The Study of the Direct Laser Deposition Productivity Increasing Effect on Forming Structure and Cracks in the Inconel 718 Alloy

Mechanical Engineering and Machine Science


Аuthors

Alymov N. R.*, Turichin G. A.**, Vildanov A. M.***, Yurchenko N. Y.****, Aleksandrov V. L.*****

St.-Petersburg State Marine Technical University (SMTU), Saint Petersburg, Russian Federation

*e-mail: sir.alymoff@yandex.ru
**e-mail: gleb@ltc.ru
***e-mail: wildam92@mail.ru
****e-mail: Yurchenko@ilwt.smtu.ru
*****e-mail: oimilist@yandex.ru

Abstract

The presented study deals with an experimental analysis the direct laser deposition (DLD) productivity increasing effect on the microstructure and susceptibility to the liquation (hot) cracking in the heat-resistant nickel alloy Inconel 718, widely used in the aerospace industry at the elevated temperatures. The two DLD regimes were compared, namely classical (bead ~2.6 × 0.75 mm, power 1.2–2.0 kW) and high-productivity (bead 8.15 × 2.3 mm, power 10–12 kW). The objective of the work consisted in establishing a quantitative relationship between the bead geometry, effective heat input and grain morphology, which determines susceptibility to the crack formation.
Methods for studying grain morphology, element distribution and the nature of defects included metallography, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD). The gas-atomized powder chemical composition complied with the ASTM B637 standard, with a noted elevated silicon content (0.31 wt.%) contributing to crack growth.
The results revealed that increasing bead geometry and powder feed rate (from ~16 to ~154 g/min) substantially alters the thermal balance of the process. A larger fraction of laser energy is being absorbed by the powder stream for heating and melting, which reduces the effective heat input into the substrate and previous layer. This suppresses the epitaxial growth of the elongated columnar grains across the interlayer boundaries and facilitates formation of a fine-grained equiaxial zone in the boundary regions. As the result, microsegregation of Nb, Si, and Mo decreases, formation of the brittle Laves phase is being limited, and susceptibility to liquation cracking is being reduced.
Elongated columnar grains with a pronounced <001> texture, creating continuous paths for segregation and crack propagation, were observed in the classical mode. A mixed structure was identified in the high-productivity mode, namely the small equiaxed grains (20–50 μm) in the central part of the bead and limited columnar growth closer to the periphery. Cracks were characteristic of classical samples, especially at еру elevated power (such as sample 1.3 at 1600 W), whereas high-productivity samples (such as sample 2.3 at 12 kW) demonstrated a minimal number of cracks and absence of lack of fusion in the internal beads.
The results revealed that both increasing bead geometry and powder feed rate reduce effective heat input into the previous layer, suppressing epitaxial growth of columnar grains and promoting formation of a fine-grained interlayer zone. All this decreases the element segregation, Laves phase formation, and liquation cracking. A simplified model of energy distribution between the powder and substrate was proposed based on the thermal balance analysis.
The obtained results contribute to the development of the domestic aerospace complex, enabling production optimization and components repair of aviation gas turbine engines and elements of space-rocket technology, enhancing their reliability, durability, and reducing production costs.

Keywords:

direct laser deposition, hot cracks, productivity increasing of laser powder cladding, nickel heat-resistant alloys, additive manufacturing in the aerospace industry

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