Abstract
A novel process of manufacturing high performance AZ31 alloy strip was put forward by combining semisolid rolling, deep rolling and heat treatment, and the effects of dynamic recrystallisation during deep rolling of semisolid slab and heat treatment on microstructure and properties of AZ31 alloy were investigated. When the casting temperature was set from 650 to 690°C and the vibration frequency was controlled at 80 Hz, AZ31 alloy strip with the cross-section size of 4×160 mm and fine near round and rosette grains was manufactured by semisolid rolling. Dynamic recrystallisation happened during deep rolling process. Dynamic recrystallisation grain size decreases with the increase in deep rolling temperature in a certain scope. With the increase in deep rolling deformation amount, the microstructure evolved gradually from coarse primary grains and twins to fine dynamic recrystallisation grains and twist primary grains as well as small regular equiaxed grains. AZ31 alloy strip with very fine equiaxed grain in size of 6 μm was obtained by deep rolling. After solution treatment at 415°C for 20 h, the saturated solid solution of AZ31 alloy strip has been formed. After solution treatment at 415°C for 20 h and aging at 230°C for 16 h, the deep rolled AZ31 alloy strips exhibit excellent mechanical properties; the ultimate tensile strength, yield strength, elongation to failure and hardness could reach 330 MPa, 204 MPa, 11 and 71 HV respectively.
Introduction
Magnesium alloy is the lightest material among applied engineering metals. Its density is about two-thirds of aluminium and is similar with plastic material. 1 Magnesium alloy also has excellent performances, such as high specific strength, good electromagnetic shielding function and good electric and heat conduction properties.2,3 Moreover, magnesium resource is very rich. Thus, magnesium alloy has a very good application prospect. The consumption of magnesium alloys in 3C, automobile and aviation takes on a rising trend in recent years. 4 However, the price of magnesium alloy product is relatively high because of the low production efficiency. To develop advanced processing techniques of magnesium alloy and improve its productivity become a hot focus at present.
Because magnesium has a hexagonal close packed crystal lattice with three independent slip systems in the matrix, it is relatively difficult to deform at the ambient temperature. During the processing of magnesium alloys, multipass annealing is usually demanded to achieve deformation process. Thus, the production cost is increased. As a short metal forming process, semisolid forming technique has many advantages such as easy forming, good microstructure and well performance as compared with solid plastic deformation. It is an ideal way to manufacture magnesium alloy materials by semisolid processing.5,6 The first key step in semisolid processing is to prepare semisolid slurry of alloys. Many advanced methods of slurry preparation have been developed in recent years, such as mechanical stirring, magnetohydrodynamic stirring, electromagnetic stirring, cooling slope plate, bubble stirring, melt conditioner direct chill, etc.7–13 Cooling slope process, which is also called new rheo casting in Japan, has obvious advantages of low cost and high efficiency, so it was highlighted and developed rapidly. 14 Motegi successfully manufactured semisolid billet in size of 50×1000 mm of aluminium alloy by this process. 15 Plato Kapranos, Grimmig, Salarfar and Haga have also carried out many research works on this technique.16–19 In the present work, in order to avoid slurry adhesion on the plate surface, the cooling slope was modified to a vibration copper plate, and semisolid rolling was achieved by combining vibration cooling slope and rolling mill. Previous studies mainly focused on microstructure evolution during the slurry preparation. Canyook and Mehrara et al. reported that the microstructure refinement during cooling slope process derives from the extrication of the heterogeneous nucleus formed on the plate surface, and the breakage of dendrites happened in the mushy zone.20,21 Guan holds the viewpoint that besides above mechanisms, eruptive nucleation happening in most of the melt also contributes to microstructure formation. 22 However, it is still short of systematic research and deep discussion on the microstructure and properties of AZ31 alloy strip processed by semisolid and deep rolling.
In the present paper, in order to manufacture high performance AZ31 alloy strip by the proposed process, effects of dynamic recrystallisation and heat treatment on microstructure and properties of AZ31 alloy prepared by semisolid and deep rolling were systematically studied; the AZ31 alloy strip with excellent microstructure and properties was successfully manufactured.
Experimental
The experimental material was self-made AZ31 magnesium alloy whose main chemical compositions are shown in Table 1. The experimental equipment of semisolid rolling was the self-designed D400 rheo rolling machine, which is shown in Fig. 1. The melt was cast onto the surface of the vibrating cooling slope at the temperature from 650 to 700°C, and then, the melt nucleated and solidified rapidly. Owing to the synergistic actions of metal flow and vibration with the frequency from 60 to 100 Hz, semisolid slurry with fine non-dendrites and remnant liquids was prepared. The semisolid slurry was directly rolled into strip with a cross-section of 4×160 mm. Because the temperature of semisolid slurry is much lower than that of the melt during conventional roll casting, the rolling speed of semisolid alloy is much higher than that of conventional roll casting. The process is expected to be developed as a high speed roll casting technique. In addition, the microstructure and mechanical properties of the alloy strip produced by semisolid rolling are better than that of conventional roll casting.23,24 A D450 rolling machine was adopted for secondary deep rolling, and the deep rolling temperature was controlled in the scope from 350 to 400°C. AZ31 alloy strip obtained by deep rolling was treated by solution and aging treatment. The scheme of heat treatment is summarised in Table 2.
Schematic diagram of semisolid rolling device
Chemical composition of AZ31 alloy
Scheme of heat treatment
AZ31 alloy strips at different rolling stages were taken for microstructure observation and mechanical property analysis. After the specimens were polished and etched by the solution of 15 mL HCl+56 mL C2H5OH+47 mL H2O, the microstructures were observed under an OLYMPUS PMG51 metallographic microscope. An SSX-550 scanning electron microscope (SEM) was adopted to analyse the distributions of the alloy elements and phases. The size of tensile specimen of the strip is shown in Fig. 2. Mechanical properties of AZ31 alloy strip were measured by a CMT5105 tensile testing machine at the room temperature.
Size of tensile sample (mm)
Results and discussion
Microstructure and properties of AZ31 alloy strip obtained by semisolid rolling
Through the experiment, semisolid rolling parameters for preparing AZ31 alloy strip were optimised. When the casting temperature was set from 650 to 690°C and the vibration frequency was controlled at 80 Hz, AZ31 alloy strip with the cross-section size of 4×160 mm was successfully manufactured by semisolid rolling. Figure 3 shows the microstructures of AZ31 alloy strip on cross and longitudinal sections. It can be found that the inner microstructures are mainly composed of fine near round and rosette grains. Owing to strong cooling rate and stirrings caused by vibration and metal flow, heterogeneous nucleation and nucleus multiplication happened during slurry preparing, and nucleation rate was greatly improved, which devotes to the fine non-dendrite microstructure formation. 22 The deformation of solid grain in the rolling gap was not obvious, and its original shape was basically maintained. This mechanism was also reported in the related studies. 22 Generally, the ultimate tensile strength of AZ31 alloy strip, which was manufactured by conventional roll casting, is from 202 to 230 MPa, and elongation is from 1·35 to 4.23,24 While the present experimental results show that the ultimate tensile strength can reach 232 MPa, the yield strength was 116 MPa, the elongation to failure was 6, and the hardness is 57 HV. Thus, the mechanical properties of AZ31 alloy strip processed by present method are better than that of conventional roll casting.23,24
Microstructures of AZ31 alloy strip on cross and longitudinal sections
Effects of deep rolling dynamic recrystallisation on microstructure and properties of AZ31 alloy strip
Figure 4 shows microstructures of AZ31 alloy strip on cross-section obtained at different deep rolling temperatures. The microstructures are mainly composed of coarse primary grains, fine equiaxied structures and twins. Some twins distribute on primary grain boundaries, and the others penetrate into or cross the primary grain matrix. It was found that dynamic recrystallisation happened during deep rolling process. The twins were mainly induced by deformation and dynamic recrystallisation. Owing to short time of deformation and recrystallisation, dynamic recrystallisation grains could not grow adequately, and the grain size of the strip hereafter deep rolling became fine, as shown in Fig. 4. Because AZ31 alloy has three plastic slipping systems, when the deformation reached 8, 25 alloys had to deform through twinning. Thus accumulative energy induced by deformation in the twins and primary grain boundaries was very high, and the driving force of dynamic recrystallisation in these regions was also very large. Under this situation, dynamic recrystallisation preferred to generate in these areas. This kind of dynamic recrystallisation mechanism leaded to non-uniform grain distribution. Grain size in the dynamic recrystallisation regions is very small, and that in other area is relatively large. Owing to the fine grain size in dynamic recrystallisation regions, deformation could continue through slipping and rotating; 26 thus, dynamic recrystallisation improves deformation ability of the alloy.
Microstructures on cross-section of AZ31 alloy strip obtained at different deep rolling temperature
It can be found in Fig. 4 that there are more deformation twins when the rolling temperature is lower, and dynamic recrystallisation grain is few. On the contrary, more dynamic recrystallisation grains can generate at higher rolling temperature. The reason is that atoms become active at high temperature, and the interactions of dislocation and twinning can easily take place, the critical deformation amount of dynamic recrystallisation decreases with the increase in deep rolling temperature.
27
When the deformation time is a constant, higher deep rolling temperature can lead to more dynamic recrystallisation grains. The final grain size depends tightly on the process parameters related to dynamic recrystallisation. Generally, the relationship of dynamic recrystallisation grain size and process parameters can be expressed by the formula
28
Figure 5 shows microstructures of AZ31 alloy strip on cross-section obtained at different rolling deformation amounts. It can be seen that many twins occurred when the rolling deformation amount reached 19·7, but there was no obvious dynamic recrystallisation grain, as shown in Fig. 5a. The deformation can induce a very large stress and cause interactions of slipping, twinning and secondary twinning. This mechanism was also reported in many studies. 29 Further deformations can provide enough energy and suitable structure for dynamic recrystallisation happening, so when the rolling deformation amount reached 30·7, many wide twins generated, and dynamic recrystallisation structures occurred around big grains and near the thin twins simultaneously, as shown in Fig. 5b. As shown in Fig. 5c, when the rolling deformation amount was improved to 38·7, dynamic recrystallisation happened in even larger area, so a large amount of primary grains were divided, and fine dynamic recrystallisation grains were maintained in the alloy. At the same time, a few initial grains take on the feature of twist deformation and are enclosed by small dynamic recrystallisation grains. In this case, alloy is refined adequately. When the rolling deformation amount reached 73·7, as shown in Fig. 5d, complete dynamic recrystallisation and grain growth took place, and the microstructure is mainly composed of small regular equiaxed grains. The reason is that the severe deformation induced a strong driving force for dynamic recrystallisation and grain growth, and dynamic recrystallisation grain growth took place. Finally, small regular equiaxed grains were maintained. Through the study, it can be concluded that under the present experimental conditions, with the increase in rolling deformation amount, alloy microstructure evolved gradually from coarse primary grains and twins to fine dynamic recrystallisation grains and twist primary grains as well as small regular equiaxed grains.
Microstructures on cross-section of AZ31 alloy strip obtained at different rolling deformation amounts
Through experimental studies, AZ31 alloy strip with very fine equiaxed grain in size of 6 μm and excellent mechanical properties was obtained. The ultimate tensile strength of the strip can reach 298 MPa, the yield strength is 164 MPa, the elongation to failure was 9, and the hardness is 68 HV. The ultimate tensile strength, the yield strength, the elongation and the hardness are separately improved by 28, 41, 50 and 19 in comparison with the AZ31 alloy strip before deep rolling, as shown in Table 3.
Mechanical property of AZ31 magnesium alloy under different process
Effect of heat treatment on microstructure and properties of AZ31 alloy strip
After AZ31 alloy strip was deep rolled, it was treated by solution and artificial aging. Figure 6 shows SEM images of AZ31 alloy strip at the solution temperature of 415°C and different solution times. It is found that the white phases in the matrix disappeared with the solution time prolonging, which indicates that the secondary phases gradually dissolved into the matrix. After solution treatment at 415°C for 20 h, most of the second phases at grain boundaries dissolved into Mg phase, and grain boundary became obscure. In this case, saturated solid solution has been formed. It exhibits an ultimate tensile strength of 302 MPa, the yield strength of 127 MPa, elongation to failure of 14 and hardness of 55 HV. The ultimate tensile strength and the elongation are separately improved by 1 and 55 in comparison with the AZ31 alloy strip before solution treatment, but the yield strength and the hardness decreased 22 and 19, as shown in Table 3.
Images (SEM) of AZ31 alloy strip at solution temperature of 415°C and different solution times
Figure 7 shows the SEM images of AZ31 alloy strip at the aging temperature of 190°C and different aging times. It is found that more second phases precipitated gradually with the aging time prolonging during aging process. When the aging time is 4 h, there is no obvious precipitated phase, as shown in Fig. 7a. When the aging time was prolonged to 8 h, a few second phases occurred along grain boundaries, as shown in Fig. 7b. While the alloy was held for 24 h at the aging temperature of 190°C, large amount second phases precipitated not only along grain boundaries but also in the grains, as shown in Fig. 7c and d. Figure 8 is the energy dispersive spectroscopy (EDS) energy spectrum analysis result of AZ31 alloy strip that was aged at 190°C for 24 h at the corresponding point A in Fig. 7d. It is found that the atom ratio of Mg and Al of the precipitated phase is ∼17∶12, so the second phase was determined as Mg17Al12, which was also confirmed as a main strengthening phase by many related studies. 30
Images (SEM) of AZ31 alloy strip at aging temperature of 190°C and different aging times
Energy dispersive spectroscopy energy spectrum analysis result of AZ31 alloy strip that was aged at 190°C for 24 h at corresponding point A in Fig. 7d
Figures 9 and 10 show the relationships of aging time and mechanical properties of the alloy at different aging temperatures. It can be seen that the tensile strength of the alloy increases first and then decreases with the increase in aging time. However, the elongation of the alloy always decreases with the increase in aging time. The reason is that small Mg17Al12 phase precipitated gradually with the time prolonging in early aging stage, so the strength of the alloy increased little by little, and the elongation decreased correspondingly. Once the aging time was longer than 16 h, Mg17Al12 phases were coarsened, and so called overaging happened. In this situation, Mg17Al12 phase strengthening effectiveness became weak, so the strength and the elongation of the alloy decreased. In addition, the aging process became faster when the aging temperature was higher, and the elongation declining was faster. After solution treatment at 415°C for 20 h and aging at 230°C for 16 h, the deep rolled AZ31 alloy strips exhibit excellent mechanical properties; the ultimate tensile strength, yield strength, elongation to failure and hardness could reach 330 MPa, 204 MPa, 11 and 71 HV, as shown in Table 3.
Relationships of aging time and tensile strength of alloy at different aging temperatures
Relationships of aging time and elongation of alloy at different aging temperatures
Conclusions
When the casting temperature was set from 650 to 690°C and the vibration frequency was controlled at 80 Hz, AZ31 alloy strip with the cross-section size of 4×160 mm and fine near round and rosette grains was manufactured by semisolid rolling. The ultimate tensile strength was 232 MPa, the yield strength was 116 MPa, the elongation to failure was 6, and the hardness is 57 HV.
Dynamic recrystallisation happened during deep rolling process. Dynamic recrystallisation grain size decreases with the increase in deep rolling temperature in a certain scope. With the increase in deep rolling deformation amount, the microstructure evolved gradually from coarse primary grains and twins to fine dynamic recrystallisation grains and twist primary grains as well as small regular equiaxed grains. AZ31 alloy strip with very fine equiaxed grain in size of 6 μm and excellent mechanical properties was obtained by deep rolling. The ultimate tensile strength of the deep rolled strip is 298 MPa, the yield strength is 164 MPa, the elongation to failure was 9, and the hardness is 68 HV.
After solution treatment at 415°C for 20 h, the saturated solid solution has been formed. The AZ31 strips exhibit an ultimate tensile strength of 302 MPa, yield strength of 127 MPa, elongation to failure of 14 and hardness of 55 HV.
After solution treatment at 415°C for 20 h and aging at 230°C for 16 h, the deep rolled AZ31 alloys exhibit excellent mechanical properties; tensile strength could reach 330 MPa, yield strength of 204 MPa, elongation to failure of 11 and hardness of 71 HV.
Footnotes
Acknowledgement
The authors thank for the supports of National Natural Science Foundation for Outstanding Young Scholars of China under grant no. 51222405, Key Project of National Natural Science Foundation of China under grant no. 51034002, Henry Fok foundation of young teachers under grant no. 132002, the Basic Scientific Research Operation of Center University under grant nos. N120602002 and N120502001, and State Basic Research Development Program of China under grant no. 2011CB610405.