Formation mechanism and evolution of surface coarse grains on a ZK60 Mg profile extruded by a porthole die

doi:10.1016/j.jmst.2019.11.037

Abstract

Abstract:

Porthole die extrusion of Mg alloys was studied by means of experimental and numerical studies. Results indicated that an inhomogeneous microstructure formed on the cross-section of the extruded profile. On the profile surface, abnormal coarse grains with an orientation of <11-20> in parallel to ED (extrusion direction) appeared. In the profile center, the welding zone was composed of fine grains with an average size of 4.19 μm and an orientation of <10-10> in parallel to ED, while the matrix zone exhibited a bimodal grain structure. Disk-like, near-spherical and rod-like precipitates were observed, and the number density of those features was lower on the profile surface than that in the profile center. Then, the formation and evolution of coarse grains on the profile surface were investigated, which were found to depend on the competition between static recrystallization and grain growth. The stored deformation energy was the factor dominating the surface structure through effective regulation over nucleation of the precipitates and recrystallization. A profile with a low stored deformation energy suppressed formation of precipitates and consequently facilitated grain growth rather than recrystallization, resulting in the formation of abnormal coarse grains. Finally, the surface coarse grains contributed detrimentally to hardness, tensile properties, and wear performance of the bulk structure.

Key words: Porthole die extrusion, Abnormal coarse grains, Stored deformation energy, Recrystallization

Jianwei Tang, Liang Chen, Guoqun Zhao, Cunsheng Zhang, Xingrong Chu. Formation mechanism and evolution of surface coarse grains on a ZK60 Mg profile extruded by a porthole die[J]. J. Mater. Sci. Technol., 2020, 47: 88-102.

Figures/Tables 20

Fig. 1. Schematic diagram of the extrusion process and sampling position.

Fig. 2. Finite element modeling of the porthole die extrusion experiment.

Fig. 3. EBSD maps of P1 at the different positions of (a) Z1, (b) Z2, (c) Z3, (d) Z4, and (e) the sampling positions.

Fig. 4. (a) Strain and (b) strain rate distribution of P1 during the porthole die extrusion.

Fig. 5. Pole figures and inverse pole figures of P1 in different positions of (a) Z1, (b) Z2, (c) Z3, and (d) Z4.

Fig. 6. Distribution of precipitates (a) in the grain interior of Z1, (b) at the grain boundary of Z1, (c) in the grain interior of Z2, and (d) at the grain boundary of Z2.

Fig. 7. Optical microstructures of the cross-section of the profiles at the (a) P1, (b) P2, (c) P3, (d) P4, (e) P5, and (f) sampling positions.

Fig. 8. Surface microstructures on the welding zones (a) P2, (b) P3, (c) P4, (d) P5 and (e) the sampling positions.

Fig. 9. Simulated results of the (a) flow stress and EBSD analysis of the local misorientation in the SCG zones of profiles (b) P1, (c) P2, (d) P3, (e) P4, and (f) P5.

Fig. 10. (a)?(c) First stage and (d)?(f) second stage of SRX nucleation, including the (a, d) EBSD maps, (b, e) local misorientation, and (c, f) inverse pole figures, where P-g indicates the parent grains, R-g represents the recrystallized grains, and Sub-g indicates the sub-grains.

Fig. 11. Pole figures and inverse pole figures of the welding zones: (a) P2, (b) P3, (c) P4, and (d) P5.

Fig. 12. Distribution and EDS maps of the second phase particles in the SCG zone of P4.

Fig. 13. Bright field TEM images of the precipitates viewed along zone axis of [[11], [12], [13], [14], [15], [16], [17], [18], [19], [20]]Mg in profiles (a?c) P4 and (d?f) P5.

Fig. 14. High resolution TEM images, corresponding FFT patterns and inverse FFT images of (a?c) a β'1 precipitate and (d?f) a β'2 precipitate.

Fig. 15. Tensile test results of the extruded profiles: (a) engineering stress?strain curves and (b) tensile properties.

Fig. 16. Fracture and schematic diagrams of the tensile samples.

Fig. 17. SEM images of the fracture morphology of (a) P1, (b) P2, (c) P3, (d) P4, and (e) P5.

Fig. 18. Hardness distribution of the extruded profiles at the (a) P1, (b) P2, (c) P3, (d) P4, (e) P5, and (f) test positions.

Fig. 19. Variation of the wear rate in the different extruded profiles of P1?P5.

Fig. 20. SEM images of the worn surfaces after the wear test of the profiles (a) P1, (b) P2, (c) P3, (d) P4, and (e) P5, and (f) schematic diagram of the sliding wear test.

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