PSI - Issue 53

Omid Emadinia et al. / Procedia Structural Integrity 53 (2024) 278–284 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction

Laser Directed Energy Deposition (L-DED), as an additive manufacturing (AM) technique, has drawn considerable attention in recent years. L-DED is defined as a “process in which focused thermal energy of the laser is used to fuse materials by melting as they are being printed” (DIN 2021). Being an in-situ powder feeding process, it allows fabricating parts without any enclosed chamber. This process can also produce components with different compositions (Bobbio, Bocklund et al. 2018), layer by layer, and even through the mixing of different powders, known as functionally graded materials (Vieira, Ferreira et al. 2021), which is challenging to realize in laser powder bed fusion (LPBF) (Wei and Li 2021). Inconel 625, a solid solution and/or precipitation strengthened nickel-based superalloy offers a rare combination of favourable tensile yield strength, fatigue strength, creep resistance, and exceptional oxidation and corrosion resistance (Dinda, Dasgupta et al. 2009). L-DED of IN625, a nickel-based superalloy has gained particular interest due to the considerable difficulty and lengthy process times required for its machining, and multiple research articles have been published regarding L-DED processing/ process optimization, mechanical properties and microstructural evolution of IN625 (Pratheesh Kumar, Elangovan et al. 2021),(Karmuhilan and Kumanan 2021) (Zafar, Emadinia et al. 2023). In several applications, L-DED suits well for replacing manual welding in repair process offering greater flexibility and improved consistency for various part geometries (Svetlizky, Das et al. 2021). L-DED has also been considered for fabricating thin-walled structure (Barragan, Rojas Perilla et al. 2021), and repair applications (Kim and Saldana 2020).However, successive printing of layers, one upon the other, leads to repetitive solidification and remelting of pre-printed layers upon subsequent layers. This layer upon layer printing of metal leads to heat accumulation in the pre-printed layers (Schnell, Schoeler et al. 2021, Song, Dong et al. 2021, Jang, Van et al. 2022). Such heat accumulation is characteristic of either conventional welding techniques (Gao, Shi et al. 2022) and the laser powered directed energy deposition technique (Gu, Du et al. 2019). This heat accumulation in the printed metal leads to a significant change in the local solidification dynamics by affecting the local solidification rate and the temperature gradient, which eventually leads to a significant variation of local grain structure across the printed component. An understanding of this variation in the microstructure and the underlying phenomena could be beneficial to achieve more resilient L-DED components. This study presents the microstructural characterization and microhardness test results of an almost thin-walled IN625 structure printed by L-DED along the build direction in an almost thin wall structure. Commercially available, gas atomized IN625 powder (MetcoClad 625® from Oerlikon), was used for this study. The powder presented a spherical morphology and a particle size ranging between 45 and 90 μ m. Figure 1 shows the morphology of the IN625 powder. The powder had a composition as shown in Table 1. The substrate was an AISI 4140 steel plate with dimensions of 150 mm × 100 mm × 7 mm. The L-DED system in the current study is a Fraunhoffer IWS COAX12V6 nozzle assembled with a KUKA 6 axis robot and connected to a Medicoat AG Disk feeder. The carrying and shielding gas is a Grade 3 Argon. The laser source is a coherent highlight FL3000 (Coherent, Santa Clare, SA, USA) fiber laser presenting a gaussian distribution with a wavelength of 1070 nm ± 10 nm, maximum power of 3000 W, operating in continuous wave (CW) mode. The IN625 sample sized 7.2 x 133.2 x 30mm (t x L x H) was printed using the laser process parameters given in Table 2. The printing sequence started from the contour followed by a zig-zag layout infilling the contour lines. 2. Materials and Methods

Table 1 Wt.% elemental composition of IN625 powder

Wt.% elemental composition

Ni

Cr

Mo

Nb

Fe

60.8

21.3

9.2

4.6

4.1

A vertical section of the printed structure was mounted and polished and then electrolytically etched using oxalic acid dehydrate for microstructural characterization. Microscopic observations were conducted using a digital microscope