Effects of Extrusion–Shear Process Conditions on the Microstructures and Mechanical Properties of AZ31 Magnesium Alloy

Effects of billet temperatures on grain sizes

The billet temperatures applied in the ES process are 370, 400 and 420 ℃, respectively. The effects of the billet temperature on the average grain sizes of Mg alloy are presented in Figure 2, and the comparison of microstructure between direction extrusion and ES is presented in Figure 3. It can be seen that the averaged grain size increases obviously with the increasing of billet temperature. For the direct extrusion process, the mean grain sizes of the AZ31 samples deformed at 370, 400 and 420 ℃ are 12.5, 14 and 15.5 µm, respectively (Figure 4). The Mg alloys are prone to dynamic recrystallization (DRX) by hot extrusion process. However, the higher temperature also results in the growth of the DRXed grains size. The grain size became larger with the increasing of the billet temperature, which indicates that the extrusion temperature is an important factor in controlling the microstructure of AZ31 Mg alloy [22, 23]. The same phenomenon of the effects of temperature on the microstructure of Mg alloys during extrusion has also been observed [7, 24, 25].

Figure 2:

Microstructure of bars with different billet temperatures: (a) 370 ℃, (b) 400 ℃ and (c) 420 ℃.

Figure 3:

Microstructure of bars at the billet temperature of 400 ℃: (a) direct extrusion and (b) ES process.

Figure 4:

Average grain sizes fabricated by ES process and direct extrusion with different billet temperatures.

For the ES process, the relationship between billet temperature and grain size is not as same linear as that in direct extrusion. As shown in Figure 4, the average grain sizes of Mg alloy fabricated by ES are much smaller than those prepared by direct extrusion and they increase slowly when the billet temperature increase from 370 to 400 ℃, but increase significantly from 400 to 420 ℃, which demonstrates that billet temperatures have significant effect on the sizes of grains. The finer microstructure in ES could be attributed to the shear deformation in ES process. The extremely large shear strain has also found in ECAE, and the shear stress has been proven to be very effective to refine the microstructure [26]. Gong et al. [27] investigated the differential speed rolling on Mg alloys and found that the grain refinement could be achieved by the intense shear deformation. The grain refinement mainly occurs in the shear bands because the main portion of the plastic deformation is thought to take place in the shear bands during rolling. A smaller grain size in the shear bands indicates that more intense plastic deformation concentrates on the shear bands during the differential speed rolling process. The results validate that ES process could refine the grains effectively if the billet temperature of billets is lower.

Effects of billet temperatures on microhardness

The Vickers’s indentation tests have been conducted at different positions in longitudinal sections of Mg alloy rods fabricated by ES processes with different billet temperatures, respectively, and the test results are shown in Figure 5. The positions 1, 2 and 3 are located at the head part, middle part and tail part of extruded rods, respectively. It can be seen that the hardness will increase with the increasing of the plastic deformation from positions 1–3 at the billet temperature of 370 ℃. The hardness at position 1 strip is about HV 65 and increase to HV 68.5 at position 3, which states that the hardness of the alloy could be improved by plastic deformation attributed to the strain hardening effect and grain refinement.

Figure 5:

Micro-hardness of different position of rods fabricated by ES process (error range: 3HV).

By comparing Figure 5, it can be seen that the hardness values decrease with the increasing of billet temperatures. This is because DRXs happen fully and recrystallized grains grow up with billet temperature increasing during the ES process [28, 29]. The mechanical properties are also consistent with the microstructure observation from Figure 2 and grain size measurement from Figure 4.

Effects of billet temperatures on tensile properties

The stress–strain curves from tensile tests are shown in Figure 6. The tensile properties, including elongation, yield strength and tensile strength, of AZ31 Mg alloy fabricated by direct extrusion and ES process with different billet temperatures are shown in Figure 7. It is obvious that strengths (yield strength and tensile strength) for AZ31 Mg alloy rods fabricated by ES are much higher than that formed by direct extrusion with about 20 % sacrifice of the elongation.

Figure 6:

Engineering stress (σ) and strain curves of the AZ31 Mg alloy processed by ES process and direct extrusion.

Figure 7:

Effects of billet temperatures on properties of Mg alloy fabricated by ES process and direct extrusion at different temperatures: (a) elongation, (b) yield strength and (c) tensile strength.

As shown in Figure 7(b), the yield strengths of Mg alloy fabricated by ES process are about 45 MPa higher than those fabricated by direct extrusion with billet temperatures of 370 and 400 ℃. But the difference decreases when billet temperature is 420 ℃. The tensile strength of Mg alloy formed by ES is higher than those fabricated by direct extrusion with the same billet temperatures, as shown in Figure 7(c). The effects of temperature on the tensile properties are complicated. Generally, the increase of the billet temperature will result in the decrease of the yield strengths and tensile strengths.

According to Hall–Petch equation [30], in the relationship between strength and average grain size, the strength increases as the average grain size is reduced. As to the effect of billet temperature, the higher the billet temperature, the coarser the microstructure, and subsequently the lower the strength. Besides, comparing to direct extrusion rods, the ES-processed rods present smaller grain size and the strengths are much higher. However, the elongation is smaller for the ES processed rods, because the elongation is not only influenced by the grain size, it is also impacted by the internal stress and dislocation density [31–33]. Further study is needed to study the stress, texture in the AZ31 extrusion rods. In addition, heat treatment is needed to explore the further increase of the plasticity in order to meet industrial application of Mg alloys.