Influence of heat treatment on microhardness and microstructure of the joining interface for Ti-24Al-15Nb-1.5Mo/TC11

Transcription

1 Advanced Materials Research Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Influence of heat treatment on microhardness and microstructure of the joining interface for Ti-24Al-15Nb-1.5Mo/TC11 Yingying Liu 1,a, Xin Lin 2,b, Wenxin Zhang 1,c,Yuanbo Zhang 1,d 1 Xi an University of Architecture &Technology, Xi an , China 2 State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an , China a wfllyy7779@163.com, b xilin@nwpu.edu.cn, c hourenze5251@163.com, d yuanbozhang@163.com Keywords: Ti-24Al-15Nb-1.5Mo/TC11, heat treatment, microhardness, microstructure Abstract: The microhardness and microstructure of Ti-24Al-15Nb-1.5Mo/TC11 dual alloy joining interface processed by different heat treatment regimes were investigated. The results show that the microhardness of the weld zone and Ti-24Al-15Nb-1.5Mo alloy side of the dual alloy processed by gradient heat treatment are higher than that of the specimen processed by balance heat treatment. Moreover, the microstructure differences between the specimens processed by gradient heat treatment and balance heat treatment are very obvious, particularly in the interface between the TC11 alloy and the weld zone. While the joining interface is processed by gradient heat treatment, the microstructure change homogeneously, and the demarcation lines are ambiguous. However, for the joining interface processed by balance heat treatment, the demarcation lines are very obvious, and the microstructure of the TC11 alloy side is bi-morphic, the α lath is short and coarse. Therefore, the interface integrity of the dual alloy processed by gradient heat treatment is better, which is an adoptable method to improve the properties of the dissimilar alloy joining interface. Introduction The manufacture technology of high weight-thrust ratio engine is necessary for the development of advanced gas turbine engine. Generally, the high weight-thrust ratio of the engine will largely be achieved by raising pressure ratios and raising turbine inlet temperatures [1], which leads to more arduous service conditions for high pressure turbine disc in the hot gas path. If this objective can be achieved without detriment to the reliability and maintainability of the engines in operation, it will be the optimal option [1]. To overcome the limitation in the temperature capability of currently available materials and achieve the higher performance, many researches have been investigated the bimetallic turbine disc. The bimetallic gas turbine disc enables the different mechanical property requirements for the bore and the rim sections to be reconciled in a single disc structure. Therefore, this is an increasing interest in the manufacture of dual alloy with different properties in different positions. The main barrier to the development of dual alloy disc is the lack of a suitable, cost effective manufacturing method that meets the component design and integrity requirements adequately. Many investigators are employed in this technology [2-8]. One of the promising methods strengthening the welded joint is near isothermal forging plus heat treatment. In this paper, this method is used to strength the welding interface of Ti 3 Al/TC11 dual alloy. It is known that the heat treatment regime has an obvious effect on the properties of the alloy [8]. Therefore, it is important for the components to adopt the suitable heat treatment method. For dual alloy disc, there are two kinds of heat treatment methods. One is gradient heat treatment, it exists temperature gradient in the different portion in the same component due to the different heat treatment regime for every alloy. The other is balance heat treatment, which adopts the same heat treatment regime for the whole dual alloy disc. In order to obtain the suitable heat treatment regime, the effects of heat treatment on microhardness and microstructure of Ti-24Al-15Nb-1.5Mo/TC11 joining interface have been investigated. 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 Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-04/03/16,00:46:21)

2 Advanced Materials Research Vols Experimental The nominal composition of TC11 titanium alloy, 42 mm in diameter, is shown in Table 1. Its phase transformation temperature was 995 C. Before welding, the TC11 alloy was preformed using air hammer into a pie about 100mm in diameter and 25mm in height. The oil furnace was used as the heating device, and it was controlled in the oxidizing atmosphere. The heating temperature of the billet was 980 C, and the goal forging temperature was above 850 C. The heating time of the specimen was calculated by 0.8 mm per minute. After deformation, the specimen was cooled in air. The microstructure after forging by air hammer is shown in Fig.1 (a), it is bi-morphic, including the equiaxed and the acicularα phases. Table 1 Chemical composition of the TC11 alloy in wt%. Alloy Ti Al Mo Zr Si TC11 Balance The Ti-24Al-15Nb-1.5Mo(at,%)alloy, 90 mm in length and width, 220mm in height. Its phase transformation temperature was about 1100 C. Before welding, the Ti-24Al-15Nb-1.5Mo alloy was also preformed using air hammer into a pie about 130mm in diameter and 25mm in height. The oil furnace was used as the heating device, and it was also controlled in the oxidizing atmosphere. The heating temperature of the billet was 1080 C; the goal forging temperature was above 850 C. The heating time of the specimen was calculated by 0.9 mm per minute. After deformation, the specimen was cooled in air. The microstructure after forging by air hammer is shown in Fig.1 (b), it consists of the α phase (bright phase) in the equiaxed or the acicular shape and the 2 β phase. Fig.1 Microstructure of the two alloys before welding (a) TC11 titanium alloy, (b) Ti-24Al-15Nb-1.5Mo alloy After deformation, the TC11 alloy billet was machined into a pie, 80mm in diameter and 20mm in height. The Ti-24Al-15Nb-1.5Mo alloy billet was machined into a ring, and its inner diameter is 80mm. Afterwards, the TC11 alloy pie and the Ti-24Al-15Nb-1.5Mo alloy ring were joined using KS55-G150 vacuum welding machine with 150 KV accelerating voltage, 18mA welding current, 2146mA focusing current and 8mm/s welding speed. The compression tests of the dual alloy modelling disc along its axial direction were carried out using a hydraulic press with controllable strain rate. The deformation temperature was 960 C, and the strain rate was 10-3 s -1. The dual alloy discs of No.1 and No.3 were deformed by 30 %, and the No.2 and No.4 were deformed by 40 %. The die temperature was kept at 930 C in deformation. In the heating and the deformation, the surfaces of dual alloy modelling discs were coated with special glass lubrication to resist oxidation. After deformation, they were cooled in air. After near isothermal forging, the Ti-24Al-15Nb-1.5Mo/TC11 dual alloy discs of No.1 and No.2 were processed by gradient heat treatment. The temperature gradient can reach 300 using the cooling block and the cooling water. The heat treatment regime was 1020 C(on Ti-24Al-15Nb-1.5Mo alloy side)/ 950 C(on TC11 alloy side)1 h, AC (air cooled) C/ less than 530 C, 1 h, AC C / 530 C, 7 h, AC. The Ti-24Al-15Nb-1.5Mo/TC11 dual alloy of No.3 and No.4 were processed by balance heat treatment, the regime was 970 C, 1h, AC C, 8h, AC. The deformation amount and heat treatment regime of each dual alloy specimen was given in Table 2.

3 2032 Materials and Processes Technologies V Table 2 Deformation amount and heat treatment regime of each dual alloy specimen Specimen Deformation heat treatment regime number amount (%) No.1 30 gradient heat treatment (Ⅰ) No.2 40 gradient heat treatment (Ⅰ) No.3 30 balance heat treatment (Ⅱ) No.4 40 balance heat treatment (Ⅱ) An optical microscope (OLYMPUS PM-G3) was used to observe the microstructure of the joining interface. The specimen was etched with a solution containing (5-10)vol.%HF+30vol.%HNO 3 in water. The microhardness was measured using HPX-1000TM microhardness tester with a load of 100g and a dwelling time of 15s. Results and discussion The microhardness of the welded joint processed by different heat treatment regimes is shown in Fig.2. It can be seen that the difference in the microhardness of the TC11 alloy side is not obvious. However, the michardness of the weld zone and the Ti-24Al-15Nb-1.5Mo alloy side processed by gradient heat treatment are higher than that of the dual alloy modeling discs processed by balance heat treatment. The reason is as the dual alloy modeling disc is processed by balance heat treatment, the aging temperature is 700 C (970 C, 1h, AC C, 8h, AC), it is higher than that of the TC11 alloy (530 C).Therefore, the α phase and the α 2 phase of the weld zone precipitated from β phase are coarse due to the heat energy. While the dual alloy modeling disc is processed by gradient heat treatment, the solution temperature and aging temperature are suitable for TC11 alloy and Ti-24Al-15Nb-1.5Mo alloy, the fine α phase and α 2 phase of the weld zone are still maintained. Consequently, at any deformation amount, the microhardness is relatively higher as the dual alloy disc is processed by gradient heat treatment. Fig. 2 Microhardness of the welded joint processed by different heat treatments Start point of HAZ on TC11 side ( µ m): No.1: 1650; No.2: 850; No.3: 1700; No.4: 1100; Start point of the weld ( µ m): 2250; End point of the weld ( µ m): No.1: 4530; No. 2: 4100; No.3: 3650; No. 4: 4600; End point of HAZ on Ti-24Al-15Nb-1.5Mo side ( µ m): No.1: 5800; No.2: 5300; No. 3: The microstructure of the welded joint processed by various heat treatment regimes are shown in Fig.3-Fig.6. It can be seen that there exists obvious differences in the microstructure of the specimens processed by different heat treatments, particularly in the interface between the TC11 alloy and the

4 Advanced Materials Research Vols weld zone. The reason is that the dual alloy disc is processed by gradient heat treatment (Fig.3 and Fig.5), the temperature in the weld zone approaches the heat treatment temperature of the Ti-24Al-15Nb-1.5Mo alloy (1020 C), so the temperature is higher for TC11 alloy close to the weld zone. Therefore, the microstructure on the TC11 alloy side close to the weld zone is basket-wave, and the acicular α phases extend from TC11 alloy side into the weld zone. Whereas, the transition of the microstructure from the weld zone to the Ti-24Al-15Nb-1.5Mo alloy side is not obvious. However, as the dual alloy discs are processed by balance heat treatment (Fig.4 and Fig.6), the microstructure on the TC11 alloy is bi-morphic, and the α lath is short and coarse. The demarcation lines of the welded joint are very obvious compared with the interfaces undergone gradient heat treatment (Fig.4(a) and Fig.6(a)), particularly in the interface between the TC11 alloy and the weld zone. Therefore, the interface integrity of the dual alloy disc processed by gradient heat treatment are better, which is a better way to improve the properties of the dissimilar alloy interface. Fig. 3 Microstructure of the joining interface of No.1 (30%, gradient heat treatment) Fig. 4 Microstructure of the joining interface of No.3 (30%, balance heat treatment) Fig.5 Microstructure of the joining interface of No.2 (40%, gradient heat treatment) Fig.6 Microstructure of the joining interface of No.4 (40%, balance heat treatment)

5 2034 Materials and Processes Technologies V Conclusions 1) The microhardness of the weld zone and the Ti-24Al-15Nb-1.5Mo alloy side of the dual alloy processed by gradient heat treatment are higher than that of the dual alloy processed by balance heat treatment. However, the difference in the microhardness of the TC11 alloy side processed by different heat treatment is delicate. 2) The microstructure differences between the specimens processed by gradient heat treatment and balance heat treatment are very obvious, particularly in the interface between the TC11 alloy and the weld zone. While the specimen is processed by gradient heat treatment, the microstructure on the TC11 alloy side close to the weld zone is basket-wave, and the acicular α phases extend from TC11 alloy side into the weld zone, the microstructure changes homogeneously and the demarcation lines are ambiguous. However, as the dual alloy specimen is processed by balance heat treatment, the microstructure on the TC11 alloy is bi-morphic, and the α lath is short and coarse. Moreover, the demarcation lines of the joining are very obvious. 3) Gradient heat treatment is an adoptable method to improve the interface integrity of the dissimilar alloy joining interface. Acknowledgements This work was financially supported by the Natural National Science Foundation of China ( ), the fund of the State Key Laboratory of Solidification Processing in NWPU (SKLSP201117), National Undergraduate Training Programs of Innovation and Entrepreneurship ( ), and Shaanxi Province Undergraduates Training Programs of Innovation and Entrepreneurship (1020). References [1] M. B. Henderson, J. Hannis, G. McColvin and G. Ogle, Proc.Conf on Advanced materials and processes for gas turbines, (ed.g. E. Fuchs et al.), Copper Mountain, CO, USA, 2002, Warrendale, PA, TMS, 2003, p [2] U. E. Klotz, M. B. Henderson, I. M. Wilcock, S. Davies, P. Janschek, M. Roth, P. Gasser, McColvin, Mater. Sci. and Technol. 21(2005) [3] D. P. Mourer, E. Raymond, S. Ganesh, J. Hyzak, The Eighth International Symp. on Superalloy, Pennsylvania, Warrendale, TMS, 1996, pp [4] Z. K. Yao, H. Z. Guo, M. L. Zhang, in: G. Lutjering and J. Albrecht (Eds.). The 10 th Word Conf.on Titanium, Hamburg, Warrendale, TMS, 2004, pp ( In Chinese). [5] Z. K. Yao, M. L. Zhang, X. M. Liang, H. Z. Guo, Trans. China weld Inst. 25(2004) ( In Chinese). [6] Z. K. Yao, X. M. Liang, H. Z. Guo, S. K. Cheng and H. Zhang, Rare Met. Mater. Eng. 34(2004) ( In Chinese). [7] Y. Y. Liu, Z. K. Yao, H. Z. Guo, Rare Met. 28(2009) ( In Chinese). [8] Y. Y. Liu, Z. K. Yao, H. H. Yang, C. Qin., H. Z. Guo, J. Aeronautic mater. (4)29(2009) 21-26( In Chinese).

6 Materials and Processes Technologies V / Influence of Heat Treatment on Microhardness and Microstructure of the Joining Interface for Ti- 24Al-15Nb-1.5Mo/TC /