Processing of Shape Memory Alloys – A

5th National Conference on "Recent Advances in Manufacturing (RAM-2015)", 15 - 17 May, 2015

Machining/Processing of Shape Memory Alloys – A Review Vishal John Mathai1, Sudhanshu Kumar2, Harshit K. Dave3, Keyur P. Desai4 1,2Research

Scholar Department of Mechanical Engg S. V. National Institute of Technology Surat – 395007, Gujarat, India Email: [email protected] [email protected]

3Assistant

Professor Department of Mechanical Engg, S. V. National Institute of Technology Surat – 395007, Gujarat, India. [email protected]

4Professor

Department of Mechanical Engg, S. V. National Institute of Technology Surat – 395007, Gujarat, India. [email protected]

Abstract: Shape Memory Alloys (SMAs), which are one type of multi functional materials, are widely used in the fields like biomedical, aerospace etc. for components or devices at macro as well as micro scale. However, machining of these materials is really difficult due to their unique properties. This paper aims in reviewing the machining as well as processing aspects of SMAs by various conventional as well as non conventional machining techniques. Literatures suggest that it is difficult to employ conventional techniques for machining SMAs. At the same time, even though these materials can be machined with relative ease by non conventional machining techniques like electro discharge machining, laser beam machining, water/abrasive water jet etc., the after effects of these processes like hardening, recast and HAZ formation etc result into degradation of the its SME of the same. So, post processing the features by chemical or electrochemical based techniques may be necessary to use them with required functional effectiveness. Keywords: Multi functional materials, Shape memory alloys, Machining, Processing

1.0 INTRODUCTION Shape memory materials (SMM) are those category of materials which have the ability to memorize or retain their previous form when they are subjected to certain stimulus in terms of thermal, mechanical, electrical, chemical, light and magnetic variations (Jani et al., 2014, Sun et al., 2012). These materials can be classified into shape memory alloys (SMA), shape memory polymers (SMP) and shape memory hybrids (SMH) (Huang et al., 2010). Out of these materials SMAs have widely used due to its relatively higher values of specific strength, corrosion resistance, wear resistance, anti fatigue properties and good actuation response (Manjaiah et al., 2014). The ability of a SMA to memorize its initial shape depends on the material composition and its ability to exhibit shape memory and super elasticity effects which happens mainly due to the martensitic transformation. The displacement of the atoms in the same direction during this transformation results in the macroscopic change in shape of the material. Another important parameter that is involved in making the material to exhibit these effects is the temperature settings at which the materials are thermo mechanically characterized. In fact, it determines the type of shape memory effect the SMA can exhibit – one way or two way. In one way shape memory effects, only the shape of the parent phase is remembered by the alloy and in two way shape memory effect, both the parent as well as the martensite shapes can be remembered (Otsuka et al., 2002). Purpose of any material is fulfilled only when it is converted into a product of use. SMAs have wide range of applications in fields like biomedical (Morgan, 2004), aerospace (Hartl and Lagoudas, 2007), material reinforcement (Sanusi et al., 2014), industrial engineering applications like fastners, sealings (Wu and Schetky, 2000) etc. Even though a wide range of SMA alloys are available in the market now a days, NiTi based, Copper based and Iron based SMAs are reported to have a relatively higher commercial viability (Huang et al., 2010). However, selection of appropriate machining process is necessary for accurate production of a feature on SMAs. Different machining processes - both conventional and non-conventional – have been reported by researchers for machining of SMAs. This paper is aimed to review the various works that deal with the machining of SMAs by conventional machining processes like Milling, Turning etc. and

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non conventional machining processes like Electro Discharge Machining, Laser Beam Machining, Electro Chemical Machining/Polishing and Water Jet/Abrasive Water Jet Machining. 2.0 MACHINING OF SMAs BY CONVENTIONAL MACHINING PROCESSES Conventional machining processes are those machining processes in which a single point or multi point cutting tool with a clearly defined shape and motion is utilized for generating features. As the tool comes into direct contact with the job surface during these types of machining processes, properties of the workpiece material as well as tool material plays an important part in the effectiveness. As the commercially used SMAs are nickel or titanium based alloys, application of conventional machining processes to machine these materials are always difficult and challenging due to the peculiarities in base material properties, resulting into very high cutting forces and tool wear (Lin et al, 2000, Weinart 2004). As the material is hard to machine the cutting time will be relatively higher for these materials (Wu et al. 1999). Apart from these issues, post processing is necessary for the features fabricated by these techniques as the chip breaking capacity is lower for these materials and burr formation is almost inevitable (Weinart and Petzoldt, 2004). Adhering of the chips to the tool is also an issue as it reduces its cutting ability resulting in to deteriorated surface generation (Wu et al. 1999, Lin et al., 2000). Further formation of affected layer due to strain hardening, cyclic hardening and variation in the micro hardness of the material may also result in degrading the shape memory capacity of the alloy (Weinart and Petzoldt, 2004). Attempts have been made by researchers to study the feasibility of tool coatings for machining SMAs. It has been reported that the machining performance can be improved significantly by employing multi layer coatings systems like TiCN/TiAlN or TiCN/TiN on the cutting tools because of reduction in chip adhering (Lin et al. 2000, Weinart et al., 2004). The concept of minimum quantity lubrication has also been reported to reduce the issue of chip adherence to the tool (Weinart and Petzoldt, 2008). Improvement of process by providing additional tool kinematics like ultrasonic vibration has also been reported for turning process. Results suggest that the surface integrity can be improved by such type of hybridization techniques (Chegini and Akbari, 2009). Concept of cryogenic cooling has also been employed to carry out turning efficiently and results suggest that even though surface roughness and cutting forces and wear characteristics like notch wear and flank wear can be reduced, the technique leads to generation of affected layer incapable of martensitic transformation, thereby resulting into the loss of SME of the material (Kaynak et al., 2013, Kaynak et al., 2014). Even though the conventional machining processes are difficult to be applied for machining SMA materials, the efficiency of machining can be improved by using the parameters at optimum range. Literatures suggest that higher cutting levels are preferable for proper machining of these materials (Weinart and Petzoldt, 2004). However, due to burr formation, preprocessing techniques like chemical etching or electro polishing is a necessity prior to its application for features fabricated by conventional machining processes. 3.0 MACHINING OF SMAs BY ELECTRO DISCHARGE MACHINING Electro discharge machining is a thermal based advanced machining process in which the material is removed from the job surface in the form of small crater by precisely controlled and repetitive sparks in the presence of a dielectric medium. The technique can be applied to any electrically conductive material irrespective of its mechanical properties. Many variants of EDM exist in the manufacturing industry now a days and the selection of a variant depends on the shape of the final feature. As SMAs are electrically conductive, the same can be machined very easily using EDM process. But, the formation of recast layer and heat affected zone on the machines surface can deteriorate the SME property of the material as the metallurgical properties of these layers will be different from the base material. This will be more evident on materials machined by EDM and is subjected to higher bending strains (Lin et al., 2001). Therefore the process parameters must be selected in such a way that the thickness of recast layer as well as heat affected zone will be lower. A higher pulse ON time is preferable for lower recast layer thickness (Hsieh et al., 2009). It has also been reported that material composition of the SMA also influences the thickness of the recast layer (Lin et al., 2005). Roughness of the feature is an important quality characteristic as it assesses the direct usability of the feature or component in a particular application. The features generated from SMAs are

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widely used in biomedical and aerospace applications where the surface quality is of high importance. For fabricating features with lower surface roughness, a lower current and higher pulse ON time is preferable (Narendranath et al., 2013). Apart from discharge parameters, other factors that may influence the surface roughness of SMAs machined by EDM are material composition (Chen et al. 2007), its thermal conductivity and melting temperature (Chen et al., 2008). Use of lower discharge conditions may yield a good surface, but it will also result in to an increase in total machining time. So, it is beneficial to do the parameter level selection based on the prime objective. A higher current condition must be employed for higher material removal rates and lower current conditions for precise machining of features (Lin et al., 2001). If both such contrasting characteristics have same level of importance, concept of simultaneous optimization of the machining process must be carried out (Manjaiah et al., 2014). 4.0 MACHINING/PROCESSING OF SMAs BY LASERS In laser beam machining or processing, a laser is used as the energy source. In this, optical energy is focused on to the job surface, resulting in to the application of highly focused, high density energy, leading to melting and evaporation of the material in a controlled manner. The process can be utilized for machining or processing metals as well as non metals. But, as in the case of electro discharge machining, effects of heat affected zone do exist in this process, which is detrimental for SMAs and quality of surface generated is also poor. Despite these limitations, the machining technique is used for fabricating medical devices like active catheters, stents etc (Tung et al., 2008). There are different types of lasers which differ in their system configuration, wave lengths and pulse durations. For cutting SMAs, short pulse lasers like femtosecond lasers are more preferable than Nd:YAG lasers or processes like milling and EDM as the thickness of affected layer under such lasers are relatively lower (Huang et al., 2004). Further, the surface quality of the cut features can also be enhanced by the use of short and ultrashort pulse lasers (Pfeifer et al. 2010). Literatures also suggest that by giving proper movements to the laser beam, the feature quality can be enhanced and the recast layer thickness can be reduced (Li et al., 2006). Apart from machining or cutting, lasers have also been used for generation of surfaces having specific properties and structures. Such process can be termed as laser surface alloying or laser engineered net shaping. These process variants can be effectively used for generating layers having specific structures. Even though NiTi SMAs have good compatibility, the presence of nickel in the alloy may cause allergic reactions with some specific body tissues. Application of engineered surfaces that differ in compositions from the base SMA material can be used in such conditions also. This has been successfully demonstrated by Man et al. (2005). They have generated porous layers having open and interconnected structures on an SMA surface with very low Ni content by laser surface alloying process. The layer is reported to have improved osteointegration characteristic making it more suitable for biomedical applications. However, it has also been reported that higher levels of porosity in such generated structures may reduce the recoverable strain (Krishna et al., 2008). 5.0 MACHINING/PROCESSING TECHNIQUES

OF

SMAs

BY

CHEMICAL/ELECTROCHEMICAL

Chemical or electrochemical based processes have the advantage of removal of material from the job surface in smallest amount, resulting in to the generation of smooth surfaces by chemical and anodic dissolution. Application of these processes comes majorly in the scope of finishing features that has been generated by other conventional and non conventional machining processes. As the material is removed from the job surface at atomic level, these processes can be used to generate features of micro level size. Researchers have reported generation of SMA foils having thickness as small as 10µm by chemical etching. However, down scaling of the feature is difficult because of side etching effect. It has also been reported that the shape recovering property of SMAs tend to degrade slightly with reduction in size of the micro feature (Chen and Wu, 1999). Generation of features in similar range by electro polishing has also been reported (Shelyakov et al., 2011). Application of backside dummy material has also been employed by researchers for generating SMA features like micro actuators by throughout electro chemical etching (Mineta et al., 2002). Another aspect in which process like electro chemical polishing is beneficial is that the

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corrosion resistance of the job surface can be improved. However, the corrosion properties do depend on the initial surface texture (Pohl et al., 2004). Further, formation of a smooth and homogenous layer of oxide on the job surface which is free from Nickel content will make it biocompatible (Simka et al., 2010). Further, material properties which are critical importance for medical applications like wettability, blood compatibility, thromboresistance etc. have also been reported to improve after electro polishing (Chu et al. 2008). Electrolytes used for the process also have an effect on achieving the final process objective. Neutral electrolytes are preferable for achieving higher material removal rate and acid electrolytes for precision machining (Lee and Shin, 2011). The duration of polishing time is also an aspect that must be considered as prolonged exposure of surface to electrolytic processing may result into wavy surface (Pohl et al., 2004). The process can be used efficiently for surface improvement; however it is preferable to reduce the roughness of highly deteriorated surfaces to a lower level by any other technique prior to electrolytic polishing (Lee and Shin, 2011). Application of Electro Chemical Machining for feature fabrication on SMAs has also been reported. To counter the issue of side etching occurs in ECM which leads to erratic feature generation, application of concept of confined etchant layer technique has been reported for effective generation of 3D micro structures (Maa et al., 2007). At the same time, proper selection of process parameters like current and duty factor is important for effective and accurate feature fabrication (Lee et al. 2010). 6.0 MACHINING OF SMAs BY WATER/ABRASIVE WATER JET TECHNIQUES Literatures suggest that water jet machining is a viable option for cutting or machining SMAs as the machined surface will be free from effect of heat. The overall processing time and operating costs are lower when compared to other processes. However, deburring or electro polishing as in the case of conventional machining processes is necessary in the case of water jet machining also. It has also been reported that the top and bottom surfaces of the cut features surface differ significantly and the cut edges suffer heavy plastic deformation during the process. Feature generation at micro level has also been reported to be difficult as the water jet blows away the feature after a certain limit (Frotscher et al., 2011). Apart from cutting, the application of water jet and abrasive water jets has been employed for generation of pockets on SMA surfaces. It has been reported that abrasive water jets are more viable than plain water jets for controlled depth milling. But, in terms of surface integrity, plain water jets perform better. So a sequential application of abrasive water jets and plain water jets has been reported to be beneficial for effective milling of SMAs (Kong et al., 2011). The use of plain water jets after abrasive water jet machining have other advantages also, it can remove any embedded abrasive on the surface and the surface cracks can also be reduced making it capable of using under fatigue loading conditions (Kong et al., 2013). 7.0 CONCLUSIONS A brief review of the machining of different shape memory alloys by conventional as well as non conventional techniques has been carried out. Machining of SMAs by conventional techniques like milling, turning, drilling are difficult to its peculiar material properties and very high tool wear. Even though thermal based non conventional techniques like Electro Discharge Machining and Laser Beam Machining are capable of machine these type of materials with relative ease, the recast layer and thermal effects may degrade the SME effect of the material. Chemical and Electrochemical based processes are a good option for processing the features generated by other machining techniques as they can improve the surface quality. Water jet and abrasive jet techniques are capable of cutting SMAs with least thermal effects; however, post processing is required in such conditions also due to burr formation. Further, it has also been understood that machining performance and machining outcomes does vary with the SMA’s composition; irrespective of the machining process. Therefore, identification of critical process parameter level is critical for efficient feature generation on SMAs. 8.0 REFERENCES 1.

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