Microstructure evaluation of shear bands of microcutting chips in AA6061 alloy under the mechanochemical effect

doi:10.1016/j.jmst.2021.02.044

Abstract

Abstract:

The mechanochemical effect on the microcutting of AA6061 alloy is studied through characterization on the chip. A pronounced reduction of machining forces and chip thickness was observed with mechanochemical effect during microcutting. Furthermore, electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) observations were performed on the chips and shear bands. The result reveals much coarser grains (24.6 μm in size) in the surfactant-affected chip than that in the surfactant-free chip (13.5 μm). Different grain orientations are induced by microcutting. {100}<001> and {110}<112> grain orientations are majority for surfactant-free chip, and {110}<001> and {100}<110> dominate most for all grain orientations for surfactant-affected chip. Additionally, since less localized shear strain and lower temperature are generated inside the shear band with mechanochemical effect, almost no recrystallization phenomena can be observed in this region. TEM analysis shows that fewer subgrains and dislocations could be observed inside the shear band of the surfactant-affected chip in comparison with the surfactant-free chip. Based on the high-resolution transmission electron microscopic (HRTEM) observations, dislocations were observed at the atomic scale. The results show that the main dislocation motion mode in shear bands of surfactant-free and surfactant-affected chips are dislocation climb and dislocation glide, respectively.

Key words: Mechanochemical effect, Microcutting, Shear band, Chip formation, Dislocation

Jiayi Zhang, Yan Jin Lee, Hao Wang. Microstructure evaluation of shear bands of microcutting chips in AA6061 alloy under the mechanochemical effect[J]. J. Mater. Sci. Technol., 2021, 91: 178-186.

Figures/Tables 9

Fig. 1. (a) Experimental setup (Fc and Ft are the cutting and thrust forces, respectively), (b) the tested cutting and thrust forces of the two chips during microcutting (Chip A: without mechanochemical effect; Chip B: with mechanochemical effect), and the SEM microstructures of (c) Chip A and (d) Chip B (the insert images are side view of chips after ion thinning).

Fig. 2. EBSD orientation distribution color maps of (a) Chip A and (b) Chip B (the yellow wireframe representing the chips region), the inverse pole figure is inserted: red for {001}, blue for {111}, and green for {101} direction. The ODF maps of the chips: (c) Chip A and (d) Chip B.

Table 1 Volume fractions of grain orientations in the chips.

Chip	Volume fraction (%)
Chip	{100}<001>	{110}<112>	{110}<001>	{100}<110>	{112}<111>	Un-recognisable
A	31.7	33.1	1.7	2.4	5.2	25.9
B	2.8	4.3	40.9	26.7	4.1	21.2

Fig. 3. The TEM observations for shear bands on the chips: (a) the local microstructure of Chip A, (b) the amplified bright field image of red dotted line frame of (a), (c) the weak beam dark field image of (b) and (d) the selected subgrain inside shear band of (c).

Fig. 4. The TEM observations for shear bands on the chips: (a) the local microstructure of Chip B, (b) the amplified bright field image of red dotted line frame of (a), (c) the local amplified bright field image of (b) and (d) the weak beam dark field image of (c).

Fig. 5. The average misorientation (θ) maps of (a) Chips A and (b) Chip B.

Table 2 The related constants of recrystallization kinetics for the studied alloy [39].

δD₀	γ	Q	k	R	T
9.9 × 10^-14 m³/s	0.324 J/m²	100 kJ/mol	1.38 × 10^-23 J/K	8.314 J/(K·mol)	660 K (Chip A); 541 K (Chip B)

Fig. 6. The relationship between grain boundary rotation time (t) and misorientation angle (θ) for chips.

Fig. 7. The microstructure of selected shear band zones: (a) Chip A and (c) Chip B, and HRTEM observations for dislocation regions: (b) local amplification of (a) and (d) local amplification of (c). The schematic diagrams of (e) dislocation climb and (f) dislocation glide, which shows the different dislocation motion types in shear bands.

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