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Research Papers

Chip Fracture Behavior in the High Speed Machining of Titanium Alloys

[+] Author and Article Information
Xueping Zhang

School of Mechanical Engineering,
Shanghai Jiao Tong University,
Shanghai 200240, China
e-mail: zhangxp@sjtu.edu.cn

Rajiv Shivpuri

Department of Integrated Systems Engineering,
The Ohio State University,
Columbus, OH 43210

Anil K. Srivastava

Department of Manufacturing Engineering,
University of Texas Rio Grande Valley,
Edinburg, TX 78539

1Corresponding author.

Manuscript received July 23, 2015; final manuscript received December 30, 2015; published online March 25, 2016. Assoc. Editor: Laine Mears.

J. Manuf. Sci. Eng 138(8), 081001 (Mar 25, 2016) (14 pages) Paper No: MANU-15-1368; doi: 10.1115/1.4032583 History: Received July 23, 2015; Revised December 30, 2015

Machining of titanium alloy is a severe fracture procedure associated with localized adiabatic shearing process. Chip segmentation of titanium alloy is usually characterized with adiabatic shear band (ASB) and localized microfracture evolution process. ASB has been recognized as the precursor of fracture locus due to its sealed high strain intensity. Besides strain intensity, stress triaxiality (pressure-stress states) has also been identified as a significant factor to control fracture process through altering critical loading capacity and critical failure strain. The effect of stress triaxiality on failure strain was traditionally assessed by dynamic split Hopkinson pressure bar (SHPB), quasi-static tests of tension, compression, torsion, and shear for finite element (FE) analysis. However, the stress triaxiality magnitudes introduced by these experiments were much lower than those generated from the high speed machining operation due to the fact that ASBs in chip segmentation are usually involved in much higher strain, high strain rate, high stress, and high temperature associated with phase transformation. However, this aspect of fracture evolution related with stress triaxiality and phase transformation is not well understood in literature. This paper attempts to demonstrate the roles of stress triaxiality and phase transformation in chip segmentation especially in the high speed machining of titanium alloy in FE framework. Johnson–Cook (JC) failure model is calibrated by addressing the characteristics of stress triaxiality and phase transformation associated with high speed machining. This research confirms that the selection of failure criterion parameters incorporated the effects of stress triaxiality and the alpha–beta phase transformation is indispensible to successfully predict fracture behavior during chip segmentation process in the high speed machining of titanium alloys.

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Figures

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Fig. 1

The FE model for the machining of titanium alloys

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Fig. 2

Void nucleation and growth inside a shear band in machining of titanium Ti–6Al–4 V alloy: (a) nucleation of voids within a shear band; (b) growth of voids; (c) elongation and rotation of voids; and (d) coalescence [38]

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Fig. 3

Stress–strain curve with progressive damage degradation [32]

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Fig. 4

Forces and stresses acting on isolated chip segmentation during machining

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Fig. 5

Stress triaxiality in chip segmentation of titanium alloy during machining at the speed of 5 m/s: (a) t = 1.188 × 10−4 s with element information and (b) t = 1.188 × 10−4 s without element information

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Fig. 6

Stress triaxiality distribution in one cycle of chip segmentation during machining of titanium alloy at speed of 21.8 m/s: (a) t = 2.40 × 10−5 s with element information; (b)t = 2.44 × 10−5 s with element information; (c) t = 2.40 × 10−5 s; (d) t = 2.44 × 10−5 s; (e) t = 2.48 × 10−5 s; (f) t = 2.52 × 10 −5 s; (g) t = 2.56 × 10−5 s; and (h) t = 2.60 × 10−5 s

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Fig. 7

Temperatures and beta phase fraction in chip segmentation during machining titanium alloy at speed of 21.8 m/s at the instant of t = 2.40 × 10−5 s: (a) temperature distribution and (b)beta phase fraction

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Fig. 8

Temperatures and beta phase fraction in chip segmentation during machining titanium alloy at speed of 5 m/s at the instant of t = 1.188 × 10−4 s: (a) temperature distribution and (b)beta phase fraction

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Fig. 9

Strains in chip segmentation during machining titanium alloy at speed of 5 m/s: (a) t = 1.188 × 10−4 s with element information and (b) t = 1.188 × 10−4 s without element information

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Fig. 10

Strains in chip segmentation during machining titanium alloy at speed of 21.8 m/s: (a) t = 2.40 × 10−5 s with element information and (b) t = 2.40 × 10−5 s without element information

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Fig. 11

Failure strain versus stress triaxiality under different loading conditions [40]

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Fig. 12

Temperature dependent mechanical state and phase transformation for titanium alloy: (a) tension reduction in area of Ti–6Al–4V samples with a microstructure of A: equiaxed α; B: deformed, partially globularized Widmanstatten α; C: β-annealed and water quenched acicular α; and D: β-annealed and air cooled Widmanstatten α with extensive grain-boundary α from Semiatin et al. [49] and Matsumoto et al. [50]; (b) temperature dependence β volume fraction and α/β microstructure for Ti–6Al–4V from Semiatin et al. [45]

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Fig. 13

Temperature dependence failure strain titanium Ti–6Al–4V alloy: (a) failure strains of Ti–6Al–4V samples with a microstructure of A: equiaxed α; B: deformed, partially globularized Widmanstatten α; C: β-annealed and water quenched acicular α; and D: β-annealed and air cooled Widmanstatten α with extensive grain-boundary α at the temperature range of 725–980 °C; (b) failure strains of Ti–6Al–4V samples with microstructures of A and D at temperature range of 725–825 °C

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Fig. 14

Fracture evolution process in chip segmentation in machining titanium alloy at speed of 21.8 m/s, (a) t = 0 s; (b) t = 3.5 × 10−7 s; (c) t = 7.0 × 10−7 s; (d) t = 1.05 × 10−6 s; (e) t = 1.4 × 10−6 s; (f) t = 1.75 × 10−6 s; (g) t = 2.1 × 10−6 s; (h) t = 2.45 × 10−6 s; (i) t = 2.8 × 10−6 s; (j) t = 3.15 × 10−6 s; (k) t = 3.85 × 10−6 s; and (l) the experimental chip at speed of 21.8 m/s from Sutter and List [19]

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Fig. 15

Temperatures in chip segmentation during fracture evolution in the machining of titanium alloy at speed of 21.8 m/s, (a) t = 0 s; (b) t = 3.5 × 10−7 s; (c) t = 7.0 × 10−7 s; (d) t = 1.75 × 10−6 s; (e) t = 2.1 × 10−6 s; and (f) t = 2.45 × 10−6 s

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