Static and fatigue strength of a die cast aluminium alloy under different feeding conditions

The paper investigates the influence of porosity and casting defects on the static and constant-amplitude fatigue strength of a die cast aluminium alloy. Three batches of specimens, differing for the sprue-runner design and consequently for content and type of defects, are tested in “as cast” conditions. Defects consist in gas and shrinkage pores as well as cold fills, dross and alumina skins. Casting defects are observed to significantly lower the static and fatigue properties of the material. While for the static characteristics the decrease is progressive with the porosity range, for the fatigue strength the decrease is most significant from the lowest to the middle porosity range.The batches are classified with regards to the porosity level, as the metallurgical defects are not detectable a priori through X-ray examination. However, content and size of metallurgical defects are observed to increase together with the porosity level. SEM observation of the fracture surfaces proved the important role played by dross, alumina skins and, above all, cold fills on the fatigue fracture