1 5 A D V A N C E D M A T E R I A L S & P R O C E S S E S | J A N U A R Y / F E B R U A R Y 2 0 2 3 round and square feedstock. These machines run off the same FSW platforms Bond has been providing to customers for several years, but now create a hybrid AFSD machine capable of FSW, AFSD/FSAM, and machining with temperature and force feedback controls complementing displacement control processing. Generically, the variable process parameters are tool geometry, spindle rotational speed, filler feed rate, and tool traverse rate. The tool design has been shown to have a large impact on the deposited layer thickness, and thus the volumetric deposition rate. The next most significant process parameter controlling the volumetric deposition rate is the traverse rate. The maximum traverse speed for a defect-free deposition is a function of tool design, filler feed rate, and spindle rotational speed. While the spindle rotational speed is the most significant parameter in controlling the temperature for a given tool, the filler feed rate is the most significant parameter in controlling the pressure for a given tool plunge depth. Hence, the interdependent process parameters need to be optimized for a in comparison to fusion-based additive manufacturing techniques, e.g., laser powder bed fusion (L-PBF) and electron beammelting (EBM). The lower severity of thermal cycles reduces the residual stresses within the deposition and substrate, thus, residual stress induced distortion is minimized[23]. The quality of the desired microstructure and fabrication rates are established through flexible process parameters such as spindle geometry, spindle rotation speed, filler feed rate, and tool traversing rate. COMPARISON TO OTHER PROCESSES It is important to note that although AFSD has the advantage of providing increased deposition rates in an open atmosphere (certain materials like Ti alloys require shielding) with low energy there are certain disadvantages. Finish machining is required when using AFSD since components are generally more near net-shape when compared to other AM methods. Figure 3 compares the build accuracy and build rate of AFSD, directed energy deposition (DED), and powder bed fusion (PBF). The current nascent state of the AFSD technology is commercially available from MELD Manufacturing and is constrained to square cross-section feedstocks. The process is controlled by operator written, traditional CNC G-code via the commercial MELD Manufacturing software, which translates the machine code into 3-axis movements and deposition rates. During operation, the software generated timestamped machine feedback at 1 Hz is subsequently analyzed for temporal and material consumption examination. Tabulated position data for the machine head and linear actuator are determined by position sensors located in the AFSD machine, which are used to determine absolute distance and material consumption. Although not equipped to provide feedback control, force measurements are recorded at 1 Hz via electrical current required to maintain feedstock deposition velocity. Companies such as Bond Technologies provide alternative AFSDmachine platforms, referred to by Bond Technologies as friction stir additive manufacturing (FSAM), machine platforms that are now capable of depositing both Fig. 3 —Comparison of build accuracy versus build rates for additive friction stir deposition, powder bed, and direct energy deposition.
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