Control of casting structure through open surface profits

Processy litʹâ, 2021, Tom 146, №4, p.30-39

Abstract

Received 26.10.2021

UDK 621.744: 539.216

On the model and metal systems, the possibility of controlling the structure of the casting through the open surface of the riser with the use of a vibrating crystallizer immersed in the near-surface layers of the melt in a mold with low thermophysical characteristics has been studied. Physical modeling carried out on an alloy of organic substances: camphene – 90 %, trіcycline – 10 %, made it possible to observe and fix the nucleation, growth of crystals on a stationary steel mold, immersed 8-10 mm under the mirror of the melt. As well as the nucleation, growth and separation of crystals from a crystallizer vibrating with a frequency of 50 Hz and an amplitude of 0.5 mm, and their settling to the bottom of the mold under the action of gravity with the formation of a "cone of deposition", due to which the appropriate conditions are created for the formation of a fine-crystalline structure at solidification. The verification of the regularities established during physical modeling was carried out on aluminum grade A7 using similar vibration modes and a casting mold made of vologran, the thermal conductivity of which is 0,16 W/(m·K), the heat capacity is 1,185·103 J/(kg·K). Two series of experiments carried out under conditions of natural cooling from a temperature of 670 0C in air with a stationary and vibrating crystallizer, showed that the treatment of the solidifying metal with a vibrating crystallizer promotes a 16-fold decrease in the average size of macrograins in comparison with castings obtained by volumetric solidification in the Vologran mold. in the absence of a crystallizer. The presence of an immovable crystallizer in the melt leads to a decrease in the density of the lower part of the casting by 0.42 %, while a vibrating one leads to an increase in its density by 1.2 %. The use of a vibrating mold provides an increase in strength characteristics by a factor of 1.87 both when receiving castings in a Vologran mold without a mold, and when it is stationary. In this case, the plasticity of the samples increases by more than 3 times. The results of experimental studies are explained from the standpoint of the theory of nucleation and growth of crystals.