The Influence of Copper Addition on Crack Initiation and Propagation in an Al-Si-Mg Alloy During Cyclic Testing

The effect of copper (Cu) addition up to 3.2 wt.% on crack initiation and propagation in an Al-Si-Mg cast alloy was investigated using in-situ cyclic testing in the as-cast condition. A novel combination of digital image correlation, electron backscatter diffraction, and scanning electron microscopy was used to investigate crack initiation and propagation behaviour during in-situ cyclic testing. The results showed that Cu-rich intermetallic compounds with the addition of Cu up to 1.5 wt.% do not affect the fatigue behaviour of these alloys, and that crack propagation in these cases is trans-granular and trans-dendritic. However, increasing the concentration of the Cu retained in the primary α-Al matrix in solid solution and Cu-containing precipitates delayed crack propagation during cyclic testing. The results showed that strain accumulation was highest at the grain boundaries; however, the crack preferred to propagate along or across primary α-Al dendrites due to the relatively lower mechanical strength of the matrix compared to the eutectic and intermetallic phases. Moreover, the addition of Cu of more than 3.0 wt.% to Al-Si-Mg alloys changes the fatigue behaviour such that an almost static failure occurs, with limited crack propagation.