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

This paper reports investigations of 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 in the sprue-runner design and consequently in content and types of defects, were tested in 'as-cast' conditions. Defects consisted of gas and shrinkage pores as well as cold fills, dross and alumina skins. Casting defects were observed to reduce significantly the static and fatigue properties of the material. Whereas for the static characteristics the decrease was progressive with the porosity range, for the fatigue strength the decrease was most significant from the lowest to the middle porosity range. The batches were classified with regard to the porosity level, as the metallurgical defects were not detectable through X-ray examination. The content and size of metallurgical defects were observed to increase together with the porosity level. Scanning electron microscopy (SEM) observation of the fracture surfaces demonstrated the important role played by dross, alumina skins and, above all, cold fills on the fatigue fracture.