Enhanced Surface Integrity From Cryogenic Machining of AZ31B Mg Alloy: A Physics-Based Analysis With Microstructure Prediction

Abstract

The use of magnesium (Mg) alloy has been continuously on the rise with numerous expanded application in transportation/aerospace industries due to their lightweight and other areas, such as biodegradable medical implants. It was shown recently that machining can be used to improve the functional performance of Mg-based products/components, such as corrosion resistance, through engineered surface integrity. In this paper, the behavior of AZ31B Mg alloy in cryogenic machining was discussed firstly. The surface integrity can be significantly improved by introducing the ultrafine grained (UFG) layer due to the severe plastic deformation (SPD) effect during cryogenic machining. The mechanisms of microstructure evolution and plastic deformation were analyzed based on the experimental findings in literature. A physics-based constitutive model involving material plasticity and grain refinement is developed based on both slip and twinning mechanisms and successfully implemented in a finite-element (FE) analysis with multiple cutting passes to predict the microstructure evolution by nanocrystalline grain refinement and other improvement of the surface integrity in the cryogenic machining of AZ31B Mg alloy. With a more quantitative assessment, the FE model results are further discussed for grain refinement, changes in microhardness, residual stresses, and slip/twinning mechanism with the apparent SPD taking place due to rapid cryogenic cooling.

Enhancement of stress corrosion cracking of AZ31 magnesium alloy in simulated body fluid thanks to cryogenic machining

“…These results agree with those obtained by Pu et al reporting a wider nanocrystalline region in cryogenically machined AZ31B alloy than in the dry machined counterparts [45]. The wider extension of both the SPD and nano-crystalline layer in the case of cryogenic cooling can be explained as follows: during machining, the material is subjected to high strain at very high strain rate inducing the fragmentation of original grains by dislocation movements [56]. With the application of liquid nitrogen, this effect is emphasized since the temperature is greatly reduced.…”

Development of a simulation model to study tool loads in pcBN when machining AISI 316L

“…Direct measurement of such loads is often impossible, and therefore, in recent years, the finite element modelling technique has become an important numerical tool for simulating the machining process. Constitutive models describing the material flow behaviour as a function of strain hardening, strain rate hardening, and thermal softening are required in order to capture the plasticity behaviour of the workpiece, during the conditions encountered in the machining process [13,32]. In finite element simulations, there are two types of formulations, Eulerian and Lagrangian.…”

Cryogenic milling and formation of nanostructured machined surface of AISI 4340

“…Strain hardening and grain refinement may be the other contributing factors to the increases in the hardness on the surface and sub-surface of the cryogenic machined parts [13]. From the Zenner-Holloman equations, there is a relation between grain size, strain rate, and temperature as shown in equation 3…”

“…The predicted forces showed good agreement with the experimental data, and the simulated chip shape matched well with the experimental data. Shen et al [13] investigated the different tool edge radii on predicting grain size, micro-hardness, and residual stress using a physics-based material model for magnesium alloy. These studies indicate that applying cryogenic cooling or a larger edge radius could increase the surface hardness.…”