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This study aims to investigate surface roughness, microstructure, and mechanical properties of overhead thin-wall structures of stainless steel(SS316L) fabricated by cold metal transfer (CMT)-based wire + arc additive manufacturing (WAAM). In the first stage, single-layer bead experiments were carried out in flat and overhead positions utilizing Box-Behnken experimental design with a range of process parameters (i.e., wire feed rate, travel speed, and weave amplitude). To study the effect of individual process parameters on the bead geometry and identify a process window, analysis of variance(ANOVA) is performed using the bead cross-section measurement data. For single layer bead experiments in flat and overhead position, out of all process parameters, the weave amplitude is the most significant parameter on bead width, whereas travel speed is most significant parameter for bead height. Based on single-layer bead experiments, process parameters for thin wall deposition were identified. In the second stage, two thin-walls were deposited with wire feed rates of 1000 and 1500 mm/min in the overhead position. The surface roughness was measured using cloud point data acquired from the coordinate measuring machine(CMM). The deposited structure with the wire feed rate of 1500 mm/min resulted in better surface quality. It was also observed that, microstructure was composed of austenite and dendritic delta ferrite. The microstructure changed as the deposition height increased. The average microhardness value was measured 183 HV and 187.4 HV for the overhead structures. Average tensile properties of the SS316L overhead structures were comparable to that of SS316L fabricated by other WAAM processes. 

This paper proposes a conceptual architecture of digital twin with human-in-the-loop-based smart manufacturing (DH-SM). Our proposed architecture integrates cyber-physical systems with human spaces, where artificial intelligence and human cognition are employed jointly to make informed decisions. This will enable real-time, collaborative decision-making between humans, software, and machines. For example, when evaluating a new product design, information about the product’s physical features, manufacturing requirements, and customer demands must be processed concurrently. Moreover, the DH-SM architecture enables the creation of an immersive environment that allows customers to be effectively involved in the manufacturing process. The DH-SM architecture is well fitted to those relatively new manufacturing processes, such as metal additive manufacturing, since they can benefit from using digital twins, data analytics, and artificial intelligence for monitoring and controlling those processes to support non-contact manufacturing. The proposed DH-SM will enable manufacturers to leverage the existing cyber-physical system and extended reality technologies to generate immersive experiences for end users, operators, managers, and stakeholders. A use case of wire + arc additive manufacturing is discussed to demonstrate the applicability of the proposed architecture. Relevant development and implementation challenges are also discussed.