A notebook (NB) computer component was manufactured from AZ91D Mg alloy by a die-casting process. After chemical conversion treatment, a discoloration was noted on the component surface. The source of this discoloration has been studied in detail by scanning electron microscopy, energy dispersive spectroscopy, and spark atomic absorption spectroscopy. The corrosion resistance was also measured by potentiodynamic polarization, hydrogen evolution and salt spray testing. The formation mechanism for the discoloration which was caused by the residue left behind by excess mold release agent sprayed during the die-casting was discussed in detail. After chemical conversion treatment, the residual-baked mold release agent was apparent on the component surface as “white ash.” Consequently, it degraded seriously both the appearance and the corrosion resistance of the manufactured component.

In this paper, microstructures and properties of die casting components with various thicknesses made of AZ91D alloy have been investigated by means of scanning electron microscope (SEM), transmission electron microscope (TEM), high-resolution transmission electron microscope (HRTEM), etc. It is concluded that mechanical properties of the die casting components mainly depend on grain size of α-Mg phase. At the same time, however, the voids formed by entrapping air during die casting process have a remarkable detrimental effect on the mechanical properties. To what extent the effect is caused is determined by quantity, size and shape of the voids, which are related to the thicknesses and position of the component in die. Nanometer particles found on the wall of the voids are magnesium oxide compounds, which are formed when oxygen from the entrapped air reacts with magnesium during die casting process. The compounds have no obvious effect on the mechanical properties even though there is possibly volume change present to form other compounds in the course of the components’ being used.

By means of surface mechanical attrition treatment (SMAT), a nanocrystalline (NC) surface layer about 40 μm was synthesized on an AZ91D Mg alloy. The microstructural features of the surface layer produced by SMAT were systematically characterized by optical microscopy (OM) observations, X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) investigations, and hardness measurement was also carried out in order to examine the hardness variation along the depth. Experimental results show that the microstructure is inhomogeneous along the depth. In the region from top surface to about 10 μm in depth, the grain size increases from about 50 nm to 100 nm. In the adjacent region of about 10–40 μm in depth, the grain size increases from about 100 nm to 400 nm. The grain refinement can be attributed to the activity of dislocation and occurrence of dynamic recrystallization. Moreover, emergence of stacking faults in the grain interior is a novel discovery. After the SMAT, the micro-hardness of the surface layer was enhanced significantly compared with that of the original sample.

A component for a mobile phone, made of AZ91D alloy, is manufactured by die cast. After chemical conversion treatment, it was found that there were some bright spots on the component surface. The formation mechanism of the bright spots was studied by means of SEM and metallgraphic microscope. The corrosion resistance of coatings was measured by potentiodynamic polarization test. The reason of bright spots formation is that the coarse primary α is dissolved into a dent during acid activation process because of little netted β and little aluminum. Besides, the coatings on bright spots are thin, the dimensions of primary α grains are large, and the surfaces are smooth relatively, so they are prone to reflecting light and become bright spots.