Enhancing strength and ductility of AZ31 magnesium alloy sheets by the trapezoid extrusion

Introduction

Magnesium alloy is one of the lightest metallic materials relevant for significant advances in the automotive industry and electronic products due to their low density, high specific strength, etc.^1,2 Wrought magnesium alloys mostly focus on strips and sheets productions, which can be expanded to fabricate the structural components. However, Mg alloys generally exhibit low ductility at room temperature, and thus, their applications are greatly limited. This is due to the limited number of active slip systems in hexagonal close packed crystalline structure.^3,4 Mg alloy sheets normally give rise to a strong basal texture where the c axis of grains is predominantly aligned parallel to the sheet normal during the severe plastic deformation processes, such as the extrusion and the rolling, which results in a poor deformation capability and a stronger anisotropy.^5,6

It is well known that the grain orientation distribution (texture) plays a critical role in meliorating mechanical properties and formability of the hexagonal metals and alloys. ⁷ Texture control should be considered as an effective way to improve the formability in the primary processing.^8–10 It is of considerable interest to develop the thermomechanical response and deformation mechanism guidelines for designing the relevant devices. Extrusion processing has been applied as an economical way to produce sheets fabrication for the structural components. ¹¹ However, conventionally extruded Mg alloy sheets exhibit poor mechanical properties due to the strong basal texture and limited number of available plastic deformation modes.

Many different technologies, e.g. equal channel angular pressing, cross-rolling and different speed rolling, were performed to randomise the texture of magnesium alloy sheets, which were characterised by the inclination direction, the tilted angles and the texture intensity of the basal plane.^12–15 Different speed rolling process was extensively used to modify the final texture and thus improved the formability for the thin sheet fabrication.^16–18 The velocity between the top and bottom rolls was different, which created the asymmetric shear deformation throughout the whole thickness of the sheets. ¹⁹ This differential speed processing is expected to introduce a single extrusion processing step of strain paths associated with the novel integrated processing in one pass. Differential speed extrusion, which has similar deformation mode with different speed rolling, could thus be developed to improve the corresponding mechanical properties. However, reports on differential speed extrusion for Mg alloys in literature are limited to our best knowledge. In this present work, the trapezoid extrusion (TE) die with special structure was designed. The parallel flow passage length at top and bottom surfaces was different so that the velocity in TE process at near surface region was thus different. Thus, large numbers of asymmetric shear deformation was introduced. Moreover, a suitable constitutive model of the TE was also applied to investigate the influence of the shear deformation on the microstructure and properties of AZ31 magnesium alloy sheets.

Experimental

The starting billet of as cast AZ31 (Mg–3·07 wt-Al–0·78 wt-Zn–0·38 wt-Mn) was homogenised at 430°C for 2 h. The extrusion was conducted by the conventional extrusion (CE) die and the TE die with an extrusion ratio of 101∶1 at 20 mm s⁻¹ extrusion rate. Figure 1 shows the scheme of flow passage in CE and TE dies. The parallel flow passage length at top and bottom surfaces in the TE die was different. The extrusion temperature was 430°C. The extruded sheets were of 56 mm width (transverse direction) and 1 mm thickness (normal direction).

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1

Scheme of a CE die and b TE die

Dog bone tensile samples of 12 mm in gage length, 6 mm in width and 1 mm in thickness were machined from the sheets with various directions tilting 0, 45 and 90° to the extrusion direction (ED) respectively. Tensile tests were performed on a CMT6305-300KN universal testing machine at the initial strain rate of 10⁻³ s⁻¹ at room temperature. The analysis of (0002) pole figures was determined using X-ray diffraction (Rigaku D/Max 2500). For the TE sheets, the pole figures were measured at top surface and bottom surface.

Results and discussion

To examine the microstructures in detail, optical micrographs of the CE sample and the TE sample at top surface, midlayer (normal direction, 0·5 mm in thickness was removed from the surface) and bottom surface are shown in Fig. 2 respectively. It can be seen that the CE sheet was homogeneous and composed of equiaxed dynamically recrystallised (DRX) grains of ∼12 μm. It can be found that a considerable grain refinement could be accomplished by the TE process. The TE samples exhibited quite different microstructure throughout the thickness direction. For the top surface microstructure of the TE sample, as shown in Fig. 2b, characterised by finer DRX grains of ∼6 μm. The midlayer microstructure of the TE sheets was formed by DRX grains of ∼8 μm. Meanwhile, the bottom surface microstructure of the TE sheets was composed of fine DRX grains of ∼7 μm. Thus, TE sheets have finer DRX grains under this shear strain path.

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2

Optical micrographs of AZ31 alloy sheet

Figure 3 shows the (0002) basal texture of CE sample, TE sample at top and bottom surfaces respectively. The sheets developed the basal texture and < > direction in most grains was orientated parallel to ED after the hot extrusion. The results reveal that the basal texture of the TE sample weakens and tilts toward ED compared with CE one. The basal texture intensity has decreased from 22·6 to 18·9 (at top surface) and 14·7 (at bottom surface) respectively. The DRX grains at near surface rotated and oriented to away from the basal texture owing to the asymmetric shear deformation. It can be seen that the basal texture of top microstructure is larger than that of the bottom microstructure in TE sheets. This is consistent with the fact that the top microstructure has finer DRX grains under this shear strain path and the flow velocity of the top surface is faster. Meanwhile, the effective strain of the top surface is larger compared with other areas of TE sheets. The TE process has imposed the maximum asymmetric shear deformation. It makes the basal texture rotate towards the imposed shear direction in one pass during the hot extrusion process. The tilted weak texture was thus obtained resulting from this particular TE processing. Apparent inclination of (0002) basal texture is observed at the both top and bottom surfaces throughout the sheet thickness direction.

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3

(0002) pole figures of AZ31 sheets

Figure 4 shows mechanical responses of CE and TE samples measured at different directions tilting 0, 45 and 90° to ED at room temperature. There is a significant difference in the mechanical behaviour of the two sheets. The TE sheet shows the superior strength and ductility for each direction compared with those of CE sheet. The average values of the ultimate tensile strength (UTS), the 0·2 proof stress (YS) and the uniform elongation E_u are summarised in Table 1. The average values of mechanical properties given from the values of three directions can be expressed as  = (M₀°+2M₄₅°+M₉₀°)/4. The average value of the E_u increases from 19·7 to 21·8. The average value of the YS increases from 156·3 to 181·3 MPa and that of the UTS increases from 327·2 to 340·3 MPa simultaneously. Figure 5 shows the SEM fracture images of CE and TE samples obtained by both the secondary and the backscattered electron patterns. The notable feature is that a large amount of plastic dimples and cleavage planes appears. Some dimples are inside the cleavage planes, which belong to the ductile failure. ²⁰ The CE fracture surface has less and shallower dimples, while those of TE samples show cruder. This obviously evinces the flow passage in the TE processing results in good plasticity. It was consistent with mechanical responses as above mentioned. Improvement of strength and ductility is attributed to refined grains and tilted weak basal texture. The CE sheets with the strong basal texture are unfavourable for basal slip during the plastic deformation. ²¹ During TE process, the extra shear strain deformation is different throughout the sheet thickness direction and would activate non-basal slip systems. The DRX grains in this sheet exhibited a tilted basal texture with the c axis inclined towards ED. The intense interactions of multiple deformation models would bring about the rotation of basal texture. Further investigation of deformed microstructural structure is required to clarify the deformation mechanism of the present extruded Mg alloys.

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4

a true stress–strain curves and b mechanical properties of CE samples and TE samples with various directions tilting 0, 45 and 90° to ED at room temperature

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5

Secondary and backscatter SEM fracture images of AZ31 sheets a 0°, c 45° and e 90° of CE sample and b 0°, d 45° and f 90° of CE sample

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Table 1

Average values of UTS, YS and E_u of CE and DSE AZ31 sheets at room temperature

Samples	UTS/MPa	YS/MPa	Eu/
CE	327·2	156·3	19·7
TE	340·3	181·3	21·8

Conclusions

The present work has evidenced that the mechanical responses of AZ31 alloy sheet processed by TE process were outstandingly enhanced compared with those of CE process. (0002) basal texture intensity of TE sheets has been weakened due to the simple shear deformation introduced throughout the sheet thickness direction. This enforces the crystallographic reorientation of basal planes to tilt toward ED. Trapezoid extrusion process was much more effective in the grain refinement. A remarkable enhancement in the strength and ductility has been obtained by TE process.