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2 工學碩士學位論文 Nitinol 형상기억합금의마이크로전해가공및전해연마의가공특성에관한연구 A Study on the Micro Electrochemical Machining and Electropolishing Characteristics for Nitinol Shape Memory Alloys 2008년 2월 仁荷大學校大學院機械工學科 ( 固體및生産工學專攻 ) 辛敏貞

3 지도교수이은상 이논문을석사학위논문으로제출함

4 이論文을辛敏貞의碩士學位論文으로認定함 2008年 2月日 主審김재도 ( 印 ) 副審이은상 ( 印 ) 委員조명우 ( 印 )

5 A Study on the Micro Electrochemical Machining and Electropolishing Characteristics for Nitinol Shape Memory Alloys by Min-Jung Shin A THESIS Submitted to the faculty of INHA UNIVERSITY in partial fulfilment of the requirements for the degree of MASTER OF MECHANICAL ENGINEERING Department of Mechanical Engineering February 2008

6 Abstract In this paper, micro-electrochemical machining and electropolishing which are non-contact machining method have been developed to nitinol shape memory alloys to make micro structures and high quality surfaces. A nearly equiatomic nickel-titanium alloy(ni-ti alloy; Nitinol) is a metal alloy composed of Ni and Ti around 50% respectively in that it possesses a so-called shape memory. This shape memory effect involves the recovery of a certain deformation induced at low temperature and as a consequence, a controlled change of shape when the alloy is heated. Nitinol can be put to use for various applications such as the antenna of artificial satellite, temperature sensor system and micro-actuator. Moreover, nitinol has been suggested for use in the biomedical applications. To obtain tendencies of micro electrochemical machining and elecropolishing for nitinol shape memory alloys, many experiments about electrochemical machining and electropolishing. In electrochemical machining experiments, tungsten carbide micro-pins used as tool, and electrolyte NaNO 3 and H2SO 4 were used. To analysis characteristics of electrochemical machining for nitinol, machined shapes are measures in terms of current density machining time, pulse on/off time. In electropolishing experiments, nitinol workpieces were rough grinded to find improving of surface roughness, and micro-burr was made on the workpieces edge to find deburring effect. Surface roughness and the size of burr were measured in turms of current density, polishing time, electrode gap. -i-

7 국문요약 현재다양한분야의산업이발달함에따라기존의기술로는이룰수없었던 첨단기기의개발이가능하게되었다. 인간의눈으로는확인할수없는초정밀 미세패턴, 반도체의고집적화, 인체삽입용의료기기, 마이크로센서등특수 한기능을가진제품의요구가증가됨에따라새로운능력을지닌소재의개발 이진행되어형상기억합금, ER/MR 유체, EAP(Electro-Active Polymer) 등의 신소재가개발되고이러한소재들의특성과산업의적용가능성에대한연구가 진행되고있다. 이중, 형상을기억하여변형된후에도특정한온도에이르면원래의형상으로 돌아가려는형상기억합금의쓰임이증가되고있다. 현재까지개발된형상기억합 금중에서도가장성능이뛰어난것으로평가받고있는 Nitinol 형상기억합금은 온도에따라형상이변하는성질과형상이변할때생기는회복응력, 형상의 변화를반복하는반복성, 초탄성성질을이용하여첨단온도센서, 치과용와이 어, 의료용스텐트, 마이크로구동부품반도체장비등여러분야에폭넓게 사용되고있다. 특히전자, 기계, 재료등의관련부품산업에서의새로운소재개발과소형화, 경량화, 고도의가공기술이절실히요구되고있어형상기억합금을이용한미세 가공에대한연구에대한요구가증가하고있다. 모바일기기등의전자기기의 소형화에따른부품의미세화와반도체의고집적화에따른고정밀의미세가 공기술, 인체에삽입되는스텐트, 척추고정기구등의료기기에대한요구가급 증하고있으며이에따라마이크로단위의미세구조물을가공하는가공기술 에대한연구가활발히이루어지고있다. 따라서본논문은 Nitinol 형상기억합금의미세가공을위하여 MMSP 의일종 인전기-화학적특수가공인전해가공및전해연마를적용하는것을검토하고 가공특성을파악하기위한실험을진행하였다. -ii-

8 미세구조물의제작을위한전해가공을위한실험으로서직경이수십~ 수백μm 에이르는텅스텐카바이드미세탐침을전극으로이용하여 Nitinol 시편의가 공형상을관찰하고전압, 가공시간, 펄스 on/off time에따른가공형상의직 경을측정하여각가공조건에따른가공특성을파악하였다. 한편첨단미세부 품에필수적으로요구되는정밀하고평활한고품질의표면품질을얻기위하 여전해연마를적용하여표면거칠기향상, 미세버의제거효과를확인하는실 험을진행하여가공시간, 전류밀도, 가공간극에따른연마특성을파악하였다. 본논문은아래와같은구성으로이루어져있다. 1) Nitinol 형상기억합금의특징분석 2) 텅스텐카바이트마이크로탐침을전극으로이용한 Nitinol 형상기억합금의 전해가공수행 3) 가공결과에영향을미치는인자인전압, 가공시간, 펄스 on.off time에따 른가공형상관찰, 측정및가공경향분석 4) 표면품질향상을위한 Nitinol 형상기억합금의전해연마가공수행 5) 전류밀도, 가공시간, 가공간극에따른표면거칠기향상및미세버의제거 효과확인, 가공특성분석 - iii -

9 Table of Contents Abstract ⅰ Abstract(In Korean) ⅱ Contents ⅳ List of Figures ⅵ List of Tables ⅷ NOMENCLATURES ⅸ Chapter 1 Introduction Background of Study Contents of Study 3 Chapter 2 Nitonol Shape Memory Alloys Shape Memory Alloys Nitinol Shape Memory Alloys 6 Chapter 3 Micro-Electrochemical machining for Nitinol Alloys Theory of Micro-Electrochemical machining Experiment for Nitinol Alloys Experiment Results 19 -iv-

10 Chapter 4 Micro-Electropolishing for Nitinol Alloys Theory and Mechanism of Micro-Electropolishing Effects of Micro-Electropolishing Experiment for Nitinol Alloys Experiment Results 39 Chapter 5 Conclusion 55 References 58 -v-

11 List of Figures Fig. 2.1 Effect of (a) Shape memory (b) Ultra-elasticity 8 Fig. 2.2 Application of shape memory alloys 8 Fig. 2.3 Chemical composition of nitinol using EDX 9 Fig. 2.4 Consitution diagram of Nitinol alloys 9 Fig. 2.5 Crystal structure of Nitinol alloys 10 Fig. 3.1 Principal scheme of micro electrochemical machining 22 Fig. 3.2 Consitution diagram of Tungsten carbide 23 Fig. 3.3 Fabrication of Tungsten carbide Micro pin 24 Fig. 3.4 Schematic diagram of micro electrochemical process for Nitinol alloys 25 Fig. 3.5 Function generator and oscilloscope for electrochemical machining 26 Fig. 3.6 The state of the micro electrochemical machining 26 Fig. 3.7 Micrograph of machined surface at 3V 27 Fig. 3.8 Micrograph of machined surface at 4V 27 Fig. 3.9 The relationship between machined shape and voltage 27 Fig Micrograph of machined surface at 120sec 28 Fig Micrograph of machined surface at 300sec 28 Fig The relationship between machined shape and machining time 28 Fig. 4.l Mechanism of Electropolishing 44 Fig. 4.2 Simple illustration of polarization curve of Electro-Polishing, AB: corrosion, dissolution(etching), BC: passive, CD: flated mirror surface(electro-polishing), DE: pitting 44 -vi-

12 Fig. 4.3 The surface roughness of stainless steel before and after electropolishing 45 Fig. 4.4 Anti-corrosion effect (a) electropolished (b) non-electropolished 45 Fig. 4.5 Electropolishing system 46 Fig. 4.6 The shape of (a) nitinol workpiece and (b) micro burr 47 Fig. 4.7 The relationship between surface roughness and current density 47 Fig. 4.8 Micrographs of surface (a)before electropolishing and applying (b)6a/cm 2 (c)12a/cm 2 (d)18a/cm 2 48 Fig. 4.9 The relationship between surface roughness and machining time 49 Fig Micrographs of surface (a)before electropolishing and after electropolishing (b)60sec (c)120sec (d)180sec (e)more than 300sec 50 Fig The relationship between surface roughness and electrode gap 51 Fig The pit mark on nitinol surface 51 Fig The relationship between burr size and polishing time in 6A/cm 2 52 Fig Machining time when the burr size is smaller than 70μm 52 Fig The burr size in terms of machining time and current density 53 Fig Micrographs of micro-burr after 120sec electropolishing with (a)3a/cm 2 (b)6a/cm 2 (c)9a/cm 2 and (d) after than 900sec at 9A/cm vii-

13 List of Table Table 2.1 Physical and mechanical property of nitinol alloy 10 Table 3.1 Physical and mechanical property of Tunsten carbide 23 Table 3.2 Experiment condition for tungsten carbide micro-pins 24 Table 3.3 Experimental conditions 25 Table 4.1 Fixed condition in experiment 46 - viii -

14 NOMENCLATURES atomic weight machining time mass of metal removed rate of metal removal volumetric removal rate elements of alloy % of element a % of element b atomic weight of element a atomic weight of element b valency of element A valency of element B Faraday's constant(96500c) electrolyte conductivity inter-electrode gap size current voltage pulse voltage overpotential anode metal density electrode feed-rate effective volumetric electrochemical equivalent -ix-

15 Chapter 1 Introduction 1.1BackgroundofStudy As high tech industries are growing, new high technology which cannot achieve through traditional machining and material techniques. Growing of industries about nano scale patterns, very large scale integration, implant for human body, and micro sensor require new material which have specific characteristics such as shape memory alloy, ER/MR fluid, EAP(Electro-Active Polymer) and techniqueal research of these new material. Shape memory alloy, one of smart material, has unique thermo-mechanical properties such as shape memory effect and pseudo elasticity. These characteristics are related to a temperature dependent phase transformation. The shape memory effect can be explained as follows: when the shape memory alloy is heated, deformation previously induced at low temperature is recovered. As a consequence, it is possible to the control change of the shape by controlling the alloy s temperature. Ni-Ti alloy, one of shape memory alloy which has most effective shape memory effect, are promising materials for actuators, medical devices and so on. Especially, Ni-Ti alloy transforms around human body temperature, so it is already widely used in medical devices such as orthodontic arch wires and coronary stents. For applying these high tech industries, precision machining technique of part is recognized in each -1-

16 development of high technology importantly. Generally, techniques of micro-machining process can be classified in to two : Micro Machining by Silicon Process(MMSP) and Micro Machining by Machine Tools(MMMT). MMMT has limitation to achieve micro-scale machining such as limit of minimize of machining tool, residual stress and demage through contact between tool and workpiece, and tool wear. Especially, material which have specific properties such as shape memory alloy are hard to manufacture through MMMT because of the above statements and these drawbacks of MMMT bring quality deterioration of specific properties.[1] Therefore, the aim of this study is to get the insight into the effect of electrochemical machining and electropolishing one of MMSP method. And their machining tendencies in terms of machining parameters of nitinol shape memory alloy. This paper repots an experimental investigation into the electrochemical and electropolishing results depend on machining parameters for nitinol. Size of micro grooves which made through electrochemical machining and surface roughness, deburring rate which achieved through electropolishing are measured in terms of machining parameters. -2-

17 1.2 Contents of Study As the demand for the micro-structure technique is increased, the use of smart material for micro-structure and micro-industrial technique is increased. So, it is very important to achieve techniqueal development for micro-industrial using smart materials and find out the optimum micro-machining process for ultra precision and high quality micro structures using smart materials such as shape memory alloys. In this study, the characteristics of electro chemical machining and electropolishing for nitinol shape memory alloy were find out. This study contains these contents as follows : 1) The characteristics of nitinol shape memory alloy. 2) Micro electrochemical machining experiments for nitinol shape memory alloy using electrode which made tungsten carbide micro-pin 3) Evaluation of machining results in terms of the machining parameter such as voltage, machining time and pulse on-off time in micro electrochemical machining. 4) Electropolishing for nitinol shape memory alloy to improve the surface quality. 5) Evaluation of improving surface roughness and deburring effect in terms of machining parameters such as current density, machining time and electrode gap. -3-

18 Chapter 2 Nitonol Shape Memory Alloys 2.1 Shape Memory Alloys Introduction High tech industry requires new materials which have specific characteristics. Therefore techniqueal researches about new metallic material are progressed and finally, shape memory alloy - which can memory and change original shape was developed. Typical metallic materials have an elastic limit. When stress less than elastic limit to metallic material and change its shape, it will be return its original shape. But if the stress is more powerful than elastic limit, the metallic material will have plastic deformation. But in the case of shape memory alloy, it have specific characteristic which remember and return its original shape: pseudo elasticity and shape memory effect, so it used to high tech industry such as biomedical application and so on. The first shape memory alloy(cu-zn alloy) was obtained in 1938 by O. Greninger and V. Mooradian, and than T. Read obtained Au-Cd alloy and In-Ti alloy which have shape memory effect. Other researchers discovered Fe-Pt, In -Cd, Fe -Ni, Ni-Al, Na-Ti, and stainlsss steel alloy which have shape memory effect. The first use of shape memory alloy is the coupling of pipe for submarine cables.[2] Application of Shape Memory Alloys -4-

19 The range of applications for shape memory which have shape memory effect and pseudo elasticity has been increasing in recent years. The most famous product is wire of woman's bra. It use very widely in present industry. The development of dental braces that exert a constant pressure on the teeth. Patented by George Andteasin who changed the formula and then formally introduced the use of Ni-Ti shape memory alloy. Harmeet D. Wlalia utilized the alloy in the manufacture of root canal files in endodonitics. There have also been limited studies on using these materials in robotics as they make it possible to create very light robot. An Ti shape memory alloy is used to make eyeglass frames under the trademark flexon. Nitinol wire is also used in robotics and in a few magic tricks, particularly those involving heat and shape shifting. Boeing, General Electric Aircraft Engines, Goodrich Corporation, NASA, and All Aippon Airways developed the Variable Geometry Chevon using shape memory alloy that reduced aircraft's engine noise.[2][20]~[23] Characteristics of Shape Memory Alloys To make shape memory effect, specific heat treatment is required. In case of Ni-Ti shape memory alloy have to be fixed and applied heat around 400~500 to remember its fixed shape. If deformation is forced, this memory effect can return its memorial shape when shape memory heated on specific temperature. This specific temperature can control through controlling its process parameters. Cu-Zn-Ni, Cu-Al,-Ni, Ag-Ni-Au-Cd, Ni-Ti alloy were developed as shape memory alloy. Ni-Ti alloy most effective shape memory effect, so generally more expensive and possess superior. -5-

20 2.2 Nitinol Shape Memory Alloys Introduction Nitinol, Ni-Ti shape memory alloy, had been obtained in 1963 by W. Buehler and his research team. Nitinol typically composed of approximately 55% nickel by weight has characteristic which can change its crystal structures in terms of temperature. At low temperature, nitinol has easy-to-change characteristic to change its crystal structures but as the temperature increases, crystal structures has strong solidarity characteristic. Making small changes In the composition can change the transition temperature of the alloy significantly. For this reason nitinol may of may not be super-elastic at room temperature. These unique properties and tailor-ability of nitinol to be used in a wide range of temperatures(-400 ~212 ) makes it suitable for many application.[24] Nitinol is possessing superior shape memory effect and mechanical properties when compared to other shape memory alloys. Its distinguished pseudo-elasticity and shape memory effect have application to variety industry such as medical devices, parts of micro sensor, parts of aerospace industry and so on Chemical and Physical Properties of Nitinol Shape Memory Alloys Although nitinol contain around 55% of Ni, it has chemical properties similar to Ti such as high hardness, strong rigidity, and excellent corrosion resistant characteristics. But it has weak point of -6-

21 crack by stress corrosion. Fig. 2.2 and 2.3 show chemical composition and Consitution diagram of nitinol shape memory alloy. Nitinol shape memory alloy has low magnetic permeability and anti magnetic properties and superior protection against dust. Table 2.1 shows Physical and mechanical properties of nitinol shape memory alloy Mechanical Properties of Nitinol Shape Memory Alloys Nitinol has advantage to repeat transformation its shape because its high tensile strength, low yield strength, high elongation, high fatigue life characteristics. But it is difficult to manufacture through traditional machining for nitinol because it is hard-to-cut material which has brittleness on room temperature. So, it is hard to obtain high quality nitinol product through traditional machining such as cutting, milling, drilling and so on required high machining technique. Therefore electro-discharge machining, water-jet cutting, laser process and electro-chemical machining process is used to make nitinol products.[1]~[5] -7-

22 (a) Fig. 2.l Effect of (a) Shape memory (b) Ultra-elasticity Fig. 2.2 Application of shape memory alloys -8-

23 Fig. 2.3 Chemical composition of nitinol using EDX Fig. 2.4 Consitution diagram of Nitinol alloys -9-

24 Fig. 2.5 Crystal structure of Nitinol alloys Table 2.1 Physical and mechanical property of nitinol alloy Melting range ( O C) Density (g/cm 2 ) Magnretic Permeability Electrical Resistivety Young's Modulus 1240~ <1.002 Yield Strength (kfg/mm 2 ) Tensile Strength (kfg/mm 2 ) Elongation Thickness (%) 15~20 82~ ~55 Hardness 65~68 3.6*10 6 Fatigue Strength (kfg/mm 2 )

25 Chapter 3 Micro-Electrochemical machining for Nitinol Alloys 3.1 Theory of Micro-Electrochemical machining Introduction Traditional machining which performs with tool contact to workpiece is difficult to apply the machine parts which need to consider functional characteristics before mechanical or structural characteristics such as micro-structure for micro machines. Micro electrochemical machining, one of non-traditional machining technique without contact between tool and workpiece can be used to achieve a desired workpiece sufrace and shape by dissolving the metal workpiece with an electro-chemical reactions. Micro electrochemical machining can machine difficult-to-cut materials which has characteristics such as high hardness, strength, tension, and heat resistant. And it can achieve micro-complex shapes without distortion, scratches, burrs, and stress. This micro electrochemical machining method can be applied in industry for cutting, deburring, drilling, and shaping the woripiece. It provides an effective alternative for manufacturing a wide rage of components such as aircraft turbine parts surgical implants, bearing cages, molds and dies, and even micro-components.[6]~[8] Basic Theory of Electrochemical Machining Fig. 3.1 shows the principle of micro electrochemical. The -11-

26 workpiece and the electrode which used as tool are in electrolyte. The workpiece is contacted to anode and electrode is contacted to cathode in electrolyte. And than the electrical power is supplied, the electrical and chemical reaction between workpiece, electrode and electrolyte is occurred. The electron moves through a negative-flow of electricity. So, the surface of workpiece which contact to anode accepts electrons and the electrode which contact to cathode gives electrons. In other words, the oxidation-deoxidation process is occurred between anode and cathode. The speed and shape of electro-chemical reaction can be controlled through regulating the electrolyte and electrical power. Electro-chemical reaction is the oxidation-deoxidation process, and this oxidation process makes oxides which is a cause of corrosion of workpiece. These corrosion can be described as follows : 3.1 At anode, the oxidation process is occurred and ions of the workpiece are ionized and dissolved in electrolyte. And than, hydrogen gas and deoxidation process are occurred at cathode. (Creation of water) (Creation of hydrogen gas) (Creation of oxalic acid) 3.2 And than, other metallic deoxidation process is occurred.[6]

27 3.1.3 Theory of General Electrochemical Machining General micro electrochemical machining has been used in turbine blade of aircraft and cutting of metal, drilling in primary using direct currents. The mass of removed metal is defined following equation (3.4) using Faraday's law in micro electrochemical machining. 3.4 Therefore, the rate of metal removal is the following equation. 3.5 Where, is the electrochemical equivalent which is the important parameter in micro electrochemical machining. When is the density of metal, the volumetric removal rate is a following equation from equation In case of alloys, "Superposition of charge" method is applied because of the amount of electrical charge required to dissolve the mass contribution of each constituent element to a defined mass of the alloy. This method is defined as following equation

28 Current efficiency is defined by mass removal rate corresponding to the specified experimental conditions as following equation (3.8). It is often convenient to express the current efficiency in terms of a percentage ratio. 3.8 For an efficiency of 100%, the entire current is used to dissolve the metal in accordance with Faraday's law. For zero efficiency, the current passes without metal dissolution. The theoretical removal rate for metal according to current density, machining time needs to be measured because micro changes of parameters which are current density, voltage, machining time much affected machinability in micro electrochemical machining Theory of Micro Pulse Electrochemical Machining Generally, it is difficult to be maintained initial inter-electrode gap size continuously and to be established small because of hydrogen gas and heat generation which are generated by electrochemical reaction in micro electrochemical machining. To solve these problems, the initial inter-electrode gap size can be established by micro unit using pulse power. The electrolyte conductivity which plays part in resistances is a important parameter which is recognized to be affected by heat increasing, generation of hydrogen gas. From Ohm's law, it can be defined as following equation

29 is the electrolyte conductivity, is the current density, is the voltage, is the inter-electrode gap size. From Faraday's law, the mass of metal removed in pulse on time is defined as following equation In this study, small inter-electrode gap size and voltage are applied to come true the micro electrochemical machining and the electrode did not be moved. Therefore, the inter-electrode gap size according to metal removal from pulse on time( ) to pulse off time( ) is defined as following equations is the coefficient determined according to workpiece and when initial condition t=0sec, the gap is S=S The inter-electrode gap size is increased in proportion to root curves according to the voltage and the machining time applied in where the result of analysis above. In pulse electrochemical machining, the inter-electrode gap size is increased due to the electrochemical dissolution as pulse on time and the electrochemical dissolution recessed as pulse off time so -15-

30 the electrolyte which is including heat generation, hydrogen gas removed and new electrolyte is applied smoothly.[10][11] The mechanism of pulse electrochemical machining is similar to general electrochemical machining, but there are pulse on-off times. Micro electrochemical machining has been processed using DC current in the beginning, but recently, pulse electrochemical machining which can continue micro inter-electrode gap, increase the accuracy of shape and make better surface roughness using pulse current than surface roughness using DC current has been researched. Micro pulse electrochemical machining can manufacture micro shape machining with high efficiency and increase dimensional accuracy and processing quality result in current convergence makes electrochemical dissolution on anode actively because it can supply electrolyte and pulse current stably, continue micro inter-electrode gap size. Merits of Pulse Electrochemical machining technology are the removal of burrs, the removal of surface residual stress, independence of workpiece's mechanical properties, non-wear of electrode.[12][13] -16-

31 3.2 Experiment for Nitinol Alloys Introduction To make micro structure using nitinol shape memory alloy, the great skills about machining technique is required because of its hard-to-cut characteristics. And it is difficult to make micro structure using micro tools because of intense tool wear. So, aim of this study is to investigate to adopt micro electrochemical machining for nitinol shape memory alloy and define machining characteristics in terms of machining parameters. As the tool electrode, tungsten carbide micro pins were used. For more minute machining, tungsten carbide pins which has 125μm diameter were machining through electrochemical machining until it had 30μm of diameter. Experiments for define tendencies of machining were performed in terms of voltage, machining time, pulse on-off time and than grooves on machined surface were made observation Fabrication of Tungsten Carbide Micro-pins Tungsten carbide which used widely for micro tool because its has high hardness and rigidity.[19] of Fig. 3.2 is Consitution diagram and table 3.1 shows physical and mechanical properties. To make micro pins which has 30μm of diameter, tungsten carbide pin which has 5mm of length and 125μm of diameter wes machined through electrochemical machining. Table 3.2 shows machining condition for tungsten carbide micro pins. Through this experiment, -17-

32 micro pins which has 30μm of diameter was obtained.(fig. 3.3) Experimental set-up Fig. 3.4 shows electrochemical machining system for nitinol shape memory alloy. Nitinol workpiece is fixed on work table and tungsten micro pin which used for electrode tool is fixed to micro-stage which has 10μm resolution to x, y, z direction to control electrode gap. To apply electrical energy and monitor the power supply, function generator and oscilloscope were used.(fig. 3.5) During electrochemical machining, it can be monitored using vision system that pertinent electrode gap and supply of electrolyte.(fig. 3.6) To define tendencies of machining characteristics of electrochemical machining for nitinol shape memory alloy, several experiments were performed in terms of machining parameters. -18-

33 3.3 Experiment Results Effect of Voltage for Micro-electrochemical Machining Grooves of machined surface in terms of voltage were measured to evaluate the effect of voltage during electrochemical machining for nitinol shape memory alloy. Tungsten carbide micro pin was fixed to micro stage and the electrode gap was controlled 100μm using vision system. In this test, machining time was 60sec, the period of pulse was 1 ms, duty factor was 50%, aqueous sodium nitrate was used for electrolyte and 1~5V of voltage were applied. Generally, duty factor is applied to divide each condition when the pulse power source and duty factor is defined the following equation When 1V and 2V of voltage were applied, there are any indications of machining result. It shows electrochemical machining was not performed. It can be analyzed when applied voltage is too low; electro-chemical reactions which make dissolution of workpiece cannot reach between electrode and workpiece. When 3V of voltage was applied, there was machining groove. It shows to achieve electrochemical machining, more than 3V of voltage have to be applied in this machining condition.(fig. 3.7) In this test, more than 4V of voltage were applied, shapes of machining surface similar to electrode were obtained. (fig. 3.8) It -19-

34 shows that to make part using electrochemical machining for nitinol shape memory alloy more than 4V of voltage is required. The size(diameter) of machined grooves were measured. Fig. 3.9 illustrates the relationship between applied voltage and size of grooves. At same machining condition, the size of groove has tendency to become increases as the applied voltage increases Effect of Machining time for Micro-electrochemical Machining To evaluate the tendency of machining result in terms of machining time, experiments of 60, 120, 180, 240, 300 sec of machining times were performed. In this test, 5V of applied voltage, 100μm of electrode gap, 50% of duty factor, 1ms aqueous sodium nitrate as electrolyte were used of the period of pulse, and Fig and 3.11 show groove on machined surface after 120sec and 300sec machining time. The size of groove after 120sec was 794 μm, and after 300sec was 927 μm. This result shows that the groove became large when electrochemical machining is performed with long machining time in same machining condition. To achieve more precision machining result, it is important to select optimum machining time. Fig shows the relationship between size of grooves and machining time. As the machining time increases, the size of grooves increases, but the increasing rate of size becomes slow after specified machining time. In this test, after 300sec the increasing rate became slow, and after 500sec, there are no more increasing of size. It is because that as the machining time increases, electrode gap become wider, so the electro-chemical

35 effect cannot reach to workpiece Effect of pulse on/off time for Micro-electrochemical Machining The experiment to evaluate machining characteristics according to pulse on-off time was performed. In this test, the period of pulse was fixed(1 ms), and 25%, 50%, 75% of duty factor were given. 3V and 5V of voltage were applied and 120sec of machining time, 100 μm of electrode gap were used. Fig shows grooves of machined surface when the applied voltage were 3V and 5V in terms of different duty factor(25%, 50%, 75%). Micro electrochemical machining did not occur when the duty factor was 25% because of short pulse on time.(fig. 3.13(a)) When the duty factor was 75% micro electrochemical machining occurred but excess electrical energy through long pulse on time became a cause of scorch mark around groove on machined surface. Fig 3.13(c) shows the scorch mark on surface after electrochemical machining with 75% of duty factor. Similar result occurred when 5V of voltage was applied. In case of 25% of duty factor was used, small indication of machining occurred on surface. And when 75% of duty factor was used, machining area was burned black and pitted surface occurred by excess electrical energy. -21-

36 Fig. 3.1 Principal scheme of micro electrochemical machining -22-

37 Fig. 3.2 Consitution diagram of Tungsten carbide Table 3.1 Physical and mechanical property of Tungsten carbide Melting range ( O C) Density (g/cm 2 ) Magnretic Permeability Electrical Resistivety 2800~ < Yield Strength (MPa) Tensile Strength (MPa) Young's Modulus ~ Hardness

38 Table 3.2 Experiment condition for tungsten carbide micro-pins Conditions Values Conditions Values Composition of electrolyte 15% H 2 SO 4 (Sulphuric acid) Voltage Machining time 1.5V 10 min Workpiece Tunsten carbide Frequency Pulse on time Electrode Copper Power supply 1kHz 500 μs HP 8116A Pulse Generator Fig. 3.3 Fabrication of Tungsten carbide micro pin -24-

39 Fig. 3.4 Schematic diagram of micro electrochemical process for Nitinol alloys Table 3.3 Experimental conditions Conditions Composition of electrolyte Workpiece Electrode Voltage Frequency Electrode gap Power supply Surface observation Values 15% Aqueous NaNO 3 Nitinol shape memory alloy Tungsten carbide micro pin 2V~5V 1 khz 100 μm HP 8116A Pulse/Function Generator An optical microscope Camscope -25-

40 Fig. 3.5 Function generator and oscilloscope for electrochemical machining Fig. 3.6 The state of the micro electrochemical machining -26-

41 Fig. 3.7 Micrograph of machined surface at 3V Fig. 3.8 Micrograph of machined surface at 4V 500 Workpiece : Nitinol Machining time : 60sec Electrode gap : 100μm 400 Pulse on/off : 500μs Size of spot (μm) Voltage (V) Fig. 3.9 The relationship between machined shape and voltage -27-

42 Fig Micrograph of machined surface at 120sec Fig Micrograph of machined surface at 300sec 900 Size of spot (μm) Workpiece : Nitinol voltage : 5V Electrode gap : 100μm Pulse on/off : 500μs Machining time (sec) Fig The relationship between machined shape and machining time -28-

43 Chapter 4 Micro-Electropolishing for Nitinol Alloys 4.1 Theory of Micro-Electropolishing Fundamental Principal of Electropolishing Electropolishing is an anodic dissolution process using an electrochemical reaction. Through electropolishing, smooth, bright and reflective surface that exhibits superior corrosion resistance can be obtained when the workpeice which connected with anode, and electrode which connected with cathode are charged in electrolyte. Electropolishing is one of non-traditional without contact between tool and workpiece so it is suitable for the polishing of both complex shapes and hardened of thin materials that can hardly be machined mechanically. The mechanism of electropolishing has not yet been fully determined, but it is usually explained as follows. As the voltage increase, the emulsion which contain high specific gravity, viscosity and electric resistance, is created by the ion eluted from workpiece. Then, the emulsion cover the depression( 凹 ) of the surface and it obstruct the elution of the depression. In result surface planarization is completed as the prominence( 凸 ) is firstly eluted.[13] During anode dissolution, a polarization curve can be obtained both the tool electrode and workpiece are positioned close together and the applied current is increased. Fig. 2.2 is the simple illustration of -29-

44 polarization curve of electropolishing. For anode potentials in the AB range, the metal surface becomes etched, when the anode potential becomes greater than B in the BC range, an oxide film may be formed suddenly on the anode. In the CD range, named the 'Plateau region' where the electropolishing affect take place, there is hardly any change in current density as the voltage increases. When the electropolishing process is carried out quickly because high current is applied, it leaves surface defects such as pits; hence, a current in the plateau region is usually used. However, it should also be pointed out that an applied current slightly higher than one in the plateau region is actually required. Therefore the plateau region in electropolishing is no the best range to achieve the best electropolishing, but the applied current density is a more important parameter.[14]~[16] Theory of Electropolishing The passivation film formed at the anode surface is a very thin oxidisation film caused by result of reaction of metal ion(+) and electrolyte ion(-). Once this passivation film is formed, electro chemical reaction is slow down and finally stopped because passivation film obstruct the electrolyte ion to reach to the metal surface. Therefore, to continue the machining by the electro chemical reaction it is desirable to remove the passivation film. The electric resistance of electrolyte and specific conductivity as follow equation

45 cm 4.2 Where, [ cm2] is the cross section of electrode, [ cm] is the gap between anode and electrode, [ Ω. cm] is the specific resistance. The electric resistance R can be defined using the specific conductivity as follow. 4.3 The specific conductivity is closely connected with the concentration of electrolyte and the specific conductivity is larger as the concentration increase. There are two kinds of electrolyte, activated and inert. In ultra precision machining the inert electrolyte is used because it makes a oxide film. When the voltage is applied to the both ends in electrolyte, the current and current density through the electrolyte are as follows. 4.4 cm2 4.5 The removal rate of electropolishing process is predicted theoretically using Faraday's law and the theoretical equation conditionisasfollows. 1) The eluted atom of metal should be confirmed. 2) The metal elution is the only from the atom state caused by the electro chemical reaction. -31-

46 3) The metal elution is the only reaction at the both ends. According to the Faraday's law, the necessary quantity of electricity to elute 1 g element which is consist of atomic value n and atomic weight is coulomb. Therefore the eluted element quantity g caused by I[A] current during t[s] time is as follow. 4.6 Where is Faraday constant. When the density of this atom is, the theoretical removal volume is as follow. mm3 4.7 Where is specific removal volume which eluted volume per unit quantity of electricity( ), which differs as the metal type. The electrochemical equivalent of metal element k is as follow equation. 4.8 When the metal to be machined is an alloy, electrochemical equivalent is as follows : 4.9 Where is each material ratio of alloy, is an electrochemical -32-

47 equivalent of each metal element. Table 2-3 shows electrochemical equivalent of principal metals. The eluted quantity and volume can be defined by using a electrochemical equivalent as follow mm But the real removal rate of electropolishing is less than the theoretical value as the various machining condition, environment, material property and etc. So the real removal rate of electropolishing is that multiply theoretical value by current efficiency The current efficiency is very closely connected with current density. The efficiency of active electrolyte such as NaCl is almost 100%, but inert electrolyte such as NaNO 3 is defends on current density and the efficiency is less than 80%. -33-

48 The advantage of machining which use electro-chemical reaction such as electrochemical machining and electropolishing is as follows: 1) The dissolution of anode metal is performed as ion unit. 2) There are no were of electrode which connected with cathode. 3) There are no physical-load through electro-chemical reaction. 4) The machining efficiency is absent from hardness of materials. The drawbacks of this electro-chemical machining is as follows: 1) Nonconductor of electricity materials cannot be machined. 2) The rate of dissolution of ion is defends on atomic characteristics of each material. -34-

49 4.2 Effects of Micro-Electropolishing Removal of Irregularity for Improving Surface Roughness Electropolishing can improve surface roughness through selective dissolution of prominence part on surface. Through electropolishing, surface roughness improves about 50~80 %. The elution by the electrochemical reaction is happened at the prominence( 凸 ) part than the depression( 凹 ) part, so the surface planarization is performed. And as not only the surface roughness but also the tolerance is important, proper parameter value should be selected as the each metal property. Fig. 4.3 shows the improved surface roughness of stainless steel Elimination of Hydrogen In case of being hydrogen on the metal surface there are two serious problems, fatigue fracture and the propagation of bacteria. The hydrogen not only being on the surface but also inside the metal can be removed by the electropolishing, so the fatigue fracture is prevented Better Corrosion Properties The corrosion of metal surface is inhibited through oxide layer which obtained through electropolishing. Electropolishing provides more excellent corrosion resistant characteristics than other surface treatment method by removing impurities, damaged layers and materials which form a nucleus of corrosion from the workpiece surface. Fig. 4.4 shows the result of corrosion resistant test using -35-

50 oxalic acid solution Luster of Surface Electropolishing is known as the excellent machining for the uniformity, brilliance and removal of metal. The Cr multi-layer produced on the surface is very similar the Cr coating layer excepting the adhesive property. So the Cr multi-layer has a strong corrosion property and brilliance property. The planarization surface machined by electropolishing cause a very high brilliance surface reflect the light Stress Discoloration and Removement of the Surface Through heat treatment, welding, electro-discharge machining, heat transformation which causes discoloration and weak point is remain on machined surface and copula. Moreover the contact between tool and workpiece remain residual stress and casting, forge is cause of work hardening. Electropolishing can remove this discoloration, residual stress and hardening The Deburring Effects Through mechanical machining such as drilling, cutting, milling and so on, micro-burr is remained on edge of workpiece. Micro deburring effect is great advantage of electropolighing to complex shape and narrow gap of metal workpieces. [17][18] -36-

51 4.3 Experiment for Nitinol Alloys Introduction For applying high-tech industry, it is necessary to obtain a purity and high precision surface of product using surface treatment methods. To improve surface quality of nitinol shape memory alloy which used for high-tech industry, electropolishing was adopted. Experiments were performed to get the insight into the effect of electropolishing and tendency in terms of machining parameters and surface treatment of nitinol alloy. Surface roughness, metal removal rate and deburring rate are measured in terms of current density, machining time, electrode gap Experimental Set-up for Micro-electropolishing In order to make an observation the effects and characteristics of electropolishing, a simple apparatus illustrated in Fig. 4.5 for improving surface roughness and deburring effect. A cathode made of copper is used for the electrode in surface experiments and tungsten carbide micro pins used in deburring experiments. And the anode which can hold the workpiece is connected with micro power controller. The micro-stage which has 10μm resolution to x, y, z direction can control location of the workpiece, so it is easy to control the electrode gap. The anodic workpiece was a nitinol workpiece cut with wire cutting machine. The cathode and anode were submerged in electrolyte of the bath, and then the current was applied. Table 4.1 shows about experimental condition. Fig. 4.6 shows the workpiece which used for this study. To obtain -37-

52 the data of surface quality, rough grinding was performed on the surface of nitinol workpices and micro burr was created. Before electropolishing, the surfce roughness was 0.8μmRa, and the size of micro burr was 700 μm. The quality of the polished surface was observed using an instrument for measuring surface roughness. And canscope, optical microscope was used for measuring of deburring rate of workpiece. -38-

53 4.4 Experiment Results Effect of Current Density for Improving Surface Roughness To analysis electropolishing effect for improving surface roughness of nitinol shape alloy, experiments were performed in terms of current density. In this test, electrolyte which composed with phosphoric acid, sulphuric acid and distilled water was used and the electrode gap was 1mm. And the surface roughness of the workpiece before electropolishing was 0.8μmRa. In this test, the polishing time was 180sec. Fig. 4.7 shows the relationship between the surface roughness of the nitinol workpiece and the current density. The surface roughness of the workpiece becomes smoother and improves as the current density increases in same conditions(machining time and electrode gap). At 3~9A/cm 2 of current density, the surface roughness was decrease abruptly. More than 9A/cm 2 of current density, rate of improving surface roughness is has slowed compare with range of 3~9A/cm 2. Fig. 4.8 shows that the surface micrographic before and after electropolishing at 6A/cm 2, 12A/cm 2, 18A/cm 2. Fig. 4.8 (a) is surface image of non-electropolished. It shows many protuberances such as tool marks, scratch and rough shape on surface of workpiece. In fig. 4.8 (b) and (c), it can be shown that protuberances of surface were removed. In fig. 4.8 (d), including current density changes in the 18A/cm 2, especially good results are obtained as shown. This result shows that if high current density is applied, the -39-

54 surface roughness of the nitinol workpiece is improved and the electropolishing effect about surface roughness can be obtained more quickly. That means the current density is an important parameter which improves the surface roughness of nitinol workpiece through electropolishing Effect of Machining Time for Improving Surface Roughness An experiment was performed with machining time and surface roughness. In this test, electrolyte which composed with phosphoric acid, sulphuric acid and distilled water was used and the electrode gap was 1mm. And the surface roughness of the workpiece before electropolishing was 0.8μmRa. In this test, the current density was 9A/cm 2. In this test, 30, 60, 90, 12, 180, 210sec of machining times were gives and than the surface roughness of each machining times was measured. The results of this test, the relationship between surface roughness is illustrated in fig The graph shows that surface roughness has tendency to become decrease as the machining time increase. In a range of 30sec~120sec, the surface roughness is improved abruptly. At 120sec of machining time, the surface roughness was about 0.5μmRa. After this, in a range of 120sec~180sec, the rate of decrease in the value of the surface roughness was very low. At 210sec the surface roughness was about 0.43μmRa. And over 210sec of machining time, abrupt decreases of the surface roughness is observed again. The surface micrographic of this test is shown in figure 4.10 (a)~(e). As the machining time is increased, protuberances on the -40-

55 surface of the nitinol workpiece are removed more clearly. More than 300sec, the damaged layers are completely removed but the thickness of the workpiece is decrease extremely. This result shows that the important of selection on machining time when surface roughness was improved and damaged layer was removed Effect of Electrode-gab for Improving Surface Roughness Electrode gab, a distance between workpiece and electrode, is important parameter which delivers electro-chemical reaction and control the flow of electrolyte. To define the tendency of improving surface roughness in terms of electrode gap, several experiments were performed. In this test, electrolyte which composed with phosphoric acid, sulphuric acid and distilled water was used and the electrode gap was 1mm. And the surface roughness of the workpiece before electropolishing was 0.8μmRa. In this test, the current density was 9A/cm 2 and machining time was 180sec. And than the surface roughness of each electrode gap(1, 2, 3 mm) was measured. Figure 4.11 shows the effect of the electrode gap to improve the surface roughness of nitinol workpiece. When the electrode gap was 1mm, the best surface roughness was obtained. The surface roughness of nitinol has tens to worse as the electrode gap increases. As electrode gap increases, the value of surface roughness was increases because the electric effect while is diminished as the electrode gap becomes wider. It shows that electrode gap is one of important parameter in electropolishing. The best result was obtained at 1mm of electrode -41-

56 gap, but when too narrow electrode gap is gives, pits on surface were formed(fig. 4.12). Generally, pits can be formed at a certain small spot where a high current density is applied in the case of a narrow electrode gap. An electro-discharge mart may occur on the surface in the case of too narrow electrode gap Micro-deburring effect through electropolishing In order to investigate the deburring effect of electropolishing, several experiments were performed. Before electropolishing, the size of burr was about 700 μm, and after electropolishing, remained burr size were measured to analysis deburring effect in terms of machining parameters. Fig shows that the time when the size of burr of workpiece was less than 70μm each levels of current density. In this test, the polishing time was 180sec, and the electrode gap was 1mm. The electrolyte was composed of phosphoric acid, sulphuric acid and distilled water. At a current density of 3A/cm 2, it took more than 700sec to removes 90% of burr. And at the current density of 6A/cm 2, it took 270sec and at 9A/cm 2, it took less than 150sec. To remove the burr completely, for burr size smaller than 70 μm, long periods of times(more than 900sec) were needed in accordance with all of parameters. This result shows that when current density is high, burr of workpiece can be removed faster than low current density. It means it is important that the size of burr which has to removed is considered when select the current density in electropolishing. The relationship burr size and machining time is shown in fig The current density was 6A/cm 2, and other conditions of the -42-

57 experiment were same with above statements. It shows that as the machining time increase, the size of burr is decrease. And finally the burr was removed completely more than 900sec. A comparison of current density, machining time and size of burr is shown in fig At high current density, the size of burr is removed rapidly more than at low current density. Fig shows the shape of micro-burr of workpiece when the current density is applied 3A/cm 2, 6A/cm 2 9A/cm 2 after electropolishing 120sec. The figure clearly shows that when high current density is applied, the size of burr is smaller than low current density in same time. Through this experiment, the importance of selection with considering about the size of burr in machining parameter such as current density and machining time is founded. -43-

58 Fig. 4.l Mechanism of Electropolishing Fig. 4.2 Simple illustration of polarization curve of Electro-Polishing, AB: corrosion, dissolution(etching), BC: passive, CD: flated mirror surface(electro-polishing), DE: pitting -44-

59 um Roughness profile, gaussian filter, cut-off 0.8 mm mm um Roughness profile, gaussian filter, cut-off 0.8 mm mm Fig. 4.3 The surface roughness of stainless steel before and after electropolishing (a) (b) Fig. 4.4 Anti-corrosion effect (a) electropolished (b) non-electropolished -45-

60 Fig. 4.5 Electropolishing system Table 4.1 Fixed condition in experiment Conditions Composition of electrolyte Workpiece Values Phosphoric acid (H 3 PO 4 ) Sulphuric acid (H 2 SO 4 ) Distilled water (H 2 0) Nitinol Ni: 51%, Ti: 49% Electrode Copper (99%) Chucking Copper (99%) Surface roughness tester Size of burr measurement Taylor Hobson Surtronic 3 + Camscope -46-

61 230 μm, 200X (a) (b) Fig. 4.6 The shape of (a) nitinol workpiece and (b) micro burr Surface Roughness (μm) Workpiece : nitinol Machining time : 180 sec Electrode gap : 1mm Electrolyte : H 2 SO 4 +H 3 PO 4 +H 2 O Current Density (A/cm 2 ) Fig. 4.7 The relationship between surface roughness and current density -47-

62 (a) (b) (c) (d) Fig. 4.8 Micrographs of surface (a)before electropolishing and applying (b)6a/cm 2 (c)12a/cm 2 (d)18a/cm 2-48-

63 Surface Roughness (μm) Workpiece : nitinol Current density : 9A/cm 2 Electrode gap : 1mm Electrolyte : H 2 SO 4 +H 3 PO 4 +H 2 O Machining Time (sec) Fig. 4.9 The relationship between surface roughness and machining time -49-

64 (a) (b) (c) (d) (e) Fig Micrographs of surface (a)before electropolishing and after electropolishing (b)60sec (c)120sec (d)180sec (e)more than 300sec -50-

65 0.65 Surface Roughness (μm) Workpiece : nitinol Current density : 9A/cm 2 Machining time : 180sec Electrolyte : H 2 SO 4 +H 3 PO 4 +H 2 O Eledctrode gap (mm) Fig The relationship between surface roughness and electrode gap Fig The pit mark on nitinol surface -51-