Study On The Micro Thixoforming Of A356 Alloy And Its Ultrasonic Assisted Mechanism

Supported by National Natural Science Foundation of China, (Project tittle:Research on the theory and method of ultrasonic assisted semi-solid micro thixoforming in meso-scale, Grant No.50775203), micro thixoforming and its ultrasonic assisted mechanism were studied in this thesis by fabricating micro-pin array structures applied in the micro-reactor for on-board fuel processing unit. Firstly, the feasibility of the micro thixoforming process was analyzed by investigating the fluidity of semi-solid alloys and was verified by experiment. Then, numerical simulation and experiments were performed to study the micro thixoforming processes of micro-pin array structure. Finally, the size effects in micro thixoforming were investigated, and ultrasonic vibration was introduced into micro thixoforming to improve the die filling of semi-solid alloy.In Chapter 1, the background and significance of this study were introduced. Then, the current research situations of micro metal forming technology and ultrasonic assisted forming technology were expatiated, and the research contents of this thesis were proposed.In Chapter 2, the thixoforming process was introduced. Then the feasibility of micro thixoforming process was analyzed by investigating the fluidity of semi-solid alloys, and the feasibility was verified by the thixoforming experiments of micro-pin array structure.In Chapter 3, the constitutive model of the A356 alloy in the semi-solid temperature range was developed considering the influences of temperature, strain and strain rate, and the validation of the constitutive model was performed. Then, the constitutive model was used to simulate the thixoforming of micro-pin array structures. The influence of process parameters (workpiece temperature, forming velocity and mold temperature) and the geometric parameters of the die (depth to diameter, die duty ratio and position of the micro hole) on thixoforming of micro-pin array structures were analyzed by DEFORM software. The analysis results provided the guidance for the design of the thixoforming process and die structure.In Chapter 4, the parameters to assess the quality of thixoformed micro-pin array structures were presented based on three typical features of micro-pins. According to the results of thixoforming experiments, the influences of the forming velocity, workpiece shape, lubrication condition and forming load on thixoformed micro-pin array structures were analyzed.In Chapter 5, the single micro-pin was thixoformed under different lubrication conditions. The size effects existing in the micro thixoforming were investigated based on two assessment parameters, including the height-diameter ratio of the formed micro-pin and the forming force. The results indicated that significant size effects existed in the micro thixoforming. With the decrease of the hole diameter, the height-diameter ratio of the formed micro-pin deceased, and the forming force increased.In Chapter 6, due to the defect of incomplete die filling of micro-pin array structure, ultrasonic vibration was adopted in the micro thixoforming. The influences of the ultrasonic vibration on the forming force, the fluidity of material and the height-diameter ratio of the formed micro-pin were investigated. The results indicated that ultrasonic vibration can decrease the forming force and increase the height-diameter ratio of the formed micro-pin.In Chapter 7, the chief work of this thesis was summarized, and the further research works were proposed.