Formation mechanism of pits on the surface of thin-wall die-casting magnesium alloy components

A notebook personal computer casing, made of AZ91D alloy, is manufactured by precision die casting. When the computer casing was kept for some time, it was found that there were some small pits on the surface of the casing. The formation mechanism of the pits was investigated by means of scanning electron microscope and fourier transform infrared spectrophotometry. It is concluded that MgO inclusions in the Mg–Al alloy are an important factor, which leads to the formation of small pits. MgO turns into Mg(OH)2 by absorbing water or steam. Because of density difference between MgO and Mg(OH)2, volume expansion takes place. A basic way of reducing pits is to cut down the content of MgO inclusions.