Abstract
Additive manufacturing of Nitinol is a promising field, as it can circumvent the challenges associated with its conventional production processes and unlock unique advantages. However, the accompanying surface features such as powder adhesions, spatters, ballings, or oxide discolorations are undesirable in engineering applications and therefore must be removed. Plasma electrolytic polishing (PeP) might prove to be a suitable finishing process for this purpose, but the effects of post-processing on the mechanical and functional material properties of additively manufactured Nitinol are still largely unresearched. This study seeks to address this issue. The changes on and in the part caused by PeP with processing times between 2 and 20 min are investigated using Nitinol compression springs manufactured by Laser Beam Melting. As a benchmark for the scanning electron microscope images, the differential scanning calorimetry (DSC) measurements, and the mechanical load test cycles, conventionally fabricated Nitinol springs of identical geometry with a medical grade polished surface are used. After 5 min of PeP, a glossy surface free of powder adhesion is achieved, which is increasingly levelled by further polishing. The shape memory properties of the material are retained without a shift in the transformation temperatures being detectable. The decreasing spring rate is primarily attributable to a reduction in the effective wire diameter. Consequently, PeP has proven to be an applicable and effective post-processing method for additively manufactured Nitinol.
Highlights
As the most widely used shape memory alloy, Nitinol exhibits a combination of characteristic thermomechanical properties that predestine the material for a range of applications
The material removal rate (MRR) characterizes the amount of material removed per unit of time and describes the productivity of the polishing process
Based on a shared design, Nitinol smart springs were manufactured both by Laser Beam Melting (LBM)
Summary
As the most widely used shape memory alloy, Nitinol exhibits a combination of characteristic thermomechanical properties that predestine the material for a range of applications. Nitinol exhibits an ordered BCC lattice structure at high temperatures, which is referred to as austenite When it is cooled beyond the transformation range, it converts to a less ordered twinned martensite structure. The characterizing transformation temperatures (TT) of the austenite (Start AS , Finish AF ) and martensite transformation (analogue MS , MF ) as well as the resulting hysteresis width react very sensitively to the chemical composition, in particular the Ni-Ti ratio in the matrix [7]. Another decisive influence is the thermal and mechanical processing involved in the selected production
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