Purification of AZ80 and Degassing of AZ91 Alloy by Ultrasonic Treatment

Abstract

The effects of ultrasonic power, treatment time and holding time on AZ80 magnesium melt purification by ultrasonic field were studied. The results indicate that ultrasonic treatment can accelerate the separation of inclusions and attain melt purification. When the magnesium alloy melt is treated with ultrasonic power 80 W at 650 °C for 60 s and holding 100 s, the best melt purification is achieved. Moreover, the effect of ultrasonic degassing on AZ91 alloy was also investigated. When the ultrasonic power is 150 W for 90 s, the hydrogen content and degassing efficiency are 9.6 cm³/100 g and 50.5%, respectively. The corresponding mechanical properties are R_m = 194 MPa, R_0.2 = 133 MPa and A = 4.8%, respectively, and the mechanisms of purification and degassing were analyzed because of the cavitation effect.

1. Introduction

Magnesium alloys have the advantages of low density, higher specific strength and stiffness, good thermal conductivity and electrical conductivity and electromagnetic shielding [1,2,3]. Thus, they have broad commercial applications in automotive, aerospace and 3C products [4,5,6,7]. With the intensive research about magnesium alloys, it has been found that the low purity of magnesium alloys has become one of the problems which limits the application of magnesium alloys [8,9]. The high contents of inclusion dramatically reduce the formability and corrosion resistance of magnesium alloys [10,11,12].

Therefore, how to improve the purity of melt is an urgent technical problem, which becomes a topic of great concern to metallurgical researchers. The magnesium alloy melt is mainly purified by chemical methods and physical methods [13,14]. Chemical purification methods mainly use flux to purification, and physical purification methods mainly include blowing purification, filtration purification, sedimentation purification and electromagnetic purification [15,16,17]. Scholars have carried out lots of research on magnesium melt purification technology in recent years [18,19,20,21]. However, compared with the research on the purification technology of aluminum melt, the research on magnesium melt purification technology is still in its initial stage. AZ80 is the deformed magnesium alloy and mainly involves purification inclusions. Therefore, this paper takes AZ80 magnesium alloy as the research object and ultrasonic treatment is used to purify magnesium alloy melt to study the melt purification.

Besides inclusions, porosities are also a bottleneck problem of magnesium alloys, and porosities is mainly caused by high hydrogen content. It is reported that the solubility of hydrogen is about 30 mL/100 g in magnesium alloys melt at 730 °C. If hydrogen is dissolved in solid metal, the hydrogen will become the solution strengthening element like some other alloying elements [22]. However, once the hydrogen content exceeds the solid solution limit, the second phase is bubbles, which enhance the formation of porosity, and severely deteriorate the mechanical and corrosion properties of the magnesium alloys [21]. Therefore, it is of great significance to remove hydrogen from the melt. However, there are few studies on this problem at present. AZ91 alloy is generally used for die casting, and the main problem is still the problem of micropores. Therefore, the degassing of AZ91 alloy was also investigated in this paper.

2. Experimental

AZ80 and AZ91 magnesium alloys were chosen as the experimental materials, and the specific chemical composition is shown in

Table 1. Chemical composition (mass%) of AZ80 and AZ91 alloy.

Element	Al	Zn	Mn	Fe	Mg
AZ80	7.8	0.524	0.19	0.003	Bal.
AZ91	9.1	0.82	0.20	0.020	Bal.

.

The experimental apparatus for AZ80 and AZ91 alloy melt by ultrasonic treatment was shown in Figure 1. The equipment included an ultrasonic treatment system, a resistance furnace, and an iron crucible. The ultrasonic system was composed of an ultrasonic generator with a frequency of 20 ± 2 kHz, a magneto-strictive transducer made of high-purity nickel sheet and a tool head. The ultrasonic system power was continuously adjustable from 0 to 2 kW. During the experiment, the temperature of the magnesium melt, the preheating temperature of the tool head (size 20 mm) and the ultrasonic treatment time was precisely controlled.

The AZ80 alloy was melted in an iron crucible with the dimensions of Φ70 mm × Φ50 mm × 120 mm. The AZ80 alloy in the iron crucible was melted in a resistance furnace with CO₂ + 0.5% SF₆ gas mixture for protection. The alloy was heated to 700 °C to make it completely molten and stirred homogeneously, and then the temperature of alloy decreased to 650 °C for 10 min. The preheated ultrasonic radiator was inserted into the melt about 20 mm below the liquid surface for 10 min.

To study the effect of ultrasonic treatment on the purification effect about AZ80 alloy melt, the selected ultrasonic power in the experiment was 0 W, 35 W, 80 W, 170 W and 220 W, respectively. The melt treated under different conditions was water-cooled immediately after treatment. The ingots prepared under different conditions were seen along the longitudinal direction, and the longitudinal sections were ground and polished, and then etched with 4% nitric acid alcohol solution.

The experimental ingots were divided into four layers from top to bottom: the top, the second, the third and the bottom layers, as shown in the macroscopic photographs Figure 2. Moreover, the distribution and content of inclusions in the longitudinal section were statistically analyzed using body microscope, optical microscope and image analysis software. The XRD and SEM samples were taken from the bottom from the ingot, and its size was 10 mm × 10 mm × 10 mm, which (A) was shown in Figure 2.

The AZ91 alloys were treated by 150 W ultrasonic at 730 °C, and the melt was inserted into 20 mm. The ultrasonic duration was 0 s, 30 s, 90 s and 150 s, respectively, and then the melt holding for 10 min was absorbed by quartz tube. Samples after machining of hydrogen content were tested by RH404 hydrogen tester, and its size was Φ5 mm × 20 mm. Finally, the melt was poured into the water-cooled copper at the temperature of 700 °C, and the microstructures and pores were observed from the center of the ingots, and its size was 10 mm × 10 mm × 10 mm. The mechanical properties were test and cross-sectional microporosity of ingots by ultrasonic treatment also were observed.

3. Results and Discussion

3.1. Effect of Ultrasonic Power on Purification of Ingot

Figure 3 shows the X-ray diffraction pattern of AZ80 alloy, and the main inclusions are MgO, including a small amount of ZnO and MnO. Figure 4 and Figure 5 show the morphology and energy spectrum analysis of inclusions in AZ80 magnesium alloy, respectively. The forms of the mainly inclusions are flake-like and block-like respectively. According to the energy spectrum analysis, the atomic percentage of Mg in the block is 52.366%, and the atomic percentage of O reaches 25.025%, so it is the oxide of Mg mainly. The energy spectrum analysis of black flakes shows that the atomic percentage of Mg is 67.445%, and the atomic percentage of O is 30.284%. Combined with the XRD results, it shows that the inclusions are mainly composed of MgO and a very small amount of Zn and Mn oxides.

Figure 6 is the SEM images of AZ80 alloy in A-zone. It can be seen the distribution of Mg is high and O element is low at the block. The O element at the black block is aggregated, but there is less Mg here. It can be determined that the block and black sheets are MgO. The MgO inclusions mainly come from several aspects: firstly, magnesium alloy melt is easy to react with oxygen to form MgO, accounting for about 80% of the total inclusions in magnesium alloy melt; Secondly, magnesium alloy melt is easy to react with water vapor in air to form MgO and H₂; In addition, the melt also reacts with N₂ in the air to form Mg₃N₂, but the reaction is very slow. In order to simplify the treatment inclusions, the raw materials in this paper are not purified with flux, but only protected with gas. Therefore, the inclusions are mainly MgO.

Figure 7 shows the relationship between the relative area fraction of inclusions at different positions of ingot cross-section and ultrasonic power for AZ80 magnesium alloy melt with different power at 650 °C, and the ultrasonic power is 0 W, 35 W, 80 W, 170 W and 220 W, respectively. The ultrasonic treatment duration is 60 s and the holding time is 60 s. It can be seen that the relative content of inclusions at the bottom of the ingot increases and then decreases with the increasing of ultrasonic power, while the relative content of inclusions at the top of the ingot first decreases and then increases. The curve shows that the ultrasonic treatment of magnesium alloy under specific power helps the inclusions in the melt settle at the bottom of the ingot and the purification effect is closely related to the ultrasonic power. It can also be seen that the ultrasonic power of 80 W has the best purification effect, and the relative area fraction of inclusions at the bottom of the ingot under this condition is as high as 95%. Therefore, the inclusions in the melt are mainly settled at the bottom of the ingot under the ultrasonic treatment. Ultrasonic power can achieve the purification effect under 80 W ultrasonic power.

3.2. Effect of Ultrasonic Treatment Duration on Purification of Ingot

Figure 8 shows the relationship between the actual area fraction of inclusions in each layer of the ingot cross-section under different ultrasonic time at 650 °C. From the Figure 4, it can be seen that the degree of purification is similar.

3.3. Effect of Holding Time on Purification of Ingot

Figure 9 shows the relationship between the relative area fraction of each layer of inclusions on the ingot cross-section and the holding time, and the AZ80 magnesium alloy melt is treated with 80 W at 650 °C. The ultrasonic duration is 60 s, and the holding time is 60 s, 100 s and 150 s, respectively. It can be seen that the relative area fraction of inclusions at the bottom of the ingot increases. With the increasing of holding time from 60 s to 120 s, the purification effect improves rapidly. However, when the holding time of melt reaches 150 s, the relative content of inclusions at the bottom decreases. The relative content of inclusions at the top of the ingot increases slightly, and the purification effect has a decreasing tendency.

Generally, the inclusions can leave to the bottom of the crucible for a long time, which can greatly improve the purification effect, but there are contrary results in the experiment. This may be because the gas around the inclusion is removed by ultrasonic treatment, which increases the effective density of the inclusion and increases the settlement speed of the inclusion. Therefore, the inclusions of bottom can be stranded by ultrasonic in a short time. At the same time, due to a long holding time, the inclusions floating on the liquid surface may also enter the melt, resulting in more inclusions in the upper part of the ingot. The inclusions that remain in the upper part of the ingot during cooling and solidification, which affects the ultrasonic purification effect.

When ultrasonic treatment is applied, fine inclusion particles rapidly collide and agglomerate into large inclusion clusters under the action of sound field and stranded in the bottom of the crucible. However, with the increasing of ultrasonic time, the concentration of inclusions in the melt decreases obviously, and the frequency of particle collision and condensation decreases significantly. At this time, the effect of melt purification will not be significantly improved if an ultrasonic wave is continuously applied.

After ultrasonic treatment, the melt needs to hold for a certain time, and the sedimentation is to separate the inclusion from the molten metal by using the different density between the inclusion and the molten metal. The density of inclusions in magnesium alloy melt is generally higher than that of magnesium alloy melt. After standing for an appropriate time, the inclusions with larger density sink to the bottom of the crucible, and the upper part is the clean molten metal. It shows that the leave velocity of inclusions is directly proportional to the square of its radius, the density difference between inclusions and liquid magnesium alloy, and is inversely proportional to the viscosity coefficient of liquid magnesium alloy. Therefore, the larger the size and density of inclusion particles, the smaller the viscosity and density of magnesium alloy liquid, and the more favorable the sedimentation speed. The agglomerates of inclusions have enough time to leave the bottom of the crucible, and the holding time has a direct impact on the purification effect of the melt. After ultrasonic treatment, the effective density of the inclusion in the melt increases, the thermal effect produced by ultrasonic reduces the viscosity of liquid metal and increases the leave speed of inclusions in the melt to a certain extent. Therefore, the inclusion can leave to the bottom of the crucible in a short time. Continuing to prolong the leave time may make the surface of the molten solution oxidized, the generated oxides sink into the melt and remain in the ingots during the subsequent cooling process, affecting the purification effect.

3.4. Effect of Ultrasonic Degassing on AZ91 Alloy

Figure 10 Shows that effect of ultrasonic treatment time on hydrogen content and degassing efficiency. With the increasing of ultrasonic treatment time, the hydrogen content in the melt gradually decreases. When ultrasonic treatment time is 90 s, the hydrogen content decreases from 19.4 cm³/100 g to 9.6 cm³/100 g, and the degassing efficiency can reach 50.5%. The degassing efficiency reduces to 26.3% by further increasing duration to 150 s.

Figure 11 shows the microstructure of AZ91 alloy at different ultrasonic durations. As shown in Figure 11a, the solidification structures have lots of coarse dendrites and porosities, and the size of porosities can reach 100 μm. When the ultrasonic duration is 30 s, the portion of coarse dendrites change into equiaxed grains but there still exists lots of small porosities, which is as shown in Figure 11b. When the ultrasonic duration increases to 90 s, the microstructure is fine equiaxed fine crystals with uniform distribution, and there are few porosities, as shown in Figure 11c. When the ultrasonic treatment time increases to 150 s, the microstructures are not obvious refinement, but lots of porosities appear in the ingots, as shown in Figure 11d.

Figure 12 shows the morphology of porosity of AZ91 alloys by different ultrasonic duration. Porosity distributions are largely changed by different treatment duration in ingots. When ultrasonic duration is 90 s, very few porosities exist in the ingots. Figure 13 shows the relationship between area fraction of porosities and hydrogen content. Therefore, the lower the amount of hydrogen, the smaller the corresponding area of porosities. When the hydrogen decreases from 19.4 cm³/100 g to 9.6 cm³/100 g, the corresponding area of porosities are from 3.09% to 0.47%, which reduces by 84.8%. The effect of ultrasonic duration on mechanical properties is shown in Figure 14. When the ultrasonic duration is 90 s, the tensile strength increases from 152 MPa to 194 MPa, which increases by 27.6%. The elongation increases from 2.8% to 4.8%, but the ultrasonic treatment had little influence on yield strength, which increases from 125 MPa to 133 MPa (improving by 6.4%). The mechanical properties decrease by further increasing duration.

Figure 15 is the cross-sectional microporosity of AZ91 alloy ingots by ultrasonic treatment, big pores in the cross-sectional can be seen without ultrasonic treatment. There are microporosities in the cross-sectional by ultrasonic treatment 90 s, which has low hydrogen content, because the melt was held for 10 min, so the refinement by ultrasonic cavitation effect is attenuated. Therefore, the big pores are the main reasons for reduction in mechanical properties by ultrasonic treatment of AZ91 alloys.

According to the research findings by Naji et al. [22], cavitation bubbles are always accompanied by expansion and compression during ultrasonic treatment. The surface areas of cavitation bubbles increase gradually during the expansion stage. The gas existing in the melt can diffuse to the bubbles through the bubble wall. In contrast, the surface areas of cavitation bubbles become smaller, and the corresponding amount of gas entering the bubble will be reduced during the compression stage. Therefore, the gas content in the dmelt are reduced. In addition, the film thickness of cavitation bubbles will affect the diffusion rate of gas. The reduction of cavitation bubbles surface areas will increase the thickness of the film. The reduction of gas concentration difference between cavitation bubbles and melt will reduce the diffusion rate of gas. When the cavitation bubbles expand, the thickness of the bubble films will decrease and the concentration difference between the gas in the bubbles and the melt will increase. When the thickness of the bubble films decreases, the diffusion rate of the gas in the melt into the bubble will be accelerated sharply. When the melt is treated with appropriate ultrasonic duration treatment, the gas diffusion process caused by the expansion and contraction of cavitation bubbles will be more active, resulting in an effective degassing effect.

Therefore, with ultrasonic treatment of the melt, cavitation bubble swelling and shrinkage of pulsating diffusion movement is very active, so the appropriate ultrasonic processing time can make the hydrogen effect better. In addition, under the ultrasonic treatment, acoustic streaming and mixing these tiny bubbles, in addition to hydrogen, combined a lot of opportunities, and can overcome the limit of fluid flow, and eventually rise to the surface, so as to achieve the hydrogen effect, because the bubble motion takes time; therefore, the melt was let stand for 10 min. However, ultrasonic has some refinement effect. The refining effect mainly comes from the expansion and contraction of cavitation bubbles which occurred during the formation of the high temperature and high-pressure reaction, and it can stimulate the melt in the dynamics of the nucleation. Therefore, in the elaboration and in addition to hydrogen, under the joint action there were improved mechanical properties. In addition, at the interface between the melt, at the same time there is a process of hydrogen absorption and hydrogen in addition to the two opposites. Therefore, when the hydrogen content is very low in the melt, ultrasonic power will increase the hydrogen content from the external.

4. Conclusions

(1)

The rapid separation of inclusions in magnesium alloy melt can be realized by ultrasonic treatment, which proves that purifying metal melt by ultrasonic field is entirely feasible.

(2)

The purification of ultrasonic melt is related to ultrasonic power, duration and standing time. The best purification effect is obtained by 80 W for 60 s at 650 °C, and the standing time is 100 s.

(3)

The hydrogen content in magnesium alloy melt can be reduced by ultrasonic treatment technology. When the ultrasonic power was 150 W for 90 s, the hydrogen content and degassing efficiency are 9.6 cm³/100 g and 50.5%, respectively. The corresponding mechanical properties are R_m = 194 MPa, R_0.2 = 133 MPa and A = 4.8%, respectively. The “area” and “shell” effects of the resonant has an efficient degassing result, and the big pores, because of high hydrogen content, are the main reason for fracture by ultrasonic treatment in AZ91 alloys.