Effect of material anisotropy on ultra-precision machining of Ti-6Al-4V alloy fabricated by selective laser melting

Abstract

Selective laser melting (SLM) is generally characterized by the brief laser-powder interaction time, highly localized heat intensity and input distributions, steep temperature gradients and high cooling rate. It was admitted that the mechanical properties of as-built SLMed components exhibit significantly anisotropic due to different heat history and microstructures in different directions. Post-processing of additively manufactured parts is generally considered an essential operation due to poor surface quality and dimensional accuracy. In this paper, comprehensive research is conducted about the influence of material anisotropy of additively manufactured Ti-6Al-4V alloys on machining performance in ultra-precision micromachining. The machining performance of SLMed Ti-6Al-4V alloys was derived from two cutting directions (parallel/perpendicular to the laser scanning direction), two machining surfaces (top/front surface) and three laser scanning strategies (0°-line scanning, 67.5°/90°- rotation scanning). Surface topography, roughness, microstructure and microhardness were assessed to characterize the material anisotropy of SLMed titanium alloy. The machining results showed that the anisotropic mechanical properties and microstructure feature of the Ti-6Al-4V alloys are the fundamental reasons for the anisotropic machining performance in terms of cutting force and surface roughness. The machining properties of SLMed Ti-6Al-4V alloys are affected by the microstructure, machining surfaces, cutting directions and cutting parameters. Both the magnitude and fluctuation of dynamic cutting force produced on the front surfaces are obviously larger than those generated on the top surfaces. Significant improvement in surface roughness ranging from Sa = 80–100 nm could be achieved along the laser scanning direction for 0°-line SLMed Ti-6Al-4V sample when cutting depth was from 1 to 15 μm at vc = 50 mm/min. In addition, some typical SLMed defects appear on the machined surface and produced chips. This research is applicable to assist manufacturers in choosing the appropriate machining method for SLMed components in ultra-precision machining.