Thermal and Thermo-Mechanical Behaviour of Additively Manufactured Hastelloy X

Abstract

Additive manufacturing (AM) is one of the emerging technologies to manufacture near-net-shape engineering components layer-by-layer using a suitable heat source. The rapid adoption of AM in the field of aerospace owes to its ability to fabricate intricate parts in one step, sidestepping multiple processing stages, thereby reducing time, costs, and material waste. The blend of AM with nickel-based superalloys enables the production of intricate and durable components for the aerospace industry. The laser powder bed fusion processing (LPBF) of Hastelloy X serves as a prime example of this capability. While Hastelloy X has a long-standing history in aerospace applications, additive manufacturing represents a novel manufacturing technique. To ensure the industrial implementation of additively manufactured Hastelloy X, validating its viability through an extensive array of experimental investigations is imperative. Hastelloy X, known for its exceptional strength, oxidation resistance, and ability to withstand high temperatures, is particularly suitable for demanding applications in aerospace, especially in gas turbine engines. This study aims to investigate the microstructural and mechanical properties of LPBF processed Hastelloy X to establish a correlation between its various microstructural states and both room-temperature and high-temperature mechanical behaviours. Chapter 1 provides a comprehensive literature review of additive manufacturing processes, including their various types, benefits, and applications in high-temperature materials. Additionally, a detailed overview of the microstructural characteristics, mechanical behaviour, texture properties, and creep behaviour of additively manufactured Hastelloy X is presented. The chapter identifies the existing knowledge gaps regarding AM processed Hastelloy X, thereby outlining the scope of the work to address these gaps. Chapter 2 provides a thorough elaboration of all the experimental details utilized in this study. Chapter 3 comprises the initial characterization of as-built LPBF Hastelloy X. Additionally, the effect of heat treatment processes in incrementally rising temperatures on as-built microstructure, texture and room temperature tensile behaviour has been explored. Elongation was found in the range of 55 -75%, outperforming the wrought counterpart. The exceptional ductility of heat-treated specimens was attributed to combined factors, including the nearly full-density specimen (designed to have no cracks and minimal porosity), the stacking fault, the dislocation-mediated plasticity caused by the formation of microbands, and the twinning-induced plasticity (TWIP) effect. In Chapter 4, the elevated temperature tensile behaviour of stress-relieved (1050°C for 1 hour) Hastelloy X ranging from 500°C to 900°C have been investigated in detail. Furthermore, the precipitation behaviour of M6C and M23C6 carbides in the above-mentioned temperature range has also been explored. The gradual decline in elongation was linked to the prevalence of intergranular fracture at higher temperatures, driven by the precipitation of carbides along the grain boundaries. Chapter 5 explores the effect of prolonged thermal ageing on microstructural evolution and mechanical behaviour. The stress-relieved Hastelloy X underwent prolonged thermal exposure at two distinct temperatures, namely 800°C and 950°C, each for a duration of 500 hours. This investigation aimed to elucidate the distribution and evolution of secondary phases within the material. A comprehensive microstructural analysis utilizing transmission electron microscopy (TEM) revealed the development of M23C6 carbides alongside topologically close-packed (TCP) phases such as μ, R, and P. The room temperature tensile tests demonstrated higher yield strength and lower elongation of the 800°C, 500 h specimen compared to the 950°C, 500 h specimen, which was attributed to the precipitation strengthening provided by the μ phase. In Chapter 6, creep and stress rupture behaviour of LPBF Hastelloy X have been investigated in the stress range of 75 – 150 MPa at 800°C, accounting for the microstructural anisotropy in two different directions in stress-relieved conditions. The investigation reveals that the origin of the threshold stress is attributed to the dynamic precipitation of carbides and TCP phases, in addition to the pre-existing Al-Ti-rich oxides. Accounting for the threshold stress, the stress exponent values were estimated to be 4.5, indicating dislocation climb as the underlying deformation mechanism. Notably, vertically oriented samples exhibit significantly superior creep and stress rupture properties compared to their horizontally oriented counterparts. This discrepancy is largely attributed to the columnar grain morphology observed in the vertical specimens, contrasting with the equiaxed morphology seen in the horizontal specimens. Chapter 7 offers a comprehensive summary and outlines future research directions based on the foundational work laid out in preceding chapters.