Research On Al-Si Alloy Melting Process Based On Nonlinear Ultrasonic Cavitation Effect

The hydrogen content of aluminum-silicon alloy has an important influence on the structure and performance after solidification,mainly in the porosity,looseness,density and strength of the ingot.Ultrasonic treatment has certain effects to improve the structure,internal pores and mechanical properties of the alloy.Due to the limit of high temperature,effect of ultrasound on the alloy cannot be directly observed.Therefore,the KM-poly tropic equation is solved by Matlab and the growth process of cavitation bubbles in the Al-Si alloy melt is numerically simulated from the perspective of ultrasonic frequency and ultrasonic power.Ultrasonic treatment of Al-Si alloy at different times and powers,combined with simulation data and experimental results,respectively analyze the mechanism of refinement mechanism,degassing mechanism and mechanical properties under ultrasound.Further use of rare earth elements(Ce,La)to modify the aluminum-silicon alloy to determine the best process formula.(1)The numerical simulation results show that when the ultrasonic frequency range is 20 kHz-40 kHz,the lower ultrasonic frequency,the better cavitation effect and higher efficiency of cavitation bubble growth.Therefore,the selected ultrasonic frequency is20 kHz.Ultrasonic treatment with different power(300W-800W)is applied to the aluminum-silicon alloy: when the applied ultrasonic power range is 300W-600 W,steady-state cavitation effect occurs;when the ultrasonic power is 700W-800 W,occurs transient cavitation effect.The experimental results show that the outgassing rate of the alloy gradually increases at 300W-500 W,the density increases by 1.47% at 500 W,and the outgassing rate is 73.6%;however,the density and degassing rate gradually decreased at 600W-800 W,which shows that the steady-state cavitation effect is conducive to the combination of cavitation bubbles and hydrogen to escape from the melt;the transient cavitation effect is not conducive to degassing.The results indicate that bubble cavitation model plays effective guidance in the degassing of aluminum-silicon alloys.(2)Different ultrasonic time and power are applied to aluminum-silicon alloy at20 kHz to observe its microstructure.It is found that the ultrasonic treatment can significantly refine the primary silicon.Its size decreases first and then increases with the increase of ultrasonic power.When ultrasonic power and time are in 500 W and 150 s,the refinement effect reach to the best: the average aspect ratio is 1.26,and its shape changesfrom polygonal block to round and granular.But ultrasonic treatment has no obvious improvement effect on the eutectic structure of Al-Si alloy.(3)The effect of different ultrasonic treatment time and power on the mechanical properties of the alloy is studied.The mechanical properties are best at 600 W and 150s:tensile strength is 198.4MPa,increased by 27.1%,and the elongation is increased by3.1%.Because ultrasonic treatment refines and changes the morphology of the primary silicon,reduce the gas content and the internal defects of the alloy,thereby improving the mechanical properties.(4)The effect of ultrasound and rare earth(Ce,La)cooperating on mechanical properties are studied.The results show that when ultrasonic power is 600 W,rare earth content is 0.2%,the mechanical properties of the alloy are better than those of any single treatment method,and the improvement of La is better than Ce.The tensile strength compared with only ultrasonic treatment are 205.9MPa and 209.7MPa,increased by5.59% and 3.68% respectively.(5)The best process parameters for refinement and degassing of Al-Si alloy are:ultrasound frequency is 20 kHz,ultrasound time is 150 s,ultrasound power is 500 W.The best process parameters to improve mechanical properties are: rare earth La content is0.2%,ultrasound frequency is 20 kHz,the ultrasonic time is 150 s,and the ultrasonic power is 600 W.