The semi-solid metal forming technology has many advantages, and the preparation of semi-solid slurry or billet is its foundation and key, requiring the microstructure of the billet to be uniform and small spherical non dendritic structure. Extensive research has been conducted both domestically and internationally on the preparation of semi-solid slurry. However, due to the influence of container size, it is difficult for large volumes of molten metal to enter the supercooled state simultaneously. There are significant temperature differences in various parts of the liquid alloy, resulting in different cooling rates in different parts, leading to inconsistent final microstructure. In view of this, this project has developed a processing device (which includes a stainless steel hollow heat absorbing circular tube), using a new type of thermal equilibrium pulping process. This process has low cost, short process, simple operation, and can quickly absorb the overheating of the inner melt of the material cylinder, improving the quenching effect of the inner melt. In addition, the heat balance treatment device was externally connected to an ultrasonic vibrator, and for the first time, the combination of ultrasonic vibration and heat balance method was applied to the aluminum alloy melt, further improving the microstructure of the slurry. The preliminary experiment used composite ultrasonic vibration thermal equilibrium method and direct thermal equilibrium method to prepare A356 aluminum alloy semi-solid slurry, and compared and analyzed its microstructure. It was found that the microstructure of the sample obtained from the former was better than that of the latter. Taking A356 aluminum alloy as the research object, semi-solid slurry of aluminum alloy was prepared using ultrasonic vibration thermal equilibrium method, direct thermal equilibrium method, and traditional casting process. The same pouring temperature, mold temperature, injection pressure, and injection speed were used for forming. By analyzing the influence of different pulping process conditions on the metallographic structure and mechanical properties of the die casting, the optimization effect of the combination of ultrasonic vibration and thermal equilibrium on the alloy structure was studied.
Graphic and textual results
Three slurry preparation processes were used in the experiment: (1) Ultrasonic vibration thermal balance method: the thermal balance treatment device was combined with an ultrasonic vibrator with a power of 1.8 kW and a vibration frequency of 20 kHz, and ultrasonic vibration was turned on to prepare semi-solid slurry; (2) Direct thermal equilibrium method: prepare semi-solid slurry using a thermal equilibrium treatment device without turning on ultrasonic vibration; (3) Traditional die casting: without using a heat balance treatment device, directly die casting is formed.
The parameters of the thermal equilibrium method slurry are that the melt treatment temperature is 630 ℃, the outer diameter of the heat absorbing circular tube is 34mm, and the wall thickness is 1.1mm. Die casting process parameters: The preheating temperature of the mold is above 250 ℃, the pouring temperature of the melt is 620 ℃, the injection pressure of the die casting machine is 400kN, and the injection speed is 0.3m/s. After the slurry die-casting is formed, the metallographic structure and mechanical properties of the die-casting parts obtained under different process conditions are compared, and some samples are subjected to T6 heat treatment (530 ℃ × 6h solid solution+80 ℃ water quenching+150 ℃ × 6h aging followed by air cooling).
After preheating the crucible resistance furnace and other tools, place the A356 aluminum alloy ingot into the furnace and heat it to 720 ℃ and keep it warm. Remove slag and gas from the melt, and prepare alloy slurry and die cast into shape according to three different processes: (1) Ultrasonic vibration thermal balance method, transfer the pre poured melt to a small crucible preheated with the furnace, measure the temperature of the aluminum liquid with a thermocouple, and when the temperature drops to 630 ℃, pour the melt into the barrel; Turn on the lifting motor with a lifting speed of 20 mm/s, and at the same time, turn on the ultrasonic vibration. Under the drive of the motor, the heat absorbing circular tube enters the material cylinder. After the heat absorbing circular tube reaches the bottom of the material cylinder, let it stand for 5 seconds and then pull it out; Measure the temperature of the aluminum liquid in the barrel, and when the temperature drops to 620 ℃, perform die casting on the semi-solid slurry; (2) Direct thermal equilibrium method, without turning on ultrasonic vibration, the heat absorbing circular tube is directly inserted into the material cylinder, and after reaching the bottom of the cylinder, it is left to stand for 5 seconds before being pulled out. Die casting is carried out when the temperature of the aluminum liquid drops to 620 ℃; (3) Traditional die casting involves transferring the pre poured melt to a small crucible preheated in the furnace, and directly forming it by die casting when the temperature drops to 620 ℃.
The thermal equilibrium pulping process uses heat absorbing components as internal coolers to absorb heat, control temperature, and produce pulp. Compared to the traditional method of directly preparing slurry based on external cooling, this process provides a means to improve the controllability and efficiency of the cooling process. When preparing semi-solid slurry using ultrasonic vibration thermal equilibrium method, the heat absorbing circular tube absorbs heat from the melt while applying ultrasonic vibration. When ultrasonic waves propagate in the melt, mechanical effects can be generated, causing micro turbulence, promoting convective heat transfer between the barrel wall, heat absorbing circular tube wall and the melt, accelerating the temperature homogenization of the melt, thereby increasing the nucleation rate and providing conditions for uniform nucleation, making the grains grow uniformly in all directions, and the metallographic structure become uniform, fine and round.
The microstructure of the semi-solid die-casting parts obtained under the process conditions of ultrasonic vibration thermal balance is the best, with the smallest average equivalent diameter of 41.3 μ m and the largest average shape factor of 0.95. The metallographic structure of traditional die-casting parts is the worst. The comprehensive mechanical properties of semi-solid die castings obtained under the process conditions of ultrasonic vibration thermal balance method are the best, while the comprehensive mechanical properties of traditional process die castings are the worst. T6 heat treatment can effectively improve the comprehensive mechanical properties of die-casting parts.
Author of this article:
Li Chao, Yang Xiangjie, Guo Hongmin
School of Mechanical and Electrical Engineering, Nanchang University
Key Laboratory of High Performance Precision Forming in Jiangxi Province
Li Chao
Engineering Department of Nanchang Vocational College
Wang Jiaxuan and Li Zhenxing
Jiangxi Jiangduan Heavy Industry Co., Ltd
This article comes from: "Special Casting and Nonferrous Alloys" magazine, "Die Casting Weekly" strategic partner
