Effect of Deformation Process on Superplasticity of Inconel 718 Alloy

Rare Metal Materials and Engineering Volume 44, Issue 2, February 2015 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2015, 44(2): 0298-0302.

ARTICLE

Effect of Deformation Process on Superplasticity of Inconel 718 Alloy Dong Hongbo,

Wang Gaochao

Nanchang Hangkong University, Nanchang 330063, China

Abstract: After grain refinement through hot-forging and į-phase precipitation and recrystallization heat-treatment processes, the high temperature tensile experiments were carried out to investigate the effect of heat treatment and deformation process on the superplasticity of Inconel 718 alloy. Two tensile processes were used as follows: the maximum m value superplastic deformation method and the strain-reduced superplasticity deformation process based maximum m value method. The results indicate the fine and homogeneous grain structure of Inconel alloy is obtained by hot forging, į phase precipitation and recrystallization heat treatment processes, and the į phase can play a role in controlling grain size during recrystallization annealing and hot deformation. Upon stretching by the above two processes at 950 ºC, the percentage elongations of Inconel 718 alloy are improved from 340% and 566%, respectively. The results show that the higher value of percentage elongation can be obtained by the strain-induced superplastic deformation process based maximum m value method. Key words: Inconel 718 alloy; superplasticity; deformation process

Inconel 718 alloy is an aged hardenable Ni-Cr-Fe based wrought superalloy. The metastable body centered tetragonal coherent precipitate ȖƎ (Ni3Nb) phase and the face-centered cubic coherent precipitate Ȗƍ (Ni3AlTi) phase are strengthening phases, and the ȖƎ phase is the major strengthening phase. The equilibrium phase corresponding to the ȖƎ phase is the orthorhombic incoherent į (Ni3Nb) phase[1,2]. Inconel 718 is an important material used for aero-engine turbine disks due to its high strength, plasticity and good fatigue resistance, corrosion resistance etc. Inconel 718 alloy is a difficult-to-deformation material due to its great deformation resistance and narrow hot-working temperature range. The higher manufacturing cost and rejection rate of complex parts of the superalloy currently restrict its application. The study[3] found that Inconel 718 alloy also exhibits some superplasticity, which provides an access to the integral forming technology of complex parts of the alloy. The ultra-fine microstructure suitable for superplastic forming can be obtained by the special thermomechanical treatment process such as large

deformation and heat treatment process[4,5]. As well known, it is difficult and costly to fine the grain of Inconel 718 alloy, which directly affects the extensive application of the superplastic forming technique in this alloy. Superplastic tensile deformation such as constant velocity or constant strain rate is the traditional experiment method used by most researchers. Lu Hongjun et al. [4] found that ultrafine-grained sheets of Inconel 718 alloy were obtained by higher cold-rolling reduction and annealing treatment. The maximum elongation of 368.2ˁ was obtained in the alloy deformed at 940 ºC with an initial strain rate of 6.1×10-4 s-1. Han Xue et al. [5] observed that the grains of Inconel 718 alloy were fined using hot deformation and multiple heat treatment, and an elongation of 467% was obtained in a tensile test at 1000 ºC and a strain rate of 1.14 h10-4 s-1. Mukhtarov et al. [6] found that nanocrystalline structure was formed in Inconel 718 bulk semi-products by multi-axis forging, and the elongation of 350% was obtained at 600 ºC under a strain rate of 1.5h10-4 s-1. In addition to refining structure only, is there any new forming

Received date: March 25, 2014 Foundation item: National Natural Science Foundation of China(51164029) Corresponding author: Dong Hongbo, Ph. D., Professor, School of Aeronautical Manufacturing Engineering, Nanchang Hangkong University, Nanchang 330063, P. R. China, Tel: 0086-791-83863032, E-mail: [email protected] Copyright © 2015, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

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technique to improve the superplasticity of the alloy? The author and coworker investigated that the superplastic ability of some alloys can be improved using the maximum m value superplastic deformation method or the strain-reduced superplasticity deformation process based the maximum m value methodˈwhich has been evidenced in the previous experiments of titanium alloys[7,8]. In this paper, combined with hot deformation and heat treatment processes, the maximum m value superplastic deformation method and the strain-reduced superplasticity deformation process based the maximum m value method were adopted to investigate the superplasticity of Inconel 718 alloy.

50 μm

Fig.1

Original microstructure of Inconel 718 alloy ĭ5f0.05

The material in this experiment was as-rolled Inconel 718 alloy bar, whose composition is (wt%) 53.51 Ni, 18.39Cr, 17.37Fe, 5.34Nb, 2.97Mo, 0.99Ti, 0.50Al, 0.35Co, 0.10Si, 0.05Mn, 0.024C, 0.02Ta, 0.008N, 0.006S, 0.004B, 0.001O, 0.0005S and 0.0008Mg. The original microstructure of the material is shown in Fig.1. Optical microstructure picture shows that the grain sizes are not uniform, and the average size of the grains is about 40 ȝm. Meanwhile twin crystal structure is visible distinctly. The free forging process was carried out on Inconel 718 alloy bar with decreasing temperature from 1050 ºC to 950 ºC. The cylindrical billets were cut from the forgings, and heat-treated in box resistor-stoves. Based on the characteristics of Inconel 718 alloy, the microstructures can be controlled by į phase precipitation and recrystallization heat-treatment. Given that the peak temperature of į phase precipitation appears at 890~900 ºC and į phase begins to dissolve at about 980 ć, the precipitation treatment of į phase and recrystallization heat-treatment are generally carried out at 890 and 950 ºC, respectively[9,10]. So the heat-treatment process used in this experiment included two processes as follows˖890 ºC/10 h + 950 ºC/1 h + air cooling and 890 ºC/10 h + 950 ºC/3 h + air cooling. The tensile specimens were machined from the cylindrical billets after heat treatment, as shown in Fig.2. The tensile experiments of high temperature were conducted using the SANS-CMT 4104 electrical universal material testing machine equipped with a high temperature heating-furnace and a computer control system, by which dynamical measuring of the strain-rate sensitivity index and automatical adjusting deformation velocity were realized during the deformation. The isothermal hot tensile tests of Inconel 718 alloy were carried out at 950, 980 and 1020 ºC. The maximum m value superplastic deformation method and the strain-reduced superplasticity deformation process based the maximum m value method were used during tensile deformation. The maximum m value superplastic deformation method

M12

1 Experiment

R2.5 20

12

44 Fig.2

Dimension of tensile specimen

can be realized by means of real-time controlling deformation velocity based on dynamical measuring of m value to obtain a higher m value at any instant during the hot deformation. In general, the higher the m value is, the better the superplasticity of the materials is. The main parameters of the maximum m value superplastic deformation method were as follows: initial tensile velocity v0 was 0.8 mm/minˈincrement value of the speed jump Ƹv was 0.09 mm/minˈand interval of the speed jump Ƹt was 6 s. The basic principle of the strain-reduced superplasticity deformation process was that a certain degree of pre-strain was firstly applied to the materials followed by holding for a period of time to refine grains and soften materials by recovery and recrystallization, and then better plasticity may be obtained during the subsequent superplastic deformation. The main parameters of the strain-reduced superplasticity deformation process were as follows: tensile velocity of pre-strain v was 0.8 mm/min, pre-strain İ was 0.2, holding time after pre-strain t were 10, 20 and 30 min. After holding, the tensile deformation then was carried out immediately by the maximum m value superplastic deformation method.

2

2.1

Results and Discussion Microstructures treatment

after

forging

and

heat

The microstructure of Inconel 718 alloy after forging is shown in Fig.3. The original grains have been broken and the grain boundaries aren't obvious. The intermetallic compound į phase has been precipitated at the grain boundary and wherein short rod or granular structure appears, and

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a

20 μm b Fig.3

Microstructure of Inconel 718 alloy after forging

the particle size distribution is uniform. The dispersed į phase in nickel base superalloys can be used to control the grain size after recrystallization. The microstructures of Inconel 718 alloy forged then treated by precipitation and recrystallization heat-treatment. are shown in Fig.4. The equiaxed fine grain is uniform and the average grain size is less 5 ȝm. Compared with the alloy after annealing at 950 ºC for 1 h, the grains of the alloy after annealing at 950 ºC for 3 h become smaller and more homogeneous. The results show the grains can be fined effectively by forging, į phase precipitation and recrystallization annealing processes, and the refine effect becomes more evident when annealing at 950 ºC for 3 h.

2.2

The maximum m value superplastic deformation method

The superplastic tensile deformation of Inconel 718 alloy was carried out at 950, 980 and 1020 ºC, by the maximum m value superplastic deformation method. The results are shown in Table 1. With the decrease of the grain size, the elongation increase gradually at the same deformation temperature. The elongation of the alloy with the same microstructure increases gradually as the deformation temperature decreases. When tensile specimens are deformed at 950 ºC, the maximum elongation to failure of the unrefined alloy is 157% (Fig.5a), and that of the alloy after forging, į phase precipitation and recrystallization annealing for 3 h is 340% (Fig.5b). The results show that Inconel 718 alloy exhibits good superplasticity during superplastic tensile deformation from 950 to 1020 ºC, and the superplasticity is improved with grain refinement.

2.3

Strain-reduced process

20 μm

Fig.4

treatment processes: (a) 890 ºC/10 h+950 ºC /1 h; (b) 890 ºC/10 h+950 ºC/3 h Table 1

Elongation of Inconel 718 alloy by using the maximum m value superplastic deformation method Elongation, G /% Temperature, T/ºC Unrefined Forged 1 h annealed 3 h annealed 950 157 232 269 340 980 130 202 258 267 1020 117 178 211 207 a Undeformed 1020 ºC, į=117%

980 ºC, į=130%

950 ºC, į=157%

b Undeformed

superplasticity deformation

The superplastic tensile deformation was carried out using the maximum m value superplastic deformation method. After the pre-strain of 0.2, the holding time was 10, 20 and 30 min. The results are shown in Table 2. When the samples with the same undeformed microstructure are deformed from 950 to 1020 ºC, the better elongation appears at 950 ºC under the same holding time.

Microstructures of Inconel 718 alloy after different heat

1020 ºC, į=207%

980 ºC, į=267% 950 ºC, į=340%

Fig.5

Tensile specimens of Inconel 718 alloy by using the maximum m value superplastic deformation method: (a) unrefined alloy and (b) the alloy annealed for 3 h

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Furthermore, the best elongation appears when holding time is 20 min at the same deformation temperature. When the samples are deformed at 950 ºC and holding time is 20 min, the maximum elongation to failure of the unrefined alloy is 177% (Fig.6a), and that of the alloy after Table 2

Experimental parameters and elongation of Inconel 718 alloy Holding Elongation, G/% Temperatu time , 1h 3h Unrefined Forged re, T/ºC t/min annealed annealed 10 149 245 403 426 20 177 274 486 566 950 30 155 238 412 431 10 138 240 246 264 20 163 253 293 329 980 30 145 229 235 258 10 130 189 208 209 20 149 201 224 266 1020 30 122 193 207 215 a Undeformed

1020 ºC, į=149%

and recrystallization heat-treatment. The results show that the grain size is tightly controlled by the pinning effect of the precipitate G phase on the grain boundary. The į phase can still played a role in controlling grain size to prevent grain growth during hot deformation. In the other hand, the precipitation process of G phase can result in ȖƎ phase depletion through the consumption of some strengthening element Nb, which can be beneficial to improve the plasticity and avoid stress concentration[11]. The temperature of G phase dissolving back into the solution is about 980 ºC, so the elongation of Inconel 718 alloy is better at 950 ºC, and the elongation decreases at 980 or 1020 ºC. Static recovery and recrystallization will occur to refine the grains and soften the material upon applying a pre-strain to Inconel 718 alloy and holding for a period of time. The experimental results show that the rational holding time after pre-strain can improve the alloy superplasticity, and the best holding time during superplastic deformation of the Inconel 718 alloy is 20 min. The rational holding time should not only have enough time to complete static recovery and recrystallization to fully soften the material, but also effectively control grain size and avoid grain growth.

4 Conclusions

980 ºC, į=163% 950 ºC, į=177%

b Undeformed 1020 ºC, į=266%

980 ºC, į=329% 950 ºC, į=566%

Fig.6

Tensile specimens of Inconel 718 alloy by the strain-reduced superplasticity deformation process: (a) unrefined alloy and (b) the alloy annealed for 3 h

forging, į phase precipitation and recrystallization annealing for 3 h is 566% (Fig.6b). The results show that the elongation of Inconel 718 alloy with the same undeformed microstructure is improved through the appropriate pre-strain and holding time, and deformation temperature, grain size and holding time after pre-strain have important influences on the superplasticity.

1) The fine and uniform structure of Inconel 718 alloy can be obtained through forging, į phase precipitation and recrystallization heat-treatment. The į phase can play a role in controlling grain size during recrystallization and hot deformation. 2) Inconel 718 alloy exhibits a good superplasticity at a wide range of temperature from 950 to 1020 ºC, and the best elongation appears at 950 ºC. 3) The maximum tensile elongation is 566% by using the strain-reduced superplasticity deformation process based maximum m value method at 950 ºC. The excellent superplasticity of Inconel 718 alloy can be obtained with this superplastic deformation process.

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