Effect of magnesium, iron (impurity) and solidification rates on the fracture toughness of Al7Si0.3Mg casting alloy

Abstract

A dynamic fracture toughness study on Al7Si0.3Mg casting alloy with cadmium trace addition and at varying magnesium and iron levels and solidification rates was carried out using an instrumented impact testing machine. With an increase in magnesium and iron contents, the load-time curves showed a change from sinusoidal to triangular shape. Even at a low iron level of 0.2%, with an increase in magnesium content from 0.32 to 0.65% the total energy absorbed to fracture E T dropped significantly by about 50%, demonstrating the enhancement of the embrittlement effect of magnesium on the alloy. At 0.32% Mg, an increase in iron content from 0.2 to 0.8% significantly decreased E T, the dynamic fracture toughness K d and the critical flaw size a cr. The embrittling effect on the alloy containing a higher level of iron (0.8%) is attributed to the presence of β phase (FeSiAl 5). A distinct brittle fracture seen on the fractured specimens and the ratio P max/ P gy of unity both further confirmed this embrittling effect. Generally, an increase in solidification rate produces higher E T, K d and a cr values, and teh effect was more pronounced at lower magnesium and iron levels than at higher levels.