ADVANCED MATERIALS & PROCESSES | SEPTEMBER 2023 21 Fig. 1 – Schematic of a cutout of a part with an advancing SBT tool in the process of producing an EIP[1,3]. associations[7–9]. One way to advance the overall maturation of the technology could be to implement a blockchain technology for part design and process documentation traceability[10]. Lacking in the metalworking industry until now has been a robust fabrication method for producing EIPs in wrought plate products machined, for example, into unitized structures. Various drilling operations have long been available for producing round, linear holes in components machined from the various product forms listed in Table 1. Producing nonlinear 3D pathways and non-round internal passages within machined parts, or hog-outs, has typically depended on assembling two or more parts manufactured from rolled plate or other product forms. SUBMERGED BOBBIN TOOL TUNNELING With the advent of submerged bobbin tool (SBT) tunneling as a new localized forming technology, the metalworking industry can now place EIPs within hog-outs in the same setups for machining operations. This new fabrication option was introduced by Burford and Mishra in early 2021 as a variant of friction stir processing (FSP)[2]. Subsequent research and development has demonstrated EIPs can be produced with SBT tunneling in plate stock as well as in cast material, forged blocks, and in wrought metal printed via the MELD process, a solid-state AM process[11]. SBT tunneling is illustrated in Fig. 1. This new FSP technique involves forming integral internal channels through specially designed bobbin toolsets. Conventional bobbin toolsets used for friction stir welding (FSW), such as described by Thomas et al.[12] and Goetze et al.[13] were not found to be adaptable to this new use for friction stir bobbin tools. Therefore, specially designed bobbin tools were developed for tunneling operations. Like a conventional bobbin tool (BT) for FSW, an SBT has two opposing shoulders spaced apart along the probe section of the tool. Unlike a conventional BT, an SBT is used to form integral subsurface channels, or EIPs, by passing the shoulder at the end of the probe through the workpiece during processing. Using these specially designed toolsets tailored for tunneling, non- linear (3D) and non-round channels have been produced in sample structures made of malleable plate materials, including aluminum 2000, 5000, 6000, and 7000 series alloys. As also shown in Fig. 1, the process involves passing the SBT toolset along a specified channel path with the tool oriented orthogonally to the part surface along the EIP path. SBT DESIGN EVOLUTION Studies show that this special form of FSP has relatively low process forces, allowing it to be deployed on computer numerical control (CNC) machining centers and friction stir-capable industrial robots, as well as on purpose- built FSP machines. This suggests SBT tunneling holds potential use in production environments for applications requiring curvilinear internal pathways for wiring, fluids, and gases, as well as producing internal spaces for the strategic placement of voids or solid materials, such as composites. An earlier form of FSP for producing internal channels for heat exchangers and similar hardware, called friction stir channeling (FSC), was introduced by Mishra et al.[14–20] Mehta and Vilaça have reviewed variants of FSC that emerged since Mishra’s original disclosure[21]. Notable related variants include a stationary shoulder approach recently advanced by Gandra et al.[22–24] Also notable, Karvinen et al. have formed channels along joint lines between similar and dissimilar metal components while joining the parts by combining FSC with FSW[25, 26]. The initial SBT development was carried out starting in late 2020 on a CNC tool room lathe equipped with a specially-designed fixture as described by Burford et al.[1,3] This approach to forming channels was pursued because of the potential bobbin tools have for reducing out-of-plane process forces compared to single-sided friction stir tools. Similar to other BT toolset designs, the opposing shoulders of SBT designs serve to contain a significant portion of stirred material generated throughout the progression of the process. As a result, process forces generated parallel to the tool’s axis of rotation are partially reacted between the opposing shoulders. Compared to single-sided tool designs having one shoulder, SBT toolsets therefore produce relatively lower out-of-plane forces that must be supported by the fabrication equipment in use. This, in turn, means that the fabrication equipment for the SBT process has reduced stiffness requirements compared to equipment for single-sided FSC methods. Also demonstrated in this work, SBT tunneling may be performed rapidly with an appropriate toolset design and process parameters. For example, well-formed channels have consistently been produced at travel speeds of 635 mm/min in AA6061-T6511 aluminum bars, and
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