The Influence of Marangoni Flows on Crack Growth in Cast Metals

In previous work with copper-based alloys, the authors showed that a necessary part of the hot-tearing process is the creation of a void in association with an inclusion and its subsequent growth to form the crack observed in the as-cast state. The work reported here is an examination of hot tearing in aluminum alloy A201 and various similar Al-based aerospace alloys to determine (1) if a similar process to that seen in copper alloys takes place; (2) the extent to which buoyancy forces influence the movement of solute-enriched liquid and so contribute to the development of voids which are subsequently observed as hot tears; and (3) the influence of the local variations in liquid/void surface tension arising due to local composition and temperature changes on interdendritic fluid flow--that is, the effects of Marangoni convection on void size and movement during solidification. Hot cracking of aluminum alloy A201 has been examined under standardized experimental conditions. In addition, experiments were conducted in which fluid flow in test castings was controlled by magnetic fields. The results from these various investigations are presented in this paper. Similar experiments are planned for reduced gravity aircraft parabolic flights. To assist in the planning and interpretation of the results, numerical modeling simulations have been developed for Al-Cu alloys.