4D+ Printing â Form and Function Design in Shape Memory Alloys
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abstract
Parts that can morph their shape, adopt properties due to external stimuli, or, components that can inherently monitor themselves, may still appear like a dream of the future. In fact, complex engineering applications in oil and gas extraction, medicine, and, aerospace may become possible by such intelligent parts with intricate three-dimensional geometry that changes with time (shape change) and tailored physical properties that change with spatial locations (functionally gradient) â a need that cannot be satisfied by existing manufacturing methods. Nonetheless, advances in 4D printing (4DP) â additive manufacturing (AM) or 3D printing of stimulus-responsive materials can soon make such intelligent building blocks a reality. A 4D-printed part can undergo a programmed change in shape or properties over time under the influence of external stimuli, such as heat, stress, magnetic field, light, or moisture, among others. Thus, 4DP enhances the engineering design space by concurrently leveraging geometric intricacies offered by existing 3D printing tools and the all-important â fourth dimensionâ , i.e. the ability to transform properties over time. In the past five years 4DP in polymers has been somewhat successful but in metals it is considerably more challenging and still in its infancy. The PIs have recently demonstrated 4D Plus printing (4D+P) â an innovative additive manufacturing approach that combines 4DP and in-situ functional grading to allow simultaneous design of form and function in 3D metal structures. As a first example, the PIs fabricated a simple geometry, nickel-titanium shape memory alloy (SMA) part by in-situ manipulation of its thermal history during selective laser melting. The 4D+ printed NiTi part demonstrated a unique combination of spatial and time-dependent stimulus response because of subtle differences in local compositions, precipitates and microstructure. 4D+P offers exciting new possibilities as it further enhances the AM design space to â beyond the fourth dimensionâ i.e. the ability to transform properties over time and space. Therefore, to enable systematic and controllable 4D+ manufacture of intelligent metal parts with tailored (or graded) properties, our overarching scientific goal is to quantitatively link AM process parameters, thermal history, composition, microstructure, and spatial properties of technologically relevant SMAs. Our research objective is to exhaustively investigate the hypothesis that, â spatial and time-dependent stimulus response in shape memory alloy parts can be engineered by effective control of spatial thermal history during AMâ . To pursue this objective and climb up the TRL ladder to level 4, we propose to 4D+ additively manufacture intelligent metallic structures of simple to intricate geometry by selective laser melting (SLM) and/ or electron beam melting (EBM); with tailored, spatial and time-dependent properties. Our research approach involves five innovations. (1) in-situ thermal monitoring combined with finite element analysis to ascertain the spatial thermal history during AM, (2) high-throughput, quantitative, composition and microstructure characterization during AM..........
