【Citation Format】
ZHANG Yuxin, ZHANG Wenda, WANG Yu, et al. Effect of Gas-Powder Composite Refining Method on the Purification Effect of ZL205A[J]. Special Casting & Nonferrous Alloys,2024,44(3):354-358.
Citation:ZHANG Y X，ZHANG W D，WANG Y，et al. Effects of gas-powder compound refining method on purification of ZL205A［J］. Special Casting & Nonferrous Alloys，2024，44（3）：354-358.
Aluminum alloy is easy to react with water and gas during the smelting process and oxidize and absorb hydrogen, resulting in metallurgical defects such as porosity and inclusions after the alloy solidifies and forms, and deteriorates the mechanical properties of the material. As an Al-Cu alloy, ZL205A aluminum alloy has excellent mechanical properties and is widely used in the production of high-end castings in the fields of automobiles and aerospace, but the melt viscosity of ZL205A alloy is large, and the fine inclusions (size < 20 μm) are difficult to remove, which cannot meet the requirements of high-quality aluminum alloy castings for high-clean melt control.
Refining the melt in the alloy production process is an effective technical means to control the hydrogen content and inclusion content. For aluminum melt, there are filtration method, static treatment method, flux method, bubble floating method, ultrasonic treatment method and vacuum treatment method. The filter method and the static treatment method have a size greater than 10 The inclusions of μm have a good removal effect, but the dehydrogenation effect is not significant; the flux method is limited by the effective contact between the flux and the melt and the physical and chemical properties of the flux, and its purification effect needs to be further improved; ultrasonic treatment is mainly based on refinement, although the hydrogenation removal effect is involved, the effective power range is limited, and it cannot process a large number of melts, so it is rarely used in production; vacuum treatment has excellent degassing effect, but can not effectively remove the impurities in the melt, and the equipment is relatively expensive, which limits its popularization and use; the rotary injection method belongs to the bubble floating method, because it has a good refining effect, no pollution and simple operationIt is easy to realize automation and other characteristics are adopted by most aluminum manufacturers. With the development of the aluminum industry and the increase in the demand for high-quality aluminum alloys in various industries, a single melt purification method cannot meet the requirements of high-quality castings for the cleanliness of aluminum melt, so various composite refining and purification technologies have been produced. At present, argon rotary injection argon combined with flux method is often used for composite refining and slag removal, and the purification effect is better than that of single rotary injection method or flux method. Zheng Weidong et al. reduced the hydrogen content in the melt to 0.09 mL/(100 g) Al by first rotary injection and then flux, while Fan Zhenzhong reduced the hydrogen content in the aluminum melt to 1.9×10-7 by adding flux first and then rotary injection combined with vacuum treatment. However, at present, the selection of rotary injection and flux compounding methods and their influence on the purification effect are still unclear.
The research team of North University of China published an article entitled "The Effect of Gas-Powder Composite Refining Methods on the Purification Effect of ZL205A" in the journal Special Casting and Nonferrous Alloys, Vol. 44, No. 3, 2024, in which the authors carried out different sequences of flux (powder) and rotary injection (argon) and rotary injection powder spraying composite refining of ZL205A alloy, and analyzed the purification effects of different gas-powder composite methods by using hydrogen detector, metallographic microscope, tensile testing machine and scanning electron microscope. The results show that the flux can increase the interfacial activity between the inclusion and the melt and reduce the viscosity of the molten aluminum, and the composite method with rotary injection argon has a great impact on the degassing and impurity removal effect of the ZL205A alloy, and the best purification effect is achieved by the rotary injection internal powder spraying process, the melt hydrogen content is reduced from 0.176 mL/(100 g) Al to 0.087 mL/(100 g) Al, and the yield strength, tensile strength and elongation of the material are increased by 19.2%, 18.3% and 100%, respectively, compared with the unrefined alloy.
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【Methodology】
The test material is a commercial ZL205A alloy, and its composition is shown in Table 1. The melting plant is a 12 kW pit melting furnace with a No. 80 clay crucible. The flux is HERA-excluded commercial aluminum alloy refining agent, and its composition is NaF, KCl, NaAlF6 and Na2SO4. The rotary injection equipment is XC(P)230-1 online rotary degasser, and the purified gas used is high-purity argon gas (volume fraction of 99.999%).
The ZL205A alloy used in each furnace test is 7 kg, which is pretreated with cleaning and drying before smelting, and the temperature of the molten aluminum is adjusted to 725~735 °C after melting, and then refined, the specific process is shown in Table 2. The following is a brief description of the different compounding methods of rotary injection and refining agent as different gas-powder composite methods. During the test, the ambient humidity is 30%~40%.
During flux refining, the amount of flux added is 0.2% of the mass of molten aluminum, and the refining time is 15 min, and the nozzle speed is 200 r/min, the refining time is 15 min, and the injection flow rate is 20 L/min. After the refining is completed, the melt is allowed to stand for 10 min.
The HYCAL MINI handheld hydrogen meter was used to measure the on-line hydrogen of the alloy melt. After the hydrogen measurement is completed, it is poured at around 720 °C and the mold is a cast iron mold preheated to 200 °C. Specimens for tissue observation and performance testing are shown in Figure 1. The metallographic samples were polished, polished, anodic coated and then cleaned, and observed by ZEISS-Imager metallographic microscope, and the grain size was counted with the help of Image J software. According to GB/T228.1-2010, the casting is processed into a tensile test rod with a diameter of φ5 mm in the gauge section, and the size is shown in Figure 2. The CMT-5105 universal testing machine was used to test the mechanical properties of the standard mechanical test rod with a tensile rate of 1 mm/min and the average of at least 3 specimens in each group. Inclusion determination (EDS) and tensile fracture observation (SEM) were performed using a ZEISS EVO MA15 scanning electron microscope with an Oxford AZtec energy dispersive analyzer.
Fig.1. Schematic diagram of the sampling position of the specimen (thickness of 30 mm)
Fig.2. Schematic diagram of the size of the tensile test rod
【Research Results】
Compared with the alloy without refining treatment, the proportion of black defects (holes and inclusions) on the metallographic surface of the refined alloy to the field of view area is significantly reduced. The size of inclusions and holes in Fig. 3b is larger than that in Fig. 3c, but the number of inclusions in Fig. 3c is significantly more than that in Fig. 3b, and the size and number of inclusions and holes in Fig. 3d are significantly smaller.
Fig.3. Microstructure of the alloy after refining treatment with unrefined and different gas-powder composite methods
After refining by different gas-powder composite methods, the grains of ZL205A alloy grew to different degrees compared with those before refining, and the average grain size increased from 93.1 μm before refining to 107.4, 117.9 and 112.6 μm, respectively. Since the inclusions in the aluminum melt are reduced after refining, and the small inclusions may be used as the basis for heterogeneous nucleation during grain nucleation, it is inferred that the phenomenon of grain size increase may be related to the decrease of heterogeneous nucleation cores in the melt.
Fig.4. Alloy anodic corrosion microstructure after refining by different composite methods
Fig.5. Statistical results of alloy grains after refining by different composite methods
Irregularly shaped defects often have inclusions and associated hole defects. After EDS analysis of each point in Figure 6, the results are shown in Table 3. There are three main types of inclusions: alumina film slag inclusions, MgO particles, spinel slag inclusions, and other types of slag inclusions.
Fig.6 Inclusion morphology and EDS analysis in Fig.3
The hydrogen content was above 0.17 mL/(100 g) Al before melt purification, and the hydrogen content was reduced to around 0.11 mL/(100 g) Al after different gas-powder composite purification processes. Among the three gas-powder composite methods, the rotary injection refining agent had the best hydrogen removal effect, and the melt hydrogen content was reduced to 0.086 mL/(100 g) Al, and the hydrogen removal rate reached 50.6%. After refining and degassing by composite method I, the proportion of inclusions and holes in the metallographic field decreased by 52.0% compared with the unrefined samples, the proportion of composite method II decreased by 42.3%, and the proportion of composite method III was the highest, reaching 77.4%.
Fig.7. Effect of gas-powder composite refining method on the degassing and impurity removal effects of ZL205A aluminum alloy melt
Combined with the metallographic photographs and the analysis of the proportion of inclusions and holes in each sample, it can be found that the refining process of the three composite methods has a positive effect on the removal of impurities and gas in the melt. From the point of view of purification effect, rotary injection refining agent > refining agent + rotary injection >rotary injection + refining agent. According to the "parasitic mechanism" of the interaction between impurities and gases, when the impurities in the molten aluminum are high and distributed in fine dispersion, the active "window" of hydrogen adsorption will be significantly increased and its comprehensive aggregation force field will be enhanced, so that the hydrogen is easy to parasitize on the inclusions, thus increasing the hydrogenation tendency of the molten aluminum, reducing the diffusion rate of hydrogen in the molten aluminum, and deteriorating the degastric conditions. In this case, if degassing measures are taken, such as composite method II., the effect is not ideal. This can be further explained by the mathematical expression of the hydrogen removal rate.
In actual production, the research and development of hydrogen removal and purification technology mainly revolves around three aspects: increasing the ratio of A and V, K value and action time t, and some advanced hydrogen removal and purification technologies and devices have been developed, but ignoring the interdependence and interaction between inclusion and hydrogen, and the difference in purification effect between composite mode I and II stems from this. The existence of a large number of inclusions will increase the viscosity μ of the molten aluminum, reduce the ratio of A to V, and reduce the diffusion coefficient D at the same time, and the diffusion velocity of hydrogen in the molten aluminum will be reduced due to the existence of inclusions, thereby reducing the efficiency of hydrogen removal. Composite method I. The flux is added before the rotary injection argon refining, although the effective contact area between the flux and the melt is limited at this time, but still dissolves and adsorbs some inclusions, on the one hand, the melt viscosity is reduced, the diffusion coefficient of hydrogen is improved, on the other hand, the reaction between the flux and the inclusion improves the activity of the contact interface between the inclusion and the melt, which helps the argon bubble to play an adsorption role for subsequent degassing and impurity, so the refining effect is better than that of the composite method II. Composite method III uses argon gas as a carrier to spray powdery refining agent into the melt, and evenly disperses it into the melt under the shear action of the high-speed rotating graphite rotor, which increases the effective contact area between the flux and the melt, further improves the refining efficiency of the flux method, and at the same time uses the adsorption of floating bubbles to discharge the inclusions and free hydrogen, giving full play to the advantages of the two refining methods, and improving the purification effect compared with the composite methods I and II.
Fig.8 Schematic diagram of purification principle
1. Inclusion particles 2. Free hydrogen 3. Large bubbles containing refining agent
4. Shear the small bubbles after crushing
After refining by composite method III, the yield strength σ0.2, tensile strength σb and elongation δ of the alloy reached 93 MPa, 233 MPa and 23%, respectively, which were 19.2%, 18.3% and 100% higher than those of the original alloy, and 6.3%, 4.3% and 48.4%, 1.6%, 7.6% and 56.5% higher than those of composite method I and composite method II, respectively. Combined with the grain size analysis of the alloy after refining, the positive effect of the improvement of aluminum melt cleanliness on the mechanical properties is stronger than the negative effect of the increase of grain size.
Fig.9. Mechanical properties of unrefined alloys and alloys after refining by different gas-powder composite methods
When the alloy is not refined and degassed, the hydrogen content of the melt is high, and the aggregated shrinkage porosity and shrinkage porosity can be observed at the tensile fracture, and the cracks germinate from the shrinkage porosity and shrinkage porosity, and the aggregated shrinkage porosity cuts the matrix, resulting in a stepped fracture pattern in the fracture. There is no obvious shrinkage porosity in the alloy purified by composite method I, the shrinkage porosity size is smaller than that of the original alloy and no aggregation phenomenon is found; the shrinkage porosity is still the source of crack initiation, but there is no obvious step-like pattern; the fracture has an uneven three-dimensional structure, and the formation of this structure is due to a large number of tensile damage inside the material during fracture, which makes the crack change locally when propagating, and the plastic deformation occurs in the higher stress concentration area, forming a depression and then being formed by the combined influence of stress distribution and other factors. Although the size of the shrinkage porosity in the fracture of the alloy purified by composite method II is larger than that of composite method I., there is no shrinkage porosity defect in the original alloy, and the fracture surface is relatively flat, indicating that the crack is less hindered in the propagation process and the mechanical properties of the material deteriorate. After the composite method III, the alloy has no obvious shrinkage porosity, and the number of shrinkage pores is further reduced and the size is reduced compared with the first three groups of specimens; the fracture surface has an uneven three-dimensional structure, but the size is reduced compared with Figure 10b; the three-dimensional structure pits are filled with many fine cracks, which are usually considered to be formed during the fracture process; there are obvious dimples in the fractures, indicating that the material may be ductile fracture caused by plastic failure.
Fig.10. Morphology of alloy tensile fractures before and after refining by different gas-powder composite methods
【Research Results】
(1) ZL205A alloy was refined, and the flux (powder) and rotary injection (argon) and rotary injection refining agent were combined and refined in different sequences, and the flux could increase the interfacial activity between the inclusion and the melt, which was helpful to improve the argon adsorption and impurity removal efficiency. The flux addition method has a significant effect on the purification effect of aluminum alloy melt, and the rotary injection refining agent process achieves the best purification effect.
(2) Rotary injection (argon + powdered flux) refining, the hydrogen content is reduced to 0.086 mL/(100 g) Al, the inclusion content is reduced to 0.177%, the hydrogen removal rate reaches 50.6%, and the yield strength, tensile strength and elongation of the material are increased by 19.2%, 18.3% and 100% respectively compared with the unrefined alloy.
(3) The hydrogen content of the ZL205A alloy without compound refining treatment is high, and shrinkage porosity and shrinkage porosity are easy to form during the solidification process, and the matrix is fragmented, and the fracture pattern is stepped shaped.