Electron beam melting (EBM) is a rapidly growing metal additive manufacturing technology due to its ability to manufacture near net shape metal parts, especially from high-value materials like Alloy 718. Defects like porosities in the EBM manufactured Alloy 718 are inevitable to some extent and are of concern as they can degrade the mechanical properties of the part. Therefore, EBM manufactured Alloy 718 is typically subjected to post-processing to improve the properties of the as-built material in order to achieve the required performance of the parts. Hot isostatic pressing (HIP) is usually employed to close the defects. Nevertheless, it is widely known that the HIP treatment cannot close the open defects, which are connected to the environment of the material.

Thus, the aim of this study was to investigate if the open defects in EBM built Alloy 718 can be encapsulated by application of a coating on the surface, to effectively close these during HIP treatment. The EBM manufactured Alloy 718 specimens were coated on two sides by high-velocity air fuel (HVAF) spraying using Alloy 718 powder and were HIPed. X-ray computed tomography analysis of the defects in the coated sample before and after HIP treatment showed that some of the defects connected to the EBM material surface were effectively encapsulated by the coating as they were closed after HIP treatment. However, some of these 'surface connected' defects were retained. The reason for such remnant defects could be their connection to the environment through the gap between the coating and the EBM material. The gap could have provided the path for argon infiltration from the uncoated sides during the HIP treatment, thereby hindering complete densification of the specimen. In addition, the lack of fusion defects were typically associated with inclusions, which could also be prevented their complete closure during HIP treatment.