Material removal mechanisms in ultra-high-speed scratching of Ti6Al4V alloy by selective laser melting

Abstract

Selective laser melting (SLM) offers advanced solutions for manufacturing high added value titanium alloy (Ti-alloy) components, owing to its capability to facilitate rapid, integrated, and customisable manufacturing of complex parts. However, surface machining is imperative for SLM-manufactured (SLM-ed) components due to the poor surface integrity. SLM-ed Ti-alloy is a typical difficult-to-machine material, conventional machining methods are difficult to realize high-efficiency and high-quality machining of SLM-ed Ti-alloy. Ultra-high-speed machining (UHSM) exhibits immense potential for enhancing machining efficiency and quality. However, the material removal mechanism of SLM-ed Ti-alloy in ultra-high-speed regions remains unclear. This study develops a single-point scratching (SPS) system to investigate material removal mechanisms across speeds ranging from 20 m/s to 220 m/s. Systematic characterisations regarding surface creation, subsurface microstructure, and chip formation were conducted using FIB and STEM techniques. The results revealed that the pile-up effect was significantly suppressed at higher speeds. The machining-deformed zone (MDZ) exhibited a “skin effect,” with plastic deformation confined to a superficial layer with a depth within 1 μm at 220 m/s. The deformation mechanism transitioned from dislocation-mediated deformation (DMD) to twin-mediated deformation (TMD) under extremely high strain rate conditions, leading to the formation of ultrafine grains with embedded twins (UGENTs) structure. Additionally, the chip removal mode progressively shift from continuous chips to segmented chips, and eventually to fragmented chips with increased scratching speed. This study provides an insight into the material removal and deformation process of SLM-ed Ti-alloy under low to ultra-high-speed deformations, and lays the theoretical basis for the high-efficiency and high-quality machining of difficult-to-machining materials.