Additively Manufactured Inconel 718 : Microstructures and Mechanical Properties

Abstract

Linköping 2018Cover image: A fracture surface of EBM IN718 after tensile test at room temperature.During the course of research underlying this thesis, Dunyong Deng was enrolled in Agora Materiae, a multidiciplinary doctoral program at Linköping University, Sweden. Printed by LiU-Tryck 2018 © Dunyong Deng AbstractAdditive manufacturing (AM), also known as 3D printing, has gained significant interest in aerospace, energy, automotive and medical industries due to its capabilities of manufacturing components that are either prohibitively costly or impossible to manufacture by conventional processes. Among the various additive manufacturing processes for metallic components, electron beam melting (EBM) and selective laser melting (SLM) are two of the most widely used powder bed based processes, and have shown great potential for manufacturing high-end critical components, such as turbine blades and customized medical implants. The futures of the EBM and SLM are doubtlessly promising, but to fully realize their potentials there are still many challenges to overcome.Inconel 718 (IN718) is a nickel-base superalloy and has impressive combination of good mechanical properties and low cost. Though IN718 is being mostly used as a turbine disk material now, the initial introduction of IN718 was to overcome the poor weldability of superalloys in 1960s, since sluggish precipitation of strengthening phases γ ′ /γ ′′ enables good resistance to strain-age cracking during welding or post weld heat treatment. Given the similarity between AM and welding processes, IN718 has been widely applied to the metallic AM field to facilitate the understandings of process-microstructure-property relationships.The work presented in this licentiate thesis aims to better understand microstructures and mechanical properties EBM and SLM IN718, which have not been systematically investigated. Microstructures of EBM and SLM IN718 have been characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM) and correlated with the process conditions. Monotonic mechanical properties (e.g., Vickers microhardness and tensile properties) have also been measured and rationalized with regards to the microstructure evolutions before and after heat treatments.For EBM IN718, the results show the microstructure is not homogeneous but dependant on the location in the components, and the anisotropic mechanical properties are probably attributed to alignment of porosities rather than texture.iii Post heat treatment can slightly increase the mechanical strength compared to the as-manufactured condition but does not alter the anisotropy. SLM IN718 shows significantly different microstructure and mechanical properties to EBM IN718. The as-manufactured SLM IN718 has very fine dendritic microstructure and Laves phases in the interdendrites, and is "work-hardened" by the residual strains and dislocations present in the material. Mechanical properties are different between horizontally and vertically built samples, and heat treatment can m...

Study on the Effect of Powder-Bed Fusion Process Parameters on the Quality of as-Built IN718 Parts Using Response Surface Methodology

“…The δ phase is a brittle phase which consumes the strength, inducing Nb in the alloy. This transformation might hinder the high-temperature applications of IN718, where characteristics like tensile and yield strengths are important [8]. From the Pareto chart in Figure 9, it is clear that the scan speed is the dominant factor in determining the percentage of γ" in the as-built samples.…”

“…IN718 is composed of chromium, molybdenum, aluminum, titanium, cobalt, niobium, and nickel, with nickel being the highest proportion. The commonly seen phases in IN718 are γ, γ', γ", δ, MC, and laves [8]. The major phase of IN718 is the gamma (γ) phase, as it is mainly comprised of Ni and forms an austenitic face-centered-cubic (FCC) matrix [9,10].…”

Influence of Heat Input on the Formation of Laves Phases and Hot Cracking in Plasma Arc Welding (PAW) Additive Manufacturing of Inconel 718

“…There are several ways to deal with the formation of the Laves phase and hot cracking. Deng et al [15] determined that a slight variation in the Nb, Ni and Ti composition of IN718 feed wire, decreases the formation of the Laves phase and improves the mechanical properties of the WAAM IN718. Xu et al [16] reported that the postdeposition homogenization heat treatment not only dissolves the Laves phase back into the matrix but improves the microstructure and elastic properties.…”

Effect of Powder Recycling on Defect Formation in Electron Beam Melted Alloy 718

“…Alloy 718 stands out for its high strength, corrosion resistance, a wide operating temperature spectrum (from cryogenic up to around 650°C), and notably good welding characteristics. [5,6] However, such high-performance materials often suffer from difficulties during fabrication. Castings, for example, typically exhibit coarse, segregated, and porous microstructures which require homogenization and densification to reach the requirements for critical components.…”