Investigation of Post-Processing of Additively Manufactured Nitinol Smart Springs with Plasma-Electrolytic Polishing

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.

Plasma‐Electrolytic Polishing as a Post‐Processing Technology for Additively Manufactured Parts

“…Fig. 12 shows an example of upskin vs. downskin surface before PeP and after identical polishing time of 20 min [9].…”

Possibilities of Duplex Plasma Electrolytic Treatment for Increasing the Hardness and Wear Resistance of a Commercially Pure Titanium Surface

“…An alternative method of modifying metal and alloy surfaces to increase a variety of performance indicators is plasma electrolytic treatment [4][5][6][7]. There are a variety of processes aimed at obtaining ceramic-like coatings by plasma electrolytic oxidation [8][9][10][11][12][13][14] and surface polishing or removal of the surface layer by plasma electrolytic polishing (PEP) [15][16][17][18][19][20][21][22]. The most diverse is the surface modification through plasma electrolytic diffusion saturation: nitriding (PEN), boriding (PEB), and nitrocarburizing (PENC) [23,24], which have found a wide application for the processing of steel products.…”

Spray electrolyte plasma polishing of GH3536 superalloy manufactured by selective laser melting

“…The existing investigation results also suggest that EPP technology has a great application value in the medical field [17][18][19]. In 2021, Stepputat et al [13,20] studied the polishing effect of EPP technology on additive manufactured parts. The application of EPP technology on the inner surface of metal pipes was studied by Radkevich et al [21,22].…”