Effect of cooling rate on precipitation during homogenization cooling in an excess silicon AlMgSi alloy

The effect of cooling rate on the precipitation process during homogenization cooling in EN AW 6082 alloy was investigated. The extent of precipitation was estimated from electrical conductivity measurements on samples cooled to room temperature at different rates. There is considerable amount of precipitation during cooling even at 1000 degrees C h(-1). The precipitating phase is predominantly the equilibrium beta-Mg2Si. The hardness decreases while the extent of precipitation increases with decreasing cooling rate, particularly below 250 degrees C h(-1), where orthogonal beta'-Mg2Si platelets, in addition to beta-Mg2Si, form. A considerable fraction of the precipitation capacity is retained at the end of homogenization cooling at rates relevant to the industrial practice. Billets respond to a subsequent preheating cycle by beta as well as beta' precipitation. The metastable beta '' phase, on the other hand, is suppressed. Cooling rates between 1000 and 150 degrees C h(-1) have no impact on the dissolution behavior during billet preheating. All billets undergo complete dissolution at 475 degrees C implying that there may be possibilities of exploring lower preheating temperatures than what is normally used in the processing of EN AW 6082 billets. (C) 2012 Elsevier Inc. All rights reserved.