Properties and Interface Characteristics of Sodium Silicate Investment Shell Hardened Through Micro-droplet Spreading of Aluminum Potassium Sulfate Solution

Sodium silicate shell for investment casting is widely used in parts production. However, the key problems with this technique are that the shell has a low green strength, fired strength, and high residual strength in the shell; as well as environmental pollution resulting from toxic gas containing ammonia from the evaporation of ammonia liquor used as a hardener during the hardening process. The former is related to uncontrollable hardening reaction process, while the latter is related to hardener characteristics. In this work, the microfluidic technique was employed to precisely control of the hardening reaction of the shells, and aluminum potassium sulfate solution served as replacement for ammonia liquor. The green, fired, and residual strength of the shell specimens and their high-temperature self-load deformation were investigated. It is found that the hardening reaction can be effectively controlled, and the hardening characteristics of the shells were obviously improved. The bending strength of the shells increases initially and then decreases at the time of micro-droplet spreading. The specimens hardened for 8 min by micro-droplet method reached the highest green strength level of 37.48 MPa, a fired strength of 10.07 MPa, and a lowest high-temperature self-load deformation value of 0.18%, about 150%, 65% higher, and 53% lower than by the current immersion method, respectively. Moreover, the fracture surfaces of the specimens were observed using scanning electron microscopy (SEM). The results reveal that the number of cracks in the sodium silicate gel film in the shell decreased significantly. The cracking tendency caused by shrinkage stress during hardening was reduced. This is due to the accurate reactant flow control provided a large number of microreactors for sodium silicate hardening. The hardener micro-droplets with an excellent monodispersity dispersed in a small volume on the surface of a continuous phase of water glass film, the volume shrinkage caused by micro-hardening can easily be compensated by the continuous phase of the liquid sodium silicate in the adjacent region and the cracking stress is partially or completely relaxed. The shrinkage trend in the process of dehydration polymerization of sodium silicate decreased and the bending strength of shell specimen improved. Microfluidic technique provides a powerful means for accurate delivery of reactant and control of gel structure during shell hardening.