DETERMINATION OF HEAT TRANSFER COEFFICIENT BETWEEN ALUMINUM CASTING AND GRAPHITE MOLD

Abstract

Graphite molds can be used to produce titanium, nickel, copper, aluminum and zinc castings. Using of the graphite molds provides a high cooling rate. Moreover no die coatings and lubricants are required. To get appropriate results of the casting process simulation in graphite molds it is necessary to know thermophysical properties of materials and boundary conditions such as interface heat transfer coefficients, but they are still unknown. The most important properties are heat transfer coefficient between casting and mold, and between mold parts and between mold and environment. The heat transfer coefficient h (iHTC – interface Heat Transfer Coefficient) was determined between cylindrical aluminium (99,99 % Al) casting and mold made of low-ash graphite. The mold was produced by milling graphite blocks on the CNC machine. The heat transfer coefficient was determined by minimizing the error function, representing the difference between the experimental and obtained by simulation temperature in the mold during pouring, solidification and cooling of the casting. The dependences of the iHTC between aluminium and graphite on the temperature of the casting surface and time elapsed from the start of pouring of the casting. Determined values of the heat transfer coefficient at metal temperatures 1000, 660, 619 and 190 °С are 1100, 4700, 700 and 100 W/(m2 ·К) respectively. Therefore, with decreasing of the melt temperature from 1000 °C (pouring temperature) to 660 °C (aluminium melting point), the heat transfer coefficient increases. After casting surface forming and lowering its temperature, the heat transfer coefficient decreases. Decrease of the iHTC at temperatures below 660 °C (lower the melting point) is associated primarily with the appearance of an air gap between the mold surface and casting surface as a result of linear shrinkage. The iHTC between the mold parts (graphite– graphite) is constant 1000 W/(m2 ·К). The heat transfer coefficient between graphite and the air environment is 12 W/(m2 ·К) at the mold surface temperature up to 600 °C.