Grain refinement and orientation of AZ31B magnesium alloy in hot flow forming under different thickness reductions

doi:10.1016/j.jmst.2017.12.007

Abstract

Abstract:

An analysis of the hot flow forming of Mg-3.0Al-1.0Zn-0.3Mn (AZ31B) alloy was conducted by experiments and numerical simulations. The effects of different thickness reductions on the microstructure and mechanical properties were investigated at a temperature of 693 K, a spindle speed of 800 rev/min and a feed ratio of 0.1 mm/rev. Thickness reductions have great influence on the uniformity of microstructure along the radial direction (RD) and the grain sizes become refined and uniform when the thickness reduction reaches 45%. The c-axes of most grains are approximately parallel to the RD, with a slight inclination towards the axial direction (AD). The best mechanical properties with UTS of 280 MPa and YS of 175 MPa near the outer surface while 266 MPa and 153 MPa near the inner surface have been achieved due to grain refinement and texture. Moreover, the material flow behavior and stress/strain distributions for single-pass reductions were studied using the ABAQUS/Explicit software. The calculated results indicate that the materials mainly suffer from triaxial compressive stresses and undergo compressive plastic strain in RD and tensile strains in other directions. The higher stress and strain rate near the outer surface lead to more refined grains than that of other regions along the RD, whereas the orientation of the maximum principal compressive stress leads to a discrepancy of the grain orientations in RD.

Key words: AZ31B, Hot flow forming, Grain refinement, Texture

Yalian Zhang, Fenghua Wang, Jie Dong, Li Jin, Conghui Liu, Wenjiang Ding. Grain refinement and orientation of AZ31B magnesium alloy in hot flow forming under different thickness reductions[J]. J. Mater. Sci. Technol., 2018, 34(7): 1091-1102.

Figures/Tables 21

Fig. 1. (a) Microstructure and (b) tensile stress-strain curve of the initial AZ31B at room temperature.

Fig. 2. Schematic diagram of hot flow forming.

Table 1 Process parameters used for flow forming.

Passes	Feed Ratio (mm/rev)	Spindle Speed (r/min)	Thickness Reduction (%)	Forming Angle
Single-pass	0.1	800	20	25°
	0.1	800	30	25°
	0.1	800	45	25°
Multi-pass	0.1	800	30/50	25°
Multi-pass	0.1	800	20/50	25°

Fig. 3. Oblique views of the AZ31B Mg alloy tubes after hot flow forming: Reduction of (a) 20%, (b) 30%, (c) 45% and (d) 20%/50%.

Fig. 4. Extracted tensile specimens (A-D) and observation regions of micrographs (a-d) from outer surface to inner surface.

Fig. 5. Simulation model used for flow forming (a) and the compressive true stress-true strain curve for a 0.1 s-1 strain rate of semi-continuous casting AZ31B alloy (b) [17].

Fig. 6. Optical microstructure after single-pass flow forming of different thickness reductions: (a-d) 20%, (e-h) 30% and (i-l) 45%. The micrographs in (a-d), (e-h), (i-l) were taken from the outer to the inner surfaces along RD in the longitudinal section as shown in Fig. 4.

Fig. 7. Optical microstructure after multi-pass flow forming of different thickness reductions: (a-d) 30%/50% and (e-h) 20%/50%. Micrographs (a-d) and (e-h) were taken from the outer to the inner surface along RD in the longitudinal section as shown in Fig. 4.

Table 2 Average grain size along RD from outer surface to inner surface of different thickness reductions.

Thickness reduction (%)	Average grain size (μm)
	Region a	Region b	Region c	Region d
20	25	39	48	40
30	22	31	45	38
45	19	21	23	19
30/50	21	29	42	29
20/50	17	21	35	28

Fig. 8. Orientation image maps and (0001) pole figures along RD in the longitudinal section of the blanks deformed using a 30% reduction: (a, d) region near the outer surface, (b, e, g) middle region and (c, f, h) region near the inner surface.

Fig. 9. Orientation image maps and (0001) pole figures near the outer surface in the longitudinal section for the blanks deformed using reductions of (a, c) 45% and (b, d) 30%/50%.

Fig. 10. Mechanical properties along the RD of the blanks deformed using single-pass flow forming of different thickness reductions: (a) UTS and YS and (b) elongation. Tensile specimens were extracted as Fig. 4.

Fig. 11. Mechanical properties along the RD of the blanks deformed using multi-pass flow forming of different thickness reductions: (a) UTS and YS and (b) elongation. Tensile specimens were extracted as Fig. 4.

Fig. 12. Material flow-velocity contours of the three principal directions for a 20% thickness reduction: (a) RD, (b) TD and (c) AD.

Fig. 13. Nodes picked along the RD for different simulation steps in the longitudinal section: (a) material beneath the roller at step 100, and (b) material after flow forming at step 200.

Fig. 14. Material flow velocity trend along the RD of the contact zone from Fig. 13 (a) P1 for different thickness reductions: (a) 20%, (b) 30% and (c) 45%.

Fig. 15. Material displacements for different thickness reductions in the longitudinal section during flow forming and an oblique view after the deformation is completed: (a) 20%, (b) 30%, (c) 45%, (d) the top end of tubes fabricated by 45% thickness reduction.

Fig. 16. Stress distributions along the RD for different thickness reductions in the deformation zone as shown in Fig. 13 (a) P2: (a) 20%, (b) 30% and (c) 45%.

Fig. 17. Strain distributions along the RD for different thickness reductions in the finished components as shown in Fig. 13 (b) P3: (a) 20%, (b) 30% and (c) 45%.

Fig. 18. The maximum principal compressive stress distribution in the longitudinal section for the blanks deformed using a 30% reduction.

Fig. 19. Strain rate distribution in the longitudinal section for the blanks deformed using a 30% reduction.

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