Research On Degassing Effect For Al-Si Alloy And Control Method Based On High-Low Vibration

Al-Si alloy is widely applied in both civil and national use due to the light quality high strength and thermal conductivity as well as good liquidity. According to the analyses,85% of dissolved gas in the aluminum alloy is hydrogen, so the gas content can be regarded as hydrogen content, when cooled to solid-state, hydrogen solubility will reduce greatly due to the high hydrogen solubility in liquid aluminum silicon alloy, thus supersaturated hydrogen will be released, forming the tiny pores dispersion within the casting. The mechanical properties of castings, fatigue resistance, corrosion resistance, and ductility would decrease apparently. Therefore, degassing is the main measure to enhance the comprehensive mechanical properties of Al-Si alloy. In this paper, following works are displayed:(1) The kinetics of hydrogen in the aluminum melt is analyzed; the regassing rule of BH122A after argon stirring is verified through the experiment in Binzhou Bohai Piston CO., LTD. The following ultrasonic vibration is determined to reduce the regassing rate.(2) Main parameters to describe sound field is introduced and the sound pressure field distribution under the tool rod is established. According to the theory of cavitation threshold, effective cavitation range is confirmed to provide the necessary conditions to establish a dynamic model describing the entire process of floating up.(3) The Rayleigh-Plesset equation that describes the motion for spherical bubble is introduced.In order to describe the entire process, Rayleigh-Plesset equation is combined with Stocks formula to establish the model.(4)Simulation of bubbles’ movement is conducted by Matlab. The variation of bubble radius variation, floating time and turbulence intensity is simulated, and it is obvious that high frequency vibration combining low frequency vibration is more beneficial to stimulate bubbles’floating up.(5) According to the needs of vibration, in order to control the GMA precisely under 10-50Hz, a dynamic modified P-I model is proposed to broaden the frequency domain in this paper, which combines quasi static modified P-I model with hysteresis phase compensation. The experiment of back loop control system based on dynamic modified P-I model and PID controller is conducted. Results show that the compensation above can reduce the hysteresis no linearization of GMA obviously. The maximum of error occurs around the turning point. The control model provides the theoretical basis and makes it possible to utilize to ultrasonic-low-frequency vibration for degassing in Al-Si alloy.Innovation points of this paper:(1) Model of bubble movement during floating is established based on Rayleigh-Plesset equation and Stocks principle;(2) Simulation of bubbles’movement is conducted by Matlab. The variation of bubble radius variation, floating time and turbulence intensity is simulated, and it is obvious that high frequency vibration combining low frequency vibration is more beneficial to stimulate bubbles’floating up.(3) A dynamic modified P-I model is proposed to broaden the frequency domain in this paper, which combines quasi static modified P-I model with hysteresis phase compensation. The experiment of back loop control system based on dynamic modified P-I model and PID controller is conducted. Results show that the compensation above can reduce the hysteresis no linearization of GMA obviously.Research Significance:(1) Kinetics of hydrogen degassing of Al-Si alloy is researched in this paper, Model of bubble movement during floating is established to describe the incentive function of vibration source to bubbles, which is meaningful to know the relation between bubble movement and degassing efficiency and effectiveness.(2) Control method of GMA proposed in this paper is effective to modify the waveform of the source of vibration, which is meaningful to improve degassing efficiency and effectiveness.