Microstructural features and final mechanical properties of the iron-modified Al-20Si-3Cu-1 Mg alloy product processed from atomized powder

Abstract

The potential piston alloy Al-20Si-5Fe-3Cu-1Mg has been experimentally extruded from rapidly solidified powder, and subsequently heat treated. The effects of adding iron to the alloy on the microstructural evolution during the solution and ageing treatment subsequent to extrusion have been examined. The study shows that iron-bearing intermetallic particles modify the recrystallization behaviour of the present alloy during solution treatment at 470 °C in a complex way, through blocking the migration of recrystallized grain boundaries from particle-depleted areas, and pinning subgrain boundaries in particle-rich areas, thus leading to a partially recrystallized duplex structure in the final product. The observed two-fold role of the intermetallic particles is a consequence of their inhomogeneity in distribution, which in turn results from the processing history of the powdered alloy. It is also observed that, in the presence of the intermetallic particles, the excessive coarsening of the silicon particles dispersed in the α-Al matrix (as occurs to the base alloy during the heat treatment) is lessened. The retained subgrain boundaries provide heterogeneous nucleation sites for precipitation occurring during ageing. Most of the precipitates are characterized by being associated with iron, and the precipitating behaviour of copper and magnesium in the present alloy with the iron addition is accordingly altered. The resultant tensile properties of the alloy at room and elevated temperatures have been assessed, with reference to those of the base AI-Si-Cu-Mg alloy. The results indicate that the present alloy with the iron addition has a fairly high hot strength up to a temperature of 300 °C, which offers an important improvement ensuring its reliable application in automotive engines.