A Study on Factors Determining Tensile Properties of Ni-Al Bronze Casting

摘要

The effect of surface roughness and microstructural factors on tensile properties of the Ni-Al bronze casting was studied to understand mechanical properties determining factors. Surface roughness of the tensile test specimen was varied from 0.1 to 2.0 mum in Ra by changing machining conditions. Microstructure of the casting was also varied to obtain different level of strengthening phase content by changing pouring temperatures. Tensile fracture elongation of the Ni-Al bronze casting decreased as much as 8 percent from 25 percent to 17 percent with increasing surface roughness from 0.1 to 2 mum in Ra value, whereas tensile and yield strength remained constant with the variation of surface roughness. Dependency of elongation on surface roughness attributed to a tensile fracture mechanism of the alloys whose fracture initiated at the very surface of the casting followed by a subsequent brittle propagation without ductile necking behavior. Tensile elongation was also affected by a microstructural factor, i.e., porosity which might be a potential initiation site of tensile fracture. Decrease in pouring temperatures resulted in an increase in porosity content inducing decrease in tensile elongation. Increase in pouring temperatures also caused increase in tensile strength due to an increase in the strengthening constituents (Fe, Ni)Al intermetallic kappa. Increase in kappa phase with increase in pouring temperature was attributed to higher cooling rate. Based on this study it could be concluded that the tensile fracture elongation was a function of surface roughness and porosity on specimen, and tensile strength was a function of microstructural feature such as kappa phase fraction. The tensile properties would be improved by decrease of surface roughness and porosity as well as increase in kappa phase.