Flow forming and heat-treatment of Inconel 718 cylinders

Accepted Manuscript Title: Flow forming and heat-treatment of Inconel 718 cylinders Authors: P. Maj, P. Błyskun, S. Kut, B. Romelczyk-Baishya, T. Mrugała, B. Adamczyk, J. Mizera PII: DOI: Reference:

S0924-0136(17)30513-7 https://doi.org/10.1016/j.jmatprotec.2017.11.010 PROTEC 15484

To appear in:

Journal of Materials Processing Technology

Received date: Revised date: Accepted date:

2-2-2017 30-10-2017 4-11-2017

Please cite this article as: Maj, P., Błyskun, P., Kut, S., RomelczykBaishya, B., Mrugała, T., Adamczyk, B., Mizera, J., Flow forming and heattreatment of Inconel 718 cylinders.Journal of Materials Processing Technology https://doi.org/10.1016/j.jmatprotec.2017.11.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Flow forming and heat-treatment of Inconel 718 cylinders P. Maj1. P.Błyskun1, S. Kut2, B.Romelczyk-Baishya1, T. Mrugała3, B.Adamczyk1 J.Mizera1 *Corresponding author. Tel.: +48 849 67 15; [email protected] 1

Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141,

02-507 Warsaw, Poland 2

Faculty of Mechanical Engineering and Aeronautics, Rzeszow University of Technology,

al. Powstancow Warszawy 12, 35-959, Rzeszow, Poland 3

Pratt & Whitney Rzeszow S.A. Hetmańska 120 35-078 Rzeszow, Poland

Abstract Flow forming is a metal processing technique that enables the fabrication of hollow axisymmetric parts with a good surface finish and high mechanical properties. Furthermore, the process enables flexible manufacturing of axisymmetric parts in one process step. In the last 20 years, the method has been significantly improved by CNC controls and stiffer constructions with advanced tooling.

The technique seems very perspective from the viewpoint of the aviation industry due

to its numerous advantages. The aim of the current study has been to analyze the mechanical properties of Inconel 718 after the flow forming process and heat treatment. Four hollow axisymmetric barrel shaped elements were manufactured using a SFC 800 V500 machine. The next step involved a standard aging heat treatment. That was followed by the materials being characterized for quality assessment. Mechanical tests and microstructure analysis were carried out before and after the heat treatment in order to achieve this. Additional profilometer scans were done to assess the surface finish of the obtained parts. The obtained material had

mechanical properties exceeding those obtained by conventional processing. Cold formed elements had very high strength even up to 1600 MPa but very low elongation of up to 4%. The ultimate tensile strength decreased to 1500 MPa but the elongation significantly increased up to 20% by additional heat treatment. Furthermore, the surface parameters were at the level of fine sanding quality (Ra >3.2 μm). Additional tests were undertaken to identify the microstructure and surface parameters. The results prove that the strengthening effect is caused mainly by very fine γ’’ and δ precipitates which are evenly distributed throughout the material. This on the other hand was as a result of precipitates nucleation on the dislocations induced during cold metal forming.

Keywords: Metal spinning, Inconel 718, flow forming, heat-treatment

1. Introduction

Metal spinning is a versatile forming method that can be used to form complex axisymmetric shapes. The principle of the method involves rotation of the spindle and deformation by a roller that exerts pressure on the formed blank. In its basic form metal spinning can be done relatively easy even with basic equipment, although only thin sheets of deformable metals can be processed. The method is now booming thanks to numerous advancements like CNC controls, advanced tooling and stiffer constructions (Wong et al., 2003). Additionally numerous modifications of the method can be used to enhance its forming possibilities even further, for

example Mori et al., (2009) used laser assisted forming process, Xia et al., (2011) nonaxisymmetric forming, and JIANG et al., (2009) ) used lateral and longitudinal tooth spinning. According to Music et al., (2010) there are three different processes that employ using metal spinning: conventional metal spinning, shear forming, and flow forming (Fig. 1). Despite similar kinetics there are significant differences that distinguish them from one another. In case of the first two, the starting material is a metal sheet. In case of flow forming, it is a tube or a barrel cylinder. Furthermore, in contrast during the conventional spinning, the thickness remains almost constant along the radius, while flow forming is used mainly to reduce the thickness of the cylinder. In this technique, the roller moves along the contours given by the element geometry. It is worth noting that the process enables very high strains of even up to 90% reported by Parsa et al., (2008). This can be especially important when combined with proper heat treatment resulting in precipitation hardening. According to Cepeda-Jiménez et al., (2011) the induced strain can strongly influence the shape and distribution of precipitates which influence the mechanical properties of the material.

Fig. 1 Three groups of metal spinning processes according to Music et al., (2010) Inconel 718 (Table 1) is a super alloy that is widely used in the aviation industry and power generators. It has very good mechanical properties of up to 600 °C, good welding properties and corrosion resistance. The high strength of the material is a result of precipitation strengthening of two phases i.e. γ’ and γ’’. The second one has a particularly significant impact on the properties of the material. The γ’’ (Ni3Nb) is a body-centered tetragonal metastable phase. It forms small disk-shaped precipitates along the main crystallographic planes (010), (001) and (100), evenly distributed in the whole volume of the material. According to Slama et al., (1997) the high strength of the γ’’ is a result of semi-coherence with the matrix and nano-twinning of the particles below the size of 100 nm. After longer exposure time in the temperature range of 750-850 °C the precipitate tends to cluster and transform into orthorhombic δ-phase. It forms needle-like

precipitates along the grain-boundaries causing reduction in elongation and trans-granular cracking reported by Zhang et al., (2011). According to Wei et al., (2014) strain has a significant effect on the nucleation and growth of δ phase in the material. Furthermore Mei et al., (2015) suggested that it is possible to enhance the mechanical properties of the material by combining cold rolling and heat treatment of Inconel 718. However, it should be noted that there is an upper limit of strain above which a drop of hardness can be observed. Table 1 Chemical Composition of Inconel 718 Ni

Cr

Fe

Nb

Mo

Ti

Al

Co

Mn

C

Si

P

Chemical composition [wt. %] Max 55.00 17.00 rest

4.75

2.80

0.65

0.20

1.00

0.35

0.08

0.35

0.015

Min

5.50

3.30

1.15

0.80

-

-

-

-

-

50.00 21.00

The study done in the current research had two major purposes. Firstly, to analyze the flow forming properties of Inconel 718 from the point of view of future applications - mostly aerospace industry requirements. The second was to analyze the influence of total strain on the microstructure and mechanical properties of heat treated flow formed elements.

2. Experimental The studied material was Inconel 718 obtained according to AMS 5596. Metal sheets were initially pressed to form cylinders according to the geometry shown in Fig. 2. Afterwards they were subjected to flow forming (Fig. 3) with 4 different thickness reductions: 35%, 40%, 55% and 65%. It is worth noting that the geometry of the rollers is not standard. The general idea of the conducted project was to combine three metal working techniques, conventional metal spinning, shear and flow forming. A compromise was established between the roller geometry in

order to avoid tool rearming of rollers. Additionally, the manuscript machine is a prototype model with built in laser that can be used for heating of the work piece. Special heat resistant materials were used which significantly increased the cost of production of additional rollers. In the current experiment, standard aging heat treatment was carried out on the material after the metal forming process. Mechanical tests and microstructure studies were done for selected specimens.

Fig. 2 The geometry of the prefabricate

Fig. 3 Flow forming process schematics

Tensile tests were carried out using mini-samples with gauge lengths of 10 mm. tensile tests were conducted using a Zwick/Roell 005 testing machine at an initial strain rate of 10-3 1/s. The tests were carried out at room temperature. For precise elongation measurement, optical non-contact displacement measurements known as the Digital Image Correlation (DIC) technique was used in the experiment according to guidelines made by Molak et al., (2007). The cylinders were tested in two radial and circumferential directions in order to characterize possible anisotropy that occurred during the forming process. Engineering stress-strain curves were analyzed for all tested samples. The dimension of the mini-samples used in the experiment is shown in Fig. 4. The samples were extracted from locations where the reduction of the thickness was highest. They had a small curvature due to the cylindrical shape. Mini-tensile samples were used in order to obtain reliable results, which reduced the impact of the geometry. Furthermore, they were grinded by sand paper to smooth out any bulges. Additionally, when analyzing the displacement field, no stress concentration was observed by DIC. Lastly, the samples cracked mostly in the middle section, which is a correct result in terms of uniaxial deformation. Supplementary hardness tests for the heat-treated samples were carried out using the Vickers method, with a load of 20 kgf and 15s loading time.

a)

b)

Fig. 4. Mini-tensile test equipment a) sample holder and b) dimension of the specimens used in the experiment

Additional transmission electron microscopy (TEM) characterization was carried out in a similar manner using a JEOL JEM 1200 EX 2 microscope, with an accelerating voltage of 120 kV. The samples (100 µm thick disks with a diameter of 3mm) were cut from heat treated sheets of Inconel 718 using wire electro-discharge machining (WEDM). The foils were next electropolished using A8 electrolyte provided by Struers.

A profilometer was used in order to analyze the quality of the cylinder surface WYKO NT 9300. The measurements are done based on white light interferometry which enables the extraction of information from the superposition of waves. Vertical Scanning Interferometry was utilized for the characterization of the cylinders surface.

3. Results

The flow formed cylinders are shown in Fig. 5. The samples vary in height due to different thickness reductions, although the initial prefabricate for all of them was the same (Fig. 5). After that the cylinder was aged at 750˚C for 8 hrs. No cracks or any defects were visible. Furthermore, the obtained surface was smooth with a metallic sheen. The area marked with the red rectangle has been subjected to further surface analysis with the optical profilometer.

Fig. 5 Four cylinders obtained and used in the experiment The material was next studied in terms of mechanical properties. Sample results are shown in Fig. 6 to illustrate the importance of the heat-treatment. The results are summarized in Table 2. As can be seen the cold formed elements, mechanical properties are quite similar in both the radial and circumferential direction. The elongation is very low - below 2.5 %. The heat treatment did not cause a significant change of Ultimate Tensile Strength (UTS) - about 1500 MPa, however a significant decrease of Yield Strength (YS) (from 1350 MPa to 1000 MPa) and increase can be seen in terms of plasticity which has a nearly constant value of 20% for all deformed samples. It is worth noting that the circumferential direction of cut samples had slightly

higher UTS but lower YS than in the radial. In the opinion of the authors such results may be caused by precipitation segregation along the radial direction. A reference sample of the nondeformed location was tested and it showed significantly lower strength in comparison to other tested samples. However, its strength was clearly improved after the heat treatment but it was significantly lower than in the deformed material. Overall this suggests that the change in mechanical properties is a combined effect of deformation and heat treatment.

Fig. 6 Sample tensile results– engineering stress-strain curves

Table 2 Tensile test results summary

Cold reduction

Direction**

0

Ref.*

Cold formed material UTS YS Elongation at [MPa] [MPa] break [%] 904 412 45

35

Cir.

1440

1372

2.1

1518

800

26

Rad.

1379

1340

3.5

1469

894

26

Cir.

1483

1354

2.8

1534

861

28

Rad.

1446

1361

2.8

1457

1040

28

Cir.

1512

1364

1.9

1502

900

26

Rad.

1492

1352

2.5

1431

912

25

Cir.

1602

1412

2.1

1542

973

21

Rad.

1456

1399

1.5

1457

1062

25

40

55

65

After heat treatment UTS YS Elongation at [MPa] [MPa] break [%] 1023 760 29

*The initial state of the material ** Direction of the sample that were cut from the cylinder

In order to analyze individual elements hardness tests were carried out along the radial crosssection in 10 mm intervals (Table 3). The measurements were done to determine the homogeneity of the obtained cylinders. Additionally, reference tests were done for the initial state (before deep drawing according to AMS 5596) material and after the aging process. The spread of results was significant especially in case of the third specimen (55% thickness reduction). Furthermore, the values started to drop after achieving 40% reduction of the cross-section. In case of most deformed sample the hardness was only of 395 which is the lowest value for all the tested cylinders. It is interesting that large differences between strength and hardness trends were

observed. Different macro and micro mechanism of deformation should be considered. The explanation of that phenomenon might come from the microstructure of the samples.

Table 3 Hardness of the heat-treated samples and reference data of the initial state Element

Hardness [HV10] Spread of values

Initial state (AMS 5596) 195

3

Heat treatment only

280

5

35%

441

10

40%

453

8

55%

420

23

65%

395

15

Microstructural tests were done to analyze the changes that occurred during the heat treatment. As can be seen in Fig.7 needle like particles (later identified by SAED TEM as δ phase) are visible in all specimens except for the initial state. The induced strain caused the precipitates to occur in the whole volume of the material not only on the grain boundaries as in the case of nondeformed specimen. Furthermore, upon a more detailed analysis it can be seen that their size decrease with strain. Both 35% and 40 % higher concentration of precipitates can be seen near the outline of grain boundaries. However, at higher deformations very fine evenly distributed precipitates can be seen in the whole volume of the material.

a)

b)

c)

d)

e)

f)

Fig. 7 Microstructure of Inconel 718 a) initial state and after heat treatment b) non-deformed sample and with a cross section reduction of: c) 35%, d) 40%, e)55%, f)65%

In order to better understand the microstructure of the tested TEM material – the bright field technique was used (Fig. 8). The tests were done for the sample with the highest deformation before and after the heat treatment. As can be seen in Fig. 8a the material had a dense network of dislocations after the flow forming process. Large clusters of dislocations were seen in the vicinity of grain boundaries and individual precipitates. However, the dislocations were not seen after the aging heat treatment (Fig. 8b). Instead, the microstructure consisted of small δ needles and fine γ’’ equally distributed throughout the material. The Selected Area Diffractions (SAED) prove the existence of γ + γ’’ superstructure. Most probably the dislocations seen in the deformed material were the nucleation sites of precipitates. This was beneficial in terms of refinement and homogeneity of the structure and caused a significant increase in strength. However, it should be noted that in the vicinity of small δ precipitates the density of γ’’ decrease was observed which may have an effect on the hardness which is lower for samples after a 55% and 65% cross section reduction. a)

b)

d)

c)

Fig 8. TEM micrograph results a) cold deformed element, b) after heat treatment with additional c) SAED images and d) an reference indexed pattern In order to assess the surface parameters profilometer tests were carried out for the sample with the highest deformation due to the most visible bands being created on its surface with the highest thickness reduction (Fig. 5). Periodical circumferential dents can be seen in Fig. 9 caused by the roller and their depth was of 10 μm. The Ra parameter was about 3.2 μm which is a standard value for cold rolling and drawing. Furthermore, according to Bewlay (2006) it is possible to obtain values of Ra up to 0.5 μm by proper parameters selection accordingly.

a)

b)

Fig. 9 The flow formed cylinder a) surface view and b) the topography along the middle of the specimen

4. Discussion The main aim of the current study was to analyze the mechanical properties of the obtained cylinders that meet the requirements of aeronautical applications. Elefeterie et al. (Elefterie et al., 2017) outlined the critical values from the tensile tests in Table 4. Table 4 The requirements for Inconel 718 in aeronautical applications according to Elefeterie et al. Reference values

Current results (room temperature)

Room temperature At 650˚C After FF*

After FF+ HT**

UTS [MPa]

1350

1000

1440-1602

1413-1542

YS [MPa]

1100 MPa

860

1354-1412

800-1062

Elongation at break [%]

12

12

1.5-3.5

21-28

Reduction of cross section [%] 8

8

0.7-1.2

15-21

FF – Flow forming process FF + HT – Flow forming and subsequent heat treatment According to the current tests, Inconel 718 had exceptionally high strength after flow forming (up to 1600 MPa) although it lacked the plasticity that is necessary in demanding applications (below 2% elongation to break). Standard aging heat treatment was carried out to improve the parameters. As a result, slightly lower strength (up to 1540 MPa) was obtained. However, a 20% elongation was achieved compared to the critical value of 12% as a control sample a nondeformed fragment of the cylinder from the top wall was prepared. As it turned out much smaller values of strength (1050 MPa) were obtained. This proves that the obtained properties are as a result of both heat treatment and flow forming process. Comparing the maximum values (c.a. 1450 MPa) provided by the manufacturer Special Metals Corporation (2007), for thin aged Inconel 718, the material obtained in the current research had a maximum UTS value of 1540 MPa. In terms of YS the material directly possesses supreme strength after cold forming. It is over three times higher than in the initial state (1400 MPa vs 400 MPa) most probably due to the work hardening and the increase concentration of dislocations. What is worth noting is that the values are very similar to each other and are independent from the reduction. The range of YS between individual samples is from 1340 to 1410 MPa. This may be the limit of stress that can be induced during the flow forming process. After the heat treatment, a significant decrease of YS can be observed. However, in this case an increase with strain can be observed ranging from 800 MPa to 1062 MPa. Additionally, the tensile samples cut in the radial direction have approximately 50 MPa higher YS than in the circumferential. This may be explained by fine deformation bands creation which was observed using OM in Fig. 6. The precipitates line along

the radial direction which introduces the spread of values in both directions. What is worth noting is that the hardness values do not coincide with the UTS which is proposed in ASTM e140. The second relation was observed in Inconel 718 after cold rolling Mei et al., (2015) observed that hardness reaches a maximum value of 454 for a 50% thickness reduction. Similar in the current research show that the highest hardness value was obtained for 40% thickness reduction. On the other hand, the lowest value (350 HV) was observed for the highest deformation - 65%. Microstructure analysis was performed in order to explain this phenomenon. Alongside the mechanical testing, a microstructural study was conducted to investigate the main structural features responsible for the observed properties. The control samples had needle shaped precipitates along the grain boundaries whereas the deformed sample had much finer particles evenly distributed throughout the whole volume. The fragmentation of the microstructure increased with the thickness reduction. However, it should be noted that for samples with 35% and 40% reduction, the grain boundaries are still distinguishable whereas for higher reduction values the microstructure became too fine to indicate them. These results were supplemented by TEM observations which provided further insights into the microstructure of the studied material. It is commonly known that dislocations and grain boundaries were the main nucleation sites due to their low activation energy. In the case of the cold formed cylinder, the material had very dense network of dislocations that clustered throughout the volume. After the aging process, small precipitates were seen in place of dislocation clusters in large quantities. Comparing the obtained results with the reference data obtained by Zhang et al., (2010) it should be noted that a saturation of γ’’ and δ can be achieved depending on the imposed strain. Additionally, according to Liu et al., (1998) cold rolling increases the ratio of δ to γ’’. This suggests that the very high deformations in flow forming change the aging kinetics and nucleation. Very fine microstructure

of the precipitates can be seen as a result of heat treatment. It should be noted that despite the high amount of δ phase the tested specimens had very high strength and maximum elongation. This is attributed to the fragmentation of the microstructure evenly in the whole volume of the material. Microstructure study revealed also a preferred direction of precipitate orientation which caused the difference in mechanical properties in the radial and circumferential direction especially for the two most deformed samples. In the author’s opinion, the observed anisotropy is also the cause of differences between UTS and the hardness which drops above the cross-section reduction of 45%.

5. Conclusions The main aim of the research was to analyze mechanical properties and microstructure of Inconel 718 obtained by flow forming metalworking technique and subsequent heat treatment. As it turned out, the forming method strongly influences the microstructure and precipitation process. The main conclusions that can be drawn from the conducted research are as follows: (1) As a result of the metalworking process very high strains were introduced into the material. This caused an increase of UTS and YS after cold forming respectively to 1500 and 1350 MPa and a decrease of elongation to ca 2.5 % depending on the cross-section reduction. Heat treatment of the material changed only UTS slightly (up to 50 MPa) and decreased the YS over 300-400 MPa; on the other hand the elongation was about ten times higher than in the cold formed material. According to Elefterie et al. (Elefterie et al., 2017) the obtained mechanical properties mostly meet the aeronautical requirements for Inconel 718 alloy. It should be noted that the YS should be increased to achieve 1100 MPa after heat treatment (in the current research it is 1050 MPa). (2) The induced strain strongly influenced the precipitation process after the heat treatment of the material. Very fine γ’’ and δ precipitates in their entirety were observed along the radial direction which resulted in the overall high strength of the material. The refinement of the microstructure was higher with cross-section reduction.

Acknowledgments Financial support of the National Center for Research and Development in the program INNOLOT CASELOT INNOLOT/I/9/NCBR/2013 is gratefully acknowledged.

6. References Bewlay, B.P., 2006. Spinning. ASM. Cepeda-Jiménez, C.M., García-Infanta, J.M., Zhilyaev, A.P., Ruano, O.A., Carreño, F., 2011. Influence of the thermal treatment on the deformation-induced precipitation of a hypoeutectic Al-7 wt% Si casting alloy deformed by high-pressure torsion. J. Alloys Compd. 509, 636–643. doi:10.1016/j.jallcom.2010.09.122 Elefterie, C.F., Guragata, C., Bran, D., Ghiban, B., 2017. Aeronautical requirements for Inconel 718 alloy. IOP Conf. Ser. Mater. Sci. Eng. 209, 12060. doi:10.1088/1757899X/209/1/012060 Jiang, S. Y., Zheng, Y. F., Ren, Z.Yi., Li, C.F., 2009. Multi-pass spinning of thin-walled tubular part with longitudinal inner ribs. Trans. Nonferrous Met. Soc. China (English Ed. 19, 215– 221. doi:10.1016/S1003-6326(08)60255-1 Liu, W.C., Xiao, F.R., Yao, M., Yuan, H., 1998. Influence of cold rolling on the precipitation kinetics of gamma double prime and delta phases in Inconel 718 alloy. October 7, 245–247. Mei, Y., Liu, Y., Liu, C., Li, C., Yu, L., Guo, Q., Li, H., 2015. Effects of cold rolling on the precipitation kinetics and the morphology evolution of intermediate phases in Inconel 718

alloy. J. Alloys Compd. 649, 949–960. doi:10.1016/j.jallcom.2015.07.149 Molak, R.M., Kartal, M., Pakiela, Z., Manaj, W., Turski, M., Hiller, S., Gungor, S., Edwards, L., Kurzydlowski, K.J., 2007. Use of micro tensile test samples in determining the remnant life of pressure vessel steels. Appl. Mech. Mater. Music, O., Allwood, J.M., Kawai, K., 2010. A review of the mechanics of metal spinning. J. Mater. Process. Technol. 210, 3–23. doi:10.1016/j.jmatprotec.2009.08.021 Parsa, M.H., Pazooki, a. M. a., Nili Ahmadabadi, M., 2008. Flow-forming and flow formability simulation. Int. J. Adv. Manuf. Technol. 42, 463–473. doi:10.1007/s00170-008-1624-0 Slama, C., Servant, C., Cizeron, G., 1997. Aging of the Inconel 718 alloy between 500 and 750 °C. J. Mater. Res. 12, 2298–2316. doi:10.1557/JMR.1997.0306 Special Metals Corporation. Inconel 718 datasheet. www.specialmetals.com Wei, X., Zheng, W., Song, Z., Lei, T., Yong, Q., Xie, Q., 2014. Strain-induced Precipitation Behavior of δ Phase in Inconel 718 Alloy. J. Iron Steel Res. Int. 21, 375–381. doi:10.1016/S1006-706X(14)60058-3 Wong, C.C., Dean, T. a., Lin, J., 2003. A review of spinning, shear forming and flow forming processes. Int. J. Mach. Tools Manuf. 43, 1419–1435. doi:10.1016/S0890-6955(03)00172-X Xia, Q., Cheng, X., Long, H., Ruan, F., 2011. Finite element analysis and experimental investigation on deformation mechanism of non-axisymmetric tube spinning. Int. J. Adv. Manuf. Technol. 59, 263–272. doi:10.1007/s00170-011-3494-0 Zhang, H.Y., Zhang, S.H., Cheng, M., Li, Z.X., 2010. Deformation characteristics of δ phase in

the delta-processed Inconel 718 alloy. Mater. Charact. 61, 49–53. doi:10.1016/j.matchar.2009.10.003 Zhang, S.-H., Zhang, H.-Y., Cheng, M., 2011. Tensile deformation and fracture characteristics of delta-processed Inconel 718 alloy at elevated temperature. Mater. Sci. Eng. A 528, 6253– 6258. doi:10.1016/j.msea.2011.04.074