Capitalizing on the potential for Mg alloys to reduce vehicle weight, a multi-national Mg-intensive front end research and development (MFERD) initiative has been established to identify and address the processing, joining and corrosion protection challenges that need to be overcome to support the fabrication of a complete Mg-intensive automotive front end assembly. The initiative identified high pressure die-cast (HPDC) AM60B and linear friction stir welding (FSW) as the preferred structural casting and joining technology, respectively, to be used for the fabrication of demonstrator sub-assemblies for component-level testing. A key fundamental aspect underpinning the successful implementation friction stir welded HPDC AM60B joints is a well-developed understanding of the extent of atmospheric corrosion damage expected to occur across the joint when boldly-exposed. The susceptibility of HPDC AM60B to localized corrosion arises from the heterogeneous microstructure and the formation of galvanic couples between the Mg matrix and the secondary intermetallic phases. Friction stir welding has been reported to significantly alter this heterogeneous microstructure across the welded joint. However definitive links between microstructure and localized corrosion susceptibility remain elusive.
Electrochemical polarization and scanning vibrating probe (SVP) measurements were made to establish links between the localized corrosion susceptibility observed across the FSW joint and the resultant variations in microstructure, as revealed by scanning electron microscopy (SEM). Microstructural parameters of interest included microhardness, grain size and the size and spacial distribution of Al-Mn intermetallic particles and the secondary Mg17Al12(β) phase. The corrosive environment used was a near-neutral 5 wt.% NaCl solution. Complementary surface analyses by X-ray photoelectron spectroscopy (XPS) of the starting surface films were made to delineate the role played by the native oxide film on the subsequent localized corrosion susceptibility. The findings based on the bulk immersion testing were subsequently validated by continuous salt fog atmospheric corrosion testing.
Of the microstructural parameters studied, only the grain size and spacial distribution of the secondary Mg17Al12 (β) phase were significantly affected across the friction stir welded joint. The stir zone exhibited a finer grain size relative to the unaffected base material, but did not exhibit a secondary Mg17Al12 (β) phase network. The combined set of electrochemical measurements (open-circuit potential, potentiodynamic polarization and SVP) consistently revealed that the stir zone was more susceptible to localized corrosion than the unaffected base material. SVP measurements on both the original topography and a flat-polished surface demonstrated that the stir zone striations had little effect on this tendency (see Figure 1). The findings thus far point to a combined alloy chemistry and structure link (centering on the fate of the secondary Mg17Al12 (β) phase network) between the localized corrosion susceptibility observed across the friction stir welded joint and the resultant variations in microstructure.