Selective laser melting of high aspect ratio 3D nickel - titanium structures for MEMS applications
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Selective laser melting of high aspect ratio 3D nickel - titanium structures for MEMS applications. P.R. Chalker, A. Clare, S. Davies, C.J. Sutcliffe And S. Tsopanos Department of Materials Science and Engineering, University of Liverpool, Liverpool, L69 3BX, United Kingdom ABSTRACT Selective laser melting has been used to build high aspect ratio, three-dimensional NiTi components for the first time. The influence of laser dwell time and raster pitch on the density of NiTi shape memory alloy parts and their resolvable feature sizes are reported. The properties of shape memory springs produced by this method are reported and the application of selective laser melted NiTi components in microelectromechanical devices is discussed. INTRODUCTION The advent of silicon-based micro-electro-mechanical systems (MEMS) has been largely borne out of processes developed for manufacturing essentially two-dimensional (2D) integrated circuits. In order to extend MEM technologies to 3D applications such as microfluidic valves, inkjet nozzles, and projector micro-mirrors for example, high aspect ratio patterning techniques were developed such as ‘LIGA’ or X-ray photoresist processing [1]. More recently a range of direct-write methodologies, such as ink-jet printing or selective laser melting [2,3] have been developed to manufacture MEM component structures. In the selective laser melting process a succession of individual part cross sections are created from powdered materials by melting them with an ytterbium fibre laser. A layer of the build powder is pre-deposited and spread uniformly by a wiper blade. The laser ‘writes’ the layer pattern in the powder bed melting the material and joining it to the previously formed build structure. By this technique, between 25 and 200 µm thickness of material can be built in each layer. This method has been applied to the rapid prototyping of a range of devices including micro-heat exchangers [4] and prosthetic implants amongst others. A potential application of SLM lies in the prototyping of three-dimensional MEMs structures, which could be built from metals, polymers or ceramics. In particular SLM can be readily applied to the manufacture of metal components from materials such as stainless steel for its corrosion resistance, or from shape memory alloys (SMAs) such as NiTi to realise thermally actuated components. This paper considers the SLM of three-dimensional NiTi structures such as helical springs. The influence of the SLM process parameters on the resulting NiTi components is discussed. EXPERIMENTAL Selective Laser Melting, a layer-based solid freeform fabrication process, has been used to fabricate metal structures from NiTi powder. The apparatus used is the MCP Realizer, which a commercial SLM workstation with a 100 W continuous wave Ytterbium Fibre
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Laser (IPG, Germany) operating with a wavelength of 1068-1095 nm. The build speed is 5cm³ dense metal per hour on average, and the build envelope is 250x250x240mm. The scanning system used is a Dual Axis Mirro
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