Optimum rolling speed and relevant temperature- and reduction-dependent interfacial friction behavior during the break-down rolling of AZ31B alloy

doi:10.1016/j.jmst.2018.03.020

Abstract

Abstract:

The present study aimed to determine the optimum rolling speed for break-down rolling of as-cast AZ31B alloy and investigated the friction behavior associated with temperature- and reduction-sensitivity at the roll/plate contact interface. Tensile testing, formability evaluation and microstructural studies relevant to different rolling speeds were performed and finally the optimum operating rolling speed (50.0 ± 0.8 m/min) was obtained. Further, the effects of rolling reduction and initial temperature were assessed on the temperature variation, lateral spread and interfacial friction behavior at optimum rolling speed. The results showed that lower rolling speed (18.0 ± 0.8 m/min) resulted in an incompletely recrystallized structure where twins occupied relatively high volume fraction. Twinning dominated the deformation at rolling speed exceeding the optimum, resulting in the local recrystallization with shear bands and coarse grains. Rolling at 50.0 ± 0.8 m/min could get the best overall tensile properties and rolling formability due to the relatively high recrystallization degree and microstructure uniformity. An inverse method has been developed to determine the interfacial friction coefficient during interaction of AZ31B alloy with roll surfaces. When rolling at the optimum speed, the interfacial friction coefficient ranged from 0.16 to 0.58, which was strongly positively correlated with the reduction but slightly positively correlated with the initial temperature. Depended on the rolling characteristics, external friction effect coefficient ranged from 1.25 to 2.35 and it exhibited positive correlation with both the initial rolling temperature and rolling reduction.

Key words: AZ31B alloy, Rolling speed, Lateral spread, Friction coefficient, External friction effect coefficient

Weitao Jia, Yan Tang, Fangkun Ning, Qichi Le, Lei Bao. Optimum rolling speed and relevant temperature- and reduction-dependent interfacial friction behavior during the break-down rolling of AZ31B alloy[J]. J. Mater. Sci. Technol., 2018, 34(11): 2051-2062.

Figures/Tables 19

Fig. 1. Metallographic structure of the edge region (a) and the internal region (b) after homogenization.

Fig. 2. Polarized light micrographs in the RD-ND plane of plates rolled at different speeds: (a) 18.0 ± 0.8, (b) 50.0 ± 0.8, and (c) 72.0 ± 0.8 m/min.

Fig. 3. Optical micrographs of plates rolled at different speeds: 18.0 ± 0.8 (a and d), 50.0 ± 0.8 (b and e) and 72.0 ± 0.8 m/min (c and f).

Fig. 4. Grain size distribution relevant to different rolling speeds, including 18.0 ± 0.8, 50.0 ± 0.8, and 72.0 ± 0.8 m/ min.

Fig. 5. Processing data about grain size, including the standard deviation, range, and maximum/minimum/mean grain size.

Fig. 6. Effect of rolling speed on edge cracks (a) and tensile properties (b), including 0.2% offset yield strength (YS), ultimate tensile strength (UTS), and elongation (EL).

Fig. 7. Temperature time-history curves of different positions during 200 °C-14% rolling.

Fig. 8. Maximum temperature difference under different process parameters: (a) temperature drop of the surface layer and (b) temperature rise of the center layer.

Fig. 9. Macroscopic morphology of plates rolled at 45% ± 3.5% with different initial temperatures (T0): (a) 250 °C, (b) 300 °C, (c) 350 °C and (d) 400 °C.

Fig. 10. Determination process and results for the non-crack critical reduction (εc).

Fig. 11. Rolling spreads under different rolling conditions.

Fig. 12. FEM simulation of the flat rolling under 400 °C-37% with different friction coefficients.

Fig. 13. Flow diagram for calculating friction coefficient at roll/plate interface.

Fig. 14. Inverse determination process of Coulomb friction coefficient under 290 °C-25% and temperature variation and rolling force changes in both experimental results and simulation results.

Fig. 15. Inverse determination of rolling friction coefficients under different conditions.

Fig. 16. Actual calculation of external friction effect coefficients under different rolling conditions.

Table 1 Different empirical calculation equations for nσ′.

Fig. 17. Correlation between the actual calculation n’σ values (as shown in Fig. 16) and the results calculated by empirical equations: (a) Eq. (12), (b) Eq. (13), (c) Eq. (14), (d) Eq. (15), (e) Eq. (16), (f) Eq. (17), (g) Eq. (18) and (h) Eq. (19).

Fig. 18. Relation between the calculated n’σ values by Stone’s equation (before and after the optimization) and actual calculation values.

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