Effect of rotary jet on melt density equivalent of die-cast aluminum alloy (ADC12)

As an important process for producing aluminum alloy parts, die-casting technology has high molding accuracy, production efficiency, and excellent surface quality. It is suitable for large-scale automated output and occupies an increasingly high position in the casting industry [1]. ADC12 is a common die-cast aluminum alloy and has always been an important structural material in the aviation, aerospace, military, and automotive industries [2]. However, in actual production, due to the influence of many factors such as smelting and casting processes, hydrogen is absorbed and diffused into the aluminum alloy melt, resulting in defects such as pores and slag inclusions in the castings, which seriously affects the performance and service life of the castings [3]. Therefore, choosing a reasonable process to remove hydrogen before casting and improving the melt quality before aluminum alloy casting is an effective way to solve the internal defects of die-castings and enhance product performance and service life. The main methods for degassing aluminum alloys are the solvent method, vacuum degassing method, ultrasonic vibration degassing method, rotary jet degassing method, etc. [4]. Among them, the rotary spray degassing method, as a kind of bubble floating method, is currently the most widely used and economical method in actual industrial production. The author of this paper studied the influence of process parameters such as rotor speed and nitrogen flow rate on the density equivalent of die-cast aluminum alloy through the rotary spray degassing experiment of aluminum alloy melt and studied its influence mechanism to obtain the optimal process parameter range.

1 Experimental materials, equipment, and methods

The experimental material is ADC12 aluminum alloy (composition see Table 1), the smelting equipment is a tower melting furnace, the aluminum degassing equipment is StrikoWestofen DC2S rotary degasser, and the measuring equipment is VN6130DV density equivalent meter.

The alloy is melted in the melting chamber, refined, and deslagged in the holding chamber, and the transfer bag is rotated for degassing and deslagging again. The aluminum tapping temperature is 710~730℃. During degassing, high-purity nitrogen (purity 99.99%) is introduced into the graphite rotor. After deslagging, it is left to stand for 10 minutes. Two portions of aluminum liquid 5 cm below the liquid surface are scooped from the degassed alloy liquid with a scoop. One portion is cooled and solidified naturally, and the sample density Datm is measured. The other portion is cooled and solidified under vacuum and the density DV is measured (the vacuum pressure is 7~9 kPa, and the vacuum time is 4 minutes). Then the density equivalent DI is calculated. The calculation formula of density equivalent is DI=(1-Dv/Datm)×100%.

2 Results and discussion

The density equivalent is to use the Archimedean buoyancy method to test the purity (density) of aluminum liquid to accurately and quantitatively reflect the comprehensive influence of hydrogen and various slag inclusions in aluminum liquid on aluminum alloy castings. In the process of preparing samples under a certain vacuum degree, the pores and shrinkage inside the aluminum liquid are magnified due to the balance of internal and external pressures. Theoretically, for absolutely pure aluminum liquid, the values of samples prepared under normal pressure and reduced pressure are the same, that is, Datm=Dv, DI=0; the purer the aluminum liquid, the closer Dv is to Datm, that is, the smaller the DI value. In other words, the smaller the density equivalent value DI, the better the quality of the aluminum liquid. It can be seen from this that the overall quality of the aluminum liquid can be accurately judged by the size of the DI value.

2.1 Effect of rotor speed on density equivalent of aluminum liquid

The nitrogen flow rate was controlled at 20 mL/min, and the rotor speed was set to 200, 300, 400, 500, and 600 r/min. The density equivalent of aluminum liquid after degassing for 4 and 6 min was measured respectively. The relationship between density equivalent and rotor speed is shown in Figure 1. It can be seen that when the rotor speed is in the range of 0-400 r/min, as the rotor speed increases, the smaller the value of density equivalent of aluminum liquid, the better the degassing effect; when the speed exceeds 400 r/min, as the rotor speed increases, the density equivalent value of aluminum liquid increases.

The principle of rotary degassing is to blow inert gas into the metal liquid through the rotor, and through the continuous rotation of the rotor, a fast-moving gas and water vortex is formed in the deep of the melt, so that the bubble size becomes small and evenly distributed, thereby achieving the purpose of floating.

The principle is shown in Figure 2. According to the partial pressure dehydrogenation principle of the floating method, the effect of gas partial pressure on solubility is used to control the hydrogen partial pressure in the melt, resulting in a large partial pressure difference between the hydrogen partial pressure in the melt and the external environment, generating a dehydrogenation driving force to discharge the gas. The degassing speed of the floating method is related to the mass transfer rate of hydrogen atoms in the aluminum liquid to the bubble surface. The larger the mass transfer coefficient, the faster the dehydrogenation speed. According to the surface renewal theory, the mass transfer rate k can be expressed by formula (2) [5].

In the formula, D is the diffusion coefficient of hydrogen in the aluminum liquid; ts is the contact time between the bubble and the aluminum liquid; Vb is the floating speed of the bubble; db is the diameter of the bubble.

It can be seen that to improve the degassing efficiency of the rotary jet, the number of bubbles should be increased as much as possible, the effective contact time between the aluminum liquid and the bubble should be improved, and the diffusion distance of hydrogen in the aluminum liquid should be reduced. Therefore, when the rotor speed is between 0 and 400 r/min, as the speed increases, the bubble diameter can be reduced, the movement speed of the bubble in the aluminum liquid can be increased, the mass transfer rate k can be increased, and the degassing efficiency can be improved. However, when the rotor speed exceeds 400 r/min, the aluminum liquid will roll, causing the aluminum liquid to be re-involved in the gas. Due to the influence of the transfer bag capacity, the breakneck speed will also cause the originally broken nitrogen bubbles to re-aggregate into large bubbles, indirectly reducing the number of bubbles in the aluminum alloy melt, and resulting in a decrease in the degassing effect. It can also be seen that the density equivalent of the aluminum liquid will be significantly lower than that of 4 min after degassing for 6 minutes, and the experimental results are consistent with the principle of the flotation method.

2.2 Effect of nitrogen flow rate on density equivalent of aluminum liquid

The graphite rotor speed was set to 400 r/min, and the nitrogen flow rate was set to 15, 20, 25, 30, and 35 mL/min. The density equivalent of aluminum liquid after degassing for 4 and 6 minutes was measured respectively. The density equivalent of aluminum liquid under different nitrogen flow rates is shown in Figure 3.

Analysis of the data in Figure 3 shows that when the nitrogen flow rate is between 0 and 20 mL/min, the smaller the value of the density equivalent of aluminum liquid is, the better the degassing effect is. On the contrary, when the nitrogen flow rate exceeds 20 mL/min, the larger the density equivalent of aluminum liquid is, the worse the degassing effect is. Figure 4 is a schematic diagram of the principle of removing slag inclusions by floating method.

It can be seen that after the Al2O3 inclusions in the aluminum liquid are adsorbed by the bubbles, the free energy of the contact surface area S between the two decreases. According to the second law of thermodynamics, the direction of the reduction of the surface free energy of the system is the direction of the process automatically proceeding [6]. Therefore, as the amount of nitrogen in the aluminum liquid increases, the Al2O3 inclusions will float to the surface of the aluminum liquid along with the bubbles adsorbed on its surface. This is one aspect of the decrease in the density equivalent value after increasing the nitrogen flow rate within a suitable range. On the other hand, as the nitrogen flow rate increases, the bubble density in the aluminum liquid increases, the influence range of the bubbles in the aluminum liquid increases, and the contact area between nitrogen and the melt also increases accordingly. According to Stokes' law, see formula (3):

From formula 2, it can be seen that the floating speed ν of the bubble in the aluminum liquid is proportional to the density ρ of the bubble in the aluminum liquid and the radius r of the bubble. Increasing the nitrogen flow rate can just increase the number of bubbles in a unit volume of aluminum liquid and increase the floating speed of the bubble. According to formula (2), the increase in bubble buoyancy will increase the mass transfer rate k of hydrogen in the aluminum liquid, thus improving the degassing effect. For the 800 kg degassing bag used in this experiment, when the nitrogen flow rate exceeds 20 mL/min, one hand, the diameter of the bubble will increase, and the rupture of large bubbles will cause the surface of the aluminum liquid to be unstable, resulting in the aluminum liquid rolling, causing the aluminum liquid to be re-involved with gas, and the density equivalent will increase; on the other hand, the increase in bubble diameter will reduce its influence range in the aluminum liquid, and the degassing effect will decrease.

2.3 Effect of other factors on the density equivalent of aluminum liquid

In addition to the rotor speed and nitrogen flow rate, the type of inert gas and the effective length of the baffle immersed in the aluminum liquid will also affect the degassing effect of the aluminum liquid. Figure 5 shows the effect of nitrogen and argon on the density equivalent of die-cast aluminum alloy (ADC12). Analysis shows that nitrogen and argon have no significant effect on the density equivalent of aluminum liquid. When the temperature is high, the degassing effect of argon is slightly better than that of nitrogen, mainly because argon is more stable than nitrogen, and the relative atomic mass of Ar is larger than that of N atoms, which is more conducive to gas discharge. Figure 6 shows the effect of the effective length of the baffle immersed in the aluminum liquid on the density equivalent of the aluminum liquid, where the long baffle is a complete baffle and the short baffle is a baffle with a burnout of more than 1/3. It can be seen from the figure that the complete baffle is more conducive to breaking the bubbles in the aluminum liquid than the baffle with a burnout of more than 1/3, increasing the number of bubbles and improving the degassing effect. In addition, the tapping temperature will also affect the density equivalent of the aluminum liquid. The higher the aluminum liquid temperature, the greater the absorption tendency of the aluminum liquid, the more serious the oxidation and burning of the aluminum liquid, and the more Al2O3 slag inclusions.

When the amount is 1.0%, the spheroidizer is FeSiMg alloy, the addition amount is 1.2%, and the composition is shown in Table 8. The tapping temperature is controlled at 1 480~1 500 ℃. The chemical composition, metallographic, and mechanical properties of the castings are tested and all meet the requirements of QT600-10 (see Table 9).

3 Conclusion

When the carbon equivalent is 4.1~4.4, 0.9%~1.2%FeSiMg alloy is added by the flushing method. As the silicon content increases, the ferrite content increases. When the silicon content reaches 4.28%, castings that meet QT600-10 can be produced.

The rotor speed has an important influence on the effect of rotary spray degassing of die-cast aluminum alloy (ADC12) melt. When the rotor speed is between 0 and 400 r/min, increasing the rotor speed will better break up the bubbles, reduce their radius, increase the number of bubbles distributed in the aluminum liquid, and increase the degassing effect; when the rotor speed is too high, the broken nitrogen bubbles will re-aggregate, and it will also cause the aluminum liquid surface to roll and entrain gas, affecting the degassing efficiency. The best rotor speed in this experiment is 400-450 r/min.

The nitrogen flow rate has an important influence on the effect of rotary spray degassing of die-cast aluminum alloy (ADC12) melt. When the nitrogen flow rate is between 0 and 20 mL/min, increasing the nitrogen flow rate will increase the number of bubbles in the aluminum liquid, increase the range of action of the bubbles on the aluminum liquid, and improve the degassing effect. When the nitrogen flow rate is too high, the diameter of the bubble increases, and the rupture of the large bubble will cause the aluminum liquid surface to be unstable, causing the aluminum liquid to roll, and the aluminum liquid to re-entrain gas, reducing the degassing effect. The best nitrogen flow rate in this experiment is 20-25 mL/min.

The effect of nitrogen and argon on the degassing effect of rotary jetting of die-cast aluminum alloy (ADC12) melt is not obvious; the length of the baffle immersed in the aluminum liquid has an important influence on the degassing effect. When the baffle burns more than 1/3 of the total length, the baffle should be replaced immediately. According to field experience, under the premise of meeting the process requirements, the aluminum tapping temperature should be reduced as much as possible to reduce the burning of aluminum liquid and the tendency of gas absorption.