Design and Development of Hybrid Metal and Polymer Additive Manufacturing System
Additive manufacturing (AM) is a layer-based manufacturing process aimed at producing parts directly from a Computer-aided design (CAD) model. There are various types of AM systems, which can be classified based on: (i) the base material being used for fabrication, such as polymers, ceramics, and metals; (ii) indirect and direct processes depending on the bonding method; and (iii) the state of the input raw material, i.e., liquid, molten, powder, and solid layer. The current research in AM processes includes technology development for the printing of multi-material parts using two or more materials such as metal, polymer, glass, ceramics, and graphene. The multi-material additive manufacturing (MMAM) processes are complex and challenging due to the significant differences in material deposition techniques, material processing temperature, or pre/post-processing methods involved for an individual material. Our work focuses on the hybrid metal/polymer printing process where liquid metal printing is achieved using a novel design of a molten metal droplet-on-demand (MMDoD) system. The metal is fed into the MMDoD system in the form of solid wire and is melted using a zero-voltage-switching circuit based induction heater. The magnetic field, eddy current density, and power transfer from the induction coil to the molten metal pool are studied using experiments, theoretical formulation, and FEM simulations. The influence of workpiece geometry on the induction heating process is also studied for solder alloy and aluminum billets. These studies show that for a given geometry of induction coil and workpiece, the power transferred to the workpiece is a non-monotonic function of the workpiece’s resistivity. Also, the heating rate of the workpiece depends on the thermal mass and the magnetic field flux in and around the workpiece. Using these studies, the resistivity of the workpiece, and the geometry of the workpiece and induction coil, can be chosen to achieve faster heating and melting of the metal. Once the raw material is in the liquid state, it can be used to generate molten metal droplets (MMDs). To generate the MMD, a novel MMDoD system is designed and developed using a thermally insulating piston and magnetostrictive actuator. Using the MMDoD mechanism, the molten metal is deposited on the printing bed surface (glass) or partially formed part (metal or polymer – PLA/ABS). To find the optimal parameters of MMD generation process, a parametric study of the MMDoD mechanism is conducted by varying the size and material (Brass, Stainless-Steel, Nickel-plated steel alloy) of the nozzle, the gap between nozzle and piston, unfiltered vs. low-pass filtered actuation pulse, and the actuation pulse amplitude. This shows the following regions where, DoD process is not achieved, and the DoD is achieved with the generation of single or multiple droplets for each actuation. The droplet size, Feret width and length, and standard deviation are measured using snapshots from the high-speed camera of the droplet formation process. The region where a single MMD is generated for each actuation of the MMDoD mechanism with the least standard deviation is most desirable for the reproducible metal AM process. A parametric study is conducted to find the optimal printing parameters of the metal AM system by varying the gap between each droplet to print the 2D connected metal lines on the substrate. Other parameters like the size of MMDs, droplet ejection rate (20Hz), and liquid metal temperature are kept fixed. The 3D metal printing can be achieved by printing these metal lines layer-by-layer. MMDoD system is extended to multi-material additive manufacturing (MMAM) system by combining it with polymer extrusion system. The designed MMAM system consists of a controller board to control the overall system, an induction heater, a computer numeric control (CNC) build platform/positioning system, MMDoD mechanism, and the polymer extruder. The system is designed and developed to print metal (Solder alloys - Sn99Cu1, Sn63Pb37, and Sn96.5Ag3.5) with polymer (PLA and ABS). To demonstrate the hybrid AM of metal and polymer, a few mechanical structures (2D planar text, hollow tube, hollow square pyramid, hollow hexagon) and electronic device (RC-LED circuit) are fabricated. The deposition of molten metal on polymer substrate leads to good bonding of metal on polymer due to remelting of the polymer surface. The working of the printed, electronic device is tested and found satisfactory. The testing is conducted by checking the electrical connectivity along the track and the functionality of the electronic device by measuring the output signal waveform. In the future, the combined metal and polymer AM system can be combined with a pick-and-place mechanism that can help achieve a rapid AM of functional 3D electronic devices.
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