B. Shalchi Amirkhiz

Microstructures and properties of Mg alloy/DP600 steel dissimilar refill friction stir spot welds

Abstract 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− 1 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 FeAl2 particles accommodate bonding of the sheets, which appears to have originated from the galvanised coating on the DP600.

Bonding mechanism and interface characterisation during dissimilar friction stir welding of an aluminium/polymer bi-material joint

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 ultrafine-grained 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 nano-scale 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.

Role of interfacial reaction on the mechanical performance of Al/steel dissimilar refill friction stir spot welds

ABSTRACT Al/steel welds were successfully fabricated by refill friction stir spot welding. The welding parameters were optimised based on the clamping ring temperature and weld strength. 85.7% of welds achieve a strength which exceeds the American Welding Society requirement when the clamping ring temperature ranges from 230 to 265°C. Cracks are formed under the pin and sleeve in the Al substrate at the Al/steel interface, which are associated with the tool sleeve plunging period and attributed to the α-Al + Al–Zn eutectic structure. The interdiffusion between Al and Zn at the steel surface produced an Al–Zn eutectic structure layer at the Al/steel interface, while part of the zinc coating materials is squeezed out of the sleeve periphery, leading to a brazing effect which contributes to weld strength. Nanoscale discontinuous Fe4Al13 and FeAl intermetallics form as a layer localised at the Al–Zn layer/steel substrate interface.

Improvement of Superplasticity in High-Mg Aluminum Alloys by Sacrifice of Some Room Temperature Formability

The mechanical properties of fully annealed Al-4.6 wt pct Mg alloys with different levels of Mn and Fe have been characterized at room and superplastic forming (SPF) temperatures. The effects of Mn and Fe on the intermetallic phase, grain structure, and cavitation were investigated and correlated to the formability at different temperatures. Although both Mn and Fe contribute to the formation of Al6(Mn,Fe) phase, which refines the grain structure by particle-stimulated nucleation and Zener pinning, their effects are different. An increasing Mn reduces the room temperature formability due to the increasing number of intermetallic particles, but significantly improves the superplasticity by fine grain size-induced grain boundary sliding. Meanwhile, the Fe makes the constituent particles very coarse, resulting in reduced formability at all temperatures due to extensive cavitation. A combination of high Mn and low Fe is therefore beneficial to SPF, while low levels of both elements are good for cold forming. Consequently, the superplasticity of high-Mg aluminum alloys can be significantly improved by modifying the chemical composition with sacrifice of some room temperature formability.