Research and development of metal casting rheological forming technology

In recent years, rheological forming technology also has a lot of research and application. University of Brunel, UK Fan et al. Studied the rheoforming by using the double helix mechanical stirring method. Due to the double helix mechanical device, the alloy melt can obtain high turbulence intensity and high periodic shear effect. Coupled with the severe stirring effect, the metal melt is broken, the nucleation rate is greatly increased, the grain is refined, and the uniform and fine spherulite or near spherulite structure can be obtained. Each stirring stroke only needs 10 ~ 30s to obtain a certain amount of high-quality semi-solid slurry. The prepared slurry directly enters the alloy injection chamber through the steering valve and is formed by the injection hammer, which realizes the connection between the forming equipment and the pulping device.

Wu Shusen of Huazhong University of science and technology improved the structure of double helix bar. The two helix bars have multiple stagger angles and multiple leads. The alloy melt can be subjected to different shear forces at different temperatures and times. The rheo diecasting test of AZ91D magnesium alloy was carried out. The semi-solid continuous casting and rolling technology has been gradually studied and applied since 2000. The rheologic continuous casting and rolling equipment for semi-solid steel materials has been successfully developed by Beijing University of science and technology. The rheologic rolling process of stainless steel (1Cr18Ni9Ti) and spring steel (60Si2Mn) has been systematically studied by electromagnetic stirring pulping. In the aspect of semi-solid magnesium alloy continuous casting and rolling, Beijing General Academy of nonferrous metals and Nanchang University have also carried out basic research. The parameters such as rolling force, rolling speed and rolling forming temperature have been studied by means of mechanical stirring. The ssrtm rheoforming process was proposed by Martinez and Flemings of the United States. Firstly, the alloy melt with low superheat was poured into the crucible, and then the alloy melt was stirred for a short time by a stirrer until it was under the liquidus When the temperature is below 2 ℃, the agitator is taken out. When the alloy melt is cooled to a predetermined solid fraction or temperature, the alloy melt is quickly poured into the pressure chamber of the die casting machine for rheological die casting. At present, we have developed relatively mature pulping equipment such as ssrtmstation, which can connect with die casting machine.

The HVSC Ube NRC 8000kN equipment developed by Yubu Co., Ltd. is a typical example of the combination of semi-solid rheo Diecasting and squeeze casting. The specific process is as follows: firstly, the alloy melt with low superheat is prepared, and a stainless steel mold with an inclined angle and upper and lower covers is set. When the alloy melt is poured into the mold, it is covered for heat preservation and air cooled to a predetermined semi-solid temperature. Then, the radial temperature field is adjusted by using the “skin effect” of high frequency heating, and the subsequent die casting and squeeze casting processes are carried out With the success of R & D, the process has been able to mass manufacture automobile related parts.

Kang Yonglin of Beijing University of science and technology developed a forced convection stirring device (FCR) to control the rotational speed, temperature and pouring temperature of the cylinder, and connected with the die casting process to realize the rheological die casting. Bai Yuelong et al. Studied the rheo diecasting technology of A356 aluminum alloy. The results show that the higher the injection specific pressure, injection speed and slurry temperature are conducive to the filling of semi-solid alloy melt; when the specific pressure is 15 ~ 25MPa, the forming temperature is 585 ~ 595 ℃, the injection punch speed and filling speed are 0 The results show that there is no obvious segregation and the microstructure is uniform and fine when the flow rate is 0.072 ~ 0.12m/s. The results show that higher injection specific pressure, slurry temperature and injection filling speed of slurry in ingate are beneficial to rheological filling. Wannasin et al. Carried out rheological die casting experiments on semi-solid slurry prepared by bubble stirring method of aluminum alloy melt. The best process parameters were obtained as follows: stirring in the slurry for about 10s by graphite bubble generator, and pouring directly into the pressure chamber for die casting when the solid fraction of semi-solid slurry was about 10%.

After studying the rheo diecasting process of aluminum alloy, sun Yongzhong thought that if the process was not controlled properly, the defects such as porosity would still occur under the rheo diecasting condition, and the matching of pouring temperature and injection speed was the main process parameter to control the porosity defects. When the pouring temperature and filling speed were low, the porosity defects decreased significantly. By studying the rheo diecasting of aluminum alloy, Wu Shusen thought that the direct rheo diecasting could be realized by directly installing the ultrasonic vibration device on the die casting equipment or preparing the semi-solid slurry in the ladle before pouring. The results show that the best filling temperature of A356 is 25 L. Zheng Peng et al. Simulated the porosity distribution and cavity filling process of A357 aluminum alloy through the simulation analysis of the rheological forming process, and carried out the experimental verification. The results show that with the increase of injection speed in a certain range, the alloy melt fills well, but if the injection speed is too high, it will produce turbulence, leading to inclusion or entrapment and other defects.

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