Abstract
Defects in electron beam melting (EBM) manufactured Alloy 718 are inevitable to some extent, and are of concern as they can degrade mechanical properties of the material. Therefore, EBM-manufactured Alloy 718 is typically subjected to post-treatment to improve the properties of the as-built material. Although hot isostatic pressing (HIPing) is usually employed to close the defects, it is widely known that HIPing cannot close open-to-surface defects. Therefore, in this work, a hypothesis is formulated that if the surface of the EBM-manufactured specimen is suitably coated to encapsulate the EBM-manufactured specimen, then HIPing can be effective in healing such surface-connected defects. The EBM-manufactured Alloy 718 specimens were coated by high-velocity air fuel (HVAF) spraying using Alloy 718 powder prior to HIPing to evaluate the above approach. X-ray computed tomography (XCT) analysis of the defects in the same coated sample before and after HIPing showed that some of the defects connected to the EBM specimen surface were effectively encapsulated by the coating, as they were closed after HIPing. However, some of these surface-connected defects were retained. The reason for such remnant defects is attributed to the presence of interconnected pathways between the ambient and the original as-built surface of the EBM specimen, as the specimens were not coated on all sides. These pathways were also exaggerated by the high surface roughness of the EBM material and could have provided an additional path for argon infiltration, apart from the uncoated sides, thereby hindering complete densification of the specimen during HIPing.
Highlights
Additive manufacturing (AM), commonly known as three-dimensional (3D) printing, is a rapidly growing technology for generating complex geometrical products layer by layer from a 3D computer-aided design (CAD) model data [1,2]
The hypothesis is that encapsulation of these surface defects by applying a coating on the Electron beam melting (EBM)-built Alloy 718 could allow hot isostatic pressing (HIPing) to effectively heal all defects present, including those that are surface-connected
The as-built EBM Alloy 718 specimens were characterized by the presence of defects
Summary
Additive manufacturing (AM), commonly known as three-dimensional (3D) printing, is a rapidly growing technology for generating complex geometrical products layer by layer from a 3D computer-aided design (CAD) model data [1,2]. An idea of encapsulating the surface-connected “open” defects through deposition of a thin film/coating on the as-built specimen surface was explored in the literature for laser-based AM techniques [13]. This approach has not yet been widely investigated, and in particular no such efforts have yet been reported in case of EBM-manufactured material, where retained surface-connected defects after HIPing can be of concern. The hypothesis is that encapsulation of these surface defects by applying a coating on the EBM-built Alloy 718 could allow HIPing to effectively heal all defects present, including those that are surface-connected. Surface roughness of the as-built specimen is measured using white light interferometry
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