Effect of Iron on the Microstructure and Mechanical Properties of an Al-7%Si-0.4%Mg Casting Alloy

Effect of iron (Fe) content on the microstructure and mechanical properties of aluminum alloys has been investigated in primary A356 and secondary 356 lost foam castings. Increasing Fe content from 0.13% (A356) to 0.47% (356) significantly increases the amount and size of Fe-rich intermetallic phases, and in particular the porosity in the microstructure. The average area percent and size (length) of Fe-rich intermetallics changes from about 0.5% and 6μm in A356 to 2% and 25μm in 356 alloy. The average area percent and maximum size of porosity also increases from about 0.4% and 420μm to 1.5% and 600μm, respectively. As a result, tensile ductility decreases about 60% and ultimate tensile strength declines about 8%. Lower fatigue strength was also experienced in the secondary 356 alloy. Low cycle fatigue (LCF) strength decreased from 187MPa in A356 to 159MPa in 356 and high cycle fatigue (HCF) strength also declined slightly from 68MPa to 64MPa.

The influence of Fe content on the mechanical properties, especially fatigue performance, of aluminum castings can be interpreted using Weibull analysis of microstructural discontinuities such as porosity and oxide films. For castings containing porosity and oxide film sizes > 100μm, a reliable prediction of fatigue properties can be readily achieved using a simple liner elastic fracture mechanics (LEFM) model and probabilistic estimates of pore and oxide film sizes in the castings.