Composite Heat Treatment Of AZ Series Magnesium Alloys And Its Strengthening And Toughening Mechanism

2023-04-28

AZ series magnesium alloy sheets are widely used in automobile and aviation industries, and the morphology and quantity of the strengthening phase Mg17Al12 have an important influence on the formability of the sheet. The rolled AZ31, AZ61 and AZ91 magnesium alloy sheets were annealed at 300 ℃ for 5 h, cryogenically treated with liquid nitrogen for 6 h, aged at 160 ℃ for 18 h, and subjected to different composite heat treatments. The results show that the main strengthening mechanisms of the as-rolled AZ-series magnesium alloys are solid solution strengthening and dislocation strengthening, and the material has high strength and poor plasticity. The long grains are broken under the temperature stress of cryogenic treatment, the yield ratio of the material decreases, the strain hardening index increases, but the plasticity does not increase much; after annealing + cryogenic + aging heat treatment, the strain hardening index increases, A large number of finely dispersed and precipitated second phases in the grain boundary and in the grain are beneficial to the initiation of different slip systems during deformation, which improves the uniform deformation ability of the material, increases the strength, and further reduces the yield-to-strength ratio. Processing is favorable.
AZ series is one of the most widely used wrought magnesium alloys at present. It has excellent properties such as high specific strength, specific stiffness, and shock absorption. It is widely used in automobiles, electronics and medical care. However, magnesium alloys have a close-packed hexagonal structure and have poor plastic deformation ability at room temperature. Usually, they need to be plastically processed after heating to a certain temperature, and they have strong anisotropy. AZ series magnesium alloys are widely used series of magnesium alloys. Because their crystal structure is close-packed hexagonal structure, in addition, the main second phase in AZ series magnesium alloys is Mg17Al12 with bcc structure. Performance has a major impact. The Mg17Al12 phase mainly precipitates at the grain boundaries and within the grains. How to improve the plastic forming ability of magnesium alloy sheet is the key direction of magnesium alloy research.

The solid solubility of Al in magnesium alloys at room temperature is about 2%, but at 437 ℃ its maximum solid solubility can reach about 12.5%, which has a strong solid solution strengthening effect and also provides the possibility for the precipitation of the second phase. With the decrease of temperature, its solid solubility further decreased. Under the condition of -150 ℃ cryogenic treatment, the change of solid solubility and volume shrinkage will promote the precipitation of the second phase. Although there have been a lot of reports on the heat treatment process of magnesium alloy sheets, there are few reports on the composite heat treatment method of magnesium alloy sheets. In the early stage, AZ31, AZ61, and AZ91D magnesium alloys were used as the research objects, and they were annealed + cryogenic treatment + aging treatment to study the effect of different heat treatment processes on the microstructure and mechanical properties of magnesium alloys. On this basis, this topic mainly analyzes the influence mechanism of its performance improvement through the microstructure, aiming to provide a reference for broadening the forming and application of magnesium alloy sheets.

1 Test materials and methods

The test materials are three kinds of magnesium alloy sheets, AZ31 (0.5 mm), AZ61 (0.5 mm), and AZ91D (0.55 mm) as rolled, and the rolling temperature is 280 °C. The chemical compositions are shown in Table 1. The three materials were annealed at 300 ℃ for 5 h in a heat treatment furnace, and the annealed sheets were immediately immersed in a liquid nitrogen container for 6 h of cryogenic treatment, and the cryogenically treated sheets were aged at 160 ℃ for 18 h. The heat-treated specimens were inlaid with dental trays, ground with sandpaper, mechanically polished, and etched with picric acid, and finally the metallographic observation was carried out. The precipitation phase and microstructure were analyzed by scanning electron microscope and transmission electron microscope, and the tensile properties at room temperature were tested.

2 Analysis of test results

2.1 Optical metallographic analysis

Figure 1 shows the metallographic microstructure of AZ61 magnesium alloy as rolled and under different composite heat treatment conditions. It can be seen that the as-rolled grains are broken and elongated along the rolling direction. After annealing at 300 ℃ for 5 h, the grains are relatively regular, and after cryogenic treatment, the grains are obviously refined. From the crystal grains treated by cryogenic treatment for 6 h, it can be clearly seen that the large-sized grains are broken into small-sized grains, and the degree of microstructure refinement is relatively large. , the average grain size changed from 36 μm before cryogenic treatment to 23 μm, and a large number of second phases were precipitated during annealing + cryogenic treatment + aging treatment. After rolling, there are slender grains inside the material. Due to the chilling effect during the cryogenic treatment, a large temperature stress is generated inside the material, and the stress is concentrated, the slender grains are easily broken, and small grains with straight and continuous grain boundaries appear. grains, and twinning, but it has little effect on small-sized grains, which is consistent with the research results of Chen Jinqiu et al. and Yi Dingguo et al.

The content of Al element in AZ61 magnesium alloy is relatively high. After cryogenic treatment, the solubility of Al element in supersaturated solid solution in the matrix decreases, but the low temperature also weakens the diffusion ability of Al element, so cryogenic treatment does not make the second phase particles more However, after the aging treatment, the second phase particles are precipitated in large quantities from the grain and the grain boundary. It can be seen that the internal stress and microstructure changes generated by the cryogenic treatment provide conditions and places for the precipitation of the second phase during the aging treatment.

Fig.1 Metallographic structure of AZ61 magnesium alloy

(a) As-rolled (b) annealed at 300 °C for 5 h without cryogenic treatment (c) annealed at 300 °C for 5 h for 6 h cryogenic treatment (d) annealed at 300 °C for 5 h for 6 h cryogenic treatment and aging treatment

2.2 Mechanical properties analysis

Room temperature tensile tests were performed on magnesium alloy sheets with different treatments to detect their mechanical properties. The tensile specimens are shown in Figure 2. The stress-strain curves after aging treatment are shown in Figure 3, and the yield-strength ratio and strain index changes are shown in Figure 4.

Figure 2 Dimensions of the tensile test piece

The yield strength of the annealed and cryogenically treated AZ61 magnesium alloy decreased compared with that of the untreated one, the yield strength ratio decreased, and the plastic deformation was stable. The elongation of the annealed and cryogenically treated specimens after annealing was improved and the tensile strength decreased slightly compared with the untreated specimens.

The elongation of the AZ61 magnesium alloy with composite heat treatment increased from 3.3% to 12.3%, and the tensile strength of the material did not decrease significantly, from the untreated 355 MPa to 312 MPa, and the yield-strength ratio decreased. This is because the internal stress and elastic energy of the material are generated by the chilling effect, and the elastic energy can be used as the driving force for the precipitation of the second phase, which promotes the precipitation of a large amount of the second phase after aging treatment. The second-phase particles hinder the dislocation movement, and at the same time, the alloys will be twinned during the plastic deformation process, which in turn promotes the dislocation slip. At the same time, some grains are broken under the action of internal stress or some rotate in the direction conducive to deformation, which greatly improves the deformation ability of magnesium alloys.

After composite heat treatment, the tensile strength of AZ91D magnesium alloy does not decrease significantly compared with that of the untreated sample, and the other treatment methods all decrease by about 50 MPa. During composite heat treatment, the elongation increases from 3.2% to 9.6%, which is the largest increase, and the strength is guaranteed to be stable. This is due to the high content of Al in the AZ91 magnesium alloy, which leads to the precipitation and dispersion of a large number of second phases, which hinders the movement of dislocations, promotes the initiation of dislocation slip, and improves the deformability of the material.

                                       (a) AZ31 magnesium alloy

                                       (b) AZ61 magnesium alloy

                                       (c) AZ91D magnesium alloy

Fig.3 Stress-strain curves of three kinds of magnesium alloys after aging treatment

(a) Variation of yield-strength ratio (b) Variation of strain hardening exponent

Fig.4 Changes of yield ratio and strain hardening exponent of magnesium alloys after different heat treatments

It can be seen from Figure 4 that the main strengthening mechanisms of the three magnesium alloys without any treatment (rolled state) are grain refinement strengthening and dislocation strengthening, the strain hardening exponent n is large, but the yield strength ratio is also large, As a result, the elongation of the material is very low; the strengthening mechanism of the samples does not change after different heat treatments, and the main mechanism is solid solution strengthening. However, with the increase of the precipitation phase, the effect of the precipitation phase gradually appeared. The value of the strain hardening exponent n increases gradually, the yield ratio decreases gradually, the elongation of the material increases greatly, and the tensile strength increases slightly. After heat treatment, the yield-strength ratios of the three magnesium alloys were significantly lower than those of the untreated ones. The yield-strength ratio of the combination of annealing + cryogenic treatment + aging treatment was reduced to about 0.57, and the uniform deformation ability of the material was significantly improved.

2.3 SEM and TEM analysis

The second phase of AZ magnesium alloy is mainly β-Mg17Al12. Due to the different Al content, the number of precipitation phases after heat treatment is also different. Among them, the number of precipitation phases in AZ91 magnesium alloy after composite heat treatment increases significantly. Figure 5 is a scanning electron microscope image of AZ91D magnesium alloy after composite heat treatment. It can be seen that the β phase is distributed in the AZ91 magnesium alloy matrix, mainly in the form of flakes or spheres, with a thickness of about 250 nm. Figure 6 shows the TEM microstructure images of AZ31 and AZ61 magnesium alloys. According to TEM observation, after annealing + cryogenic treatment + aging treatment, the second phase particles are precipitated in two forms: intragranular precipitation and grain boundary precipitation, and intragranular precipitation is the main precipitation method. There is no obvious boundary between the intragranular precipitation phase and the matrix phase, and it is evenly distributed in the matrix α-Mg; the grain boundary precipitation phase is relatively coarse, and there is a clear boundary between it and the matrix. It can be seen from Fig. 6c that after cryogenic treatment, dislocation packing occurs near the grain boundary. Figure 6d is the calibration result of the diffraction spot of the precipitated particles, and it is determined that the precipitated phase is β-Mg17Al12. Intragranular precipitation can play the role of pinning dislocations, slowing down the accumulation rate of dislocations at grain boundaries when the material is deformed, improving the storage capacity of dislocations within the material, and enabling the material to activate more slip systems. The distribution of fine dispersion inside the grains improves the uniformity of material deformation, which is the main reason for the large increase in elongation.

Fig.5 Second phase and EDS analysis of AZ91D magnesium alloy

(a) AZ31 magnesium alloy (b) AZ61 magnesium alloy (c) AZ31 magnesium alloy (d) second phase particle diffraction spot calibration

Fig.6 TEM image and diffraction spot calibration of second-phase particles

It is generally believed that AZ-based magnesium alloys with high Al content, such as AZ61 magnesium alloys, are more prone to precipitation phases, but through the composite heat treatment method, AZ31 magnesium alloys with low Al content will also precipitate a small amount of secondary phase, thereby improving the properties of magnesium alloys.

3. Conclusion

(1) After cryogenic treatment, a large temperature stress is generated inside the material, and the stress is concentrated, causing the grains to deform or rotate, the structure is refined, the larger grains are broken into fine grains, and twin crystals are generated. The grain refinement of AZ31 magnesium alloy is 47.8%; the grain refinement of AZ61 magnesium alloy is 36.1%; the grain refinement of AZ91D magnesium alloy is 20.8%.

(2) The main strengthening mechanisms of as-rolled magnesium alloys are grain refinement strengthening and dislocation strengthening. The strain hardening index n value is large, but the yield-strength ratio is also large, resulting in low material elongation; The strengthening mechanism is solid solution strengthening. With the increase of the precipitation phase, the effect of the precipitation phase gradually appeared. The value of the strain hardening exponent n increases gradually, the yield ratio decreases gradually, the elongation of the material increases greatly, and the tensile strength also increases, which effectively improves the uniform deformation ability of magnesium alloys.

(3) The magnesium alloy adopts composite heat treatment, and a large number of fine dispersed and precipitated second phase particles in the grain are conducive to the activation of more slip systems, which greatly improves the formability of the magnesium alloy. Among them, the strength is not much different from that before treatment, and the elongation of AZ31 magnesium alloy is increased by 2.5 times compared with that before treatment; the elongation of AZ61 magnesium alloy is increased by 4 times; the elongation of AZ91D magnesium alloy is increased by 3 times.

Citation format: Zhang Ju, Hu Xiaodong, Zhao Hongyang, et al. Composite heat treatment of magnesium alloys and its strengthening and toughening mechanism [J]. Special Casting and Nonferrous Alloys, 2022, 42(1): 64-68.