Constitutive modelling of Ti6Al4V alloy fabricated by laser powder bed fusion and its application to micro cutting simulation

Abstract

Although there is a rising interest in applications of metal additive manufacturing (AM) for fabrication of highly complex metallic components, poor surface quality of AM parts is an inherent limitation of this technology. Therefore, post-process machining operations are frequently performed to meet the necessary requirements for the application. When compared to wrought Ti6Al4V, the distinct metallurgical and mechanical characteristics of Ti6Al4V fabricated by laser powder bed fusion (LPBF) can lead to very different mechanical behavior, thus different machinability. Therefore, to properly perform the numerical modelling of the cutting processes, it is important to accurately define material mechanical behavior of LPBF Ti6Al4V. This study presents the experimental determination of the coefficients of the Johnson-Cook (J-C) constitutive model and the numerical simulation of orthogonal micro cutting process of hot isostatic pressed (HIP) LPBF Ti6Al4V. Besides, the results were also compared to the wrought Ti6Al4V. For that purpose, quasi-static and dynamic behaviors of wrought and LPBF Ti6Al4V were investigated by quasi-static tensile tests at several temperatures (0.001 s−1 at 20, 400, 600 °C) and at high strain rates using the split Hopkinson pressure bar (SHPB) tests (∼1000-5000 s−1 at 20, 400 °C). Orthogonal micro cutting simulations were performed using experimentally obtained J-C model of wrought and LPBF Ti6Al4V. A series of orthogonal micro cutting tests at different cutting speeds (75, 100, 150 m/min) and uncut chip thickness values (2.5, 5, 7.5, 10 μm) were performed to compare the measured forces and chip compression ratio (CCR) with those obtained by simulation. Good predictions of main cutting force and maximum CCR were obtained for both wrought and LPBF Ti6Al4V under different cutting conditions. Several critical points to be considered for better prediction of thrust force and chip geometries in future studies were highlighted.