Additive manufacturing of Ni-based superalloys: The outstanding issues
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Introduction Nickel-based superalloys constitute an important class within the broader family of superalloys, with Ni as the main alloying element. They possess a combination of outstanding mechanical and physical properties in the temperature range of 540°C–1000°C, notably tensile and creep strength as well as resistance to thermal fatigue and oxidation, making them especially suitable for gas-turbine and jet-engine components.1 Alloying elements are selectively included to improve their performance. For instance, Ti, Al, and Nb contribute to the formation of precipitation strengthening phases (Ni3Al (γ′) and Ni3Nb [γ′′]), which are formed as coherent fine precipitates following aging. Other elements (e.g., Ta, Ti, Mo, Hf, W, Cr) contribute to the formation of carbides, which assist in grain-size control and resistance to grain-boundary sliding at high temperatures. Other elements (e.g., Al, Cr, La, Y, Ce) are added to improve their oxidation and corrosion resistance, which is essential to the aforementioned applications.1,2 Generally, Inconel 718 (also called Alloy 718) has been the workhorse of the aero-engine sector, due to its combination of good weldability, forgeability, and strength up to 650°C at a reasonable cost.3 There has been increasing interest in the development of Ni-based superalloys that are capable of higher-temperature
performance, in order to increase the turbine entry temperature (TET) of gas turbines (the temperature of combustion byproduct gases entering the turbine section), which manifests in improved thermodynamic efficiency.4 As a result, newer alloys were developed to achieve this, including CM247LC, RR1000, René 41, Hastelloy-X, Waspaloy, Udimet 720, N18, and Astroloy (Table I).4 Despite their outstanding high-temperature performance, these alloys have limited weldability due to the presence of a high γ′-fraction (directly linked to the Ti + Al content), which increases the susceptibility of the alloys to cracking during postweld heat treatment (or reheating operations), also known as “strain-age cracking.”5 The alloys also become susceptible to ductility-dip cracking (DDC), which is defined as the drop in ductility at intermediate temperatures (0.4–0.7× the melting point), and is typically associated with the formation of grain-boundary carbides in Ni superalloys.6 This relationship between alloy chemistry and weld susceptibility is shown in Figure 1.7 This does not include other cracking mechanisms such as liquation and solidification cracking, which could occur during welding of alloys, regardless of the chemistry. In simple terms, additive manufacturing (AM) can be described as a multilayer/repeated welding process (see the
Moataz M. Attallah, Advanced Materials and Processing Lab, School of Metallurgy and Materials, University of Birmingham, UK; [email protected] Rachel Jennings, Advanced Materials and Processing Lab, School of Metallurgy and Materials, University of Birmingham, UK; [email protected] Xiqian Wang, Advanced Materials and Processing Lab, School of Metall
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