Abstract
Friction stir spot welding (FSSW) is a solid-state welding process, wherein the properties of a weld joint are influenced by the state of friction and localised thermodynamic conditions at the tool-workpiece interface. An issue well-known about FSSW joints is their lack of reliability since they abruptly delaminate at the weld-faying interface (WFI). This study explores the origins of the delamination of multiple lap welded aluminium alloy (AA 5754-H111) sheets joined by FSSW at different rotational speeds typically used in industry. Experimental techniques such as the small punch test (SPT), Vickers hardness test, Scanning Electron Microscopy (SEM), Scanning Acoustic Microscope (SAM), Transmission Electron Microscopy (TEM), Energy-dispersive X-ray spectroscopy (EDX) and Frequency-Modulated Kelvin Probe Force Microscopy (FM-KPFM) were employed. The experimental results revealed that a complex interplay of stress-assisted metallurgical transformations at the intersection of WFI and the recrystallised stir zone (RSZ) can trigger dynamic precipitation leading to the formation of Al3Mg2 intermetallic phase, while metallic oxides and nanopits remain entrapped in the WFI. These metallurgical transformations surrounded by pits, precipitates and oxides induces process instability which in turn paves way for fast fracture to become responsible for delamination.
Highlights
Weight reduction has become a key driver in the transport sector for improving fuel efficiency and thereby to achieve sustainability
One fundamental problem hitherto unaddressed in the literature about Friction stir spot welding (FSSW) is the interfacial delamination of the weld interfaces akin to how a coating delaminates from a substrate
The following broad conclusions were summarised based on the discussions: (i) Two distinct weld stages, namely stick-slip and steady-state welding were observed during FSSW
Summary
Weight reduction has become a key driver in the transport sector for improving fuel efficiency and thereby to achieve sustainability. Gener ally, a weight reduction of about 10% of an automobile can reduce its specific fuel consumption by 3–7% and CO2 emissions by 9 g/km [1]. Friction stir spot welding (FSSW) is emerging as a promising technology for solid-state joining of weld battery components, strand-terminal con nectors and terminals.
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