Precipitation hardening of A356 Al alloy produced by gas induced semi-solid process

Aged hardening of semisolid cast A356 Al alloy produced by gas induced semi-solid (GISS) process was studied. It was found that maximum hardness and tensile strength could be achieved from specimens aged at 165 °C for 18 h of which the average maximum hardness, the average ultimate tensile strength and the average percent elongation were 96.4 HRE, 312 MPa and 7.6%, respectively. The higher aging temperature of 195 °C for 3 h led to a slightly lower average tensile strength of 305 MPa together with a higher average elongation of 9.8%. The strain hardening exponent of specimens aged at both sets of conditions was lower than that of the as-cast specimen as well as the as-cast specimen aged at 225 °C for 15 min. The mechanical properties of the alloys in this study were comparable to those of typical thixoformed products. β″ phase was mainly responsible to the strengthening of the peak aged alloy. Elongated precipitates were formed in the specimen after prolonged aging at 195 °C for 16 h. The activation energy for the precipitation hardening process of the alloy derived in this research was 128,717 J/mol.