Experimental and numerical investigations on the process quality and microstructure during induction heating assisted incremental forming of Ti-6Al-4V sheet

Abstract

• Development of induction heating SPIF system to provide localized heating to deform Ti-6AI-4V alloy sheet. • A Nickel ball-roller forming tool has been integrated into the SPIF system to improve the surface quality. • The relation between mechanical and microstructural behaviours was investigated by analysing forming force, geometric accuracy, thickness profile and surface roughness with recrystallisation, grain refinement and micro-hardness throughout the process. • A Finite Element (FE) model was established to verify the experimental results. The FE obtained strain and strain rate history was used as input to form a constitutive model to investigate the microstructural evolution. • Arrhenius model has been established by calculating of Zener-Hollomon parameter (Z-parameter) to correlate the grain size and micro-hardness at specific regions. The conventional single point incremental forming (SPIF) process is unable to perform high geometrical accuracy and formability for the Ti-6AI-4V alloy sheet. In response, this article has proposed a reliable high-frequency induction heating-assisted SPIF system. Rapid localised heating (600 ℃ and 700 ℃ ) was integrated with a synchronized Inconel 625 Nickel alloy ball-roller forming tool to achieve high geometric accuracy and surface quality. This article also produced new insights into correlating mechanical and microstructural properties in SPIF at 600 ℃ and 700 ℃ . By investigating the mechanical properties (forming force, geometric accuracy, thickness profile), an explicit finite element (FE) simulation was established to predict the results. The output strain history from the FE simulations was used as input and integrated with electron backscatter diffraction (EBSD) and micro-hardness characterisations, to form a constitutive model (Arrhenius model) to calculate the Zener-Hollomon parameter (Z-parameter). The grain size and micro-hardness experimental results were correlated with Z-parameter calculation to predict the microstructural development at the initial, middle, and final stages of the deformation process. The mechanical results revealed that the 700 ℃ experiment performed enhanced geometric accuracy and thickness profile, with a reduced forming force. However, the surface quality is reduced as the lubricant dissipated rapidly, while the ball-roller tool effectively compensated for this behaviour by reducing the friction. At the microstructural level, 600 ℃ revealed strong strain hardening and grain deformation, and 700 ℃ revealed better grain refinement by dynamic recovery (DRV) and dynamic recrystallisation (DRX). A proportional relationship between Z parameters and grain size and a low-high-low micro-hardness profile was proposed.