TMMOB Metalurj i ve Malzeme Mühendisleri Odas ı Eğ i t im MerkeziBildir i ler Kitab ı

10719. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2018

An Investigation of the Microstructure and Mechanical Properties of Inconel 718 and Hastelloy-X Alloys Produced by Selective Laser Melting

Bertu Birkan Gül, Yeşim Nur Gülcan, Güney Mert Bilgin

Tusas Engine Industries Inc., 26003, Eskişehir, Turkey

Abstract

Selective Laser Melting (SLM) is a revolutionary process that eliminates several drawbacks of conventional manufacturing techniques with allowing rapid fabrication of the complex geometries without need of expensive tools. In aero-engine industry specialized production methods are required to produce high performance parts especially for hot section turbine components. Inconel 718 (IN718) and Hastelloy-X (HX) are nickel based super alloys that have been widely used in stationary and rotational engine components such as disks, combustion chambers, blades and vanes up to 600-700°C. In this study, IN718 and HX samples were manufactured by using SLM method in two different built directions: parallel (z-axis) and perpendicular (xy-axis) to the built direction. Microstructural investigations showed that grain orientations strictly depend on heat flow direction which the columnar grains are generally formed perpendicular to the base plate. Moreover, due to rapid cooling of melt pool, cellular microstructure was obtained after SLM for both of the alloys. Tensile tests were also performed for evaluating strength properties of SLMed specimens at room temperatures.

1. Introduction

Additive manufacturing (AM) (also described by various terms, e.g., rapid prototyping, rapid manufacturing, 3D printing, solid free form fabrication) is a revolutionary process in which complex 3D parts are built layer-by-layer directly from CAD data. AM has remarkable advantages over conventional manufacturing methods like freedom to produce complex geometries, removal of expensive tooling, short lead time, optimum material usage, manufacturing of near-net shape parts with good dimensional accuracy [1].

Selective laser melting (SLM) is a powder bed fusion based AM process where 3D parts are produced from raw material powder via laser source under inert atmosphere. In SLM, a powder layer is scanned by a laser beam. Upon irradiation, when sufficient power is applied to powder layer, then the powder melts and forms a liquid pool. Subsequently, the liquid pool solidifies and cools down rapidly and forms the cross section of a layer. After completing a layer, build platform is lowered by one layer thickness and a new powder layer is laid. The process repeats until the complete part is built. Finally, the part is removed from the built platform and if necessary, secondary operations are performed [1, 2].

The nickel based superalloy 718, commonly known as IN718, is a precipitation hardenable material that especially used for production of high temperature parts in gas turbine engines due to its superior creep and fatigue properties. IN718 is mainly strengthened by coherent disk-shaped particles in the fcc 𝛾 matrix. There is also another form of phase under overaged conditions which are undesired incoherent acicular 𝛿 particles. Hastelloy-X (HX) is a solid solution strengthened superalloy that widely used in combustion parts of the gas turbines. Besides solid solution strengthening mechanism, Hastelloy-X is also strengthened by carbide, intermetallic precipitation [3, 4].

Conventional production of complex IN718 and HX aerospace parts are inconvenient and expensive because of the subtractive fabrication nature of these manufacturing methods. Therefore, nowadays, there are many studies about SLM of IN718 and HX parts [5-10]. Although SLM provides crucial advantages on conventional methods, high cooling rates obtained in SLM process lead to formation of non-equilibrium microstructure [10].

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In this study, IN718 and HX specimens were manufactured by using SLM in both parallel (XY) and perpendicular (Z) to the built direction. Depending on the built direction mechanical behaviors and microstructural properties were evaluated and compared among each other for as-built IN718 and HX superalloys.

2. Experimental Procedure

IN718 and HX powders were used to produce as-built samples in this study. Whereas the prealloyed IN718 powders have a particle size distribution between 16-45 μm, the particle size distribution of prealloyed HX powders is 18-51 μm. IN718 and HX powders were provided by the LPW and EOS, respectively. The SEM images of IN718 and HX powders where the chemical compositions given in Table 2, are shown in Figure 1.

Figure 1. SEM images of a) IN718 powders and b)HX powders

Cylindrical, cubic and rectangular prism samples were built on XY and Z directions for utilizing tensile tests and metallographic characterization. Totally 24 samples made of IN718 and HX were produced by EOS M290 SLM machine. The blank dimensions of these samples are given in Table 1.

Table 1. Blank dimensions of samples mm Tensile Test Characterization

Z-Cylindirical 14,5 x 81 - XY-Prismatic 15 x 14,5 x 80 -

Cubic - 15 x 15 x 15

The cubic samples were manufactured in order to investigate several properties including density, hardness and microstructure. After sample production was completed, blank parts were separated from the base plate by wire-EDM, and cut surfaces were cleaned by using sand blasting. The density of the cubic samples was evaluated by image analysis method. Rockwell hardness testing was performed using Emcotest Durajet G5 machine. Cubic samples were cut by abrasive cutter before micro hardness testing and microstructural investigations in both parallel and perpendicular to the built direction. The Vickers micro hardness testing was carried out using Innovatest 400D Machine and each of

the data which is given in Table 4 and Table 5 represents an average of measurements from five tests. The samples were mounted, ground from 80 to1000 meshes for each 1 min and polished with 3 for 3 min. Samples were etched and then NIKON ECLIPSE MA200 optical microscope was used for the microstructural characterization.

Tensile test specimens were machined by milling for room temperature tensile tests conforming to ASTM E8. Tensile testing was carried out using a servo hydraulic MTS 370.10 machine.

3. Results and Discussion

The chemical compositions of IN718 and HX powders and samples can be found in Table 2. By examining this table, it can be understood that chemical compositions where not changed significantly after SLM manufacturing.

Table 2. Chemical compositions of powders used and as-built specimens

Elementswt%

IN718Powder

HXPowder

IN718As-Built

HXAs-Built

C 0.05 0.01 0.049 0.01 Mn 0.09 0.01 0.058 0.044 Si 0.05 0.15 0.049 0.1 P <0.01 <0.01 - - S <0.01 <0.005 - - Cr 19.13 20.98 19.1 21.6 Co <1 1.44 0.18 2.57 Mo 3.1 8.54 3.02 8.5 Nb 5.14 - 5.22 <0.003 Ti 0.94 0.05 0.85 0.09 Al 0.44 0.04 0.46 0.049 Fe Balance 18.25 17.6 18.4 Ni 52.86 Balance 53.2 47.4 W - 0.70 0.05 0.93

The SLM as-built microstructure of IN718 samples is shown in Figure 2 for both XY and Z direction. Melt pools which have the arc-shaped configuration can be seen in Figure 2.b) with corresponding average depth as 84.5 . On the other hand, the microstructure of SLM as-built HX samples is illustrated in Figure 3 for both XY and Z direction. Melt pools which have the arc-shaped configuration can be seen in Figure 3.b) with corresponding average depth as 98 μm. Whereas the average distance between laser tracks is around 109 μmfor IN718, this value is approximately 112.5 μm for HX. These values were determined by examining Figure 2.b) and Figure 3.b) microstructure images.

TMMOB Metalurj i ve Malzeme Mühendisleri Odas ı Eğ i t im MerkeziBildir i ler Kitab ı

10919. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2018

a) XY Direction b) Z Direction Figure 2: Optical images of SLMed IN718

microstructures.

a) XY Direction b) Z Direction Figure 3: Optical images of SLMed HX microstructures.

In this study, optical image analysis method was used to obtain the bulk densities of samples which were given in Table 3. According to relative density results, it is seen that SLM allows manufacturing of IN718 and HX parts having relative densities more than 99%. In addition, the related results show that build orientation does not affect the relative densities remarkably.

Table 3. Relative Densities (%) of IN718 and HX as-built samples

XY Direction 99.98±0.007As-Built IN718Z Direction 99.98±0.004XY Direction 99.99±0.004As-built HXZ Direction 99.98±0.008

In Table 4 and Table 5, average microhardness (HV) and average macrohardness (HRC) results are shown for as-built IN718 and HX alloys respectively. When results for IN718 and HX alloys are compared, it can be found that hardness values of IN718 alloy are greater than the hardness values of HX as expected and the difference in HV is about 50.

Table 4. Average microhardness and macrohardness values for as-built IN718 samples.

IN718 As Built

XY Z

Average Microhardness (HV) 318,3 ± 17,46 329,2 ± 7,37

Average Macrohardness (HRC)

25,4 ± 0,64 25,1 ± 4,65

On the other hand, hardness measurements were performed for samples built in both XY direction and Z

directon to be able to show the effect of built orientation on hardness values. However, results demonstrate that built orientation does not affect the hardness property of the material significantly.

Table 5. Average microhardness and macrohardness values for as-built HX samples.

HX As Built

XY Z

Average Microhardness (HV) 264,8 ± 11,02 275,6 ± 9,69

Average Macrohardness (HRC)

16,8 ± 0,96 19,6 ± 6,53

Normalized strength values for IN718 and HX samples are compared to strength values found in ASTM F3055 and AMS 5536 specifications respectively, in Figure 4. AMS F3055 specification covers additively manufactured UNS N07718 components using full-melt powder bed fusion such as selective laser melting and electron beam melting while AMS 5536 specification covers a corrosion and heat-resistant nickel alloy (UNS N06002) in the form of sheet, strip, and plate procured in inch/pound units. As it can be seen in Figure 4, both yield and ultimate tensile strength of SLMed IN718 samples are much greater than the strength values in ASTM F3055. On the other hand, when room temperature strength of SLMed HX samples are compared to strength values in AMS 5536, it was understood that yield and tensile strength for XY built SLMed samples are higher than the values in related specification. Only tensile strength was slightly lower in Z direction for HX samples. Moreover, SLMed IN718 samples have higher yield and tensile strength than SLMed HX samples. In addition, it should be emphasized that for both SLMed materials, all strength values in XY direction are greater than values in Z direction.

Figure 4. Normalized Strength values of as-built IN718 and HX alloys.

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4. Conclusions

The main purpose of this study was to investigate and to compare the metallurgical and mechanical properties of IN718 and HX alloys which are both manufactured via SLM. Therefore, cylindrical, cubic and rectangular prism samples were manufactured in two different built directions: parallel (z-axis) and perpendicular (xy-axis) to the built direction in SLM method. Within this scope, microstructural analysis, porosity analysis, tensile and hardness tests were performed. The main conclusions were summarized as follows:

• Chemical compositions of SLMed samples do not differ from chemical composition of powder used as raw material significantly.

• Microstructural investigations demonstrated that grain orientations depend on heat flow direction. Moreover, the columnar grains formed perpendicular to the base plate for both IN718 and HX materials. In addition, cellular microstructures were attained for both of the alloys.

• SLM allows manufacturing IN718 and HX parts having relative densities which are more than 99%.

• Hardness values of IN718 alloy are greater than the hardness values of HX as expected and the difference in microhardness is about 50 HV regardless of built orientation.

• Yield and ultimate tensile strength of SLMed IN718 samples fulfill the requirements of ASTM F3055 specification in both building directions.

• SLMed HX samples have superiority in yield strength in comparison to AMS 5536.

• SLMed IN718 samples have higher yield and ultimate tensile strength than SLMed HX samples independent on the building direction.

5. References

[1]Gibson, I., Rosen, D.W., Stucker, B., 2010, Additive manufacturing technologies, rapid prototyping to direct digital manufacturing, Springer. [2]Rombouts, M. (2006). Selective Laser Sintering/Melting of Iron-Based Powders (Selectief laser sinteren/smelten van ijzergebaseerde poeders).[3]Donachie, M. J., & Donachie, S. J. (2002). Superalloys: a technical guide. ASM international. [4]Geddes, B., Leon, H., & Huang, X. (2010). Superalloys: alloying and performance. Asm International.[5]Wang, F. (2012). Mechanical property study on rapid additive layer manufacture Hastelloy® X alloy by selective laser melting technology. The International Journal of Advanced Manufacturing Technology, 58(5-8), 545-551.

[6]Wang, F., Wu, X. H., & Clark, D. (2011). On direct laser deposited Hastelloy X: dimension, surface finish, microstructure and mechanical properties. Materials Science and Technology, 27(1), 344-356. [7]Choi, J. P., Shin, G. H., Yang, S., Yang, D. Y., Lee, J. S., Brochu, M., & Yu, J. H. (2017). Densification and microstructural investigation of Inconel 718 parts fabricated by selective laser melting. Powder Technology, 310, 60-66. [8]Trosch, T., Strößner, J., Völkl, R., & Glatzel, U. (2016). Microstructure and mechanical properties of selective laser melted Inconel 718 compared to forging and casting. Materials letters, 164, 428-431. [9]Vilaro, T., Colin, C., Bartout, J. D., Nazé, L., & Sennour, M. (2012). Microstructural and mechanical approaches of the selective laser melting process applied to a nickel-base superalloy. Materials Science and Engineering: A, 534, 446-451. [10]Wang, Z., Guan, K., Gao, M., Li, X., Chen, X., & Zeng, X. (2012). The microstructure and mechanical properties of deposited-IN718 by selective laser melting. Journal of Alloys and Compounds, 513, 518-52