An In Situ Study of Short Crack Initiation and Propagation During Fatigue Testing of a Hot Isostatically Pressed Al-7%Si-0.5%Mg (A357-T6) Alloy Specimen.

A hot isostatically pressed specimen of the A357 alloy in T6 condition has been tested for fatigue performance in situ. During testing, multiple small cracks were observed during the first cycle, both in proximity to and far from the stress concentration. These cracks have competed to form a propagating crack, forming multiple crack paths initially. Once the propagating crack has been established, it has chosen paths from multiple cracks that have opened around the tip to grow further. All small cracks observed to open have been attributed to bifilms, i.e., liquid metal damage. It is imperative to develop processes that minimize liquid metal damage to enhance the fatigue performance of aluminum alloy castings.