Microstructure-property development in friction stir welds of Al-Mg alloys

This work focuses on microstructural development and the structure-property relations in 5xxx-series (Al-Mg) friction stir welds, highlighting the influence of basemetal microstructural features (e.g. grain and intermetallic particle structures), and the role of the basemetal processing route on the weld microstructure. Qualitative and quantitative microstructural investigations using optical, scanning and transmission electron microscopy were performed on 5251-H34, 5754-O, and 5083-H116 welds. It was further confirmed that the spatial distribution of the constituent intermetallic particles in the basemetal influenced the microstructural features associated with the welds (e.g. the formation of the onion rings structure). The starting temper did not influence the spatial hardness distribution or the microstructure in the TMAZ. However, the microstructural heterogeneities associated with the onion rings structure were observed to create abrupt increases in the hardness, compared to a roughly uniform TMAZ hardness in the welds that did not show a clear onion rings structure. In general, grain boundary strengthening is the main strengthening mechanism, although an additional contribution from Orowan strengthening can be found.