WALL SLIP OF SEMI-SOLID A356 IN COUETTE RHEOMETERS

Wall slip of suspensions in a couette rheometer is caused by segregation of a thin layer of liquid phase adjacent to the rheometer cylinder and cup, respectively. This causes the bulk phase to slide along the walls, which means that the fluid flow velocities respective to the walls are not zero. This affects the evaluation of the rheological properties and results in apparent flow curves. Despite of the importance of understanding and controlling segregation effects, little research has been done on this subject area. Indeed in industrial casting, the die filling behaviour, and therefore the product quality, may depend on the segregation phenomena. The wall slip occurring while investigating the semi-solid aluminium alloy A356 in a couette rheometer was investigated in the present work, employing the Kiljanski method. The goal of the present work was to obtain a true flow curve of the material, eliminating the effects of wall slip. Employing the method of Kiljanski, the wall slip velocity was calculated and subtracted from the apparent velocity of the alloy. Hence, the true shear rate in the medium and the true flow curve were estimated. The wall slip effect is assumed to be negligible in a couette system with a grooved rotational rod. This assumption was set to test by comparing the flow curve calculated from a system with grooved rotational rod with the flow curve calculated with the Kiljanski method. Both methods result in identical flow curves which can be assumed to be the physically correct one.