ZL114A alloy is widely used in production of large thin-walled complex aluminum alloy castings in aviation, aerospace, automobile and other fields due to its high mechanical properties, good casting process performance and corrosion resistance. Vacuum differential pressure casting is to fill mold under low pressure and solidify under high pressure. During the whole process of solidification, alloy melt is affected by pressure field, castings with fine grains and dense structures can be obtained.
Ultrasonic treatment of melt is an important means to improve structure and properties of alloys. Ultrasonic treatment of 7050 aluminum alloy can significantly refine structure, and with increase of vibration power, effect of grain refinement is enhanced. Ultrasonic vibration is beneficial to refinement of equiaxed grain structure of ZL101 aluminum alloy, improves its strength, hardness and other properties. Appropriate ultrasonic power in aluminum alloy melt can effectively reduce gas content in sample, thereby improving compactness of structure. After ultrasonic treatment of pure aluminum, tensile strength increased from 52MPa to 72MPa, an increase of 35%, and hardness (HB) also increased from 17.2 to 19.7. At present, researchers have conducted a lot of research on the effect of ultrasonic power and solidification pressure on grain refinement, and obtained many valuable results. However, there are few reports on synergistic effect of ultrasonic power and solidification pressure on hardness of ZL114A alloy. Therefore, this subject combines vacuum differential pressure casting and introduces ultrasonic melt processing technology to explore influence of ultrasonic power-solidification pressure synergistic field on hardness of ZL114A alloy castings. Application of thin-wall precision ZL114A castings provides reference and reference.
Graphical results
Self-made VCPC-I vacuum differential pressure casting equipment was used, and an ultrasonic vibration device was introduced for test, as shown in Figure 1. When vacuuming of vacuum differential pressure casting ends, ultrasonic device is turned on, so that ultrasonic vibration is continuously applied during filling, pressure boosting, solidification and pressure holding stages until pressure is released, and ultrasonic vibration-vacuum differential pressure synergistic effect is implemented. Size of sample is φ(6, 9, 12) mm*100mm round bar, casting mold is metal mold, preheating temperature is 270℃, alloy is ZL114A, vacuum differential pressure casting ZL114A alloy samples were poured with process parameters such as vacuum degree of 20kPa, mold filling pressure difference of 35kPa, pressure holding time of 80s, ultrasonic power of 0, 300, 600, 900 W, and solidification pressure of 200kPa, 250, 300, and 350 kPa.

 

Fig.1 Schematic diagram of vacuum differential pressure casting system under ultrasonic vibration

1. Ultrasonic device 2. Upper tank 3. Metal mold 4. Partition plate 5. Lower tank 6. Cooling water outlet 7. Cooling water inlet 8. Crucible 9. Liquid riser pipe 10. Lower tank inlet pipe 11. Communication valve 12. Lower tank intake control valve 13. Vacuum pumping switch valve 14. Pressure relief switch valve 15. Vacuum pump 16. Lower tank intake switch valve 17. Lower gas storage tank 18. Upper gas storage tank 19. Upper tank intake switch valve 20 .Straight pipe switch valve 21. Upper tank intake control valve 22. Straight pipe 23. Upper tank intake pipe

 

Fig.2 Change trend of hardness of aluminum alloy samples under different ultrasonic power
Under different coagulation pressures, hardness of same part of sample with different diameters has basically same change trend with ultrasonic power. Taking a sample with a diameter of 9mm and a pressure of 350kPa as an example, when ultrasonic power is 0, hardness (HV) value of sample is 83.2; when ultrasonic power is applied to 300 W, its hardness (HV) is 86.1, an increase of 3.5%. This is because ultrasonic waves are introduced in vacuum differential pressure casting, ultrasonic waves produce strong cavitation effects and acoustic flow effects during solidification process of metal melts, cavitation effect will generate cavitation bubbles in molten metal, and cavitation bubbles will generate powerful shock waves at the moment of rapid closure or collapse, which will crush primary α-Al and eutectic Si, causing a large number of free crystals to appear in melt; in addition, acoustic flow effect generated by ultrasonic waves forces molten metal to flow between dendrites, thereby bringing free crystals to interior of molten metal. Under joint action of cavitation effect and acoustic flow effect, nucleation rate of metal melt is increased, dendrites are broken, and alloy structure is fully refined.

 

Fig.3 Diameter of 9mm sample under different ultrasonic power of 350kPa Microstructure

 

Fig.4 Effect of solidification pressure on sample hardness

 

Fig.5 Metallographic structure of samples with a diameter of 9 mm under different solidification pressures
During crystallization and solidification process of castings, high-pressure field always exists. Driving force that pressure causes aluminum alloy melt to flow to feeding zone through narrow channel between solidified dendrites is called squeeze seepage effect, and squeeze seepage capacity of vacuum differential pressure can be calculated as follows:

 

where, ΔG is metal capacity of squeeze seepage, m3; pc is crystallization pressure, Pa; ps is static pressure of metal liquid acting on squeeze seepage surface, Pa; pg is interdendritic gas in squeeze seepage layer μ is dynamic viscosity of molten metal, Pa s; ΔL is depth of squeeze seepage, m; F is area of squeeze seepage, m2; Δτ is time of squeeze seepage, s; K is permeability coefficient, m2 .
In conclusion
(1) Under synergistic effect of ultrasonic power and solidification pressure, as ultrasonic power increases, hardness of sample increases first and then decreases. When ultrasonic power reaches 600W, regardless of solidification pressure, hardness of sample reaches maximum value.
(2) When ultrasonic power and coagulation pressure act synergistically, hardness of sample increases with increase of coagulation pressure, and reaches maximum value at 350kPa; when ultrasonic power is greater than 600W, coagulation pressure has a greater influence on hardness.