High temperature air oxidation behavior of Hastelloy X processed by Electron Beam Melting (EBM) and Selective Laser Melting (SLM)

Highlights

• The initial microstructure of the AM alloys was strongly dependent on the build procedure, parameters, composition and heat treatment (EBM versus SLM).

• Faster oxidation kinetics were observed for the alloys with high Mn and Si contents. The alloys with high Mn and Si contents formed an outer oxide scale consisting of MnCr2O4 on top of a Cr₂O₃ layer with underlying SiO₂ precipitates.

• In the presence of a MnCr₂O₄ layer, a finer Cr₂O₃ grain microstructure and larger Mn depletions were observed in the underlying alloy.

• The Cr depletion induced the dissolution of Mo-rich carbides underneath the oxide layers.

• The significant spallation of the EBM1 alloy was associated with presence of a continuous SiO₂ layer along with carbide dissolution/oxidation at grains boundaries and the large elongated grain alloy microstructure.

Abstract

The microstructure and oxidation behavior of Laser Beam Melted, Electron Beam Melted and wrought Hastelloy X were studied at 950 °C in dry air, 50 h cycles, up to 1000 h. The variability of processing parameters and powder compositions strongly impacted the oxidation and spallation behavior. The alloys with high Mn and Si were associated with higher oxidation rates, finer Cr₂O₃ grain microstructure and increased spallation was observed for the alloys with large elongated grain microstructures (aspect ratio of 10 ± 3).

Introduction

The rapid development of Additive Manufacturing (AM) processes, such as Selective Laser Melting (SLM) or Electron Beam Melting (EBM), is driven by the growing need for manufacturing high performance components with complex geometries and minimum material wastage. AM uses computer-aided design to build objects layer-by-layer in contrast with casting processes where numerous subsequent operations are needed in order to obtain the desire shape object geometry. SLM and EBM are Powder Bed Fusion (PBF) methods involving a local melting and rapid solidification of the powder [1]. The solidification microstructure is governed by the ratio between the temperature gradient, the solidification rate and under-cooling △T [2]. Usually SLM and EBM processes involve fast cooling rates inducing a preferred crystallographic direction for solidification along [100] for face centered and body centered cubic materials and non-equilibrium solidification conditions [1], [3], [4]. For EBM, the build temperature can be modified allowing additional manipulation of the resulting alloy microstructure. However, without understanding of the coupling between alloy metallurgy and AM parameters, different alloy mechanical properties can be obtained. For example, for SLM and EBM alloys, interdentritic precipitation was observed to be detrimental for the mechanical properties of several materials (such as IN718 or Ti-6Al-4V) [5], [6], [7]. Control of AM parameters [8], [9], [10] and post-processing of AM parts (heat treatment, HIP-treatment, polishing) [11], [12], [13], [14], [15], [16] are being studied in order to achieve comparable mechanical properties with the respective wrought materials.

Ni-based alloys are used for their combined corrosion resistance and high temperature strength. They are strengthened by precipitates and/or by elements forming a solid solution. Hastelloy X® is a Ni-based alloy solution and carbide strengthened (presence of Mo-rich carbides) [17], [18], [19] and is widely used in gas turbine engines environments for its high temperature properties. The oxidation behavior of wrought Hastelloy X was studied in the past under isothermal conditions [20], [21], [22] and under cyclic conditions in air between 760 °C and 1150 °C for times up to 10,000 h [23]. The oxidation behavior was also studied in air + 10 % H₂O at 950 °C [19] and under carburizing isothermal (650 °C) [24] and cyclic conditions (1000 °C) [25]. In general, at these temperatures, wrought Hastelloy X was observed to form an outer (Ni,Fe,Cr)₂O₄ or MnCr₂O₄ spinel, if the alloys contains sufficient Mn, along with an inner protective Cr₂O₃ scale and partial SiO₂ underlayer. In a turbine environment, the alloy is exposed to thermal cycling conditions which can induce the spallation of the protective scale [26].

While studies focused on the mechanical properties and microstructure of AM Hastelloy X [27], [28], [29], [30], [31], [32], the oxidation behavior of AM alloys has received little attention in the literature [33], [34], [35], [36], [37]. One preliminary study was found on the oxidation behavior of Hastelloy X made by EBM and SLM after isothermal and 100 h cyclic exposure in air at 950 °C [38]. Higher oxidation and spallation rates for (Mn, Si)-containing AM materials were found and were associated with the presence of Mo, Si-rich carbides at grain boundaries [38]. Parabolic isothermal oxidation behavior of SLM IN718 after 100 h exposure in still air at 850 °C was observed and increasing spallation of the oxide film with increasing laser volumetric energy density [33]. Similar isothermal oxidation kinetics were observed for SLM and EBM IN 718 after 48 h between 600 and 800 °C in air [37] and after 100 h at 800 °C in air [35]. In [35], roughness of EBM IN 718 was considered to evaluate the oxidation rate constant. Higher oxidation kinetics of as-fabricated IN 718 than wrought IN 718 were measured after 1000 h exposure in air + 10% H₂O between 650 and 750 °C due to the formation of Fe-rich oxide nodules on EBM IN 718 [34]. Finally, similar short-term oxidation kinetics were seen between as-fabricated, Hot Isostatic Pressed (HIP) and traditionally hot-rolled Ti-6Al-4V after up to 500 h isothermal exposure at 600 °C [36]. In most of the studies, the combined influence of alloy composition, microstructure and exposure conditions (thermal cycling) on the oxidation behavior were not covered.

The objective of this paper is to evaluate the oxidation behavior of three different AM Hastelloy X materials under isothermal and cyclic oxidation conditions. Thermogravimetric analyses were performed to study the early stage oxidation behavior after 8 and 72 h exposure at 950 °C. Simultaneously, the specimens were thermally cycled with a hold time of 50 h in a furnace at the same temperature for a duration of up to 1000 h to study their cyclic oxidation behavior. The effect of composition (Mn and Si contents) and alloy microstructure on the Cr₂O₃ scale microstructure and oxidation kinetics will also be discussed.