Scalable laser powder bed fusion processing of nitinol shape memory alloy
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Research Letter
Scalable laser powder bed fusion processing of nitinol shape memory alloy Ian McCue, Christopher Peitsch, Tim Montalbano, Andrew Lennon, Joseph Sopcisak, Morgana M. Trexler, and Steven Storck, Research and Exploratory Development Department, The Johns Hopkins University Applied Physics Laboratory (JHU/APL), Laurel, MD 20723, USA Address all correspondence to Steven Storck at [email protected] and Morgan M. Trexler at [email protected] (Received 16 June 2019; accepted 16 September 2019)
Abstract The authors report on pulsed laser powder bed fusion fabrication of nitinol (NiTi) shape memory materials. The authors first performed singletrack laser parameter sweeps to assess melt pool stability and determine energy parameters and hatch spacing for larger builds. The authors then assessed the melt pool chemistry as a function of laser energy density and build plate composition. Brittle intermetallics were found to form at the part/build plate interface for both N200 and Ti-6-4 substrates. The intermetallic formation was reduced by building on a 50Ni–50Ti substrate, but delamination still occurred due to thermal stresses upon cooling. The authors were able to overcome delamination on all substrates and fabricate macroscopic parts by building a lattice support structure, which is both compliant and controls heat transfer into the build plate. This approach will enable scalable fabrication of complex NiTi parts.
Introduction Shape memory alloys (SMAs) are a unique class of functional materials with the ability to convert thermal energy into mechanical work by recovering their shape upon an increase in temperature. Nitinol (NiTi) is by far the most popularly used SMA due to its excellent mechanical properties, biocompatibility, and corrosion resistance.[1–4] In addition, NiTi exhibits superior shape memory and superelastic effects, capable of restoring large strains up to 8% by heating and unloading, respectively.[5] Owing to these physical and functional properties, NiTi has found application in a variety of industries such as sensing and actuation (aerospace), dampening (automotive), implants, and fixtures (biomedical).[1,2,5] However, further advancing functionality in these applications will likely require components with complex geometries, and NiTi is challenging to process, shape, and form. NiTi’s high ductility makes it difficult to machine, limiting parts to simplistic form factors such as wires, plates, bars, and tubes.[1,2,5,6] Furthermore, the wellknown shape memory effect is highly sensitive to Ni:Ti composition, impurities (carbon and oxygen), and secondary phase precipitates (Ti-rich and Ni-rich), which can be altered during high-temperature machining or fabrication processes.[5] All of these factors make NiTi, a perfect candidate for additive manufacturing (AM), adding SMAs to the rapidly growing topic area of 4D printing: complex three-dimensional (3D) parts that can undergo controlled shape changes in response to external stimuli.[1,2,4,6–13] There is huge potential to develo
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