New Sol-Gel Coatings to Improve Casting Quality

Sand casting is one of the oldest casting methods. The greatest advantage of
sand casting is that almost any metal can be poured in the sand and there is
almost no limit on the size, shape, or weight of the part. Sand casting provides
the most direct route from pattern to casting. Among the limitations involved in
sand casting is the need for fettling and machining in order to finish the castings,
especially castings having rough surfaces, veinings, or finnings, and other surface
defects. In this regard, this project deals with the understanding and reduction or
elimination of the occurrence of these mentioned defects on the castings made in
sand moulding materials by use of novel refractory coatings. The project involves the development of novel foundry coatings using the SOL-GEL Technology. The idea was to harness the possibilities offered by this technology to produce environmentally-friendly refractory foundry coatings. This process is a novel application for foundry coating development. In the approach for this research project, intensive efforts were made to understand casting technology, the behaviour (mechanical and thermal) of moulding materials (at ambient and elevated temperatures), foundry coating technology and purposes, mould-metal reactions, metallurgy, modelling and simulations of casting processes, and surface metrology of castings. In the experimental approach, several coating formulations were prepared both with nanoparticle refractory materials (TiO2, ZnO) and microparticle refractory materials (filter dust, zircon, china clay, olivine, aluminium silicates). However, the nanoparticle refractory materials proved incompatible with the sand grains for foundry coating production. Therefore, the bulk of the work was centred on the micro-sized refractory materials which showed positive results. The coatings were extensively characterized experimentally. The wet properties (density, oBaumé parameter, viscosity) and dry properties (penetration, coverage, layer thickness, moisture, permeability) as they apply to foundry industry, were measured and presented. The results show that the particle size, the particle size distribution, and the percentage solid content play a significant role in the rheology of the coatings. The percentage solid content is directly proportional to the density, oBaumé parameter, viscosity, coverage, layer thickness and inversely proportional to the penetration, moisture content and permeability. In this project, a new Strength Testing Machine (STM) for moulding materials was built and calibrated in-house for the determination of the mechanical properties of moulding materials. Coated moulding materials were tested for strengths (tensile, bending and compression) using the new STM, to determine the effect of the coatings on the strengths of the moulding materials. The results reveal that the coatings degrade the strengths in the tensile and bending modes, but not significantly, while the compression strength was increased using the coatings. Thermal degradation examinations were performed on cores coated with sol-gel containing coatings and those coated with the same coating without sol-gel component to understand the effect of the sol-gel component on the thermal stability. The observations made lead to the conclusion that lower amounts of sol-gel component will enhance the thermal stability of the moulding material. Numerical modelling and simulations of the casting processes were performed to understand and predict defect areas in the castings. The coating layer thickness was modelled for simulation purposes. The thermal behaviour of the moulding materials was also simulated. The simulation results show that the effect of the coating on the moulding materials is thermal insulation. The simulation results agree with the experimental results. In another context, the likely surface reactions of the moulding materials and the metal were predicted and these depend on the free energy of formation of the products, CO₂/CO ratio and the temperature. Many casting experiments were performed in both horizontally- and verticallyparted moulds using coated cold box and furan cores with uncoated cores as control. Many industrial tests were also performed. The cores were coated by dipping at different times in filter dust sol-gel coating with different solid contents, commercial coatings, and commercial coatings containing various amounts sol-gel component as additive. The coating application on moulds was by spraying. The surface quality of the castings shows that the filter dust sol-gel coating improved the surface quality of the castings and the results are comparable with the investigated commercial coatings. However, this coating formulation was dropped due to the fact that the filter dust is not a conventional material and it is not in continuous supply. The surface of the castings made with cores coated with the commercial coatings containing sol-gel component as an additive shows that better surface quality of castings were obtained with lower amount of sol-gel additive. In another context, the casting surface quality obtained by spraying the vertically-parted mould reveals that the coatings can enhance the surface quality and also influence the metallostatic pressure effects. Multilayer application by spraying proved more satisfactory. The thermal measurements during the casting experiments shows that the coatings have a thermally insulating effect on the cores, in concordance with the numerical results. The surface metrology applied on the surface of the castings shows that the castings made with coated cores have lower surface roughness than the castings made with uncoated cores. In this project, it was also shown that optical metrology with white light is applicable to the evaluation of the surface texture of castings with a medium magnification, say 5×. Finally, this project has successfully applied sol-gel technology in the development of foundry coatings and the results show positive potentials.