Materials Science Forum Vols. 558-559 (2007) pp 529-532 online at http://www.scientific.net © (2007) Trans Tech Publications, Switzerland Online available since 2007.10.15
Effect of Initial Lamellar Structure on Globularization of Hot Multi-Forged ELI grade Ti-6Al-4V Alloy Jin Young Kim1, In Ok Shim1, Soon Hyung Hong2 1
Technology R&D Center, Agency for Defense Development, Yusong P.O. Box 35-5, Daejeon 305-600, Korea 2
Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusong-dong, Yusong-gu, Daejeon 305-701, Korea E-mail:
[email protected]
Keywords: Ti-6Al-4V, lamellar structure, globularization, multi-step forging
Abstract. The effect of initial lamellar structure of β heat treated Ti-6Al-4V alloy on the globularization behavior during the multi-step forging was investigated. Specimens of different lamellar thicknesses were upsetted and stretched by side pressing repeatedly, i.e. multi-step forging, at the sub-transus temperature to break down the lamellar structure. The microstructural changes after multi-step forging were analyzed in the light of globularization behavior. The results showed that the thick lamellar structure was more difficult to be transformed to homogeneous equiaxed structure than thin lamellar structure. Introduction The α+β titanium alloy with lamellar microstructure generally has the superior fatigue properties and creep resistance to the alloy with equiaxed or bimodal structure [1,2]. But due to the poor ductility of this lamellar structure, the equiaxed and bimodal structures are preferred in most of industrial applications. So, the conversion of lamellar structure to the equiaxed or bimodal structure is one of the key steps in the titanium semi-product fabrication process. This conversion process, called globularization, is known to occur by the dynamic/static recrystallization during the subtransus deformation and the post thermomechanical treatment, and is closely related to the initial microstructural features and processing parameters [3,4]. The objective of this research is to investigate the effects of initial lamellar structure on the globularization of Ti-6Al-4V during multistep forging. After the multi-step forging of the specimens with different lamellar thicknesses, their microstructural changes from initial lamellar to globular α were investigated. In addition, it was compared with microstructural changes of the specimens subjected to only heat treatment at the same temperature without multi-step forging. Materials and Experimental Procedures Mill annealed Ti-6Al-4V ELI (Extra Low Temperature Interstitial) grade alloy was used as the starting material. Its chemical composition in wt% was Tβ 5.96 aluminum, 3.88 vanadium, 0.10 iron, 0.001 carbon, 0.097 oxygen, 0.006 nitrogen, AC FC AC 0.0076 hydrogen with the balance being titanium and its β transus temperature was Time Ι. β homogenization II. Deformation 975oC. The processing of β heat treatment and multi-step forging used in this experiment were Fig. 1 Schematic illustration of β heat treatment schematically shown in Fig. 1. and multi-step forging used in this experiment. Specimens of 40 mm diameter and 60 mm height were prepared from the 200 mm round bar billet. After β annealing (BA), either air cooling (AC) or furnace cooling (FC) was employed to All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 143.248.110.26-29/02/08,12:59:59)
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produce thin and thick lamellar structures respectively prior to deformation. In the case of FC, the specimens were cooled to 700oC with a cooling rate of 1oC/min in the furnace and then air cooled. Multi-step forging consisting of repetitive and alternate upsetting and side pressing were carried out to β heat treated specimens between two flat dies under the non-isothermal conditions. The dies heated at 350oC were operated by hydraulic press with the moving ram speed of 5mm/s. Specimens were heated at 940oC for 30 min prior to deformation and upsetted to 40mm height. Then stretching was undertaken by pressing billets laterally to 36mm thickness with rotating the sides until the original height of 60 mm was restored. Intermediate heating was given between the side pressing steps. A series of upsetting and stretching was repeated 3 times yielding the accumulated strain of ~2.4. In order to characterize the microstructural state just before forging at forging temperature, small pieces of β homogenized specimens were heated to forging temperature (940oC), and then either water quenched or air cooled. Microstructures were observed by optical microscopy and scanning electron microscopy, and analyzed quantitatively by an image analyzer. Results and Discussion The microstructures of β annealed specimen followed by different cooling rates are shown in Fig. 2. Microstructure of air cooled specimen (hereafter, BAAC) consisted of thin α lamellar and thick grain boundary α layer. For furnace cooled specimen (hereafter, BAFC), the thicknesses of both lamellar α and grain boundary α increased due to the slow cooling rate. The microstructures after multi-step forging for BAAC and BAFC are shown in Fig. 3. In BAAC (a) (b) (Fig. 3a), microstructure exhibited the bimodal structure of equiaxed α and interparticle lamellar. But, in BAFC (Fig. 3c), elongated α particles were observed although its thickness was nearly the Fig. 2 Microstructures of β annealed billets: same with the diameter of equiaxed α particles in BAAC. Measured mean grain sizes and fractions (a) BAAC (cooling rate ~102 oC/min), o of α globules are listed in Table 1. It is known that (b) BAFC (cooling rate 1 C/min). one of the primary factors deciding the size of α particle after thermomechanical treatment (TMT) is the initial α lamellar thickness prior to hot working [2]. However, the comparison between Fig. 3(a) and (b) revealed that, after multi-step forging, the diameter of equiaxed α particles in BAAC was nearly the same with the thickness of elongated α particles in BAFC even with very different initial α lamellar thickness (Fig. 2), but only the different aspect ratio was resulted. In addition, while recrystallized equiaxed α grains were evenly distributed in BAAC (Fig. 3b), the traces of (a) (b) lamellar colony boundaries remained and lamellarlike structure still existed at the interior of deformed colony in the case of BAFC (Fig. 3d). To analyze the different globularization behavior with the initial lamellar structures, BAAC and BAFC specimens were reheated at the same temperature (i.e. 940oC) for 30 min, and then (c) (d) either water quenched (WQ) or air cooled (AC) without multi-step forging. After water quenching, the shape and volume fraction of α lamellar are preserved, and matix β transformed to martensite. Therefore, information on the microstructural state just before multi-step forging can be obtained. Fig. 3 Microstructures after multi-step Through air cooled microsrtuctures, the effect of forging: (a) and (b) for BAAC, (c) and (d) multi-step forging on the microstructural change for BAFC. during subsequent air cooling can be analyzed.
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Table 1 Thickness and fraction of α lamellar and α globules before and after multi-step forging
Thickness of α lamellar (µm) Fraction(%) of α lamellar Fraction (%) of α globule Average min. diameter (µm) of α globule Average max. diameter (µm) of α globule Average aspect ratio of α globule
BAAC ~1 92 55 ~7 ~11 1.5
BAFC ~8 91 57 ~8 ~22 2.6
Remark Before TMT
After TMT
For the purpose of comparison, histograms showing (a) (b) the lamellar thickness and volume fraction of α phase at each processing stage are constructed in Fig. 5. From the Fig. 5, it can be seen that the lamellar thicknesses in BAAC and BAFC changed in different ways during reheating and cooling. (Aforementioned, the microstructural status at the (c) (d) reheating stage can be obtained by the water quenched specimens.) In BAAC, the lamellar thickness remained nearly constant during the reheating, but slightly increased during the air cooling. On the contrary, in BAFC, the lamellar Fig. 4 SEM microstructures after reheating thickness decreased rapidly during reheating but followed by water quenching or air increased during the air cooling, still smaller than cooling: (a) BAAC-WQ, (b) BAFC-WQ, initial thickness. These observations strongly indicate that the different globularization behavior (c) BAAC-AC, (d) BAFC-AC. of multi-step forged BAAC and BAFC (Fig. 3), i.e. (a) equiaxed α globules in BAAC and elongated ones in BAFC and (b) the diameter of the former was nearly the same as the thickness of the latter, could be ascribed to changes in α lamellar thickness and volume fraction during reheating of β annealed alloy. As listed in Table 2, the microstructure of BAAC consisted of higher (~52%) volume fraction of thin (~1µm) α lamellar during reheating but, in BAFC, the fraction of α lamellar decreased to low (~37%) level and α lamellar became thicker (~3µm) than those of BAAC. It is known that β phase of bcc structure is softer than α plate of hcp at the reheating temperature [5]. In BAFC, thick β layers were embedded between thick α lamellar plates (Fig. 4b). Under this condition, β layers are likely to deform preferentially and therefore only small portion of the applied stress is to be transferred to α lamellar during deformation, resulting in insufficient plastic deformation of α lamellar for recrystallization. Eventually, the final microstructure of BAFC after multi-step forging exhibited the traces of deformed colonies and α particles of high aspect ratios inside these colonies as shown in Figures 3c and 3d.
Fig. 5 Thickness and fraction of α lamellar or α globules at various processing stages.
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Table 2 Thickness and fraction of α lamellar or α globules at various processing stages BAAC
BAFC
WQ AC WQ AC ~1 ~1.3 ~3 ~4.5 Thickness of initial α lamellar (µm) 52 * 37 60 Fraction of initial α lamellar (%) * The volume fraction of α in BAAC after air cooling without forging could not be measured since α phase transformed from β phase during air cooling could not be distinguished from the initial α phase. Conclusions The globularization of ELI grade Ti-6Al-4V alloy with different initial lamellar structures during multi-step forging was investigated. After multi-step forging, the alloy with initial thin lamellar structure exhibited equiaxed recrystallized α globules, but elongated α globules with high aspect ratio was dominant in the alloy with initial thick lamellar structure. However, the diameter of equiaxed α globules in the former was comparable to the thickness of elongated α globules in the latter. This different globularization behavior was found to be mainly ascribed to the changes in thickness and fraction of α lamellar during reheating prior to forging which might influence the stress and strain distribution during forging. And the elongated α globules resulted from the insufficient deformation of α lamellar for globularization by the preferential deformation in soft β phase of higher volume fraction during forging. References [1] S.L. Semiatin, V. Seetharaman and I. Weiss, in Advances in the Science and Technology of Titanium Alloy Processing, edited by I. Weiss, R. Srinivasan, P.J. Bania, D. Eylon and S.L. Semiatin, The Minerals, Metals & Materials Society, 1997, pp 3~73 [2] G. Lütjering and J.C. Williams: Titanium (Springer, Germany 2003), pp177~232 [3] S.L. Semiatin, J.F. Thomas, Jr., and P. Dadras: Metallurgical Transactions A Vol.14A (1983), p.2363 [4] I. Weiss, F.H. Froes, D. Eylon and G.E. Welsch: Metallurgical Transactions A Vol.17A (1986), p.1935 [5] I. Weiss and S.L. Semiatin: Materials Science & Engineering A 243(1998), p46
