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Despite demonstrating a strong antibacterial property, surfaces machined and coated by nano-silver powder mixed electrical discharge machining (PMEDM) are faced with challenges relating to regulation of silver distribution as well as the spattered layer. These are among the main challenges that inhibit PMEDM’s suitability for medical implant applications, if the antibacterial coating is required. This study aims to solve these problems by introducing ultrasonic vibrations while machining Ti-6Al-4V workpiece surfaces using powder mixed electrical discharge machining involving both sinking and milling operations. During machining, the concentration of powder suspended in the dielectric fluid, the vibration amplitude as well as tool electrode size are varied. Results show that ultrasonic vibration has a capability to not only enhance the silver uniformity on the surface, but to also prevent the occurrence of a spattered layer, both of which are significantly affected by the vibration amplitude. Furthermore, for ultrasonic vibration assisted PMEDM, the tool electrode size plays an important role in the re-solidification process of the molten material since it influences the turbulence intensity, the occurrence of cavitation as well as the dielectric fluid’s flow rate in the machining gap.

An increase in geriatric population coupled by a growing incidence of different vascular ailments including peripheral vascular and coronary artery diseases imply that the medical devices market will continue to enlarge. As demand for minimally invasive surgical procedures and treatments increase, so does the significance of the abilities of nitinol, including shape memory, superelasticity, high fatigue resistance, corrosion resistance and biocompatibility which have revolutionized the medical device manufacturing sector. In order to fully exploit the capabilities of nitinol, sound and application oriented machining processes have to be selected for optimum efficiency. This is especially so for nitinol since it is a hard material to machine owing to its phase transformation and work hardening. Though stand alone research on processing nitinol is available, vital comparison information regarding to strengths and limitations of different machining processes applied in micromachining nitinol is lacking. This study presents on steps in a nitinol processing chain and offers a juxtaposition between different nitinol manufacturing, forming, machining and post processing methods including pointing out how they may affect final part performance. Different performance measurement parameters have been used to categorize the processes and a suitability analysis has been made which can offer a guide on when to use which method.

Previous studies have revealed the potential of powder mixed electrical discharge machining (PMEDM) with regards to concurrently machining part geometry and coating an antibacterial layer on medical devices. This study is aimed at further demonstrating this potential. In order to do so, the PMEDM process was varied by adding different concentrations of silver nano-particles into the dielectric fluid and used to machine Ti-6Al-4V. Afterwards, the resulting machined and coated surfaces were characterized with regards to surface integrity, the coating layer's thickness, microhardness and chemical elements as well as antibacterial property. Material removal rate, tool wear and pulse signals were also analysed in order to give an insight on process feasibility. From both qualitative and quantitative results, it could be established that the surfaces machined and coated by PMEDM method have demonstrated a significant reduction of not only the amount of S. aureus bacteria, but also the number of bacterial clusters on the coating layer's surface. Moreover, the coating layer's silver content, which depends on the powder concentration suspended in the dielectric fluid, plays a vital role in the antibacterial property. As compared to surfaces without silver, surfaces containing approximately 3.78% silver content showed a significant decrease in both bacterial numbers and clusters, whereas a further increase in silver content did not result in a considerable bacterial number and cluster reduction. Regarding the machining performance, as compared to EDM without powder, machining time is remarkably decreased by using the PMEDM method.

Most nitinol medical applications are hinged on its superelasticity and shape memory-two unique properties that are dependent on nitinol's phase transformation between a martensitic phase and an austenitic phase. Since these transformations are thermomechanical in nature, establishing the influence of thermal processing on nitinol's phase-transformation behaviour is vital as this can help in predicting changes and/or tuning its mechanical properties to fit specific applications. This study uses differential scanning calorimetry to investigate the influence of micro-electrical discharge machining (micro-EDM) on nitinol's phase-transformation behaviour. For conclusive analysis, a relatively a thermal Jet-ECM process is used as a reference for the as-received material, whereas Laser, a more commercially established medical-grade nitinol machining process, is used to provide comparative analytical aid. From the results, it can be clearly shown that high discharge energies in micro-EDM do indeed have the potential to significantly alter nitinol's transformation behaviour, including reducing thermal hysteresis and resulting in the occurrence of an unusual three-peak phenomenon in the endothermic reverse transformation on heating.

After reporting on the ability of micro-EDM to significantly alter the transformation behaviour of Nitinol whereby increasing discharge energy reduces thermal hysteresis and results in a three-peak reverse phasetransformation on heating, this study helps to further characterize the Nitinol micro-EDM process. This is by closely varying discharge energy so as to establish the boundary conditions for the three peak transformation behaviour as well as establish the influence of arcing on the mechanical properties of Nitinol. Samples machined using micro-EDM and jet-ECM are analysed using differential scanning calorimetry as well as tensile testing with five loading and unloading cycles after which the samples are loaded till failure. Moreover, discharge pulses are used to analyse arcing. From the results, it is not only possible to conclusively identify and establish arcing as the main phenomenon behind the three peak transformation behaviour, but also that the thermal damage caused by arcing results in a high residual strain, reduced elongation to failure, loss of machining accuracy and a reduction in upper and lower plateau stresses. It is also evident that if the discharge energy is carefully controlled to avoid arcing, it can be increased over a significant range (from ≈3.4 mJ to ≈130.2 mJ in this study) without significantly altering the phase transformation behaviour of Nitinol, which is very closely linked with its shape memory and superelasticity.