ABSTRACT

Additive manufacturing (AM) allows for the fabrication of complex near net shape components, leading to new design opportunities and significant savings, for example by decreasing tooling and materials cost or by accelerating prototype development. New Ni-based alloy heat exchangers with thin-walled areas exposed to hot gases will, however, suffer from corrosion degradation, and the high temperature oxidation performance of AM alloys needs to be addressed. Alloy N06002 (Ni-22Cr-18Fe-9Mo) fabricated by electron beam melting (EBM) and selective laser melting (SLM) along with EBM-fabricated N07247 and modified N07247 were cyclic oxidation tested at 950°C in air and humid air for durations ranging from 100 h to 5000 h. Increased spallation rates were observed initially for the AM N06002 alloys in comparison with wrought N06002 alloy because of the AM alloy specific microstructures. After ~2000 h, the situation was reversed with significant spallation observed for the wrought HX alloy only. Low mass gains were observed for the EBM N07247 alloys at 950°C, but thick oxide scales were observed locally due to the presence of oxidized cracks. Massive oxidation of these cracks led to fast oxidation rates at 1100°C.

INTRODUCTION

Additive manufacturing (AM) provides the unique opportunity to fabricate complex near net shape components leading to significant saving by accelerating component development through fast prototyping, customization, or by decreasing the cost of tooling and materials [1]. The large-scale production of high-temperature Ni-based components has not been achieved yet, because of the difference in microstructure between wrought/cast materials and similar alloys fabricated by AM. For electron beam melting (EBM) and selective laser melting (SLM), the microstructure is highly dependent on the deposition parameters and is often anisotropic due to thermal gradients during the building process [2], Therefore, qualification of AM Ni-based components, requires generation of mechanical and oxidation data relevant for in-service applications. So far, many studies have focused on microstructure, tensile and fatigue properties of alloys such as N07718 [3], N06625 [4], N06002 [5-6] or N07247 [7], but long-term properties such as creep and cyclic oxidation are scarce [8-9]. This paper compares the cyclic oxidation behavior at 950°C in static air of AM N06002, EBM N07247 (MarM247) and EBM modified N07247 (CM247), alloys commonly used in gas turbine components, with the cyclic oxidation behaviors of similar wrought alloys. For the EBM N07247 and EBM Mod N07247, data at 1100°C will also be presented and discussed.

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