Abstract
3D printing results in anisotropy in the microstructure and mechanical properties. The focus of this study is to investigate the structure, texture and phase evolution of the as-printed and heat treated IN718 superalloy. Cylindrical specimens, printed by powder-bed additive manufacturing technique, were subjected to two post-treatments: homogenization (1100 °C, 1 h, furnace cooling) and hot isostatic pressing (HIP) (1160 °C, 100 MPa, 4 h, furnace cooling). The Selective laser melting (SLM) printed microstructure exhibited a columnar architecture, parallel to the building direction, due to the heat flow towards negative z-direction. Whereas, a unique structural morphology was observed in the x-y plane due to different cooling rates resulting from laser beam overlapping. Post-processing treatments reorganized the columnar structure of a strong {002} texture into fine columnar and/or equiaxed grains of random orientations. Equiaxed structure of about 150 µm average grain size, was achieved after homogenization and HIP treatments. Both δ-phase and MC-type brittle carbides, having rough morphologies, were formed at the grain boundaries. Delta-phase formed due to γ″-phase dissolution in the γ matrix, while MC-type carbides nucleates grew by diffusion of solute atoms. The presence of (Nb0.78Ti0.22)C carbide phase, with an fcc structure having a lattice parameter a = 4.43 Å, was revealed using Energy dispersive spectrometer (EDS) and X-ray diffractometer (XRD) analysis. The solidification behavior of IN718 alloy was described to elucidate the evolution of different phases during selective laser melting and post-processing heat treatments of IN718.
Highlights
Selective laser melting (SLM) process is the most widely used process for additive manufacturing (AM) of complex and customized parts in different industries, such as aerospace and automotive sectors and in the medical field [1]
Parimi et al [18] investigated the variation in the deposition path and the laser power on the texture, grain structures and morphology development of different phases using unidirectional versus bidirectional building strategies [18]
The surface morphology of a non-etched gas atomized particle and microdendritic structure of a mounted, sectioned and polished IN718 powder specimen are shown in Figure 4a,b, respectively
Summary
Selective laser melting (SLM) process is the most widely used process for additive manufacturing (AM) of complex and customized parts in different industries, such as aerospace and automotive sectors and in the medical field [1] In this method, laser beam selectively melts the surface of a metallic powder to create a fully-dense (close to 99.7%) solid structures with mechanical properties comparable to those achieved by wrought or casting processes, depending on the initial powder composition and process parameters [1,2]. Parimi et al [18] investigated the variation in the deposition path and the laser power on the texture, grain structures and morphology development of different phases using unidirectional versus bidirectional building strategies [18] In both strategies, columnar grains oriented along the moving heat source were produced. The generation of such information is a critical tool for the optimized design of the SLM process of superalloys as well as identification of the critical information on mechanical properties of SLM manufactured parts
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