Microstructure Characterization of Alloy 625 Deposited on Nickel Foam using Air Plasma Spraying

Abstract

Sandwich constructions offer many advantages over traditional stiffened shapes including light weight, high strength to weight ratio, and high stability. The basic concept of sandwich structures consists of two thin skins (faces) and a lightweight thicker core. The skin materials may be metallic, wood, or composite laminates, while cores can be made from a variety of materials including metallic and nonmetallic honeycombs, cellular metallic or polymeric foams. [1,2] Foam structures are attractive due to their outstanding mechanical and physical properties such as relatively low specific weight, high specific strength, high stiffness and gas permeability. [3,4] Due to their excellent performance, they are finding widespread use in aerospace, marine and offshore industries, and civil structures. A variety of new applications would become possible if foam core sandwich structures capable of operating at high temperatures and in corrosive environments were available. Ni foams have been available for several years [5,6] and more recently foams of high temperature alloys have been developed. [7] Ni-based superalloys are well known for their good combination of creep strength, yield strength, tensile strength and high service temperature, and so would be attractive candidates for the skin material. However, the bonding of thin sheets of such alloys to the foam core, and the fabrication of complex shapes are challenging manufacturing issues.In recent years there has been a growing worldwide interest in the use of thermal spray processes such as air plasma spraying (APS) to deposit protective superalloy coatings onto the surfaces of engineering components to improve their performance under severe conditions. APS is a well developed industrial process capable of depositing dense coatings of up to a few millimeters in thickness on complex shaped substrates. APS coatings are built up from splats, consisting of columnar and cellular grains, which result from the impact, flattening, and solidification of completely or partially melted powder particles on the substrate. The cooling rate during solidification of the splats is very high (∼ 10 4 -10 8°C /sec) [8] which may lead to formation of unusual amorphous or other metastable phases not found in conventionally manufactured materials. Individual splats are connected to each other by mechanical and chemical bonding, forming a lamellae structure exhibiting anisotropic behavior, with mechanical properties parallel to the substrate (longitudinal) different than those through the coating thickness (transverse). Microstructural features also include defects such as pores and microcracks. Due to the entrainment of the surrounding air into the plasma jet, the existence of oxide phases in APS metal coatings is not unexpected and a number of studies have addressed the existence of an oxide phase in the final microstructure. [8,9] Due to high velocity and temperature gradients in the plasma plume, changes in process parameters can result in significant changes in the characteristics of t...

Comprehensive Structural, Surface-Chemical and Electrochemical Characterization of Nickel-Based Metallic Foams

“…The term Inconel refers to a family of alloys with slightly different compositions and contains Ni as the main element, up to 25% of Cr, Fe, Mo, and Nb, and other metals in varied amounts . The other elements are added to accomplish specific characteristics, such as corrosion resistance, grain structure, high temperature stability, strength hardness, elasticity, and ductility. ,− Previous research on these acid resistant materials focused on bulk materials that were true alloys. Since Inconel and FE2 foams are not true alloys, their behavior in strongly acidic solutions cannot be easily related to the behavior of analogous bulk materials.…”

Powder bed binder jet printed alloy 625: Densification, microstructure and mechanical properties

“…Additionally, one EDS scan spectrum and results (relative error of EDS can be as high as 2%) of the as-received feedstock powder are given in Table 1, respectively, showing agreement within the relative error with the manufacturer-provided values (Table 1). Powder XRD (Figure 2c) showed that the main peaks are related to the prevalent fcc γ-Ni phase [16]. Since Mo and Nb were present in the matrix as solid solution hardening alloying elements, no characteristic peaks were detected for these elements.…”

Grain growth and pore elimination in Inconel 625 deposited by APS

“…The mechanical performance of this type of sandwich structure depends on the properties of the coating since it carries almost all of the loads while the substrate acts as a low density supporting component. Previous results showed that the microstructural characteristics of the coating affect its mechanical properties [4,5]. For example, porosity in the coatings could exponentially decrease the yield and ultimate tensile strength of the structure.…”

“…The microstructural characteristics of the as-sprayed Inconel 625 were investigated by the authors and results were given in details elsewhere [4]. Since the microstructural features of the as-sprayed alloy Inconel 625 differ from those of the conventionally manufactured material, its response to the conventional heat treatment schedules may result in completely different mechanical properties compared to those expected for conventionally manufactured alloys.…”