Surface Modification of Additively Manufactured Parts for End-Use Surgical Instruments

Abstract

Additive manufacturing (AM) is a fast developing industry, with the aerospace, automotive and medical industries readily embracing the new technology. AM provides numerous benefits for design and production, however, one of the major problems preventing metal AM from overtaking conventional machining and casting processes is the high surface roughness of AM parts. In the medical industry, additive manufacturing is revolutionising medical devices with custom implant designs, and there is similar potential for complex surgical tools. These specialized parts require well finished surfaces that are able to be easily cleaned to prevent the spread of infection as well as being aesthetically pleasing or functional. In this thesis mass finishing and post-processing of additively manufactured parts were examined with the aim of producing end-use surgical instruments via additive manufacturing. Using the selective laser melting method, designed 17-4 PH stainless steel samples with various surface orientations and types were printed for investigating various surface finishing methods. 2D stylus profilometry was used to quantify the surface roughness and a scanning electron microscope was used to observe the sample surfaces. Energy dispersive X-ray spectroscopy was used to measure the surface elemental composition to investigate the contamination of the surfaces throughout post-processing. The initial surface roughness of the samples was very high (Ra=13.5±2.0μm, Wa=4.0±1.5μm, Pv=103.8±17.7μm: inclined surface). Chemical polishing methods using hydrochloric acid and hydrofluoric + nitric acid solutions were shown to be relatively ineffective at significantly reducing the surface roughness. Mechanical mass finishing processes, abrasive blasting and centrifugal disc finishing, were also investigated. Of the abrasive blasting processes white oxide vapour blasting produced the smoothest surfaces (Ra=2.1±0.4μm, Wa=3.2±1.1μm, Pv=25.3±4.8μm: inclined surface), but still not comparable to machined surfaces. Centrifugal disc finishing with ceramic media reduced roughness significantly, but the external radii significantly increased congruently and internal surfaces were unaffected by this process. After white oxide blasting, centrifugal finishing for 4 hours and performing a final glass bead blast, the smoothest surface was obtained (Ra=0.6±0.1μm, Wa=0.9±0.3μm, Pv=6.9±1.5μm: inclined surface). The order of these operations was also of significance as white oxide blasting after centrifugal finishing resulting in rougher surfaces. Contamination with aluminium oxide particles from white oxide blasting was able to be removed by glass bead blasting and then using a citric acid passivation to reduce the glass particle contamination. Wire electric discharge machining (a common process to remove AM parts from the build platform) of wrought Ti6Al4V and 17-4 PH stainless steel showed high amounts of copper and zinc present on the surface. Removal of these contaminants was attempted using acidic solutions. Titanium wire-cut surfaces responded only to a hydrofluoric and nitric acid solution. However, for stainless steel wire-cut surfaces, citric acid was found to reduce the levels appropriately, but hydrofluoric acid also outperformed citric acid by completely removing the contaminants. A process was determined to produce end-use surgical instruments. After printing, the parts should be removed from the build plate via wire electric discharge machining. The supports should then be broken and the surfaces with scaffold support attached should be machined/linished to flatten this extremely rough surface. White oxide vapour blasting then centrifugal finishing should be used to cut down the remaining surfaces before the parts are heat treated. After heat treatment critically dimensioned surfaces should be machined and then the part should be glass bead blasted to remove the oxide scale and provide the final finish. A citric acid cleaning procedure then passivates the surface and reduces surface contaminants. When designing and manufacturing a part in this way, the process should be adapted to the key specifications of the part and its surface.