Additive Friction Stir Deposition (AFSD), often referred to in industrial practice as AFM for complex hollow dies, is an emerging solid-state manufacturing technology well suited for producing large, intricate metal components with internal cavities and challenging geometries. Unlike conventional fusion-based additive processes, AFSD builds parts without melting the feedstock. Instead, a rotating tool plasticizes a metallic feed material through frictional heating and severe shear deformation, then deposits the softened material layer by layer onto a substrate. This solid-state mechanism helps reduce common defects such as porosity, solidification cracking, and distortion, which are especially problematic in complex die structures.For hollow dies, dimensional stability and internal integrity are critical. Traditional manufacturing methods often rely on extensive machining from forged blocks or multi-piece assembly, both of which can be time-consuming, material-intensive, and expensive. AFSD offers a more efficient alternative by enabling near-net-shape fabrication of the die body, including thick sections, curved walls, and internal channels that would otherwise require multiple machining and joining operations. Because the process occurs below the melting point, it can also preserve more favorable microstructural characteristics, leading to improved mechanical performance and reduced residual stress.One of the main advantages of AFSD for complex hollow dies is design flexibility. Engineers can integrate weight-saving cavities, cooling passages, and functional features directly into the printed structure. This is particularly valuable for tooling applications where thermal management affects service life and production quality. By tailoring the internal geometry, heat dissipation can be improved and localized thermal fatigue reduced. In addition, the process supports the repair and rebuilding of worn die sections, extending tool life and lowering replacement costs.However, the successful application of AFSD to complex hollow dies requires careful control of process parameters, including rotational speed, traverse speed, feed rate, temperature, and tool geometry. These factors influence material flow, bonding quality, surface finish, and geometric accuracy. For hollow structures, support strategy and path planning are also important to prevent collapse, distortion, or incomplete filling in internal regions. Post-processing such as machining, heat treatment, and inspection is usually necessary to achieve final tolerances and ensure structural reliability.Overall, AFSD presents a promising route for manufacturing complex hollow dies with reduced lead time, lower waste, and improved performance. Its ability to create dense, robust, and customizable metallic structures makes it an attractive option for advanced tooling applications where traditional methods face limitations.