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Invar is a 36 % nickel iron alloy that has a low thermal expansion, compared to other metals and alloys, at temperatures ranging from room temperature up to approximately 230°C. Invar alloy is ductile, easily weldable and its machinability is similar to that of austenitic stainless steel. Due to the low thermal expansion of Invar, it is used for shadow masks for display devices such as UHDTV -organic light-emitting diode. In this study, micro-pulse electrochemical machining (MPECM), which is a non-contact ultra-precision machining method, was developed to manufacture Invar sheets; optimum parameters of MPECM were defined and the basic MPECM experiments were carried out on an Invar sheet. The optimum parameters were determined with pulse-on time and duty-ratio analysis. The experimental results show that MPECM is hard to control. Therefore, using ultrashort charging times and very high pulses, it is possible to achieve a successful anodic dissolution at a very small electrode gap. Hence, a longer pulse-on time and a small electrode gap may provide the scope for further improvement of the machining accuracy by controlling the localization effect. Furthermore, the machining depth and MPECM efficiency were investigated with respect to various parameters and pulse-on time, considering different duty-ratio conditions. Keywords: Invar alloy, electrochemical machining, surface characteristics Invar je 36 % zlitina niklja in`eleza, ki ima v primerjavi z vsemi kovinami in zlitinami, nizek koeficient toplotnega raztezka v obmo~ju sobne temperature pa do okoli 230°C. Invar zlitina je duktilna, primerna za enostavno varjenje in njena obdelovalnost je podobna avstenitnemu nerjavnemu jeklu. Zaradi nizkega koeficienta termi~nega raztezka, se zlitina Invar uporablja v maskah za sen~enje displejev v OLED-UHD TV (angl.: Organic Light Emitting Diode -Ultra High Definition TeleVision) napravah. V tej {tudiji je bila razvita metoda mikropulzne elektrokemi~ne obdelave kovin (angl. MPECM), ki je brezkontaktna ultraprecizna tehnologija mehanske obdelave. Izvedeni so bili osnovni poskusi na zlitini Invar. Dolo~eni so bili optimalni parametri postopka. Eksperimentalni rezultati ka`ejo, da je obdelovalnost z MPECM te`ko nadzorovati. Izjemno kratki~asi, z zelo visokimi napetostnimi impulzi, omogo~ajo nastanek pogojev za uspe{no anodno raztapljanje kovine pri zelo majhni re`i elektrode. Zato se pri zelo visokem {tevilov impulzov in manj{em razmiku elektrod, lahko zagotovi nadaljnje izbolj{anje to~nosti obdelave z MPECM. Nadalje je bila raziskana obdelovalna globina in obdelovalnost z MPECM pri razli~nih procesnih parametrih (napetost-~as-storilnost).

Ultra-precision products which contain a micro-hole array have recently shown remarkable demand growth in many fields, especially in the semiconductor and display industries. Photoresist etching and electrochemical machining are widely known as precision methods for machining micro-holes with no residual stress and lower surface roughness on the fabricated products. The Invar shadow masks used for organic light-emitting diodes (OLEDs) contain numerous micro-holes and are currently machined by a photoresist etching method. However, this method has several problems, such as uncontrollable hole machining accuracy, non-etched areas, and overcutting. To solve these problems, a machining method that combines photoresist etching and electrochemical machining can be applied. In this study, negative photoresist with a quadrilateral hole array pattern was dry coated onto 30-µm-thick Invar thin film, and then exposure and development were carried out. After that, photoresist single-side wet etching and a fusion method of wet etching-electrochemical machining were used to machine micro-holes on the Invar. The hole machining geometry, surface quality, and overcutting characteristics of the methods were studied. Wet etching and electrochemical fusion machining can improve the accuracy and surface quality. The overcutting phenomenon can also be controlled by the fusion machining. Experimental results show that the proposed method is promising for the fabrication of Invar film shadow masks.

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