Influence of burnishing process on surface integrity of inconel 718 fabricated by laser powder bed

Abstract

Keywords: Additive Manufacturing; Surface Integrity; Roller Burnishing

Introduction

Inconel 718 is a material found in the group of nickel-chromium-based super alloys, with high yield and tensile strength, high creep resistance, high oxidation and corrosion resistance and it shows good behavior against extremely demanding conditions such as high pressure and temperature [1-4]. Due to these properties, Inconel 718 is

widely used in aerospace, defense industry, automotive and energy sectors, etc. [5, 6]. Machining, which is one of the traditional manufacturing methods, is widely used in the shaping of Inconel 718. However, machining of this alloy seems difficult because it causes micro-welding and stacked edge formation at the tooltip and chip interface [7]. It is also classified as “hard-to-cut materials” [8]. This situation

*Corresponding author. *E-mail address: amamertkaya@gmail.com This paper was recommended for publication in revised form by Regional Editor in Cheif Ahmet Selim Dalkilic Published by Yıldız Technical University Press, İstanbul, Turkey Copyright 2021, Yıldız Technical University. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

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paved the way for the additive manufacturing method, which has recently become widespread and is widely used in many industries. Additive manufacturing has a unique advantage such as high geometric complexity, reduction of production steps, low material consumption, reduction of lead time, and low production cost [9-11]. LPBF, one of the additive manufacturing methods, is widely used in the production of Inconel 718 [12]. But it also has some disadvantages like surface roughness, tensile residual stress, porosity, and cracks [13-15]. First of all, it is aimed to eliminate or reduce these disadvantages with production parameter optimization. Even in productions carried out at optimum parameters, surface quality, and mechanical properties may not be at the desired level. This situation makes post-processing necessary [16]. Post-processes are selected according to the material used, the desired mechanical properties, and surface quality. Roller burnishing is an easy-to-use, cost-effective post-process that creates compressive residual stress on the part surface as well as increasing the surface quality [17]. In the literature, Raaj et al. [18] applied low plasticity burnishing post-processing at different pressures (10, 20, 30, 40 MPa) after grinding to Inconel 718 material produced by electron beam additive manufacturing. They reported that at the highest pressure, the surface hardness increased by 25%, the porosity decreased by 89%, the surface roughness decreased by 99%, and a compressive residual stress of 1300 MPa occurred on the surface. Yaman et al. [19], we applied a roller burnishing process with two burnishing conditions (dry, air), with two burnishing forces (250, 750 N) and two burnishing speeds (350, 1050 RPM) to as built, sandblasted, and heat treated Inconel 718 parts manufactured with LPBF. As a result, the lowest surface roughness was achieved after sandblasting + roller burnishing post-processing with a decrease of 98%, and the highest wear resistance on the surface was 55% after heat treatment + roller burnishing. Shinoda et al. [20] reported that after preheating at 200°C on the Inconel 718 part produced with LPBF, the creep life of the part increased by 2.09 times and the lower creep elongation increased by 1.4%. Yıldırım et al. [21] turned Inconel 625 in different cutting environments such as dry, minimum amount of lubrication, nanofluids, cryogenic cooling, and combinations of these cooling methods. They reported that the best results were cryogenic with nano-fluid technique to improve turning performance in the results evaluated according to tool-chip interface temperature, surface roughness, and tool wear. In this study, roller burnishing was applied to Inconel 718 parts fabricated by LPBF under different conditions after turning. The roller burnishing process was performed at 300 N, 600 N, and 900 N burnishing forces under dry, preheated at 200°C and cryogenic burnishing conditions. The effects of these conditions on the surface roughness, microhardness, and microstructure were examined and XRD measurements were presented. Also, it is aimed to reveal the effect of severe plastic deformation on surface integrity, which occurs because of roller burnishing under

different conditions applied to Inconel 718 parts produced with LPBF.

Materials And Methods

Material and Material Manufacturing Parameters In this study, Ø20 x 80 mm Inconel 718 nickel-based super alloy cylinders produced by LPBF process on ENAVISION 250 3D Additive Manufacturing Machine were used. Process parameters are presented in Table 1. In this machine, these production parameters are the optimum production parameters determined for Inconel 718 [19, 22].

Table 1. Parameter set used in the manufacturing of specimens by LPBF method Laser Spot Diameter, d (μm)

CNC Turning, Roller Burnishing, and Heat Treatment Processes The cylinder specimens were machined on a Doosan Puma GT2100 CNC turning center before the roller burnishing process was applied. DNMG 11 04 04-MF 1105 coated carbide tools used in machining and the cutting speed was 60 m/min, the depth of cut (αp) 0.2 mm, feed rate (f) 0.1 mm/rev, under dry burnishing condition. After turning, in the turning direction, the roller burnishing process was applied under three different burnishing forces and three burnishing conditions. The parts were heated to 200°C with using a 7-step programable furnace capable of reaching high temperatures (0-1200°C), and 5000 W power (23 A/Phase) under the atmosphere. Then they were roller burnished with 300 N, 600 N, and 900 N burnishing forces (Figure 1(a).). The same procedures were repeated for the dry parts at room temperature. In addition, these processes were repeated by cooling with liquid nitrogen during the process to reveal the effect of cryogenic conditions on burnishing (Figure1(b).). Other burnishing parameters are taken as constant throughout the process; the burnishing speed was 30 m/min, the feed rate was 0.1 mm/rev. For the roller burnishing operations, YAMASA SX8 single-ball roller burnishing tool was used. Roller

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(a) (b) (c) (d) Figure 1. Roller burnishing process (a) dry condition, (b) cryogenic condition and (c) cryogenic condition with infrared camera image of -11°C (d) preheat condition with infrared camera image of 200°C.

burnishing forces were determined by deducting a certain ratio from the maximum force to be applied to Inconel 718 for the selected roller burnishing tool. Optris Infrared Camera Temperature measurements during the roller burnishing process were made with the Optris PI 400 infrared camera with ±2% sensitivity and a resolution of 329x288 pixels. A sample was raised to a constant temperature using an oven, and the accuracy of the temperature was confirmed by measuring with both a thermocouple and a camera at the same time. As a result of calibration, the emissivity value was found to be 0.67 and was applied for temperature measurement. Figure 1(c) shows the infrared camera image taken while roller burnishing is applied to the part where the ambient temperature is -11 °C in the cryogenic condition and the part temperature drops to -45 °C. Figure 1(d) shows the infrared camera image taken while roller burnishing is applied to the piece heated to 200°C in the oven. Characterization of Samples A 3D Keyence VHX-6000 optical microscope was used to examine the surface topography, surface roughness, and microstructure of etched parts. To examine the surface and subsurface layer after the roller burnishing process, parts were cut with a diamond cutting disk followed by cold-mounted, ground, and polished. The Bruker Smart Apex II Quazar X-Ray Single Crystal Diffraction Device was used for the X-ray diffraction (XRD) measurements of the polished parts. Cu Kα cathode with a wavelength of 1.54 𝐴 was used in XRD measurements carried out within the scope of phase analysis, and the current and voltage values were chosen as 40 mA and 40 kV, respectively. The polished surface was etched using a solution of 15 ml HCl, 10 ml Glycerol, and 5 ml HNO3 to review the microstructure. Etched parts were also examined with the FEI (PHILIPS) XL30 SFEG SEM scanning electron microscope. Future-Tech FM310e device was used for hardness measurements. The hardness of each part was determined following the ASTM E 384 standard with an average of 5 measurements, a load residence time of 15 s, and a test load of 100 gf.

Results And Discussions

Surface Roughness The surface roughness of the Inconel 718 parts manufactured with LPBF was measured Sa=23 µm and Sz=177.69 µm after production. With the turning parameters given in the experimental procedure, the surface roughness of the parts reduced to Sa=1.7 µm and Sz=23.75 µm. Both the turning process and the roller burnishing process were carried out in parallel with the build direction. In Figure 2 the surface roughness values obtained as a result of the different processes applied in this study are shown. It was observed that the surface roughness decreased as the burnishing force increased on the roller-burnished surface under all burnishing conditions. The highest decrease in surface roughness was measured after the roller burnishing process performed at 900 N and dry conditions. Surface roughness values measured as Sa= 0.794 µm and Sz= 6.458 µm. As the burnishing force increased, the penetration depth of the process on the surface increased, which resulted in the flattening of the surface [23]. When the burnishing conditions are examined between each other, the surface roughness of the workpiece in the cryogenic burnishing condition is higher than the other conditions. Surface roughness was measured as Sa=1.012 µm and Sz=12.294 µm after the roller burnishing process was applied with 300 N burnishing force under cryogenic burnishing conditions. The reason for this is that the workpiece undergoes maximum plastic deformation with cryogenic burnishing, and as a result, it becomes difficult to machining, causing the surface roughness to be higher compared to other burnishing conditions [24]. For a similar reason, a better surface roughness was obtained compared to dry cutting conditions, since the preheat conditions applied to the part facilitate the machinability of the part. Microstructure The roller burnishing process causes plastic deformations, reductions in grain sizes, and orientations near the surface of the part [25]. Figure 3. shows the SEM photographs of the post-production state and turned Inconel 718 parts produced with LPBF parts at the same magnification. The microstructure on the surface and below is similar. Also, in the turned part no orientation or grain reduction

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Figure 2. Surface roughness values of LPBF Inconel 718 parts with roller burnishing process applied in different parameters and topography images. was observed. In Figure 4. a reduction in grain size is clearly seen on all surfaces after the roller burnishing process was applied. Grain size reduction increases the grain boundary area per unit volume. This means more grain boundary slippage and contributes to the overall deformation. The grain size reductions on the surface are clearly visible in the microstructures. The increase in plastic deformation due to these reductions can be explained as the reason for the increase in surface hardness [23]. As a result of detailed

characterization on the surface after the roller burnishing process, a thin layer of white color was observed. Studies in the literature have shown that this structure is a deformed ultra-fine-grained or nanocrystalline structure [26]. In the roller burnishing process, it was observed that the affected layer thickness increased with the increase of the burnishing force. After roller burnishing was applied with a force of 300 N under cryogenic conditions, the affected layer was measured at about 2 µm. When the force was increased to

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(a) (b) Figure 3. SEM images of (a) as built and (b) turned LPBF Inconel 718 parts.

Figure 4. SEM images of roller burnished parts with different burnishing forces and different burnishing conditions. 900 N, the affected layer increased to 3.3 µm. The reason for this is that the area undergoing permanent plastic deformation increases with the increase of forces exceeding the yield point of the material [27, 28]. When the SEM images were examined after the roller burnishing process performed with the same burnishing force under different burnishing conditions, it was

observed that the depth of the part affected by the process increased as the temperature increased. While the affected layer depth as a result of roller burnishing performed at 900 N and cryogenic conditions was 3.3 µm, this value increased to 4 µm when performed under dry conditions with the same force. When the same process was applied to the preheated part, the affected layer depth increased to

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XRD Results And Discussion

Although the roller burnishing operation applied to the parts did not cause a new phase formation, it was observed that it caused a change in the widths and intensities of the peaks in the (111) and (200) planes. In addition, shifts were observed in the peak angles. Figure 6. shows the XRD results of roller-burnished LPBF Inconel 718 parts under 300 N, 600 N, and 900 N burnishing forces and preheat, dry, and cryogenic burnishing conditions. The difference is most pronounced between the (111) and (200) planes. While as built has larger intensity at (200) plane, all roller burnished has larger intensity at (111) plane. When the (111) plane is examined, higher intensity and wider peaks are seen compared to the as-built one. Wider ɣ peaks indicate smaller grain structures [32]. The reason for the increase in the peak width is the grain reduction due to the increase in the force acting between the roller burnishing tool and the surface of the part when the force is high [33]. Increasing grain size is associated with microhardness.

Conclusion

This study presents the effect of the roller burnishing process with various parameters and under various conditions applied to Inconel 718 parts manufactured with LPBF. The following can be concluded from the present study; • The roller burnishing process used after turning is a very effective post-processing operation to decrease surface roughness for the Inconel 718 parts manufactured with LPBF. Depending on the burnishing force, the surface roughness can be lowered between 46% and 54% under dry conditions. • It has been observed that as the temperature of the treated part increases, which depends on roller burnishing conditions, the depth of the affected layer increases. As a matter of fact, the deepest affected layer was detected after the process carried out under the preheat burnishing condition. In this condition, at 900N burnishing force, the affected layer depth was 4.3 µm. • The increase in roller burnishing force caused an increase in surface microhardness under all roller burnishing conditions. The hardest surface was obtained under the cryogenic burnishing condition, which increased by 21% compared to the as-built state. • From the XRD pattern of Inconel 718 parts manufactured with LPBF, it has been observed that the burnishing force and burnishing conditions remarkably affect intensity at (111) and (200) planes. The roller burnishing process with 900N under preheat conditions resulted in an increase in the intensity of the (111) plane by up to 28% compared to the plane of the as-built state.

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Acknowledgements

Financial support from TUBITAK (The Scientific and Technological Research Council of Turkey) under Project number 118R039 is greatly acknowledged.

Data Availability Statement

The authors confirm that the data that supports the findings of this study are available within the article. Raw data that support the finding of this study are available from the corresponding author, upon reasonable request.

Conflict Of Interest

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethics

There are no ethical issues with the publication of this manuscript.

References

1. Anderson M, Thielin A-L, Bridier F, Bocher P, Addit Manuf Technol 2021;1:592. Savoie J. δ Phase precipitation in Inconel 718 and [15] Bandyopadhyay A, Bose S. Additive manufacturing. associated mechanical properties. Mater Sci Eng A CRC Press; 2019. [CrossRef] 2017;679:48−55. [CrossRef] [16] Sunay N, Mert K, Kaynak Y. Chemical post-pro-
2. Slama C, Abdellaoui M. Structural characteri- cessing methods for enhancing surface properties of zation of the aged Inconel 718. J Alloys Compd parts fabricated by additive manufacturing: a review. 2014;306:277−284. [CrossRef] Sigma J Eng Nat Sci 2020;38:2027−2042.
3. Li R, Yao M, Liu W, He X. Isolation and determina- tion for δ, γ′ and γ ″phases in Inconel 718 alloy. Scr ing. Int J Mach Tools Manuf 2000;40:1603−1617. Mater 2002;46:635−638. [CrossRef] [CrossRef]
4. Ergene B. Simulation of the production of Inconel 718 and Ti6Al4V biomedical parts with different relative Jahagirdar A, Joshi S, et al. Exploring grinding and densities by selective laser melting (SLM) method. J burnishing as surface post-treatment options for Fac Eng Archit Gazi Univ 2022;37:469−484. [CrossRef] electron beam additive manufactured Alloy 718.
5. Baicheng Z, Xiaohua L, Jiaming B, Junfeng G, Surf Coat Technol 2020;397:126063. [CrossRef] Pan W, Chennan S, et al. Study of selective laser [19] Yaman N, Sunay N, Kaya M, Kaynak Y. Enhancing melting (SLM) Inconel 718 part surface improve- Surface Integrity of Additively Manufactured ment by electrochemical polishing. Mater Des Inconel 718 by Roller Burnishing Process. Procedia 2017;116:531−537. [CrossRef] CIRP 2022;108:681−686. [CrossRef]
6. Kaynak Y, Tascioglu E. Post-processing effects on the surface characteristics of Inconel 718 alloy fabri- K, Kakehi K. Effects of substrate preheating on cated by selective laser melting additive manufactur- mechanical properties of in 718 processed by selec- ing. Prog Addit Manuf 2019:1−14. [CrossRef] tive laser melting. SSRN 2022;4017319. [CrossRef]
7. Feyzi T, Safavi SM. Improving machinability of Inconel 718 with a new hybrid machining tech- formance of nanofluids, cryogenic and hybrid nique. Int J Adv Manuf Technol 2013;66:1025−1030. cooling in turning of Inconel 625. Tribol Int [CrossRef] 2019;137:366−378. [CrossRef] Sigma J Eng Nat Sci, Vol. 42, No. 2, pp. 335−343, April, 2024 343
8. Taşcıoğlu E, Kaynak Y, Sharif S, Pıtır F, Suhaimi MA. Machining-induced surface integrity of Inconel 718 Y. Parts Diamond Burnishing Process Regimes alloy fabricated by powder bed fusion additive man- optimization Made of INCONEL 718 Alloy via ufacturing under various laser processing parame- Selective Laser Sintering Method. 2020 IEEE ters. Mach Sci Technol 2022;26:49−71. [CrossRef] 10th International Conference Nanomaterials:
9. Hassan AM. The effects of ball-and roller-bur- Applications & Properties (NAP). IEEE; nishing on the surface roughness and hardness of 2020:02SAMA01-1-02SAMA01-5. [CrossRef] some non-ferrous metals. J Mater Process Technol [29] Grzesik W, Żak K. Producing high quality hardened 1997;72:385−391. [CrossRef] parts using sequential hard turning and ball burnishing
10. Kaynak Y, Tascioglu E. Finish machining-induced operations. Precision Eng 2013;37:849−855. [CrossRef] surface roughness, microhardness and XRD analysis [30] Kaynak Y, Karaca H, Jawahir I. Cutting speed depen- of selective laser melted Inconel 718 alloy. Procedia dent microstructure and transformation behavior of Cirp 2018;71:500−504. [CrossRef] NiTi alloy in dry and cryogenic machining. J Mater
11. Hamadache H, Laouar L, Zeghib N, Chaoui K. Eng Perform 2015;24:452−460. [CrossRef] Characteristics of Rb40 steel superficial layer under [31] Kaynak Y, Tobe H, Noebe R, Karaca H, Jawahir I. ball and roller burnishing. J Mater Process Technol. The effects of machining on the microstructure and 2006;180:130−136. [CrossRef] transformation behavior of NiTi Alloy. Scr Mater
12. Olugbade TO, Lu J. Literature review on the mechan- 2014;74:60−63. [CrossRef] ical properties of materials after surface mechan- [32] Jia Q, Gu D. Selective laser melting additive ical attrition treatment (SMAT). Nano Mater Sci manufacturing of Inconel 718 superalloy parts: 2020;2:3−31. [CrossRef] Densification, microstructure and properties. J
13. De Lacalle LL, Lamikiz A, Sánchez J, Arana J. Alloys Compd 2014;585:713−721. [CrossRef] The effect of ball burnishing on heat-treated steel [33] Varin RA, Czujko T, Wronski ZS. Nanomaterials and Inconel 718 milled surfaces. Int J Adv Manuf for solid state hydrogen storage. Springer Science & Technol 2007;32:958−968. [CrossRef] Business Media; 2009. [CrossRef]

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