Materials Engineering Department
BS in Materials Engineering
Segregation of alloying elements during casting of forging billets may persist to the final forged component leading to unacceptable surface appearance. Determination of the elements or compounds that segregate is the essential first step to solving this problem. Differential Scanning Calorimetry (DSC) was used to determine a composition profile of the macrosegregation occurring at the subsurface of an as-cast 7050 aluminum alloy billet. Data collection was carried out by separating 10 to 20 mg samples from the outer centimeter along the radius of the billet, as well as samples from the bulk interior of the billet for comparison with the nominal alloy composition. DSC analysis was performed using platinum capsules on the Exstar DSC6000, from 20°C to 600°C at a rate of 5°C/min over the temperature range of interest. DSC scans showed heat flow peaks for incipient melting and crystallization transformations for Al2CuMg (S-phase). Size comparison of DSC peaks provided data about the relative masses at each distance from the surface. The partial heat of fusion of samples of several depth profiles was used to determine relative mass fractions as compared to a bulk sample. A sharp increase in S-phase concentration was observed near the surface and a drastic decrease at approximately 3 mm from the surface, followed by a return to bulk alloy values after 5 mm due to shrinkage-induced flow in the casting process.