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Joining of ZEK100 Mg alloy and Zn coated DP600 steel sheets is studied using refill friction stir spot welding (RFSSW). The RFSSW process involves a tool with independently moving sleeve and pin components, which rotate at a constant speed and penetrate into only the top sheet. Welds could be achieved, which exceeded 4.7 kN shear strength, using the following process parameters: 1800 rev min 21 tool speed, 3.0 s welding time and 1.5 mm of penetration into the upper ZEK100 sheet. Scanning electron (SEM) and transmission electron microscopy (TEM) are used to characterise the Mg/steel interface. It is revealed that a continuous layer of FeAl 2 particles accommodate bonding of the sheets, which appears to have originated from the galvanised coating on the DP600.

Defect-free dissimilar Al/zinc coated steel and Al/AlSi coated steel welds were successfully fabricated by refill friction stir spot welding. However, Al alloy and uncoated steel could not be welded under the same welding condition. Al-Zn eutectic layer formed at the Al/zinc coated steel interface showed non-uniformity in thickness and nanoscale intermetallic (IMC) produced was discontinuous. The bonding formation between the Al-Zn layer and the surrounding materials was attributed to a liquid/solid reaction mechanism. Bonding formation at Al alloy and AlSi coated steel interface was attributed to a solid/solid reaction mechanism, as the joining process did not involve with melting of base metals or AlSi coating materials. Kissing bond formed at the weld boundary acted as a crack initiation and propagation site, and the present study showed that weld strength of Al 5754/AlSi coated steel was greatly influenced by properties of original IMC layer.

In the present study, detailed microstructural characterisation of a friction-stir-welded joint between AA5059 alloy and high-density polyethylene was carried out using field-emission scanning and high-resolution-transmission electron microscopy analysis. The structural features indicate large numbers of macro-, micro-and nano-mechanical interlocks between the Al-fragments and melted/re-solidified polymer matrix at the stir zone of the joint, with a 30-nm thick semi-crystalline aluminium structure layer elevated in levels of O, and traces of C at the interface. An ultrafinegrained structure with an average cell size of <100 nm was formed for the embedded Al-alloy fragments in the polymer matrix, as a result of low-temperature severe plastic deformation during friction-stir welding process. The interfacial chemical reactions assisted by generation of nanoscale pores inside the metal surface at the interface and secondary Van der Waals bonding are suggested as the main joining mechanisms, leading to significant improvements in the mechanical properties.