Additive manufacturing of Inconel 718 using solid-state bonding

Abstract

The capability for manufacturing large-scale metallic components is a sign of a country's productive capacity. Further capability improvement is limited by factors such as quality control during the casting (solidification defects), the high tonnage requirement on equipment and the high energy consumption of the processes. In this thesis, a new manufacturing method has been proposed for the construction of large or extra-large components, based on a layer-by-layer solid-state bonding method. The advantages of the newly proposed method lie in improved quality control and a lower tonnage capacity requirement for the equipment. For this novel method, the bonding quality between each layer is the key to the final product achieving high performance in service. In this thesis, the bonding has been conducted at different scales: at a small-scale using bar samples and a Gleeble testing machine, and at a larger scale using three-layer plates under forging conditions to first demonstrate the construction concept. At the Gleeble scale, the bonded samples, as well as reference samples without a bonding interface, have been tested to evaluate bonding interface quality, while excluding the microstructural effects. From microstructure characterization, the bonding mechanisms have been revealed as being a mix of oxide film fracture, metal extrusion and microstructure-evolution-induced interface grain boundary migration processes. Based on these, a four-stage model has been developed to quantitatively describe the bonding process and predict the bonding quality. At the larger plate scale, the bonding was conducted under practical industrial conditions. The effects of vacuum, ageing heat treatment, and location have been evaluated through tensile tests at both room and high temperatures. Based on the proposed bonding mechanism and model, the results from the larger plate forging trial show agreement between model and practice. Due to the complexity of the various mechanisms dominating the bonding process, it is important to understand the role of grain boundaries at the bonding interface. To identify this role, bi-crystalline samples, as well as polycrystalline samples showing typical grain boundary migrated and non-migrated behaviour across the bonding interfaces were manufactured and tested so that the role of interface grain boundary could be unambiguously determined.