Evaluation of dynamic development of grain structure during friction stir welding of pure copper using a quasi in situ method

doi:10.1016/j.jmst.2019.01.018

Abstract

Abstract:

By employing a quasi in situ method, we investigated the dynamic evolution of the grain structure considering the material flow, strain, and strain rate in the friction stir welding of pure copper. The tool ‘stop action’ and rapid cooling were employed and a brass foil was used as a marker to show the material flow path. The grain structure along the material flow path was characterised using electron backscatter diffraction. Static recrystallization occurs for the work-hardened base material in the preheating stage in front of the tool. In the acceleration flow stage, grains are significantly refined by plastic deformation, discontinuous dynamic recrystallization, annealing twinning during the strain-induced boundary migration and slight continuous dynamic recrystallization. In the deceleration flow stage, due to a strain reversal, the grain first coarsens, and is thereafter refined again. Finally, the hot-deformed material in the shoulder-affected zone is ‘frozen’ directly whereas that in the probe-affected zone undergoes significant annealing; thus, the recrystallized microstructure and 45°-rotated cube texture are obtained in the probe-affected zone.

Key words: Friction stir welding, Grain structure, Material flow, Pure copper, EBSD

X.C. Liu, Y.F. Sun, T. Nagira, K. Ushioda, H. Fujii. Evaluation of dynamic development of grain structure during friction stir welding of pure copper using a quasi in situ method[J]. J. Mater. Sci. Technol., 2019, 35(7): 1412-1421.

Figures/Tables 14

Fig. 1. Horizontal and longitudinal cross-sections of the keyhole showing the rotation flow zone by the tool traversing two dissimilar copper alloys (800 rpm-100 mm/min).

Fig. 2. Schematic of the experiment.

Fig. 3. Material flow features, strain and strain rate distributions on the keyhole horizontal planes [23]: (a) and (c) are the material flow on the 0.5-mm-plane and the corresponding strain, material flow velocity and strain rate, respectively; (b) and (d) are the material flow on the 2.5-mm-plane and the corresponding strain and strain rate, respectively.

Fig. 4. Microstructural features of the base material (a-d) and the preheating stage (e-h) (x = -7, 0.5-mm-plane): (a, e) EBSD orientation map; (b, f) grain boundaries map; (c, g) misorientation distribution; (d, h) (001) PF. Note: the welding direction (WD) is the same as the rolling direction (RD) of the base material.

Fig. 5. Variations in the grain size, LABs fraction and TBs fraction with respect to the accumulative true strain at the acceleration stage: (a) 0.5-mm-plane; (b) 2.5-mm-plane. Note: the error bars are based on multiple measurements.

Fig. 6. Variations in the grain structure on the 0.5-mm-plane at the acceleration stage shown by the EBSD orientation maps (a-c) and grain boundary maps (d-f): (a, d) ε=0.06, x = -4; (b, e) ε=0.92, x = -2; (c, f) ε=1.45, x = -0.5.

Fig. 7. Variations in the grain structure on the 2.5-mm-plane at the acceleration stage shown by the EBSD orientation maps (a-d) and grain boundary maps (e-h): (a, e) ε=0.05, x = -3; (b, f) ε=0.10, x = -2.5; (c, g) ε=0.32, x = -2; (d, h)ε≈1.03, x =-1.5.

Fig. 8. Mechanisms of the grain structure evolution at the acceleration stage: (a, d) IPF maps of the local microstructures; (b, e) the corresponding grain boundary maps and (c, f) KAM maps; (g-i) grain boundary maps showing perfect twins evolving into common grains; in the grain boundary maps, the blue, red and green lines represent HABs, LABs and Σ3 TBs, respectively; in the KAM maps, the black lines represent HABs.

Fig. 9. Orientation relationship between the recrystallized grains and the deformed grains (x = -2.5, 2.5-mm-plane): (a) EBSD image quality map of the recrystallized grains (orientation spread<2°); (b) (001) PF in the SD and SPN; (c) inverse PF in the normal direction; (d) EBSD image quality map of the deformed grains (orientation spread>2°); (e) (111) PF in the SD and SPN; (f) inverse PF in the shear direction; red lines denote LABs, blue lines denote HABs and green lines denote Σ3 TBs.

Fig. 10. Microstructural features at the high-velocity flow stage: (a, c) the 0.5-mm-plane, x = 0; (b, d) the 2.5-mm-plane, x = 1; (a, b) grain boundary map; (c, d) misorientation distribution.

Fig. 11. Variations in the statistical grain size, LABs fraction and Σ3 TBs fraction with respect to the accumulative true strain at the deceleration stage on the 0.5-mm-plane. Note: the error bars are based on multiple measurements.

Fig. 12. Variations in the grain structure at the deceleration stage on the 0.5-mm-plane shown by the EBSD orientation maps (a-d) and grain boundary maps (e-h): (a, e) ε=1.62, x = 1; (b, f) ε=2.02, x = 2; (c, g) ε=2.54, x = 3; (d, h) ε=2.69, x = 4.

Table 1 Variations in the statistic grain size, LABs fraction and Σ3 TBs fraction at the annealing stage on the 2.5-mm-plane.

x value (mm)	2	4	6	8
Grain size (μm)	1.49	1.64	1.82	2.03
LABs fraction	0.383	0.156	0.109	0.092
Σ3 fraction	0.047	0.243	0.317	0.367

Fig. 13. Grain structures (a1, b1) and corresponding orientation distribution functions (a2, b2) at the annealing stage (2.5-mm-plane): (a1, 2) x = 2; (b1, 2) x = 8. (Note: the R-cube here denotes the 45° rotated cube).

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