Research Papers

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

[+] Author and Article Information
Ninggang Shen

Department of Mechanical and
Industrial Engineering,
University of Iowa,
Iowa City, IA 52242

Hongtao Ding

Department of Mechanical and
Industrial Engineering,
University of Iowa,
Iowa City, IA 52242
e-mail: hongtao-ding@uiowa.edu

Zhengwen Pu, I. S. Jawahir

Institute for Sustainable Manufacturing (ISM),
University of Kentucky,
Lexington, KY 40506

Tao Jia

GE Global Research
Niskayuna, NY 12309

1Corresponding author.

Manuscript received February 5, 2016; final manuscript received July 5, 2016; published online January 31, 2017. Assoc. Editor: Guillaume Fromentin.

J. Manuf. Sci. Eng 139(6), 061012 (Jan 31, 2017) (13 pages) Paper No: MANU-16-1093; doi: 10.1115/1.4034279 History: Received February 05, 2016; Revised July 05, 2016

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.

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Fig. 1

Microstructure in the machined surface and chip produced under condition Cryo-Re70: (a) optical microscopy, (b) SEM micrograph, (c) AFM image of machined surface, and (d) optical micrograph of chip (raw images adopted from Ref. [15])

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Fig. 2

Dislocation density-based plasticity model predictions for Mg Alloy AZ31B compared with flow stress measurement under various conditions from Ref. [54] collected by a TSHB apparatus

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Fig. 3

Multipass cryogenic machining simulation via advantedge

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Fig. 4

Flowchart of the material subroutines in advantedge

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Fig. 5

Surface and chip meshing in a two-pass simulation (condition Cryo-Re30): machined surface in (a) the first pass and (b) the second pass; (c) serrated chip formation and adaptive remeshing in the chips (color contours in plastic equivalent strain)

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Fig. 6

Comparison of simulated results from 1-pass and 2-pass cut for condition Dry-Re70: (a) equivalent plastic strain, (b) total dislocation density, (c) grain size, and (d) change of hardness

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Fig. 7

Simulated temperature field and cutting force histories compared with the experimental measurement for condition Cryo-Re30 (experimental data adopted from Ref. [15])

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Fig. 8

Simulated fields under condition Cryo-Re30: (a) equivalent plastic strain, (b) total dislocation density, and (c) grain size

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Fig. 9

Simulated profiles of (a) equivalent plastic strain, (b) total dislocation density, (c) grain size, and (d) machined subsurface micrographs from Ref. [15]

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Fig. 10

Simulated microhardness distribution for condition Cryo-Re30 in (a) machined surface and (b) chip

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Fig. 11

Simulated microhardness profiles compared with the experimental measurements in Ref. [15]

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Fig. 12

Simulated circumferential residual stress fields for conditions Cryo-Re70

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Fig. 13

Simulated residual stress profiles in depth compared with measurements in Ref. [15]

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Fig. 14

Comparison of the predicted twinning distribution with the microstructure beneath the machined surface of Cryo-Re30 (raw images adopted from Ref. [15])




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