Pyrolysis defects in aluminum lost foam castings

摘要

Casting defects related to liquid pyrolysis products from the pattern can occur in aluminum lost foam castings. Often, their formation is most severe when the metal velocity during mold filling is greater than some critical value. Consequently, inadequate time is available for transport of liquid polystyrene products into the refractory coating. The results of a number of tests confirm the relationship between metal velocity and various pyrolysis- related defects, such as internal porosity, surface defects, folds, and blisters. Their severity is related to the model for mold filling in the aluminum lost foam casting process that has been developed and reported in the last several years. Both metal velocity and the morphology of the metal front during filling are related to defect formation. Auger and metallographic analyses were used to help characterize the pyrolysis-related defects in aluminum castings. These results confirm the presence of a two-part interface between the defect and 356 aluminum alloy; one part is an aluminum-magnesium oxide and the second part is a carbon-based residue from the pyrolysis products.