Fundamental Investigation on Polishing of Internal Structures Made by Laser-based Powder Bed Fusion

This study aims at utilizing a self-developed hybrid polishing system to establish polishing capabilities of electropolishing (EP), abrasive fluid polishing (AFP), multiple polishing in different sequences and innovative hybrid polishing for the internal structures prepared by laser-based powder bed fusion (L-PBF). By studying polishing effects on various inner surfaces, the relationships between polishing processes and material removal mechanisms of L-PBF surface features are established considering microstructural differences of surface features. The thesis starts with a comprehensive introduction of the project and then a literature review outlining L-PBF process, application of typical L-PBF internal structures, surface characteristics, advantages and limitations of current polishing methods for L-PBF inner surfaces. Breakthroughs in the surface finish of L-PBF internal structures are required in order to improve surface quality to meet the specific requirements of product performance. It is notified that EP and AFP exhibit complementary advantages in the polishing of internal structures among various polishing technologies. In the chapter 3, different types of inner surfaces for fundamental investigation and typical internal structures for application development are designed, and prepared by L-PBF using 316L stainless steel and Ti6Al4V powders. Then, un-sintered powders and sintered area are characterized as common surface features on L-PBF top, face up, side and face down surfaces considering the differences of morphology and microstructure. Meanwhile, an innovative hybrid polishing system which could carry out EP, AFP, their multiple polishing and hybrid polishing is established for the surface improvement of the L-PBF inner surfaces and internal structures. In the chapter 4, a polishing mode consisting of two-step EP is developed by using different potentials and polishing time for L-PBF 316L stainless steel and Ti6Al4V inner surfaces after parametric study in the developed polishing system. Based on material removal characteristics of surface features, the effectiveness and high efficiency of the two-step EP are demonstrated. Considering polishing characteristics of AFP, the inlet design of polishing chamber is improved and the material removal process of various L-PBF internal surfaces are discussed by analyzing the evolution of surface morphology, roughness and microstructure on cross sections in the chapter 5. In the chapter 6, multiple polishing in different sequences and hybrid polishing based on EP and AFP are investigated for L-PBF inner surfaces. It is found that L-PBF inner surfaces after single polishing can be further improved by multiple polishing in different sequences because of the complementary characteristics of EP and AFP in removing L-PBF surface features. In addition, hybrid polishing can perform EP and AFP simultaneously without interfering with each other, showing great potential in improving polishing efficiency of L-PBF inner surfaces. In the chapter 7, different polishing processes are applied to three typical L-PBF internal structures. The superiority of multiple polishing in different sequences and hybrid polishing for L-PBF inner structures is verified. Overall, it is confirmed that the self-developed innovative hybrid polishing system is capable of polishing various L-PBF internal structures with reliable results. Fundamental research on material removal characteristics of L-PBF surface features during polishing provides a theoretical basis for the applications of multiple and hybrid polishing based on EP and AFP. High efficiency and cost- effectiveness make the hybrid polishing system a strong candidate for industrial implementation in the surface finish of L-PBF internal structures.