IDENTIFYING SOURCES OF GAS CAUSING POROSITY DEFECTS IN LOST FOAM ALUMINUM CASTINGS

摘要

During metal filling, existence of gaseous pyrolysis products complicates the understanding of the gas porosity defect formation in lost foam aluminum castings (LFAC). In this study, the sources causing gas porosity defects in the LFAC were identified through a series of experiments. First, real time X-Ray technology was used to visualize the formation of the gas porosity caused by foam decomposition products. Colors and morphologies of the gas pores were studied visually to set up a stereotype of the gas porosity caused by foam pyrolysis products. The color and morphology of the gas pores in other LFACs agreed with the proposed stereotype. This indicates that the gas porosity in LFAC is predominantly caused by foam pyrolysis gas. Second, formation of gas pores caused by air entrapment during the metal filling of a sand mold was also visualized using real time X-Ray. The color and morphology of air pores and the metal filling velocity in sand molds were compared with those of the lost foam mold. The results indicated that air entrapment in lost foam castings is very unlikely. In addition, the surface of the gas pores in the LFAC was studied with SEM (Scanning Electric Microscope) and EDX (Energy Disperse X-ray). Except for aluminum and/or magnesium oxides, coating debris was detected. This provides further evidence that the gas pores in the LFAC is predominantly pyrolysis product related. Results from this study will help to understand the formation of gas porosity defects in the LFAC and minimize the efforts to eliminate the defect.