Call number: LD2668 .T4 1951 M377 / Master of Science

Thesis (Ph. D.)--OGI School of Science & Engineering at OHSU, 2004. / Includes bibliographical references.

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Plasma electrolytic Polishing (PeP) is a non-conventional technology for the surface treatment of electrically conductive materials. It is an effective machining technique for cleaning and polishing of metals and considered as a more environmentally friendly alternative to the electropolishing process. The electropolishing process uses aggressive media such as acids, whereas in PeP, acids or toxicants are replaced by low concentrated water solutions of various salts. In PeP, high DC voltage is applied to the electrodes in the aqueous electrolyte solution, which establishes a thin steam-gas layer around the surface of the work piece resulting in the generation of plasma. From the previous research, it is found that the formation of stable plasma generally takes place between 180-370 volts, where it results in better surface conditions. The aim of this study is to analyse the behaviour of current according to different voltages and their effects on surface roughness and material removal rate (MRR) of stainless steel in Plasma electrolytic Polishing process.

Plasma electrolytic Polishing (PeP) is a non-conventional technology for the surface treatment of electrically conductive materials. It is an effective machining technique for cleaning and polishing of metals and considered as a more environmentally friendly alternative to the electropolishing process. The electropolishing process uses aggressive media such as acids, whereas in PeP, acids or toxicants are replaced by low concentrated water solutions of various salts. In PeP, high DC voltage is applied to the electrodes in the aqueous electrolyte solution, which establishes a thin steam-gas layer around the surface of the work piece resulting in the generation of plasma. From the previous research, it is found that the formation of stable plasma generally takes place between 180-370 volts, where it results in better surface conditions. The aim of this study is to analyse the behaviour of current according to different voltages and their effects on surface roughness and material removal rate (MRR) of stainless steel in Plasma electrolytic Polishing process.

Additive manufacturing of Nitinol is a promising field, as it can circumvent the challenges associated with its conventional production processes and unlock unique advantages. However, the accompanying surface features such as powder adhesions, spatters, ballings, or oxide discolorations are undesirable in engineering applications and therefore must be removed. Plasma electrolytic polishing (PeP) might prove to be a suitable finishing process for this purpose, but the effects of post-processing on the mechanical and functional material properties of additively manufactured Nitinol are still largely unresearched. This study seeks to address this issue. The changes on and in the part caused by PeP with processing times between 2 and 20 min are investigated using Nitinol compression springs manufactured by Laser Beam Melting. As a benchmark for the scanning electron microscope images, the differential scanning calorimetry (DSC) measurements, and the mechanical load test cycles, conventionally fabricated Nitinol springs of identical geometry with a medical grade polished surface are used. After 5 min of PeP, a glossy surface free of powder adhesion is achieved, which is increasingly levelled by further polishing. The shape memory properties of the material are retained without a shift in the transformation temperatures being detectable. The decreasing spring rate is primarily attributable to a reduction in the effective wire diameter. Consequently, PeP has proven to be an applicable and effective post-processing method for additively manufactured Nitinol.

In many cases, the functional performance of additively manufactured components can only be ensured by finishing the functional surfaces. Various methods are available for this purpose. This paper presents a procedure for selecting suitable processes for finishing laser beam melting additive–manufactured parts which is ultimately based on technological knowledge. It was experimentally proven that the use of several consecutive finishing processes is beneficial to achieve better surface quality. One finishing process chain was particularly effective (namely particle blasting/vibratory grinding/plasma electrolytic polishing) and the technological limits of this method were investigated in this study. The optimal parameters for this process combination ensured a surface roughness Sa < 1 µm.

A novel, environmentally friendly, fast, and flexible polishing process for Nitinol parts is presented in this study. Nitinol samples with both superelastic and shape memory properties at room temperature were investigated. The chemical contamination and surface roughness of superelastic Nitinol plates were examined before and after plasma electrolytic polishing. The shift in phase transformation temperature and tensile strength before and after the polishing process were analysed using Nitinol wire with shape memory properties. The obtained experimental results were compared to the data obtained on reference samples examined in the as-received condition. It was found that plasma electrolytic polishing, when the right process parameters are applied, is capable of delivering Nitinol parts with extremely high surface quality. Moreover, it was experimentally proven that plasma electrolytic polishing does not have a negative impact on functionality or mechanical properties of polished parts.