Numerical and experimental study on the hot cross wedge rolling of Ti-6Al-4V vehicle lower arm preform

Cross wedge rolling (CWR) has unique advantages in the production of shaft preforms with refined grains and improved mechanical properties. Considering the sensitivity of Ti-6Al-4V(TC4) alloy to heat treatment temperature, the effect of different initial deformation temperatures (IDTs) on the forming quality, mechanical properties, and microstructure evolution of the TC4 alloy lower arm preforms in CWR forming was studied in this work. The flow stress curves of TC4 alloy in the two-phase region were obtained by isothermal compression experiments. The Arrhenius constitutive model was established and applied to DEFORM-3D finite element (FE) software to simulate the CWR forming process of TC4 alloy lower arm preforms. The forming quality of TC4 alloy parts was compared and analyzed by 3D FE simulation and experiment. And their mechanical properties at room temperature were tested by tensile test. The results showed that the rolled part has well forming quality (no steps and necking defects) and higher geometric dimension accuracy at the IDT 885°C. Moreover, with the increase of IDT, the radial force and torque in the rolling process decrease. In addition, there were no internal defects in the parts rolled by different IDTs, because the die gap reduces the number of alternating cycles of tensile-compressive stress in the rolled workpieces. Compared with the initial state, the microstructure was refined. When the IDT is 885 °C, the ultimate tensile strength (UTS), yield strength (YS), and elongation (EI) of the parts were 987 MPa, 924 MPa, and 16.8%, respectively, which was able to ensure the mechanical performance requirements of the lower arm preform. The results provide theoretical guidance for the actual production of lower arm preform by CWR.

preforms in CWR forming were studied in this work. The flow stress curves of TC4 alloy in the 20 two-phase region were obtained by isothermal compression experiments. The Arrhenius 21 constitutive model was established and applied to DEFORM-3D finite element (FE) software to 22 simulate the CWR forming process of TC4 alloy lower arm preforms. The forming quality of TC4 23 alloy parts was compared and analyzed by 3D FE simulation and experiment. And their mechanical 24 properties at room temperature were tested by tensile test. The results showed that the rolled part has 25 well forming quality (no steps and necking defects) and higher geometric dimension accuracy at the 26 IDT 850°C. Moreover, with the increase of IDT, the radial force and torque in the rolling process 27 decrease. In addition, there were no internal defects in the parts rolled by different IDTs, because the 28 die gap reduces the number of alternating cycles of tensile-compressive stress in the rolled workpieces.

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Compared with the initial state, the microstructure was refined. When the IDT is 885 °C, the ultimate 30 tensile strength (UTS), yield strength (YS) and elongation (EI) of the parts were 987 MPa, 924 MPa 31 and 16.8 % respectively, which was able to ensure the mechanical performance requirements of the 32 lower arm preform. The results provide theoretical guidance for the actual production of lower arm 33 preform by CWR.

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Wheeled armored vehicle plays an extremely important role in the modern battlefield, 40 anti-terrorism, peacekeeping and other fields. The number of its equipment is also increasing according 41 to strategic needs. The lower arm is the key component of wheeled armored vehicle suspension. 42 Ti-6Al-4V (TC4) has the advantages of light weight, high strength, strong corrosion resistance, making 43 it an ideal material in the aviation industry and military industry [1][2][3]. As the preferred material for the 44 lower arm of wheeled armored vehicle, TC4 alloy not only contributes to reducing the weight of the 45 vehicle, but also ensures that it can meet the requirements of service life under harsh road conditions.

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Most of the forging of lower arm preforms are produced by free forging and precision forging. Due to 47 the large forging force, the free forging process is easy to form eccentricity, bending or crack when 48 forging shaft parts. The precision forging process also has the disadvantage of expensive equipment 49 and low production efficiency [4][5][6]. Cross wedge rolling (CWR) is a new near-net forming process, 50 which can reduce the processing cost of shaft parts and improve their quality [7].

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Steel products with good quality can be obtained by selecting suitable die parameters and process 52 parameters during CWR [8,9]. It is necessary for us to further explore and research how to control the 53 surface quality and internal quality of TC4 alloy shaft parts in the rolling process. Li

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The mechanical properties of titanium alloy are closely related to the microstructure 79 characteristics, and the evolution of its microstructure will affect the flow behavior of the material [24]. 80 TC4 alloy is sensitive to hot processing parameters. Different heat treatment conditions and 81 deformation processing parameters can regulate the size, morphology and volume fraction of the phase.

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The equiaxed microstructure with an average grain size of 1.9μm was obtained by multi-directional 83 isothermal forging (MDIF) of TC4 alloy by Zhang et al. [25], and the mechanism of grain refinement 84 was studied. The tensile strength, yield strength and elongation of the alloy after grain refinement were greatly improved at room temperature and 400 °C. Zhai et al. [26] studied the effects of α phase content 86 and morphology on the microstructure and mechanical properties of TC4 alloy during multiple heat 87 treatment processes by experimental method. Wang et al. [27] established the 88 rate/temperature/microstructure constitutive model of TC4, and successfully predicted the evolution 89 law of β-phase volume fraction and grain size during the process of hot ring rolling. Li et al. [28] 90 studied the effects of IDT, area reduction and rolling speed on the volume fraction of α phase in TC6 91 alloy during CWR by FEM and experimental method. Therefore, it is necessary to study the 92 corresponding law between the microstructure characteristics and mechanical properties of TC4 alloy 93 during hot CWR.

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The hot deformation behavior of TC4 alloy with bimodal microstructure was first studied by 95 isothermal hot compression method, and the constitutive equation of TC4 alloy was established for FE 96 simulation. Secondly, the thermodynamic coupling numerical simulation of CWR process of TC4 alloy 97 lower arm preform was carried out by using software Deform, and the accuracy of FEMs were verified.  Figure 1 shows the microstructure of initial TC4 alloy bar. The microstructure has globular 106 primary α phase and lamellar secondary α phase. The chemical composition of the raw material used 107 in this experiment is shown in Table 1 The Gleeble-1500D thermo-simulation machine was used to obtain isothermal compression data.

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The deformation temperatures and strain rates were set at 850°C, 900°C, 950°C, and 0.1, 1, 10 s -1 , 115 respectively. After the test, the temperature of the specimen was immediately brought down to room 116 temperature by water cooling.

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The true stress-strain curves of the TC4 alloy at different strain rates and temperatures are shown 120 in Fig. 2. The flow stress decrease with the increase of temperature, and differently, the flow stress 121 increases with the increase of strain rate. The true stress-strain curves were all in the α+β two-phase 122 field, and the stress increases rapidly to a peak at low strain and then decreases to the steady state, 123 which were more prominent at higher temperatures and lower strain rates. At relatively low strain rates, The correlation coefficient (RR) and average absolute relative error (AARE) were used to evaluate 140 the accuracy of the equation, as follows: In the equations, The FEM for the CWR study is shown in Fig. 4. As the geometrical model of the test specimens 154 and the rolling dies were symmetrical, the boundary conditions of the FEM were set to be symmetrical 155 relative to the center plane. The following assumptions were made in the course of this study. (1) 156 Because the deformation can be ignored, the roll dies and guide plates considered as rigid bodies.   Fig. 6a   In Fig. 12a-b, the periodic contacts between the workpiece and the surface of die gap area 239 during rolling were recorded by smearing pink paint evenly on the surface of the die gap area. 240 This is because the rolled workpiece before entering the gap area did not form a standard circular 241 section but form an oval section with a larger size. The oval cross-section contacts with the surface 242 of the roll cavity, and the contact scratches can be recorded after rolling. 243 circumferential stress (Stress-Theta) have obvious orthogonal distribution. The radial stress is 282 tensile stress along the long-axis direction of the elliptical section and compressive stress along the 283 short-axis direction. The distribution of circumferential stress is exactly opposite to the radial stress. 284 With the decrease of IDT, the tensile and compressive stress distribution of radial stress tends to be 285 uniform. The circumferential compressive stress transfers to the center of the section, and its 286 distribution also tends to be uniform. The axial stress (Stress-Z) at the center of the cross-section at 287 different IDTs is shown as tensile stress. In the process of IDT decreasing from 945 °C to 855 °C, the 288 axial tensile stress range gradually expands from the central region to the outer surface. Finally, the 289 axial stress of the section is all tensile stress.

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The axial strain (Strain-Z) distribution has no significant change. The workpiece material flows to the 301 intersection area of the long-axis and the outer surface under the combined action of the above strain, 302 and the ovality is more obvious than other IDT conditions. As the IDT decreases, the radial strain 303 annulus area near the outer surface gradually becomes homogeneous, and the tendency of the material 304 flowing from the outer surface to the center tends to be the same everywhere. When the IDT decreases 305 to 885 °C, the circumferential tensile strain gradually disappears and the circumferential compressive

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The force condition of the workpiece is particularly complex in the CWR process. Large plastic 316 deformation often occurs along the axis and diameter direction of the workpiece. In order to 317 achieve large plastic deformation, the mill needs to be able to provide sufficient rolling force and

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The more the number of alternating cycles in the center of the workpiece, the greater the trend of 342 internal defects will occur. The stress in the CWR process increases with the decrease of IDT, which 343 means that the stress state of material is more serious in the rolling process at relatively low 344 temperature. When the IDT is high, the CWR process will make the grain boundary of the material  At the same IDT, the UTS and YS in the middle are higher than those in the core, this may be due 373 to the different degrees of deformation of the microstructure along the radial direction from the surface 374 to the center of the part. Because the effective stress and strain gradually decrease from the contact 375 surface along the radial direction to the core position ( Fig. 15 and Fig. 16), the deformation of the 376 microstructure according to Fig. 22a-h is also decreasing. The thickness of lamellar α phase at the 377 middle is smaller than that at the core. The α phase of the microstructure in the middle and core was 378 refined, which improves the UTS and YS properties of the parts with different IDTs.

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From the volume fraction of the equiaxed α phase (fα_e) in Fig. 20a and b, the fα_e decreases with 380 the increase of IDT. Because the volume fraction of the transition from primary α phase to β phase 381 increases with the increase of temperature [28]. The fα_e at the CSP corresponding to the different 382 IDTs are lower than that at the MSP. According to Fig. 21a and b, the plastic temperature rise level at 383 the two points is nearly close. The MSP is close to the mold, and the temperature drop is faster than the 384 CSP. The deformation of the MSP during rolling was larger than that of the CSP, and the lamellar α 385 phase undergoes equiaxed transformation, which promotes the increase of the fα_e.