Al-Si alloy has good mechanical properties and casting properties, and is suitable for various casting processes to produce high-performance castings. ZL114A alloy is widely used in automobiles, high-speed rail and aerospace fields due to its excellent corrosion resistance and specific strength. However, during solidification process, coarse needle-like eutectic Si is easily formed in ZL114A alloy matrix. These irregular eutectic Si structures have a serious splitting effect on alloy, causing stress concentration and seriously deteriorating mechanical properties. In recent years, aerospace aluminum alloy castings have become more complex, thin-walled, and integrated, which has put forward new demands for casting and forming processes and methods. Therefore, it is current research focus to achieve densification and complete forming of castings while improving microstructure of ZL114A alloy.
Commonly used method for refining eutectic Si in industry is to modify and refine aluminum alloys by adding modifiers containing elements such as Na, Sr, and Ba. Research has found that deterioration effectiveness of commonly used Na salt is only 0.5 to 1 hour, making it unable to be applied to large-scale continuous production. Studying influence of Na, Sr and other modified elements on density of ZL101 alloy, it was found that larger dense shrinkage pore areas will be formed in alloy structure modified by Na salt, while more dispersed pores will be generated in alloy sample treated with Sr. Al-Si alloy salt modifiers are easy to use, but effective time for modification is insufficient, and there is a problem of insufficient densification due to inhalation.
Physical fields such as magnetic fields, ultrasound, and mechanical vibration have attracted attention because they are efficient, non-polluting, and can effectively refine alloy structure. Application of a magnetic field during casting process has a significant impact on structural morphology changes of primary Si and eutectic Si in Al-Si alloys. Lorentz force generated by external magnetic field will cause forced convection and repeated oscillation of melt. It has remarkable effects in refining grains, reducing casting segregation defects and improving surface quality of aluminum alloy castings. It has advantages that other melt processing technologies such as chemical and physical fields cannot match. When researchers used strong magnetic fields to control solidification structure of hypereutectic Al-Si alloys, they found that morphology of primary Si crystals of alloys was significantly refined under strong magnetic fields, changing from plate-like to massive. When a rotating magnetic field was applied to Al-Si alloy, it was found that size of originally long strips of primary Si became smaller after external rotating magnetic field was applied, and formation of granular silicon phase was induced.
Solidification process, structure and properties of differential pressure casting of aluminum alloy castings were studied under different pressures. It was found that pressure has driving effect of causing aluminum alloy liquid to flow to feeding zone through narrow channels between solidified dendrites, which has a good feeding effect, making alloy structure grains refined and structure dense. With further research and application of vacuum differential pressure casting, it has been strongly confirmed that casting process of vacuum filling and solidification under high pressure is suitable for production of large and complex thin-walled parts. On this basis, physical fields are introduced for collaborative casting research. Using ultrasonic treatment of vacuum differential pressure ZL114A alloy castings, significant changes were found in morphology and growth orientation of eutectic Si. Increase in ultrasonic power causes eutectic Si to change from coarse flakes to fine, uniformly distributed short rod-shaped eutectic Si. When applying ultrasonic vibration and increasing solidification pressure, eutectic Si refinement effect is more significant. When an alternating magnetic field is applied in vacuum differential pressure casting process, aluminum alloy melt will be affected by synergistic effect of electromagnetic force and solidification pressure. Forced convection of melt caused by electromagnetic force is coupled with squeezing and percolation effect generated by pressure field, which further improves microstructure of casting and thereby improves mechanical properties of casting.
Taking ZL114A alloy as object, combining characteristics of alternating magnetic field and vacuum differential pressure casting, ZL114A alloy samples were prepared by cooperatively casting alternating magnetic field and graded pressure, analyzed morphology changes of eutectic Si phase, studied morphology and mechanical properties of eutectic Si in ZL114A alloy under action of alternating magnetic field to provide a reference for obtaining thin-walled complex aluminum alloy castings with dense structure and excellent performance.
Graphical results
Self-developed VCPC vacuum differential pressure casting equipment is used, and an alternating magnetic field generator is introduced on this basis. Equipment and process curves are shown in Figure 1. At the end of vacuuming, alternating magnetic field emitter is turned on to continuously apply alternating magnetic field during filling, voltage boosting and pressure holding stages, then turned off until pressure relief stage to implement alternating magnetic field-vacuum differential pressure synergy. Mold is cast in a sand mold, mold material is phenolic resin coated sand, and pouring temperature is 720℃. Preheating temperature is set to 120℃, and the test process parameters are shown in Table 2. ZL114A alloy sample was cast by vacuum differential pressure casting, and vacuum degree was 20kPa during test. During test, 6 types of magnetic induction intensity were set, and optimal magnetic induction intensity was first determined under graded pressurization pressure difference of 135kPa. Then set 5 different graded pressurization pressure differences, see Table 3, and determine optimal graded pressurization pressure difference under ideal magnetic induction intensity.
Metallographic specimens and tensile specimens were taken from same position of alternating magnetic field-vacuum differential pressure collaborative casting alloy specimen. After grinding and polishing, metallographic specimen was etched with HF with a volume fraction of 0.5%, and corrosion time was 10 ~15s. Morphology of eutectic Si phase in sample was observed using a Quanta 200 scanning electron microscope. WDW-50Y microcomputer-controlled electronic universal testing machine was used to test mechanical properties of tensile specimens.

Table 1 Main chemical composition of ZL114A alloy (%)

 

Figure 1 VCPC type vacuum differential pressure equipment and process curve
(a) VCPC type vacuum differential pressure casting equipment: 1. Pressure plate 2. Electromagnetic coil 3. Long bolt 4. Insulation block 5. Mold 6. Casting 7. Resistance furnace 8. Crucible 9. Liquid riser tube 10. Lower pressure tank 11. Middle partition 12. Upper pressure tank
(b) Ideal process curve of vacuum differential pressure staged pressure casting

Table 2 Test process parameters for different magnetic induction intensities

Table 3 Process parameters of different graded pressurization and pressure difference tests
When graded pressurization pressure difference is 135kPa, morphology of ZL114A aluminum alloy eutectic Si under different alternating electromagnetic induction intensities is shown in Figure 2. It can be seen from Figure 2a that when no magnetic field is applied, eutectic Si precipitates along grain boundaries and appears in coarse flakes and blocks. During vacuum differential pressure casting process, Si elements in melt still tend to be enriched toward front edge of solidification interface, and eventually aggregate at primary α-Al grain boundaries to form agglomerated eutectic Si. Segregation of elements in alloy can easily split matrix and destroy continuity, micropores can easily appear at boundaries between bulk Si phase and aluminum grains. Magnetic field-pressure synergistic field has a significant impact on microstructure of vacuum differential pressure casting ZL114A alloy. Magnetic field forces aluminum melt to convection, disrupting solute distribution at solidification front, causing dendrites to precipitate before breaking. While grains are refined, eutectic Si phase precipitates discontinuously. After magnetic field treatment, eutectic Si transforms from a coarse agglomerate to a filamentous structure. Under 24mT magnetic field and 160kPa graded pressure differential pressure field, eutectic Si takes the shape of discrete ellipsoids, which effectively reduces splitting effect of Si relative to ZL114A alloy matrix.

 

Figure 2 Morphology of ZL114A alloy eutectic Si in different alternating magnetic induction intensities under the composite field process
(a)0mT (b)8mT (c)24mT (d)40mT

 

Figure 3 Morphology of ZL114A alloy eutectic Si in different graded pressurization pressures under composite field process
(a)60kPa (b)110kPa (c)160kPa

 

Figure 4 Mechanical properties of ZL114A alloy under different magnetic induction intensities

 

Figure 5 Fracture morphology of ZL114A alloy under different alternating magnetic induction intensities
(a)0mT (b)8mT (c)24mT (d)40mT
It can be seen that as magnetic induction intensity increases, tensile strength of ZL114A alloy first increases significantly and then decreases, while elongation fluctuates greatly. Among them, alloy sample obtained maximum tensile strength at 24mT, which was 302.67MPa, which was approximately 39.2% higher than when no magnetic field was applied. At the same time, elongation reached 3.8%, an increase of 153.3%. This is because after magnetic field treatment, grain size in alloy is significantly reduced, and eutectic Si morphology changes from block to filament. Grain refinement and Si phase morphology transformation improve mechanical properties of alloy. In order to further clarify tensile fracture behavior of ZL114A alloy treated with magnetic field under vacuum differential pressure, fracture morphology of alloy is shown in Figure 5. As can be seen from Figure 5a, fracture surface of ZL114A alloy without alternating magnetic field treatment is distributed with a large number of tearing edges, cleavage steps and blocky smooth cleavage surfaces, and only a few shallow dimples appear, showing characteristics of quasi-cleavage brittle fracture. It can be seen from Figure 5b that local distribution of fine dimples on cross section of sample treated with 8mT magnetic field increases. As magnetic field is further strengthened, it can be seen in Figure 5c that number of cleavage steps and cleavage surfaces decreases, dimples increase significantly, and a large number of tearing edges appear. It is confirmed that application of alternating magnetic field effectively suppresses brittle fracture trend of ZL114A alloy and turns it to ductile fracture. Under influence of a magnetic field, forced convection occurs in melt, broken phase precipitates dendrites, grains are refined, growth and distribution of Si phase are improved. At the same time, thermal effect of magnetic field causes local melt overheating, affecting solidification and feeding of casting. It can be seen from Figure 5d that exposed secondary dendrite arms in fracture surface are attributed to microscopic shrinkage cavities in casting. Micropores in structure can easily become source of alloy cracks, resulting in a significant decrease in tensile strength of alloy after treatment with excessive magnetic induction intensity.

 

Figure 6 Mechanical properties of ZL114A alloy under different pressurization pressure differences

 

Figure 7 Fracture morphology of ZL114A alloy under different pressurization pressure differences
(a)60kPa (b)110kPa (c)160kPa