Microstructure analysis of silicon sol investment casting shell

High quality precision castings with smooth surface and clear edges and corners need to be manufactured by high quality investment casting shell.In the process of investment casting shell making, silica sol has become a common water-based binder in investment casting because it does not need chemical hardening, is easy to mix into high powder liquid ratio of high quality coating, and the shell made of high temperature strength, high temperature deformation resistance.

1. Brief introduction of technical terms

Some technical terms are understood differently in the field of chemistry than in the field of foundry.For the convenience of discussion, terms in the field of colloidal chemistry are used uniformly, and a narrow sense of understanding is adopted.

1.1 colloid

Colloid is not a special substance, nor is it the nature of matter, but a special state of matter existence.The colloidal system is characterized by a certain size of dispersed particles, and there is an obvious physical interface between the colloidal particles and the dispersing medium, so the colloidal system is a two-phase or multi-phase heterogeneous dispersion system.

1.2 sol

According to the definition of colloid, as long as the particle sizes of different aggregated dispersed phases are between 1 nm and 100 nm, the colloid system can be formed in different dispersive media.Except that the dispersing phase and the dispersing medium are both gases and cannot form the colloid system, the other 8 dispersing systems can form the colloid system.When the dispersion medium is solid, it is called solid sol;When the dispersion medium is a gas, it is called an aerosol;When the dispersing medium is liquid, it is called liquid sol or sol.

1.3 gel

Gels are also called gelatins.Colloid particles in the sol under certain conditions can be connected to each other, forming a spatial network structure, the structure gap is full of liquid as a dispersion medium, no matter how much liquid, will lose the liquidity of the dispersion system is called gel.

1.4 Capillarity

The continuous dispersing phase forms the solid skeleton of the gel, and the continuous dispersing medium forms the fluid part of the gel. The particle size of the gel makes the gel have the capillary microstructure.Capillary phenomenon refers to the phenomenon that liquid rises or falls in a vertical tube due to the difference of adhesion between the liquid and the tube wall and the cohesion of the liquid itself.

1.5 gel

The process in which a certain concentration of sol forms gel under appropriate conditions is called gelation.During the gelation process, the silica sol formed capillary network structure by hydrogen bond at the early stage of drying.In the late drying period, the hydroxy group dehydrates, making the hydrogen bond form silicon-oxygen bond and the colloidal particles grow in three dimensions.

2. The formation process of microstructure of investment casting shell

Silicosol coating is a mixture of silica sol, refractory powder and a small amount of functional auxiliaries.In essence, the process of shell making by silica sol investment casting is the gelation of silica sol coating, and then the gel changes from wet gel to dry gel.During the drying process, a series of physical and chemical changes take place in the internal microstructure of the shell due to the action of capillary force. The process can be divided into four stages.

2.1 Sol-gel transition stage

After the silica sol coating is infiltrated into the mold, due to the evaporation of water on the surface of the coating, there will be a humidity difference between the surface and the interior, and the interior moisture will continue to diffuse to the surface in the form of liquid water driven by the humidity difference.At the same time, the colloidal particles of the silica sol are continuously precipitated out in the colloidal dispersion state. The precipitated silica colloidal particles are bonded by hydrogen bonds to form a skeleton and form a connected capillary structure. Finally, the gel completely loses fluidity and becomes a gel, and the gel at this time is a wet gel.

2.2 Volume shrinkage and drying stage

Silica sol silica colloidal particles form a solid skeleton of the gel, which is filled with water as a dispersing medium.There are four forms of water in silica sol gel: the first is the water directly adsorbed on silica colloidal particles by hydrogen bonding, which is in the form of hydroxyl;The second is the polarized water layer adsorbed by hydrophilic groups around the colloidal particles, which is in the form of hydrated hydroxyl groups.The third is the water wrapped in micropores by silica colloidal network structure, which is free water form, but its movement is blocked.The fourth is water in the gel gap, which is free water.The silica sol coating gel starts with a very soft framework, and the capillary tension in the system is small.With the process of water evaporation and gel skeleton shrinkage, the mechanical strength of silica skeleton increases gradually.The structure of the gel is fixed when the rigid strength of the silica aggregates in the skeleton prevents the skeleton from contracting any further.

2.3 Skeleton aging stage

When the drying reaches the critical point, the rate of water evaporation begins to decrease further, and the shrinkage of the solid skeleton has stopped.At this stage, the liquid transfer in the hole is still mainly carried out by the flow mode, accompanied by the diffusion of steam.Because most of the evaporation of the liquid is still on the surface of the gel, that is, the surface of the shell.

2.4 Table dry and dry stage

The drying process is not uniform at the micro level, and lumpy thin layers of liquid are distributed unevenly on the inner wall of the pores during the skeleton aging stage.With the process of water evaporation, the distance between the outer surface and the evaporation surface in the hole becomes farther and farther, and the driving force of evaporation becomes smaller and smaller, so that the flow of liquid gradually disappears, and the distribution of liquid on the outer surface slowly presents a discontinuous state. At this time, the drying enters the surface dry stage.

3. Verification of microstructural model of molded shell

3.1 Comparison of characteristics between silica sol shell and ethyl silicate shell

The capillary tension is proportional to the surface tension of the liquid.The liquid in the pores of the silica sol shell is mainly water.The liquid in the capillary pore of the ethyl silicate shell is a mixture of alcohol and water.The surface tension of the mixture of alcohol and water is smaller than that of water. Correspondingly, the capillary tension P is smaller, and the pore size of the gel is larger.The larger pore size makes the ethyl silicate shell have better air permeability than the silica sol shell.

3.2 FS Ⅰ and FS Ⅲ quick-drying type colloidal silica shell

FS Ⅰ type quick dry silica sol is a aluminium modified silica sol, gelation time short than unmodified silica sol.In the process of gel drying, due to the high supersaturation of silica colloidal particles, a large number of colloidal cores can be produced. Because the modified colloidal cores are not easy to grow up, they can form a gel with larger than the surface energy. The gel framework is compact, so the density of the shell is better.FS Ⅲ type quick dry silicon sol is added in the FS Ⅰ type quick dry silicon sol organic high polymer.The addition of high polymer greatly reduces the probability of shell cracking or cracking.

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Post time: Mar-26-2021