Laser assisted micro-milling of titanium alloy

Abstract

The interest in applying Additive Manufacturing (AM) technology has grown due to its ability to produce complex parts with high flexibility, serving as an alternative to conventional manufacturing processes. However, the poor surface quality and limited dimensional accuracy of AM parts often necessitate post-processing such as machining, grinding, and polishing. Titanium, specifically Ti6Al4V alloy, is frequently used in AM technology. This study compares the machinability of AM Ti6Al4V parts produced by Electron Beam Melting (EBM) with extruded Ti6Al4V parts, focusing on cutting forces, specific cutting energy, burr formation, and surface quality in the micro-milling process. Despite the higher hardness of EBM Ti6Al4V, no significant difference in cutting forces was observed at chip thicknesses between 7.4 μm and 37.3 μm. However, at chip thicknesses below 7.4 μm, EBM parts exhibited lower cutting forces and specific cutting energies, and finer surface roughness. Both materials formed continuous wavy-type burrs of comparable size during micro-milling. The study also underscores the significance of Laser-Assisted Machining (LAM) in reducing machining forces and increasing Material Removal Rate (MRR). Instead of the traditional LAM method, a pico-second laser (USPL) was used to pre-structure the Ti6Al4V parts, impacting uncut chip thicknesses during micro-milling. A kinematic model was developed to understand the influence of workpiece structuring on uncut chip thicknesses, identifying structure density and depth as critical parameters. Experimental tests showed a significant reduction in cutting forces with pre-structured workpieces, without notable changes in surface roughness. The orientation of structure lines relative to the helix angle affected the surface roughness. Pre-structuring led to controlled subsurface damage and less material removal during milling, resulting in better machinability.