Effect of inclusions on the tensile properties of Al-7% Si-0.35% Mg (A356.2) aluminium casting alloy

The present study was performed on an A356.2 alloy. Two types of initial materials were used, i.e. fresh and recycled. A total of 13 operations representing those normally applied in aluminium foundries were simulated under dry atmospheric conditions (humidity similar to 15%-20%). The molten metal was cast into test bars which were T6 tempered prior to tensile testing. The results show that holding the liquid metal for a long time, i.e. 72 h at 735 degrees C reads to sedimentation of most inclusions towards the bottom of the melting crucible. However, a change in the surrounding humidity may cause absorption of hydrogen and, hence, a large amount of porosity. Degassing using dry argon injected into the liquid metal th rough a rotary impeller (speed similar to 160 r.p.m) appears to be the best technique for inclusion removal. The efficiency of this process is significantly improved when it is coupled with filtration using ceramic foam filters (10 and 20 p.p.i). A linear relationship between alloy ductility and logarithm of percentage inclusions has been established. Owing to decohesion between the inclusions/oxide films and the surrounding matrix, cracks are easily initiated at their interfaces, leading to unpredicted failure. (C) 1998 Chapman & Hall.