Semi-solid forming combines the advantages of casting to produce complex part geometries with improved mechanical properties. Lower process temperature and laminar flow of the melt lead to less shrinkage porosity and entrapped pores in the finished parts [1–3].
The most promising part quality is expected by utilizing “semi-solid forging” (SSF), one variant of the semi-solid process routes. In this process, a semi-solid billet is forged to near net shape by applying a high forming pressure on the complete surface of the workpiece which is maintained until complete solidification. The shear rate–dependent viscosity of semi-solid metals allows producing geometrically complex parts in one single forming operation using significantly lower forces than in conventional hot forging [4, 5]. As a result of these technological and economical potentials, semi-solid forming of aluminum alloys [6, 7], magnesium alloys [8, 9], zincaluminite alloys  and steels [11, 12] have been carried out and part of them are introduced to industrial production.
However, the semi-solid processing behavior of titanium alloys which are widely used in many fields has not been investigated. Most recently, some of present authors [13–15] reported that the semi-solid microstructure and mechanical behavior of titanium alloy were quite different from the conventional one. The objectives of this work were to comprehensively study and explain microstructure evolution and mechanical behavior of Ti-7Cu alloy (Ti-Al-7Cu-Si) after semi-solid forging at different temperatures. Through a series of studies, the influence of processing temperature and deformation ratio on microstructure of Ti-7Cu alloy in a semi-solid state were studied by forging tests, and the mechanical properties of Ti-7Cu alloy after semi-solid forging were performed by tensile and hardness tests, which could reinforce the primary processing theory.
Materials and processing
Ti-7Cu alloy is a typical eutectoid α + Ti2Cu-type burn-resistant Ti alloy [9, 10]. The melting point of Ti2Cu is 990°C. If the deformation temperature rises over 990°C, Ti-7Cu alloy will transform to the semi-solid state. The alloy used in this paper is vacuum melted ingot and the microstructure is shown in Figure 1. After conventional break-up, the ingot is forged to bars with a diameter of 40 mm, and then reforged to 25 mm at 1,000, 1,050 and 1,100°C, respectively.
Macrostructure and mechanical testing
Microstructures of Ti-7Cu alloy after forging were analyzed by optical microscopy (OM, OLYMPUS GX71). Phase Analysis was determined by X-ray diffraction technique (XRD, XRD-1700).
The mechanical properties of the Ti-7Cu alloys after semi-solid forging were evaluated by room temperature tensile tests. Samples for tensile testing were machined from the center of forged bar, and the sizes are shown in Figure 2. The machined specimens were wet polished using waterproof emery paper up to 1500 #. Tensile tests were performed at a nominal strain rate of 4.2 × 10−3 s−1 using an Instron System at room temperatures. After tensile testing, the fracture surface features were examined using scanning electron microscopy (SEM, JSM-6700). Vickers hardness at 10 kg load was measured by MH-5 Digimatic Vickers hardness tester and the dwell time is 30 s. A total of five indents were taken for each sample and average value of hardness was reported here.
Results and discussion
Microstructure of Ti-7Cu alloy after forging at different temperatures
The results of XRD test are shown in Figure 3. Only diffraction peaks of α-Ti matrix and Ti2Cu were found after SSF, which reveals that no new phase is formed and the only precipitate phase is Ti2Cu. Figure 4 shows the optical micrograph (OM) images of Ti-7Cu alloy forged at different semi-solid temperatures. It can be seen that α-Ti grains grew obviously with the increase of semi-solid forging temperature. The grain sizes of the forged specimens are listed in Table 1. All semi-solid forged specimens have smaller grain sizes than the specimens prior to forging (500 μm) (as shown in Figure 1) and the grain sizes tend to increase with the increase of forging temperature, which suggests that dynamic recrystallization occurs during forging, and grain refinement is obtained. The researches of semi-solid deformation behavior of Al alloys  and Mg alloys  indicate that the semi-solid deformation is also controlled by the plastic deformation of solid particles (PDS) when the deformation performs in semi-solid state near the solidus. In this paper, the same result is obtained that plastic deformation is also the main deformation mechanism in test temperature which leads to recrystallization and results in grain refinement.
It also can been seen from Figure 4 that precipitation of Ti2Cu occurred both within grains and on grain boundaries in samples forged at 1,000°C (Figure 4(a)). Furthermore, more Ti2Cu tended to precipitate on grain boundaries in samples forged at higher temperatures, which can be clearly seen in Figure 5. As a result, more acicular Ti2Cu precipitate on grain boundaries and form precipitation zones adjacent to grain boundaries after forged at 1,100°C (Figures 4(c) and 5(c)). The microstructure difference suggested the distribution of Ti2Cu precipitation was significantly affected by temperature and more Ti2Cu precipitations occurred on grain boundaries at higher semi-solid forging temperature.
Microstructure of Ti-7Cu alloy after forging at different temperatures
Figure 6 shows the microstructure of specimens forged at 1,050°C with different forging ratios. It is obvious that the grain size is refined with the increase of forging ratio. The measured grain size of specimens and the percentage reduction of grain size are listed in Table 1. For forging at 1,050°C, it can be seen that the grain size decreases with the increase of forging ratio and grain size reduced about 49% at forging ratio of 75% compared with that of as-received sample, which suggests that higher forging ratio promotes dynamic recrystallization and results in significant grain refinement. The grains of Ti-7Cu alloy change rounder and finer gradually with the development of recrystallization, which plays an important role in improving the semi-solid formability.
Stress–strain curves reflect the internal relationship between stress and deformation conditions and also reveal the evolution of microstructure and mechanical property of materials. The room temperature tensile stress–strain curves of Ti-7Cu alloys with different forging conditions are shown in Figure 7. As seen from Figure 7, all the stress–strain curves of the Ti-7Cu alloys exhibit a similar feature that the stress increases with the increasing of strain after a sharp increase. After the peak stress, stress–strain curves reach to a steady stress, which attributes to a result of a combination of work-hardening effect. The work-hardening effect resulting from the increasing of dislocation density is a dominant factor leading to a rapid increase in stress. Furthermore, almost no obvious heterogeneous deformations were found from stress–strain curves, which suggested that no obvious necking zones were observed during tensile tests (as shown in Figure 8), resulting in low ductility.
Figure 9(a) and (b) shows the tensile strength and hardness of as-received alloy and the semi-solid forged alloy, respectively. It is notable that the semi-solid forged specimens exhibit higher strengths and hardness than that of the as-received specimens. However, the strengths and hardness decrease with the increase of semi-solid forging temperature, especially after forged at 1,050°C. In comparison, the influence of forging ratio on the strength of forged specimens is slight. These results can be reasonably explained by the dependence of grain size on the temperature and the forming ratio. It is well known that, as a kind of hcp metal, titanium exhibits a strong dependence of strength and hardness on grain sizes due to the lack of slip systems . Therefore, the high strength of the forged specimens is attributed to grain refinement caused by hot forging.
Figure 9(c) shows the elongation of as-received alloy as well as forged alloys. All forged specimens exhibit low elongation compared with that of the as-received alloy. The elongation decreased gradually with the increase of semi-solid forging temperature, but it increases with the increase of forging ratio at the same forging temperature. This reveals that forging ratios have significant effect on the ductility. The experimental results show that the method of semi-solid forging can be employed to improve the tensile properties of Ti-7Cu alloy by controlling temperatures and forging ratios.
Figure 10 illustrates the SEM fractographs of the room temperature tensile specimens after SSF with different forging temperatures. All specimens after semi-solid forging display a brittle fracture mode and a typical intergranular fracture surface (Figure 9(a)–(c)), where the delamination between precipitates and matrix is the principal mechanism for the nucleation of microcracks. When these microcracks propagate and reach to the coarse grain boundaries, the cracks are deflected following the grain boundaries, which result in intergranular fracture [19, 20].
Forging temperature and forging ratio have significant effect on the microstructure and mechanical properties of Ti-7Cu alloy in semi-solid state. Dynamic recrystallization occurs during semi-solid forging and the grain grows obviously with the increase of forging temperature. Forging ratio can promote the process of dynamic recrystallization, which causes decrease of grain size. The variation in grain size with different forging conditions is the main reason for the difference in strength and hardness. Furthermore, the research results of Al alloy show that the defects could provide nucleation sites for Ti2Cu phase. It is persuadable that the effective nucleation sites would remove to the grain boundaries at high temperatures in semi-solid state resulting in precipitates zones. The decrease in ductility was also observed in OM fractography as shown in Figure 10. Cleaved grains which are characteristic of decreased ductility were observed in the alloy after forging at 1,100°C (Figure 10(c) and (f)), which is attributed to the precipitate zone formed along the grain boundaries. And this is the principal mechanism for the nucleation of microcracks and intergranular fracture (Figure 11).
In general, the above experimental results and discussions indicate that the method of semi-solid forging can be employed to improve the tensile properties, especially the ductility of Ti-7Cu alloy by controlling forging ratio. In addition, within the test regions, the increase of the forging ratio can facilitate dynamic recrystallization, which is favorable to formability during semi-solid processing.
The effect of forging temperature and forging ratio in semi-solid state on microstructure and tensile property of Ti-7Cu alloy were investigated by forging tests at temperature of 1,000–1,100°C and forging ratio range from 45 to 75%. The conclusions are as follows:
The dynamic recrystallization occurred during semi-solid forging and grain refinement was attained. Forging temperature affects the distribution of Ti2Cu precipitates. As the forging temperature increased, more Ti2Cu tended to precipitate on grain boundaries which form precipitates zone and coarsen the grain boundaries. Forging ratios promoted dynamic recrystallization, and caused decreasing in grain size.
High strengths were obtained for all semi-solid forged Ti-7Cu alloys, which attributed to grain refinement caused by dynamic recrystallization.
The differences in ductility are associated with the distribution of the Ti2Cu precipitates associating from the different forging temperature and forging ratio. The extensive formation of precipitates in the grain boundary regions with low forging ratio leads to a significant reduction in ductility at room temperature and causes intergranular fracture.
The authors acknowledge financial supports of The Project of 973 Program (grant no. 2007CB613807), Natural Science Basic Research Plan of China (grant no. 0520119), Special Fund for advanced technology Research of Central Colleges (0009-2014G2310019).
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About the article
Published Online: 2015-07-07
Published in Print: 2016-06-01
Funding: The Project of 973 Program (grant no. 2007CB613807), Natural Science Basic Research Plan of China (grant no. 0520119.