EFFECT OF NATURAL AGING AND PREAGING ON SUBSEQUENT PRECIPITATION PROCESS OF AN AL-MG-SI ALLOY WITH HIGH EXCESS SILICON

The mechanism of the influence of natural aging (that occur's between solution heat treatment and artificial aging) and preaging immediately after quenching on the subsequent precipitation process in an Al-0.95 Mg-1.55Si-0.3 Mn-0.1 Zr alloy was studied using differential scanning calorimetric analyses and microhardness measurements. The results show that natural aging has a distinct influence on the subsequent precipi tation process. It is proposed that the formation of stable silicon cl usters during natural aging is the major reason for such an effect. Pr eaging at 120 or 150 degrees C for 5 min is effective in suppressing t he clustering process, and thus inhibiting the deleterious effect of n atural aging. The decrease in hardness dire to the natural aging in th e alloy studied is not as marked as that reported for balanced Al-Mg2S i alloys. II is proposed that a high excess silicon content reduces th e detrimental effect of natural aging. (C) 1997 The Institute of Mater ials.