Review on Microstructural Evolution and Mechanical Behavior of Ti-6al-4v

1. INTRODUCTION

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Titanium alloys have splendid properties such as loftier forcefulness at high temperatures, depression density, and high corrosion resistance. They are used in biomedical, aerospace, automotive parts, defense industry, marine, and chemical industries (Leyens and Peters, 2003Leyens, C., Peters, K. (2003). Titanium and titanium alloys: fundamentals and applications. John Wiley & Sons, Weinheim. ; Cui et al., 2011Cui, C., Hu, B., Zhao, L., Liu, S. (2011). Titanium alloy production applied science, market prospects and industry development. Mater. Design 32 (3), 1684-1691. https://doi.org/10.1016/j.matdes.2010.09.011. ). Although titanium is costly to use compared to alternative materials, it is indispensable in some applications due to these withstand properties (Beal et al., 2006Beal, J.D., Boyer, R., Sanders, D. (2006). Forming of Titanium and Titanium Alloys. Volume 14B, Metalworking, Sheet Forming ASM Handbook. ).

Blend Ti-6Al-4V with α-β microstructure is the most used titanium alloy in industry, accounting for approximately 75-85% of the global titanium product (Mosleh et al., 2018Mosleh, A., Mikhaylovskaya, A., Kotov, A., AbuShanab, Due west., Moustafa, Due east., Portnoy, Five. (2018). Experimental Investigation of the Effect of Temperature and Strain Rate on the Superplastic Deformation Behavior of Ti-Based Alloys in the (α+ β) Temperature Field. Metals eight (x), 819-835. https://doi.org/10.3390/met8100819. ). This material is unremarkably a sail formed by a cold or hot plastic deformation procedure. However, the forming of complex-shaped parts is plush and problematic due to the high yield forcefulness and low elastic modulus. Notably, the formability of Ti-6Al-4V at room temperature is low and limited. Increasing the temperature increases the formability, reduces springback, increasing the function geometrical accurateness (Fan et al., 2013Fan, X.1000,. Yang, H., Gao, P.F. (2013). Prediction of constitutive behavior and microstructure evolution in hot deformation of TA15 titanium alloy. Mater. Design 51, 34-42. https://doi.org/10.1016/j.matdes.2013.03.103. ; Liu et al., 2017Liu, Z., Li, P., Geng, L., Liu, T., Gao, H. (2017). Microstructure and texture development of TA32 titanium alloy during superplastic deformation. Mater. Sci. Eng. A 699, 71-80. https://doi.org/10.1016/j.msea.2017.05.082. ). The forming process parameters such as temperature and strain rate significantly affect menstruation beliefs, microstructural characteristics, and mechanical properties of titanium alloys (Özturk et al., 2013Özturk, F., Ece, R.Due east., Polat, Northward., Koksal, A., Evis, Z., Polat, A. (2013). Mechanical and microstructural evaluations of hot formed titanium sheets by electric resistance heating process. Mater. Sci. Eng. A 578, 207-214. https://doi.org/x.1016/j.msea.2013.04.079. ; Zong et al., 2015Zong, Y., Liu, P., Guo, B., Shan, D. (2015). Springback evaluation in hot five-bending of Ti-6Al-4V alloy sheets. Int. J. Adv. Manuf. Technol. 76, 577-585. https://doi.org/10.1007/s00170-014-6190-z ; Quan et al., 2015Quan, G., Luo, K., Liang, J., Wu, D., Mao, A., Liu, Q. (2015). Modelling for the dynamic recrystallization evolution of Ti - 6Al - 4V alloy in two-phase temperature range and a broad strain rate range. Comput. Mater. Sci. 97, 136-147. https://doi.org/10.1016/j.commatsci.2014.10.009. ; Kopec et al., 2018Kopec, One thousand., Wang, Thou., Politis, D.J., Wang, Y., Wang, Fifty., Lin, J. (2018). Formability and microstructure evolution mechanisms of Ti6Al4V blend during a novel hot stamping procedure. Mater. Sci. Eng. A 719, 72-81. https://doi.org/10.1016/j.msea.2018.02.038. ). Depending on the temperature, the name of the process varies, such as at low temperature it is drawing, at the intermediate temperature it is hot forming or hot stamping, and at very high temperature information technology is superplastic forming (SPF) (Liu et al., 2002Liu, One thousand., Lin, Z., Bao, Y., Cao, J. (2002). Eliminating Springback Error in U-Shaped Part Forming past Variable Blankholder Force. J. Mater. Eng. Perform. 11, 64-lxx. https://doi.org/x.1007/s11665-002-0009-z. ). In terms of ductility and springback behavior, it is generally preferred to form Ti-6Al-4V alloy at temperatures in a higher place 540 ^oC and below the β-trans temperature. In hot forming, the temperature is considered betwixt 750 ^oC and 890 ^oC and SPF above 900 ^oC. Compared to SPF, the procedure temperatures of hot stamping are considerably lower, making it a higher potential for the forming industry. All the same, decreasing the forming temperature increases the springback effect (Liu et al., 2002Liu, G., Lin, Z., Bao, Y., Cao, J. (2002). Eliminating Springback Error in U-Shaped Part Forming by Variable Blankholder Force. J. Mater. Eng. Perform. 11, 64-lxx. https://doi.org/ten.1007/s11665-002-0009-z. ).

Many studies have been carried out on the deformation behavior at high temperatures of Ti-6Al-4V alloy (Brooks, 1982Brooks, C.R. (1982). Heat treatment, structure and backdrop of Nonferrous Alloys. American Society for Metals, Metals Park (OH). ; Welsch et al., 1993Welsch, G., Boyer, R., Collings, Due east.Due west. (1993). Materials backdrop handbook: Titanium alloys. ASM International. ; Semiatin et al., 1999Semiatin, Fifty., Seetharaman, V., Weiss, I. (1999). Catamenia behavior and globularization kinetics during hot working of Ti-6Al-4V with a colony alpha microstructure. Mater. Sci. Eng. A 263 (2), 257-271. https://doi.org/ten.1016/S0921-5093(98)01156-3. ; Picu and Majorell, 2002Picu, R.C., Majorell, A. (2002). Mechanical behavior of Ti-6Al-4V at high and moderate temperatures-Part 2: constitutive modeling. Mater. Sci. Eng. A 326 (two), 306-316. https://doi.org/10.1016/S0921-5093(01)01508-viii. ; Fan et al., 2017Fan, Ten.Thousand., Yang, H., Gao, P.F., Zuo, R., Lei, P.H., Ji, Z. (2017). Morphology transformation of primary strip α phase in hot working of two-phase titanium alloy. Trans. Nonferrous Met. Soc. Cathay 27 (6), 1294-1305. https://doi.org/10.1016/S1003-6326(17)60150-X. ; Wang et al., 2017Wang, X., Wang, L., Luo, 50., Liu, X., Tang, Y., Li, X., Fu, H. (2017). Hot deformation behavior and dynamic recrystallization of cook hydrogenated Ti-6Al-4V blend. J. Alloys Compd. 728, 709-718. https://doi.org/10.1016/j.jallcom.2017.09.044. ). Quan et al. (2015)Quan, G., Luo, Grand., Liang, J., Wu, D., Mao, A., Liu, Q. (2015). Modelling for the dynamic recrystallization development of Ti - 6Al - 4V blend in ii-stage temperature range and a wide strain rate range. Comput. Mater. Sci. 97, 136-147. https://doi.org/ten.1016/j.commatsci.2014.ten.009. performed a series of Ti-6Al-4V blend compressions with a summit reduction of lx% in a temperature range of 750 - 1050 °C and a strain rate range of 0.01-10 due south^−one on a Gleeble-3500 thermo-mechanical simulator. They found that the formability of titanium alloys could increase by increasing the temperature and could decrease by increasing the strain rate in the lower temperature region. The strain required for the same corporeality of dynamic recrystallization (DRX) volume fraction in a constant strain rate increased with decreasing temperature, in contrast, for a constant temperature, it increased with increasing strain rate. Still, they found that the elongation to failure more strongly depends on the strain rate than does the deformation temperature for the Ti6Al4V alloy. Mosleh et al. (2018)Mosleh, A., Mikhaylovskaya, A., Kotov, A., AbuShanab, W., Moustafa, E., Portnoy, Five. (2018). Experimental Investigation of the Result of Temperature and Strain Charge per unit on the Superplastic Deformation Behavior of Ti-Based Alloys in the (α+ β) Temperature Field. Metals eight (10), 819-835. https://doi.org/ten.3390/met8100819. investigated the effect of temperature and strain rate on the superplastic deformation behavior of Ti-based alloys in the (α+β) temperature field alloys. They plant that 850 ^oC and 0.001 s⁻¹ are the optimum superplastic temperature and strain rate for maximum elongation for the Ti6Al4V alloy. Typically, an increase of the tensile strength value was observed every bit the strain charge per unit increased and the temperature decreased. Increasing temperature further led to a decrease in the elongation to failure, due to the β-phase fraction increase. Kopec et al. (2018)Kopec, M., Wang, K., Politis, D.J., Wang, Y., Wang, L., Lin, J. (2018). Formability and microstructure evolution mechanisms of Ti6Al4V alloy during a novel hot stamping process. Mater. Sci. Eng. A 719, 72-81. https://doi.org/10.1016/j.msea.2018.02.038. studied the formability of the Ti-6Al-4V alloy on a novel hot stamping process. They found the elongation ranging from 30% to lx% could be achieved at temperatures ranging from 750 to 900 °C, respectively, and qualified parts tin be formed successfully at 750- 850 °C. The hardness of the material after deformation first decreased with the temperature due to recovery and afterward increased mainly due to the phase transformation. Öztürk et al. (2013)Özturk, F., Ece, R.Eastward., Polat, N., Koksal, A., Evis, Z., Polat, A. (2013). Mechanical and microstructural evaluations of hot formed titanium sheets by electrical resistance heating process. Mater. Sci. Eng. A 578, 207-214. https://doi.org/x.1016/j.msea.2013.04.079. used the electric resistance heating method to investigate the event of temperature on the mechanical properties of the Ti-6Al-4V sheets. The yield stress and tensile stresses decrease with an increase in the temperature considerably, and the total elongation is raised significantly above 500 °C.

All the same, few references can be plant focusing on the springback behavior of the hot-stamped Ti-6Al-4V blend. In terms of production time and toll, keeping the springback phenomenon to a minimum and achieving the required mechanical functioning is vital. Zamzuri et al. (2013)Zamzuri, H., Ken-Ichiro, M., Tomoyoshi, M., Yuya, Y. (2013). Hot stamping of titanium alloy sail using resistance heating. Vestn Nos Magnitog State Tech. Univer. 5, pp. 12-15. have applied the resistance heating to the hot hat-shaped bending of Ti-6Al-4V alloy sheets. They plant that using resistance heating above 370 °C is effective in preventing the springback and oxidation of the titanium blend canvas. The angle load at a heating temperature of 880 °C reduced from half-dozen.5 kN at room temperature to 1.viii kN. Zong et al. (2015)Zong, Y., Liu, P., Guo, B., Shan, D. (2015). Springback evaluation in hot 5-bending of Ti-6Al-4V alloy sheets. Int. J. Adv. Manuf. Technol. 76, 577-585. https://doi.org/ten.1007/s00170-014-6190-z investigated the springback beliefs of Ti-6Al-4V alloy via the V-bending process, and they found that the rising temperature finer softens and facilitates the flow behavior. The springback angle was eliminated when Ti-6Al-4V was treated at around 750 °C and held for 8-10 min.

Therefore, the paper aims to evaluate the springback behavior of the Ti-6Al-4V alloy deformed on the U-bending process at room temperature and temperature ranges between 350-950 ^oC. The springback angles of the deformed sheets were measured. Tensile and hardness tests were performed on the specimens taken from the upper and side-wall surfaces of the plain-featured sheets. Microstructural analysis was also conducted on upper and side-wall specimens. The effect of the deformation temperature on the springback beliefs and mechanical properties of the Ti-6Al-4V alloy is then critically examined related to the microstructural modifications.

2. MATERIALS AND METHODS

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In this written report, Ti-6Al-4V alloy sheets with 1 mm thickness were investigated. The chemical limerick of the canvass is given in Table i.

Table ane. Chemical composition of Ti-6Al-4V (wt.-%)

Composition	Al	V	Fe	O	Due north	H	C	Ti
Content	five.9	4	0.09	0.xiv	0.01	0.002	0.01	Bal.

In the as-received condition, Ti-6Al-4V alloy microstructure with slightly elongated grains of h.c.p. α (low-cal) and intergranular b.c.c. β (dark) two-phase structure was observed under an optical microscope (Fig. one).

Effigy 1. The microstructure of Ti-6Al-4V alloy as received.

Ti-6Al-4V sheets were primarily cutting into blanks with a size of 500×400×one mm³ in the specimen training procedure, and the blanks' length management was along the rolling direction. Then the blanks were heated to temperatures of 400 ^oC, 500 °C, 600 °C, 700 °C, 800 °C, 900 °C, and thou °C for 10 min in a conventional furnace. At the finish of this process, the heated blanks were straight transferred to the press and subsequently formed. The transfer fourth dimension from the furnace to the cold dice was 5 s, and the temperature drop was around 50 ^oC. The hot-forming process was carried out on a 200 ton hydraulic press with 10 mm/s punch speed. The blanks were formed and held for i min in the dies. The experimental setup is shown in Fig. 2a, in which the punch and die were made by a tool steel.

Figure ii. Experimental setup of (a) hydraulic press and (b) die and punch used for hot forming.

The exam specimens, as shown in Fig. 2b, were taken from the lesser and the side-wall of the formed U blank. The microstructure evolution of the samples was observed through optical microscopy. The samples were prepared by conventional metallographic procedures for microstructural characterization of titanium, including hot mounting, grinding, and polishing. The samples were polished using 600, 800, 1200, 2000, and 2500 SiC papers, and the terminal polishing was carried out using a three µm diamond polishing solution and water-based lubricant. The samples were etched in Kroll etchant for xv-20 south, wiped with a cotton ball, and thoroughly washed and dried. Microstructure images were captured using a Nikon Eclipse MA100 optical microscope.

The tensile and hardness tests were performed at room temperature. The dimensions of tensile exam specimens according to the ASTM E8 standards were given in Fig. 2b. Tensile tests were performed at 10 mm/min abiding strain rate on UTEST 25 tons universal tensile testing machine. The Vickers hardness values of the specimens afterward the hot-forming were measured through a Metkon Duroline-Yard make hardness tester at room temperature with 6 measurements per specimen. Measurements were performed using a 0.5 kg load for 10 s dwelling fourth dimension.

The images of the U-profiles were taken with an Olympus digital camera white lite scanning system to decide the springback behavior. A wireframe model was generated from the digital images, and texture mapping was done using the RapidForm software. The scanned data was registered into the CAD information to brandish the data sets by using the software. The springback beliefs of the U-profiles was divers by the side-wall bending, having the most disquisitional effect on dimensional accuracy and thus affecting the subsequent assemblies (Zong et al., 2015Zong, Y., Liu, P., Guo, B., Shan, D. (2015). Springback evaluation in hot 5-bending of Ti-6Al-4V alloy sheets. Int. J. Adv. Manuf. Technol. 76, 577-585. https://doi.org/x.1007/s00170-014-6190-z ). The springback amount was measured past the angle (θ) between the deformed shape of the bare without and later springback (Fig. 3).

Figure 3. The location of specimens taken from formed part.

3. RESULTS AND DISCUSSION

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three.1. Mechanical properties

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The room temperature tensile properties and microhardness of the test specimens, which were cut from the bottom and the side-wall surface of the formed parts, produced through the hot-forming process at different temperatures, are presented in Fig. 4. The trend of curves shows that the forming start temperature has a significant influence on the tensile forcefulness and hardness of the formed parts. Increasing the forming temperature leads to a reduction of the tensile strength and hardness in the exact parallel. As it tin can be seen, there are differences in the tensile strength between the side-wall and the bottom surface specimens after 450 ºC. Considering of the gap between the die, punch, and sheet, the contact time of the side-wall surface is delayed compared with the bottom surface of the formed component, leading to a reduced heat transfer. The bottom surface of the punch contacts the sheet before than the side-wall surfaces of the canvas during hot forming and rapid cooling starts from higher temperatures primarily at the bottom surface. Too, deformation hardening occurs due to pinch by punch and dice increase at the bottom surface of the bare. Furthermore, the actual contact area is smaller on the side-wall surface than the bottom surface, pressed between die and punch. For these reasons, a decrease in the tensile strength and hardness related to the temperature was realized more prominently in the side-wall surface above 450 ^oC.

Effigy 4. The effect of forming start temperature on tensile strength and microhardness of formed Ti-6Al-4V alloy parts in the lesser and side-wall surfaces.

The material, which gradually softens up to 750 ^oC, the tensile strengths of the bottom and the side-wall specimens were slightly decreased well-nigh 7% with increasing forming temperature from 450 ^oC to 750 °C. A similar change occurred in the microhardness. After 750 ^oC, undergoes a phase alter above this temperature, producing an increase in the tensile strength and microhardness. Tensile strength, which reached a minimum value of effectually 1010 MPa at 750 ^oC, increased virtually 4% in the bottom specimens and three% in the side-wall specimens until 950 °C. Beyond 750 °C, the microhardness increased more slightly compared to the strength in all specimens.

3.two. Microstructure evaluation

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The result of the forming beginning temperatures on the microstructural changes is investigated in this study. The microstructure of the samples with forming commencement temperatures ranging betwixt 350 to 550 °C are given in Fig. 5. The microstructure images of bottom and side-wall samples are very similar, and no significant difference is noticed regarding the phases found or their grain sizes. The microstructure of the room temperature formed part consisted of primary α phase and β phase, and the average length of the lamellar α stage was about 3-7 µm (Fig. 5a). The microstructure of the samples with 350 °C and 450 °C forming offset temperatures did not differ noticeably from the starting structure due to the small-scale amount of β phase that exists at this temperature. Fig. 5a-c show the uniform duplex microstructure composed of lamellar α phase together with the β phase. Information technology can be observed that the lamellar construction (Fig. 5a-c) has transformed into an equiaxed structure (Fig. 5d) with the increment in temperature. Withal, a decrease in the tensile force and microhardness was also observed at 550 ^oC (Fig. four).

Figure five. Microstructure images of the samples with (a) room temperature, (b) 350 °C, (c) 450 °C, and (d) 550 °C forming offset temperatures.

On the other hand, the microstructure evolution of Ti-6Al-4V blend during forming processes at elevated temperatures includes three mechanisms: recovery, recrystallization, and phase transformation. Consequently, the micro constituents and microstructures varied in several types, and two types of α, primary and secondary (or transformed β), are present. The main α is nowadays during prior hot working, and the secondary α is produced past transformation from β (Brooks, 1982Brooks, C.R. (1982). Heat treatment, structure and properties of Nonferrous Alloys. American Lodge for Metals, Metals Park (OH). ). The α phase in these conditions has unlike appearances and peradventure acicular or lamellar, platelike, serrated, martensitic, basket weave or Widmanstätten. In this study, the microstructure samples of the bottom surface of hot-formed blanks at 650 °C, 750 °C, 850 ^oC, and 950 °C are given in Fig. 6, and the samples of the side-wall surface in Fig. 7. The microstructure images and mechanical tests (Fig. four) clearly show that most of these mechanisms and micro constituents took place when the temperature was college than 650 °C. The microstructural changes show that different mechanisms occur in the bottom and side-wall surfaces of the formed function. The microstructure of the samples taken from the formed part at 650 ^oC consists of elongated α grains (low-cal) in a matrix with transformed β grains (dark) for the bottom and side-wall surfaces (Fig. 6a and 7a). Also, the grain size of chief α increased in some places, and a fibroid-grained equiaxed structure chosen globular was formed within an intergranular stage. Globularization is deformation-induced and occurred at kinks in the lamellae and some of the prior-beta grain boundaries with the loss of coherency of α/β interfaces (Fan et al., 2017Fan, X.Thousand., Yang, H., Gao, P.F., Zuo, R., Lei, P.H., Ji, Z. (2017). Morphology transformation of master strip α stage in hot working of ii-phase titanium blend. Trans. Nonferrous Met. Soc. China 27 (6), 1294-1305. https://doi.org/ten.1016/S1003-6326(17)60150-Ten. ). A critical strain is required for the initiation, and increasing temperature increases the globularization rate (Semiatin et al., 1999Semiatin, 50., Seetharaman, V., Weiss, I. (1999). Flow behavior and globularization kinetics during hot working of Ti-6Al-4V with a colony alpha microstructure. Mater. Sci. Eng. A 263 (2), 257-271. https://doi.org/x.1016/S0921-5093(98)01156-3. ). The critical strain rate was exceeded in this study, and globular α has slightly occurred in a higher place 650 ^oC.

Figure six. Microstructure images of the bottom surface samples with (a) 650 °C, (b) 750 °C, (c) 850 °C, and (d) 950 °C forming start temperature.

Effigy 7. Microstructure images of the side-wall surface samples with (a) 650 °C, (b) 750 °C, (c) 850 °C and (d) 950 °C forming commencement temperature.

It can be observed that increasing the forming temperature increased the transformed β containing acicular α (Fig. 6b). Acicular α is the well-nigh common transformation product, a result of nucleation and growth on crystallographic planes of the prior β matrix (Welsch et al., 1993Welsch, G., Boyer, R., Collings, E.Westward. (1993). Materials properties handbook: Titanium alloys. ASM International. ). It is possible to see that acicular α occurred in colonies at the bottom surface samples of blanks formed at 750 °C (Fig. 6b). This formation slightly took place in the side-wall surface samples at a higher temperature of 850 °C (Fig. 7c). Information technology is thought that this difference occurs because the bottom surface of the punch contacts the blank before than the side-wall surfaces during forming, and the rapid cooling starts from higher β rates in the bottom surface samples. Fig. 6b besides shows the homogeneously distributed dynamic recrystallized (DRX) equixial α grains with 2-5 µm in size in the sample deformed at 750 °C forming beginning temperature. DRX was generally occurred in the α phase due to the depression content of the β phase in microstructure at 750 ^oC (Wang et al., 2017Wang, X., Wang, Fifty., Luo, Fifty., Liu, X., Tang, Y., Li, X., Fu, H. (2017). Hot deformation beliefs and dynamic recrystallization of melt hydrogenated Ti-6Al-4V alloy. J. Alloys Compd. 728, 709-718. https://doi.org/10.1016/j.jallcom.2017.09.044. ). The tensile strength and microhardness results also support this, showing the lowest value at 750°C, due to the softening issue of recrystallization.

The samples formed at 850 °C forming start temperature exhibited both equiaxed and elongated α structures, the amount of globular α increased in the intergranular β stage. As the temperature increases, β phase transformation increases, and the volume fraction and grain size of DRX grains too increase (Fig. 6c). Individual grains elongated in the direction of the metal flow or plastic deformation.

The microstructure of the formed function at 950 °C forming start temperature indicates a coarse transformed β structure (Fig. 6d and Fig. 7d). At loftier temperature and low strain rate, a long time exposure tin result in dynamic grain growth. At 950 ºC, the structure consists of stable α and β phases in the shape of colonies of parallel α-stage lamellae in primary β-phase grains (Fig. 6d and Fig. 7d). This feature microstructure is called "basket weave" or Widmanstätten. The Widmanstätten structure frequently nucleates at the α-allotromorphous and includes lamellar α+β colonies surrounded by untransformed thin β phase. The width of the α plates increased drastically compared with room temperature formed role grain width, from well-nigh three-7 µm to 55-120 µm in the bottom surface samples (Fig. 8a) and 120-350 µm in the side-wall surface samples (Fig. 8b). Due to the later contact with the blank and tools, slower cooling (cooling in the air) of the side-wall surface samples takes identify and pregnant grain coarsening occurred compared to the lesser surface samples.

Figure 8. Microstructure images with grain size measurements of (a) the bottom and (b) the side-wall surface samples formed at 950 °C temperature.

three.3. Springback beliefs

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The springback angles of Ti-6Al-4V blend formed parts at dissimilar forming start temperatures are given in Fig. nine. By comparing the profile of U-angle parts formed at dissimilar temperatures, it is found that the springback bending was reduced significantly with the increment in the forming start temperature. The bending decreased from 24° to 15° between the room temperature and 850 ^oC forming outset temperature. Zong et al. (2015)Zong, Y., Liu, P., Guo, B., Shan, D. (2015). Springback evaluation in hot v-bending of Ti-6Al-4V alloy sheets. Int. J. Adv. Manuf. Technol. 76, 577-585. https://doi.org/x.1007/s00170-014-6190-z and Kopec et al. (2018)Kopec, Grand., Wang, K., Politis, D.J., Wang, Y., Wang, L., Lin, J. (2018). Formability and microstructure development mechanisms of Ti6Al4V alloy during a novel hot stamping process. Mater. Sci. Eng. A 719, 72-81. https://doi.org/10.1016/j.msea.2018.02.038. reported that the Ti-6Al-4V exhibits good ductility and material softening during the hot deformation procedure at temperatures between 700-850 °C and the springback angle was reduced between these temperatures. The stage transformation of Ti-6Al-4V alloy betwixt these temperatures is dominated by the fact that there is ii-phase alter as the temperature is increased: dynamic recrystallization and β phase transformation. Picu and Majorell (2002)Picu, R.C., Majorell, A. (2002). Mechanical beliefs of Ti-6Al-4V at high and moderate temperatures-Part II: constitutive modeling. Mater. Sci. Eng. A 326 (2), 306-316. https://doi.org/x.1016/S0921-5093(01)01508-8. have described that the ascent in temperature causes the increase in the rate of β stage transformation and causes menstruation stress drop. So, the deformation at high temperatures lowers the mechanical strength and reduces the amount of springback for the formed sheet because of their lower yield strength to elastic modulus ratio (Welsch et al., 1993Welsch, K., Boyer, R., Collings, E.W. (1993). Materials properties handbook: Titanium alloys. ASM International. ). However, with the increase in forming start temperature from 850 ^oC to 950 ^oC, the springback angle increased to xix^o because of the formation of the Widmanstatten microstructure as stated in a higher place.

Effigy nine. The issue of forming start temperature on springback angles of formed Ti-6Al-4V alloy parts.

4. CONCLUSIONS

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In this report, the effect of the forming start temperature on mechanical backdrop, microstructure, and springback beliefs of the bottom and side-wall surface specimens of the U-contour formed parts were investigated. Based on the test results and microstructural analyzes, the following conclusions were derived:

• There is no significant difference noticed in the microstructure evolution and mechanical properties until 550 °C forming commencement temperature betwixt the bottom and side-wall surfaces. All the same, the springback angle decreased by 24%.

• Increasing the forming showtime temperature increased the transformed β and globular α within the intergranular phase. In addition, dynamic recrystallization was found to announced during forming of Ti-6Al-4V alloy at 750 ^oC forming commencement temperature. By increasing the forming kickoff temperature from 550 ^oC to 750 °C, the hardness of the bottom surface specimens decreased by 7.4%, and the side-wall samples decreased past 3%. The tensile forcefulness of the formed parts decreased past 60 MPa in the bottom surface specimens and 40 MPa in the side-wall specimens, and the springback bending decreased by xi% between these temperatures.

• After heating to 950 °C temperature, significant phase transformation and grain coarsening occurred. The microstructure of the lesser and side-wall surface samples was fully transformed into a Widmanstätten structure. The tensile force and hardness distribution demonstrated the aforementioned tendency and indicated an increase of about 3%, and the springback bending was likewise increased past 26.vii% due to the phase transformation between the 850 ^oC and 950 ^oC deformation temperatures.

ACKNOWLEDGMENTS

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This work is part of a project supported past The Commission of Scientific Research Projects of Bursa Uludag University (Project No: OUAP(MH)-2015/11). The authors would similar to give thanks the department for its valuable support.

REFERENCES

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Aggravate, J.D., Boyer, R., Sanders, D. (2006). Forming of Titanium and Titanium Alloys. Volume 14B, Metalworking, Canvas Forming ASM Handbook.

Brooks, C.R. (1982). Oestrus treatment, structure and properties of Nonferrous Alloys. American Society for Metals, Metals Park (OH).

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Fan, Ten.G,. Yang, H., Gao, P.F. (2013). Prediction of constitutive beliefs and microstructure evolution in hot deformation of TA15 titanium blend. Mater. Design 51, 34-42. https://doi.org/10.1016/j.matdes.2013.03.103.

Fan, X.G., Yang, H., Gao, P.F., Zuo, R., Lei, P.H., Ji, Z. (2017). Morphology transformation of primary strip α phase in hot working of two-phase titanium blend. Trans. Nonferrous Met. Soc. People's republic of china 27 (6), 1294-1305. https://doi.org/10.1016/S1003-6326(17)60150-Ten.

Kopec, K., Wang, G., Politis, D.J., Wang, Y., Wang, L., Lin, J. (2018). Formability and microstructure evolution mechanisms of Ti6Al4V blend during a novel hot stamping procedure. Mater. Sci. Eng. A 719, 72-81. https://doi.org/10.1016/j.msea.2018.02.038.

Leyens, C., Peters, M. (2003). Titanium and titanium alloys: fundamentals and applications. John Wiley & Sons, Weinheim.

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