PSI - Issue 42

Aditya Pandey et al. / Procedia Structural Integrity 42 (2022) 1017–1024

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Pandey et al. / Structural Integrity Procedia 00 (2019) 000–000

behaviours (Gaur et al. (2018); Gaur et al. (2015), Zhong et al. (2019)). Deng et al. (Deng et al. (2018)) examined the directional mechanical characteristics of SLM IN718, revealing that the horizontally printed specimens have lower ductility than the vertically printed samples but greater tensile strength. Schneider et al. (Schneider et al. (2018)) studied the e ff ect of post processing on the mechanical properties and found that the heat-treated samples have better strength than the as-built ones. Pei et al. (Pei et al. (2019)) did the series of comparison between SLM IN718 and forged IN718 and found that the forged IN718 had good fatigue performance than the SLM printed IN718. Johnson et al. (Johnson et al. (2017)) performed the fatigue test on Laser Engineered Net Shaping Deposited Inconel 718 samples and found that the AM processed samples had lower fatigue life than the wrought IN718. Zhang et al.(Zhang et al. (2021)) investigated the molten pool dimensions and area-specific microstructure and found that the microstructural variation in build direction more significant than the longitudinal direction. Lee at al. (Lee et al. (2016)) did the fluid flow modelling and investigated the temperature gradient (G) and growth rate (R) at molten pool edge and tail, found that the G is higher at edge and lower at tail where R is higher at tail and lower at edge. Even though many studies related to the fatigue analysis of IN 718 alloy do exist but the general consensus and the understanding is still dubious mainly due to widely scattered observed data. The purpose of this work is to study the e ff ect of heat treatment on the mechanical behavior of the IN718 alloy printed using SLM process. In addition to the experimental work comprising of several tensile and fatigue tests, an attempt has also been made to develop a numerical model to validate the experimentally observed characteristics..

2. Methods

2.1. Material

In the current study, the commercially available gas atomized IN718 powder supplied by Renishaw was used for coupon preparation and the chemical composition of the alloy powder are listed in Table 1. The metal powder was spray-dried for 10 hours to eliminate moisture content to enhance the formability before printing.Figure 1a. shows the majority of the powder particles were spherical in shape. The IN718 powder size distribution showed an average particle diameter of 32 µ m and a standard deviation of 0.52 µ m, as shown in Figure 1c .

Table 1. Chemical composition of IN718 powder (Wt.%). Elements Ni Cr Nb Mo

Ti

Al

Co Mn

Si

C

S

O Fe

Wt. % 50-55 17-21 4.75-5.5 2.80-3.30 0.65-1.15 0.2-0.8

≤ 1 ≤ 0.35 ≤ 0.36 0.02-0.05 ≤ 0.01 ≤ 0.03 Bal.

2.2. Sample fabrication and heat treatment

An EOS AM250 SLM machine equipped with an Ytterbium fiber laser having an output power range of 280 W was used for the purpose in this study. The substrate was initially preheated up to 80 °C and the entire process was performed in a controlled inert atmosphere to reduce the porosity and to protect powder particle to react with atmosphere. Test samples were printed along laser travel direction and 90 °to the deposition direction (horizontal built). ASTM E8 and E466 standards have been used to prepare the tensile and fatigue test specimens, as shown in Figure 2. The building plane is specified as the X-Y plane, and the built direction as the Z-axis (Figure 3a). A bidirectional scanning strategy was used to fabricate the samples with a 67°rotation in the X-Y plane after each layer, as displayed in Figure 3b (Deng et al., 2018).Table 2 lists the SLM process parameters used to fabricate the samples. The as-printed samples were the heat treated according to the SAE Standard: Solution treatment (980°C, 1h / air cooling) + double aging (720°C, 8h / furnace cooling at 55°C / h to 620°C, 8h / air cooling) (Schneider et al. (2018))

Table 2. Sample fabrication process parameters. Parameters Laser Type

Laser power

Scan speed Hatch spacing Spot radius Layer thickness

Value

Ytterbium fiber laser

280 Watt

1000 mm / sec

0.12 mm 0.13 mm 30 µ m