Effect of Ultrasonic Nanocrystal Surface Modification on the Microstructure and Martensitic Transformation of Selective Laser Melted Nitinol

Abstract

Nitinol has significant potential for biomedical and actuating-sensing devices, thanks to its functional properties. The use of selective laser melting (SLM) with Nitinol powder can promote novel applications aimed to produce 3D complex parts with integrated functional performances. As the final step of the production route, finishing processing needs to be investigated both for the optimization of the surface morphology and the limit alteration of the Nitinol functional properties. In this work, the effect of an advanced method of surface modification, ultrasonic nanocrystal surface modification (UNSM), on the martensitic transformation and microstructure of SLM built Ni50.8Ti49.2 (at.%) was investigated. Scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry indicated that the UNSM process can generate stress-induced martensite, at least partially suppressing the martensitic transformation. The microhardness profile indicates that the UNSM process can affect the mechanical properties of the SLMed Nitinol sample in a range of up to approximately 750 μm in depth from the upper surface, while electron backscatter diffraction analysis highlighted that the initial austenitic phase was modified within a depth below 200 μm from the UNSMed surface.

Additively Manufactured NiTi and NiTiHf Alloys: Estimating Service Life in High-Temperature Oxidation

“…It is well reported that the richer Ti is, the more oxidation happens [22,27,43,44]. As reported in our previous works [28,[45][46][47][48] different SLM process parameters or post processing result in different compositions, microstructures, and thermomechanical behaviors of the as-fabricated NiTi alloys. Such a variation in composition and microstructure of as-fabricated alloys can differently affect the oxidation resistance.…”

Biomedical Alloys and Physical Surface Modifications: A Mini-Review

“…UNSM is a method that applies ultrasonic impacts to generate nanostructured surfaces. During UNSM, the ultrasonic waves are passed through a tungsten carbide tip at high frequencies (~20 kHz) and transposed onto a sample surface (see Figure 4 ) [ 129 ]. The UNSM is unique in that it can produce precise hierarchical surface patterns by accurately adjusting the operation parameters.…”

Ultrasonic Surface Rolling Process: Properties, Characterization, and Applications

“…In the UNSM process, the primary function of the spherical roller tip is to create a mechanical impact and produce the SPD. In the case of USRP, the primary function of the spherical roller tip is to perform DR. Scholars adopted UNSM techniques on different steel grades [90][91][92][93][94], titanium alloys [95][96][97][98], aluminum alloys [99][100][101], and magnesium alloys [102,103], nickel alloys [89,104], copper alloys [105,106], shape memory alloys [107,108] high entropy alloys [109][110][111] and thus observed a significant enhancement in surface mechanical properties and surface integrity. In addition, there are advancements in USNM techniques, such as electropulsing-assisted UNSM and laser-assisted UNSM [112,113].…”