Development of laser powder bed fusion and heat treatment parameters for additively manufactured Ni51.5Ti48.5 shape memory alloys, The

Abstract

Local thermodynamic control in Laser-Powder Bed Fusion (L-PBF) Additive Manufacturing (AM) provides the opportunity to exploit Process-Structure-Property (PSP) relationships to drive local composition changes in certain materials. This is particularly useful for Shape Memory Alloy (SMA) systems such as Nickel Titanium (NiTi or NiTiNOL), where subtle processing changes have been shown to produce dramatic composition and transformation temperature changes. These characteristics, combined with the capability to produce complex geometries, provide a platform for transforming the design process for materials utilized in the medical and aerospace industries. To date, research on AM SMA's has focused on Ni50.8Ti49.2 (or lower Ni) compositions with challenges in processing, composition control, and functional performance that has limited commercial interest. Due to in-process Ni-vaporization the resulting as-built compositions with this feedstock can be lower than desired for medical device components. This work takes a different approach by beginning with Nickel-rich Ni51.5Ti48.5 powder feedstock with the intent to achieve compositions in as-built parts near Ni50.8Ti49.2 via Ni-vaporization and post-process heat treatments, and characterizes the process-structure-property relationships in as-built and heat-treated parts. Through a systematic production strategy, the relationships between build parameters and heat input metrics, geometry, defect formation mechanisms, and microstructural characteristics are elucidated with recommendations for processing considerations which minimize defects and maximize functional performance. It is found that embrittlement caused by increased Ni-composition results in a higher susceptibility to thermal-stress induced periodic cracking, which can be minimized either by increased heat input (e.g., Volumetric Energy Density (VED)), decreased layer time, or a combination thereof which is geometrically dependent. It is also found that residual effects of increased heat-input occur, including overmelting, dislocation annihilation and transformation temperature changes affected by both Ni-vaporization and secondary phase formation, which are also geometrically dependent. Further, an extensive post-processing heat treatment study is conducted to determine direct-age and two-step age conditions which can produce elevated-temperature functional compressive superelasticity. It is found that through preceding a 400C 4h heat treatment step with a 200C 12h pre-age step to drive Ni-clustering, conditions can be achieved with compressive strength exceeding 2GPa and 5-7% recoverable strain, and tensile strength near 800MPa.