples are cooled in air, but that is in thinner acicular form and distributes disorderly when the samples are quenched in water. In addition, primary a grain size quenched in water is smaller than that cooled in air. However, cooling mode has little effect on primary a content and primary a aspect ratio. (4) Whether the samples are deformed by one-pass or two-pass isothermal local loading, the close morphology of microstructure are obtained. Primary a grain size of samples deformed by two-pass isothermal local loading are grater than that of samples deformed by one-pass isothermal local loading.
Acknowledgements The authors would like to gratefully acknowledge the support of Natural Science Foundation for Key Program of China (No. 50935007), National Basic Research Program of China (No. 2010CB731701), the 111 Project (B08040) and Fund of the state key Laboratory of Solidification Processing of NWPU (KB201027).