Laser and Surface Processes of NiTi Shape Memory Elements for Micro-actuation
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INTRODUCTION
IN the last few years, the trend of miniaturization of components and products has been evidently and strongly introduced in several industrial fields, such as biomedical, electronics, aerospace, and mechanics ones.[1,2] This trend can be also directly observed in the machining of small features by means of different production technologies. In general, micromachining can be associated to a large collection of technologies, all of them focused in the direction of miniaturization of the single feature to be produced and eventually in the assembly of the complete or more complex products. One more attractive application field is represented by actuation, in which smart and functional materials, such as shape memory alloys (SMAs), are able to find an interesting role and application. SMAs are intermetallic materials, exhibiting a reversible martensitic transformation (MT) by a simple thermal loop across two main solid phases, stable at two different critical temperature ranges.[3,4] The most well-known and used SMA is the near-equiatomic NiTi alloy; at high temperature it shows a body-centered cubic lattice (B2, austenite), while at lower temperatures ADELAIDE NESPOLI, CARLO ALBERTO BIFFI, ELENA VILLA, Researchers, and AUSONIO TUISSI, Senior Researcher, are with the National Research Council (CNR), Institute for Energetics and Interphases (IENI), Corso Promessi Sposi 29, Lecco, Italy. Contact e-mail: [email protected] BARBARA PREVITALI, Professor, is with the Department of Mechanical Engineering, Politecnico di Milano, Via La Masa, 1 Milan, Italy. Manuscript submitted November 27, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
it transforms to a monocline lattice (B19’, martensite). After aging at specific temperatures, this transition may occur in two steps in association with a trigonal phase, called R-phase. Thanks to this reversible phase transformation this alloy is employed in the actuation field to produce mechanical work.[2–6] In fact, when a SMA element is in the martensite state, it can be easily deformed (detwinned) by applying an external load. Upon heating above the austenite finish temperature, the material recovers its original shape producing a certain amount of stroke easily exploitable to produce linear or rotational motion. In the actuator field a relatively fast heating of the SMA component can usually be induced by the Joule effect, while the cooling process requires more time due to the material diffusivity properties as well as heat exchange phenomena which may interfere with the dynamic response of the actuator. Normally, cooling is achieved by natural convection and, therefore, the actuator dimensions play an important part in the duration of one single thermo-mechanical loop. This can be positively associated to the trend of the current miniaturization of components.[1,2] When compared with the current microtechnology, SMAs present the highest power-to-weight ratio as a function of the actuator weight.[6–10] Moreover, one of the main benefits in scaling down is that new mechanical desi
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