The effect of solution heat treatment and quenching rates on mechanical properties and microstructures in AlSiMg foundry alloys

Four common AlSiMg foundry alloys have been solution heat treated at 813 K, quenched, and immediately aged at 423 K for up to 240 minutes. The mechani cal properties are found to be related to the amount of Mg and Si in the al loys. A high strength is obtained after only 60 minutes of solution heat tr eatment, indicating that the solid solution is rapidly saturated on ME and Si. The ductility is very much related to the amount of silicon present and the refinement of the silicon crystals within the eutectic areas, since si licon crystals are observed to crack when load. is applied. Thus, a well-mo dified structure is the best way to obtain high ductility. Reduced quenchin g rates after solution heat treatment lead to a lower strength, since a low er number of hardening beta'-Mg2Si precipitates are formed. The ductility o f alloys with 0.6 wt pet Mg is increased with a reduced quenching rate. A m ore ductile matrix corresponding to the lower amount of hardening precipita tes can explain this. Alloys with 0.2 wt pet Mg remain relatively unchanged . A hypothesis that may explain this phenomenon is the precipitation of bri ttle silicon or formation of coarse Mg2Si within the dendrites.