Abstract
The effect of various factors on pressure tightness of Cu-8%Sn alloy cylindrical castings solidified from one direction and from both directions was investigated. Darcy's law was applied to the measurement of pressure tightness, that is, a pressure of nitrogen was applied to one side of test pieces 1mm thick cut from the castings. The rate of leakage from the other side was proportional to the pressure, so pressure tightness could be represented by leakage coefficient calculated from Darcy's law. Microscopic observation showed that in the test pieces with a large leakage coefficient continuous microporosity dispersed at dendrite cell boudary, and in that with a small leakage coefficient most of porosity dispersed between secondary dendrite arms although some dispersed at the dendrite cell boundary. Leakage did not occur in the columnar grain zone, but it occcured in some of the equiaxed grain zone. There was no relation between leakage and the equiaxed grain size. Pressure tightness was related to G/Δθf where G and Δθf are temperature gradient and partial solidification time. Leakage occured when G/Δθf was below 0.019°C/cm⋅sec for the castings solidified from one direction and 0.027°C/cm⋅sec for those solidified from both directions. Leakage also occurred when porosity became more than 4%, irrespective of the solidification conditions.
