Micro-Electrical Discharge Machining of Nitinol for medical applications

Scripts Precision and Microproduction Engineering, Band 13
1. Auflage 2019
ISBN: 978-3-95735-107-4
Verlag: Wissenschaftliche Scripten

Buch, Englisch, Band 13, 174 Seiten, Format (B × H): 140 mm x 210 mm, Gewicht: 300 g

Reihe: Scripts Precision and Microproduction Engineering

Referat
Gegenstand dieser Arbeit ist die Bearbeitung von Nitinol für medizinische Anwendungen mittels Mikrofunkenerosion (Mikro-EDM) ermittelt. Dabei wurden die generelle Möglichkeit und das Potential dieser Bearbeitung untersucht. Dazu gehört die Analyse von Prozesseffekten auf die Oberflächeneigenschaften, auf das Verhalten der martensitischen Phasenumwandlung sowie auf die mechanischen Eigenschaften von Nitinol. Auf Grund der Eigenschaften Formgedächtniseffekt und Superelastizit¨at wird Nitinol häufig für medizinische Anwendungen eingesetzt. Diese Arbeit gibt einen Einblick in die Auswirkungen des Bearbeitungsprozesses auf diese beiden speziellen Eigenschaften. Es konnte gezeigt werden, dass Mikro-EDM das Potenzial hat, thermische Effekte zu implementieren, die das Verhalten der Nitinolphasenumwandlung verändern können. Dazu gehören die Reduzierung der Hysterese sowie das Auftreten eines Dreispitzenphänomens bei der umgekehrten endothermen Phasenumwandlung beim Erwärmen. Als Hauptursache für dieses Dreispitzenverhalten konnte das Auftreten von Lichtbögen ermittelt werden. Darüber hinaus konnte gezeigt werden, dass es durch systematische Variation ausgewählter Prozessparameter möglich ist, Nitinol mit vernachlässigbaren thermischen Einflüssen zu bearbeiten. Unter der Voraussetzung einer Lichtbogen freien Bearbeitung war es zudem möglich, eine Erhöhung der Entladungsenergie zu erreichen, ohne wesentlich das Umwandlungsverhalten des Materials zu beeinflussen. Dies erlaubt die Abtragrate, die Elektrodenverschleißrate und die Oberflächenqualität zu optimieren und gleichzeitig unerwünschte Prozesseffekte zu minimieren.



Abstract
Within the scope of this work, micro-electrical discharge machining (micro-EDM) of medical-grade nitinol has been undertaken and examined for suitability and capability. This includes analysing process effects on nitinol’s surface characteristics, martensitic phase transformation behaviour as well as mechanical properties. Consequently, an insight on
how the process affects nitinol’s shape memory and superelasticity, two unique properties that make nitinol a key application material in the medical field, is given. It can be shown that micro-EDM has the potential to impart thermal effects that alter nitinol’s phase transformation behaviour. These include reducing hysteresis as well as resulting in the occurrence of a three-peak phenomenon during the reverse endothermic phase transformation on heating. It was also possible to establish arcing as the main cause of this three-peak behaviour. Moreover, it can be shown that by carefully controlling vital micro-EDM machining parameters, it is not only possible to machine nitinol parts with negligible thermal-related influences, but also that, as long as arcing is avoided, an increase in discharge energy does not significantly affect the material’s phase transformation behaviour. This can therefore aid in optimising the material removal rate, tool wear rate and surface quality, while minimising unwanted process effects.

Autoren/Hrsg.

Mwangi, James Wamai

Schubert, Andreas

Inhaltsverzeichnis

1 Introduction

2 State of the art
2.1 Nitinol’s fundamentals
2.1.1 Nitinol’s discovery and development
2.1.2 Nitinol’s unique properties: Shape Memory and Superelasticity
2.1.3 Nitinol’s thermomechanical and phase transformation behaviour
2.1.4 Diffusional transformations in nitinol
2.1.5 Factors affecting nitinol’s transformation behaviour
2.1.6 Nitinol’s suitability for medical applications
2.1.7 Significance of nitinol processing on its medical applicability
2.1.8 Shape setting nitinol

2.2 Nitinol processing and its significance to medical applications
2.2.1 Nitinolmanufacturingmethods
2.2.2 Forming nitinol
2.2.3 Nitinol micromachining methods
2.2.4 Post processing nitinol

2.3 Fundamentals of Micro-Electrical Discharge Machining
2.3.1 Micro-EDM process principle
2.3.2 EDMvariants
2.3.3 Micro-EDM pulse generator technology

2.4 Micro-EDMof nitinol
2.4.1 Influence of machining parameters on MRR, TWR and surface quality
2.4.2 Influence of machining parameters on HAZ
2.4.3 Influence of micro-EDM machining parameters on biocompatibility

2.5 Summary on state of the art


3 Aims of this study

3.1 Process characterization and optimisation

3.2 Justification of the study


4 Experimental implementation

4.1 Material selection
4.1.1 Workpiecematerial
4.1.2 Toolmaterial selection

4.2 Micro-EDM machining methodology
4.2.1 Applied micro-EDM machining technology
4.2.2 Investigating nitinol’s machinability by micro-EDM
4.2.3 Pulse analysis formicro-EDMof high aspect ratio nitinol bores
4.2.4 Pulse analysis criteria for calculating actual micro-EDM discharge energies
4.2.5 Investigating the influence of dielectric fluid and tool electrode on nitinol’s surface integrity
4.2.6 Heat Affected Zone analysis

4.3 Phase transformation based thermal analysismethodology
4.3.1 Procedure for machining samples used in testing phase transformation behaviour
4.3.2 Differential Scanning Calorimeter measurement procedure

4.4 Mechanical properties analysismethodology

4.5 Micro-EDM process optimisation criteria for machining nitinol for medical applications


5 Results and discussion

5.1 Fundamentals of micro-EDM of medical grade nitinol
5.1.1 Micro-EDM’s pulse formation characteristics
5.1.2 Effect of machining conditions on pulse formation

5.2 Machining parameter influence on micro-EDM of nitinol
5.2.1 Effect of varying discharge energy on micro-EDM of nitinol
5.2.2 Effect of varying open circuit voltage on micro-EDM of nitinol
5.2.3 Effect of dielectric fluid and tool electrode on micro-EDM of nitinol

5.3 Surface integrity analysis ofmicro-EDMedmedical grade nitinol
5.3.1 Micro-EDMed surface roughness analysis
5.3.2 Elemental surface composition analysis

5.4 Heat affected zone analysismicro-EDMed nitinol

5.5 Analysis of the phase transformation behaviour of micro-EDMed nitinol
5.5.1 Phase transformation behaviour of low energy micro-EDMed nitinol
5.5.2 Phase transformation behaviour of high energy micro-EDMed nitinol
5.5.3 Surface effects that could alter transformation behaviour

5.6 Micro-EDM process optimisation for machining nitinol for medical applications
5.6.1 DSC analysis to establish the onset of the three-peak phase transformation behaviour
5.6.2 Pulse analysis for characterization of the three-peak phase transformation behaviour
5.6.3 Characterization of the arcing phenomenon
5.6.4 Analysis of MRR, TWR and surface quality for process optimisation

5.7 Effect of Micro-EDM on the mechanical properties of nitinol


6 Conclusions and recommendations

Leseproben

Foreword from the editor

The book series titled, “Precision and Microproduction Engineering” presents, in an irregular sequence, results of research related to recent developments in the sectors of Precision Manufacturing and Surface Technology. These results are based on the research of the Professorship “Micromanufacturing Technology” of Chemnitz University of Technology as well as the“Functional Surfaces and Microsystem Manufacturing”department of the Fraunhofer Institute for Machine Tools and Forming Technology, both of which form the “Competence Center Micromanufacturing and Surface Technologies - KoMOT”. This thirteenth volume of the series is dedicated to the topic of Electrical Discharge Machining (EDM). EDM is an important manufacturing technology especially when it comes to the need for machining very hard, or hard to machine electrically conducting materials, a need which is often combined with the demand for achieving tiny and/or precise features. In this volume, the focus is set on micromachining a nickel-titanium based shape memory alloy. The dissertation submitted by James Wamai Mwangi aims for the analysis of influences of micro-electrical discharge machining on shape memory alloys, specifically the nickeltitanium alloy usually referred to as Nitinol. The presented work considers the influence of material and surface property changes, that are inflicted by the machining process, on the mechanical properties as well as the phase transformation behavior of Nitinol. Moreover, the influence of micro-EDM’s thermal impact on the material is investigated. In this regard, different machining conditions were considered and the influence of the micro-EDM process was compared with influences related to other suitable nitinol machining processes. This work contains challenging scientific tasks and includes experimental as well as analytical work in the field of engineering sciences. The results show the impact of the micro-EDM machining process, mainly of the thermal influences, on the properties of Nitinol, the roughness and the chemical composition of the surface layer and the stress-strain-behavior of the machined material. In addition, investigations are done to establish how the thermal impact of the machining process leads to an altering of the phase transformation temperatures and the thermal hysteresis of the material.Generally speaking, this thesis supports the systematic design of appropriate micro-EDM processes for machining of Nitinol in the context of medical and microstructured applications.

Prof. Dr.-Ing. Andreas Schubert
Chemnitz, June 2019