Quantitative characterisation of correlations between casting defects and mechanical strength of Al-7Si-Mg alloy castings

An Omniment3 Instrument image analyser has been used for quantitative characterisation of casting defects, The concepts of the casting defect density and area fraction were proposed for the first time to quantify the effect of casting defects on the strength properties of castings. To complement the quantitative metallography, a conventional three point bend strength, i.e. ultimate bend strength (UBS), test was also conducted on specimens using three different runners. SEM was then applied for microstructure observation on the fracture section of the three point bend test specimens. In order to reveal the correlations between strength properties and casting defect density/area fraction in the specimens, the correlation coefficients are defined. Based on the quantitative characterisation of casting defects and strength test results, it is confirmed that there exist strong correlations between the strength properties of castings and the casting defect porosity. A correlation coefficient with a value close to 1 indicates that casting defect distribution across the castings is more uniform with fewer concentrated defects and higher correlation coefficients characterise more reliable castings. However, it is stressed that the final strength properties of the castings are not only manifested by the density and area fraction of casting defects but also influenced by the defect distribution features and the defect morphology. Concentrated casting distribution of defects such as shrinkage pores and casting defects with a sharp shape will generate stress concentration and damage the strength properties. Among the castings made using three different runner types, it was found that a casting defect distribution present in a casting plate made using a vortex flow runner (VR) was more uniform than those in casting plates made using rectangular runner (RR) and triangular runners (TR).