ADVANCED MATERIALS & PROCESSES | SEPTEMBER 2026 35 BEYOND WIRE AND TUBE: SINTER-BASED ADDITIVE MANUFACTURING OF NiTi SHAPE MEMORY ALLOYS VIA LITHOGRAPHY-BASED METAL MANUFACTURING A look at two application fields where a sinter-based, photopolymer process enables new NiTi geometries beyond those previously accessible to wire and tube. Lucas Vogel,* Tarang Mehta, and Andreas Baum, MetShape GmbH, Pforzheim, Germany Carlo Burkhardt, Pforzheim University, Pforzheim, Germany The clinical and industrial applications of nickel-titanium (NiTi) alloys have developed around a limited set of semi-finished forms: drawn wire, laser-cut tube, and rolled sheet. Established devices such as self-expanding stents, stone-retrieval baskets, orthodontic archwires, and actuator springs are configured according to the geometries these processes can produce, rather than those an application might require[1]. This practice results from the sensitivity of NiTi to its thermal and compositional history: The functional response depends on a narrow compositional window, and the dominant fabrication routes were adopted because they preserve it. This article describes a sinter-based additive approach, presents current material data, and examines two application fields in which the combination of geometry and measured properties is already productive. It closes with a question the data directly pose: whether requirements inherited from wrought semi- finished products are the appropriate yardstick for every application in which NiTi could serve. A SINTER-BASED ROUTE IN BRIEF Lithography-based metal manufacturing (LMM) is a sinter-based additive process, metallurgically closer to metal FEATURE injection molding than to laser powder-bed fusion (Fig. 1)[2]. A feedstock of metal powder, approximately 55% by volume, is suspended in a photosensitive binder and cured layer by layer via projected UV light to form a green part. Solvent and thermal debinding remove the binder, and sintering densifies the part to near-full density. Because the material is sintered rather than melted, the process avoids melt-pool phenomena, including the preferential evaporation of nickel that alters the Ni:Ti ratio. It requires no support structures, involves no loose metal powder during printing, and uses no tooling, since the geometry is defined in computer-aided design software. The same process applies to stainless steels (316L, 17-4PH), titanium alloys such as Ti6Al4V and NiTi, as well as to other metals that can be sintered. THE CHEMICAL ORIGIN OF GEOMETRIC CONSTRAINTS The principal constraint in NiTi manufacturing is chemical rather than geometric. Functional behavior is governed by the austenite finish temperature, Af, which varies with composition: A change of approximately 0.1 at.% in the Ni:Ti ratio shifts the martensite-start temperature by 10-15°C. Any thermal step that alters composition, whether by nickel evaporation, interstitial uptake, or precipitation of secondary phases, can move a device outside its functional window. ASTM F2063, the reference chemistry for medical NiTi limits carbon, oxygen, and nitrogen to below 0.05 wt% each and total interstitials to below 0.25 wt%[3]. These limits are metallurgically motivated: carbon precipitates as TiC and oxygen as the Ti4Ni2O× intermetallic. Both phases embrittle the alloy and bind titanium from the matrix, depleting Ti and shifting Af. The established routes were selected to preserve this chemistry, and each constrains geometry accordingly. Wire drawing produces the work-hardened, fine-diameter material used in guidewires and braids, but its geometry is one-dimensional; function is obtained by coiling or bending the wire into a secondary shape. Laser cutting of drawn tube yields the two-dimensional mesh of self-expanding stents and retrievers but cannot depart from the cylindrical Fig. 1 — Parts produced by LMM, illustrating the feature resolution and geometric complexity achievable in a single-step process without tooling or support structures. Parts shown span multiple alloy systems and are not all NiTi. *Member of ASM International
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