Postprocessing challenges in metal AM: Strategies for achieving homogeneous microstructure in Ni-based superalloys

Abstract

Additive manufacturing (AM) is an approach in which the component is manufactured in a layer-by-layer manner. It is a technique in which a 3D model of the component is divided into thin slices and the component is then built by depositing the material layer by layer. Thus, it is called AM. The process in its early years was known as rapid prototyping as it was used to produce visualization models and prototypes from polymers. With the advent of technology, the process can now be employed on biomaterials and metals. Due to this flexibility in the materials that can be processed with AM, the process has been widely employed in aerospace, automobile, medical and dental, and defence applications. Metal AM processes are distinguished depending on the heat source employed (laser beam, electron beam, or electric arc), feedstock material (wire or powder), and feed system. Based on these factors, AM processes are categorized as powder bed fusion (PBF), directed energy deposition (DED), and binder jetting (BJ). PBF is a technique in which the metal powder is heated/melted using a laser beam or electron beam. DED is a technique in which the feed material (powder or wire) is fed through a nozzle and is melted using laser/electron beam or electric arc and deposited layer by layer. BJ involves a print-head selectively depositing a liquid binding agent onto a thin layer of powder particles followed by sintering. PBF is widely used for manufacturing complex geometries because of high-dimensional accuracy and capability for topology optimization. PBF has the capability to create internal passages in high-value components such as gas turbine blades, which is not possible with other metal AM techniques. Therefore, PBF is chosen over other AM processes because of its ability to manufacture intricate geometries. Inconel 718 is a high strength superalloy of nickel and exhibits thermal stability at elevated temperatures around 600°C and has excellent resistance to wear, oxidation, creep resistance and superior fatigue life. IN718 has been used as the main structural material for aircraft parts such as turbine blades, guide vanes, engine manifold and combustion chamber. To fabricate parts with complex geometries and merging components to reduce the number of parts, AM presents a feasible approach. (Ni3 (Al, Ti)) and (Ni3Nb) are the primary strengthening phases for IN718. Apart from these, IN718 has other phases such as phase (Ni3Nb), Laves phase (Fe, Cr, Ni, Si)2 (Nb, Mo), and carbides (M23C6). These phases are termed as detrimental phases as they deteriorate the mechanical properties and reduce the fatigue and creep resistance of the component. In AM, the composition of phases formed varies from cast components because of rapid heating and cooling. The microstructure and composition of the phases in the as-printed component are dependent on the process parameters such as laser power, scan speed, scan strategy, and hatch spacing etc. Columnar microstructure is preferred with strengthening phases and for high-temperature applications. The processing parameters affect components characteristics such as part density, surface roughness, residual stress, etc. Parameters employed during processing have their individual effects on the component’s properties. Scan length of the laser is directly proportional to the residual stress

that is, smaller the scan length, lower is the residual stress. With an increase in the laser power, the part density increases whereas with an increase in scan speed reduces part density. Therefore, there is a need of parameter optimization to maintain the build quality. Similarly scan strategies affects the heat distribution and flow rate during the process and thereby control the texture, residual stresses, and microstructure of the component. X-Y scan (90-degree rotation) and rot scan (67-degree rotation) provides with minimum residual stress, strong texture, and columnar grain structure. But AM by virtue of the process has rapid cooling and in the case of complex geometries has variable cooling rates, leading to certain variations in the microstructure. Microstructure evolution is mainly determined by the ratio of temperature gradient. Crystal theory suggests that growth of the crystal will take place by an edgewise extension of the more closely packed planes rather than by growth normal to any particular lattice plane. The temperature gradient during processing is influenced by the processing parameters. For AM Inconel 718, the as-printed microstructure contains brittle laves phase formed due to microsegregation of Nb and Ti. Thus, a heat treatment strategy is required to dissolve undesirable phases back in the matrix, relieving residual stresses and homogenize microstructure. Generally, the as-printed component is subjected to number of treatments namely homogenization treatment (1080°C for 1.5 hours), solution treatment (980°C for 1hour) and double ageing (720°C and 620°C for 8 hours each). In addition to these, HIP (hot-isostatic pressing) at 1180°C for 100–150MPa is also used to reduce the porosity level of the component. It is important to design a heat treatment cycle based on the properties of the as-printed component. It is required so that a certain heat treatment temperature and duration are achieved, as required to maintain a balance in the phases available in the processed component. The aim of the work is to propose a set of processing parameters and heat treatment strategies to obtain an IN718 AM component suitable for high-temperature applications with and strengthening phases and columnar grain structure. The work will also look at the applicability of intrinsic heat treatment as a part of the AM process. © 2023 Elsevier Ltd. All rights reserved.