July_August_AMP_Digital

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 U L Y / A U G U S T 2 0 1 9 1 7 related to changes in the local micro- structure and alloy chemistry caused by differences in the temperature his- tory during processing. By controlling the local microstructure and chemistry in NiTi SMAs, it is not only possible to achieve properties that can be manip- ulated using an external stimulus (tem- perature or stress), but also to provide location-specific properties [4] . Further, 4D printing of metallic functional ma- terials has the potential to go beyond NiTi SMAs and provide fabrication flexibility for other types. These in- clude SMAs such as high temperature and ferromagnetic versions, as well as magnetostrictive materials and other magnetic systems with properties that are microstructurally and chemically sensitive. INGREDIENTS OF 4D PRINTING To achieve 4D printing in metal- lic functional materials, it is first neces- sary to identify feasible regions in the alloy-process space that result in suc- cessfully fabricated parts. Feasible re- gions in this space can be identified in terms of their printability [5] , which is a global indicator for the suitability of al- loy-process parameter combinations for printing a component free of macro- scopic defects. Printability characteristics of com- monly used AMmaterials such as SS316, Ti-6Al-4V, and Inconel 718 are well es- tablished in industry and academia. However, when a new material is to be used as a feedstock for AM, lengthy and costly trial and error experimental pro- cedures are inevitable to find the op- timal processing parameters that will yield defect-free printed parts. In an ef- fort to expedite process parameter se- lection and optimization, this research team has established a computational methodology for predicting the print- ability of alloys during laser powder bed fusion (LPBF) AM [6] . The printabili- ty metric is defined as a hyper volume in the processing parameter space. Specifically, regions in the laser power vs. scanning speed space are first iden- tified and associated with single tracks that are free of major defects. The print- ability region is then outlined in terms A dditive manufacturing (AM) or 3D printing is a manufacturing technique in which complex geometries can be fabricated in a layer-by-layer fashion. Yet the flex- ibility brought about by AM goes beyond the ability to produce com- ponents with intricate shapes. One of the most attractive capabilities of AM is the potential to produce parts and components with engineered proper- ties. 4D printing, for example, enables fabrication of complex objects––gen- erally built from functional materials, e.g., shape memory polymers and (re- versibly swelling) hydrogels that trans- form over time (the fourth dimension) when subjected to external stimuli. AM also enables fabrication of functionally graded materials, where spatial tailor- ing of the properties of the fabricated part is possible by controlling feedstock and/or process conditions. In contrast to polymeric materi- als, 4D printing of metallic function- al materials is of special interest due to the capacity for self-assembly and multi-functionality, with the added benefit of higher actuation capability. For example, AM has been shown as a viable method to fabricate NiTi shape memory alloy (SMA) parts, overcoming challenges suchas lowmaterial removal rates and high tool wear associated with conventional approaches [1] . SMAs are a special class of alloys that, when deformed, have the ability to return to their undeformed shape upon heat- ing or removal of the load. This shape change, which can reach as high as 10% in NiTi SMAs, is realized through a re- versible martensitic transformation be- tween a cubic parent phase and a mono- clinic product phase. NiTi SMAs are re- markably different from conventional engineering alloys due to this large re- versible shape change capability, with the added benefits of superior biocom- patibility, high specific strength, and corrosion and wear resistance. These features have successfully enabled in- tegration of NiTi SMAs in a plethora of applications such as sensors, actua- tors, stents and orthodontic archwires, eyeglass frames, vibration dampers, pumps, micro valves, miniature grip- pers, and more [2] . The physical and functional re- sponses of AM NiTi SMA parts greatly depend on the processing parameters used during fabrication [3] . Small differ- ences in processing parameters result in amplified variations in transformation temperatures, strains, and mechani- cal strength, allowing tunability on de- mand. These functional changes are