Microstructure-mechanical properties modeling for a novel surface milling-burnishing process

Abstract

A novel surface milling-burnishing process (SMBP) was proposed for surface strengthening of metals. The microstructural evolution, nano-hardness, and macro-tensile properties of SMBP-treated Ti-6Al-4V alloy were investigated through experimental and finite element (FE) modeling combined methods. A user-defined subroutine combining JCM-ms plasticity model and modified JMAK dynamic recrystallization model was proposed to predict grain refinement, while the Hall-Petch theory and a nanoindentation hardness-yield strength relationship (HN = 4.6σ_y) were established to model the surface nanoindentation hardness. The simulated distributions of grain refinement layer (GRL), plastic flow angle, grain size, and nanoindentation hardness exhibit good consistency with the experimental SEM, EBSD, and nanoindentation measurements. The SMBP-treated sample generates an ultrafine-grained surface layer (40–50 µm thick, with grain size <1 µm), achieving a peak hardness of 5.8 GPa which is 38 % higher than that of untreated material. SMBP enables active control of stress states (tension or compression) along the feed-direction through tool movement, while normal-direction stress states remain consistent with varying speeds. Moreover, microstructure-dependent FE simulations of tensile testing were proposed. The microstructural evolution in both GRL and plastic deformation layer (PDL) enhanced the macro-tensile strength of SMBP-treated samples through combined grain refinement and dislocation strengthening mechanisms. Compared to the unstrengthened sample, SMBP-treated specimens exhibited 6.1 %, 8.1 %, and 8.1 % improvements in tensile strength, yield strength, and elongation, respectively. This work establishes a novel SMBP for surface strengthening of Ti-6Al-4V alloy, along with a novel theoretical modeling approach for predicting microstructure and mechanical behaviors.