Development of Novel Surface Finishing Processes for Additively Manufactured Metal Parts

Abstract

Poor surface quality is one of the drawbacks of metal parts made by various additive manufacturing (AM) processes. They normally possess high surface roughness and different types of surface irregularities. Post-processing operations are needed to reduce the surface roughness to have ready-to-use parts. Among all the surface treatment techniques, electrochemical surface finishing methods have the highest finishing efficiency. However, there are challenges with electropolishing in terms of reducing surface roughness of metals parts made via AM. Firstly, parts made with AM have both small-scale surface roughness and large-scale surface waviness. Electropolishing is only suitable for the reduction of micro-scale surface roughness while it is difficult to use the method to remove meso- to macro-scale surface waviness. In addition, it is still challenging to use electropolishing to reduce the surface roughness of internal channels of additively manufactured parts, benefiting from the promising feature of AM to produce parts with complex internal geometries. Finally, how to improve process sustainability is another question that needs to be addressed, since hazardous and corrosive chemicals are always used for the technique. To address the aforementioned problems, novel approaches were developed, incorporating both modeling and experimental investigations. Analytical and numerical models were constructed to explore the mechanisms of electropolishing and to understand the surface evolution during the process. The results offer valuable insights that can guide the design of experiments and foster the development of novel processes. The first experimental study focuses on using hybrid surface finishing technique to reduce meso-/macro- surface waviness. A novel surface finishing technique combining electrochemical polishing, ultrasonic cavitation and abrasive finishing was designed. Experiments were conducted on both electropolishing and hybrid finishing and the results were compared. While similar optimal arithmetic mean height values (Sa ≈ 1 μm) are achieved for both processes, the arithmetic mean waviness values (Wa) obtained from hybrid finishing are much less than those from sole electropolishing after the same processing time. The second experimental investigation aims at electropolishing internal channels. For doing this, a novel cathode tool was invented and fabricated using polymer 3D printing. Electropolishing was conducted on both straight and curved channels with different curvatures. Preliminary experiments demonstrated a maximum surface roughness Sa reduction, from 10.86 ± 0.50 μm to 1.44 ± 0.46 μm. Apart from this, electropolishing failure mechanisms were explained and design optimization was conducted through numerical simulation. The investigations show that the method is promising in reducing surface roughness of internal channels. In addition, experimental trials were also conducted to improve the sustainability of the surface finishing processes, including using greener electrolytes for electropolishing, and developing shear thickening polishing. Both alcohol-salt electrolyte system and deep eutectic solvent electrolyte were investigated, demonstrating effective surface roughness reduction. Shear thickening polishing using the corn starch slurry was also explored. In spite of some promising results, the process was not repeatable due to numerous influencing factors.