The Effect of Grain Structure on Casting Durability Assessment in Al-Si Alloys

Casting durability may be assessed by several methods. In industrial powertrain castings engineering design is linked to the assessment of casting properties in particular high cycle fatigue performance. In the past porosity has been shown to be the most deleterious microstructural constituent in Al-Si cast alloys. Porosity is nucleated by oxide biflims and evolves during solidification due to segregated hydrogen gas and/or liquid feeding difficulties in the mushy zone during solidification. Porosity and oxide films have been reported to control casting durability as assessed through the fatigue staircase and calculated −3σ plots.The authors will show in this work that grain structure can also play a major role in terms of controlling fatigue life. Specifically the presence of columnar grains in non grain refined casting structures can lead to low and unpredictable fatigue lives. The mechanisms of fatigue failure due to grain structure are reviewed and the use of grain refiner (in-mold process) to improve fatigue performance and counteract undesirable structure are discussed.