Study of Anisotropic Material Behavior for Inconel 625 Alloy at Elevated Temperatures

Study of Anisotropic Material Behavior for Inconel 625 Alloy at Elevated Temperatures

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Available online at www.sciencedirect.com

ScienceDirect Materials Today: Proceedings 18 (2019) 2760–2766

www.materialstoday.com/proceedings

ICMPC 2019

Study of Anisotropic Material Behavior for Inconel 625 Alloy at Elevated Temperatures C. Anand Badrisha, Nitin Kotkundea*,Omkar Salunkea, Swadesh Kumar Singhb a

b

Department of Mechanical Engineering, BITS Pilani, Hyderabad, Telangana, 500078, India Department of Mechanical Engineering, Gokaraju Rangaraju Institute of Engineering and Technology, Hyderabad, Telangana, 500090, India

Abstract Deformation behaviour of materials can be precisely analysed by determining anisotropic behaviour and mechanical properties. Hot tensile deformation behaviour of Inconel 625 alloy has been examined from Room Temperature to 6000C with strain rates ranging from 0.1 s-1 to 0.0001s-1 at an interval of 200 0C. At eclectic variety of temperature’s and strain rate various material properties such as yield stress, tensile strength, % total elongation and strain hardening exponent have been carried out and evaluated. Additionally, anisotropic material properties of Inconel 625 alloy mainly; Lankford coefficient (R), normal anisotropy (RN), planer anisotropy (∆R), in-plane anisotropy (AIP) and anisotropic index (δ) have been studied. Influence of temperature and strain rate showed substantial disparity in material properties. From the experimental observation, yield stress, ultimate tensile stress decreases and % elongation increases with increasing the temperature. In-plane anisotropy value is the highest at room temperature, and replicates large variation of tensile yield strengths in three orientation. Presence of low values of anisotropic index indicates very less elongation anisotropy with rise in deformation temperature for Inconel 625 alloys. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization. Keywords: Inconel 625 alloy,tensile flow test, Anisotropic material properties.

1. Introduction In contemporary engineering the main challenge is to increase material capability to withstand various types of mechanical demands in extreme environmental conditions which has let to advent of studying alternative materials. To meet the demands in aerospace, automobile and petrochemical industries [1] assorted alloys with contrasting chemical composition has been evolved and more notability is given to Nickel based alloys to meet applications * Corresponding author. Tel.: +91 9010451444; fax: +91 40 66303998. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization.

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often called super alloys, and among which Inconel 625 alloy can be accentuated. In post weld cracking at elevated temperatures up to 7000C Inconel 625 alloy exemplify exceptional mechanical properties, corrosion resistance with high strength and weldability. Inconel alloy owns superior mechanical properties such as high yield and ultimate tensile strengths, good creep and rupture strengths and high resistance to fatigue and corrosion at elevated temperature. This results in expansive applicability in aerospace, nuclear, marine and chemical industries [2]. In addition to good behavior under extreme environmental conditions, this material exhibits high degree of formability and shows better weldability than many alloyed highly nickel-based alloys. It is also used in functional prototypes and high temperature turbine parts [3]. Narrow forming temperature range, more distortion range and complex microstructure has aided investigation of high temperature flow behavior of Inconel 625 alloy. From literature, it is conveyed that hot deformation behavior of Ni-Fe-Cr superalloys is significantly affected by initial grain texture, the morphology of formed secondary phases, followed heat treatments and process parameters like deformation temperature and strain rate [4]. To assess mechanical properties of materials which incorporate elongation, tensile strength, yield strength and modulus of elasticity can be accomplished by fulfilling tensile testing. Consequently, it is key to fulfill tensile testing to evaluate mechanical and metallurgical phenomena. The directionality of the material properties is defined as Anisotropy (R) and related to the variance of atomic spacing within crystallographic orientations [5]. The flow characteristics of a metal with respect to orientation is mainly dependent on crystal texture i.e. crystal anisotropy. Plastic anisotropy of a rolled sheet metal is more oftenly portray in terms of strain ratio by Lankford coefficient. The mechanical properties of sheet metals vary considerably, depending on the base metal, alloying elements present, processing, heat treatment. The Lankford anisotropy coefficient (r), strain rate coefficient (n), strain rate sensitivity (m) and the yield stress ( ), ultimate tensile strength ( ), ductility has the strongest influences on formability [6-7]. Anisotropy (r) is defined as the directionality of properties and it is combined with the discrepancy of atomic or ionic spacing within crystallographic directions. In sheet metal forming point of view, the sheet texture affects crystal anisotropy and crystal anisotropy is the dependence of flow characteristics of a material with respect to direction [8]. The Lankford anisotropy coefficient depends on the in-plane direction. In orthogonal anisotropy three R-values are determined. These values are denoted as 0, R45, and R90 respectively [9-10]. The objective of study is mainly focused flow stress behavior study at wide range of temperatures Room Temperature to 6000C and strain rates from 0.1 s-1 to 0.0001s-1 and effect of anisotropy on flow stress behaviour and mechanical properties, tensile test specimens have been evaluated. 2. Material and Experimental details To illustrate appropriate elastic and plastic variables associated to mechanical behaviour of materials, tensile test is customary engineering method which is frequently carried out. The tensile test specimen dimensions as per sub sized ASTM E08/E8M-11 standard as shown in Fig.1. In order to discern the effect of anisotropy on flow stress behaviour and mechanical properties, tensile test specimens are organized in Rolling direction, Normal direction and transverse direction as shown in as shown in Fig.1. To obtain high surface finish and least distortion, specimens are prepared by wire cut electric discharging machine.

Fig.1: Tensile test specimen

The test material used in the present experimental work is Inconel 625 alloy sheet of thickness 1 mm with nominal composition (wt. %) as mentioned in Table 1.

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Table 1: Chemical composition of Inconel 625 alloy (wt. %) Element

Ni

Cr

Nb

Mo

Ti

Al

Fe

wt.%

62.495

20.739

3.291

9.489

0.168

0.065

3.314

Element

C

Si

wt.%

0.024

0.103

Mn 0.125

P

S

Co

Cu

0.002

0.003

0.085

-

Tensile test are carried out on Universal Testing machine which is regulated by means of computer, as shown in Fig.2. It has a maximum load capacity of 50 kN and heating capacity of two zone split furnace is from room temperature up to 12000C with ± 3°C accuracy. Samples are first heated to their deformation temperature at 20°C/min, and heat preservation time is 5 minutes in order to obtain a uniform temperature. Three samples were tested in each test setting and average values were reported. High temperature contact type extensometer was used for tensile test experiments. To record load vs displacement data and to transform it into true stress-true strain curve a computer control system is used. To achieve exponential increase of actuator speed and to obtain constant true rates a feedback control system is fortified to Universal testing machine. The experiments were carried out from Room Temperature to 6000C with an interval of 2000C with variable range of slow strain rates from 0.1 s-1 to 0.0001s-1. The true stress vs true strain data is attained from experimental test setup. To implement exponential increase in cross head speed for constant strain rate alteration in software has been taken up.

Fig.2: Universal Testing machine (UTM) of 50kN capacity with zone split heating furnace with different orientations of a sheet.

3. Results & Discussions 3.1 Flow Behavior and Material Properties of Inconel 625 Alloy The stress strain curve is symbolized for dual distinctive background shown in fig. 3(a&b), Initial one shows change in temperature at particular strain rate and another shows alteration of strain rates at room temperature. It is perceived that that temperature has substantial impact on flow stress behavior.Consequence of strain rate is imperceptible in all three rolling directions. It is observed that flow stress is decreasing with increase in temperature and influence of strain rate is insignificant with variation in flow stress. The distinct calculated material properties are cited in Table 2. Furthermore it is predicted that yield stress and ultimate tensile stress is decreasing with increase in temperature. It is found out that there is decrease in yield stress and ultimate stress which is around 45% and 20% and same trend is carried out in all three orientations. As the temperature increases the percentage in

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elongation also increases from 40% to 60%. From all the cases it is observed that yield stress, ultimate tensile stress and elongation is higher in all three orientations as shown in Fig.3.The material becomes more flowable at high temperature due to decrease in yield stress and ultimate tensile strength which is suitable for complex part forming processes. 1000

0.1/s 0.01/s 0.001/s 0.0001/s

1000

800

Stress ϭ (MPa)

True Stress ϭ (MPa)

800

1200 20 ºC 200 ºC 400 ºC 600 ºC

600

400

600

400

200

0 0.0

200

0.1

0.2

0.3

0.4

0.5

0.6

0 0.0

0.7

0.1

0.2

0.3

0.4

0.5

0.6

0.7

True Strain ɛ

True Strain ɛ (a)

(b)

Fig. 3: True stress-strain curve of Inconel 625 alloy (a) with deviation of temperatures at strain rate of 0.001 s-1 in RD (b) with deviation of strain rates at RT in RD 1000

Yield Stress (MPa)

450

400

RD ND TD

950

Ultimate Stress (MPa)

RD ND TD

500

900 850 800 750 700 650

350

600

0

100

200

300

400

500

600

0

100

200

Temperature ( C)

400

500

600

700

(b)

(a) 60

0.75

Strength Hardening Exponent n

RD ND TD

55

% elongation

300

Temperature (0C)

0

50

45

40

RD ND TD

0.70 0.65

0.60

0.55 0.50

35 0.45

30

z

0

100

200

300

400

Temperature (0C) (c)

500

600

0

100

200

300

400

500

600

Temperature (0C)

(d)

Fig. 4: Distinct material properties of Inconel 625 alloy at strain rate of 0.001 s-1: (a) Yield stress (σy) (b) Ultimate stress (σut) (c) Elongation (%) (d) Strength hardening exponent (n)

The dependence of the flow stress on the level of strain is determined as Strain-hardening coefficient (n). The flow stress increases rapidly with strain when material is having high n value and promotes more strain to regions with lower strain and flow stress. Indication of good workability of the material is found when n value is high, which intern leads to drastic contrast between yield strength and ultimate tensile strength. The calculation of n value is depend on the Hollomon power law. It has been found out that n value is largely reliant on temperature and strain rate for many structural materials. Fig. 4 (a-d) shows the deviation of n value with respect to temperature. From Fig.

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4(d) and Table 2 it is perceived that increase in n value increases temperature and identical trend has been extended in all three orientations. In present scenario all RD values are marginally higher when compared with other two directions of the sheet. It has been concluded that workability of the Inconel 625 alloy enhances at higher temperature with slow strain rate based on above diversified material properties. Table 2: Material properties of Inconel 625 alloy at assorted temperatures Temp. (0C) RT 200 400 600

Orientation RD ND TD RD ND TD RD ND TD RD ND TD

Yield Strength (MPa) 812.07 790.64 704.06 691.73 648.42 604.29 609.50 641.39 668.25 573.67 547.43 514.23

Ultimate tensile strength (MPa)

Elongation (%)

Strain hardening exponent (n)

978.55 931.91 902.23 852.79 834.20 845.29 805.18 793.42 762.39 721.12 690.58 663.76

39.65 37.22 35.67 42.99 45.05 43.60 46.57 47.32 45.29 50.99 52.14 54.86

0.568 0.542 0.518 0.681 0.685 0.693 0.738 0.821 0.872 0.686 0.693 0.742

3.2. Anisotropic properties of Inconel 625 Alloy. The directionality of the material properties is defined as Anisotropy (R) and related to the variance of atomic spacing within crystallographic orientations. In sheet metal forming point of view, the sheet texture alter crystal anisotropy, which is further depended on flow characteristics of a material with respect to direction. Plastic anisotropy of a rolled sheet metal is typically characterized in terms of strain ratio by Lankford coefficient (R). Lankford coefficient is a measure of anisotropy and it is also called as the ‘resistance to thickness change’. It is mathematically represented based on width plastic strain , to thickness plastic strain of rolled sheet specimen. (1) = Normal anisotropy (RN) is the main influencing parameter of the maximum drawability of sheet. A material with a high RN value can be experience less thinning during a deep drawing operation than a material having a smaller RN value, provided that their flow characteristics are identical. A high RN value allows deeper parts to be drawn and in shallow, smoothly contoured parts a high value may reduce the chance of wrinkling or ripples in the part. Therefore for a deep drawing operation, a suitable material must have an RN value, the average or normal anisotropy ( N) value along the three rolling direction is given by =

(2)

Where, R0, R45 & R90 are plastic strain ratio along RD, ND & TD orientations to rolling direction respectively. It is observed from Table 3 that normal anisotropy variation is inconsistent with increase in temperature. The variation is negligible with increase in temperature. It indicate that deep drawability may not be improved at higher temperatures.

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Table 3: Anisotropic properties of Inconel 625 alloys at different test temperatures Temperature °C

Lankford coefficient

Normal anisotropy (RN)

Planer anisotropy (ΔR)

In-plane Anisotropy ( )

Anisotropy index (δ)

(R0)

(R45)

(R90)

RT

0.8515

0.9686

0.9230

0.2585

0.0696

0.0433

1000C

0.4980

0.1948

0.7490 1 1. 1655

0.8848

-0.0297

0.0609

0.0378

2000C

0.8256

0.8518

1.3906

0.9800

-0.0979

0.0541

0.0337

3000C

0.6808

0.7916

1.4011

0.9163

-0.1194

0.0496

0.0324

4000C

1.0869

0.6612

0.7043

0.9784

-0.1061

0.0409

0.0314

5000C

1.7148

0.7997

0.4318

0.9365

-0.0559

0.0341

0.0303

6000C

1.2510

0.9394

0.9861

1.8038

-0.0212

0.0295

0.0275

Fig.5 (a & b) shows in-plane anisotropy (AIP) and anisotropic index ( ) of Inconel 625 alloys varying with test temperature. The AIP and value is highest at room temperature which replicates large variation of tensile yield and ultimate strength in three direction. As yield strength is also influenced by the texture in material, the decrease inplane anisotropy will be definitely observed with rise in temperature. As deformation progresses, texture will be reformed by sliding of grain boundary and grains rotation. This is the main reason for the reduction of in-plane anisotropy (AIP) and anisotropic index at a high deformation temperature.

(a) (b) Fig.5: Effect of deformation temperature on (a) in-plane anisotropy (AIP) (b) anisotropic index ( ) The planer anisotropy (ΔR) is assert by the difference of strain ratio in three orientation of sheet which is responsible for formation of ears in the drawn cups and uneven thinning. If the magnitude of the planar anisotropy parameter is large, the orientation of the sheet with respect to the die or the part to be formed will be important. In such cases, asymmetric forming and earing will be observed. As the magnitude of the value increases, the ear heights increase. Therefore for deep drawing operations, suitable materials must have smaller Δr value it is expressed as per Eq.3. ∆

=

(3)

The other alternative parameter such as in-plane anisotropy (AIP) and anisotropic index Anisotropy is very sensitive to measure along thickness direction in thin sheets. This behavior of metal properties is called as inplane anisotropy (AIP). It is mathematically represented based on yield strength ( ) in different rolling direction as =

(4)

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where, is tensile yield strength at 00 orientation to rolling direction, is tensile yield strength at 450 orientation 0 to rolling direction, and is tensile yield strength at 90 orientation to rolling direction. For isotropic metal, AIP = 0, as = = . Increase in the values of AIP, indicates increase in extent of anisotropic nature the third observation for the anisotropic nature of metal is anisotropic index to calculate degree of anisotropy by % elongation in 00 to rolling direction and 900 to rolling direction, represented as =

(%

)

(%

)

(%

)

(%

)

0≤

<1

(5)

Where, (% ) is % elongation at 00 orientation to rolling direction, and (% ) is % elongation at 900 orientation to rolling direction. Decrease in anisotropic index δ is observed with increase in deformation temperature for Inconel 625 alloys. Presence of low values of anisotropic index δ indicates very less elongation anisotropy with rise in deformation temperature. 4. Conclusion The current study is associated with material and anisotropic properties of Inconel 625 alloy at exalted temperatures. Variation of temperature influences Flow stress behavior of Inconel 625 alloy significantly than strain rate variation. Yield stress and ultimate tensile is highly influenced with increase in temperature. Consequential increase in % total elongation and strain hardening exponent establish better workability at higher temperature. Inconel 625 alloys exhibit anisotropic behavior with high in-plane anisotropy (AIP) and Anisotropic index δ values at room temperature with significant decrease with rise in temperature. Acknowledgement The financial support received for this research work from Science and Engineering Research Board (SERB – DST ECR) Government of India, ECR/2016/001402 is gratefully acknowledged. References [1] R.G.Narayanan and U.S Dixit, Metal Forming Technology and process Modelling, McGraw Hill Education (India) Pvt. Ltd(2013). [2] R.C.Reed, The Superalloys Fundamentals and Applications, Cambridge University Press, (2006). [3] C.Madu and Sodeinde, Development trends and applications of advanced engineering materials O.A (2004). [4] D. R. Roamer and C J Tyne, The Minerals, Metals & Material Society, 1997 [5] B Plunkett,O Cazacu, F Barlat,International journal of plasticity,24 (5),(2008). [6] D. R. Roamer, C J Tyne, The Minerals, Metals & Materials Society, 1997. [7] R.C.Hall, Journal of Basic Engineering, ASM, (1967) 511-516. [8] A. Thomas, M. El-Wahabi , J.M. Cabrera, J.M. Prado, J. Mat. Pro. Tech. 177, (2006) 469–472. [9] S.S.Kumar, S.Raghu, Bhattacharjee, P.Prasad, A.Rao, Borah,J. Mat. Sci.: Mat. in Electronics, 50 (19), (2015) 6444-6456,ISSN 0022-2461. 10] K. S.Prasad, S. K. Panda, S. K. Kar, M. Sen, S. V. S. N. Murty, S. C. Sharma, J. Mat. Eng. Perf. 26:4, (2017) 1513–1530.