Theoretical and experimental investigation of micro milling for additive manufactured titanium alloy

Abstract

Additive Manufacturing (AM) has garnered significant attention due to its ability to produce intricate and complex profiles, lattice structures, and internal and thin-walled structures. Selective Laser Melting (SLM) has emerged as a leading AM method for metals due to its exceptional adaptability in fabricating complex metallic parts, offering flexibility in shapes and geometries. This is especially beneficial for titanium alloys, as the rapid cooling inherent in SLM transforms the stable Ti6Al4V phase into α' martensite within a hexagonal close-packed (HCP) crystal structure. The improved mechanical and microstructural properties of Ti6Al4V produced by SLM have garnered significant interest among metal machining researchers. Consequently, the study of SLM-fabricated Ti6Al4V and its machining characteristics is a focal point in advanced manufacturing. Despite the significant improvements in the mechanical and microstructural properties of Ti6Al4V produced by SLM, challenges remain in achieving the required surface finish and precise geometry for critical applications, underscoring the importance of post-processing techniques. Micro-milling is a promising method for producing intricate parts with high accuracy, but it presents challenges such as deteriorated surface quality, burr formation, and accelerated tool wear. These issues necessitate careful investigation of influences of machining parameters on micro-milling performances and also the machining effects on the microstructure changes on the machined surface. This thesis aims to address these machining difficulties by integrating micro-milling with SLM technology, leveraging the advantages of both methods. The performance of micro-milling was evaluated in terms of surface roughness, tool wear, chip morphology, and burr formation through comparative experimentation on SLM Ti6Al4V and wrought Ti6Al4V. The results reveal the micro-milling mechanism and demonstrate superior machinability of SLM Ti6Al4V. A 3D finite element model (FEM) was developed to simulate the shearing process, incorporating complexities such as tool rotation and interactions between the cutting tool and workpiece surfaces. This model helps in understanding the impact of machining parameters on surface defects, chip morphology, and cutting forces of SLM titanium alloys and is successfully validated with experimental results. On the other hand, this study explores the application of a magnetic field to enhance machining performance. The application of an external magnetic field aligns paramagnetic particles, improving thermal conductivity and reducing surface roughness by 22%. Scanning Electron Microscopy (SEM) images confirm fewer surface defects and reduced tool wear in micro-milling SLM Ti6Al4V with a magnetic field. These insights provide valuable information for applying magnetic fields in the micro-milling of additive manufactured parts, advancing the field of precision machining. Overall, this thesis advances the integration of micro-milling with SLM technology and develops a validated 3D finite element model, with suggesting a magnetic field assistance into micro-milling of SLM Ti6Al4V to resolve machining difficulties, offering significant contributions to understanding the machining mechanism of micro-milling of AM, and further increasing the surface quality of AM parts.